MEMS Technologies for Optical and Bio-Medical Applications

Transcription

1 MEMS Technologies for Optical and Bio-Medical Applications Dr. Veljko Milanović Dr. Daniel T. McCormick Adriatic Research Institute Berkeley, CA Adriatic Research Institute, 2005

2 Outline Motivations and Background General Application Examples Some micromirror examples Additional optical MEMS components Bio-photonics (Medical Imaging) ARI-MEMS

3 Why Optical MEMS Advanced lithography techniques allow the realization of precision, aligned optical components at the scale of optical wavelengths MEMS devices and actuators have the ability to tune over wide ranges with sub optical wavelength accuracy. Optical MEMS promise to revolutionize a wide range of application areas including: Telecommunications, displays, optical imaging

4 BSAC Motivation: cm 3 Multisensor Optical Transceiver

5 SALT Project Objectives Free-space laser communication between mobile nodes using MEMS transceivers θ div : Laser beam divergence D θ t R θ div 1mrad D 5 m R 5 km

6 Applications: Automotive Collision Avoidance HUD head-up display Demand for miniature and low-cost laser beam-steering systems High speed scanning/refresh rate Large deflection angles desirable

7 Beam-steering OXC Binary Switching -N 2 switches required Analog mirrors more complex implementation 1DoF or 2DoF mirrors 2N Scaling easier to achieve larger N Lucent, C-speed, Xros, etc.

8 Image Projection Texas Instrument s DLP (DMD) Technology: individual mirrors each represent a single pixel Silicon Light Machines GLV Technology: 1-D array of light valves to form an addressable diffraction grating

9 Micro Photonics UC Berkeley micromirror Fresnel Zone Plate and Laser Diode (UCLA) Microvision, Inc. displays Clip-on and virtual retinal displays

10 Outline Motivations and Background General Application Examples Some micromirror examples Additional optical MEMS components Bio-photonics (Medical Imaging) ARI-MEMS

11 Surface MEMS 1-D Micromirror Structure: comb-drive actuators Support Frame Mirror Surface Electrostatic Combdrive Combdrive Linkage Frame Hinge Torsion Hinges Substrate Hinge Conant et al, MOEMS99

12 Lucent 2DoF Micromirror: parallel plate

13 Optical Phased Arrays Of Micromirrors Dual mode µmirror design Movable mirror comb electrodes Torsional Hinge Static top comb electrodes Static bottom comb electrodes Courtesy of O. Solgaard Phased arrays of micromirrors allow directional control and optical phase correction for precision optical spatial and spectral scanning Applications: Spectroscopy Fiber optics Free space optical communication 70 Reference mirror Moving mirror Intensity Vertical Comb Distance (mm) SEM of phased array for spatial switching. Far-field pattern of scanning mirror array with phase correction

14 Outline Motivations and Background General Application Examples Electrostatic actuators overview Some micromirror examples Additional optical MEMS components Bio-photonics (Medical Imaging) ARI-MEMS

15 2x2 MEMS Fiber Optic Switches 2x2 MEMS Fiber Optic Switches with Silicon Sub-Mount for Low-Cost Packaging Shi-Sheng Lee, Long-Sun Huang, Chang-Jin CJ Kim and Ming C. Wu Electrical Engineering Department, UCLA , engineering IV Building, Los Angeles, California Mechanical and Aerospace Engineering Department Figure 3. SEM of the torsion mirror device. Figure 1. SEM of the 2x2 MEMS fiber optic switch.

16 Magnetic Parallel Assembly Parallel assembly of Hinged Microstructures Using Magnetic Actuation Yong Yi and Chang Liu Microelectronics Laboratory University of Illinois at Urbana-Champaign Urbana, IL Figure 1. (a) An SEM micrograph of a Type I structure. The flap is allowed to rotate about the Y- axis. (b) Schematic cross-sectional view of the structure at rest; (c) schematic cross-sectional view of the flap as H ext is increased. Solid-State Sensor and Actuator Workshop Hilton Head 1998 Figure 2. (a) SEM micrograph of a Type II structure. (b) Schematic cross-sectional view of the structure at rest; (c) schematic crosssectional view of the structure when H ext is increased.

17 Outline Motivations and Background General Application Examples Some micromirror examples Additional optical MEMS components Overview of design methodology Bio-photonics (Medical Imaging) ARI-MEMS

18 MEMS Based Optical Coherence Tomography MEMS Based OCT high resolution images non-invasive imaging rapid scanning speed in vivo imaging potentially low cost Provide a new tool for a wide variety of research and clinical applications in diagnostics as well as treatment. OPTICAL BIOPSY MEMS Scanning Device McCormick, Tien et al, Cornell University, BSAC

19 Development of Fast Micromirror Scanners with Large Angle Static Deflection Dr. Veljko Milanović Adriatic Research Institute, 2005 Berkeley, CA

20 Outline High Aspect Ratio MEMS Vertical combdrives in Multilevel SOI- MEMS Gimbal-less micromirrors Tip-tilt-piston actuation Latest devices / results High fill-factor array development Conclusions and demonstrations

21 High Aspect Ratio MEMS (HARM) SOI-MEMS Height/width > ~3:1 Height/space > ~3:1 Increased capacitance for actuation and sensing Low-stress structures single-crystal Si only structural material Highly stiff in vertical direction isolation of motion to wafer plane flat, robust structures Thermal Actuator 2DoF Electrostatic actuator Comb-drive Actuator

22 MEMS Mirrors in SOI Courtesy Robert A. Conant, BSAC O.Solgaard Stanford DRIE etching of SOI materials High-quality, flat, and stiff mirrors High-speed deflection Electrostatic actuators High force, Dual-mode and twosided possible, All-conductive structure Flexible Process Phased arrays, 2-D arrays, Through-the-wafer interconnects Combs require accurate alignment Yield, Reliability, Maximum deflection Front-side processing Improved reliability and yield, controlled damping, standard wafers, integration

23 STEC Micromirror Vertical Comb-drives Conant et al, HH2000 Conant et al, HH2000

24 Outline High Aspect Ratio MEMS Vertical combdrives in Multilevel SOI- MEMS Gimbal-less micromirrors Tip-tilt-piston actuation Latest devices / results High fill-factor array development Conclusions and demonstrations

25 New possibilities with multi-level beams Upper Front surface of SOI device layer Low Lower Upper Back surface of SOI device layer (a) Front surface of SOI device layer Upper 1 High High Middle 1 Back surface of SOI device layer (b) 3 masks + 1 backside mask gave 3 level beams With taller beams, 4 types of beams result from same set of masks Upper and Lower beam overlap New beam Middle for highest compliance

26 Vertical combdrives in Multilevel beam SOI-MEMS (ARI-MEMS) Lower and Upper beams can be placed at minimum spacing that is DRIE aspect-ratio limited Lower and Upper beam masks self-aligned by same top mask Pre-engaged combfingers for linear force operation from 0 Volts Vertical comb-drives for 2D rotation and piston motion Upper beam torsional support mirror Lower beams Upper beams V. Milanović et al, MOEMS 02, Aug. 2002

27 Fabrication: masks and backside steps All masks prepared before DRIE steps Front-side two oxide masks One embedded oxide mask Back-side thick-resist mask Trench and Align masks DRIE Backside mask Backup mask Upper beams Lower beam Middle beam High beam SOI wafer with 4 masks in place for etching steps Milanović, V. JMEMS, Feb. 2004

28 Fabricated 1DoF Mirror with Independent Pistoning Control Self aligned combdrives Compliant low aspect-ratio beams Isolated combdrives Opposing actuation combdrives Mirror surface Down combs Up combs GND A B A B V 1 V 2 V 4 V3 500 µm V. Milanović et al, MOEMS 02, Aug. 2002

29 Multiple modes of actuation in Multilevel Beam SOI-MEMS combined in a single device Down combs Up combs V 2 V 1 GND B-B V 4 Upper High Lower GND V 3 A-A V 2 V 3 GND A-B V 4 V 1 GND A -B Demonstrated bi-directional rotation, pure rotation (no lateral or vertical motion) and pure pistoning motion V. Milanović et al, MOEMS 02, Aug. 2002

30 1D Rotators Generally designed for one-sided pure rotation to 10 mechanical, 20 optical beam deflection <=100 V actuation Designs from 2 khz to 5 khz resonant freq. 600 um round mirror, 30um thick Optical beam deflection [deg] Peak-to-peak resonant scanning at 4447 Hz Peak static scanning Applied voltage [V]

31 Two-axis rotation and independent pistoning gimbal-based device Pure pistoning vertical actuators (a) (b) V 4 V 3 GND V V 3 2 V V 3 2 V 2 V 1 GND Microlens V 1 V 4 V1 200 µm V 1 Limited speed due to gimbal inertia Angle[degree] % Inner mirror % Outter mirror (c) Y-Y X-X Kwon, et al, Hilton Head GND V Driving Voltage [V] Kwon, et al, Optical MEMS 2002

32 Outline High Aspect Ratio MEMS Vertical combdrives in Multilevel SOI- MEMS Gimbal-less micromirrors Tip-tilt-piston actuation Latest devices / results High fill-factor array development Conclusions and demonstrations

33 Concept of Gimbal-less 2D scanners Kyoto device Rotation actuator A Rotation actuator B Y-axis X-axis Backside etched cavity Rotation actuator B Rotation actuator A Rotation actuator A Outside linkage Inside linkage Rotation actuator A Micromirror plate Rotation transformer Axis of rotation (x-axis) Rotation transformer

34 1 st generation Kyoto device SEM V 2 GND V 1 V 1 V 2 BACKSIDE CAVITY 200 µm Milanović et al, MEMS, Kyoto, Japan, Jan. 2003

35 Two-axis scanning paradigm shift No more gimbals problematic isolation technologies Both axes can have arbitrary frequencies Inherently allows more degrees of freedom Tip, Tilt, Piston Variety of actuators can be attached (rotators, bi-dir. rotators, pure vertical pistons ) Problem is broken down into modules easy design Actuators can be very long effectively increasing frequency (and cost) as desired

36 Characterization of a typical device X-axis 4.46 khz resonance Y-axis 4.56 khz resonance Optical beam deflection [deg] Deflection from mirror on x-axis Deflection from mirror on y-axis Deflection from actuators x-axis Deflection from actuators y-axis -10 shuriken layout allows arbitrarily long actuators for each axis DC actuation voltage [V] Milanović et al, Spec. Issue IEEE JSTQE, May-Jun. V. Milanović, 2004 UCB EE245 Sp 05, Apr. 27 th, 2005

37 Ultra high-speed applications: Reducing reflector inertia Separate SOI wafer for micromirrors Multilevel fabrication Pedestal Trusses for support Thin reflector plate Custom metallization Custom-size reflectors The largest, 2.0 mm reflectors with Al metal coating assembled into ARI gimbal-less two-axis scanners and sealed with a anti-reflection (AR) coated quartz lid. A 1.2 mm reflector with Au metal coating assembled into ARI gimbal-less two-axis scanner and sealed with a antireflection (AR) coated quartz lid.

38 Low-inertia Micromirror Assembly Assembly of ARI 1.2 mm MEDIE and 2.0 mm BIGGIE with MEMS PI custom tool

39 Actuators for Special Size Mirrors: B-type, L-type, M-type B-type device: small die and smaller voltages Most economic and largest angle design L-type device: small die and large voltages Most economic high speed design M-type device: large die and large voltages Highest speed design

40 Mirror diameter vs. Resonant Freq. M 2.0 mm mirror B L Simulation results vs. several measured points 1.2 mm mirrors

41 Breaking up the problem Small elements instead of one large actuator Higher frequency response Better reflector flatness Issues with >96% fill-factor negligible Severe increase in compexity Packaging, control, calibration Universal optical element?

42 Low-mass reflector fabrication/transfer Multilevel fabrication Pedestal Trusses for support Thin reflector plate

43 Low-mass reflector fabrication/transfer Extended pads for characterization 15 µm Truss sidewall Bonding using optical epoxy Micromanipulation with MEMS PI custom made instruments (collab. with C. Keller) V. Milanović et al, Sensors and Actuators Workshop, Hilton Head, SC, Jun. 2004

44 Low-mass reflector fabrication/transfer 500 µm 500 µm Bonding using optical epoxy Micromanipulation with MEMS PI custom made instruments (collab. with C. Keller) V. Milanović et al, IEEE JSTQE, May 2004.

45 Conclusions Technologies in place to make arrays 10 khz update rate for tip-tilt-piston Actuators continue to improve performance, decrease size Thin micromirrors easy to fabricate Assembly always a challenge Assembling in batch promising

46 Device model and characterization Typical device response as a 2 nd order mechanical system, in this example with f n = 3800 Hz and Q~ Norm. response magnitude Magnitude Phase Response phase [deg] Frequency [Hz] Small signal Force response

47 Complete Model in 3 parameters Control voltage Electrostatic force (F) Mechanical system response Micromirror position v(t) K [ v(t ] 2 1 ) H ( s) = s 2 2 K2 ωn + 2 ζ ω s + ω n 2 n p(t) Natural freq. Damping ratio Gain factor ωn / 2π ζ K1*K2 [x10-4 ] Device Hz Device Hz Device Hz Critical aspect of model freq. doubling e.g. : v(t) = sin (wt) => F ~ sin (2wt)

48 The High-Q Problem Step responses in open loop, no control: Actuator vs. actuator w/ bonded mirror Mechanical Deflection Angle [deg] Time [s] HFF800 w/ bonded micromirror HFF800 actuator only

49 Closed-Loop PD Control Desired position Device model Micromirror voltage from Fig. 1c position PD v(t ) Plant : Axis d (t) + p(t ) - controller of scanner PSD sensor Step response settling times < 100 µs measured in closed-loop schemes with optical feedback Optical Deflection Angle [deg] Closed loop PD control w/ optical feedback Open loop Time [ms]

50 Open-loop Problem Norm. response magnitude Magnitude Phase Response phase [deg] Frequency [Hz] The simple solution is to LPF sharply somewhere well before ω n /2 Butterworth, Bessel filters Speed is limited and can never compete with closed loop response Type of filter still critical, i.e. group delay response

51 Inverse System Solution Inverse Sys. Final Response Orig. System H 1 ( s) p p 1 ( s + p )( s + p ) Works great (but only for small signals)

52 Open-Loop Inverse System Square Root Control Desired position voltage d(t) System inverse ~ 1 ( s ) H Amplifier ( K K ) v(t) Device model from Fig. 1c Plant : Axis of scanner Micromirror position p(t) 40 6 Command / Actuation voltage [V] Actuation voltage waveform Optical deflection on PSD Optical Deflection Angle [deg] Time [ms] 0

53 Summarized Results Settling Time LPF Butterworth LPF Bessel LPF Bessel ISR CLPD [µ s] Order = 6 Order = 6 Order = 24 ω p = ω n / 3.5 ω p = ω n /2.7 ω p = ω n /1.3 Device Device Device N/A

54 Vector projection demo Driven from laptop audio port (<1 Volt, low power) or USB port Two channel driver for X-axis and Y-axis Scans any desired vector or point-to-point drawings at high refresh rates (a) (b) (c) (d) (e) (f) (h) (i) (j) (k) (g) Milanović et al, Optical MEMS 2004, Aug. 2004

55 MirrorcleDraw Software MirrorcleDraw software runs on the Matlab platform and allows the user to control a two-axis micromirror scanner via soundcard port or a DAQ card. User can input text, shapes, drawings, functions, etc, and apply various digital filters to the prepared waveform which is then sent at a chosen refresh-rate to the device. MirrorcleDraw software s graphic user interface (GUI.) User views the input drawing in blue, a filtered drawing in red, and the predicted micromirror response in green (based on system ID.)