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Appl Phys Lett 96 211103

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  • 1. APPLIED PHYSICS LETTERS 96, 211103 2010 An all optically driven integrated deformable mirror device V. Mathur,1,a S. R. Vangala,1 X. Qian,1 W. D. Goodhue,1 B. Haji-Saeed,2 and J. Khoury2 1 Department of Physics and Applied Physics, Photonics Center, University of Massachusetts, Lowell, Massachusetts 01854, USA 2 Air Force Research Laboratory/Sensors Directorate, Hanscom Air Force Base, Massachusetts 01731, USA Received 13 March 2010; accepted 26 April 2010; published online 24 May 2010 We demonstrate a technique for actuating micromirrors vertically cascaded on wafer fused GaAs-GaP photodiodes. Unlike traditional actuation schemes, the electrostatic drive of the individual capacitive actuators is addressed optically in this device. Vertical mirror displacements of up to 500 nm were observed using interferometry while addressing the photodetectors with a 5 mW optical signal. Microlenses were used to address a 900 pixel device with patterned conductive pillars and thin film load resistors for each actuator-detector element. This approach can enable realization of faster and denser adaptive optics wave front corrector arrays. © 2010 American Institute of Physics. doi:10.1063/1.3430568 Deformable micromirrors are the key-enabling compo- In this letter, we demonstrate an integrated MEMS de- nent in a variety of optical systems such as switches in fiber vice utilizing patterned thin film resistors in parallel with optics communication,1 image production in digital light micromirrors suspended over a photodetector array using processors,2 and phase correctors in adaptive optics semiconductive pillars. This device allows operation in the systems.3 Traditional actuation schemes electrostatic,4 near infrared IR regime by utilizing a wafer fusion tech- magnetic,5 and thermal6 require integrated electrical cir- nique for fabricating GaAs photodiodes on a transparent GaP cuitry to address each actuator individually. These generally substrate. The mirror displacement is demonstrated to be a involve sophisticated fabrication techniques and drive up function of the incident laser power and is modulated opti- manufacturing costs particularly in high pixel density appli- cally. We also demonstrate that the current device developed cations. A technique utilizing one of the traditional actuation for image correction applications may also be used as an methods but addressed optically is thus the need of the hour. optically addressable ON/OFF switch. The pixels in the array For instance, in adaptive optics systems, Shack–Hartmann- are designed such that each mirror-detector element is in type feedback loops require optically addressable wave front parallel with the others, thus requiring only a single electrical correctors to achieve faster image correction. An earlier re- bias for the whole device as opposed to our earlier devices. ported attempt7 in this regard directly utilized radiation pres- Figure 1 shows three-dimensional 3D schematic of the final sure from focused laser beams to impart momentum to mi- device consisting of cascaded silicon nitride Si3N4 micro- croactuators. A significant drawback of this technique is the mirrors on a wafer fused photodiode array. The photodiodes very high optical intensity required to produce small dis- are addressed individually by focusing a control laser signal placements. For example, a focused beam of intensity individually through a cascaded microlens array. A low stress 600 mW/ mm2 is required to rotate an actuator by half a Si3N4 piston motion spring plate mirror design developed degree. In another reported technique, high frequency optical earlier11 with response time in microseconds, requiring low pulses8 were utilized to excite microstructures. This tech- actuation voltages is used here. Each individual mirror is nique finds limited applications due to the dimensional con- fabricated incorporating a patterned tantalum nitride TaN straints it imposes on the structures to operate near their thin film resistor in the megohms range. Specially developed structural resonance. SU-8 resist polyaniline PANI doped is then utilized to In one of our first attempts at developing an all optically bond and electrically connect the micromirrors to the photo- addressed microelectromechanical system MEMS diodes. Figure 2 shows a single pixel cross section of the deformable-mirror device,9 a metalized mylar membrane was device. The cascaded microlens focuses the input control la- suspended over an insulating photoresist grid on a photocon- ductive substrate. Optically addressing the substrate initiated a higher effective electrostatic drive between its front surface Phase correction and the suspended membrane, thus actuating it. The sensitiv- Micro Mirrors Resistors(Backside) ity of these devices was improved10 by replacing the sub- strate with an InGaAs photodetector array. However, to con- trol the impedance between the suspended membrane and detector, it was necessary to bias them with a very high fre- Gallium Phosphide Gallium Micro lens quency ac, or a combination of both ac and dc. It was later Arsenide array proposed that using semiconductive posts would allow volt- Photodetectors age distribution across the detector and mirror using just a dc bias. Control laser FIG. 1. Color online A 3D schematic of the mirror detector array ad- a Electronic mail: vaibhavdeniro@gmail.com. dressed via microlenses. 0003-6951/2010/96 21 /211103/3/$30.00 96, 211103-1 © 2010 American Institute of Physics Downloaded 24 May 2010 to 129.63.234.140. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
  • 2. 211103-2 Mathur et al. Appl. Phys. Lett. 96, 211103 2010 Phase Correction Coated side wall Gold + (a) (b) SU-8 SU-8 TaN Resistor Si3N4 Mirror Ohmic contact N - I GaAs P GaP Micro lens Laser FIG. 4. Scanning electron microscope images of the final MEMS device a Spring plate mirrors with surrounding spiral TaN resistors b A portion of FIG. 2. Color online Cross-section schematic of a single pixel of the the wafer fused GaAs–GaP PIN mesa array with patterned conductive SU-8. device. tops were then gold coated to form highly reflective mirrors. ser beam on the GaAs photodetectors through the transparent This metal deposition was done at an angle to ensure that the GaP back substrate. GaP was chosen as the support substrate sidewalls of an etched opening connecting the resistor side of due to its high optical transparency to the control laser near the mirror to the front side, was also gold coated Fig. 3 v . IR wavelength regime. The metal coated mirror top and PIN To fabricate the detector array, a GaAs wafer with epitaxi- diode mesa act as the upper and lower electrodes of a capaci- ally grown p-i-n structure was bonded to a GaP substrate at tive actuator, respectively. The generated photocurrent passes 700 ° C in a custom designed furnace and fixture Fig. 3 vi . vertically through the semiconductive pillars causing a volt- The GaAs substrate was subsequently removed by a combi- age drop across the thin film resistor in turn actuating the nation of lapping, polishing and wet etch. The transferred mirror. p-i-n layers were then patterned and wet etched to form the Figures 3 i –3 x illustrate the steps involved in fabricat- GaAs p-i-n mesas. The etch was monitored closely to stop in ing the top mirror and bottom detector array. First, a low the half micron thick GaAs P region. A five layer Ni/Ge/Au/ stress plasma enhanced chemical vapor deposition recipe12 Ni/Au metal scheme was then deposited to form ohmic con- was used to deposit 600 nm of silicon nitride on an indium tacts on the N side of each mesa and a single surrounding phosphide InP substrate. The TaN sheet resistivity contact on the bottom P side Fig. 3 viii . In the final step, a 1.4 k / sq was then sputtered on lithographically pat- proprietary technique for doping and patterning SU-8 2010 terned photoresist AZ1512 and lifted off to form spiral resis- Microchem Corp. with a conductive polymer PANI was tors Fig. 3 ii . Metal contact pads were then patterned fol- used to form two micron tall pillars on the mesas Fig. 3 ix . lowing a similar process at the resistor ends Fig. 3 iii . Figure 4 shows the scanning electron microscope images of Spring plates were then defined lithographically Fig. 3 iv portions of the final device with 900 pixels. The individual in the center and a reactive ion etcher used to etch the Si3N4 pixel size including the resistor was 200 microns, with ac- down to the InP. The actuators were then released by etching tual mirror area of 66 66 m2. The existing fill factor of away the InP selectively using HCl. The released actuator 10% Fig. 4 a can be further improved by shrinking the resistor size by using a higher resistivity material. The peri- odicity of the p-i-n mesas and mirrors was 250 microns, and Si3N4 GaP the surrounding conductive SU-8 pillars were 10 m wide InP GaAs PIN (vi) Wafer fusion Fig. 4 b . TaN GaAs After fabricating the GaAs p-i-ns their breakdown and Metal Doped SU-8 photoresponse characteristics were measured using a 830 nm Photoresist laser source. As the photodetectors operate in series with the patterned load resistor, their breakdown voltage sets an upper (i) Si3N4 PECVD deposition (vii) Lapping and polishing limit on the resulting voltage drop across the actuator. Since a voltage drop of 10 to 15 V is desirable across these actua- tors, the p-i-ns were designed to have reverse breakdown voltages in excess of 20 V. Figure 5 shows the I-V charac- (ii) TaN sputtering (viii) Wet etch and metal deposition 100.0 3mW 830nm Laser Back illumination through microlenses Photocurrent (Microamps) (iii) Resistor pads metal deposition (ix) Conductive SU-8 patterning 75.0 Top illumination after wafer fusion Top illumination before wafer fusion 50.0 Dark (iv) Si3N4 RIE etch (x) Samples cascaded 25.0 0.0 (v) Mirror release, metal deposition 0 10 20 30 Voltage (Volts) FIG. 5. I-V curves illustrating the photo response of optically addressed FIG. 3. Color online Micromirror and detector array fabrication steps. GaAs PINs before and after wafer fusion. Downloaded 24 May 2010 to 129.63.234.140. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
  • 3. 211103-3 Mathur et al. Appl. Phys. Lett. 96, 211103 2010 830nm Laser Frame grabber LabVIEW Dark Light 600 Oscilloscope Iris BS + Mirror displacement (nm) Photodiode - Device 400 He-Ne Laser BS BS 200 Moving mirror FIG. 6. Color online Interferometry and optical addressing setup for the MEMS device beam splitter: BS . 0 teristics of the p-i-n diodes under no illumination dark , top 0 2 4 6 Optical power (mW) illumination P-side on top before wafer fusion, and illumi- nation through the GaP substrate P-side on bottom after FIG. 7. Interferograms showing fringe shift in shining light on the 2 M resistor MEMS device inset . Mirror displacement vs optical intensity of wafer fusion. A high photocurrent of about 80 A was ob- the MEMS device with 800 k resistors. served on the first sample before wafer fusion on shining a 3 mW laser signal. The junction was found to deteriorate device, the TaN resistors were patterned to have a lower slightly after the high temperature wafer fusion, and the pho- resistance of 800 k . The control laser intensity was var- tocurrent dropped to 20 A when again addressed directly ied from 1–5 mW and the displacement measured in steps of from the top. On the wafer fused test sample, the p-i-ns were quarter wavelengths = 632 nm as shown in Fig. 7. The addressed through the GaP substrate and cascaded micro- mirror displacement was found to saturate close to 500 nm lenses focal length 2 mm . The photocurrent further dropped corresponding to a 5 mW control laser power. to 10 A due to additional optical losses , but was still In summary, a technique for optically addressing and sufficient for actuation, provided a high value load resistor. actuating micromirrors using cascaded photodiodes has been In the devices tested here, the TaN resistances were varied demonstrated. The device can be used as a simple ON/OFF upto 2 M to ensure a voltage drop of at least 10 V across optical switch or as a phase corrector device in adaptive op- the mirror. To monitor the mirror displacement as a function tics. The design proposed here offers several advantages over of optical power, an inverted microscope was modified to other addressing techniques as it eliminates the need for include standard interferometry as shown in Fig. 6. The complex circuitry, and may be adapted for several other ma- MEMS device was then placed upside down and the micro- terial systems. lens sample aligned carefully on top. A 830 nm Newport LQA-830 control laser mounted on the microscope was then 1 L. Y. Lin, E. L. Goldstein, and R. W. Tkach, IEEE J. Sel. Top. Quantum used to address approximately 100 photodiodes uniformly Electron. 5, 1 1999 . 2 and simultaneously via the microlenses. Note: The actual L. J. Hornbeck, White paper on DLP & MEMS technology, http:// power focused after the microlens on each individual detec- focus.ti.com/pdfs/dlpdmd/107_DLP_MEMS_Overview.pdf. 3 A. Tuantranont and V. M. Bright, IEEE J. Sel. Top. Quantum Electron. 8, tor was not measured . A helium neon laser probe beam 632 33 2002 . nm from a Michelson interferometer setup was focused on 4 H. Toshiyoshi and H. Fujita, J. Microelectromech. Syst. 5, 231 1996 . 5 an individual micromirror from under the microscope. The C. H. Ji and Y. K.Kim, J. Lightwave Technol. 21, 584 2003 . 6 reflected interference beam was captured using a frame grab- A. Jain, H. Qu, S. Todd, and H. Xie, Sens. Actuators, A 122, 9 2005 . 7 ber, and a portion of it reflected away using a beam splitter. J. M. Zanardi, P. O. Vaccaro, T. Fleischmann, T. Wang, K. Kubota, T. Aida, T. Ohnishi, A. Sugimura, R. Izumoto, M. Hosoda, and S. Nashima, An iris was then used to select only the innermost fringe Appl. Phys. Lett. 83, 3647 2003 . from this reflected beam and focused on a photodetector. 8 J. E. Graebner, S. Pau, and P. L. Gammel, Appl. Phys. Lett. 81, 3531 Quantitative data on the mirror displacement was obtained 2002 . 9 by monitoring the intensity of this zeroth order fringe on an B. Haji-saeed, R. Kolluru, D. Pyburn, R. Leon, S. K. Sengupta, M. Testorf, W. D. Goodhue, J. Khoury, A. Drehman, C. L. Woods, and J. Kierstead, oscilloscope. The first device tested consisted of 2 M re- Appl. Opt. 45, 2615 2006 . sistors, and the mirror was found to displace abruptly even 10 J. Khoury, K. Vaccaro, C. L. Woods, B. Haji-saeed, S. K. Sengupta, C. with very low intensity control laser signal 1 mW . This Armiento, W. D. Goodhue, J. Kierstead, and A. Davis, Proc. SPIE 6368, can be attributed to a significant contribution to the total 11 636804 2006 . parallel resistance by the semiconductive SU-8 posts, result- G. Griffith, B. Haji-saeed, S. K. Sengupta, W. D. Goodhue, J. Khoury, C. L. Woods, and J. Kierstead, IEEE Photonics Technol. Lett. 19, 173 ing in a large voltage drop. The mirror operated as a simple 2007 . ON/OFF switch in this case, as confirmed from the dark and 12 A. Tarraf, J. Daleiden, S. Irmer, D. Prasai, and H. Hillmer, J. Micromech. light interferogram in the inset of Fig. 7. In our second test Microeng. 14, 317 2004 . Downloaded 24 May 2010 to 129.63.234.140. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp