ISSN: 2689-1204

Research Article Journal of Applied Material Science & Engineering Research Microfabrication of 3D Free-Forms with Textured Surface Morphology in Monocrystalline Silicon using Grayscale Technology

Isman Khazi, Andras Kovacs, Ulrich Mescheder*

Institute for Microsystems Technology (iMST), Faculty of Mechanical *Corresponding author & Medical Engineering, Furtwangen University, Furtwangen, Germany Ulrich Mescheder, Institute for Microsystems Technology (iMST), Faculty of Mechanical & Medical Engineering, Furtwangen University Furtwangen, Germany Submitted: 06 Oct 2020; Accepted: 12 Oct 2020; Published: 05 Nov 2020

Abstract Grayscale lithography involving a controlled polymerization of the photoresist, by a gradual modification of the exposure intensity is combined with reactive ion etching process having an etching selectivity of 1:1 (i.e. for the used photoresist and monocrystalline silicon wafer), in order to microfabricate 3D free-forms with complex geometry having a textured surface into monocrystalline silicon wafer. The microfabrication of complex 3D free-forms is exemplified by a geometry of convex micro-lens having concentric rings on its surface representing the surface texture and a concave depression at its tip. The 3D geometry is exposed in positive AZ4562 photoresist having a thickness of 3.8 µm by grayscale lithography using direct writing laser. The exposed 3D geometry in the photoresist is successfully transferred into monocrystalline silicon wafer using the optimized reactive ion etching process with a selectivity of 1:1 and an

anisotropic factor close to 1 in SF6+CHF3 etching environment. The results show that the complex geometry exposed in the photoresist was successfully transferred into the silicon substrate with a pattern transferability with < 0.2 µm deviation between the convex micro-lens diameter in the exposed photoresist and the transferred geometry into silicon wafer. Therefore, the results show the efficacy of the proposed technique even for the pattern transferring of 3D microstructures having a textured surface using the proposed grayscale technology.

Keywords: Grayscale Lithography, Direct Writing Laser, Gray is complex to precisely model it for the realization of arbitrary Scale Technology, Reactive Ion Etching, 1:1 Pattern Transfer, 3D shapes, thickness and sizes, and it is still under research [1, 5]. Free-Forms, 3D Microstructures with Textured Surface, Micro- Furthermore, the masked-based GSL has been reported for Lenses fabrication of 3D photoresist structures showing a gradient in their height; however, the requirement for the multiple masks and strict Introduction alignment needed to expose layouts makes this approach The technological requirement for the microfabrication of 3D complicated, expensive and time consuming for the 3D exposures structures in order to realize various applications in micro-optics [1, 3]. Besides, the use of GSL using direct writing laser (DWL) and micro-photonics such as micro-lenses arrays (for e.g. used in has been proven to be a promising technique for high throughput mobile cameras), patterned substrates for LEDs, diffractive optical microfabrication of 3D microstructures at both micro-and elements, blazed gratings, waveguides, Fresnel lenses, functionalized nanoscale dimensions [1, 6-8]. The advantage of maskless exposure smart surfaces etc. with micro-and nanometer dimensions is undeniable with the possibility of modulation of the laser exposure intensity [1-4]. There are existing methods for the microfabrication of 3D (i.e. the dose to clear) makes the grayscale exposure relatively simpler microstructures such as high-precision mechanical milling with to expose 3D microstructures in photoresists. polishing, thermal reflow of the photoresists, mask-based grayscale lithography (GSL) etc. However, the approach of mechanical Furthermore, by combining the GSL with subsequent etching or milling is limited in terms of throughput and its resolution, which molding step, it is possible to transfer the 3D exposed pattern from hinders the scalability for microfabrication of 3D microstructures. the photoresist into the underlying substrate (for e.g. by anisotropic While, the thermal reflow method of photoresist, indeed results in DRIE or RIE etching, referred in this case as grayscale technology- realization of 3D geometries in photoresists; however, the system GST) or in molding materials (i.e. by molding) [6, 9]. Although the J App Mat Sci & Engg Res, 2020 www.opastonline.com Volume 4 | Issue 4 | 150 efficacy of using GSL for the microfabrication of simple 3D which was subsequently soft-baked at 100°C for 50 seconds on a geometries such as convex and concave micro-lenses, micropyramids hotplate. The layout for the 3D micro-lens geometry with etc. have been successfully reported wherein, the modulation of combination of convex lens having concave tip was made in Clewin the laser intensity was gradual; thereby, resulting in smooth (version 5.2.4) layout editor software, wherein 117 layers were used gradient surface profile of the exposed 3D microstructures [1, 6, to define the 3D geometry. The CAD layout geometry is shown in 7]. However, the feasibility of pattern transferring of complex 3D Figure 1 (a), wherein each color represents a specific layer and geometries i.e. large gradient of geometry variation on surface of during the GSL exposure, each layer is exposed with a specific 3D microstructures (i.e. textured surface) is still needed to be intensity corresponding to the required dose to clear for the needed investigated. One such example of 3D structure with complex geometry as shown in the schematic in Figure 1b. The arrows modulations of its surface morphology is the moth-eye geometry represent the depth of exposure, which is well-controlled by the which is used for e.g. as the anti-reflection structures on the surface intensity of the laser used. of micro-lens array as recently reported by Masoto et al. [10]. Wherein, they used a complex approach to fabricate these 3D microstructures combining series of steps involving ion beam to irradiate the glassy carbon substrate with oxygen gas to generate the anti-reflection pattern, which is followed by UV curable resin molding for fabrication of molds with anti-reflection patterns. Consecutively, the mold was used for structuring the surface of molded micro-lens array. Primarily, the irradiation technique to get well-define structures is challenging and secondarily, the process gets complex as several process steps are involved for the microfabrication of 3D microstructures with textured surface. However, by use of GSL the 3D complex microstructures such as the moth-eye geometry can be exposed with ease in the positive photoresists by appropriate CAD layouts and well-controlled modulation of laser intensity.

Therefore, the proposed idea in this paper is to show the preliminary proof of concept for the feasibility of using GST to expose and (a) etch the 3D patterns with textured surface morphology, i.e. 3D microstructures with having specific surface morphology modulations into c-Si. For the proof of concept, 3D complex geometry is exemplified by combination of convex micro-lens having a concave depression at its tip and intentional concentric rings on the surface of the convex micro-lens, which is primarily exposed in AZ4562 positive photoresist using GSL, and subsequently, the exposed pattern is transferred into underlying c-Si using RIE process having an etching selectivity 1:1 with a degree of anisotropy close to 1. Apart from this, such 3D microstructures with textured surface morphology can also be used as metrology test structures for the accurate estimation of the anisotropy factor (Af) owing to the sharp transition of geometry on its surface. As reported in [6,

7], Af is a critical parameter for the accurate 1:1 transfer of the pattern from the 3D structured photoresist to the underlying monocrystalline silicon wafer (c-Si) using RIE.

Experimental 4-inch (100) monocrystalline silicon (c-Si) wafers were initially (b) coated with adhesion promoter TI-Prime (Microchemicals) (spin Fig. 1. (a) CAD Layout with 117 layers, each color indicates a speed: 4000 rpm, acceleration: 2000 rpm/s and time: 20 s) followed specific layer for the GSL exposures using DWL. The middle by brief soft-bake at 100°C for 50 seconds. Following, the wafer circle (50 μm diamater) corresponds to the concave depression at was coated with AZ4562 positive photoresist to get a thickness of the tip of convex micro-lens, and (b) schematic showing the 3.8 µm (spin speed: 6000 rpm, acceleration: 2000 rpm/sec and concept of GSL in general and the modulation technique to time: 30 seconds). The wafer was left for 2 hours to rehydrate, expose a convex micro-lens with a concave depression at its tip.

J App Mat Sci & Engg Res, 2020 www.opastonline.com Volume 4 | Issue 4 | 151 DWL66FS from Heidelberg Instruments was used to make the transition of the grayscale exposure from convex to concave grayscale exposures. The calibration curve correlating the exposure geometry can be seen in the middle of the profile, which has a depth depth with the gray value (GV) of the DWL device was made for of 1 μm. the coated AZ4562 with 3.8 µm thickness (explanation and procedure can be found in [6, 7]). Following which the GSL exposures were made to expose the convex micro-lens having concentric rings on its surface with a concave depression at its tip. The exposed wafer was developed in AZ315B (diluted with DI H2O in 1:4 volumetric ratio) and was hard-baked on hotplate at 115°C for 1 minute. Subsequently, the wafer was etched in RIE with optimized process for 1:1 selectivity as described in [6]. The 3D exposed structures in photoresist and the subsequent transferred geometry in c-Si were characterized using profilometer (Veeco Dektak 150), optical microscope and scanning electron microscope (JEOL5400).

Results and Discussion The grayscale exposures were done by modulating the intensity of the laser in the Heidelberg Instruments DWL66FS device. The particular laser intensity used can be further divided into 0 to 127 (a) gray values (GV), which can be in principle further divided to much more GV by controlling the overlapping exposure of the laser width in order to get a fine transition of the 3D surface. Furthermore, each GV corresponds to a fine increment in the level of laser intensity and hence, determines the required dose to clear. This required dose to clear, which is needed to define the profile of the 3D micro structures is determined from a calibration curve experimentally obtained for the used photoresist, wherein the correlation between the GV and the “depth of exposure” is obtained. The 3D convex micro-lens with concentric rings and concave tip was initially designed in a CAD layout as shown in Figure 1(a). Wherein, a specific GV was assigned to its each individual layer (the concept is explained in [6,7]) and the grayscale exposures were carried out. Figure 2(a) shows the optical microscope image of the exposed convex micro-lens with concentric rings on its surface and a concave depression at its tip in AZ4562 positive photoresist after development (b) and post-bake step. The concentric rings on the micro-lens Fig. 2. (a) Convex micro-lens with concave depression at its tip, corresponds to each individual grayscale exposure transition from exposed in AZ4562 positive photoresist; post development and one layer to the adjacent layer. For proof of concept, the step size hard bake step, with a diameter of 468.32 nm, and (b) micro-lens transferred into c-Si by RIE process in SF +CHF enviroment, was intentionally assigned large, in order to emulate the surface 6 3 texture on the 3D microstructure and to see the efficacy of the with a diameter of 467.95 nm. proposed pattern transfer technique i.e. using GST into the c-Si Subsequently, the wafer with 3D microstructured photoresist was substrate. The diameter of the exposed 3D micro-lens is 468.32 μm etched in RIE (Oxford Instruments Plasmalab 100) in an and the height is 3.8 μm. The 2D profile of the exposed micro-lens environment of SF +CHF , having an etching selectivity of 1:1 and in photoresist measured using a profilometer can be seen in Figure 6 3 the degree of anisotropy close to 1. The etching recipe and the 4, wherein it is depicted by solid yellow line. Moreover, the gradual

J App Mat Sci & Engg Res, 2020 www.opastonline.com Volume 4 | Issue 4 | 152 optimization of the etching process to get an etching selectivity of 1:1 for the used positive photoresist AZ4562 and c-Si substrate can be found in our previous paper [6]. The etching time was controlled in order to etch exactly 3.8 µm simultaneously in c-Si and AZ4562 photoresist. Post RIE, the wafer was characterized by an optical microscope as shown in Figure 2(b), wherein the concentric rings on the surface corresponding to different layers i.e. emulating textured surface of 3D microstructure is successfully transferred into c-Si from the 3D structured photoresist profile as shown in Figure 2(a). Furthermore, the concave depression at the tip of the micro-lens is also transferred into c-Si from the photoresist. The measured diameter of the micro-lens post RIE is 467.950 µm; thereby, showing a reduction of ca. 185 nm on either side, which can be attributed to Af, which is given by equation 1, wherein rl and rv refers to the lateral etch rate and the vertical etch rate, respectively. The calculated Af for the used process conditions are computed using the dimension of the concave tip from Figure 4, and it is found to be ca. 0.95. Therefore, the reduction in the lateral dimension of the micro-lens post RIE can be reduced by improving

Af to reach 1, by optimizing the RIE process conditions.

Furthermore, the etched micro-lens was characterized in SEM as Fig 3 (a) and (b) SEM micrographs of the micro-lens transferred shown in the micrographs in Figure 3(a-b). The overview of the in c-Si by RIE having an etching selectivity of 1:1. The pattern micro-lens within the etched square area is shown in Figure 3(a) transferability successully resulted in the replication of the concave depression from the grayscale exposed AZ4562 and the magnified micrograph in Figure 3(b) shows the concentric photoresist into the c-Si. rings along with concave depression at the tip of the convex micro- lens, which is transferred from the photoresist into c-Si during the RIE process. The 2D profile of the etched micro-lens measured using profilometer is shown in Figure 4, which is depicted by black dashed lines. The comparison of the 2D profiles of the micro-lens exposed in photoresist and transferred into c-Si by RIE shows good overlapping both in terms of the grayscale profile defining the convex micro-lens and also for the concave depression at its tip with a depth of 1 μm.

Thereby, showing the feasibility of pattern transferring of grayscale exposed 3D microstructures with texture surface morphology in photoresist into c-Si using the proposed grayscale technology. This proof of concept opens new realms of possibilities for the microstructuring of complex 3D microstructures such as moth-eye like structures with well-defined surface texture Fig. 4. Comparision of the 2D profile of the convex micro-lens morphology with controlled dimensions. with concave tip measured using profilometer in exposed AZ4562 photoresist and into c-Si post RIE. J App Mat Sci & Engg Res, 2020 www.opastonline.com Volume 4 | Issue 4 | 153 Conclusion References The concept of exposing 3D microstructures with textured surface 1. Grushina (2019) Direct-write grayscale lithography. Advanced Optical Technologies 8: 163-169. morphology using grayscale lithography using direct laser writing tool is presented. The feasibility of transferring the exposed 3D 2. Kick A, D Helmer, F Kotz, B Rapp (2020) Generation of multi- level microstructures using a wavelength-selective photoresist structured photoresist into the monocrystalline silicon wafer using and mask-less grayscale lithography. Microfluidics, BioMEMS, optimized reactive ion etching with an etching selectivity of 1:1 and Medical Microsystems 11235: 3. and degree of anisotropy close to 1 is proposed. As a proof of 3. P Kusar, S Jessenig, G Eilmsteiner (2020) Single mask step concept convex micro-lens with textured surface having concentric etching of Fabry–Pérot etalons for spectrally resolved imagers. rings, pertaining to the large step-size used and having a concave J Micromech Microeng 30: 85004. tip is exposed in positive photoresist AZ4562 with a thickness of 4. X Li, ZJ Tan, NX Fang (2020) Grayscale stencil lithography 3.8 µm. The exposed pattern was transferred into the underling for patterning multispectral color filters. Optica 7: 1154. monocrystalline silicon wafer using the optimized reactive ion 5. R Kirchner, H Schift (2019) Thermal reflow of polymers for etching. The 3D convex micro-lens profile along with the textured innovative and smart 3D structures: A review. Materials surface and the concave tip was successfully transferred from the Science in Semiconductor Processing 92: 58-72. exposed photoresist into the c-Si wafer. The deviation in the 6. I. Khazi, U. Muthiah, and U. Mescheder (2018) 3D free forms diameter of the convex micro-lens was < 0.2 µm. Therefore, the in c-Si via grayscale lithography and RIE,” Microelectronic Engineering 193: 34-40. proposed grayscale technology can be used to microfabricate 3D microstructures with functionalized textured surfaces. 7. F. Lima, I. Khazi, U. Mescheder, A. C. Tungal, and U. Muthiah (2019) Fabrication of 3D microstructures using grayscale lithography. Advanced Optical Technologies 8: 181-193. Acknowledgment 8. T Mortelmans, Dimitrio Kazazis, Vitaliy A Guzenko, The authors would like to thank Ms. Shanedonia SJ and Mr. Celestino Padestea, Thomas Braun (2020) Grayscale e-beam Gnanasekar Jayaraj, studying M.Sc. Smart Systems at Furtwangen lithography: Effects of a delayed development for well- University for the grayscale exposures done in the framework of controlled 3D patterning. Microelectronic Engineering 225: 111272. their semester scientific project. 9. Morgan B, J McGee, R Ghodssi (2007) Automated Two-Axes Optical Fiber Alignment Using Grayscale Technology. J Microelectromech Syst 16: 102-110,. 10. M Nakamura, I Mano, J Taniguchi (2019) Fabrication of micro- lens array with antireflection structure. Microelectronic Engineering 211: 29-36.

Copyright: ©2020 Ulrich Mescheder, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

J App Mat Sci & Engg Res, 2020 www.opastonline.com Volume 4 | Issue 4 | 154