{"paper_id":"32d78683-9df7-4364-a94b-dd6c2cd790e1","body_text":"License and Terms: This document is copyright 2024 the Author(s); licensee Beilstein-Institut.\nThis is an open access work under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0). Please note that the reuse,\nredistribution and reproduction in particular requires that the author(s) and source are credited and that individual graphics may be subject to special legal provisions.\nThe license is subject to the Beilstein Archives terms and conditions: https://www.beilstein-archives.org/xiv/terms.\nThe definitive version of this work can be found at https://doi.org/10.3762/bxiv.2024.67.v1\nThis open access document is posted as a preprint in the Beilstein Archives at https://doi.org/10.3762/bxiv.2024.67.v1 and is\nconsidered to be an early communication for feedback before peer review. Before citing this document, please check if a final,\npeer-reviewed version has been published.\nThis document is not formatted, has not undergone copyediting or typesetting, and may contain errors, unsubstantiated scientific\nclaims or preliminary data.\nPreprint Title Performance optimization of microwave-coupled plasma-based ultra-\nlow energy ECR ion source for silicon nano-structuring and\napplication\nAuthors Joy Mukherjee, Safiul A. Mullick, Tanmoy Basu and Tapobrata Som\nPublication Date 22 Nov. 2024\nArticle Type Full Research Paper\nORCID® iDs Joy Mukherjee - https://orcid.org/0000-0002-5387-2617; Safiul A.\nMullick - https://orcid.org/0000-0002-0985-8914\n\n1 \nPerformance optimization of microwave-coupled plasma-based ultra-low 1 \nenergy ECR ion source for silicon nano-structuring and application 2 \n    Joy Mukherjee1, Safiul Alam Mullick2, Tanmoy Basu3, Tapobrata Som1,4* 3 \n1SUNAG Laboratory, Institute of Physics, HBNI, Sachivalaya Marg, Bhubaneswar – 751 005, 4 \nIndia 5 \n2Rabindra Mahavidyalaya, University of Burdwan, Hooghly, West Bengal, 712 401 6 \n3Centre for Quantum Engineering, Research and Education, TCG Centers for Research and 7 \nEducation in Science and Technology, Kolkata, West Bengal, 700 091, India 8 \n4Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai – 400 9 \n085, India 10 \n 11 \nAbstract: 12 \nThis literature presents a comprehensive optimization of key parameters crucial for generating 13 \nion beams in a microwave-coupled plasma -based ultra -low energy Electron Cyclotron 14 \nResonance (ECR) ion source, generally used  for nano -structuring on solid surfaces. The 15 \ninvestigation focuses on developing, accelerating, and extracting Ar + ions from a magnetron 16 \n(microwave) coupled plasma cup utilizing three-grid ion extraction composed of molybdenum. 17 \nThe study systematically examines the dependence of ion beam current on critical parameters, 18 \nsuch as gas pressure, magnetron power, extraction voltage, and ion energies. Additionally, the 19 \ninfluence of extraction voltage on beam current is investigated for different ion energies. The 20 \nvariation of beam current  as a function of ion energy is explored under constant magnetron 21 \ncurrent and extraction voltage at various conditions. The Gaussian nature of the beam profile 22 \nis scrutinized and elucidated within the context of grid extraction -based ion sources. Plasma 23 \nphysics principles are employed to interpret the observed variations in ion current density 24 \n\n2 \n(beam current) with various parameters.  The corresponding ion -induced nanopatterning on 25 \nsilicon, using the optimized beam current, is explored in detail. Furthermore, the research 26 \ndelves into the temporal evolution of the surface topography of silicon followed by off-normal 27 \nincidences (60º and 72.5º) is Ar-ion extracted at 450 eV Ar-ions. The changes in the optical  28 \nproperty, resulting from nano-patterned surfaces, investigated using UV-VIS spectroscopy, is 29 \ncorrelated the with dimension of nano patterning.  This manuscript highlights the potential 30 \napplications arising from these findings, emphasizing the transformative impact of low energy 31 \ninert ion induced nano-patterning technologies. 32 \nKeywords: Ultra -low energy  ECR-based ion source, Optimization of ion current , Surface 33 \ntopography, TEM, UV-VIS spectroscopy 34 \n*Corresponding author email: tsomiop@gmail.com 35 \nIntroduction: 36 \nThe ion source serves as a fundamental component in numerous scientific and industrial 37 \napplications, playing a crucial role in generating charged particles (ions) . Following ion 38 \nproduction, various systems harness these energetic ions for diverse purposes, spannin g 39 \nmaterial science, high -energy physics, medical applications, and agricultural science [1–5]. 40 \nPresently, energetic ions find application in various surface treatments such as nano-patterning, 41 \nsputter etching, controlled defect formation, and more [6,7]. Particularly, ultra-low energy ion 42 \nbeam proves exceptionally valuable for the precise modification of 2D layers [8], ion-induced 43 \nnano-patterning on semiconductor surfaces[9]. Over the past few decades, ion -induced nano-44 \npatterning and nanoscale functi onalizations have garnered significant interest, owing to their 45 \nbroad applications in DNA origami  [10], tuning wettability [11], electrical and magnetic 46 \nanisotropy [12,13], isolated dot formation  [14], nanoscale plasmonic array[15], field emission 47 \n[16], etc. Thus, the ion sources generate enormous possibilities for material modifications both 48 \n\n3 \nphysically and chemically.  Further, diversity exists in energetic ion production mechanisms. 49 \nThe fundamental process of producing ions is the collision of atoms with ions or electrons  50 \nwhich may be either elastic or inelastic. In elastic collisions, the internal energy of the colliding 51 \nparticles does not change. Ionization, stripping, electron capture , and excitation of atoms due  52 \nto collisions are examples of inelastic collisions. The free electrons colliding with atoms also 53 \nproduce ions. Electrons in the gas are heated by  the inductively coupled method and then 54 \nacquire enough energy to generate plasma. Due to several drawbacks in such Townsend 55 \ndischarge, these sources are not used now a days. In recent days, compact broad-beam ion 56 \nsources are widely used in scientific laboratories  to generate ions . Depending upon the 57 \nmechanism of production of various ions using gaseous plasma, the ion sources can be 58 \nclassified in three ways; direct current (dc) operated ion sources, radio frequency (microwave) 59 \nion sources, and microwave ion sources.  In the past few decades, DC ion sources were 60 \ncommonly used in the above activities [17–19]. These DC ion sources consist of a hot cathode 61 \nor filament,  which is not especially useful  in case s of reactive gas discharge; hence , their 62 \nlifetime is limited [20,21]. Moreover, the beam current produced by those ion sources is not 63 \nsuitable for modern-day applications. In material science as well as surface science 64 \napplications, the ion source should be mobile and adaptable to the vacuum system , having a 65 \nlonger lifetime. Further, the ion source should produce a relatively high beam current (i.e., 66 \ncapable of forming a high density of plasma) with lower maintenance. To address this 67 \nchallenge, Electron Cyclotron Resonance (ECR) based ion sources were develope d  [22,23]. 68 \nECR ion source  is one of the most preferred ion sources for the easy production of ions for 69 \ndifferent energies and charge states. Since the discharge is maintained  in the quartz cup via a 70 \nstrong electric field generated in the cavity , the ECR-based ion sources equipped with 71 \nmicrowave cavities neither contain any filament nor any type of electrode . The high plasma 72 \ndensity within a quartz cup is confined by solenoid magnets surrounding it, creating a multi -73 \n\n4 \ncusp magnetic field. However, careful attention is required for the microwave coupling to the 74 \nplasma cup to minimize microwave-reflected power. Mechanical adjustments to the resonator 75 \nlength and waveguide are made to ensure minimal reflection of microwave power. 76 \nAdditionally, maintaining the  necessary magnetic field strength is crucial for sustaining the 77 \nplasma. The ion source's compact design is user-friendly and capable of producing a high beam 78 \ncurrent density for a single or multi -grid extraction system. As a result, the extracted beam 79 \ncurrent is influenced by magnetron power, plasma pressure, and extraction voltage. 80 \nFurthermore, the beam current varies with different ion energies. 81 \nThis article focuses on optimizing the beam current generated by a cost -effective 82 \nmicrowave-based Electron Cyclotron Resonance (ECR) ion source and subsequent  83 \ndevelopment of nanoscale patterns on the surface of silicon. The relationship between the beam 84 \ncurrent and various parameters is extensively examined and elucidated. Experimental 85 \nparameters, spanning from plasma generation to ion beam extraction, are systematically 86 \noptimized for the study of low -energy Ar -ion-induced nanostructures on silicon. The 87 \ndependence of the extracted ion beam on plasma pressure, magnetron power, and extraction 88 \ngrid voltage is documented for different ion energies. Additionally, the manuscript establishes 89 \nthe relationship between ion beam current and the ion energy.  Irradiation of  p-type single 90 \ncrystal Si (100) surface at off-normal angles (60º and 72.5º) by a 450 eV Ar-ion results in the 91 \nwell-defined formation of nanoscale ripple patterns.  The prominence of ripple structures 92 \nincreases with prolonged irradiation time, while bombardment at 72.5º with the same ion beam 93 \nparameters leads to the coarsening of nanostructures . Cross -sectional transmission electron 94 \nmicroscopy (TEM) measurements confirm the formation of nanostructure as observed from 95 \natomic force microscopy images (AFM). The thickness of the amorphous thin layer is well 96 \nagreement with Monte Carlo Simulation (SRIM)[24]. The article further investigates and 97 \nexplains the optical response  (by UV-VIS spectrophotometer) of the nano-patterned surfaces 98 \n\n5 \nwith the dimension s of nano-patterning (i.e., wavelength and rms roughness) . The potential 99 \napplications of such na no-patterned silicon surfaces are enlightened. This article underscores 100 \nthe versatility of an optimized broad-beam ultra -low energy ion source, specifically in the 101 \ncontext of optimization of inert Ar-ion and subsequent ion-induced silicon nano-patterning. 102 \n 103 \nDescription of the ion source: 104 \n 105 \nFigure 1:  Block diagram  consisting of the component of high vacuum plasma ion source 106 \nequipped with UHV chamber.  107 \nFigure 1 illustrates the block diagram of the magnetron -coupled ultra -low energy 108 \nElectron Cyclotron Resonance (ECR) ion source to provide a comprehensive visualization of 109 \nthe entire setup. The schematic representation in Figure. 1 elucidates the process of extracting 110 \nan ultra-low energy ion beam. An microwave source (magnetron) is connected to the ceramic 111 \nplasma cup via a waveguide. The gas inlet system facilitates the filling of the plasma cup with 112 \ngas through a capillary tube. The intense electric field generat ed by the microwave source 113 \n(magnetron) induces gas breakdown (discharge), leading to the formation of a highly intense 114 \nplasma. The produced plasma is confined and sustained by a permanent magnet positioned near 115 \nthe ceramic plasma cup (made of Al2O3). For the extraction and focusing of the beam, a gridded 116 \nelectrostatic lens, commonly referred to as an Einzel lens, is employed. The shape and size of 117 \nthe beam are contingent on the extraction voltage applied at the grid and the corresponding ion 118 \n\n\n6 \nenergy. The dir ected beam impacts the silicon target  kept in an ultra -high vacuum (UHV) 119 \nwithin the target chamber. A faraday cup, connected to a multimeter, measures the beam 120 \ncurrent, and the corresponding ion fluence is expressed in terms of irradiation time. 121 \nThe sample  holder, located in a UHV chamber, is connected to a 5 -axes (x, y, z, θ, φ) 122 \nmanipulator system, offering movement and rotation in all possible directions. The sample is 123 \ntransferred to the ion source using a load -lock system. A later discussion provides a c ross-124 \nsectional view of the system. 125 \n 126 \nFigure 2: Cross-sectional (schematic) view of microwave coupled ultra-low energy ion beam 127 \nsystem. 128 \nThe cross-sectional view of the high vacuum microwave based ECR ion source mentioned 129 \nabove is shown in Figure 2. The entire system consists of a magnetron-coupled ion source, a 130 \nUHV target chamber with a cylindrical cavity and load lock, a 5-axes manipulator (PREVAC 131 \nTechnologies), and a gas inlet system. This type of magnetron-coupled ion source was first 132 \ndeveloped by Anton et.al [23]. The ion source is fitted in the cylindrical cavity of the UHV 133 \n\n\n7 \ntarget chamber. The inner diameter of the plasma cup is around 52 mm, where the plasma is 134 \ngenerated. The cup is surrounded by water-cooled magnets made up of neodymium-iron-boron, 135 \nwhich produces a multi-cusp field to confine the plasma.  The 2.45 GHz magnetron-based 136 \nmicrowave source is attached to the backside of the ion source , as shown in Figure 2. The 137 \ndimension of the cylindrical resonator (waveguide) is chosen in such a way that it can produce 138 \nmaximum beam current. To generate plasma in the ceramic cup (known as plasma cup), a gas 139 \nis inserted into it through a capillary tube attached to the gas chamber. A pressure of 10-4 mbar 140 \nis maintained for sustaining plasma by adjusting a needle valve attached to the gas chamber. 141 \nThe entire length of the ion source is around 130 cm. The extraction of ion beams is 142 \naccomplished by a three-grid ion optics system, as seen in Figure 2. Wide-ranging extraction 143 \nvoltage is applied to the grid to enable the extraction of an intense beam with different 144 \ndiameters. The circular perfection of the beam shape is evident from observations on the front 145 \nplate attached to the ultra-high vacuum (UHV) c hamber. In this configuration, the beam 146 \ncurrent, specific to a given ion energy, can be finely adjusted based on magnetron power, 147 \nworking pressure, and extraction voltage. Consequently, optimizing the dependence of beam 148 \ncurrent on these parameters is a wor thwhile pursuit, driven by the need to comprehend the 149 \nunderlying scientific principles. Additionally, the ion current (beam current) is influenced by 150 \nthe extracted ion energy and the position of the target. Hence, a comprehensive investigation 151 \ninto the intricate relationship between ion current and the mentioned parameters emerges as a 152 \ncompelling topic in the current scientific context. 153 \n 154 \n 155 \n 156 \n 157 \n\n8 \nOptimization of ion current on various plasma parameters: 158 \n 159 \nFigure 3:  Variation of beam current with (a)–(c) plasma pressure, (d)–(f) microwave 160 \n(magnetron) power for different ion energy.  161 \nThe variation of beam current with plasma pressure and magnetron ( microwave) power for 162 \ndifferent ion energies are investigated and presented in Figure 3.  Figure 3(a)-(c) demonstrates 163 \nthat the beam current decays almost exponentially with the increase in plasma pressure. The 164 \nion current is maximum at plasma pressure 1.5 × 10 -4 mbar irrespective of ion energies. This 165 \nindicates that the minimum base pressure required for generating plasma is 1.5 × 10-4 mbar. It 166 \nis also evident that for the same plasma pressure, the beam current is maximum for the highest 167 \nion energy. The gas pressure inside the plasma cup is directly proportional to the number of 168 \ngas molecules present. At low plasma pressure, the mean free path of gas molecule s is larger 169 \ndue to the lesser density of gas molecules, which allows the produced ions to traverse a longer 170 \ndistance without collision. This increases the ionization efficiency , and hence, with fewer no 171 \nof collisions, the probability for recombination of the ions is very low. Consequently, a large 172 \nnumber of ions are extracted, intensifying the beam current. The entire phenomenon can be 173 \n\n\n9 \nsummarised through the equation, λ = (σ.n)-1, where λ is the mean free path of the ions, σ is the 174 \ncross section for recombination, and n is the density of the ions inside the plasma [25–27]. The 175 \nmean free path of the ions, determined by the recombination cross section and density of 176 \nplasma, plays a key role in quantifying the ion current.  177 \nFurther, the conversion of the gas to plasma is governed by a magnetron (microwave) 178 \nsource and therefore, the ion current or plasma density must depend on magnetron power. To 179 \nunderstand that, the variation of ion current with microwave power is recorded  at different 180 \nplasma pressures and ion energies, as presented in Figure 3 (d)- (f). In general, the plasma 181 \ndensity (n) depends on the microwave frequency (ω) as n = \n𝐸𝑅𝐹𝜔2\n𝜀     where ERF is the microwave 182 \npower (energy) and ε is the minimum energy required for ion-electron pair generation [25]. The 183 \nmagnitude of ε is different for different gases. A non -zero magnitude of ε signifies the 184 \nminimum energy needed to generate plasma, commonly referred to as ionization energy, which 185 \nis supplied by magnetron power . It is evident from the above Figure (3(d)-(e)) that up to the 186 \nmicrowave power equal to a critical value, the formation of plasma is forbidden, resulting in a 187 \nzero-beam current. With the increase of magnetron power beyond the magnitude ε, the beam 188 \ncurrent increases almost linearly with the input magnetron power, since the plasma density (n) 189 \nis directly proportional to the microwave power (ERF). The beam current reaches saturation at 190 \na specific microwave power level, which varies based on the ion energy. At sufficiently high 191 \nmicrowave power, the plasma density (beam current) is high, and the rate of generation and  192 \nrecombination of the ion-electron pair is equal, resulting in a saturation of beam current a s 193 \nobserved in Figure. 3(d)-(f). Further, the cut off  power also depends on ion energy. At 194 \nsufficiently low ion energy, the microwave power required for generating plasma is high. With 195 \nhigher ion energies microwave power required for ion -electron pair generation is also less 196 \nwhich is obvious from the above discussion. 197 \n\n10 \n 198 \nFigure 4:  Variation of (a)-(c) beam current with ion extraction voltage at different ion 199 \nenergy;(d) beam current with ion energy; (e) beam current with the target position. 200 \nThe extraction of developed Ar-ion beam was governed by a combination of three-grid 201 \n(concave) ion optics system [28–30]. The beam current and the beam profile depend on the 202 \npotential applied at the grid and the target position. The change in beam current with the ion 203 \nextraction voltage recorded for different ion energies is presented in Figure. 4 (a)-(c).  Initially, 204 \nthe beam current increases linearly with the applied extraction voltage since more ions are 205 \nextracted with a higher extraction voltage. Irrespective of ion energy, t he beam current is a 206 \nmaximum extraction voltage of 200 eV. With further increases in extraction voltage, the beam 207 \ncurrent decreases rapidly. This is governed by two major phenomena, firstly, the space charge 208 \neffect, i.e., a high extraction voltage , creates an electrostatic field that repels the subsequent 209 \nions, causing the beam to spread out ; therefore, the current density reduces. Secondly, during 210 \nthe extraction of the ions, the application of  a high extraction voltage leads to the collision of 211 \nthe ions with residual gas molecules, which also causes a significant decrease in beam current 212 \n\n\n11 \ndue to ion-electron recombination. Generally, the extraction voltage is kept fixed to maintain 213 \nthe shape of the beam, essential for uniform irradiation of samples.  214 \nThe dependence of beam current upon ion energy is also investigated in Figure 4(d). It is clear 215 \nfrom Figure 4(d) that the beam current increases almost linearly with the increase in ion energy 216 \nat a fixed extraction voltage and microwave power. When the applied extraction voltage is 200 217 \neV and the ion energy is less than 200 eV, then the number of ions that can overcome the barrier 218 \nof 200 eV is less, causing a low beam current. With the increase in ion energy, the ions achieve 219 \nsufficient energy to overcome the barrier of 200 eV; hence, the beam current increases. On the 220 \nother hand, for a particular ion energy, lowering the extraction voltage also results to a lowering 221 \nof beam current as observed from the above Figure 4 (a)-(c). Therefore, to maintain a proper 222 \nbeam shape and adequate beam current , the extraction voltage and ion energy are to be 223 \nprecisely optimized. Further, the variation of beam current with the target position, known as 224 \nthe beam profile, is also presented in Figure 4(e). The beam profile is Gaussian in nature for 225 \nconcave grid (molybdenum) beam extraction optics. Such a beam profile precisely ensures the 226 \ntarget position is for maximum beam current. 227 \nNano-structuring on Si surface by 450 eV Ar-ion bombardment: 228 \nSubsequently, after a detailed optimization of the ultra-low energy Ar-ion beam, the surface 229 \ntopography of Si (investigated by AFM) after the off-normal bombardment of 450 eV Ar- 230 \nion at different incidence angles and time is investigated. Figure 5 represents the surface 231 \nmorphology of Si surface after 450 eV Ar-ion bombardment at different incidence angles. 232 \nThe arrow on the right-hand side indicates the direction of  the ion beam concerning the 233 \nsurface normal. The irradiation of silicon surface with 450 eV Ar-ion at an angle of 55º 234 \nleads to no development of surface morphology, and presented in Figure 5(a). However, at 235 \nan ion incidence angle of 58º, the evolution of surface morphology starts, although no 236 \nprominent ripple structure is observed (Figure 5(b)). On the other hand, the bombardment 237 \n\n12 \nof 450 eV Ar-ion on the Si surface for an hour at an angle of 60º concerning surface normal 238 \nleads to the formation of a well-defined nanoscale ripple pattern as observed in Figure 5(c). 239 \nThe growth of the ripple becomes more prominent with the increase in bombardment time. 240 \nThe amplitude of the ripple grows larger with longer bombardments of Ar-ions. 241 \n 242 \nFigure 5: AFM image (2D and 3D) of the evolution of surface morphology by 450 eV Ar-ion 243 \nbombarded at different incidence angles and time. The arrow indicates the ion beam direction. 244 \nTo visualize the growth of the ripple, the 3D AFM images are presented along with the 245 \n2D images. It is clear from the height scale associated with the images, that the ripple height 246 \nincreases with bombardment time. The first Fourier transform (FFT) image of nano-patterned 247 \nsurface is inset at the right lowest corner of each image.  In the present case, the fluence is 248 \nreplaced by irradiation time. The quality and the growth of the nano -structures are 249 \nquantitatively discussed in Figure 6, where a detailed variation of ripple wavelength, rms 250 \nroughness, and the power spectral density is discussed. 251 \n\n\n13 \nFigure 5(g) represents the cross-sectional transmission electron microscopy (TEM) image of 252 \n450 eV Ar-ion bombarded Si surface at an angle of 60º with a time of 3 hours. The presence of 253 \nAr-ion-induced surface corrugation in terms of ripple-like nanostructure is evidenced in Figure. 254 \n5(g). Although the amplitude of the ripples is not sufficiently large, the observed ripple 255 \nwavelength of around 31 nm from the TEM image,  is consistent with that of AFM data 256 \n(presented in Figure 6(e)). However, in addition to the ripple-like nanostructure, an ultra-thin 257 \namorphous layer formation occurs due to Ar -ion bombardment. The thickness of the 258 \namorphous layer is around 1.5 nm, which is consistent with the penetr ation depth of the Ar-259 \nions (1. 2 nm), estimated by Monte Carlo Simulation  (Fig 5(f))  [24]. Therefore, the 260 \ntopographical image is consistent with the cross-sectional image, indicating a clear signature 261 \nof ripple-like nano-structure formation. 262 \n 263 \n\n\n14 \nFigure 6: Variation of (a) – (c) surface height modulation of nano -patterned silicon surface; 264 \n(d) ripple wavelength and (e) rms roughness with irradiation time.  Variation of power spectral 265 \ndensity of nanopatterned silicon surface on (g) parallel and (h) perpendicular direction.  266 \n  Figure 6 (a)-(c) indicate the variation of the surface profile of the nano-patterned AFM 267 \nimages shown in Figure 5. The height profile is the direct evidence of variation of ripple 268 \namplitude with irradiation time. The increase in ripple height with irradiation time is shown in 269 \nFigure 6 (a)-(c). Further, the fluctuation in ripple height or amplitude generally termed rms 270 \nroughness, is also investigated in Figure 6(d).  It is clear from Figure 6(d), that rms roughness 271 \nincreases linearly with the irradiation time (fluence). Further, the ordering of the nanostructure 272 \nwith the bombardment time , examined in terms of ripple wavelength, is presented in Figure 273 \n6(e). From Figure 6(e) initially, t he ripple wavelength increases as the bombardment time  274 \nincreases from one hour to two hours. With a further increment in irradiation time, the change 275 \nof ripple wavelength is negligible, i.e., a saturation of ripple wavelength is observed.  The 276 \ndegree of similarity between two spatial morphologies is generally quantified by auto -277 \ncorrelation length or self-correlation length, as presented in Figure 6(f). It is clear from Figure 278 \n6(f), that the auto-correlation length decreases with bombardment time. This indicates a more 279 \nordered ripple st ructure is found to develop with higher irradiation time.  To understand the 280 \ngrowth of the ripple structure, the power spectral density factor along the parallel and 281 \nperpendicular direction of the developed ripple is presented in Figure 6 (g)-(h). The prominent 282 \npeak present in Figure 6(g) indicates, the development of ripple structure along the x direction 283 \n(parallel) with a particular wavevector ( kx). Besides, the absence of a ripple wavevector in 284 \nperpendicular mode is evidenced in Figure 6(h). Therefore, 450 eV Ar-ion bombardment on 285 \nSi, leads to well-defined parallel mode ripple formation at off-normal incidence.  286 \n\n15 \n 287 \n  Figures 7: AFM images (2D and 3D) of the evolution of nano -structure on Si surface with 288 \ntime under 450 eV Ar-ion bombardment at an angle of 72.5º. 289 \n Figure 7 illustrates the surface topography after 450 eV Ar-ion bombarded the silicon surface 290 \nat an angle of 72.5º with different bombardment time. Corresponding 3D AFM images are also 291 \npresented along with 2D surface topography. Generally, the transformation of well -defined 292 \nnano ripples into nano-facets occurs at this near-grazing angular region[31]. In the present case, 293 \nalthough no prominent nano-facet formation is observed, a clear signature of transformation of 294 \nripple structure towards nano-facets is seen. At sufficiently large bombardment time (10 hours), 295 \nnano-facets like structures with larger dimensions are developed, although the facets are not 296 \nwell organized. It is also evident that the rms roughness is also increasing with bombardment 297 \ntime.   298 \nSurface nano-structuring by energetic ion bombardment is a consequence of ion-beam-299 \ninduced off-normal (60º  and 72.5º ) sputtering of surface atoms and their consecutive 300 \nredistribution [9,32,33]. During ion bombardment, the unequal radius of curvature of the surface 301 \n\n\n16 \nensures unequal deposition of energy at different points on the surface, which results in unequal 302 \nsputtering at those points. This generates surface instability, and consequently , the surface 303 \natoms are redistributed to stabilize the surface. These two effects jointly trigger nano-pattern 304 \nformation on the surface. A first theoretical model was proposed by Bradley and Harper  [34], 305 \nbased on curvature-dependent sputtering of surface and near-surface atoms. Later, Carter and 306 \nVishnyakov introduced the concept of redistribution of surface atoms [35]. In present days, 307 \nseveral experiments have been carried out to understand other factors that contribute to nano-308 \npattern formation, such as preferential and differential sputtering, the role of surface and beam 309 \nimpurities, the effect of chemical compound formation, and compound ion irradiation. In cases 310 \nof ultra-low energy ion bombardment, the rate of sputtering is lower compared to medium-311 \nenergy ion bombardment; therefore, in this case, mass redistribution of the surface atoms plays 312 \na key role. Being inert, the reaction between Ar -ion and Si atoms is forbidden, ensuring the 313 \nabsence of the chemical aspect of pattern formation. However, the native silicon oxide layer 314 \npartially sputters out with  the bombardment of Ar -ion. In the present case , the sputtering of 315 \nelemental Si atoms takes place along with the Si atoms in oxide form. This is also a key factor 316 \nin generating surface instability. The surface morphology largely varies due to different 317 \namounts of near-surface mass transport by the surface-confined ion -enhanced viscous 318 \nflow[36]. Here in the present case, up to an ion incidence angle of 58º, the surface remains 319 \nunstable under 450 eV Ar-ion bombardment. Due to such sputtering , a well -defined ripple 320 \nformation is found after  an hour of 450 eV Ar-ion bombardment . With the increase in 321 \nbombardment time, Si atoms in both elemental and oxide form sputter more, and a well-ordered 322 \nripple is obtained. However, due to the presence of the ripple, the surface becomes anisotropic. 323 \nThe consequence of such an anisotropic nature of the surface is investigated and discussed in 324 \nthe upcoming section. 325 \n 326 \n\n17 \nApplication of nano-patterned Si surface:  327 \n 328 \nFigure 8: (a) UV-VIS-IR spectra of pristine and nano-patterned surfaces, (b) variation of rms 329 \nroughness and reflectivity with ripple wavelength. 330 \nThe optical response of pristine and Ar-ion-induced nano-patterned silicon surfaces are 331 \ninvestigated through UV-VIS reflectivity & presented in Figure 8. Figure 8(a) depicts the 332 \nchange in reflectivity of the silicon surface due to  the presence of nano -pattern. With the 333 \nincrease of bombardment time , reflectivity decreases drastically. Further the change in 334 \nreflectivity in UV region with respect to rms roughness and ri pple wavelength is also 335 \ninvestigated in Fig 8(b). It is clear from the Fig 8(b), that the reflectivity decreases with the 336 \nincrease in ripple wavelength. Further, variation of the rms roughness of the Ar -ion irradiated 337 \nnano-patterned Si surfaces is nominal. In general, the presence of nano-patterns on the surface 338 \nenhances the reflection of UV-VIS light, due to light trapping by multiple reflections [37–39]. 339 \nThe presence of a well-defined ripple pattern, formed due to Ar-ion bombardment (for three 340 \nhours), leads to the development of a well-defined nanopattern on the silicon surface and hence 341 \nthe reflectivity is minimal compared to the other two surfaces. The change in reflectivity 342 \ndepends on the change in the electronic structure as well as surface topography of the material. 343 \nSuch a change in electronic structure can contain several factors, like the change in chemical 344 \n\n\n18 \nnature or impurity incorporation on the surface, and amorphization of the surface. Ar-being 345 \ninert causes no chemic al modification of the silicon surface, along with the absence of the 346 \ntrapped Ar-ion on silicon surface (concluded from TEM image in Fig 5(g)), particularly in this 347 \nlower energy regime. Therefore, i n the present case, the amorphization due to ion beam 348 \nsputtering, the amorphization of the silicon surface, and the nanostructure formation on the 349 \nsurface, together change the electronic density of the material, causing a lowering in 350 \nreflectivity. The tailoring of the reflectivity by developing nano-structure is widely applicable 351 \nfor anti-reflective (ARC) coating and photovoltaic device applications [40,41].  352 \n The formation of nano -structure on the silicon surface by inert ion bombardment is a 353 \nconsequence of ion-induced instability on the surface by the interplay between sputtering and 354 \nmass redistribution of surface atoms [42,43]. During ion bombardment, the sputtering of the 355 \nnative silicon oxide layer  along with the bulk silicon takes place. The rate of sputtering of 356 \nsilicon oxide and the eleme ntal silicon is different which initiates the instability at the initial 357 \ntime of bombardment. Further, the instability is enhanced by differences in the sputtering yield 358 \nof silicon in native oxide and elemental form. These two effects are jointly responsib le for 359 \ndeveloping the nano-pattern on the surface. With the increasing bombardment time, the rate of 360 \nsputtering of Si in elemental and compound form increases, and a well -defined periodic 361 \nstructure is observed. Further, during sputter erosion, the native oxide layer is mostly removed, 362 \nforming a silicon ripple structure . Further, the exposure of the nano -patterned silicon surface 363 \nto air during optical measurement ensures the formation of non -uniform silicon oxide on the 364 \nnano-patterned silicon surface. Due to such preferential spatial formation of silicon oxide, the 365 \nchange in reflectivity is triggered.  Therefore, nano -patterned silicon surfaces can be an 366 \nalternative for memory devices.  367 \n 368 \n 369 \n\n19 \nConclusion:  370 \nIn this comprehensive study, the intricacies of an ultra-low energy magnetron-based Electron 371 \nCyclotron Resonance (ECR) ion source  are studied  systematically by exploring  optimal 372 \nparameters to achieve stable and intense beam currents. The cost -effectiveness and versatility 373 \nof this ion source make it particularly noteworthy, offering a practical solution for generating 374 \nreasonable beam currents. Notably, the ion source opera tes within an ultra -high vacuum 375 \nenvironment, rendering it valuable for both implantation and deposition processes. Our 376 \nmeticulous investigation into the ECR-based ultra-low energy ion source lays the groundwork 377 \nfor ion beam-induced nano-structuring and layer-wise material modification, affording precise 378 \ncontrol over ion penetration depth and fluence. The manuscript emphasizes an intriguing 379 \nalternative perspective by highlighting the in-depth optimization of low energy ion source and 380 \ninert ion -induced nano -patterning as a viable approach for ARC coating . 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