A MEMS Design Project

Debby Chang, Randall Evans, Caleb Knoernschild

under Jungsang Kim, Ph.D.December 10, 2005

Duke University

Overview• Design overview• First Test: manual movement of the gear

– Design of gears– Test setup for gears– Test requirements for gears– Test results for gears

• Second Test: manual actuation of the mirror– Design of the mirror– Test setup for mirror– Test requirements for mirror– Test results for mirror

• Final Test: actuation of comb drives– Design of comb drive– Test setup for comb drive– Test requirements for comb drive– Test results for comb drive

Design Overview

Design Overview

Design Overview

Backside View

y

x

Design Overview

• 3 Subsystems– Linear Actuator– Gears and Ratchet– Mirror and Mirror

Arm

Systems

Overview• Design overview• First Test: manual movement of the gear

– Design of gears– Test setup for gears– Test requirements for gears– Test results for gears

• Second Test: manual actuation of the mirror– Design of the mirror– Test setup for mirror– Test requirements for mirror– Test results for mirror

• Final Test: actuation of comb drives– Design of comb drive– Test setup for comb drive– Test requirements for comb drive– Test results for comb drive

Error 1

Missing

Anchor Anchor

• The large gear is not anchored!

“Where are the big gears??”

Gear Test

• Does the gear perform its function when manually stimulated.– Procedure: stimulate the gear by placing

a probe against one of the teeth, then pushing tangentially

Testing-Materials

• Micromanipulator with a glass pipette probe mounted vertically

• 100x+ camera capable of recording video

• light source mounted vertically

Gear Test Result

Overview• Design overview• First Test: manual movement of the gear

– Design of gears– Test setup for gears– Test requirements for gears– Test results for gears

• Second Test: manual actuation of the mirror– Design of the mirror– Test setup for mirror– Test requirements for mirror– Test results for mirror

• Final Test: actuation of comb drives– Design of comb drive– Test setup for comb drive– Test requirements for comb drive– Test results for comb drive

Mirror and Mirror Arm

Components Simulated Values

Theoretical Values

Mirror Arm Mass 2.54118e-10 kg -

Mirror Arms Spring Constant

0.00269 N/m 0.00254 N/m

Length of Mirror 75.5 um 75.5 um

Length of Arms 467 um 467 um

Length of Mirror Arm

543 um 543 um

y

x

Mirror Design

• SEM Image + Coventor Design

Test Procedure

• First test: Measure angle change based upon translation

Error?

• Mirror flaw in the connection of the mirror arm to mirror. The top stopper has a .5um gap.

• Still Connected! (most of the time)

.5 um gap

Top “stop”

Bottom “stop”

Error 2

• No release holes were designed into a large planar mirror, so the first three chips, released in HF for 4 minutes, may not have fully released the mirrors

• To overcome this, we released 2 extra devices for a total of 15mins in HF to ensure complete release– One device that was released flipped

over onto the gel (see backside picture)

First Release Results

Second Release Results

Mirror Fracture Strength

• Fracture Strength of Polysilicon =1.55 GPa (LaVan et. al )

• Cross sectional area of one mirror arm

A = WxD= 3um * 1.5 um= 4.5 e-12 m^2

• Force required to break the mirror arms

F=Fracture Strength/2*AreaF= 0.014N

• Mirror Weight = 1.1 e-9 kg = 1.1e-8N

A =Cross Sectional Area

“Size and Frequency of Defects in ” LaVan et. Al.

Broken Arms

Mobile mirror release problems

Overview• Design overview• First Test: manual movement of the gear

– Design of gears– Test setup for gears– Test requirements for gears– Test results for gears

• Second Test: manual actuation of the mirror– Design of the mirror– Test setup for mirror– Test requirements for mirror– Test results for mirror

• Final Test: actuation of comb drives– Design of comb drive– Test setup for comb drive– Test requirements for comb drive– Test results for comb drive

Design of Comb Drive

y

x

• Expectations– Designed to

move 20 um at 40 Volts, BUT …

– Flaw only allows ~ 14 um movement

– Hope to see 14 um movement

Design of Comb Drive

~14um

Test setup for Comb Drive

• DC Characteristics– Apply a DC voltage and observe movement

under microscope or camera– Use feature sizes of components on device to

estimate the comb drive movement

• AC Characteristic– Test varying voltages and frequencies– view under microscope or camera to observe

motion– Look for movement of drive – resonance

frequency occurs at largest drive displacement response

Comb Movement and Elasticity

Test Requirements for Comb Drive

• Microprobe tip• For DC Characterization

– 120 Volt, 2.5 Amps Power supply– Microscope– CCD viewing equipment

• For AC Characterization– 5 Vpp max function generator– 10x amplifier– Microscope– CCD viewing equipment

Test Results for Comb Drive

• Tests were not successful due to several design errors

– Contact Pad Flaw

– Comb Tooth Motion due to Poly 0 ?

Error 3

• Ground Electrode electronically isolated from poly 0 wire• Poly 1 pattern placed inside Anchor 1

Probe

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Poly 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Poly 1

Poly 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Poly 1

Poly 1

Contact Pad

Substrate

Poly 0

Oxide 2

Substrate

Poly 0

Oxide 1

Poly 1

Oxide 2

Poly 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Poly 1

Oxide 2

Poly 2

Poly 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Poly 1

Oxide 2

Poly 2

Poly 2

Poly 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Poly 1

Oxide 2

Poly 2

Poly 2

Poly 1

Contact Pad

Substrate

Poly 0

Substrate

Poly 0

Oxide 1

Poly 1

Oxide 2

Poly 2

Top View

Error 4

• Free comb seems to be driven to substrate instead of driven to engage more with corresponding teeth.

• Movable inner comb does not move horizontally very much

Comb Teeth Design Flaw

• Comb drive does not move as designed because of Poly 0• Poly 0 layer leaks charge to nitride layer which forces movable teeth into the substrate

C1 C1

C2

++++++++++++++++++++++++++++++++++++++++++++++++

C2 C2

C1 C1 C1 C1 22

11

2

2

1

Cx

R

Cx

R

CVx

F

But there are twice as many C2

capacitances

Comb Teeth Design Flaw

112

w

hLNC o

o

o

d

LwNC 2

2

Groundh

w1

w2

do

L is intersecting teeth length L

Comb Teeth Design Flaw

Nw

hLNC o 22

11

N

d

LwNC

o

o 22

Ground

h-Δ

w1

w2

do-Δ

L is intersecting teeth length

Comb Teeth Design Flaw

19

822

22

2

1

21

2

2

2

12

1

222

22

1111

2

ww

d

LNw

dN

w

L

R

R

Nd

LwN

d

Lw

dC

dR

Nw

LN

w

hL

hCh

R

CVx

F

o

o

oo

o

o

o

o

oo

oo

• Possible temporary solution – reduce built up charge by grounding substrate through package

and silicon paste