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

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 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 ComponentsSimulated Values Theoretical Values Mirror Arm Mass e-10 kg- Mirror Arms Spring Constant N/m N/m Length of Mirror75.5 um Length of Arms467 um Length of Mirror Arm 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*Area F= 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

Contact Pad Substrate Poly 0 Substrate Poly 0 Oxide 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 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 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 C C2 C1 But there are twice as many C2 capacitances

Comb Teeth Design Flaw Ground h w1 w2 dodo L is intersecting teeth length L

Comb Teeth Design Flaw Ground h-Δ w1 w2 do-Δdo-Δ L is intersecting teeth length

Comb Teeth Design Flaw Possible temporary solution – reduce built up charge by grounding substrate through package and silicon paste