Advancing Measurements by Light • www.polytec.com 1

Improving Performance of MEMS Designs

Using Dynamic Characterization

Eric Lawrence MEMS Business Development

Manager

Advancing Measurements by Light • www.polytec.com 2

Polytec Introduction

Contents

Introduction to Laser Vibrometry

•Polytec Micro System Analyzer (MSA-500)

•Application: MEMS Comb Drive

•Application: MEMS mmirror

•Application: MEMS / NEMS Cantilever

•Application: Wafer Level Testing Pressure Sensor

•New Ultra High Frequency Vibrometer

Laser Doppler Vibrometers

For non-contact vibration measurements

Laser Surface Velocimeters

For surface speed and length measurements

White Light Interferometers

For surface topography measurements

Advancing Measurements by Light • www.polytec.com

Optical Measurement Solutions

Fast, accurate visualization and analysis of structural vibration

Automotive

Health Monitoring

MEMS

Aerospace

Data Storage

Polytec Scanning Vibrometer

Tools for Vibration Analysis

Micro Electro Mechanical

Structures (MEMS)

Diverse tools available to measure wide range physical properties (shape, dimension, film thickness, time response, stress, roughness, stiction, resonant frequency, environmental response…)

High spatial resolution, accuracy and precision required

Fast response times often require high speed measurement techniques

Spatial complexity (mm – nm) of MEMS challenging for conventional techniques

Wide range of performance criteria among different devices

Handling and environmental requirements

Fast measurement speed is critical for high volume production testing

Reliable techniques that allow scientists and engineers to effectively communicate physical properties

Challenges and Requirements for MEMS Testing

Why optical measurement of MEMS dynamics?

MEMS usually involve active moving elements for sensing and

actuation

Electrical test can prove if a device is working or not, but……

Electrical testing can’t determine exact behavior of device

Need highly sensitive, non-invasive , real-time measurement

Laser Doppler Vibrometry

Motivation

Frequency Modulated signal

40 MHz ± fD

Bragg cell f0 ± fD

f0

f0 + 40 MHz

He-Ne Laser <1mw (633nm)

x(t) v(t)

Photo-detector

Measurement Beam

Reflected Beam

Δ fD = 2V/λ

Laser Doppler Vibrometry

Voltage ~

Velocity

Voltage ~

Displacement

FM Doppler signal

AM electrical signal

Controller Photo-detector system

FFT Spectrum

Signal Demodulation

Laser Doppler Vibrometry

The interferometer is coupled via a fiber into the microscope (MSA)

A scanning mirrors allows to scan the whole surface point by point

Scanning Laser Doppler Vibrometer

sequential measurement at all points. Excitation for all points

Vibration Time Signal

Vibration Spectrum

Scanning Laser Doppler Vibrometer

Advantages of Laser Doppler Vibrometer

Advancing Measurements by Light • www.polytec.com

• Real Time Measurement: Fast signal-based measurements from

broadband excitation, can measure transient response

• High Resolution: Displacement resolution down to picometer

• High lateral resolution: Laser spot focused down to 700 nm

• High frequency bandwidth: DC to 24 MHz (1.2 GHz)

• High accuracy: Doppler technique highly accurate and linear

• Can do difficult measurements on range of materials, under

required environmental conditions, i.e. thru glass into a vacuum

chamber, calibration independent of these factors

• Probe Station Integration: Integrates with commercially available

probe stations for wafer level testing

In-plane: Strobe Video Microscopy

Additional Strobe Video Microscopy

Capability for in-plane motion

In-Plane motion of MEMS

Comb drives

Gyroscopes

Accelerometers

Automatically acquires strobe image sets

Pattern Matching to measure displacement

Software tools for analyzing response

Topography: White Light Interferometry

Additional Topography Measurement Capability

Topographical Measurement of:

Step-Height

Shape (Curvature, Flatness)

Roughness

Form Parameters (Dimensions,

Angle, Radius)

Combines powerful tools for precise 3D

analysis of structural vibration and

surface topography:

MSA-500 Micro System Analyzer

•Scanning Laser Vibrometry for fast measurement

and 3D visualization of out-of-plane deflection

shapes.

•Strobe Video Microscopy for capturing and

analyzing in-plane motion.

•White Light Interferometry for mapping surface

topography.

Applications

Advancing Measurements by Light • www.polytec.com

In-plane actuator driven by electrostatic pulling force

Restoring force from

bifold springs

Natural Frequency

given by:

Substituting for Keff and Meff:

Where E is Young’s Modulus of Elasticity, w is

spring width, r is the density, L is the spring

length and Aeff is the effective area of comb drive. eff

eff

M

K

0

eff3

23

AL

)1( wE

r

d

2

2

1V

g

nhεF

Example: Comb Drive Resonator

Example: Comb Drive Resonator

Setup:

Measurements were performed with MSA system at our lab in Tustin, CA

Device + Fixture placed under microscope and positioned into place

Relatively easy setup for placing chip and locating P6 Comb Drive to be measured

Vibration Isolated Table to minimize background motion

Device driven from built-in waveform generator and amplifier

Example: Comb Drive Resonator

Frequency Response:

Set up a grid of approximately 700 measurement points

80 Volt Burst Chirp Excitation to 2 MHz, 25600 Lines FFT

Measurement time each spot 12.8 ms (chirp response)

Frequency Response Function measured for each point

Automatically Scan measurement for all points

F1 =.0138 MHz

0.344 MHz

0.407 MHz

0.453 MHz

0.527 MHz

0.590 MHz

0.929 MHz

1.169 MHz

1.345 MHz

1.493 MHz 1.614 MHz

Example: Comb Drive Resonator

1610 KHz

13.8 KHz

526 KHz Resonance frequency peaks selected by graphical interface tool

Operational Deflection shapes displayed for each frequency corresponds to a unique mode

Fundamental rigid body resonance at 13.8 KHz

Example: Comb Drive Resonator

•Settling time dynamics of whole

mirror (3D image of time

sequence)scanning vibrometry

Tilt Motion Direction

Hinge

Axis

•Because the heart of the projector system is the DMD mirror array – a thorough understanding of the dynamic motion of the mirrors is critical to gauge performance of current as well as future technology directions…

Dynamic Response of Mirror Array Courtesy Rick Oden, Texas Instruments

23

• Hermetically sealed Micro-opto-electro-mechanical system(MOEMS)…

• Massive array of 16mm (older) or 12.7mm mirrors are used as light deflectors (modulators)… • Arrays up to 2.2 million mirrors are currently in production…

• Each mirror has a hidden hinge over which it twists upon… • Tilts of the pixels are ±10 or ±12 degrees…

Mirror

Hinge

Hinge Post (support)

Yoke/Beam

Drive Electronics and Interconnects…

Spring Tips

Example: Texas Instruments mdisplay

Hinge Axis

(pivot axis)

• System focuses laser spot through

microscope objective onto surface of

interest…

• Reflected laser spot is sent to the

interferometer to compare against for

Doppler frequency shift…

• For this optical set-up, the minimum

beam waist for the laser signal is

approximately 1mm.

Example: Texas Instruments mdisplay

Hinge Axis

(pivot axis)

• As mirror transitions from one state to

another(‘-’ to ‘+’ for example), the MSA system can acquire a time domain response of this point on the mirror…

Example: Texas Instruments mdisplay

Hinge Axis

(pivot axis) • As we all know – three points are necessary to determine a plane. Similarly, to build up a time development of the mirror – several points must be inspected over the mirror to render reliable data…

Example: Texas Instruments mdisplay

27

Tilt Motion Direction

Hinge

Axis

Roll Motion Direction

Sag Motion Direction

• There are three primary directions

of motion that characterize these micro-mirrors… • With these three base motions in mind, MatLab is used as a processing and graphical user interface (GUI) to obtain the time developed dynamics of the mirrors…

Example: Texas Instruments mdisplay

28

• The base motion directions above are shown for a single mirror. Ability is implemented to acquire and process data on several mirrors to provide dynamics where we can compare results mirror-to-mirror…

Example: Texas Instruments mdisplay

29

• Image above (left) shows one of the processing/visualization windows within MatLab. • In this case, a (3x3) array of mirrors are shown with their corresponding tilt, roll and sag axis time developed dynamics in the right side of figure.

Example: Texas Instruments mdisplay

Properties

Length X 0.225 mm

Width 3.5e-002 mm

Thickness 4.e-003 mm

Material Si

Volume 3.15e-005 mm³

Mass 7.308e-011 kg

Nodes 988

Elements 900

Model for Modal Response of Cantilever based on mechancial parameters

Example: Cantilever

Base Excitation:

Piezo used to provide external base excitation, transmitted directly to cantilever

Excited from built-in waveform generator

More info at: http://www.physikinstrumente.com

Picma Piezo from Physik Instrumente

Resonant

frequency

[kHz] ±20%

Blocking

force [N @

120 V]

Max.

displacement

[µm @ 120 V]

Dimensions

A x B x L

[mm]

Part #

135 290 8 ±20% 3 x 3 x 9 P-883.10

Piezo

Actuator

Cantilever + Substrate

Example: Cantilever

Frequency Response:

Swept sine measurement to 2000 KHz

Measurement time each spot 12.8 milliseconds (chirp response)

Frequency Response Function measured for each point

Measurement laser spot directed at the end of the cantilever

Example: Cantilever

1st Bending Mode:

Experimental Data: Model:

Discrepancy: -29%

Example: Cantilever

2nd Bending Mode:

Experimental Data: Model:

Example: Cantilever

Discrepancy: -23%

Torsion Mode:

Experimental Data: Model:

Example: Cantilever

Discrepancy: -49%

Measurement on poly3 400 um cantilever showing resonance at 18.45 KHz and damping factor 0.10 (squeeze film damping)

Example: Cantilever Array

PARTEST: Testing Methods for Determination of Production Relevant

Parametersin MEMS on Wafer Level

http://www.memunity.org/par-test.htm

Example: Wafer level testing

Electrostatic Electrodes

• No mechanical contact to wafer

• Force applied to conductors,

semiconductors and dielectric

materials

• Realized –3 dB frequency bandwith

300 kHz

• Integrated distance measurement

• Wafer level test possible

• Electrodes transparent (ITO)

Micropositioner Probe „card“

Elctrostatic Excitation

Example: Wafer level testing

0 200 400 600 800 10000

1

2

3

4x 10

-4

f [kHz]

v [m

/s]

• Pressure sensors with quadratic membrane regular dies have the 2nd/3rd mode at the same frequency values

Example: Wafer level testing

Example: Wafer level testing

mean( EIE) = 0.08µm

std( EIE) = 0.04µm

0.05

0.1

0.15

0.2

0.25

Peak error

max. EIE

Red spots show bad dies

Wafer map with Classification

EIE

[µm

]

EIE: Estimated Identification Error

Example: Wafer level testing

UHF-120 Ultra High Frequency Vibrometer

42

Technical Specifications UHF-120

Ultra High Frequency Vibrometer

Example: SAW Filter Measurement

Ultra High Frequency Vibrometer

Example: SAW Filter 262 MHz

Ultra High Frequency Vibrometer

•Polytec MSA unique, all-in-one optical

measurement solution for 3D vibration

measurement plus topography

measurement

•Real-time, broadband measurement

with frequency response in milliseconds

•Highly Sensitive measurement with

resolution down to picometer level

• Well supported by engineers

knowledgeable with MEMS applications

and necessary requirements for testing

Advancing Measurements by Light • www.polytec.com

Conclusion