Femtosecond laser drilling controlled with laser-generated ultrasound pressure

Femtosecond laser drilling controlled with laser-generated ultrasound pressure | 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 Femtosecond laser drilling controlled with laser-generated ultrasound pressure YOSHIO HAYASAKI, TAKUMA MIURA This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4931402/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 20 Nov, 2024 Read the published version in Applied Physics B → Version 1 posted 9 You are reading this latest preprint version Abstract Laser drilling of glass using tightly focused femtosecond laser pulses while monitoring laser-generated sound is demonstrated, aiming laser drilling controlled by laser-generated sound. The amount of laser ablation was found to have a monotonical relation to the intensity of the sound pressure. It was also found that when the laser pulses were focused on the glass surface, the sound pressure increased in the initial stage of the laser drilling and then declined as the hole became deeper. These behaviors were the result of increasing ablation caused by surface roughening and loss of sound propagation through the hole, respectively. It was further found that the movement of the objective lens (OL) toward the target material at an appropriate constant speed created a hole with a large depth and narrow entrance (a high aspect ratio); that is, the lens movement changed the performance of the laser drilling. A simple method for moving the lens using laser-generated sound was adopted in this study. The axial position of the OL was controlled by maximizing the sound pressure at each pulse irradiation to obtain a hole with a high aspect ratio, which was the same as the maximum hole depth obtained by the iterative experiments in the constant-speed control of the OL. More sophisticated control methods should be developed according to the given applications. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Mechanical drilling [1], ultrasonics [2], waterjets and abrasive waterjets [3], electrolytic techniques [4], electrical discharges [5], and lasers [6] have been used to produce holes for joining composite materials in aerospace, automotive, and many other industries, and recently for conduction between substrates and layers in electronic devices. In the fabrication of composite materials, cracks, burrs, and delamination occur simultaneously, and tool wear also occurs; thus, an appropriate machining method is required. In addition, the increasing sophistication and miniaturization of electronic devices require hole processing at ever-decreasing sizes. Laser processing, which can process microholes in various materials, was used to meet this requirement. Furthermore, ultrashort pulsed lasers produce smaller heat-affected zones, making them useful for drilling microholes in various materials, including metals [7], semiconductors [8], and dielectrics [9]. Hole drilling in metal has been applied to the fabrication of nozzles for fuel injectors in automobile engines, and it has been demonstrated that hole drilling using an ultrashort pulsed laser in metal can achieve high reliability in nozzle fabrication [10]. In semiconductors, through-silicon vias (TSVs) with high aspect ratios for electrical connections between three-dimensional (3D) silicon integrated circuit (IC) chips have been successfully fabricated using a femtosecond laser-tailored Bessel beam [8]. In dielectrics, ultrashort pulsed laser drilling was applied to drill high-quality, high-aspect-ratio holes in polymers [11], alumina ceramics [12], and SiC [13]. The sharpness of the hole edges and high aspect ratio are important structures for hole drilling. To process the desired hole by laser drilling, it is necessary to select the laser parameters based on the material. The laser parameters, such as wavelength, pulse energy, pulse duration, repetition rate, scanning speed, spot size, and focal position, should be investigated experimentally. The drilled holes should be observed to verify that the desired processing has taken place. There are two types of observation methods: in-process monitoring, which is performed during the machining process, and post-process monitoring, which is performed after the machining process. The in-process monitoring provides real-time feedback of the processing parameters. Thus, the optimal parameters for processing unknown materials can be defined using a smaller number of prior experiments than the laser parameter optimization experiments we have done so far. The in-process monitoring can be performed by optical and acoustic methods [14]. The optical methods are based on interferometry, laser confocal microscopy, white-light interferometry [15], optical coherence tomography [16], X-rays, thermal imaging cameras, direct observation of the processing trace with an imager [17], and observation of plasma emission during focusing [18]. The interferometric techniques used include Michelson interferometry [19], Mach-Zehnder interferometry [20], Fabry-Perot interferometry [21], and FBG fiber lasers [22]. Interferometric techniques that acquire high-frequency vibrations without contact have been used for photoacoustic imaging [23]. Optical coherence tomography (OCT) observation is used for in-process monitoring of line-shaped beams [24]. In the method of directly observing the ablated hole using a CCD camera, it was possible to observe the shape change of the hole at high resolution during laser processing [17]. In addition, it has been reported that the method of observing plasma emission during focusing using a CCD camera allowed high ablation efficiency to be maintained by controlling the focus position based on the acquired emission intensity [18]. An important requirement is that the area around the measurement point must be transparent. Optical methods are extremely effective because of their high speed, high repetition rate, non-contact, spectroscopic, multi-point, parallel observations, and quantitative nature. With an acoustic method, pressure waves generated by laser irradiation [25] were detected. The fine approach is to detect the sound waves generated at deep sites in optically opaque materials; therefore, this approach has been used as a complementary approach to the above optical methods. Acoustic methods are implemented using a microphone, hydrophone, and acoustic emission [26]. The acoustic emission involves directly contacting the target and acquiring sound waves propagating inside the target. It has been used to detect cracks in various metal structures such as bridges [27] and aircraft [28]. In addition, measurements were performed during the processing of sound waves related to the laser focal position during laser material removal processing [29]. Methods based on acoustic emission have the advantage of small acoustic impedance differences, which reduce the attenuation of sound waves and the acquisition of sound waves in the high-frequency range. In the hydrophone technique, the measurement device is placed underwater, and sound waves propagating through the water are observed. Laser-excited sound sources are capable of remote sound source generation and are used for underwater communication and imaging of marine environments. Femtosecond laser filament formation has a significant effect on imaging. Therefore, studies on the dynamics and characteristics of femtosecond laser filament formation in water using hydrophones have been conducted [30,31]. It was confirmed that femtosecond laser-generated ultrasound is broadband [32]. Sound waves are acquired using a microphone and are used to measure the structure and function of living organisms [33]. In the field of laser processing, an autofocus system for direct laser interference patterning [34] and analysis of metals [35-38] and graphene [39] have been reported. Some observations of sound waves with a microphone were conducted to analyze the mechanism of nonlinear absorption of femtosecond lasers in air [40, 41]. The microphone technique acquires sound waves that propagate through the air, which has the advantage that contact with the material is not needed and the technique is not dependent on the shape of the material. In addition, the speed of sound waves propagating through air is slower than the speed of sound waves propagating inside a material or in water, and this has the advantage of being able to observe sound waves with small wavelengths. In the present study, laser drilling of a glass sample was investigated as a subject of in-process monitoring of ultrasound pressure and feedback control for femtosecond laser processing. The ultrasound excited by a focused femtosecond laser was observed using a microphone. The axial position of an objective lens (OL) was controlled. In Sec. 2, two methods for controlling the focal point are described: constant-speed control and sound-driven control. In Sec. 3, the experimental setup is presented. In Sec. 4, first, the basic characteristics of laser drilling of glass using constant-speed control of an OL are described. Next, the characteristics using sound-driven control are investigated, and interesting and effective features are discussed through comparing the results with those obtained using constant-speed control. We know that there are many laser parameters in laser processing and many types of procedures for regulating them, even if it is only for ultrasound in-process monitoring and feedback control of the laser parameters. This study is still in the initial step of developing new effective laser processing methods by applying simple positional control of the OL using ultrasound to a simple laser drilling application. Axial control of the focal point 2.1 Constant-speed control The axial position of the focal point should be carefully controlled to achieve the desired processing geometry. Focal point control is usually implemented by the axial movement of an OL. We now consider the simplest control procedure to compare the control procedure using laser-produced sounds when a hole is processed by multiple laser pulses. The axial position of the OL is denoted as P OL and is described as a function of the number of pulses irradiated on the target material, n : P OL = f ( n ). (1) A suitable f ( n ) can be determined through many iterative experiments. First, we define a simple control method in which the OL is moved in the depth direction at regular intervals as a linear function, P OL = a 0 + a 1 n , (2) where a 0 (µm) is the initial axial position (in our experiments, a 0 = 0.0 µm when the laser beam is focused on the surface), and a 1 (µm/pulse) is the amount of movement per pulse irradiation. The positive direction is defined as the increasing depth direction. We named this control method the constant speed control. 2.2 Sound-driven control Next, we consider the axial position control of the OL using the sound generated by laser pulse irradiation of a target material. As a preliminary experiment, the fundamental properties of laser-generated sound were investigated. Figure 1 (a) shows the sound pressure and diameter of the fabricated shallow hole versus the axial position of the OL. The sound pressure is detected by a microphone as a voltage. Each hole fabricated with the sound generation was formed by a single laser pulse irradiation with a pulse energy of 2.85 µJ. The greatest sound pressure was obtained when the laser pulse was focused on the surface, and a shallow hole with the maximum diameter was observed. The procedure used to obtain this graph is summarized as follows. The diameter was obtained from the pixel values of an image captured from above the sample using a custom-made transmission microscope built into the laser processing machine. The image was subjected to brightness adjustment and a Gaussian filter for denoising. The area darker than the surroundings was considered the fabricated hole, and its diameter was obtained. The horizontal axis represents the amount of movement of the OL from the origin along the sample optical axis. Then, the axial position of 0 was determined based on the image-focusing plane, which was matched to the laser-focusing plane in advance. The positive direction was the direction in which the OL moved toward the sample. The ultrasound pressure was defined as the maximum value of the first peak, as shown in Fig. 1 (b). The laser was irradiated with a pulse energy of 23.4 µJ. Furthermore, the sound pressure decreased as the laser-drilled hole became deeper, as shown in Fig. 1 (c), because the diameter of the processed hole was much smaller than the wavelength of the sound, and the ultrasound propagated to the surroundings and through the hole with large loss. The depth of the hole was obtained using an optical microscope to observe a side view of the sample, which will be described in the next section, and image processing for Gaussian filtering and dark area extraction, as described above. From these experimental results, it can be concluded that the sound pressure increased when strong laser ablation occurred. Furthermore, the magnitude of laser ablation cannot be determined using only the absolute sound pressure because it depends on the depth of the hole. Therefore, the following simple procedure was derived to continuously control the focus position using laser-generated sound. P OL is the sum of the displacements d s ( n ) after the \(\:n\) th pulse irradiation: $$\:{P}_{OL}={\sum\:}_{n}{d}_{s}\left(n\right)$$ 3 . In the simple procedure we adopted, d s ( n ) is obtained from the sound pressure p s ( n ) at the n -th pulse as follows: $$\:{d}_{s}\left(n\right)={\alpha\:}_{s}\{{p}_{s}\left(n\right)-{p}_{s}\left(n-1\right)\}$$ 4 , where \(\:{\alpha\:}_{s}\) is a coefficient for converting from the sound pressure change to the displacement. We called this calculation procedure sound-driven control. There are several ways to obtain the sound pressure, for example, an amplitude of a specific frequency, a combination of multiple frequencies, and a temporal summation of the sound. We do not know which is the best at present, so we selected to pick out the first peak because we observed that its magnitude monotonically depends on the amount of laser ablation. In addition, we also considered a calculation procedure in which the focal position was obtained from the sound pressure. This will be the subject of future research. Experimental setup Figure 2 shows the experimental setup. The light source was a femtosecond laser (Amplitude Laser, Tangerine) with a center wavelength of l = 1028 nm and pulse width of 129 fs. The laser pulse was focused on a sample with an objective lens (OL) having a numerical aperture (NA) of 0.55 (Sigmakoki, EPLE-50). The OL was moved along the optical axis using a piezo actuator (Sigmakoki, SFS-OBL-1). A sample was placed on a computer-controlled two-dimensional motorized stage (MS; Physik Instrument, L-738). The excited sounds were detected by a condenser microphone (ACO, TYPE7118) operating in the frequency range from 10 Hz to 200 kHz. The output signals were acquired by a computer via a preamplifier (ACO, TYPE4116), a signal amplifier (ACO, TYPE6030), and an oscilloscope (National Instruments, PXIe-5162). The sound frequency was obtained by fast Fourier transform (FFT) on the computer. The sampling frequency of the A/D converter was 400 kHz. A custom-made optical microscope was used to observe a side view of the sample. The microscope was composed of a CMOS image sensor (IS; Imaging Source, DMK33UX174), a white LED illuminator, and 2x microscope optics composed of two lenses. A side view of the hole was observed as a dark area, and the structural features were obtained from image processing. The depth and diameter of the hole were obtained by adjusting the contrast and applying a Gaussian filter to the captured images. Experimental results 4.1 Constant-speed control of OL The OL was moved at a constant speed of a 1 , and the optimum speed was experimentally investigated. a 1 ranged from 0 to 70 nm/pulse under a 0 = 0.0 µm. The pulse energy was set to 23.4 µJ. The sample was a crown glass plate (Matsunami, S1111). Figure 3 shows the sound pressure, and the depth and diameter of the hole at a 1 = 0 nm/pulse. In the initial process of the ablation up to approximately 10 pulses, the surface of the glass was initially flat and became rough as the pulse irradiation proceeded; then, the photon absorption gradually increased, and the ablation rate increased at the same time. Consequently, the laser-excited sound pressure increased. In the next process, the pressure became smaller with an increasing number of pulse irradiations (with increasing hole depth). This was because of a decrease in the amount of ablation due to the mismatch between the focal point and the material surface caused by surface changes in the ablation and reflections on the surface of the hole, as well as the fact that sound with a wavelength larger than the size of the ablated hole did not escape. The sound became bigger again from the 32nd pulse to the 103rd pulse and at the 182nd pulse. These increases of the sound were issued since the side of the hole was ablated. The depth became constant after ~ 300 pulses. This was because the phase of the laser changed with depth due to refraction inside the glass and reflection at the hole surface. When the depth of the hole was constant, the sound pressure was approximately 0.001 V, which was the resolution of the microphone. The diameter of the processed hole remained constant after 55 pulses. Fig༎3. Sound pressure, and diameter and depth of the microhole versus the number of pulses when the OL was fixed ( a 1 = 0 nm/pulse). The side image of the processed hole was improved the contrast. Figure 4 shows the sound pressure, and diameter and depth of the hole at a 1 = 20 and 50 nm/pulse. The dashed line indicates the OL position. The sound pressure decreased more slowly at a1 = 20 nm/pulse than at a 1 = 0 nm/pulse because the movement of the OL continued to cause greater ablation. From the 480th pulse to the 660th pulse, the sound pressure appeared again after it had disappeared. It was generated by ablation on the side of the hole. When a 1 = 50 nm/pulse, a larger sound pressure caused by the ablation on the side of the hole and the entrance of the hole was observed. The diameter of the processed hole increased from around the 330th pulse, as shown in the graph. After the 730th pulse, no sound was observed, although the hole was internally processed. It is considered that the sound due to internal processing was attenuated due to the difference in acoustic impedance with the glass. The depth of the hole when a 1 = 50 nm/pulse was greater than the depth of the hole at 20 nm/pulse up to the 34th pulse, and thereafter, there was no significant difference between the two cases. At approximately the 500th pulse, hole processing was stopped in both cases. The final depth of the hole fabricated with the constant-speed control was greater than that with the zero-speed control ( a 1 = 0 nm/pulse). It was found that it was effective to move the focal point closer to the material to deliver the laser energy to the deepest part of the hole. It was also found that the hole became deeper when the speed was high in the initial stage of laser irradiation (less than 10 pulses), and as processing progressed, the ablation rate decreased; thus, the OL control speed needed to be slowed down. The highest aspect ratio hole was obtained at a speed of 20 nm/pulse, which was obtained through many experimental trials to fabricate a deep hole with a large aspect ratio. These results indicate that the position of the OL should be carefully controlled in addition to the pulse energy according to the target material and the required performance metrics, such as quality and speed. 4.2 Sound-driven control of OL Figure 5 shows typical behaviors of the sound pressure and depth and diameter of the hole under the sound-driven control. The behavior of the hole fabricated with sound-driven control with α s = 5 µm/V, shown in Fig. 5 (a), was similar to that with the fixed focal point (0.0 nm/pulse), shown in Fig. 2 . This was caused by the small movement of the focal point due to the small value of α s . When α s = 25 µm/V, as shown in Fig. 5 (b), for up to 100 pulses, the focal point was vibrating; then, the ablated areas were moved near the bottom of the hole and inside the material according to the changes of the focal point. A large sound was detected when the laser pulse was irradiated on the bottom of the hole, but a small sound was detected when it was irradiated inside the material. The vibrations were attenuated after 101 pulses. The attenuation behavior of the sound pressure with increasing hole depth was almost the same as that for constant speed control with α 1 = 20 µm/pulse, at which the hole with the highest aspect ratio was formed. When α s was large, the OL may not have been controlled at the proper position due to the large amount of movement of the OL. When α s was small, little change was observed in the position of the OL even as processing progressed. Therefore, it is necessary to find the appropriate value of α s in the sound-driven position control through experiments. Figure 6 shows the depth and diameter of the microhole and the aspect ratio versus the OL speed. The speed was a 1 = 0–70 nm/pulse in the constant-speed control, and the average speed in each case of α s = 5 to 35 µm/V in the sound-driven control. As shown in Fig. 6 (a), the depth increased with increasing a 1 to 30 nm/pulse in the constant-speed control, because the movement of the focal point was effective for the beam to reach the bottom of the hole. However, the depth decreased at speeds of more than 50 nm/pulse. Then the speeds were faster than the ablative speed of the hole, and accordingly, the internal processing happened, as can be seen from the camera observation. The sound-driven control that maximizes the sound automatically avoided the internal processing and effectively irradiated the laser beam to the bottom of the hole by giving an appropriate α s . As shown in Fig. 6 (b), the hole diameter slightly increased (was almost constant) for a 1 = 0 to 20 nm/pulse, increased for a 1 = 20 to 40 nm/pulse, and was nearly constant for a 1 = 40 to 70 nm/pulse in the constant-speed control. The reason for the increase was that the side wall near the entrance of the hole was ablated by the conical shape of the focused beam. This was because the focus position became so deep that the ablation did not occur above a 1 = 40 nm/pulse. In the sound-driven control, the hole diameter exhibited small changes as the parameter α s was varied from 5 to 25 µm/V. However, the hole diameter became large for α s = 35 µm/V, because some pulses were focused inside the material and ablated a large area on the material surface, as shown in Fig. 4 (b). As shown in Fig. 6 (c), the largest aspect ratio was obtained at α 1 = 20 nm/pulse in the constant-speed control. In the sound-driven control, the highest aspect ratio was at α s = 25 µm/V. When α s was increased, a deep hole was formed, but the diameter of the hole was extended. Therefore, an appropriate α s must be determined. Conclusion We performed laser drilling of glass using tightly focused femtosecond laser pulses while monitoring the laser-generated sound. In the experiments conducted toward the goal of developing laser drilling controlled with laser-generated sound, we found that the laser ablation intensity is monotonically related to the sound pressure. When the laser pulses were focused on the glass surface, the sound pressure increased in the initial stage of laser drilling and then decreased as the hole became deeper. The number of pulses in the initial stage depended on the pulse energy, and it was several to 10 pulses in the experiments described in this paper. The increase in the sound pressure was derived from the increase of the ablation caused by roughening the glass surface. The decrease in the sound pressure was caused by the loss of sound propagation through the hole whose diameter was much smaller than the wavelength of the sound. We found that an axial movement of the OL toward the target material changed the laser drilling conditions. The constant-speed control, which is a simple method, made a deeper hole than focusing a laser beam on a glass surface with the OL. However, when the speed was high, the laser pulse was focused on inside the material at a position deeper than the bottom of the hole, and the processing stopped, resulting in internal processing. Simultaneously, the side wall of the hole was ablated, and the hole had a small aspect ratio. Through these experiments, we found that the hole with the highest aspect ratio was processed at 20 nm/pulse; however, the hole drilling for 50 nm/pulse was more effective than that for 20 nm/pulse in the initial several pulses. This indicates that more careful control of the OL will be effective due to the condition of the target material at each pulse irradiation. As the most significant result of this research, we found that the sound-driven control used to maximize the sound pressure at each pulse irradiation obtained a hole with a high aspect ratio the same as the maximum hole depth obtained by the iterative experiments in the constant-speed control of the OL. In the trial with the hole having the highest aspect ratio, the optimal focal position was sought while repeating the up-and-down movement of the OL in the first several pulses, and the OL was moved toward the target material. In this paper, a simple method in which the movement of the OL was calculated from the sound pressures generated by the two previous pulse irradiations was adopted; however, a more sophisticated method will need to be developed in the future. In future research, we will examine whether it is effective to control the laser processing with not only the sound pressure but also the sound frequency. The sound frequency distribution for the 100th pulse was almost identical to that of the first pulse. In the frequency region detected by a 200 kHz microphone, no specific distribution according to the processed structures was observed. According to Webster's Horn equation [ 43 ], which describes sound waves along a rigid axisymmetric tube, the resonant frequency of an air column with a 6 µm diameter and a 20 µm length is on the order of 10 MHz. Therefore, to observe the frequency modulation caused by the hole structure, it is necessary to measure the sound in the resonant frequency region. Declarations Funding. This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Photonics and Quantum Technology for Society 5.0” (Funding agency: QST). 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Zhao, Z. Ge, Y. Chen, L. Lin, N. Zhang, and W. Liu, “Megahertz ultrasonic source induced by femtosecond laser irradiation of graphene foam,” Opt. Laser Tech. 151 , 108077 (2022). D. V. Kartashov, A. V. Kirsanov, A. M. Kiselev, A. N. Stepanov, N. N. Bochkarev, Y. N. Ponomarev and B. A. Tikhomirov, “Nonlinear absorption of intense femtosecond laser radiation in air,” Opt. Express. 14 , 7552–7558 (2006). S. Q. Wu, J. S. Liu, S. L. Wang, and Y. Zeng, “Experimental investigation on photoacoustic emission from femtosecond-laser-induced air plasma,” Indian J. Phys. 88 , 329–332 (2014). H. Hu, X. Wang, N. Zhang, and P. Wang, “Generation of multiple stress waves in silica glass in high fluence femtosecond laser ablation,” Appl. Phys. Lett. 97 , 061117 (2010). E. Eisner, “Complete solutions of the “Webster” horn equation,” J. Acoust. Soc. Am. 41 , 1126–1146 (1967). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 20 Nov, 2024 Read the published version in Applied Physics B → Version 1 posted Reviews received at journal 09 Sep, 2024 Reviews received at journal 08 Sep, 2024 Reviewers agreed at journal 27 Aug, 2024 Reviewers agreed at journal 26 Aug, 2024 Reviewers agreed at journal 26 Aug, 2024 Reviewers invited by journal 21 Aug, 2024 Editor assigned by journal 20 Aug, 2024 Submission checks completed at journal 18 Aug, 2024 First submitted to journal 17 Aug, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4931402","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":352469466,"identity":"b6c1261b-a6ba-4ad2-9be4-af20a181e74a","order_by":0,"name":"YOSHIO HAYASAKI","email":"data:image/png;base64,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","orcid":"","institution":"Utsunomiya University","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"YOSHIO","middleName":"","lastName":"HAYASAKI","suffix":""},{"id":352469468,"identity":"f591a692-f328-45e8-aa77-aa342a2ca0fd","order_by":1,"name":"TAKUMA MIURA","email":"","orcid":"","institution":"Utsunomiya University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"TAKUMA","middleName":"","lastName":"MIURA","suffix":""}],"badges":[],"createdAt":"2024-08-18 01:08:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4931402/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4931402/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00340-024-08355-1","type":"published","date":"2024-11-20T15:58:05+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":64479842,"identity":"5d0d5896-3bb2-4d17-bdd6-7de9ad6afbe9","added_by":"auto","created_at":"2024-09-13 16:13:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":201418,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Sound pressure and diameter of the fabricated microhole versus the axial position of the OL. The sound pressure is detected by the microphone as a voltage. The horizontal axis indicates the amount by which the OL is moved toward the sample. The highest sound pressure was observed at this time. (b) An example of a temporal trace of the sound pressure. (c) Sound pressure versus the depth of a microhole.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4931402/v1/b6afc1bc68d7b80d5077bebe.png"},{"id":64479838,"identity":"e3f1c98b-c489-4223-b08a-c97d41ff19d5","added_by":"auto","created_at":"2024-09-13 16:13:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":58814,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental setup. M: mirror, L: lens, OL: microscope objective lens, IS: CMOS image sensor, MS: motorized stage.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4931402/v1/c8081c36154b19dd026df912.png"},{"id":64479839,"identity":"b494c581-255f-4ebb-97d5-aaf307fbf56d","added_by":"auto","created_at":"2024-09-13 16:13:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":148546,"visible":true,"origin":"","legend":"\u003cp\u003eSound pressure, and diameter and depth of the microhole versus the number of pulses when the OL was fixed (\u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e = 0 nm/pulse). The side image of the processed hole was improved the contrast.\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4931402/v1/42dac8991d2bc412c2ed73ec.png"},{"id":64479841,"identity":"71c755ac-a67a-452e-9acd-6b78e01379c6","added_by":"auto","created_at":"2024-09-13 16:13:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":161434,"visible":true,"origin":"","legend":"\u003cp\u003eSound pressure, diameter, and depth of the microhole versus the number of pulses when the pulse energy was 23.4 mJ. (a) 20 and (b) 50 nm/pulse. The contrast of the image of the processing hole was adjusted to improve visibility. The scale bar is 10 µm.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4931402/v1/6c116052b0780172ce4ea6cc.png"},{"id":64479840,"identity":"90f3832a-6f61-4df0-b48d-7d307d6e91d2","added_by":"auto","created_at":"2024-09-13 16:13:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":149202,"visible":true,"origin":"","legend":"\u003cp\u003eSound pressure, diameter, and depth of the microhole versus the number of pulses in the case of sound-driven control. (a) \u003cem\u003ea\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e = 5 μm/V and (b)\u003cem\u003e a\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e = 25 μm/V. The contrast of the image of the processing hole was adjusted to improve visibility.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4931402/v1/b7c282ef57c4d28e77353e6e.png"},{"id":64479837,"identity":"5591775b-c67f-4e53-a1f9-07bbcfa48521","added_by":"auto","created_at":"2024-09-13 16:13:03","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":102686,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Depth, (b) diameter, and (c) aspect ratio versus the movement speed of the OL.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4931402/v1/89d29a42b02714da15a6b06c.png"},{"id":69835226,"identity":"57d2433c-1ceb-4702-aefc-a822628d364e","added_by":"auto","created_at":"2024-11-25 16:13:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1234499,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4931402/v1/cb20b86c-e064-4647-81e1-ba652d741066.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Femtosecond laser drilling controlled with laser-generated ultrasound pressure","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMechanical drilling [1], ultrasonics [2], waterjets and abrasive waterjets [3], electrolytic techniques [4], electrical discharges [5], and lasers [6] have been\u0026nbsp;used\u0026nbsp;to produce holes for joining composite materials in aerospace, automotive, and\u0026nbsp;many other\u0026nbsp;industries, and recently for conduction between substrates and layers in electronic devices. In the fabrication of composite materials, cracks, burrs, and delamination occur simultaneously, and tool wear\u0026nbsp;also occurs; thus, an appropriate machining method is required. In addition, the increasing sophistication and miniaturization of electronic devices require hole processing at ever-decreasing sizes. Laser processing, which can process microholes in various materials, was used to meet this requirement. Furthermore, ultrashort pulsed lasers produce smaller heat-affected zones, making them useful for drilling microholes in various materials, including metals [7], semiconductors [8], and dielectrics [9]. Hole drilling in metal has been applied to the fabrication of nozzles for fuel injectors in automobile engines, and it has been demonstrated that hole drilling using an ultrashort pulsed laser in metal can achieve high reliability in nozzle fabrication [10]. In semiconductors, through-silicon vias (TSVs) with high aspect ratios for electrical connections between three-dimensional (3D) silicon integrated circuit (IC) chips have been successfully fabricated using a femtosecond laser-tailored\u0026nbsp;Bessel beam [8]. In dielectrics, ultrashort pulsed laser drilling was applied to drill high-quality, high-aspect-ratio holes in polymers [11], alumina ceramics [12], and SiC [13].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;The sharpness of the hole edges and high aspect ratio are important structures for hole drilling. To process the desired hole by laser drilling, it is necessary to select the laser parameters based on the material. The laser parameters, such as wavelength, pulse energy, pulse duration, repetition rate, scanning speed, spot size, and focal position, should be investigated experimentally. The drilled holes should be observed to verify that the desired processing has taken place.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;There are two types of observation methods: in-process monitoring, which is performed during the machining process, and post-process monitoring, which is performed after the machining process. The in-process monitoring provides real-time feedback of the processing parameters. Thus, the optimal parameters for processing unknown materials can be defined using a smaller number of prior experiments than the\u0026nbsp;laser parameter optimization experiments we have done so far.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;The in-process monitoring can be performed by optical and acoustic methods [14]. The optical methods are based on interferometry, laser confocal microscopy, white-light interferometry [15], optical coherence tomography [16], X-rays, thermal imaging cameras, direct observation of the processing trace with an imager [17], and observation of plasma emission during focusing [18]. The interferometric techniques used include Michelson interferometry [19], Mach-Zehnder interferometry [20], Fabry-Perot interferometry [21], and FBG fiber lasers [22]. Interferometric techniques that acquire high-frequency vibrations without contact have been used for photoacoustic imaging [23]. Optical coherence tomography (OCT) observation is used for in-process monitoring of line-shaped beams [24]. In the method of directly observing the ablated hole using a CCD camera, it was possible to observe the shape change of the hole at high resolution during laser processing [17]. \u0026nbsp;In addition, it has been reported that the method of observing plasma emission during focusing using a CCD camera allowed high ablation efficiency to be maintained by controlling the focus position based on the acquired emission intensity [18]. \u0026nbsp;An important requirement is that the area around the measurement point must be transparent. Optical methods are extremely effective because of their high speed, high repetition rate, non-contact, spectroscopic, multi-point, parallel observations, and quantitative nature.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;With an acoustic method, pressure waves generated by laser irradiation [25] were detected. The fine approach is to detect the sound waves generated at deep sites in optically opaque materials; therefore, this approach has been used as a complementary approach to the above optical methods. Acoustic methods are implemented using a microphone, hydrophone, and acoustic emission [26]. The acoustic emission involves directly contacting the target and acquiring sound waves propagating inside the target. It has been used to detect cracks in various metal structures such as bridges [27] and aircraft [28]. In addition, measurements were performed during the processing of sound waves related to the laser focal position during laser material removal processing [29].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Methods based on acoustic emission have the advantage of small acoustic impedance differences, which reduce the attenuation of sound waves and the acquisition of sound waves in the high-frequency range. In the hydrophone technique, the measurement device is placed underwater, and sound waves propagating through the water are observed. Laser-excited sound sources are capable of remote sound source generation and are used for underwater communication and imaging of marine environments. Femtosecond laser filament formation has a significant effect on imaging. Therefore, studies on the dynamics and characteristics of femtosecond laser filament formation in water using hydrophones have been conducted [30,31]. It was confirmed that femtosecond laser-generated ultrasound is broadband [32]. Sound waves are acquired using a microphone and are used to measure the structure and function of living organisms [33]. In the field of laser processing, an autofocus system for direct laser interference patterning [34] and analysis of metals [35-38] and graphene [39] have been reported. Some observations of sound waves with a microphone were conducted to analyze the mechanism of nonlinear absorption of femtosecond lasers in air [40, 41]. The microphone technique acquires sound waves that propagate through the air, which has the advantage that contact with the material is not needed and the technique is not dependent on the shape of the material. \u0026nbsp;In addition, the speed of sound waves propagating through air is slower than the speed of sound waves propagating inside a material or in water, and this has the advantage of being able to observe sound waves with small wavelengths.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; In the present study, laser drilling of a glass sample was investigated as a subject of in-process monitoring of ultrasound pressure and feedback control for femtosecond laser processing. The ultrasound excited by a focused femtosecond laser was observed using a microphone. The axial position of an objective lens (OL) was controlled. In Sec. 2, two methods for controlling the focal point are described: constant-speed control and sound-driven control. In Sec. 3, the experimental setup is presented. In Sec. 4, first, the basic characteristics of laser drilling of glass using constant-speed control of an OL are described. Next, the characteristics using sound-driven control are investigated, and interesting and effective features are discussed through comparing the results with those obtained using constant-speed control. We know that there are many laser parameters in laser processing and many types of procedures for regulating them, even if it is only for ultrasound in-process monitoring and feedback control of the laser parameters. This study is still in the initial step of developing new effective laser processing methods by applying simple positional control of the OL using ultrasound to a simple laser drilling application.\u003c/p\u003e\n\u003ch3\u003eAxial control of the focal point\u003c/h3\u003e\n\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Constant-speed control\u003c/h2\u003e \u003cp\u003eThe axial position of the focal point should be carefully controlled to achieve the desired processing geometry. Focal point control is usually implemented by the axial movement of an OL. We now consider the simplest control procedure to compare the control procedure using laser-produced sounds when a hole is processed by multiple laser pulses. The axial position of the OL is denoted as \u003cem\u003eP\u003c/em\u003e\u003csub\u003eOL\u003c/sub\u003e and is described as a function of the number of pulses irradiated on the target material, \u003cem\u003en\u003c/em\u003e:\u003c/p\u003e \u003cp\u003e \u003cem\u003eP\u003c/em\u003e \u003csub\u003eOL\u003c/sub\u003e = \u003cem\u003ef\u003c/em\u003e(\u003cem\u003en\u003c/em\u003e). (1)\u003c/p\u003e \u003cp\u003eA suitable \u003cem\u003ef\u003c/em\u003e(\u003cem\u003en\u003c/em\u003e) can be determined through many iterative experiments. First, we define a simple control method in which the OL is moved in the depth direction at regular intervals as a linear function,\u003c/p\u003e \u003cp\u003e \u003cem\u003eP\u003c/em\u003e \u003csub\u003eOL\u003c/sub\u003e = \u003cem\u003ea\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e + \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u003cem\u003en\u003c/em\u003e, (2)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003ea\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e (\u0026micro;m) is the initial axial position (in our experiments, \u003cem\u003ea\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.0 \u0026micro;m when the laser beam is focused on the surface), and \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e (\u0026micro;m/pulse) is the amount of movement per pulse irradiation. The positive direction is defined as the increasing depth direction. We named this control method the constant speed control.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Sound-driven control\u003c/h2\u003e \u003cp\u003eNext, we consider the axial position control of the OL using the sound generated by laser pulse irradiation of a target material. As a preliminary experiment, the fundamental properties of laser-generated sound were investigated. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) shows the sound pressure and diameter of the fabricated shallow hole versus the axial position of the OL. The sound pressure is detected by a microphone as a voltage. Each hole fabricated with the sound generation was formed by a single laser pulse irradiation with a pulse energy of 2.85 \u0026micro;J. The greatest sound pressure was obtained when the laser pulse was focused on the surface, and a shallow hole with the maximum diameter was observed. The procedure used to obtain this graph is summarized as follows.\u003c/p\u003e \u003cp\u003eThe diameter was obtained from the pixel values of an image captured from above the sample using a custom-made transmission microscope built into the laser processing machine. The image was subjected to brightness adjustment and a Gaussian filter for denoising. The area darker than the surroundings was considered the fabricated hole, and its diameter was obtained. The horizontal axis represents the amount of movement of the OL from the origin along the sample optical axis. Then, the axial position of 0 was determined based on the image-focusing plane, which was matched to the laser-focusing plane in advance. The positive direction was the direction in which the OL moved toward the sample. The ultrasound pressure was defined as the maximum value of the first peak, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). The laser was irradiated with a pulse energy of 23.4 \u0026micro;J.\u003c/p\u003e \u003cp\u003eFurthermore, the sound pressure decreased as the laser-drilled hole became deeper, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c), because the diameter of the processed hole was much smaller than the wavelength of the sound, and the ultrasound propagated to the surroundings and through the hole with large loss. The depth of the hole was obtained using an optical microscope to observe a side view of the sample, which will be described in the next section, and image processing for Gaussian filtering and dark area extraction, as described above.\u003c/p\u003e \u003cp\u003eFrom these experimental results, it can be concluded that the sound pressure increased when strong laser ablation occurred. Furthermore, the magnitude of laser ablation cannot be determined using only the absolute sound pressure because it depends on the depth of the hole. Therefore, the following simple procedure was derived to continuously control the focus position using laser-generated sound. \u003cem\u003eP\u003c/em\u003e\u003csub\u003eOL\u003c/sub\u003e is the sum of the displacements \u003cem\u003ed\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e(\u003cem\u003en\u003c/em\u003e) after the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:n\\)\u003c/span\u003e\u003c/span\u003eth pulse irradiation:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{P}_{OL}={\\sum\\:}_{n}{d}_{s}\\left(n\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e.\u003c/p\u003e \u003cp\u003eIn the simple procedure we adopted, \u003cem\u003ed\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e(\u003cem\u003en\u003c/em\u003e) is obtained from the sound pressure \u003cem\u003ep\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e(\u003cem\u003en\u003c/em\u003e) at the \u003cem\u003en\u003c/em\u003e-th pulse as follows:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{d}_{s}\\left(n\\right)={\\alpha\\:}_{s}\\{{p}_{s}\\left(n\\right)-{p}_{s}\\left(n-1\\right)\\}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\alpha\\:}_{s}\\)\u003c/span\u003e\u003c/span\u003eis a coefficient for converting from the sound pressure change to the displacement. We called this calculation procedure sound-driven control. There are several ways to obtain the sound pressure, for example, an amplitude of a specific frequency, a combination of multiple frequencies, and a temporal summation of the sound. We do not know which is the best at present, so we selected to pick out the first peak because we observed that its magnitude monotonically depends on the amount of laser ablation. In addition, we also considered a calculation procedure in which the focal position was obtained from the sound pressure. This will be the subject of future research.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Experimental setup","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the experimental setup. The light source was a femtosecond laser (Amplitude Laser, Tangerine) with a center wavelength of \u003cem\u003el\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1028 nm and pulse width of 129 fs. The laser pulse was focused on a sample with an objective lens (OL) having a numerical aperture (NA) of 0.55 (Sigmakoki, EPLE-50). The OL was moved along the optical axis using a piezo actuator (Sigmakoki, SFS-OBL-1). A sample was placed on a computer-controlled two-dimensional motorized stage (MS; Physik Instrument, L-738). The excited sounds were detected by a condenser microphone (ACO, TYPE7118) operating in the frequency range from 10 Hz to 200 kHz. The output signals were acquired by a computer via a preamplifier (ACO, TYPE4116), a signal amplifier (ACO, TYPE6030), and an oscilloscope (National Instruments, PXIe-5162). The sound frequency was obtained by fast Fourier transform (FFT) on the computer. The sampling frequency of the A/D converter was 400 kHz. A custom-made optical microscope was used to observe a side view of the sample. The microscope was composed of a CMOS image sensor (IS; Imaging Source, DMK33UX174), a white LED illuminator, and 2x microscope optics composed of two lenses. A side view of the hole was observed as a dark area, and the structural features were obtained from image processing. The depth and diameter of the hole were obtained by adjusting the contrast and applying a Gaussian filter to the captured images.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Experimental results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Constant-speed control of OL\u003c/h2\u003e \u003cp\u003eThe OL was moved at a constant speed of \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e, and the optimum speed was experimentally investigated. \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e ranged from 0 to 70 nm/pulse under \u003cem\u003ea\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.0 \u0026micro;m. The pulse energy was set to 23.4 \u0026micro;J. The sample was a crown glass plate (Matsunami, S1111). Figure\u0026nbsp;3 shows the sound pressure, and the depth and diameter of the hole at \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0 nm/pulse. In the initial process of the ablation up to approximately 10 pulses, the surface of the glass was initially flat and became rough as the pulse irradiation proceeded; then, the photon absorption gradually increased, and the ablation rate increased at the same time. Consequently, the laser-excited sound pressure increased. In the next process, the pressure became smaller with an increasing number of pulse irradiations (with increasing hole depth). This was because of a decrease in the amount of ablation due to the mismatch between the focal point and the material surface caused by surface changes in the ablation and reflections on the surface of the hole, as well as the fact that sound with a wavelength larger than the size of the ablated hole did not escape. The sound became bigger again from the 32nd pulse to the 103rd pulse and at the 182nd pulse. These increases of the sound were issued since the side of the hole was ablated. The depth became constant after ~\u0026thinsp;300 pulses. This was because the phase of the laser changed with depth due to refraction inside the glass and reflection at the hole surface. When the depth of the hole was constant, the sound pressure was approximately 0.001 V, which was the resolution of the microphone. The diameter of the processed hole remained constant after 55 pulses.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFig༎3. Sound pressure, and diameter and depth of the microhole versus the number of pulses when the OL was fixed (\u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0 nm/pulse). The side image of the processed hole was improved the contrast.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the sound pressure, and diameter and depth of the hole at \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;20 and 50 nm/pulse. The dashed line indicates the OL position. The sound pressure decreased more slowly at a1\u0026thinsp;=\u0026thinsp;20 nm/pulse than at \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0 nm/pulse because the movement of the OL continued to cause greater ablation. From the 480th pulse to the 660th pulse, the sound pressure appeared again after it had disappeared. It was generated by ablation on the side of the hole. When \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;50 nm/pulse, a larger sound pressure caused by the ablation on the side of the hole and the entrance of the hole was observed. The diameter of the processed hole increased from around the 330th pulse, as shown in the graph. After the 730th pulse, no sound was observed, although the hole was internally processed. It is considered that the sound due to internal processing was attenuated due to the difference in acoustic impedance with the glass. The depth of the hole when \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;50 nm/pulse was greater than the depth of the hole at 20 nm/pulse up to the 34th pulse, and thereafter, there was no significant difference between the two cases. At approximately the 500th pulse, hole processing was stopped in both cases.\u003c/p\u003e \u003cp\u003eThe final depth of the hole fabricated with the constant-speed control was greater than that with the zero-speed control (\u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0 nm/pulse). It was found that it was effective to move the focal point closer to the material to deliver the laser energy to the deepest part of the hole. It was also found that the hole became deeper when the speed was high in the initial stage of laser irradiation (less than 10 pulses), and as processing progressed, the ablation rate decreased; thus, the OL control speed needed to be slowed down. The highest aspect ratio hole was obtained at a speed of 20 nm/pulse, which was obtained through many experimental trials to fabricate a deep hole with a large aspect ratio. These results indicate that the position of the OL should be carefully controlled in addition to the pulse energy according to the target material and the required performance metrics, such as quality and speed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Sound-driven control of OL\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows typical behaviors of the sound pressure and depth and diameter of the hole under the sound-driven control. The behavior of the hole fabricated with sound-driven control with \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;5 \u0026micro;m/V, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a), was similar to that with the fixed focal point (0.0 nm/pulse), shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. This was caused by the small movement of the focal point due to the small value of \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e. When \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;25 \u0026micro;m/V, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b), for up to 100 pulses, the focal point was vibrating; then, the ablated areas were moved near the bottom of the hole and inside the material according to the changes of the focal point. A large sound was detected when the laser pulse was irradiated on the bottom of the hole, but a small sound was detected when it was irradiated inside the material. The vibrations were attenuated after 101 pulses. The attenuation behavior of the sound pressure with increasing hole depth was almost the same as that for constant speed control with \u003cem\u003eα\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;20 \u0026micro;m/pulse, at which the hole with the highest aspect ratio was formed. When \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e was large, the OL may not have been controlled at the proper position due to the large amount of movement of the OL. When \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e was small, little change was observed in the position of the OL even as processing progressed. Therefore, it is necessary to find the appropriate value of \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e in the sound-driven position control through experiments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the depth and diameter of the microhole and the aspect ratio versus the OL speed. The speed was \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0\u0026ndash;70 nm/pulse in the constant-speed control, and the average speed in each case of \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;5 to 35 \u0026micro;m/V in the sound-driven control. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a), the depth increased with increasing \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e to 30 nm/pulse in the constant-speed control, because the movement of the focal point was effective for the beam to reach the bottom of the hole. However, the depth decreased at speeds of more than 50 nm/pulse. Then the speeds were faster than the ablative speed of the hole, and accordingly, the internal processing happened, as can be seen from the camera observation. The sound-driven control that maximizes the sound automatically avoided the internal processing and effectively irradiated the laser beam to the bottom of the hole by giving an appropriate \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b), the hole diameter slightly increased (was almost constant) for \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0 to 20 nm/pulse, increased for \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;20 to 40 nm/pulse, and was nearly constant for \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;40 to 70 nm/pulse in the constant-speed control. The reason for the increase was that the side wall near the entrance of the hole was ablated by the conical shape of the focused beam. This was because the focus position became so deep that the ablation did not occur above \u003cem\u003ea\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;40 nm/pulse. In the sound-driven control, the hole diameter exhibited small changes as the parameter \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e was varied from 5 to 25 \u0026micro;m/V. However, the hole diameter became large for \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;35 \u0026micro;m/V, because some pulses were focused inside the material and ablated a large area on the material surface, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b).\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c), the largest aspect ratio was obtained at \u003cem\u003eα\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;20 nm/pulse in the constant-speed control. In the sound-driven control, the highest aspect ratio was at \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;25 \u0026micro;m/V. When \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e was increased, a deep hole was formed, but the diameter of the hole was extended. Therefore, an appropriate \u003cem\u003eα\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e must be determined.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe performed laser drilling of glass using tightly focused femtosecond laser pulses while monitoring the laser-generated sound. In the experiments conducted toward the goal of developing laser drilling controlled with laser-generated sound, we found that the laser ablation intensity is monotonically related to the sound pressure. When the laser pulses were focused on the glass surface, the sound pressure increased in the initial stage of laser drilling and then decreased as the hole became deeper. The number of pulses in the initial stage depended on the pulse energy, and it was several to 10 pulses in the experiments described in this paper. The increase in the sound pressure was derived from the increase of the ablation caused by roughening the glass surface. The decrease in the sound pressure was caused by the loss of sound propagation through the hole whose diameter was much smaller than the wavelength of the sound.\u003c/p\u003e \u003cp\u003eWe found that an axial movement of the OL toward the target material changed the laser drilling conditions. The constant-speed control, which is a simple method, made a deeper hole than focusing a laser beam on a glass surface with the OL. However, when the speed was high, the laser pulse was focused on inside the material at a position deeper than the bottom of the hole, and the processing stopped, resulting in internal processing. Simultaneously, the side wall of the hole was ablated, and the hole had a small aspect ratio. Through these experiments, we found that the hole with the highest aspect ratio was processed at 20 nm/pulse; however, the hole drilling for 50 nm/pulse was more effective than that for 20 nm/pulse in the initial several pulses. This indicates that more careful control of the OL will be effective due to the condition of the target material at each pulse irradiation.\u003c/p\u003e \u003cp\u003eAs the most significant result of this research, we found that the sound-driven control used to maximize the sound pressure at each pulse irradiation obtained a hole with a high aspect ratio the same as the maximum hole depth obtained by the iterative experiments in the constant-speed control of the OL. In the trial with the hole having the highest aspect ratio, the optimal focal position was sought while repeating the up-and-down movement of the OL in the first several pulses, and the OL was moved toward the target material. In this paper, a simple method in which the movement of the OL was calculated from the sound pressures generated by the two previous pulse irradiations was adopted; however, a more sophisticated method will need to be developed in the future.\u003c/p\u003e \u003cp\u003eIn future research, we will examine whether it is effective to control the laser processing with not only the sound pressure but also the sound frequency. The sound frequency distribution for the 100th pulse was almost identical to that of the first pulse. In the frequency region detected by a 200 kHz microphone, no specific distribution according to the processed structures was observed. According to Webster's Horn equation [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e], which describes sound waves along a rigid axisymmetric tube, the resonant frequency of an air column with a 6 \u0026micro;m diameter and a 20 \u0026micro;m length is on the order of 10 MHz. Therefore, to observe the frequency modulation caused by the hole structure, it is necessary to measure the sound in the resonant frequency region.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding.\u003c/strong\u003e This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), \u0026ldquo;Photonics and Quantum Technology for Society 5.0\u0026rdquo; (Funding agency: QST).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eAcknowledgments.\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eDisclosures.\u003c/strong\u003e The authors declare no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eData availability.\u003c/strong\u003e Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eSupplemental document.\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eD. 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Am. \u003cstrong\u003e41\u003c/strong\u003e, 1126\u0026ndash;1146 (1967).\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":"applied-physics-b","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aphb","sideBox":"Learn more about [Applied Physics B](http://link.springer.com/journal/340)","snPcode":"340","submissionUrl":"https://submission.nature.com/new-submission/340/3","title":"Applied Physics B","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4931402/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4931402/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLaser drilling of glass using tightly focused femtosecond laser pulses while monitoring laser-generated sound is demonstrated, aiming laser drilling controlled by laser-generated sound. The amount of laser ablation was found to have a monotonical relation to the intensity of the sound pressure. It was also found that when the laser pulses were focused on the glass surface, the sound pressure increased in the initial stage of the laser drilling and then declined as the hole became deeper. These behaviors were the result of increasing ablation caused by surface roughening and loss of sound propagation through the hole, respectively. It was further found that the movement of the objective lens (OL) toward the target material at an appropriate constant speed created a hole with a large depth and narrow entrance (a high aspect ratio); that is, the lens movement changed the performance of the laser drilling. A simple method for moving the lens using laser-generated sound was adopted in this study. The axial position of the OL was controlled by maximizing the sound pressure at each pulse irradiation to obtain a hole with a high aspect ratio, which was the same as the maximum hole depth obtained by the iterative experiments in the constant-speed control of the OL. More sophisticated control methods should be developed according to the given applications.\u003c/p\u003e","manuscriptTitle":"Femtosecond laser drilling controlled with laser-generated ultrasound pressure","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-13 16:12:59","doi":"10.21203/rs.3.rs-4931402/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2024-09-09T17:59:41+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-08T19:20:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"16652311350823525110510720120392833593","date":"2024-08-27T13:36:56+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"81924519995651340865349235167168844672","date":"2024-08-26T17:11:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"285743091228199222297175919275195022914","date":"2024-08-26T09:26:38+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-21T09:00:04+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-20T16:30:14+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-19T03:05:51+00:00","index":"","fulltext":""},{"type":"submitted","content":"Applied Physics B","date":"2024-08-18T01:01:44+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"applied-physics-b","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aphb","sideBox":"Learn more about [Applied Physics B](http://link.springer.com/journal/340)","snPcode":"340","submissionUrl":"https://submission.nature.com/new-submission/340/3","title":"Applied Physics B","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"ea6c16ce-ccba-426c-9ec2-fdf216295d9d","owner":[],"postedDate":"September 13th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-11-25T16:08:11+00:00","versionOfRecord":{"articleIdentity":"rs-4931402","link":"https://doi.org/10.1007/s00340-024-08355-1","journal":{"identity":"applied-physics-b","isVorOnly":false,"title":"Applied Physics B"},"publishedOn":"2024-11-20 15:58:05","publishedOnDateReadable":"November 20th, 2024"},"versionCreatedAt":"2024-09-13 16:12:59","video":"","vorDoi":"10.1007/s00340-024-08355-1","vorDoiUrl":"https://doi.org/10.1007/s00340-024-08355-1","workflowStages":[]},"version":"v1","identity":"rs-4931402","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4931402","identity":"rs-4931402","version":["v1"]},"buildId":"WrCJVZZCHTDjtuVLN7oU0","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}