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Microfabrication of 3D Free-Forms with Textured Surface Morphology in Monocrystalline Silicon Using Grayscale Technology

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Microfabrication of 3D Free-Forms with Textured Surface Morphology in Monocrystalline Silicon Using Grayscale Technology 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
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