What are MEMS? - UFRGS

What are MEMS?

• MEMS =

Micro-Electro-Mechanical-

Systems

• Batch fabricated like integrated

circuits with mechanical

structures

• Silicon That Moves

• An Enabling Technology

MEMS - Changing real world phenomena…

Light

Sound

Temp

Pressure& Flow

Speed

Proximity

AnalogDomain

DigitalDomain

AnalogDomain

Digital Enables:

Better Quality

More Dynamic Range

Post-processing / Enhancement

Signal Integrity

Storage / Duplication

Mixing / Editing

Transmission

Optional Compression

Online Sales

Airbag Safety Systems:

The Beginning of the MEMS Growth Curve

• Crash Detection and Airbag Deployment System– System discriminates impacts on vehicle and

determines whether to deploy airbags and which airbags to deploy

• Other Passive Safety Systems– Front satellite sensors– Side satellite sensors– ECU sensors– Rollover detection sensors– Occupant Position Sensing

• MEMS Sensors Involved– High-g Accelerometers– Pressure Sensors– Angular Rate Sensors– Low-g Accelerometers

MEMS Test - What really IS different?

From a pure electrical test perspective not much is different.

•Mixed signal analysis, precision DC measurement, wave form generators,

digital subsystem

•Standard Tester requirements

•Ported to several commercial testers

MEMS Test - What really IS different?

From a mechanical perspective things are very different.

It’s silicon that MOVES!

•Part must be “handed off” to test fixture and clamped rigidly

•Precisely aligned

•No parametric shift

•Electrical parametric measure in the presence of the mechanical stimulus

•Maintain contact during mechanical and thermal flows

•Multi Axis test requirement

•Shake, Flip or Rotate

•Reference instrumentation

•Mechanical Noise or Vibration concerns

How Do MEMS Accelerometers Work?• Single axis accelerometer in silicon has the same components

– Left / Right (X-axis)

XLeft RightM a s s

Proof MassSuspension

Spring

Suspension

Spring

Motion

How Do MEMS Accelerometers Work?• All moving parts are suspended above the substrate

– Sacrificial layer removed from below moving parts during fabrication

1st MEMS Accelerometer Test System

United States Patent 5895858

MEMS Test Challenges "Most semiconductor industry engineers have a mechanical background or an

electrical background, which is sufficient to test any type of semiconductor

product," notes Sami.

"However, since MEMS consist of both electrical and mechanical components, the

testing become complex and not many have the technical training required to

perform the tests."

To overcome this issue of customers' ignorance, test vendors have begun to focus

on educating end users on the importance of MEMS testing as well as the

benefits they are likely to obtain. Once customers recognize the true value of

testing, the adoption rates are expected to increase significantly.

Frost and Sullivan - MEMS Test Equipment Market Responds to Need for Low-Cost Products and End-User Education

•Fault modeling

•Fault simulation paradigm.

•Test generation

•Design: blocks with dynamics of 1st and 2nd order.

Designing for Test MEMs-based Systems

Electromechanical structures fault modeling

Functional fault modeling

•Circuit described through the I/O transfer function

•Several electrical, mechanical and electromechanical structures can be

modeled by 1st and 2nd order differential equations

•Basic building blocks: 1st and 2nd order transfer functions

•Faults in inner components of the transfer function

•I/O fault model described as deviations in the transfer function

21 nd

• Step input

•Time domain, transient analysis

dTp

22

2

.2)(

nn

n

pLss

sH

eOS

21%

•n is the system undamped natural frequency and is the damping ratio

•Td is the delay time to regime, Tp the peak-time, OS the overshoot percent.


Functional modeling

Original transfer function:

ps

gsT

)(1

pps

ggsT

nf

11 )(

'g' represents faults in the gain g, 'p' insertion of a deviation in the pole

frequency, and 'n' deviations in the degree of the equation.

Faulty transfer function:


Fault models for 1st order structures

bassa

KsT

2

0

2 )(

bbsaasa

kKsT f

)()1()(

2

0

2


Fault models for 2nd order structures

Original transfer function:

Faulty transfer function:

•Transient Response Analysis Method (TRAM)

•Use of compact test vectors

•Test vectors confirmed through fault simulation (FS)

•Fault dictionary based on the circuit's time response

Test vector generation

FS for the “Pole deviations”

1st order system

FS for the Order deviation

1st order system

FS for the “Gain deviation”

1st order system

The fault impact is influenced by poles and zeros positioning and ratio

FS for a 2nd order Butterworth-like configuration

FS for a 2nd order Chebyshev-like configuration.

Butterworth

Chebyschev

Fault simulatin for 2nd order structures

Application example:

Acceleration signal processing

Pendulous accelerometer

No acceleration

input

Pickoff circuitry

Proof mass

Silicon structures

•Sensor responds to input acceleration by deflecting the pendulum by an

angle proportional to the acceleration.

•The pickoff and electronics module sense the deflection and convert it to a

proportional electrical output signal.

Application example:

Acceleration signal processing

Pickoff circuitry

Proof mass

Acceleration input

Displacement

Open-loop accelerometer model:

P

A(s)

1/(Js2+Cs+Kc)

Ti(s)

KPO(s)

PO(s)

KOA(s)

VPO(s)V0(s)

A(s): Laplace tranform of the acceleration

P: Pendulosity

Ti(s): Laplace transform of the driving torque

J: moment of inertia of pendulum

C: damping torque coeficient

Kc: pendulum elastic restraint

Kpo: pickoff circuitry transfer function

Vpo(s): Laplace transform of pickoff output voltage

Koa: output amplifier transfer function

Vo(s): Laplace transform of sensor output voltage

Proposal for device modeling

•Sensor performance improved: negative feedback and ordinary low-pass filtering

•Feedback: torquer acting on the pendulum to minimize the deflection

•Filtering: 1st order admittance, where 1/RC is the time constant and KLP the DC gain


Pendulosity

1/(Js2+Cs+Kc) KPO(s) KAO(s)P + 1/(Rt+RL+Lts)

Kt

VPO(s)PO(s)A(s) Ti(s) Te(s)

Tr(s)

IO(s)

Pendulum

Dynamics Pickoff Conductance of

Load & TorquerServo Amp

(1 + RCs)/KLP

Vo(s)


•Sensor performance improved: negative feedback and ordinary low-pass filtering

•Feedback: torquer acting on the pendulum to minimize the deflection

•Filtering: 1st order admittance, where 1/RC is the time constant and KLP the DC gain

Pendulosity

1/(Js2+Cs+Kc) KPO(s) KAO(s)P + 1/(Rt+RL+Lts)

Kt


Tr(s)

IO(s)

Pendulum

Dynamics Pickoff Conductance of


(1 + RCs)/KLP

Vo(s)


•1st and 2nd order dynamic structures

•Pendulous output voltage Vo(s) forces a current Io(s) through the torquer

•Io(s) causes a restoring torque Tr applied back to the pendulum

•High loop gains and negative feedback produce nearly equal feedback and input

•Current Io is a direct measure of the input acceleration

2nd order block:

test vector: acceleration step A(s)

output response: known waveform whose shape depends on gain and poles

1st order blocks:

test vector: voltage

output response: known 1st order and 2nd order waveforms

Device testing: open-loop configuration

Pendulosity

1/(Js2+Cs+Kc) KPO(s)P +


Pendulum

Dynamics

Pickoff

IO(s)

Low-pass filterConductance of


Vo(s)

KAO(s) 1/(Rt+RL+Lts) (1 + RCs)/KLP

VOs(s)

Transient response for 10% deviations

in the moment of inertia

Different peak times and overshoots

Transient response for 10% deviations in

the danping torque coefficient.

Different peak times and overshoots

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What are MEMS? - UFRGS