Challenges in fabrication of high aspect ratio electrostatic comb-drive microactuator using one-step X-ray lithography
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ORIGINAL ARTICLE
Challenges in fabrication of high aspect ratio electrostatic comb-drive microactuator using one-step X-ray lithography Rahul Shukla1,2 • Harindra Kumar Kannojia1,6 • C. Mukherjee2,3 • P. Ram Sankar4 • B. S. Thakur5 A. K. Sinha1,2 • Dhananjai Pandey6
•
Received: 19 May 2020 / Revised: 16 September 2020 / Accepted: 31 October 2020 Ó Institute of Smart Structures & Systems, Department of Aerospace Engineering, Indian Institute of Science, Bangalore 2020
Abstract A High Aspect Ratio (HAR) electrostatic comb-drive microactuator of polymethyl-methacrylate (PMMA) is designed to deliver nearly 40 lm uniaxial displacement at 25 V DC. The HAR comb-drive microstructures are fabricated via one-step X-ray lithography (OXL). Polyimide-Au X-ray mask is fabricated and used to pattern 200 lm, 500 lm and 800 lm thick PMMA by deep X-ray lithography (DXRL), followed by drying/release and selective metallization for the development of HAR electrostatic microactuator. The release was optimized with the help of various low-surface-tension liquids. 200 lm and 500 lm thick microstructures were successfully released whereas, 800 lm structures could not be released due to the higher depth and associated capillary force. In addition, non-uniform distribution of Au on the side walls of overlapped region of comb fingers, during metallization, resulted in uneven distribution of electrostatic force followed by shortcircuiting of HAR (i.e. 40) microactuator. These are the potential issues in the fabrication of HAR microstructures and devices by OXL and discussed in details in this paper. Keywords MEMS Comb-drive High aspect ratio Deep X-ray lithography One-step X-ray lithography PMMA Electrostatic microactuator
Introduction
& Rahul Shukla [email protected] 1
Synchrotron Utilization Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
2
Homi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India
3
High Energy Optics Section, Laser Technology Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
4
Design and Manufacturing Technology Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
5
Indus Operation Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
6
School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi 221005, India
Microfabrication is introduced in a number of technologies like CMOS-MEMS (Tang et al. 1990; Nguyen and Howe 1993), laser micromachining (Borbely et al. 2006), microEDM, surface micro-machining, bulk micromachining (Azarnaminy 2005; Lobontiu and Garcia 2005; Zhang 2012), and X-ray LIGA (Ehrfeld and Mu¨nchineyer 1991; Christenson and Guckel 1995; Guckel 1996; Saile et al. 2009). All the techniques listed above except Deep Reactive Ion Etching (DRIE) and X-ray LIGA have limitations like low aspect ratio, tapering on the side walls of the structures and low-precision repeatability (Guckel 1996; Saile et
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