NOVEL SPATIAL LIGHT PHASE MODULATORWITH BI-DIRECTIONAL TILT-PISTON MICROMIRROR ARRAY

Authors: Fujitsu Laboratories Ltd.: S. Yamashita, T. Yamamoto, M. Kawai The Japan Aerospace Exploration Agency: M. Mita CIRMM, Institute of Industrial Science, The University of Tokyo: H. FujitaFujita Adamant Kogyo Company Ltd., JAPAN: M. Yano

(Reference: Transducers & Eurosensors 07/ 3B3.4) Instructor: Dr. Cheng Hsien Liu

Reported by:Student// Jiuun-Hwa (Tony) HorngDate: April/ 16th/ 2008

Class Presentation_I

AgendaIntroductionState-of-the-Art of Proposed novel SLPMConcept of Optical Wavefront Control Operation Principle of SLPMSchematic of SLPMDevice Design & Fabrication ProcessExperimental Results & Conclusions References

IntroductionOptical MEMS based on scanning mirrors are widely commercialized, spatial light phase modulator (SLPM) will play crucial roles in such applications as: Scanning DisplayFiber Switch and Couplingoptical wavefront controlfemto-second pulse shapingtunable filtersBarcode Readeretc.

MEMS-based SLPM shows good performance in terms of control flexibility

These devices reported so far, need relatively complicated fabrication process; This Report proposed a novel SLPM which realizes a simple fabrication process

AgendaIntroductionState-of-the-Art of Proposed Novel SLPMConcept of Optical Wavefront Control Operation Principle of SLPMSchematic of SLPMDevice Design & Fabrication ProcessExperimental Results & Conclusions References

Concept of Optical Wavefront ControlMEMS-based SLPM shows good performance in terms of control flexibility because the micromirror array can form smooth phase profile by bidirectional rotation and translation motionconventional liquid crystal phase shifterMEMS-Based SLPM

Schematic of Proposed Novel SLPMA novel structure of one dimensional SLPM that consists of:1) micromirror array on an SOI wafer2) ITO (Indium Tin Oxide) electrodes on a glass plate

Principle of OperationEach micromirror can exhibit bidirectional single-axis rotation, as well as up- and down translation motion

Silicone on Insulator (SOI) Wafer

Silicone on Insulator (SOI) Technology:Most popular Produced MethodSIMOX processSmart Cut processSIMOX - Separation by IMplantation of OXygen - uses an oxygen ion beam implantation process followed by high temperature (1100-1175 deg C) annealing to create a buried SiO2 layerThe Smart Cut method developed by the French firm Soitec which uses ion implantation followed by controlled exfoliation to determine the thickness of the uppermost silicon layer.400-600C reaction

Indium Tin Oxide (ITO) Electrode/Front ITO Process

AgendaIntroductionState-of-the-Art of Proposed Novel SLPMConcept of Optical Wavefront Control Operation Principle of SLPMSchematic of SLPMDevice Design & Fabrication ProcessExperimental Results & Conclusions References

Device Design: Math. ModelTorsion bar was designed so that the ratio of kt / kp had the optimized valueThe ROC was 6.1 m in case of type A, whereas 0.4 m in case of type B. The estimated driving voltages to rotate 0.1 degree were 55 V and 240 VThese results indicate that the thinned torsion bar is effective for decreasing the driving voltage and avoiding mirror warpageThe gap between the mirror and ITO electrode was set the gap 3 m to match the wavelength of 1.55 m widely used in optical communication (designed 3 times larger than the half of the wavelength of the input optical beam)

Device Design: Modal AnalysisThe resonant frequency of mode 1 is 122 kHz and that of mode 2 is 147 kHz; that means high speedresponse below 10 s can be realized.

It is estimated that the maximum rotation angle of 2.0 degree and displacement of 0.8 m can be achieved with driving voltage up to 250 V. (the results of modal analysis using Intellisuite)

Fabrication Process: MicroMirror Array SOI wafer: 9.5-m-Si / 4-m-BOX/ 400-m-substrate The fabrication process of the micromirror array: With Two step DRIE process, the torsion bar thickness was decreased from 9.5 m to 2 m. (The thinned suspension decreases driving voltage to avoid warpage of the mirror surface) The micromirror array composed of 11 mirrors. The size of two types micromirror is 12 or 28m (W) 300 m (L) and that of the torsion bar is 2 m (W) 30 m (L).titanium tungsten

Fabrication Process: DRIE Process DRIE stands for Deep Reactive Ion Etching and is a dry etch micromachining method used to create deep, steep-sided holes and trenches in wafers The use of ICP (inductively coupled plasma) allows for extremely high silicon etch rates using standard Cryo. and Bosch processes: Cryogenic is a single step process done at cryogenic temperatures

Fabrication Process: ITO Wires

AgendaIntroductionState-of-the-Art of Proposed Novel SLPMConcept of Optical Wavefront Control Operation Principle of SLPMSchematic of SLPMDevice Design & Fabrication ProcessExperimental Results & Conclusions References

Experimental ResultsFig.10) Observed interference patterns, and calculated its rotation angle and displacement from the interval of these patterns. (wavelength of the light=408 nm)

Fig.11) Measured the temporal response of the mirror to a triangular wave by laser Doppler vibrometer (LDV). Measured the displacement of the edge in width and themiddle in length of the mirror, and converted into angle

ConclusionsThis Paper states a novel structure of SLPM that; its mirror can exhibit bidirectional single-axis rotation, as well as up- and down- translation motion. 2) Investigated the optimum dimentions for realizing the device; Found that the structure having thinned torsion bars could avoid the warpage of the mirror surface. 3) The device was fabricated by quite a simple process, and 4) verified its motion experimentally. The measured data for both tilt and piston motions were in good agreement with the simulated data.

What have learnt from this paper;The State of the art of SLPM TechnologyAn typical MEMS development process-- Conceptual study-- Device Design and Simulation-- Fabrication Process settlement-- Experimental Design and data analysisSome popular and emerging MEMS related technology learnt in the class has been reviewed, which are including; SOI, BOX, ITO electrode and DRIEetc.

Reference

O. Solgaard and K. Yu, Microoptical phased arrays for spatial and spectral switching, Proc. of SPIE, vol. 4755, pp. 1-9, 2002 Volume 32, pp.103-115, Minghsin Journal, August 2006, , SPECIAL REPORT SOI Wafer Technology for CMOS ICs Robert Simonton, President, Simonton Associates Wikipedia for: DRIE, SOI, BOX, ITO electrode, SLPM