Investigation of Surface Morphology Evolution During the Borosilicate Glass Etching By Using ICP-RIE For Microfluidics Applications

Investigation of Surface Morphology Evolution During the Borosilicate Glass Etching By Using ICP-RIE For Microfluidics Applications | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Investigation of Surface Morphology Evolution During the Borosilicate Glass Etching By Using ICP-RIE For Microfluidics Applications Duygu NUHOĞLU, Meryem SARIGÜZEL, Cihat TAŞALTIN, Ilke GÜROL, Esra ZAYIM This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7923195/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Dry etching techniques are extensively used in the fabrication of silicon-based electronic components; however, the increasing use of glass substrates in microfluidic applications, due to their chemical stability, biocompatibility, optical transparency, electrical insulation, and cost-effectiveness, has created a growing demand for efficient dry etching of glass. This work presents the etching of borosilicate glasses using inductively coupled plasma reactive ion etching (ICP-RIE) with fluorine-based gas chemistries for use in microfluidic applications. Microfluidic structures were patterned via photolithography technic with micropillar were patterned using a titanium/gold stack and a nickel hard mask. The effects of various gas mixtures on the etch rate and surface morphology were systematically investigated. Optical emission spectroscopy (OES) was employed to monitor plasma characteristics, while surface analyses and etch depth were carried out using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and profilometry, respectively. The results highlight how different gas compositions influence etch behavior and surface morphology, offering valuable insight for optimizing dry etching processes in glass-based microfluidic device fabrication and emphasize the importance of selecting appropriate gas ratios to achieve uniform microstructures with minimal surface defects. Borofloat etching Plasma etching C4F8 plasma Optical emission spectroscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction One of the most important steps in Micro-Electro-Mechanical System (MEMS) fabrication is choosing the substrate. Although silicon wafers remain of great importance in MEMS due to their crystalline structure and compatibility with established processes, glass substrates offer distinct advantages that make them increasingly attractive for advanced applications. The chemical and thermal stability of glass supports durability in aggressive environments, and large-area availability provides scalability for applications such as lab-on-chip platforms. Furthermore, glass enhances biomedical performance in microfluidics owing to its inherent optical transparency, excellent electrical insulation, and biocompatibility, allowing for real-time examination. Within microfluidic systems, particle manipulation and separation can be achieved through specialized designs. For instance, deterministic lateral displacement (DLD) designs separate particles based on size, shape, and fluid flow, using microchannels filled with a regular array of micropillar obstacles [ 1 ]. The optical transparency of the glass further facilitates accurate detection of particle separation. Taken together, these properties highlight the clear advantages of glass as a MEMS substrate, enabling functionalities that extend beyond those offered by traditional substrates. Another important step is etching, which has become a critical technique for high-resolution MEMS fabrication in hard materials, including silicon and glass [ 2 ]. Besides the advantages, the etching of glass is more complex than that of silicon due to its amorphous structure. Wet etching methods, such as those based on potassium hydroxide [ 3 ], provide limited capability for production high-aspect-ratio features, which are critical in many microfabrication applications. In contrast, dry etching enables directional etching and improved control over feature dimensions [ 4 ], but it requires careful optimization of plasma chemistry, process parameters, surface morphology, and masking materials for successful application. Even though dry etching processes have been predominantly developed and optimized based on silicon, their adaptation to glass poses distinct challenges, as glass remains relatively underexplored and demands further technological advancement. Conventional Reactive Ion Etching (RIE) processes, one of the dry etching techniques, the substrate power and coil power are coupled to each other often resulting in etch non-uniformity across the substrate. However, in an Inductively Coupled Plasma (ICP) RIE system, the substrate power and the coil power are independent of each other. This feature allows outstanding regulation over plasma density (controlled by coil power), energy of etchant ions (controlled by substrate power) and generates stable plasma under low pressures necessary for removal of etching byproducts. The ICP RIE has separate power sources for the generation of radicals and ions (ICP – coil power) as plasma form and the acceleration of the ions (RF bias – substrate power), enabling them to etch a wide range of materials, including silicon, glass, and LiNbO 3 , etc. During these processes, plasma is typically generated from fluorine-containing gases such as octafluorocyclobutane (C4F8) and sulfur hexafluoride (SF6) [ 5 ]. The C₄F₈ gas dissociates in the plasma phase like CFₓ to produce fluorocarbon species, which can deposit on the surface as Teflon-like polymer layers, providing sidewall protection during etching. Furthermore, the incorporation of Ar and O₂ serves to enhance physical sputtering and control surface polymerization, thereby improving etch selectivity [ 6 ], [ 7 ]. In fluor-based plasma etching, maintaining a balance between ion-induced erosion and surface passivation is critical to achieving uniform etching. Excessive ion bombardment can cause to sputter-effect-induced erosion, leading to surface roughening and material redeposition, while insufficient ion energy limits etch efficiency. A well-balanced condition enables steady etching by sustaining the interplay between sputter-effect-induced erosion and chemical reactions, ensuring uniform and reproducible surface morphology. The ICP RIE of especially on glass, contains a combination of physical ion bombardment by ions and chemical reactions driven by fluorine radicals. Because amorphous materials like borosilicate glass, composed mainly of boron, sodium, aluminum, and potassium oxides, exhibit strong chemical bonds and low sputter reveal, high-energy ions are required to initiate material removal. Schematic illustration of plasma-assisted glass etching with ICP RIE is given in Fig. 1 . The application of RF bias power over plasma accelerates positively charged ions toward the substrate, enhancing anisotropic etching. Fluorine radicals react with substrate constituents to form volatile halogen compounds such as SiF₄ and BF₃, which are evacuated from the chamber, easily removed from the surface. However, stray particles, non-volatile halogen products like AlF₃, NaF, and KF which are physically sputtered from the surface [ 8 ], and incomplete reactions may also lead to surface contamination or etch profile irregularities. Failure to remove these non-volatile products can lead to coat the etching equipment or redeposit on the surface being etched by causing micro-masking and significantly reducing the etch rate [ 9 ]. Therefore effective control of plasma parameters and gas composition is critical for achieving clean, anisotropic etch profiles in glass substrate. To achieve precise and deep glass etching, the use of a sacrificial metal layer as a hard mask is vital, as it provides the necessary selectivity and structural integrity during the etching process. Chromium or nickel (Ni) thin films have been used as hard mask, because the polymeric thin films like photoresist has not enough selectivity to etch tens of microns of glass surface. Further, high aspect ratio devices requiring deep plateaus (> 10 µm) demand thick Ni hard masks for high-selectivity etching. E-beam evaporated Ni films exhibit significant strain and adhesion issues, particularly at thicknesses above 200 nm thickness, with resulting delamination from the device [ 10 ]. To address these challenges, electroplating provides a low-stress alternative, enabling thick, vertical Ni masks with sufficient selectivity for energetic ion etching, preserving surface integrity for further processing [ 10 ]. While the optimization of hard mask materials, such as electroplated Ni, is necessary to achieve high-selectivity and deep glass etching, another critical consideration lies in the surface-related challenges that can significantly influence etch performance and device reliability. The surface damage can be categorized as; a) deposited teflon-like fluorocarbon top passivation layer, b) roughening of the substrate surface, c) re-deposited of etched products like non-volatile substances (micro-masking), d) lattice distortion due to sputter effect of etching. It was known that surface damages categorized as c) and d), called this phenomenon plasma-induced surface damage (PISD). PISD reduces the etch rate of glass since rough surface and micro-masking effect, also it is very difficult to remove these residues on a rough surface [ 11 ]. The etching process is a mixture between physical bombardment and chemical reaction, and the relationship between the increase of PISD formation and the dominance of physical etching has been established. Given the complexity of PISD and its impact on etch performance, in-situ diagnostic techniques become key insight into plasma–surface interactions. Among these, Optical Emission Spectroscopy (OES) has emerged as a widely used for plasma-based etching processes due to its simplicity, non-intrusive nature, and capability for real-time monitoring [ 5 ]. The technique relies on detecting the spectrum of radiation emitted by excited species after energy absorption, known as the emission spectrum. By analyzing these emissions, OES provides valuable information on chamber situation, plasma composition, and surface reaction byproducts. OES captures signals from plasma neutrals such as free radicals, owing to their relatively low excitation thresholds compared to ions for understanding and controlling plasma-assisted etching [ 12 ]. In this work, borofloat glass substrates patterned with microfluidic column structures were etched using ICP-RIE with various SF₆, C₄F₈, Ar, and O₂ gas mixtures. Real-time OES was employed to monitor plasma species, while SEM, EDS, profilometry, and optical microscopy were used to analyze surface morphology, confirm PISD formation, and relate it to changes in surface morphology and etch rate. The aim of this study is to elucidate the relationship between plasma composition, etch behavior, and PISD formation, providing insights for the optimization of glass etching processes. The results show that gas chemistry strongly affects etch rate, surface morphology, and residue formation, revealing the mechanisms of PISD and its impact on glass processing. These insights provide practical guidelines for optimizing glass etching, enabling more reliable fabrication of microfluidic devices. It was found that the effective control of plasma gas composition was crucial for achieving clean, deep and stable etch profiles in glass substrates. 2. Materials And Methods 2.1 E xperimental Setup A microfluidic system pattern including obstacles like a micropillar-shaped was designed using Tanner Tools L-Edit drawing program. The microfluidic structure was presented in Fig. 2 . The DLD microchannel was designed using three sequential segments of periodic cylindrical micropillar arrays with diameters of 30, 20, and 10 µm, respectively. This configuration, known as a cascade array [ 13 ], enables stepwise variation of the critical diameter along the flow path. Each array is tilted with respect to the main flow direction so that particles larger than the segment-specific critical diameter (Dc) are forced into the displacement mode, while smaller particles follow the streamlines. The progressive reduction in micropillar diameter and inter-post spacing produces a stepwise decrease in Dc along the channel, providing a coarse-to-fine separation cascade that enhances resolution for progressively smaller particles as they travel downstream. Borofloat33 glass substrates (0.7 cm thickness, composition 81% SiO2, 13% B2O3, 2% Al, 4% Na2O/K2O) were purchased from SCHOTT Glass, were patterned by photolithography using a high-resolution laser lithography system (DWL66 + by Heidelberg Instruments, with 10 mm write head mode). The positive tone and 3.3 µm thick Az4533 UV-sensitive photoresist (PR) was used, developed after exposing with Az400K developer, which was supplied from MicroChemicals. Metal layers were coated with the physical vapor deposition technique with the Nanovak NVEB-600 electron beam evaporation system. The sacrificial metal layers, such as titanium (Ti) and gold (Au), were wet etched with TiW etch 200 and TechniEtchTM ACl2, respectively (MicroChemicals). Ni, which is used as a protective mask in etching materials such as glass, is coated with the electrodeposition technique, and the content of the bath used contains Ni sulphate (NiSO4.6H2), Ni chloride (NiCl2.6H2O) and boric acid (H3BO3) was purchased from SigmaAldrich [ 14 ]. Plasma etching was carried out by varying the process parameters such as gas chemistries, gas flow ratio, process pressure under by keeping the constant top and bottom electrode powers. An Oxford Cobra100 ICP-RIE system was used to etch a 10-20-30 µm diameter micropillar-shaped microfluidic pattern on glass substrate. Plasma generation was controlled by a 13.56 MHz top electrode power. Further, the bottom electrode power was supplied by a 4 MHz RF generator to induce the negative DC self-bias voltage. The substrate backside temperature of the wafer chuck during etching was held constant temperature by circulating the cooling ethylene glycol using a chiller. The etching chamber was evacuated by using a large capacity turbo molecular pump during the process which makes it possible to reduce collision between reaction products and the surface of the substrate. Also, owing to this characteristic of the etcher, it is possible to etch materials like glass which produce nonvolatile reaction products. The oxygen cleaning protocol was performed between experiments to remove possible polymer deposition off the sidewalls of the plasma reactor chamber for 10 minutes, minimize contamination, and preserve repeatability from run to run. Furthermore, OES was carried out to investigate the species present in the plasma composition. A Verity Instrument (Model SD1024GH) spectrometer was used to record optical response from 300 nm to 800 nm with a low resolution (0.5 nm) every 0.5 seconds. Spectra View™ was used to post-process the collected data. The spectrometer was connected to the ICP RIE reactor chamber through an optical fiber cable attached to an external viewing port along the reactor wall, schematic illustration of the system was given in Fig. 3 . After completing of ICP RIE process, the etching depths of the substrates were measured using a Bruker DektakXT stylus profilometer. The surface morphology was initially examined with an optical microscope, followed by detailed images of scanning electron microscopy (SEM) by using FEG 250 Field Emission. Elemental analyses, Energy Dispersive X-Ray Spectroscopy (EDS), of the island-like structures observed on the surface were conducted via JEOL JSM-6510 LV - Oxford Instruments INCA-EDS. 2.2 Sample Preparation The detailed process flow of microfluidic fabrication on glass was shown in Fig. 4 . Substrates were cleaned with Isopropyl Alcohol (IPA), acetone and, rinsed with deionized water respectively, then dried with nitrogen gas flow. First, a stack of 50 nm Ti, 50 nm Au layers was deposited onto substrates, using electron beam evaporation (Fig. 4 : a)). Subsequently, PR layer was deposited on the metal layer via spin-coating and structured with maskless photolithography to open windows for the fabrication of a microfluidic pattern (Fig. 4 : b)). The PR application and exposure parameters were given in Table 1 . Table 1 The PR application and exposure parameters PR Application Parameters Exposure Parameters Dehydration 150 o C – 1 minutes Write head 10 mm Hexamethyldisilane (HMDS) vapor treatment 40 seconds Write laser 355 nm, Diod Laser Power 65 mW Spin coating 3000 rpm – 1 minutes Laser Intensity 80% Prebake 100 o C – 1 minutes Exposure Develop 1:3 Az400K – 1 minutes The Ti sub-layer coating ensured good adhesion between the glass and Au film, and Au film served as a seed layer for the electroplating of Ni for the next steps. The metal-coated and PR patterned glass substrates were used as the working electrode, Ni plate anode as a counter electrode into the Ni chloride-based electrodeposition bath under 10.25 mA current. The thickness of the Ni hard mask was kept at 4 µm for high selective etching and deposited on the PR patterned substrates (Fig. 4 : c)). At the end of processes, remained PR was cleaned with acetone; Ti and, Au was removed by wet etching in areas outside of Ni (Fig. 4 : d), e)), respectively. The blue region shown in Fig. 2 represents the area that will remain unetched. Hence, the final layers illustrated in Fig. 4 :e) serve as a protective coating against ICP-RIE during the patterning of glass microstructures. The schematic representation of the desired microfluidic structure shown in Fig. 2 , obtained after etching under different recipes, is presented in Fig. 4 : f). Figure 5 presents the real sample images of prepared microfluidic structure on the borosilicate glass substrates, whose design and fabrication were completed, taken under laboratory conditions prior to the etching process. The core objective of this study was to systematically investigate the influence of different plasma compositions on glass etching; therefore, the ICP-RIE chamber parameters summarized in Table 2 were employed to develop and compare four distinct gas mixtures, designated as Recipes A, B, C, and D. ICP power at 2000 W, table temperature (chuck temperature) at 20 O C, helium “back-side” pressure at 1.33 Pa, and RF bias power at 300 W was kept constant to eliminate any other variables. The experimental study began with Recipe A, which was optimized to establish the baseline etching conditions. To examine the combined effect of C₄F₈ and O₂, Recipe B was then introduced. Subsequently, Recipe C was designed by further increasing the C₄F₈ flow rate in order to isolate its influence. Table 2 ICP RIE process parameters for glass etching ID Gas Flow Rates (sccm) RF bias (W) ICP Power (W) Pressure (Pa) SF 6 C 4 F 8 Ar O 2 A 8 - 50 - 300 2000 0.667 (5 mTorr) B 8 5 50 50 300 2000 0.667 (5 mTorr) C 8 25 50 50 300 2000 0.667 (5 mTorr) D - 40 - 20 300 2000 1.33 (10 mTorr) As shown in Table 2 , in Recipes A–C, all contain Ar gas, which contributes to plasma formation, while in Recipe D, the removal of Ar resulted in unstable discharge at 5 mTorr. Argon, as an inert noble gas, possesses abundant metastable states that enhance electron-impact ionization and facilitate plasma ignition even under low-pressure conditions. The presence of Ar ensures a high density of energetic metastable atoms, which play a crucial role in sustaining the discharge through stepwise ionization and secondary electron emission at the cathode [ 15 ], thereby lowering the effective breakdown voltage and stabilizing the plasma at reduced pressures. In contrast, oxygen is an electronegative gas: it readily captures free electrons to form negative ions, which reduces the population of free electrons available to sustain the avalanche ionization. This effect leads to higher breakdown voltages, lower electron densities, and more difficulty in maintaining plasma [ 16 ]. This electron loss increases the effective breakdown voltage and reduces plasma stability at low pressures [ 16 ]. Consequently, in Recipe D, the absence of Ar metastables and the electron attachment behavior of O₂ resulted in insufficient plasma generation at 5 mTorr. To overcome this, the chamber pressure was raised to 10 mTorr to reduce the required breakdown voltage and enable stable plasma ignition. Hence, the removal of argon from the recipes was intended to deliberately reduce ion bombardment and thereby minimize the formation of plasma-induced surface damage (PISD). 3. Results and Discussion 3.1 Surface Morphology and Compositional Analysis Optical microscope images of the glass surfaces etched for 60 minutes with different plasma recipes were captured at a magnification of 20x and were presented in Fig. 5 . The images show that plasma interaction significantly damages the surface, with the damage’s extent and shape varying with the gas compositions. In Recipe A (SF₆ + Ar), the presence of Ar promotes a sputter-effect-like process, which causes PISD, Fig. 6 : a). When O 2 and C₄F₈ were added to the initial recipe to form Recipe B (SF₆ + Ar + O 2 + low C₄F₈), dark, scattered crater-like features were observed, as shown in Fig. 6 : b). Using Recipe C (SF₆ + Ar + O 2 + high C₄F₈), in which the amount of C₄F₈ is increased, the microscope images revealed a more densely populated with overlapping structures Fig. 6 : c). In particular, dark and scattered spot regions suggest particle redeposition effects, whereas under conditions such as Recipe D (O 2 + higher C₄F₈) (Fig. 6 : d), the surface appears more uniformly modified with reduced large-scale defect formation. These observations suggest that the plasma–surface interaction, including ion bombardment, chemical etching, and redeposition phenomena, strongly depends on the process chemistry, with specific gas composition significantly influencing PISD formation. SEM images of the glass surface after 60 minutes with different plasma recipes were shown in Fig. 7 . Spherical to irregular particle-like agglomerates scattered across the surface were observed in Fig. 7 : a). This feature suggests redeposition of the etched material caused by sputter-effect-induced erosion. Such behavior is attributed to the high Ar concentration in Recipe A, which enhances the sputtering effect during etching. In Fig. 7 : b), crater-like features observed in optical microscopy (Fig. 6 : b)) were also confirmed by SEM analysis. Unlike Recipe A, where the high Ar content induced strong sputter-effect-induced erosion and noticeable material redeposition, the addition of C₄F₈ and O₂, forming Recipe B, noticeably suppressed the etching process and modified the plasma chemistry. In particular, it was known that the introduced oxygen radicals participated in the surface oxidation of the substrate, resulting in the formation of a passivating SiOₓF γ layer that further altered the near-surface reactions and contributed to etch suppression [ 17 ]. So that, the introduction of O₂ reduced the density of reactive F radicals responsible for material removal, thereby diminishing the overall etch rate. Hence, etching was limited, and the surface became passivated rather than actively etched despite continuous plasma exposure. Consequently, when using an O 2 containing gas mixture for etching, it is important to choose the optimal O/F ratio, which, for the reasons mentioned above, affects both the etching rate and etched structures. Etching was repeated with Recipe C, where only the C₄F₈ concentration was increased while the other gases were kept constant and SEM images was given in Fig. 7 : c). Recipe C led to pit structures, also caused significant sidewall polymerization with increased C₄F₈ content, forming a Teflon-like top passivation layer, as mentioned in the introduction, which is one of the factors responsible for PISD formation. This behavior indicates that the additional C₄F₈ enhanced the generation of CFₓ fragments and free F radicals, restoring the balance between surface passivation and etching. As a result, the plasma environment shifted back toward an etching dominant regime rather than surface passivation, enabling material removal under controlled conditions. SEM images of the glass samples etched with Recipe D are presented, where the C₄F₈ concentration was further increased, the O₂ content was reduced, and both Ar and SF₆ were removed from the gas mixture, In Fig. 7 (d). Under these conditions, the etching process produced a smooth and uniform surface without noticeable defects. The elimination of Ar suppressed sputter-effect-induced erosion that could lead to over-etching or micro-masking. The increased amount of C₄F₈ facilitated the generation of CFₓ fragments, promoting a stable balance between surface passivation and chemical etching. By increment of C₄F₈, the plasma environment became more chemically controlled, allowing steady material removal and yielding a highly uniform etched morphology. Table 3 EDS analysis of the glass surface after 60 minutes of etching Element Atomic % Recipe A Recipe B Recipe C Recipe D O 49.17 36.70 25.72 27.58 Si 25.41 23.88 19.48 24.63 Na 8.01 9.43 14.55 10.33 Al 1.58 1.44 4.12 3.89 K 0.49 0.49 0.32 1.13 F 11.94 27.18 32.49 32.44 Cl 3.39 0.89 0.48 - Ni - - 2.84 - The quantitative EDS analysis (Table 3 ) clarified that the surface components were highly dependent on plasma compositions, correlating with the observed morphological changes under different plasma chemistries. In addition to the substrate-related elements (Na, K, and Al) originating from the glass matrix, the surface also exhibited notable amounts of fluorine (F), chlorine (Cl), and Ni, indicating the effects of plasma-induced reactions and material redeposition. In Recipe A, the agglomerated regions observed in SEM corresponded to localized enrichment of F and Cl species. The detection of Cl is attributed to residues from the chloride-based electrolyte used during the electrodeposition of the Ni hard mask, which may remain as adsorbed or entrapped salts on the surface. The persistence of Cl signals in Recipes A–C suggests incomplete removal of these chloride species, which could remain attached to the surface or embedded within redeposited particulates. With the introduction of O₂ and a small amount of C₄F₈, Recipe B, fluorine deposition on the surface increased significantly, while oxygen content decreased. This compositional shift indicates the formation of a fluorocarbon-rich surface layer, characteristic of a polymerization-dominant regime. The simultaneous reduction in Cl content implies that the modified plasma chemistry partially suppressed chlorine-based redeposition. In Recipe C, the fluorine concentration reached its maximum ratio, and oxygen dropped to its minimum, suggesting the dominance of fluorination and polymer accumulation. The detection of Ni is associated with the sputtering and redeposition of the Ni hard mask, while the SEM-observed spherical-like structures and the high F content confirm the presence of a Teflon-like fluorocarbon polymer layer. This layer plays a crucial role in sidewall protection and surface passivation during plasma exposure. In contrast, Recipe D exhibited no detectable Cl or Ni signals, while maintaining a high F content. This finding suggests that the C₄F₈-rich plasma facilitated effectively encapsulating and removing residual metallic and halogen contaminants. Overall, the EDS results demonstrate a clear transition from oxide-dominated and chloride contaminated surfaces in Recipe A to fluorocarbon-passivated and contamination free surfaces in Recipe D. These compositional trends, particularly the evolution of the O/F ratio and the presence of polymeric fluorocarbon phases, are consistent with the morphological evidence of a shift from etching- to polymerization-dominant plasma regimes. The glass substrates were subjected to plasma etching under the specified process conditions, and the resulting etch depths were measured using a profilometer. The etch rates were then calculated from the ratio of etch depth to etch time (etch depth / etch time). Figure 8 presents the variation of etch rate as a function of etch time for the different plasma recipes, and the etching depths measured as a result of etching was written in the graph. The etch behavior observed in this figure is consistent with the morphological and compositional changes identified by SEM and EDS analyses, highlighting the evolution of the plasma–surface interaction regime with changing gas chemistry. In Recipe A, the etch rate decreased markedly with time, reaching an etch depth of only 24.8 µm after 120 min. SEM images revealed island-like surface features and particle redeposition, while EDS analysis confirmed the presence of Cl residues and relatively low F incorporation. These findings suggest that the etching process was limited by redeposition of non-volatile species and contaminants formed during sputter-effect-induced erosion. With the addition of O₂ and a small amount of C₄F₈, Recipe B, the etch rate decreased even further compared with Recipe A during 90 min, 19.1 µm. The suppression of etching was attributed to oxygen-assisted polymerization, leading to surface passivation rather than material removal. Further increasing the C₄F₈ concentration, Recipe C, reinitiated etching and enhanced the overall etch depth to 40.09 µm after 150 min. The simultaneous detection of Ni and Cl peaks indicates redeposition of sputtered Ni from the hard mask and residual halogen species, caused in reduction of etch rate over time during etching with Recipe C. In Recipe D, where Ar and SF₆ were removed while maintaining high C₄F₈ and O₂ concentrations, the etch rate stabilized at the highest value among all recipes, reaching a total etch depth of 50.25 µm in 150 min. SEM observations confirmed a smoother and more uniform surface morphology with smaller pit structures, and EDS analysis showed the absence of Cl and Ni residues. These results indicate that optimized C₄F₈/O₂ mixtures effectively suppress contamination, prevent micro-masking, chemically controlled etching with improved surface quality and maximum etch depth. Among all investigated conditions, Recipe D demonstrated the most favorable etching performance, characterized by smooth surface, and impurity-free morphology, making the etched micropillar structures promising candidates for use in microfluidic platform applications. Finally, cross-sectional SEM images of the micropillar area on the sample that were etched for 150 minutes using Recipe D and obtained an etching depth of 50.25 µm were presented in Fig. 9 . The etching process was carried out at relatively high ICP power level as given in Table 2 (2000 W) to ensure stable plasma generation and maintain a sufficient ion flux for deep glass etching. Under such energetic conditions, partial erosion of the Ni hard mask was inevitable. Nevertheless, the selected gas chemistry effectively suppressed the formation of micro-masks and redeposited particles on the surface, maintaining smooth feature profiles throughout the process. The measured etch depth of approximately 47.6 µm was in good agreement with the profilometry data presented in Fig. 8 . The micropillar array exhibits well-defined sidewalls and a uniform height distribution, indicating efficient by-product evacuation and controlled surface passivation. However, the slight rounding and surface degradation observed near the top regions of the pillars can be attributed to localized glass impurities and partial PISD. The imperfections which revealed minor compositional variations and the presence of fluorine-rich residues associated with localized polymer deposition and redeposition phenomena. The optimized C₄F₈/O₂ mixture promoted the formation of a thin, uniform fluorocarbon film that provided sidewall protection while allowing sufficient ion-assisted removal of surface polymers. This equilibrium between passivation and etching minimized surface roughening and prevented excessive PISD and underlying glass. Furthermore, in a detailed focus on upper region of the micropillars, the erosion of the Ni hard mask was obvious. This observation could be explained as table temperature fluctuation of the surface. So that the table temperature was identified as a critical factor influencing plasma–surface interactions. As indicated in Table 2 , the process operated under high ICP powers, and despite the use of auxiliary stabilization systems (e.g., pressure and temperature control), minor table temperature fluctuations were unpreventable. These variations locally modified the reaction kinetics and polymer desorption rates, contributing to the slight non-uniformities observed in the upper regions of the micropillars. Overall, the etch rate results are consistent with the SEM and EDS analyses, confirming that uniform glass etching is achieved when plasma parameters and gas composition are optimized to balance polymer formation and ion bombardment, as demonstrated by Recipe D. 4.2 Optical Emission Spectroscopy Analysis The OES spectra in Fig. 9 show the emission lines corresponding to different gas compositions employed during ICP-RIE of borosilicate glass. These spectra were obtained under 500 W ICP power, 0 W RF bias power and 1.33 Pa pressure. Although the etching processes were carried out at higher powers (2000 W ICP and 300 W RF bias power), the significantly increased signal magnitudes at such conditions hindered reliable detection and interpretation of plasma recipe [ 18 ]. The spectra presented in Fig. 10 indicated that the optical activity varied significantly with the applied gas chemistry. In Recipe A, the process chemistry supplied fluorine from SF₆ for the formation of volatile compounds such as SiF₄ and BF₃, while Ar provided energetic ions responsible for physical sputtering, and the observed peaks around 419–421 nm corresponding to S₂ [ 19 ] and F₂ [ 19 ] confirm the dissociation of SF₆. In addition, the spectra were dominated by intense Ar emission lines at 750.5 [ 20 ] − 763.5 nm [ 12 ] and relatively weak fluorine peaks at 685.9 [ 12 ] − 704 nm [ 20 ] indicating that the plasma was primarily sustained by Ar excitation with a limited population of reactive F radicals. The high Ar fraction, however, was promoted sputter-dominant etching behavior, which led to micro-masking and reduced efficiency. This interpretation is consistent with the SEM observations showing agglomerated surface regions and with the EDS results, which confirmed Cl redeposition originating from the Ni electroplating bath of the hard mask. Consequently, Recipe A operated in a sputtering-dominant regime, where physical ion bombardment prevailed over chemical etching and strong Ar peaks were evidence of this behavior. In Recipe B, the addition of O₂ and a small amount of C₄F₈, altered the plasma chemistry; the F emission lines became weaker which was explained as consumption of F ions by O 2 component [ 17 ]. Simultaneously, the emergence of a C₂ emission at 388.05 nm [ 19 ] confirmed the onset of fluorocarbon fragmentation and polymer formation, which supports the SEM results. In Recipe C, further increasing the C₄F₈ concentration enhanced both the etch rate and the polymerization tendency. The spectrum showed emission bands of C₂ became more prominent, yet the fluorine emission lines remained partially suppressed. The SEM images showed Teflon-like fluorocarbon structures, and EDS detected a high F concentration, together indicating a polymerization-dominant regime. However, uncontrolled polymer buildup eventually limited the etch rate and contributed to localized etch suppression and mask redeposition, as evidenced by Ni and Cl signals in EDS. To eliminate Ar-related micro-masking observed in Recipes A, B, and C, Ar and SF₆ were removed and the plasma consisted solely of a C₄F₈/O₂ mixture, Recipe D. The absence of Ar emission lines in the spectrum allowed clear detection of molecular bands corresponding to O₂ and C, C 2 , C₃ emissions around 387–516 nm peaks [ 19 ]. The coexistence of carbon, oxygen, and weak fluorine emissions is consistent with the formation of a thin, uniform fluorocarbon polymer layer that provided effective sidewall passivation without excessive buildup. SEM observations revealed smooth micropillar structures with minimal surface damage and the highest etch depth (≈ 47.6 µm). EDS analysis confirmed the absence of Ni and Cl contamination, verifying that the C₄F₈/O₂ plasma established a balanced regime between sputtering-induced erosion and ion-assisted removal with polymer deposition. 4. Conclusion Systematic investigation of the optical microscopy, SEM, EDS, and OES results indicated a strong correlation between surface morphology, etching performance, and plasma composition during borosilicate glass etching. OES provided complementary insights into the plasma–surface dynamics. In Recipes A–C, strong Ar emission lines inhibition the weak F peaks, confirming a sputter-dominant regime characterized by limited chemical activity and significant redeposition, which was also evidenced by Cl and F-rich residues in the EDS results. The addition of O₂ and C₄F₈ in Recipe B altered the plasma chemistry but remained constrained by strong Ar excitation, resulting in PISD formation and micro-masking effects. Although increasing the C₄F₈ content in Recipe C improved etch rate, it also intensified polymer formation and partial mask redeposition, as reflected by pronounced Ar peaks in OES. In contrast, Recipe D, which eliminated Ar and SF₆, exhibited balanced C₂, C₃, and O₂ emissions, resulting a chemically active plasma where polymerization and etching processes coexisted in equilibrium. This optimized condition yielded the smoothest surface morphology with achieving the most stable and highest etch rate among all conditions, the highest etch depth, and contamination-free results. Beyond process optimization, this work provides a unique integrated understanding of how plasma diagnostics and surface analyses converge to explain etch behavior and PISD formation. The correlation between OES and surface EDS result clarifies that Cl and Ni related residues originate under Ar rich plasmas, whereas C₄F₈/O₂ mixtures minimization such contamination by establishing a carbon-rich environment that enables self-limiting polymer passivation. This study is among the pioneering works that systematically efforts to directly correlate in-situ OES plasma diagnostics with etched surface EDS analysis to uncover the underlying mechanisms of PISD formation and suppression during glass etching. This framework elucidates the plasma–surface interactions responsible for redeposition and etch-stop phenomena, providing a comprehensive basis for developing clear surfaces, contamination-free ICP-RIE processes in future microfabrication technologies. Declarations Author Contribution Authors’ Contribution All authors reviewed the manuscript. All authors contributed to the discussion and gave helpful feedback. CT, IG: Experiments planning and conduction; DN, EZ: Main text preparation; DN, CT, MS: Photolithography, mask deposition, ICP-RIE process; DN, CT: SEM investigation; DN: Sample profilometry; DN: OES investigation; DN: Literature investigation, the representation of the results and feedback; CT: Head of the research group, consulting, and discussion. Acknowledgments The authors would like to thank Şirin Say and Şeyma Soykan for their assistance with sample preparation in the laboratory. The authors gratefully acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) due to the financial support for the Project 22AG030 and 120N816. References Davis JA, Inglis DW, Morton KJ, Lawrence DA, Huang LR, Chou SY, Sturm JC, Austin RH. (2006) Deterministic hydrodynamics: Taking blood apart. Proc Natl Acad Sci ;103:14779–84. https://doi.org/10.1073/pnas.0605967103. Choi SS, Kim D, Park MJ. (2004) Fabrication of Double Aperture for Nearfield Optical Trapping. 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1","display":"","copyAsset":false,"role":"figure","size":73542,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of plasma-assisted glass etching with ICP RIE\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/f0ea3784d1ddde09c6054961.png"},{"id":96919605,"identity":"5f8929fd-5c9d-4d1e-a4fe-d2b6314804e6","added_by":"auto","created_at":"2025-11-27 14:14:11","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":107537,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the microfluidic design\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/899de28fc7e446246f5ba38a.png"},{"id":96897334,"identity":"db026a4f-c3f0-4ec9-bec9-cc416235572d","added_by":"auto","created_at":"2025-11-27 10:36:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":83338,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the ICP RIE and spectrometer system\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/385b372dc3645ea1b3a5e0f3.png"},{"id":96920332,"identity":"4cad85df-afea-4e5a-a64a-a3c1f95a92ab","added_by":"auto","created_at":"2025-11-27 14:15:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":55875,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of sample preparation and etching. a) Ti/Au multilayer deposition on cleaned borosilicate glass, b) Photoresist coating and patterning, c) Electrodeposition of Ni layer, d) Remove photoresist by acetone, e) Removing of sacrificial metal layers by etchant, f) Etching of borosilicate glass using ICP-RIE of glass\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/650810f46cfe4f61701afff3.png"},{"id":96919613,"identity":"4b5dbb91-9d96-454c-9d56-b4f212e0cb3b","added_by":"auto","created_at":"2025-11-27 14:14:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":13571,"visible":true,"origin":"","legend":"\u003cp\u003eReal sample image of prepared microfluidic structure\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/251f9b5d2329fdea8124d75b.png"},{"id":96897344,"identity":"03820283-3384-4d65-94fc-66caef6bbc5e","added_by":"auto","created_at":"2025-11-27 10:36:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":163009,"visible":true,"origin":"","legend":"\u003cp\u003eOptical microscope images of the glass surfaces after 60 minutes of etching with Recipe \u003cstrong\u003ea)\u003c/strong\u003e A, \u003cstrong\u003eb)\u003c/strong\u003e B, \u003cstrong\u003ec)\u003c/strong\u003e C, \u003cstrong\u003ed)\u003c/strong\u003e D\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/b1018680e7ca59c0b4f760f3.png"},{"id":96897340,"identity":"a43f3c6d-de5f-42e9-a0c7-3f50bbc9f151","added_by":"auto","created_at":"2025-11-27 10:36:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":313961,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of the glass surface after 60 minutes of etching under Recipe \u003cstrong\u003ea)\u003c/strong\u003e A, \u003cstrong\u003eb)\u003c/strong\u003e B, \u003cstrong\u003ec)\u003c/strong\u003e C, \u003cstrong\u003ed)\u003c/strong\u003e D\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/ce71b40aaccfdc3a84569b93.png"},{"id":96897348,"identity":"8f735a78-e012-4117-9593-98fb85ff82e5","added_by":"auto","created_at":"2025-11-27 10:36:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":77649,"visible":true,"origin":"","legend":"\u003cp\u003eEtch time versus etch depth results of all recipes\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/684458b474402165e8f6a274.png"},{"id":96920546,"identity":"6f334b7e-de4f-4a9e-ba66-ec650ef8028c","added_by":"auto","created_at":"2025-11-27 14:15:15","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":176418,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional SEM image of etched surface with Recipe D\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/d55a8d9f389050eac317ec55.png"},{"id":96897350,"identity":"aa42a715-9116-4ac5-be83-c609958c705a","added_by":"auto","created_at":"2025-11-27 10:36:15","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":281094,"visible":true,"origin":"","legend":"\u003cp\u003eOES of different gas chemistries\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/915f649e3c5de3e15c2b4741.png"},{"id":97144619,"identity":"bfffd84c-2e1e-4103-ad2b-9697773a03e0","added_by":"auto","created_at":"2025-12-01 10:11:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1998690,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7923195/v1/f83fe753-dbc0-4489-a457-48839bd50ddc.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Investigation of Surface Morphology Evolution During the Borosilicate Glass Etching By Using ICP-RIE For Microfluidics Applications","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eOne of the most important steps in Micro-Electro-Mechanical System (MEMS) fabrication is choosing the substrate. Although silicon wafers remain of great importance in MEMS due to their crystalline structure and compatibility with established processes, glass substrates offer distinct advantages that make them increasingly attractive for advanced applications. The chemical and thermal stability of glass supports durability in aggressive environments, and large-area availability provides scalability for applications such as lab-on-chip platforms. Furthermore, glass enhances biomedical performance in microfluidics owing to its inherent optical transparency, excellent electrical insulation, and biocompatibility, allowing for real-time examination. Within microfluidic systems, particle manipulation and separation can be achieved through specialized designs. For instance, deterministic lateral displacement (DLD) designs separate particles based on size, shape, and fluid flow, using microchannels filled with a regular array of micropillar obstacles [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The optical transparency of the glass further facilitates accurate detection of particle separation. Taken together, these properties highlight the clear advantages of glass as a MEMS substrate, enabling functionalities that extend beyond those offered by traditional substrates.\u003c/p\u003e\u003cp\u003eAnother important step is etching, which has become a critical technique for high-resolution MEMS fabrication in hard materials, including silicon and glass [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Besides the advantages, the etching of glass is more complex than that of silicon due to its amorphous structure. Wet etching methods, such as those based on potassium hydroxide [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], provide limited capability for production high-aspect-ratio features, which are critical in many microfabrication applications. In contrast, dry etching enables directional etching and improved control over feature dimensions [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], but it requires careful optimization of plasma chemistry, process parameters, surface morphology, and masking materials for successful application. Even though dry etching processes have been predominantly developed and optimized based on silicon, their adaptation to glass poses distinct challenges, as glass remains relatively underexplored and demands further technological advancement.\u003c/p\u003e\u003cp\u003eConventional Reactive Ion Etching (RIE) processes, one of the dry etching techniques, the substrate power and coil power are coupled to each other often resulting in etch non-uniformity across the substrate. However, in an Inductively Coupled Plasma (ICP) RIE system, the substrate power and the coil power are independent of each other. This feature allows outstanding regulation over plasma density (controlled by coil power), energy of etchant ions (controlled by substrate power) and generates stable plasma under low pressures necessary for removal of etching byproducts. The ICP RIE has separate power sources for the generation of radicals and ions (ICP \u0026ndash; coil power) as plasma form and the acceleration of the ions (RF bias \u0026ndash; substrate power), enabling them to etch a wide range of materials, including silicon, glass, and LiNbO\u003csub\u003e3\u003c/sub\u003e, etc. During these processes, plasma is typically generated from fluorine-containing gases such as octafluorocyclobutane (C4F8) and sulfur hexafluoride (SF6) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The C₄F₈ gas dissociates in the plasma phase like CFₓ to produce fluorocarbon species, which can deposit on the surface as Teflon-like polymer layers, providing sidewall protection during etching. Furthermore, the incorporation of Ar and O₂ serves to enhance physical sputtering and control surface polymerization, thereby improving etch selectivity [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. In fluor-based plasma etching, maintaining a balance between ion-induced erosion and surface passivation is critical to achieving uniform etching. Excessive ion bombardment can cause to sputter-effect-induced erosion, leading to surface roughening and material redeposition, while insufficient ion energy limits etch efficiency. A well-balanced condition enables steady etching by sustaining the interplay between sputter-effect-induced erosion and chemical reactions, ensuring uniform and reproducible surface morphology. The ICP RIE of especially on glass, contains a combination of physical ion bombardment by ions and chemical reactions driven by fluorine radicals. Because amorphous materials like borosilicate glass, composed mainly of boron, sodium, aluminum, and potassium oxides, exhibit strong chemical bonds and low sputter reveal, high-energy ions are required to initiate material removal. Schematic illustration of plasma-assisted glass etching with ICP RIE is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe application of RF bias power over plasma accelerates positively charged ions toward the substrate, enhancing anisotropic etching. Fluorine radicals react with substrate constituents to form volatile halogen compounds such as SiF₄ and BF₃, which are evacuated from the chamber, easily removed from the surface. However, stray particles, non-volatile halogen products like AlF₃, NaF, and KF which are physically sputtered from the surface [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], and incomplete reactions may also lead to surface contamination or etch profile irregularities. Failure to remove these non-volatile products can lead to coat the etching equipment or redeposit on the surface being etched by causing micro-masking and significantly reducing the etch rate [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Therefore effective control of plasma parameters and gas composition is critical for achieving clean, anisotropic etch profiles in glass substrate.\u003c/p\u003e\u003cp\u003eTo achieve precise and deep glass etching, the use of a sacrificial metal layer as a hard mask is vital, as it provides the necessary selectivity and structural integrity during the etching process. Chromium or nickel (Ni) thin films have been used as hard mask, because the polymeric thin films like photoresist has not enough selectivity to etch tens of microns of glass surface. Further, high aspect ratio devices requiring deep plateaus (\u0026gt;\u0026thinsp;10 \u0026micro;m) demand thick Ni hard masks for high-selectivity etching. E-beam evaporated Ni films exhibit significant strain and adhesion issues, particularly at thicknesses above 200 nm thickness, with resulting delamination from the device [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. To address these challenges, electroplating provides a low-stress alternative, enabling thick, vertical Ni masks with sufficient selectivity for energetic ion etching, preserving surface integrity for further processing [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. While the optimization of hard mask materials, such as electroplated Ni, is necessary to achieve high-selectivity and deep glass etching, another critical consideration lies in the surface-related challenges that can significantly influence etch performance and device reliability. The surface damage can be categorized as; a) deposited teflon-like fluorocarbon top passivation layer, b) roughening of the substrate surface, c) re-deposited of etched products like non-volatile substances (micro-masking), d) lattice distortion due to sputter effect of etching. It was known that surface damages categorized as c) and d), called this phenomenon plasma-induced surface damage (PISD). PISD reduces the etch rate of glass since rough surface and micro-masking effect, also it is very difficult to remove these residues on a rough surface [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The etching process is a mixture between physical bombardment and chemical reaction, and the relationship between the increase of PISD formation and the dominance of physical etching has been established. Given the complexity of PISD and its impact on etch performance, in-situ diagnostic techniques become key insight into plasma\u0026ndash;surface interactions. Among these, Optical Emission Spectroscopy (OES) has emerged as a widely used for plasma-based etching processes due to its simplicity, non-intrusive nature, and capability for real-time monitoring [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The technique relies on detecting the spectrum of radiation emitted by excited species after energy absorption, known as the emission spectrum. By analyzing these emissions, OES provides valuable information on chamber situation, plasma composition, and surface reaction byproducts. OES captures signals from plasma neutrals such as free radicals, owing to their relatively low excitation thresholds compared to ions for understanding and controlling plasma-assisted etching [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this work, borofloat glass substrates patterned with microfluidic column structures were etched using ICP-RIE with various SF₆, C₄F₈, Ar, and O₂ gas mixtures. Real-time OES was employed to monitor plasma species, while SEM, EDS, profilometry, and optical microscopy were used to analyze surface morphology, confirm PISD formation, and relate it to changes in surface morphology and etch rate. The aim of this study is to elucidate the relationship between plasma composition, etch behavior, and PISD formation, providing insights for the optimization of glass etching processes. The results show that gas chemistry strongly affects etch rate, surface morphology, and residue formation, revealing the mechanisms of PISD and its impact on glass processing. These insights provide practical guidelines for optimizing glass etching, enabling more reliable fabrication of microfluidic devices. It was found that the effective control of plasma gas composition was crucial for achieving clean, deep and stable etch profiles in glass substrates.\u003c/p\u003e"},{"header":"2. Materials And Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 \u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eE\u003c/span\u003experimental Setup\u003c/h2\u003e\u003cp\u003eA microfluidic system pattern including obstacles like a micropillar-shaped was designed using Tanner Tools L-Edit drawing program. The microfluidic structure was presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The DLD microchannel was designed using three sequential segments of periodic cylindrical micropillar arrays with diameters of 30, 20, and 10 \u0026micro;m, respectively. This configuration, known as a cascade array [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], enables stepwise variation of the critical diameter along the flow path. Each array is tilted with respect to the main flow direction so that particles larger than the segment-specific critical diameter (Dc) are forced into the displacement mode, while smaller particles follow the streamlines. The progressive reduction in micropillar diameter and inter-post spacing produces a stepwise decrease in Dc along the channel, providing a coarse-to-fine separation cascade that enhances resolution for progressively smaller particles as they travel downstream.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBorofloat33 glass substrates (0.7 cm thickness, composition 81% SiO2, 13% B2O3, 2% Al, 4% Na2O/K2O) were purchased from SCHOTT Glass, were patterned by photolithography using a high-resolution laser lithography system (DWL66\u0026thinsp;+\u0026thinsp;by Heidelberg Instruments, with 10 mm write head mode). The positive tone and 3.3 \u0026micro;m thick Az4533 UV-sensitive photoresist (PR) was used, developed after exposing with Az400K developer, which was supplied from MicroChemicals. Metal layers were coated with the physical vapor deposition technique with the Nanovak NVEB-600 electron beam evaporation system. The sacrificial metal layers, such as titanium (Ti) and gold (Au), were wet etched with TiW etch 200 and TechniEtchTM ACl2, respectively (MicroChemicals). Ni, which is used as a protective mask in etching materials such as glass, is coated with the electrodeposition technique, and the content of the bath used contains Ni sulphate (NiSO4.6H2), Ni chloride (NiCl2.6H2O) and boric acid (H3BO3) was purchased from SigmaAldrich [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e\u003cp\u003ePlasma etching was carried out by varying the process parameters such as gas chemistries, gas flow ratio, process pressure under by keeping the constant top and bottom electrode powers. An Oxford Cobra100 ICP-RIE system was used to etch a 10-20-30 \u0026micro;m diameter micropillar-shaped microfluidic pattern on glass substrate. Plasma generation was controlled by a 13.56 MHz top electrode power. Further, the bottom electrode power was supplied by a 4 MHz \u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eRF\u003c/span\u003e generator to induce the negative DC self-bias voltage. The substrate backside temperature of the wafer chuck during etching was held constant temperature by circulating the cooling ethylene glycol using a chiller. The etching chamber was evacuated by using a large capacity turbo molecular pump during the process which makes it possible to reduce collision between reaction products and the surface of the substrate. Also, owing to this characteristic of the etcher, it is possible to etch materials like glass which produce nonvolatile reaction products. The oxygen cleaning protocol was performed between experiments to remove possible polymer deposition off the sidewalls of the plasma reactor chamber for 10 minutes, minimize contamination, and preserve repeatability from run to run.\u003c/p\u003e\u003cp\u003eFurthermore, OES was carried out to investigate the species present in the plasma composition. A Verity Instrument (Model SD1024GH) spectrometer was used to record optical response from 300 nm to 800 nm with a low resolution (0.5 nm) every 0.5 seconds. Spectra View\u0026trade; was used to post-process the collected data. The spectrometer was connected to the ICP RIE reactor chamber through an optical fiber cable attached to an external viewing port along the reactor wall, schematic illustration of the system was given in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAfter completing of ICP RIE process, the etching depths of the substrates were measured using a Bruker DektakXT stylus profilometer. The surface morphology was initially examined with an optical microscope, followed by detailed images of scanning electron microscopy (SEM) by using FEG 250 Field Emission. Elemental analyses, Energy Dispersive X-Ray Spectroscopy (EDS), of the island-like structures observed on the surface were conducted via JEOL JSM-6510 LV - Oxford Instruments INCA-EDS.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Sample Preparation\u003c/h2\u003e\u003cp\u003eThe detailed process flow of microfluidic fabrication on glass was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Substrates were cleaned with Isopropyl Alcohol (IPA), acetone and, rinsed with deionized water respectively, then dried with nitrogen gas flow.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFirst, a stack of 50 nm Ti, 50 nm Au layers was deposited onto substrates, using electron beam evaporation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e: a)). Subsequently, PR layer was deposited on the metal layer via spin-coating and structured with maskless photolithography to open windows for the fabrication of a microfluidic pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e: b)). The PR application and exposure parameters were given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThe PR application and exposure parameters\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003ePR Application Parameters\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003eExposure Parameters\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDehydration\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e150 \u003csup\u003eo\u003c/sup\u003eC \u0026ndash; 1 minutes\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eWrite head\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10 mm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eHexamethyldisilane (HMDS)\u003c/p\u003e\u003cp\u003evapor treatment\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e40 seconds\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eWrite laser\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e355 nm, Diod\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLaser Power\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e65 mW\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpin coating\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3000 rpm \u0026ndash; 1 minutes\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLaser Intensity\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e80%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePrebake\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e100 \u003csup\u003eo\u003c/sup\u003eC \u0026ndash; 1 minutes\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003eExposure\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDevelop\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1:3 Az400K \u0026ndash; 1 minutes\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe Ti sub-layer coating ensured good adhesion between the glass and Au film, and Au film served as a seed layer for the electroplating of Ni for the next steps. The metal-coated and PR patterned glass substrates were used as the working electrode, Ni plate anode as a counter electrode into the Ni chloride-based electrodeposition bath under 10.25 mA current. The thickness of the Ni hard mask was kept at 4 \u0026micro;m for high selective etching and deposited on the PR patterned substrates (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e: c)). At the end of processes, remained PR was cleaned with acetone; Ti and, Au was removed by wet etching in areas outside of Ni (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e: d), e)), respectively. The blue region shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e represents the area that will remain unetched. Hence, the final layers illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e :e) serve as a protective coating against ICP-RIE during the patterning of glass microstructures. The schematic representation of the desired microfluidic structure shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, obtained after etching under different recipes, is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e: f).\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents the real sample images of prepared microfluidic structure on the borosilicate glass substrates, whose design and fabrication were completed, taken under laboratory conditions prior to the etching process.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe core objective of this study was to systematically investigate the influence of different plasma compositions on glass etching; therefore, the ICP-RIE chamber parameters summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e were employed to develop and compare four distinct gas mixtures, designated as Recipes A, B, C, and D. ICP power at 2000 W, table temperature (chuck temperature) at 20 \u003csup\u003eO\u003c/sup\u003eC, helium \u0026ldquo;back-side\u0026rdquo; pressure at 1.33 Pa, and RF bias power at 300 W was kept constant to eliminate any other variables. The experimental study began with Recipe A, which was optimized to establish the baseline etching conditions. To examine the combined effect of C₄F₈ and O₂, Recipe B was then introduced. Subsequently, Recipe C was designed by further increasing the C₄F₈ flow rate in order to isolate its influence.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eICP RIE process parameters for glass etching\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eID\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e\u003cp\u003eGas Flow Rates (sccm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eRF bias (W)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eICP Power (W)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003ePressure (Pa)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSF\u003csub\u003e6\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eC\u003csub\u003e4\u003c/sub\u003eF\u003csub\u003e8\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eAr\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e2000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.667 (5 mTorr)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e2000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.667 (5 mTorr)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e2000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.667 (5 mTorr)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e2000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1.33 (10 mTorr)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAs shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, in Recipes A\u0026ndash;C, all contain Ar gas, which contributes to plasma formation, while in Recipe D, the removal of Ar resulted in unstable discharge at 5 mTorr. Argon, as an inert noble gas, possesses abundant metastable states that enhance electron-impact ionization and facilitate plasma ignition even under low-pressure conditions.\u003c/p\u003e\u003cp\u003eThe presence of Ar ensures a high density of energetic metastable atoms, which play a crucial role in sustaining the discharge through stepwise ionization and secondary electron emission at the cathode [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], thereby lowering the effective breakdown voltage and stabilizing the plasma at reduced pressures. In contrast, oxygen is an electronegative gas: it readily captures free electrons to form negative ions, which reduces the population of free electrons available to sustain the avalanche ionization. This effect leads to higher breakdown voltages, lower electron densities, and more difficulty in maintaining plasma [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. This electron loss increases the effective breakdown voltage and reduces plasma stability at low pressures [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Consequently, in Recipe D, the absence of Ar metastables and the electron attachment behavior of O₂ resulted in insufficient plasma generation at 5 mTorr. To overcome this, the chamber pressure was raised to 10 mTorr to reduce the required breakdown voltage and enable stable plasma ignition. Hence, the removal of argon from the recipes was intended to deliberately reduce ion bombardment and thereby minimize the formation of plasma-induced surface damage (PISD).\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Surface Morphology and Compositional Analysis\u003c/h2\u003e\u003cp\u003eOptical microscope images of the glass surfaces etched for 60 minutes with different plasma recipes were captured at a magnification of 20x and were presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The images show that plasma interaction significantly damages the surface, with the damage\u0026rsquo;s extent and shape varying with the gas compositions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn Recipe A (SF₆ + Ar), the presence of Ar promotes a sputter-effect-like process, which causes PISD, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e: a). When O\u003csub\u003e2\u003c/sub\u003e and C₄F₈ were added to the initial recipe to form Recipe B (SF₆ + Ar\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;low C₄F₈), dark, scattered crater-like features were observed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e: b). Using Recipe C (SF₆ + Ar\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;high C₄F₈), in which the amount of C₄F₈ is increased, the microscope images revealed a more densely populated with overlapping structures Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e: c). In particular, dark and scattered spot regions suggest particle redeposition effects, whereas under conditions such as Recipe D (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;higher C₄F₈) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e: d), the surface appears more uniformly modified with reduced large-scale defect formation. These observations suggest that the plasma\u0026ndash;surface interaction, including ion bombardment, chemical etching, and redeposition phenomena, strongly depends on the process chemistry, with specific gas composition significantly influencing PISD formation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSEM images of the glass surface after 60 minutes with different plasma recipes were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Spherical to irregular particle-like agglomerates scattered across the surface were observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e: a). This feature suggests redeposition of the etched material caused by sputter-effect-induced erosion. Such behavior is attributed to the high Ar concentration in Recipe A, which enhances the sputtering effect during etching. In Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e: b), crater-like features observed in optical microscopy (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e: b)) were also confirmed by SEM analysis. Unlike Recipe A, where the high Ar content induced strong sputter-effect-induced erosion and noticeable material redeposition, the addition of C₄F₈ and O₂, forming Recipe B, noticeably suppressed the etching process and modified the plasma chemistry. In particular, it was known that the introduced oxygen radicals participated in the surface oxidation of the substrate, resulting in the formation of a passivating SiOₓF\u003csub\u003eγ\u003c/sub\u003e layer that further altered the near-surface reactions and contributed to etch suppression [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. So that, the introduction of O₂ reduced the density of reactive F radicals responsible for material removal, thereby diminishing the overall etch rate. Hence, etching was limited, and the surface became passivated rather than actively etched despite continuous plasma exposure. Consequently, when using an O\u003csub\u003e2\u003c/sub\u003e containing gas mixture for etching, it is important to choose the optimal O/F ratio, which, for the reasons mentioned above, affects both the etching rate and etched structures. Etching was repeated with Recipe C, where only the C₄F₈ concentration was increased while the other gases were kept constant and SEM images was given in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e: c). Recipe C led to pit structures, also caused significant sidewall polymerization with increased C₄F₈ content, forming a Teflon-like top passivation layer, as mentioned in the introduction, which is one of the factors responsible for PISD formation. This behavior indicates that the additional C₄F₈ enhanced the generation of CFₓ fragments and free F radicals, restoring the balance between surface passivation and etching. As a result, the plasma environment shifted back toward an etching dominant regime rather than surface passivation, enabling material removal under controlled conditions. SEM images of the glass samples etched with Recipe D are presented, where the C₄F₈ concentration was further increased, the O₂ content was reduced, and both Ar and SF₆ were removed from the gas mixture, In Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(d). Under these conditions, the etching process produced a smooth and uniform surface without noticeable defects. The elimination of Ar suppressed sputter-effect-induced erosion that could lead to over-etching or micro-masking. The increased amount of C₄F₈ facilitated the generation of CFₓ fragments, promoting a stable balance between surface passivation and chemical etching. By increment of C₄F₈, the plasma environment became more chemically controlled, allowing steady material removal and yielding a highly uniform etched morphology.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEDS analysis of the glass surface after 60 minutes of etching\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eElement\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"5\" nameend=\"c6\" namest=\"c2\"\u003e\u003cp\u003eAtomic %\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eRecipe A\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRecipe B\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eRecipe C\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eRecipe D\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e49.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e36.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e25.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e27.58\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25.41\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e23.88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e19.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e24.63\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNa\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e9.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e14.55\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e10.33\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAl\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e1.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3.89\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eK\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e0.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.32\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.13\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e11.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e27.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e32.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e32.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCl\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e0.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNi\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.84\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe quantitative EDS analysis (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) clarified that the surface components were highly dependent on plasma compositions, correlating with the observed morphological changes under different plasma chemistries. In addition to the substrate-related elements (Na, K, and Al) originating from the glass matrix, the surface also exhibited notable amounts of fluorine (F), chlorine (Cl), and Ni, indicating the effects of plasma-induced reactions and material redeposition. In Recipe A, the agglomerated regions observed in SEM corresponded to localized enrichment of F and Cl species. The detection of Cl is attributed to residues from the chloride-based electrolyte used during the electrodeposition of the Ni hard mask, which may remain as adsorbed or entrapped salts on the surface. The persistence of Cl signals in Recipes A\u0026ndash;C suggests incomplete removal of these chloride species, which could remain attached to the surface or embedded within redeposited particulates. With the introduction of O₂ and a small amount of C₄F₈, Recipe B, fluorine deposition on the surface increased significantly, while oxygen content decreased. This compositional shift indicates the formation of a fluorocarbon-rich surface layer, characteristic of a polymerization-dominant regime. The simultaneous reduction in Cl content implies that the modified plasma chemistry partially suppressed chlorine-based redeposition. In Recipe C, the fluorine concentration reached its maximum ratio, and oxygen dropped to its minimum, suggesting the dominance of fluorination and polymer accumulation. The detection of Ni is associated with the sputtering and redeposition of the Ni hard mask, while the SEM-observed spherical-like structures and the high F content confirm the presence of a Teflon-like fluorocarbon polymer layer. This layer plays a crucial role in sidewall protection and surface passivation during plasma exposure. In contrast, Recipe D exhibited no detectable Cl or Ni signals, while maintaining a high F content. This finding suggests that the C₄F₈-rich plasma facilitated effectively encapsulating and removing residual metallic and halogen contaminants. Overall, the EDS results demonstrate a clear transition from oxide-dominated and chloride contaminated surfaces in Recipe A to fluorocarbon-passivated and contamination free surfaces in Recipe D. These compositional trends, particularly the evolution of the O/F ratio and the presence of polymeric fluorocarbon phases, are consistent with the morphological evidence of a shift from etching- to polymerization-dominant plasma regimes.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe glass substrates were subjected to plasma etching under the specified process conditions, and the resulting etch depths were measured using a profilometer. The etch rates were then calculated from the ratio of etch depth to etch time (etch depth / etch time). Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents the variation of etch rate as a function of etch time for the different plasma recipes, and the etching depths measured as a result of etching was written in the graph. The etch behavior observed in this figure is consistent with the morphological and compositional changes identified by SEM and EDS analyses, highlighting the evolution of the plasma\u0026ndash;surface interaction regime with changing gas chemistry. In Recipe A, the etch rate decreased markedly with time, reaching an etch depth of only 24.8 \u0026micro;m after 120 min. SEM images revealed island-like surface features and particle redeposition, while EDS analysis confirmed the presence of Cl residues and relatively low F incorporation. These findings suggest that the etching process was limited by redeposition of non-volatile species and contaminants formed during sputter-effect-induced erosion. With the addition of O₂ and a small amount of C₄F₈, Recipe B, the etch rate decreased even further compared with Recipe A during 90 min, 19.1 \u0026micro;m. The suppression of etching was attributed to oxygen-assisted polymerization, leading to surface passivation rather than material removal. Further increasing the C₄F₈ concentration, Recipe C, reinitiated etching and enhanced the overall etch depth to 40.09 \u0026micro;m after 150 min. The simultaneous detection of Ni and Cl peaks indicates redeposition of sputtered Ni from the hard mask and residual halogen species, caused in reduction of etch rate over time during etching with Recipe C. In Recipe D, where Ar and SF₆ were removed while maintaining high C₄F₈ and O₂ concentrations, the etch rate stabilized at the highest value among all recipes, reaching a total etch depth of 50.25 \u0026micro;m in 150 min. SEM observations confirmed a smoother and more uniform surface morphology with smaller pit structures, and EDS analysis showed the absence of Cl and Ni residues. These results indicate that optimized C₄F₈/O₂ mixtures effectively suppress contamination, prevent micro-masking, chemically controlled etching with improved surface quality and maximum etch depth.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAmong all investigated conditions, Recipe D demonstrated the most favorable etching performance, characterized by smooth surface, and impurity-free morphology, making the etched micropillar structures promising candidates for use in microfluidic platform applications. Finally, cross-sectional SEM images of the micropillar area on the sample that were etched for 150 minutes using Recipe D and obtained an etching depth of 50.25 \u0026micro;m were presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The etching process was carried out at relatively high ICP power level as given in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (2000 W) to ensure stable plasma generation and maintain a sufficient ion flux for deep glass etching. Under such energetic conditions, partial erosion of the Ni hard mask was inevitable. Nevertheless, the selected gas chemistry effectively suppressed the formation of micro-masks and redeposited particles on the surface, maintaining smooth feature profiles throughout the process. The measured etch depth of approximately 47.6 \u0026micro;m was in good agreement with the profilometry data presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. The micropillar array exhibits well-defined sidewalls and a uniform height distribution, indicating efficient by-product evacuation and controlled surface passivation. However, the slight rounding and surface degradation observed near the top regions of the pillars can be attributed to localized glass impurities and partial PISD. The imperfections which revealed minor compositional variations and the presence of fluorine-rich residues associated with localized polymer deposition and redeposition phenomena. The optimized C₄F₈/O₂ mixture promoted the formation of a thin, uniform fluorocarbon film that provided sidewall protection while allowing sufficient ion-assisted removal of surface polymers. This equilibrium between passivation and etching minimized surface roughening and prevented excessive PISD and underlying glass. Furthermore, in a detailed focus on upper region of the micropillars, the erosion of the Ni hard mask was obvious. This observation could be explained as table temperature fluctuation of the surface. So that the table temperature was identified as a critical factor influencing plasma\u0026ndash;surface interactions. As indicated in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the process operated under high ICP powers, and despite the use of auxiliary stabilization systems (e.g., pressure and temperature control), minor table temperature fluctuations were unpreventable. These variations locally modified the reaction kinetics and polymer desorption rates, contributing to the slight non-uniformities observed in the upper regions of the micropillars. Overall, the etch rate results are consistent with the SEM and EDS analyses, confirming that uniform glass etching is achieved when plasma parameters and gas composition are optimized to balance polymer formation and ion bombardment, as demonstrated by Recipe D.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Optical Emission Spectroscopy Analysis\u003c/h2\u003e\u003cp\u003eThe OES spectra in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e show the emission lines corresponding to different gas compositions employed during ICP-RIE of borosilicate glass. These spectra were obtained under 500 W ICP power, 0 W RF bias power and 1.33 Pa pressure. Although the etching processes were carried out at higher powers (2000 W ICP and 300 W RF bias power), the significantly increased signal magnitudes at such conditions hindered reliable detection and interpretation of plasma recipe [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe spectra presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e indicated that the optical activity varied significantly with the applied gas chemistry. In Recipe A, the process chemistry supplied fluorine from SF₆ for the formation of volatile compounds such as SiF₄ and BF₃, while Ar provided energetic ions responsible for physical sputtering, and the observed peaks around 419\u0026ndash;421 nm corresponding to S₂ [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] and F₂ [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] confirm the dissociation of SF₆. In addition, the spectra were dominated by intense Ar emission lines at 750.5 [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] \u0026minus;\u0026thinsp;763.5 nm [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] and relatively weak fluorine peaks at 685.9 [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] \u0026minus;\u0026thinsp;704 nm [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] indicating that the plasma was primarily sustained by Ar excitation with a limited population of reactive F radicals. The high Ar fraction, however, was promoted sputter-dominant etching behavior, which led to micro-masking and reduced efficiency. This interpretation is consistent with the SEM observations showing agglomerated surface regions and with the EDS results, which confirmed Cl redeposition originating from the Ni electroplating bath of the hard mask. Consequently, Recipe A operated in a sputtering-dominant regime, where physical ion bombardment prevailed over chemical etching and strong Ar peaks were evidence of this behavior. In Recipe B, the addition of O₂ and a small amount of C₄F₈, altered the plasma chemistry; the F emission lines became weaker which was explained as consumption of F ions by O\u003csub\u003e2\u003c/sub\u003e component [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Simultaneously, the emergence of a C₂ emission at 388.05 nm [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] confirmed the onset of fluorocarbon fragmentation and polymer formation, which supports the SEM results. In Recipe C, further increasing the C₄F₈ concentration enhanced both the etch rate and the polymerization tendency. The spectrum showed emission bands of C₂ became more prominent, yet the fluorine emission lines remained partially suppressed. The SEM images showed Teflon-like fluorocarbon structures, and EDS detected a high F concentration, together indicating a polymerization-dominant regime. However, uncontrolled polymer buildup eventually limited the etch rate and contributed to localized etch suppression and mask redeposition, as evidenced by Ni and Cl signals in EDS. To eliminate Ar-related micro-masking observed in Recipes A, B, and C, Ar and SF₆ were removed and the plasma consisted solely of a C₄F₈/O₂ mixture, Recipe D. The absence of Ar emission lines in the spectrum allowed clear detection of molecular bands corresponding to O₂ and C, C\u003csub\u003e2\u003c/sub\u003e, C₃ emissions around 387\u0026ndash;516 nm peaks [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The coexistence of carbon, oxygen, and weak fluorine emissions is consistent with the formation of a thin, uniform fluorocarbon polymer layer that provided effective sidewall passivation without excessive buildup. SEM observations revealed smooth micropillar structures with minimal surface damage and the highest etch depth (\u0026asymp;\u0026thinsp;47.6 \u0026micro;m). EDS analysis confirmed the absence of Ni and Cl contamination, verifying that the C₄F₈/O₂ plasma established a balanced regime between sputtering-induced erosion and ion-assisted removal with polymer deposition.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eSystematic investigation of the optical microscopy, SEM, EDS, and OES results indicated a strong correlation between surface morphology, etching performance, and plasma composition during borosilicate glass etching. OES provided complementary insights into the plasma\u0026ndash;surface dynamics. In Recipes A\u0026ndash;C, strong Ar emission lines inhibition the weak F peaks, confirming a sputter-dominant regime characterized by limited chemical activity and significant redeposition, which was also evidenced by Cl and F-rich residues in the EDS results. The addition of O₂ and C₄F₈ in Recipe B altered the plasma chemistry but remained constrained by strong Ar excitation, resulting in PISD formation and micro-masking effects. Although increasing the C₄F₈ content in Recipe C improved etch rate, it also intensified polymer formation and partial mask redeposition, as reflected by pronounced Ar peaks in OES. In contrast, Recipe D, which eliminated Ar and SF₆, exhibited balanced C₂, C₃, and O₂ emissions, resulting a chemically active plasma where polymerization and etching processes coexisted in equilibrium. This optimized condition yielded the smoothest surface morphology with achieving the most stable and highest etch rate among all conditions, the highest etch depth, and contamination-free results. Beyond process optimization, this work provides a unique integrated understanding of how plasma diagnostics and surface analyses converge to explain etch behavior and PISD formation. The correlation between OES and surface EDS result clarifies that Cl and Ni related residues originate under Ar rich plasmas, whereas C₄F₈/O₂ mixtures minimization such contamination by establishing a carbon-rich environment that enables self-limiting polymer passivation. This study is among the pioneering works that systematically efforts to directly correlate in-situ OES plasma diagnostics with etched surface EDS analysis to uncover the underlying mechanisms of PISD formation and suppression during glass etching. This framework elucidates the plasma\u0026ndash;surface interactions responsible for redeposition and etch-stop phenomena, providing a comprehensive basis for developing clear surfaces, contamination-free ICP-RIE processes in future microfabrication technologies.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAuthors\u0026rsquo; Contribution All authors reviewed the manuscript. All authors contributed to the discussion and gave helpful feedback. CT, IG: Experiments planning and conduction; DN, EZ: Main text preparation; DN, CT, MS: Photolithography, mask deposition, ICP-RIE process; DN, CT: SEM investigation; DN: Sample profilometry; DN: OES investigation; DN: Literature investigation, the representation of the results and feedback; CT: Head of the research group, consulting, and discussion.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003e\u003cu\u003eAcknowledgments\u003c/u\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to thank Şirin Say and Şeyma Soykan for their assistance with sample preparation in the laboratory. The authors gratefully acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) due to the financial support for the Project 22AG030 and 120N816.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eDavis JA, Inglis DW, Morton KJ, Lawrence DA, Huang LR, Chou SY, Sturm JC, Austin RH. 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(2024) Optical emission spectroscopy as a method for evaluating the change in Si etching structures profile in ICP SF6/C4F8 plasma: Microstructures. J Vac Sci Technol A; 42:063001. https://doi.org/10.1116/6.0003809.\u003c/li\u003e\n\u003cli\u003eGoyal A, Hood V, Tadigadapa S. (2006) High speed anisotropic etching of Pyrex\u0026reg; for microsystems applications. J Non-Cryst Solids;352:657\u0026ndash;63. https://doi.org/10.1016/j.jnoncrysol.2005.11.063.\u003c/li\u003e\n\u003cli\u003eLi X, Ling L, Hua X, Fukasawa M, Oehrlein GS, Barela M, Anderson HM. (2003) Effects of Ar and O2 additives on SiO2 etching in C4F8-based plasmas. J Vac Sci Technol Vac Surf Films;21:284\u0026ndash;93. https://doi.org/10.1116/1.1531140.\u003c/li\u003e\n\u003cli\u003ePark JH, Lee N-E, Lee J, Park JS, Park HD. (2005) Deep dry etching of borosilicate glass using SF6 and SF6/Ar inductively coupled plasmas. 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J Vac Sci Technol Vac Surf Films;32:041302. https://doi.org/10.1116/1.4880800.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"plasma-chemistry-and-plasma-processing","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Plasma Chemistry and Plasma Processing](https://www.springer.com/journal/11090 ","snPcode":"11090","submissionUrl":"https://mc.manuscriptcentral.com/pcpp","title":"Plasma Chemistry and Plasma Processing","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Borofloat etching, Plasma etching, C4F8 plasma, Optical emission spectroscopy","lastPublishedDoi":"10.21203/rs.3.rs-7923195/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7923195/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDry etching techniques are extensively used in the fabrication of silicon-based electronic components; however, the increasing use of glass substrates in microfluidic applications, due to their chemical stability, biocompatibility, optical transparency, electrical insulation, and cost-effectiveness, has created a growing demand for efficient dry etching of glass. This work presents the etching of borosilicate glasses using inductively coupled plasma reactive ion etching (ICP-RIE) with fluorine-based gas chemistries for use in microfluidic applications. Microfluidic structures were patterned via photolithography technic with micropillar were patterned using a titanium/gold stack and a nickel hard mask. The effects of various gas mixtures on the etch rate and surface morphology were systematically investigated. Optical emission spectroscopy (OES) was employed to monitor plasma characteristics, while surface analyses and etch depth were carried out using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and profilometry, respectively. The results highlight how different gas compositions influence etch behavior and surface morphology, offering valuable insight for optimizing dry etching processes in glass-based microfluidic device fabrication and emphasize the importance of selecting appropriate gas ratios to achieve uniform microstructures with minimal surface defects.\u003c/p\u003e","manuscriptTitle":"Investigation of Surface Morphology Evolution During the Borosilicate Glass Etching By Using ICP-RIE For Microfluidics Applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-27 10:36:10","doi":"10.21203/rs.3.rs-7923195/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-13T13:45:48+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-09T08:26:18+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-23T00:13:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"137811422628703543407052240055040951237","date":"2025-11-22T23:55:10+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"36106902867485188153581398116334239917","date":"2025-11-20T15:50:21+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-20T14:11:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-23T01:08:56+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-23T01:08:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"Plasma Chemistry and Plasma Processing","date":"2025-10-22T11:48:00+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"plasma-chemistry-and-plasma-processing","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Plasma Chemistry and Plasma Processing](https://www.springer.com/journal/11090 ","snPcode":"11090","submissionUrl":"https://mc.manuscriptcentral.com/pcpp","title":"Plasma Chemistry and Plasma Processing","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"e35d12b8-f031-4f85-97ab-ac7583ab0183","owner":[],"postedDate":"November 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-14T14:12:06+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-27 10:36:10","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7923195","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7923195","identity":"rs-7923195","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}