mems technologies for optical and bio-medical...

V. Milanović, UCB EE245 Sp’05, Apr. 27th, 2005

MEMS Technologies for Optical andBio-Medical Applications

Dr. Veljko MilanovićDr. Daniel T. McCormick

Adriatic Research InstituteBerkeley, CA

http://www.adriaticresearch.org

Outline

• Motivations and Background– General Application Examples– Some micromirror examples

• Additional optical MEMS components• Bio-photonics (Medical Imaging)• ARI-MEMS

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…

BSAC Motivation: cm3 Multisensor Optical Transceiver

SALT Project Objectives

Free-space laser communication between mobile nodes using MEMS transceivers

RD tθ≈θdiv: Laser beam divergence

D 5 mθdiv

Applications: Automotive

• Demand for miniature and low-cost laser beam-steering systems

• High speed scanning/refresh rate

• Large deflection angles desirable

HUD – head-up display

Collision Avoidance

Beam-steering OXC

• Analog mirrors – more complex implementation– 1DoF or 2DoF mirrors

• 2N Scaling – easier to achieve larger N

• Lucent, C-speed, Xros, etc.

• Binary Switching -N2 switches required

Image Projection

Texas Instrument’s DLP (DMD)Technology:individual mirrors each represent asingle pixel

Silicon Light Machines’ GLVTechnology:1-D array of light “valves” to form an addressable diffraction grating

Micro Photonics

UC Berkeley micromirror

Microvision, Inc. displays Clip-on and virtual retinal displays

Fresnel Zone Plate and Laser Diode(UCLA)

Outline

Surface MEMS 1-D Micromirror Structure: comb-drive actuators

Torsion Hinges

Support Frame

Mirror Surface

Electrostatic Combdrive

Substrate Hinge

Frame Hinge

Combdrive Linkage

Conant et al, MOEMS99

Lucent 2DoF Micromirror: parallel plate

Optical Phased Arrays Of Micromirrors

Dual mode µmirror design Static top comb

electrodes

Static bottom comb electrodes

Movable mirror comb electrodes

TorsionalHinge

Moving mirror

Vertical Comb

Reference mirror

SEM of phased array for spatial switching. Far-field pattern of scanning mirror array with phase correction

-1 -0.5 0 0.5 1

Distance (mm)

• Phased arrays of micromirrors allow directional control and optical phase correction for precision optical spatial and spectral scanning

• Applications:SpectroscopyFiber opticsFree space optical communication

Courtesy of O. Solgaard

Outline

• Motivations and Background– General Application Examples– Electrostatic actuators overview– Some micromirror examples

2x2 MEMS Fiber Optic Switches2x2 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. WuElectrical Engineering Department, UCLA

63-128, engineering IV Building, Los Angeles, California 90095-1594Mechanical and Aerospace Engineering Department

Figure 1. SEM of the 2x2 MEMS fiber optic switch.

Figure 3. SEM of the torsion mirror device.

Magnetic Parallel Assembly

Solid-State Sensor and Actuator WorkshopHilton Head 1998

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 Hext is increased.

Figure 2. (a) SEM micrograph of a Type II structure. (b) Schematic cross-sectional view of the structure at rest; (c) schematic cross-sectional view of the structure when Hext is increased.

Parallel assembly of Hinged Microstructures Using Magnetic ActuationYong Yi and Chang Liu

Microelectronics LaboratoryUniversity of Illinois at Urbana-Champaign

Urbana, IL 61801

Outline

• Additional optical MEMS components• Overview of design methodology• Bio-photonics (Medical Imaging)• ARI-MEMS

MEMS Based Optical Coherence Tomography

MEMS Based OCT

• high resolution images• non-invasive imaging• rapid scanning speed• in vivo imaging• potentially low costProvide a new tool for a wide variety of research and clinical

applications in diagnostics as well as treatment.

“OPTICAL BIOPSY”

MEMSScanningDevice

McCormick, Tien et al, Cornell University, BSAC

Development of Fast Micromirror Scanners Development of Fast Micromirror Scanners with Large Angle Static Deflectionwith Large Angle Static Deflection

Dr. Veljko Milanović

veljko@adriaticresearch.org

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

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

2DoF Electrostatic actuator

Thermal Actuator

Comb-drive Actuator

MEMS Mirrors in SOI

Courtesy Robert A. Conant, BSAC

• DRIE etching of SOI materials• High-quality, flat, and stiff

mirrors• High-speed deflection• Electrostatic actuators

• High force, Dual-mode and two-sided possible, All-conductive structure

• Flexible ProcessPhased arrays, 2-D arrays, Through-the-wafer interconnects

• Combs require accurate alignment

Yield, Reliability, Maximum deflection

• Front-side processingImproved reliability and yield, controlled damping, standard wafers, integration

O.SolgaardStanford

STEC Micromirror – Vertical Comb-drives

Conant et al, HH2000

Conant et al,HH2000

Outline

New possibilities with multi-level beams

• 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

Back surface of SOI device layer

Front surface of SOI device layer

Middle

er Hig

Back surface of SOI device layer

Front surface of SOI device layer

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 beamtorsional support

mirror

Lowerbeams

Upperbeams

Upper beamtorsional support

mirror

Lowerbeams

Upperbeams

V. Milanović et al, MOEMS’02, Aug. 2002

Fabrication: masks and backside steps

Backup maskBackside mask

Trench and Align masks

• All masks prepared before DRIE steps• Front-side two oxide masks• One embedded oxide mask• Back-side thick-resist mask

SOI wafer with 4 masks in placefor etching steps

Milanović, V. JMEMS, Feb. 2004

Lower beam

Upper beams

High beam

Middle beam

Fabricated 1DoF Mirror with Independent Pistoning Control

500 µm

Up combs

Down combs

Mirror surface

• Self aligned combdrives• Compliant low aspect-ratio

beams• Isolated combdrives• Opposing actuation

combdrives

Multiple modes of actuation in Multilevel Beam SOI-MEMS – combined in a single device

Up combsDown combsV2 V1

A-A’

B-B’

A’-B

A-B’

Upper HighLower

Demonstrated bi-directional rotation, ‘pure’ rotation (no lateral or vertical motion) and ‘pure’ pistoning motion

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

Applied voltage [V]0 20 40 60 80 100 120

Peak-to-peak resonantscanning at 4447 Hz

Peak staticscanning

Applied voltage [V]0 20 40 60 80 100 120

Peak-to-peak resonantscanning at 4447 Hz

Peak staticscanning

Two-axis rotation and independent pistoning

gimbal-based device

Kwon, et al, Optical MEMS 2002

-10 0 10 20 30 40 50 60 70 80 90

Driving Voltage [V]

% Inner mirror % Outter mirror

Limited speed due to gimbal inertia

200 µm200 µm

Pure pistoning vertical Pure pistoning vertical actuatorsactuators

X-X’Y-Y’

(a) (b)V4

Microlens

Kwon, et al, Hilton Head 2002.

Outline

X-axis

Y-axis

Rotation actuator A

Rotation actuator A’

Rotation ac

tuator B

Rotation ac

tuator B

Backside

etched cavity

X-axis

Y-axis

Rotation actuator A

Rotation actuator A’

Rotation ac

tuator B

Rotation ac

tuator B

Backside

etched cavity

Concept of Gimbal-less 2D scanners – Kyoto device

Rotationactuator A

Rotationactuator A’Micromirrorplate

Rotationtransformer

Axisof rotation

(x-axis)

Insidelinkage

OutsidelinkageRotation

actuator ARotation

actuator A’Micromirrorplate

Rotationtransformer

Axisof rotation

(x-axis)

Insidelinkage

Outsidelinkage

200 µm

V 2V 1

BACKSIDECAVITY

1st generation Kyoto device SEM

Milanović et al, MEMS, Kyoto, Japan, Jan. 2003

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

Characterization of a typical device

X-axis 4.46 kHz resonanceY-axis 4.56 kHz resonance

DC actuation voltage [V]0 20 40 60 80 100 120 140

25Deflection from mirror on x-axisDeflection from mirror on y-axisDeflection from actuators x-axisDeflection from actuators y-axis

“shuriken” layout allows arbitrarily long actuators for each axis

Milanović et al, Spec. Issue IEEE JSTQE, May-Jun. 2004

‘Ultra’ high-speed applications:Reducing reflector inertia

Separate SOI wafer for micromirrorsMultilevel fabrication

PedestalTrusses for supportThin reflector plate

Custom metallizationCustom-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 anti-reflection (AR) coated quartz lid.

Low-inertia Micromirror Assembly

Assembly of ARI1.2 mm MEDIE and 2.0 mm BIGGIEwith MEMS PIcustom tool

www.memspi.com

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

Mirror diameter vs. Resonant Freq.

2.0 mm mirror

1.2 mm mirrors

Simulation results vs. several measured points

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?

Low-mass reflector fabrication/transfer

Multilevel fabricationPedestalTrusses for supportThin reflector plate

15 µm Truss sidewallExtended pads

for characterization

Bonding using optical epoxyMicromanipulation with MEMS PI custom made instruments (collab. with C. Keller)

V. Milanović et al, Sensors and Actuators Workshop,Hilton Head, SC, Jun. 2004

Bonding using optical epoxyMicromanipulation with MEMS PI custom made instruments (collab. with C. Keller)

V. Milanović et al, IEEE JSTQE, May 2004.

500 µm

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

Frequency [Hz]0 2000 4000 6000 8000

MagnitudePhase

Device model and characterization

• Typical device response as a 2nd order mechanical system, in this example with fn = 3800 Hz and Q~60

Small signalForce response

ωωζω

+⋅⋅⋅+⋅

=[ ]21 )(tvK ⋅)(tv )(tp

Controlvoltage

Electrostaticforce (F)

Mechanical systemresponse

Micromirrorposition

Natural freq. Damping ratio Gain factorωn / 2π ζ K1*K2 [x10-4]

Device 1 6000 Hz 0.005 5.7Device 2 4400 Hz 0.008 5Device 3 2326 Hz 0.137 6.5

‘Complete’ Model in 3 parameters

Critical aspect of model – freq. doublinge.g. : v(t) = sin (wt) => F ~ sin (2wt)

Time [s]-0.0005 0.0000 0.0005 0.0010 0.0015 0.0020

HFF800 w/ bonded micromirrorHFF800 actuator only

The High-Q Problem

Step responses in open loop, no control:Actuator vs. actuator w/ bonded mirror

Time [ms]-0.2 0.0 0.2 0.4 0.6 0.8 1.0

8Closed loop PD control w/ optical feedbackOpen loop

)(td )(tp

Desired positionvoltage

Device modelfrom Fig. 1c

Micromirrorposition

PSDsensor

PDcontroller

‘Plant’: Axisof scanner+ -

)(tv)(td )(tp

Micromirrorposition

PSDsensor

PDcontroller

‘Plant’: Axisof scanner+ -

Closed-Loop PD Control

Step response settling times < 100 µs measured in closed-loop schemes with optical feedback

Open-loop Problem

• The simple solution is to LPF sharply somewhere well before ωn /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

Frequency [Hz]0 2000 4000 6000 8000

MagnitudePhase

Inverse System Solution

( )( )21

211 )(psps

ppsH++

⋅⋅−

Inverse Sys.

Final Response

Orig. System

Works great (but only for small signals)

Time [ms]-0.05 0.00 0.05 0.10 0.15 0.20 0.25

Actuation voltage waveformOptical deflection on PSD

)(td)(tp

Micromirrorposition

‘Plant’: Axisof scanner

)(tv)(~ 1 sH −

System inverse

( ) 121

Amplifier

)(td)(tp

Micromirrorposition

‘Plant’: Axisof scanner

)(tv)(~ 1 sH −

System inverse

( ) 121

Amplifier

Open-Loop ‘Inverse System Square Root’ Control

Settling Time LPF Butterworth LPF Bessel LPF Bessel ISR CLPD[µ s] Order = 6 Order = 6 Order = 24

ωp = ωn / 3.5 ωp = ωn /2.7 ωp = ωn /1.3

Device 1 376 440 292 128 104Device 2 510 476 280 96 92Device 3 1750 1050 790 284 N/A

Summarized Results

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

Milanović et al, Optical MEMS 2004, Aug. 2004

(a) (b) (c) (d) (e)

(h) (i) (j) (k)

(a) (b) (c) (d) (e)

(h) (i) (j) (k)

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.)

mems technologies for optical and bio-medical...

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