A bulk-micromachined corner cube retroreflector with piezoelectric micro-cantilevers
© Park and Park; licensee Springer. 2013
Received: 28 September 2013
Accepted: 25 November 2013
Published: 18 December 2013
A piezoelectrically actuated corner cube retroreflector (CCR) has been investigated for free space optical communications. The proposed CCR consisted of two mutually orthogonal bulk-micromachined mirror assembled with piezoelectrically actuated horizontal mirror. The vertical mirrors were fabricated by using anisotropic wet-etching of double silicon-on-insulator (SOI) wafer and horizontal mirror was supported by two stress-compensating and one actuating lead zirconate titanate (PZT) micro-cantilevers. The fabricated CCRs exhibited angular displacement of 1.87° at 5 volts and switching times of 276 µ s. It also exhibited a good cut-off frequency of 2.5 kHz which can be digitally modulated up to about 5 kb/s.
KeywordsCorner cube retroreflectors (CCR) Optical mirrors Piezoelectric actuators Micro-cantilevers Anisotropic silicon etching
A corner cube retroreflector (CCR) has been developed as an optical passive transmitter in wireless optical communication with low power consumption . While the CCR does not have a light source, it can transmit the data to the source by digitally modulated reflection of the incident light. It is comprised of two mutually orthogonal vertical mirrors and horizontal mirror with the magnetic or electro-static actuator. The actuator is utilized to form the angular displacement of the horizontal mirror. The electrostatic actuators have been commonly used due to their simple working principles and structures [2–5]. However, the electro-static actuator still needs high driving voltage to obtain the large angular displacement. Two mutually orthogonal vertical mirrors for the CCRs have successfully been fabricated using surface micromachining technique [2–5]. However, the alignment of two mirrors has been limited due to the curvature of the fabricated mirrors from the asymmetric film stresses and a manual assembling of two mirrors. In order to improve the flatness and alignment of the mirrors, bonded silicon-on-insulator (BSOI) with structurally-assisted and assembled or self-assembled structure was utilized [3, 4]. While they have presented good feasibility, it is not easy to obtain the accurate angular alignment to form mutually orthogonal mirror surfaces. In this study, a silicon bulk micromachined CCR was investigated with ultra-low voltage operation and negligible power consumption . It was comprised of the bulk-micromachined silicon vertical mirror and silicon nitride horizontal mirror with piezoelectric cantilever actuator. For achieving good surface roughness, accurate angular alignment, and mass productivity of the vertical mirror, a new fabrication process was developed using a double-SOI wafer and anisotropic KOH etching technique. For obtaining a large displacement at low induced voltage and minimizing the initial angular displacement of the horizontal mirror, the piezoelectric micro-cantilever actuator and supports were new1y applied.
Figure 2 (e - f) shows the fabrication process for the horizontally actuated mirror. Firstly, low stress SiNx layer of 1 μ m in thickness was deposited on a silicon substrate and then Ti/Pt bottom electrode, PZT, and Pt top electrode were sequentially deposited to have a thickness of 20 nm / 120 nm, 500 nm, and 100 nm, respectively. The PZT film was formed using spin-casting and annealing processes. The supporting and actuating PZT cantilevers were defined through the dry etching of Pt/PZT/Pt thin film. The horizontal mirrors with a size of 150 ×150 μ m2 or 250 ×250 μ m2 and torsional meander spring with 5 μ m in width were formed by using low stress SiNx layer and Au was then deposited on the horizontal mirror by using lift-off technique. The SU-8 holders with thicknesses of 100 μ m were patterned to accurately align and hold the fabricated vertical mirror. Finally, the PZT cantilevers and the horizontal mirror were released by using KOH wet etching technique. The fabricated cantilevers have width and length of 70 μ m and 100 μ m, respectively. Finally, the proposed MEMS CCR was fabricated by aligning and inserting the vertical mirror manually into the micro-holder formed on the horizontal mirror as shown in Figure 2 (h).
Silicon bulk micromachined CCRs have been presented for free space optical communications. They were comprised of two vertical silicon mirrors and one piezo-electrically actuating horizontal mirror. The fabricated vertical mirror exhibited an accurate angular alignment of three mutually orthogonal reflective surfaces by using anisotropic wet etching technique of (110) Si wafer. The fabricated horizontal mirror with PZT cantilever actuator exhibited large angular displacement and low switching voltage for on-off keying. The alignment of the horizontal mirror with the vertical mirror was significantly improved by applying the supporting PZT cantilevers and meander springs.
The authors are grateful to acknowledge the support from the Basic Science Research Program (2010-0024618) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Korea.
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