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A bulk-micromachined corner cube retroreflector with piezoelectric micro-cantilevers

A bulk-micromachined corner cube retroreflector with piezoelectric micro-cantilevers 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 μs. It also exhibited a good cut-off frequency of 2.5 kHz which can be digitally modulated up to about 5 kb/s. Keywords: Corner cube retroreflectors (CCR); Optical mirrors; Piezoelectric actuators; Micro-cantilevers; Anisotropic silicon etching Introduction improve the flatness and alignment of the mirrors, bonded A corner cube retroreflector (CCR) has been developed silicon-on-insulator (BSOI) with structurally-assisted and as an optical passive transmitter in wireless optical com- assembled or self-assembled structure was utilized [3,4]. munication with low power consumption [1]. While the While they have presented good feasibility, it is not easy CCR does not have a light source, it can transmit the to obtain the accurate angular alignment to form mutu- data to the source by digitally modulated reflection of the ally orthogonal mirror surfaces. In this study, a silicon bulk incident light. It is comprised of two mutually orthogonal micromachined CCR was investigated with ultra-low volt- vertical mirrors and horizontal mirror with the mag- age operation and negligible power consumption [6]. It netic or electro-static actuator. The actuator is utilized was comprised of the bulk-micromachined silicon vertical to form the angular displacement of the horizontal mir- mirror and silicon nitride horizontal mirror with piezo- ror. The electrostatic actuators have been commonly used electric cantilever actuator. For achieving good surface due to their simple working principles and structures roughness, accurate angular alignment, and mass produc- [2-5]. However, the electro-static actuator still needs high tivity of the vertical mirror, a new fabrication process was driving voltage to obtain the large angular displacement. developed using a double-SOI wafer and anisotropic KOH Two mutually orthogonal vertical mirrors for the CCRs etching technique. For obtaining a large displacement at have successfully been fabricated using surface micro- low induced voltage and minimizing the initial angu- machining technique [2-5]. However, the alignment of lar displacement of the horizontal mirror, the piezoelec- two mirrors has been limited due to the curvature of tric micro-cantilever actuator and supports were new1y the fabricated mirrors from the asymmetric film stresses applied. and a manual assembling of two mirrors. In order to Findings The PZT micro-cantilever was utilized as an actuator for the horizontal mirror to obtain large angular dis- *Correspondence: jaepark@kw.ac.kr Department of Electronic Engineering, Kwangwoon University, 447-1 Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of Korea © 2013 Park and Park; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 2 of 6 http://www.mnsl-journal.com/content/1/1/7 placement. However, the PZT cantilever has an initial the fabrication procedure to obtain the two mutually bending due to its asymmetric film stress in multilayer orthogonal reflective surfaces with accurate angular align- structure. The initial bending of PZT cantilever intro- ment. A double-SOI wafer with a silicon spacer was duces the angular misalignment between the horizontal used to fabricate the cross shaped vertical silicon mir- and vertical mirrors. Since there is difficulty in the con- ror. The double-SOI wafer was comprised of two silicon trol of the residual stress in the deposition of PZT thin wafers with thickness of 300 μm, buried oxide layers film, it might be serious problem for the MEMS CCR of 1 μm in thickness, and a silicon spacer with thick- because the colinear differential scattering cross section ness of 30 μm. On the double-SOI wafer, silicon nitride (CDSCS) of MEMS CCR is affected by the radius of curva- was deposited as a masking layer for KOH wet etch- ture and angular misalignment of mirrors [5]. Especially, ing. Firstly, top and bottom SiNx layers were sequen- the maximum angular misalignment of mirror should be tially patterned using the same mask with parallel lines below 0.055° to communicated over free space and 0.11° to <111> direction. In order to obtain the symmetri- or more angular misalignment is sufficient to switch off cally formed vertical comb structure with high aspect the CCR. Therefore, the stress compensated structure ratio, the double-SOI wafer was etched down to 300 μm of the PZT cantilevers was investigated for the hori- as far as by using KOH solution and the buried oxide zontal mirror. Figure 1 presents the schematic drawing was used as an etch stop layer. The KOH etchant was of the proposed horizontal mirror with the PZT micro- optimized to have 40 % of concentration and 70°C of wt cantilevers for MEMS CCR. As shown in Figure 1 (a), processing temperature to minimize surface roughness the horizontal mirror was equally suspended by two sup- of the fabricated silicon mirror structure. To obtain the porting and one actuating PZT cantilevers with torsional cross shaped vertical mirror, the etched double-SOI wafer meander springs. These two supporting PZT cantilevers was carefully sawed in the perpendicular direction of the were utilized to improve the angular alignment of the formed structures and finally rotated by 90°. In order to horizontal mirror by compensating the initial bending of improve the reflectivity of the vertical mirror, 800 Å of the actuating PZT cantilever after the fabrication. Thus, gold thin film was sputtered on the vertical silicon mirror the proposed CCR has three mutually orthogonal mir- surfaces. rors to reflect the incident light to the source as shown in Figure 2 (e - f) shows the fabrication process for the Figure 1 (a). When the angular misalignment is occured horizontally actuated mirror. Firstly, low stress SiNx layer by actuating cantilever as shown in Figure 1 (b), the of 1μm in thickness was deposited on a silicon substrate incident light is scattered away from the source. There- and then Ti/Pt bottom electrode, PZT, and Pt top elec- fore, the proposed MEMS CCR can transmit the on-off- trode were sequentially deposited to have a thickness of keyed digital siginal to the source as a passive optical 20 nm / 120 nm, 500 nm, and 100 nm, respectively. The transmitter. PZT film was formed using spin-casting and annealing The vertical mirror should have two mutually orthog- processes. The supporting and actuating PZT cantilevers onal reflective surfaces with accurate angular alignment were defined through the dry etching of Pt/PZT/Pt thin and good surface roughness. Figure 2 (a - d) shows film. The horizontal mirrors with a size of 150×150 μm Figure 1 Schematic drawing of the proposed CCR with horizontal mirror using PZT cantilever actuator for large deflection at low induced voltage and supports for minimizing an initial tilting angle: (a) on-state and (b) off-state. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 3 of 6 http://www.mnsl-journal.com/content/1/1/7 Figure 2 Fabrication sequences of proposed MEMS CCR with silicon cross shaped vertical mirrors and horizontal mirrors with PZT cantilevers: (a) deposition of SiNx on a double SOI wafer, (b) formation of mask for KOH wet etching, and (c) KOH wet etching, (d) dicing and rotation, (e) deposition of SiNx/Ti/Pt/PZT/Pt on a silicon wafer, (f) formation of PZT cantilevers, horizontal mirrors, torsional hinge springs, and SU-8 micro-holder, (g) KOH wet etching for release, and (h) dicing and assembly of the vertical mirrors onto the horizontal mirrors. or 250×250 μm and torsional meander spring with 5 Finally, the proposed MEMS CCR was fabricated by align- μm in width were formed by using low stress SiNx layer ing and inserting the vertical mirror manually into the and Au was then deposited on the horizontal mirror by micro-holder formed on the horizontal mirror as shown using lift-off technique. The SU-8 holders with thick- in Figure 2 (h). nesses of 100 μm were patterned to accurately align and Figure 3 shows scanning electron micrograph (SEM) hold the fabricated vertical mirror. Finally, the PZT can- pictures of the fabricated MEMS CCR. It is comprised of tilevers and the horizontal mirror were released by using four CCRs where each device works independently. The KOH wet etching technique. The fabricated cantilevers fabricated vertical silicon mirror with 300 μm(length) × have width and length of 70 μm and 100 μm, respectively. 300 μm (height) × 30 μm (thickness) were well aligned Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 4 of 6 http://www.mnsl-journal.com/content/1/1/7 Figure 3 SEM pictures and photomicrograph of the fabricated MEMS CCR with two supporting and one actuating PZT cantilevers mounted on a PCB substrate: (a) top view of four MEMS CCRs, (b) side view of vertical mirror assembled on horizontal mirror with PZT cantilever, (c) SU-8 holder to align the vertical mirror, and (d) MEMS CCR on PCB test jig. with the cross angle of 90°. The angular misalignment large angular displacement of approximately 2.29° with of two vertical mirrors measured by optical microscope radius of curvature of 4.16 mm due to its residual stress, was bounded within 0.32° and the surface roughness the angular misalignments (δ) were within 0.28° and measured by AFM was within 3.523 nm rms (root- 0.13° for the horizontal mirrors with area of 150×150 2 2 mean-square). Figure 4 presents the surface topograph μm and 250×250 μm , respectively, due to the use of fabricated horizontal mirrors. The surface roughness of two supporting cantilevers. The smaller angular mis- and radius of curvature of fabricated horizontal mirror alignment can be achieved by optimizing the length of measured by Nanofocus uSurf 3D non-contact profiler cantilevers or residual stress of PZT cantilevers. The were approximately 5.75 nm rms and 54.6 mm, respec- horizontal mirror exhibited an angular misalignment tively. While the fabricated PZT cantilever exhibited below 0.05° through the FEM simulation using Coventor- Figure 4 Surface topograph of fabricated horizontal mirrors using PZT cantilever actuator with area of 150×150 μm (a) and 250× 250 μm (b) by using Nanofocus uSurf 3D non-contact profiler. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 5 of 6 http://www.mnsl-journal.com/content/1/1/7 Figure 7 The measured frequency responses of the fabricated Figure 5 The simulated (dotted line) and measured (symbol) MEMS CCRs with two different horizontal mirrors with diameters angular displacement of the fabricated MEMS CCRs with the of 150 μm and 250 μm. 2 2 horizontal mirrors of 150×150 μm and 250×250 μm . Ware at the overall residual stress of 200 MPa in PZT the off-state and on-state of the fabricated CCR. As shown cantilevers. in Figure 6 (b) and (c), there is a clear difference between Figure 5 shows the angular displacement of the fab- the“on”and “off”states. InordertodetectaHe-Nelaser ricated MEMS CCRs with two different horizontal mir- beam reflected from the CCR, a silicon photodiode was rors. The fabricated CCRs with the horizontal mirrors utilized as a receiver. The installed distance between the 2 2 of 150×150 μm and 250×250 μm exhibited the angu- CCR and receiver was 50 cm because the fabricated CCR lar displacement of 1.37° and 1.87° at 5 volts, which are had large angular misalignment. As shown in Figure 7, good enough for on-off keying, respectively. As shown in the output voltage at the photodiode was significantly Figure 5, the measured angular displacement was also in decreased as the driving frequency increases above 1 kHz. good agreement with the simulated one. It might be attributed to the finite switching time, so that The fabricated CCR was demonstrated to check the the mirror does not undergo full angular displacement feasibility as a passive optical transmitter. Firstly, the and creates reduced peak-to-peak voltage. The 3-dB cut- reflective pattern was observed to confirm that the three off frequencies were approximately 2.5 kHz and 1 kHz for mirrors were well aligned orthogonally to each other. the CCR with the horizontal mirrors of 150×150 μm and Figure 6 (a) shows the reflective pattern of an unactuated 250×250 μm , respectively. It also exhibited good switch- CCR with diagonal illumination (λ = 632.8 nm). As shown ing characteristics with an off-to-on-state transition of in Figure 5, the patterns exhibit similar “star” patterns due 163 μs and on-to-off-state transition of 276 μsatarect- to six effective reflective regions [3]. Figure 6 (b) and (c) angular input voltage and switching frequency of 10 V and present the captured images by CMOS image sensor in 1 kHz, respectively. Figure 6 Photograph of the reflective pattern of the fabricated MEMS CCR from diagonally illuminated lay (λ = 632.8 nm) (a),and captured images in off-state (b) and on-state (c) by using a CMOS image sensor. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 6 of 6 http://www.mnsl-journal.com/content/1/1/7 Conclusions Silicon bulk micromachined CCRs have been presented for free space optical communications. They were com- prised 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. Competing interests The authors declare that they have no competing interests. Authors’ contributions JYP and JP carried out the design & simulation and drafted the manuscript. JP carried out the fabrication of MEMS device and experimental measurements. Both authors read and approved the final manuscript. Acknowledgements 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. Received: 28 September 2013 Accepted: 25 November 2013 Published: 18 December 2013 References 1. Kahn J, Katz RH, Pister K (2000) Emerging challenges: mobile networking for smart ´ dust. J Commun Netw 2:188–196 2. Chu PB, Lo NR, Berg EC, Pister KSJ (1997) Optical communication using micro corner cube reflectors In: Proceedings of IEEE Micro Electro Mechanical Systems Workshop, pp 350U355. IEEE, Piscataway, NJ 3. Zhou L, Kahn JM, Pister KSJ (2003) Corner-cube retrore ectors based on structure-assisted assembly for free-space optical communication. J Microelectromech Syst 12:233–242 4. Hong YK, Syms RRA, Pister KSJ, Zhou LX (2005) Design, fabrication and test of self-assembled optical corner cube reflectors. J Microelectromech Syst 15:663–672 5. Zhu X, Hsu VS, Kahn JM (2002) Optical modeling of mems corner-cube retroreflectors with misalignment and nonflatness. IEEE J Select Top Quantum Electron 48:26–32 6. Park JC, Park JY, Y WJ, Kim DH, Park J (2011) Silicon bulkmicromachined piezoelectically actuated corner cube retro reflector In: 16th Int Conf on Solid-State Sensors, Actuators and Microsystems, Transducers, pp 1578–1582. IEEE, Piscataway, NJ doi:10.1186/2213-9621-1-7 Submit your manuscript to a Cite this article as: Park and Park: A bulk-micromachined corner cube journal and benefi t from: retroreflector with piezoelectric micro-cantilevers. Micro and Nano Systems Letters 2013 1:7. 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Micro and Nano Systems Letters Springer Journals

A bulk-micromachined corner cube retroreflector with piezoelectric micro-cantilevers

Park, Jongcheol; Park, Jae
Micro and Nano Systems Letters , Volume 1 (1) – Dec 18, 2013
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Springer Journals
Copyright
Copyright © 2013 by Park and Park; licensee Springer.
Subject
Engineering; Circuits and Systems; Electrical Engineering; Mechanical Engineering; Nanotechnology; Applied and Technical Physics
eISSN
2213-9621
DOI
10.1186/2213-9621-1-7
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Abstract

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 μs. It also exhibited a good cut-off frequency of 2.5 kHz which can be digitally modulated up to about 5 kb/s. Keywords: Corner cube retroreflectors (CCR); Optical mirrors; Piezoelectric actuators; Micro-cantilevers; Anisotropic silicon etching Introduction improve the flatness and alignment of the mirrors, bonded A corner cube retroreflector (CCR) has been developed silicon-on-insulator (BSOI) with structurally-assisted and as an optical passive transmitter in wireless optical com- assembled or self-assembled structure was utilized [3,4]. munication with low power consumption [1]. While the While they have presented good feasibility, it is not easy CCR does not have a light source, it can transmit the to obtain the accurate angular alignment to form mutu- data to the source by digitally modulated reflection of the ally orthogonal mirror surfaces. In this study, a silicon bulk incident light. It is comprised of two mutually orthogonal micromachined CCR was investigated with ultra-low volt- vertical mirrors and horizontal mirror with the mag- age operation and negligible power consumption [6]. It netic or electro-static actuator. The actuator is utilized was comprised of the bulk-micromachined silicon vertical to form the angular displacement of the horizontal mir- mirror and silicon nitride horizontal mirror with piezo- ror. The electrostatic actuators have been commonly used electric cantilever actuator. For achieving good surface due to their simple working principles and structures roughness, accurate angular alignment, and mass produc- [2-5]. However, the electro-static actuator still needs high tivity of the vertical mirror, a new fabrication process was driving voltage to obtain the large angular displacement. developed using a double-SOI wafer and anisotropic KOH Two mutually orthogonal vertical mirrors for the CCRs etching technique. For obtaining a large displacement at have successfully been fabricated using surface micro- low induced voltage and minimizing the initial angu- machining technique [2-5]. However, the alignment of lar displacement of the horizontal mirror, the piezoelec- two mirrors has been limited due to the curvature of tric micro-cantilever actuator and supports were new1y the fabricated mirrors from the asymmetric film stresses applied. and a manual assembling of two mirrors. In order to Findings The PZT micro-cantilever was utilized as an actuator for the horizontal mirror to obtain large angular dis- *Correspondence: jaepark@kw.ac.kr Department of Electronic Engineering, Kwangwoon University, 447-1 Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of Korea © 2013 Park and Park; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 2 of 6 http://www.mnsl-journal.com/content/1/1/7 placement. However, the PZT cantilever has an initial the fabrication procedure to obtain the two mutually bending due to its asymmetric film stress in multilayer orthogonal reflective surfaces with accurate angular align- structure. The initial bending of PZT cantilever intro- ment. A double-SOI wafer with a silicon spacer was duces the angular misalignment between the horizontal used to fabricate the cross shaped vertical silicon mir- and vertical mirrors. Since there is difficulty in the con- ror. The double-SOI wafer was comprised of two silicon trol of the residual stress in the deposition of PZT thin wafers with thickness of 300 μm, buried oxide layers film, it might be serious problem for the MEMS CCR of 1 μm in thickness, and a silicon spacer with thick- because the colinear differential scattering cross section ness of 30 μm. On the double-SOI wafer, silicon nitride (CDSCS) of MEMS CCR is affected by the radius of curva- was deposited as a masking layer for KOH wet etch- ture and angular misalignment of mirrors [5]. Especially, ing. Firstly, top and bottom SiNx layers were sequen- the maximum angular misalignment of mirror should be tially patterned using the same mask with parallel lines below 0.055° to communicated over free space and 0.11° to <111> direction. In order to obtain the symmetri- or more angular misalignment is sufficient to switch off cally formed vertical comb structure with high aspect the CCR. Therefore, the stress compensated structure ratio, the double-SOI wafer was etched down to 300 μm of the PZT cantilevers was investigated for the hori- as far as by using KOH solution and the buried oxide zontal mirror. Figure 1 presents the schematic drawing was used as an etch stop layer. The KOH etchant was of the proposed horizontal mirror with the PZT micro- optimized to have 40 % of concentration and 70°C of wt cantilevers for MEMS CCR. As shown in Figure 1 (a), processing temperature to minimize surface roughness the horizontal mirror was equally suspended by two sup- of the fabricated silicon mirror structure. To obtain the porting and one actuating PZT cantilevers with torsional cross shaped vertical mirror, the etched double-SOI wafer meander springs. These two supporting PZT cantilevers was carefully sawed in the perpendicular direction of the were utilized to improve the angular alignment of the formed structures and finally rotated by 90°. In order to horizontal mirror by compensating the initial bending of improve the reflectivity of the vertical mirror, 800 Å of the actuating PZT cantilever after the fabrication. Thus, gold thin film was sputtered on the vertical silicon mirror the proposed CCR has three mutually orthogonal mir- surfaces. rors to reflect the incident light to the source as shown in Figure 2 (e - f) shows the fabrication process for the Figure 1 (a). When the angular misalignment is occured horizontally actuated mirror. Firstly, low stress SiNx layer by actuating cantilever as shown in Figure 1 (b), the of 1μm in thickness was deposited on a silicon substrate incident light is scattered away from the source. There- and then Ti/Pt bottom electrode, PZT, and Pt top elec- fore, the proposed MEMS CCR can transmit the on-off- trode were sequentially deposited to have a thickness of keyed digital siginal to the source as a passive optical 20 nm / 120 nm, 500 nm, and 100 nm, respectively. The transmitter. PZT film was formed using spin-casting and annealing The vertical mirror should have two mutually orthog- processes. The supporting and actuating PZT cantilevers onal reflective surfaces with accurate angular alignment were defined through the dry etching of Pt/PZT/Pt thin and good surface roughness. Figure 2 (a - d) shows film. The horizontal mirrors with a size of 150×150 μm Figure 1 Schematic drawing of the proposed CCR with horizontal mirror using PZT cantilever actuator for large deflection at low induced voltage and supports for minimizing an initial tilting angle: (a) on-state and (b) off-state. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 3 of 6 http://www.mnsl-journal.com/content/1/1/7 Figure 2 Fabrication sequences of proposed MEMS CCR with silicon cross shaped vertical mirrors and horizontal mirrors with PZT cantilevers: (a) deposition of SiNx on a double SOI wafer, (b) formation of mask for KOH wet etching, and (c) KOH wet etching, (d) dicing and rotation, (e) deposition of SiNx/Ti/Pt/PZT/Pt on a silicon wafer, (f) formation of PZT cantilevers, horizontal mirrors, torsional hinge springs, and SU-8 micro-holder, (g) KOH wet etching for release, and (h) dicing and assembly of the vertical mirrors onto the horizontal mirrors. or 250×250 μm and torsional meander spring with 5 Finally, the proposed MEMS CCR was fabricated by align- μm in width were formed by using low stress SiNx layer ing and inserting the vertical mirror manually into the and Au was then deposited on the horizontal mirror by micro-holder formed on the horizontal mirror as shown using lift-off technique. The SU-8 holders with thick- in Figure 2 (h). nesses of 100 μm were patterned to accurately align and Figure 3 shows scanning electron micrograph (SEM) hold the fabricated vertical mirror. Finally, the PZT can- pictures of the fabricated MEMS CCR. It is comprised of tilevers and the horizontal mirror were released by using four CCRs where each device works independently. The KOH wet etching technique. The fabricated cantilevers fabricated vertical silicon mirror with 300 μm(length) × have width and length of 70 μm and 100 μm, respectively. 300 μm (height) × 30 μm (thickness) were well aligned Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 4 of 6 http://www.mnsl-journal.com/content/1/1/7 Figure 3 SEM pictures and photomicrograph of the fabricated MEMS CCR with two supporting and one actuating PZT cantilevers mounted on a PCB substrate: (a) top view of four MEMS CCRs, (b) side view of vertical mirror assembled on horizontal mirror with PZT cantilever, (c) SU-8 holder to align the vertical mirror, and (d) MEMS CCR on PCB test jig. with the cross angle of 90°. The angular misalignment large angular displacement of approximately 2.29° with of two vertical mirrors measured by optical microscope radius of curvature of 4.16 mm due to its residual stress, was bounded within 0.32° and the surface roughness the angular misalignments (δ) were within 0.28° and measured by AFM was within 3.523 nm rms (root- 0.13° for the horizontal mirrors with area of 150×150 2 2 mean-square). Figure 4 presents the surface topograph μm and 250×250 μm , respectively, due to the use of fabricated horizontal mirrors. The surface roughness of two supporting cantilevers. The smaller angular mis- and radius of curvature of fabricated horizontal mirror alignment can be achieved by optimizing the length of measured by Nanofocus uSurf 3D non-contact profiler cantilevers or residual stress of PZT cantilevers. The were approximately 5.75 nm rms and 54.6 mm, respec- horizontal mirror exhibited an angular misalignment tively. While the fabricated PZT cantilever exhibited below 0.05° through the FEM simulation using Coventor- Figure 4 Surface topograph of fabricated horizontal mirrors using PZT cantilever actuator with area of 150×150 μm (a) and 250× 250 μm (b) by using Nanofocus uSurf 3D non-contact profiler. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 5 of 6 http://www.mnsl-journal.com/content/1/1/7 Figure 7 The measured frequency responses of the fabricated Figure 5 The simulated (dotted line) and measured (symbol) MEMS CCRs with two different horizontal mirrors with diameters angular displacement of the fabricated MEMS CCRs with the of 150 μm and 250 μm. 2 2 horizontal mirrors of 150×150 μm and 250×250 μm . Ware at the overall residual stress of 200 MPa in PZT the off-state and on-state of the fabricated CCR. As shown cantilevers. in Figure 6 (b) and (c), there is a clear difference between Figure 5 shows the angular displacement of the fab- the“on”and “off”states. InordertodetectaHe-Nelaser ricated MEMS CCRs with two different horizontal mir- beam reflected from the CCR, a silicon photodiode was rors. The fabricated CCRs with the horizontal mirrors utilized as a receiver. The installed distance between the 2 2 of 150×150 μm and 250×250 μm exhibited the angu- CCR and receiver was 50 cm because the fabricated CCR lar displacement of 1.37° and 1.87° at 5 volts, which are had large angular misalignment. As shown in Figure 7, good enough for on-off keying, respectively. As shown in the output voltage at the photodiode was significantly Figure 5, the measured angular displacement was also in decreased as the driving frequency increases above 1 kHz. good agreement with the simulated one. It might be attributed to the finite switching time, so that The fabricated CCR was demonstrated to check the the mirror does not undergo full angular displacement feasibility as a passive optical transmitter. Firstly, the and creates reduced peak-to-peak voltage. The 3-dB cut- reflective pattern was observed to confirm that the three off frequencies were approximately 2.5 kHz and 1 kHz for mirrors were well aligned orthogonally to each other. the CCR with the horizontal mirrors of 150×150 μm and Figure 6 (a) shows the reflective pattern of an unactuated 250×250 μm , respectively. It also exhibited good switch- CCR with diagonal illumination (λ = 632.8 nm). As shown ing characteristics with an off-to-on-state transition of in Figure 5, the patterns exhibit similar “star” patterns due 163 μs and on-to-off-state transition of 276 μsatarect- to six effective reflective regions [3]. Figure 6 (b) and (c) angular input voltage and switching frequency of 10 V and present the captured images by CMOS image sensor in 1 kHz, respectively. Figure 6 Photograph of the reflective pattern of the fabricated MEMS CCR from diagonally illuminated lay (λ = 632.8 nm) (a),and captured images in off-state (b) and on-state (c) by using a CMOS image sensor. Park and Park Micro and Nano Systems Letters 2013, 1:7 Page 6 of 6 http://www.mnsl-journal.com/content/1/1/7 Conclusions Silicon bulk micromachined CCRs have been presented for free space optical communications. They were com- prised 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. Competing interests The authors declare that they have no competing interests. Authors’ contributions JYP and JP carried out the design & simulation and drafted the manuscript. JP carried out the fabrication of MEMS device and experimental measurements. Both authors read and approved the final manuscript. Acknowledgements 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. Received: 28 September 2013 Accepted: 25 November 2013 Published: 18 December 2013 References 1. Kahn J, Katz RH, Pister K (2000) Emerging challenges: mobile networking for smart ´ dust. J Commun Netw 2:188–196 2. Chu PB, Lo NR, Berg EC, Pister KSJ (1997) Optical communication using micro corner cube reflectors In: Proceedings of IEEE Micro Electro Mechanical Systems Workshop, pp 350U355. IEEE, Piscataway, NJ 3. Zhou L, Kahn JM, Pister KSJ (2003) Corner-cube retrore ectors based on structure-assisted assembly for free-space optical communication. J Microelectromech Syst 12:233–242 4. Hong YK, Syms RRA, Pister KSJ, Zhou LX (2005) Design, fabrication and test of self-assembled optical corner cube reflectors. J Microelectromech Syst 15:663–672 5. Zhu X, Hsu VS, Kahn JM (2002) Optical modeling of mems corner-cube retroreflectors with misalignment and nonflatness. IEEE J Select Top Quantum Electron 48:26–32 6. Park JC, Park JY, Y WJ, Kim DH, Park J (2011) Silicon bulkmicromachined piezoelectically actuated corner cube retro reflector In: 16th Int Conf on Solid-State Sensors, Actuators and Microsystems, Transducers, pp 1578–1582. IEEE, Piscataway, NJ doi:10.1186/2213-9621-1-7 Submit your manuscript to a Cite this article as: Park and Park: A bulk-micromachined corner cube journal and benefi t from: retroreflector with piezoelectric micro-cantilevers. Micro and Nano Systems Letters 2013 1:7. 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com

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Published: Dec 18, 2013

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