micro/nanosystems technology · moems derive their functionality from the miniaturization of: •...

Micro/Nanosystems TechnologyWagner / Meyners 1

Micro/Nanosystems Technology

Prof. Dr. Bernhard Wagner

Dr. Dirk Meyners

Optical MEMS


Optical MEMS - MOEMS

MOEMS: Micro-Opto-Electro-Mechanical Systems

MOEMS derive their functionality from the miniaturization of:

• Optics

• Electronics

• Mechanics

Microactuators with

optical functionality

Mechanics

ElectronicsOptics

MOEMS


Optical MEMS - Classification

Optical MEMS

Micro-Optics

Alignment

Components

Lenses

Lense Arrays

Beam Shaping (LED)

Display Backlighting

Fiber coupling

Beam Homogenizer

Fixed

Structures

V-grooves

Connectors

Benches

Gratings

Optical Couplers

WDM devices

Optical Spectroscopy

Structural

Components

Packaging

Beam Steering

Fiber-Guides

Hermetic VacuumEncapsulation

Optical connections

MOEMS

Moving

Elements

Mirrors

Shutters

Filters

Attenuators

Gratings

Lenses

Phase modulation

Switches

Scanner (LIDAR…)

Displays

Optical focusing

Fabry-Perot

Tunable WDM devices


MEMS mirrors for optical switching & scanning

Typical mechanical forms

• Rotational and translational

• 1D and 2D

• Torsional and bending springs

• Gimbal-mounted and gimbal-less

Beam deflection

• Static, quasistatic

• resonant

Movable optical mirrors

• MEMS actuators (electrostatic,

magnetic, thermal, piezoelectrical)

• Reflective mirror plate

• Optional: integrated sensor for

position detection

M. Bao, Analysis and design principles of MEMS devices, 2005

H. Specht, MEMS-Laser-Display-System […], Diss., 2010


MEMS mirrors - examples

Microvision Inc.

Lemoptix SA Fraunhofer ISIT

Texas Instruments

Innoluce

Fraunhofer IPMS

Fraunhofer ISIT


Digital Light Processing

DMD: Digital Mirror Device

reflective Spatial Light Modulator (SLM)

2D array of switching micromirrors

each mirror represents an image pixel

individual control of each mirror

DLP market introduction 1996

20 years development timewww.dlp.com

http://www.dlp.com/

http://www.dlp.com/


Operation of micromirror device

gray scales:

binary pulse width modulation (PWM)

10 bit 1024 gray scales

digital operation:

on-state and off-state angles

are defined by mechanical contact

flat state is not used

contrast ratio: > 1000:1

Hornbeck 1997


Digital micromirror array

View of landed mirrors

mirror size: 14 x 14 µm

resonant frequency: > 100 kHz

switching time: ~ 15 µs

Illustration of micromirrors operation states

Douglas 2003


Double layer mechanical structure

separate layers for mirrors and hinges

mirror is covering the hinges

fill factor > 90%


Electrostatic torque generation

bias voltage concept:

V2 = (Vb – Va)2

Vb : bias voltage high equal forces on both sides (instable equlibrium)

Va : address voltage imbalance by small address voltage

2

2

02

1

)/(

z

V

dA

dF

dAdAdFxTe

electrostatic torque:

x: lateral distance from torsion axis

z: gap between electrodes

A: area

F

z

x


DMD process flow: 6 mask layers

CMP: chemical mechanical polishing

provides flat surface for mirror fabrication

spacer 1: sacrificial layer

UV-hardened photoresist

hinge metal: Al-alloy: Al98.8Si1.0Ti0.2

~ 60 nm thick, sputtered

oxide hinge mask: masking layer for RIE of Al

PECVD deposited SiO2

yoke metal: Al alloy

mask #1 #2

#3

#4

post-processing on CMOS wafers

all process steps at T < 400°C


TAlumimum RIE: stop on sacrificial resist layer

simultaneously for yoke and hinge

spacer 2: sacrificial layer

UV-hardened photoresist

mirror: Al-alloy

for long-term reliable operation

#5 #6

partial sawing: into Si substrate

release etch:

plasma etch of sacrifical layer

coating with anti-stiction layer:

lowers surface energy of contacting surfaces

separation of chips by breaking


SEM pictures

completed device: FIB cross-section

focused ion beam preparationdevice without mirror

address

electrode

yoke

hinge


DMD products

SVGA: 800 x 600 mirrors

XGA: 1024 x 768 mirrors

SXGA: 1280 x 1024 mirrors

HDTV: 1920 x 1080 mirrors

30 x 15 mm2 chip with

more than 2 Mio mirrors

challenge for yield engineering!


Further spatial light modulator (SLM) applications

Microlithography:

SLM as programmable reflective photomask

Mask manufacturing for micro lithography

Maskless optical direct writing for

microlithography

PCB fabrication

stepper with 200x

reduction opticsMicronic AB, Fraunhofer IPMS 2005

digital printing (photofinishing)

data storage

microscopy

spectroscopy

….


Spatial optical phase modulator

Adaptive optics

dynamic control of optical phase

wavefront correction

200 x 240 mirror array (Aluminum)

mirror size: 40x40µm

vertical displacement: 0 - 450 nm

analog operation:

continuous mirror movement < /2

Applications:

Ophthalmology

Astronomy

Microscopy

Fraunhofer IPMS


Flying-spot laser projection displays

Deflection of laser in x- and y-direction by

one two-axes mirror or

two single-axis mirrors

Laser source(s)

monochrome laser or

red, green and blue lasers for full-color display

modulated (MHz-range) with the image data

Advantages:

format-free

projection on curved surfaces

torsional micromirror:

large angle deflection at high frequency

extremely high mechanical load on mirror and hinge material


Two-axis scanning micromirrors

micromirror with

two-axis comb drivesflying-spot RGB-laser projection

with two-axis micromirror


Single mirror vs. double mirror

Single two-axes mirror with gimbal suspension

easier optical design

mirror control more difficult

(cross-coupling issues)

Projection with two single-axis mirrors

optical design more difficult

individual mirror control

static mirror

(gimbal)


Projection operational modes

Raster scan: vertical slow deflection with frame rate (fver = 60 Hz, sawtooth)

horizontal fast deflection (sinusoid, resonant)

fhor = Nvert∙ 60 Hz for uni-directional writing

fhor = Nvert∙ 30 Hz for bi-directional writing

e.g. SVGA resolution: fhor = 36 kHz for uni-directional writing

fhor = 18 kHz for bi-directional writing

Lissajous scan: fast deflection in both axes (sinusoid, resonant)

SVGA raster scan Lissajous projection


Advantage of laser sources

… compared to UHP lamps

or LED sources

expanded color space

higher brightness

higher lifetime

www.novalux.comRGB VCSEL

Vertical Cavity Surface Emitting Diode Laser


Modelling torsional mirrors – Modal Analysis Model

• Rectangular/circular scanning mirror

• Rectangular flexure beams

• Desired mode: torsional

• Vertical, horizontal, rocking modes are parasitic

Differential equation (without damping and external

forces) for the single-degree-of-freedom rigid body

system with independent variable 𝜂:

𝜂 + 𝜔02𝜂 = 0

General solution is a harmonic oscillation

𝜂 = 𝐴 sin(𝜔0𝑡 + 𝜑)

𝜔0: natural frequency, φ: phase, 𝐴: amplitude

Assumptions

• Entire bending takes place at flexures

• Mirror is rigid

• Mass of flexures is negligible

• Very low damping H. Urey, Torsional MEMS Scanner design for high-resolution display systems, 2002


Modelling torsional mirrors – Modal Analysis

𝑇

2𝑎

2𝑏

𝐿𝑓

𝑘𝜃 = 𝑎𝑏3 16

3− 3.36𝑏

𝑎1 − 𝑏4

12𝑎4for 𝑎 ≥ 𝑏

Paralllel combination of two springs:

𝑘𝑡 = 2𝑘𝜃



V. Kaajakari, Practical MEMS, 2009



(horizontal and rocking modes are

parasitic


Modelling torsional mirrors – Material stress

In general stress is a distributed force on an external or internal surface of a body.

Here the maximum stress on the flexures is a function of the twisting moment T and of

the form and dimensions of the cross section.

The maximum shearing stresses on the rectangular flexures due to torsion are at the

middle points of the long sides:

The maximum calculated stress should be smaller than the yield stress of the material

𝜏𝑚𝑎𝑥 =3𝑇

8𝑎𝑏21 + 0.6095 𝑏

𝑎+ 0.8865 𝑏

𝑎

2− 1.8023 𝑏

𝑎

3+ 0.91 𝑏

𝑎

4, 𝑎 ≥ 𝑏


Modelling torsional mirrors – FEM

For complex geometries the analytical approach is inapplicable

→ FEM (Finite Element Method) for modal and stress analysis

FEM is a numerical method for finding approximate solutions to boundary value

problems for partial differential equations using the calculus of variations

Modal analysis for a gimbal-mounted 2D MEMS scannerGimbal mode Mirror mode

U. Hofmann, Fraunhofer ISIT


Resolution of projected imageOptical requirements: resolution of projected image

Angularly scanned Resolution:

𝑁 =𝜃

∆𝜃=𝑤

𝛿N is the total number of elemental

points in the focused image. The

diffraction-limited spread is expressed

sin ∆𝜃 ≈𝛿

𝑓=𝑎𝐹𝜆

𝑓=𝑎𝜆

𝐷 𝐹 = 𝑓 𝐷 is the f-number of the

converging cone focussed to the spot

with spotsize 𝛿 = 𝑎𝐹𝜆. For small

diffraction angles (Δ𝜃 ≈ sin Δ𝜃) the

basic expression for angularly

scanned resolution is

𝑁 =𝜃𝐷

𝑎𝜆The aperture shape factor 𝑎 accounts

for the differing distributions of the

focused spot (depends on aperture

shape and illumination).


Resolution of projected image

Resolution is determined by diffraction from mirror aperture, i.e. mirror diameter D

Number of resolvable spots N

a

D

a

DN

optmech 4

mech mechanical angle zero-to-peak

opt full optical scan angle

wavelength

a mirror shape factor, depends on

aperture shape and illumination conditions

a 1…2resolution is proportional to D-product

D is full-excursion of mirror edge

demand for high resolution limits miniaturization

Example: SVGA (Nhor= 800), = 650nm , a=1.3

mechD = 170 rad∙µm = 9.7 deg∙mm

optD = 39 deg∙mm


Mirror deformation

2

52

max 217.0m

mech

Et

Df

Optical imaging system keep aberrations at minimmum

Demand: Mirror deformation: < /10

Dynamic deformation: forces due to mirror oscillation with frequency f

maximum mirror deviation from linearity

D5-dependency determines upper limit of mirror size

large mirrors must have large thickness tmoptimum mirror diameter: Ø 1mm

Static deformation: induced by intrinsic material stress or thermal stress

mirror material: single-crystalline silicon

reflection layer: Aluminum (R > 95% in visible range)

dielectric interference coatings for higher R


Electrostatic comb drives for 2-axis actuation

fixed combs and moving combs in two planes

linear force-angle relation

torque:

Movable comb

Fixed comb

silicon mirror plate: Ø 1.0 mm

silicon torsion hinge:

appox. square cross-section

mirror/hinge thickness: 30-60 µm

2

2

1V

CT

U. Hofmann

Fraunhofer ISIT

comb drives for

x- and y-direction



Comb capacitors for driving and sensing

• Stacked vertical comb electrodes

• Nonlinear capacitance characteristic

• FEM model of the tilted mirror with variable angular deflection



Nonlinear capacitance characteristic of comb electrodes


Electrostatic comb sensor for 2-axis detection

Detection of mirror angular deflection

• Capacitive sensing

• Moving comb electrodes

→ Time varying capacitance as a function of angular deflection

• Applying a DC bias voltage

→

• Charge mode amplifier / transimpedance amplifier

• Analog filtering and amplification


Electrostatic comb sensor for 2-axis detection

Charge mode amplifier

• Transfer function

• Amplification in the passband


Micromirror cross-section

poly-

silicon 2

poly-

silicon 1

fixed comb

electrodes

movable

combssubstrate

mirror with relective coating


Electrostatic micromirror with waferlevel package

Three wafer stack:

borosilicate glass cap waferdeep cavity (~ 0.5 mm)

MEMS mirror wafer

Mirror diameter ~ 1.5 mm

Si bottom wafer with getter

vacuum encapsulation

resonant operation (Q > 100.000)

low-power drive



Hermetically sealed vacuum packaging of MEMS mirrors

• Wafer Level Packaging

→ Mass producible at low cost

• High Q-factors (Q = 40.000…100.000)

→ Low power consumption and large scan angles

• No contamination by fluids or particles, no risk of condensing moisture

→ Longterm stability

MEMS mirror wafer packaged on wafer levelMEMS mirror cross-section with tilted optical window

Fraunhofer ISIT



Fabrication of glass cap wafers with tilted windows

Step 1:

Anodic bonding of silicon islands

to both sides of a glass wafer

Step 2:

Anodic bonding of a silicon wafer

with cavities to the glass wafer

Step 3:

Reflow of the glass in a vacuum oven

Step 4:

Removing the silicon parts by etching

Fraunhofer ISIT


Scanning Laser Display

Development Board

Altera FPGAStratix III

Image MemorySRAM

512k x 36

Phase / Delay

DACScanner Phase

DetectionLaser

Capacitivesensing

XY

MirrorControl

(2D)

ADC

DACVoltageAmplifier

MemoryControl

System overview

Capacitive sensing provides a feedback for

• stable phase controlled scanner operation

• synchronization of MEMS scanner deflection and laser modulation

Fraunhofer ISIT


Scanning Laser Display – 2D MEMS mirror

Fraunhofer ISIT

General properties

• Gimbal mounted mirror

• Vacuum packaging

• Torsional springs

• Electrostatic actuation

• Capacitive sensing

Technical data

• Mirror diameter 1mm

• Horizontal scan frequency 18kHz

• Vertical scan frequency 600Hz

• Optical scan angle 60°

• Chip dimensions (W x L x H) 5 x 7 x 3 mm3

• Power consumption 0.1mW


Mirror diameter: 1 mm

Resolution: 1024 x 512 Pixel

Scan angles: 60 x 60 degrees

Drive voltage: 50 V, peak to peak

Applications: Picoprojectors for smart phones, VR/AR displays

Automotive head-up displays

Scanning Laser Display

Fraunhofer ISIT


MEMS scanner applications

~ 1 mm diameter

• Picoprojectors

• Gesture recognition

• 3D Lidar cameras

~ 2 – 7 mm diameter

• Laser-Phosphor headlights

• Lidar range sensors

for autonomous cars

> 7 mm diameter

• Laser welding

• Laser cutting

• Laser micro structuring

• 3D printer

Fraunhofer ISIT

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