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Surface Micromachining II

Dr. Thara SrinivasanLecture 4

Picture credit: Sandia National Lab

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Lecture Outline

• Reading• From reader: Bustillo, J. et al., “Surface Micromachining of

Microelectromechanical Systems,” pp. 1552-56, 1559-63.

• Problem set #1 due; problem set #2 on website

• Today’s Lecture• Lateral Resonator Process Flow (from Lecture 3)• MUMPS Foundry and Design Rules• Sandia and Texas Instruments Processes • MEMS Test Structures• Microstructure Release and Surface Passivation

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5Lateral Resonator Process Flow

bumper

electrostatic comb drive

shuttle

spring suspension Shuttle with attached combs are spring-suspended 2 µm above ground plane poly

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Lecture Outline

• Today’s Lecture• Lateral Resonator Process Flow• MUMPS Foundry and Design Rules• Sandia and Texas Instruments Processes• MEMS Test Structures• Microstructure Release and Surface Passivation

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5MultiUser MEMS Process

• Microelectronics Center of North Carolina, MultiUser MEMS Process (MUMPS), now owned by MEMSCAP, France.• Three-level polySi surface

micromachining prototyping and foundry service

• 8 photomasks• $4,900 for 1 cm2 die area

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MUMPS Micromotor

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5MUMPS Process Flow I

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MUMPS Process Flow II

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5MUMPS Process Layers

• Layer properties• Thickness• Stress

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MUMPS Masks

• Mask conventions• Light field: draw features that will stay through fabrication• Dark field: draw holes to be cut out

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• Minimum feature size• Determined by MUMPS’ photolithography precision• Violations results in missing (unanchored), under/oversized,

or fused features• Use minimum feature only when absolutely necessary

nominal min feature min spacepoly0, 1, 2, hole0, poly1_poly2_via 3 µm 2 2anchor1, 2 3 3 2dimple 3 2 3metal 3 3 3hole1, hole2 4 3 3holem 5 4 4

MUMPS Minimum Features

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MUMPS Design Rules

C. Cut-in D. Cut-out

A. Enclosure B. Spacing

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5Design Rule Summary

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Design Rule Example

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5Design Rule Example

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Stringers and Planarization

• Sidewall stringers

• Planarization

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5Lecture Outline

• Today’s Lecture• Lateral Resonator Process Flow• MUMPS Foundry and Design Rules • Sandia and Texas Instruments Processes • MEMS Test Structures• Microstructure Release and Surface Passivation

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Sandia SUMMiT Process

2 mechanical layers

3 mechanical layers

1 mechanical layer

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5Sandia SUMMIT Process

• Sandia Ultraplanar Multilevel MEMS Technology (SUMMiT) is a 5-layer polysilicon process• 14 masks, up to 240 process

steps; most complex poly surface micromachining process

• 1 ground plane/electrical interconnect layer

• 4 mechanical layers• Residual film stress < 5 MPa • Device topography is

planarized using chemical-mechanical polishing (CMP)

4-poly process stack

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SUMMIT Devices

Comb drive microengine actuates hinged mirror

through gear transmission

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5Digital Micromirror Display

• Texas Instruments DMD• 2-D array of optical

switching pixels on silicon substrate.

• Pixel is a reflective micromirror supported on a central post

• Post is mounted on lower metal platform, yoke, suspended by torsional hinges from posts anchored to substrate.

• 2 electrodes under yoke are used to tilt mirror ±10°

• Component in >17 projector brands

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Digital Micromirror Display

16 µm

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DMD Fabrication

Maluf

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Lecture Outline

• Today’s Lecture• Lateral Resonator Process Flow• MUMPS Foundry and Design Rules• Sandia and Texas Instruments Processes• MEMS Test Structures• Microstructure Release and Surface Passivation

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5Thin Films Mechanical Properties

• Mechanical properties which are critical • Adhesion• Residual stress, σ • Stress gradient, Γ• Pinhole density• Density • Mechanical strength

• Young’s modulus, Ε • Fracture strength• Fatigue

• Need for on-wafer measurement• Local measurement of film properties• Difficult to handle and align small structures

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Residual Stress• Origins of residual stress, σ

• Growth processes• Non-equilibrium deposition

– Grain morphology change• Gas entrapment• Doping

• Thermal stresses• Deposition, Coefficient of

thermal expansion mismatch• Annealing

• Stress gradient• Variation of residual stress in

the direction of film growth• Can warp released structures

in z-direction

A bad day at MCNC! (1996)

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5Stress Measurement

• Wafer curvature method (Tencor Flexus)• Compressive stress

makes wafer convex, tensile stress makes wafer concave.

• Optically measure deflection of wafer before and after film is deposited

σ = E’ T2

6Rt

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MEMS Test Structure: Stress

• Clamped-clamped beams (bridges)• Compressive stress causes buckling• Arrays with increasing length are used

to determine critical buckling load • Only compressive stress is measurable

2LEI

cr ≈σ

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5MEMS Test Structure: Stress

• Vernier pointers• Expansion or contraction of

beams causes deflection of pointer, read on vernier

• Single structure indicates compressive or tensile stress

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Stress Gradient Measurement• Beam cantilevers

• Strain gradient Γ causes beams to deflect up or down

• Assuming linear Γ [L-1], z = ΓL2 / 2

• Spiral cantilevers

compressivetensile

+

–

Krulevitch Ph.D.

L.S. Fan Ph.D.

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5Young’s Modulus

• Definition: slope of stress-strain curve in elastic region [N/m² = Pa]

• σ = Eε ε = ∆L / L

• On-chip measurement• Resonating structures

140-190 GPaPolysilicon73 GPaSilicon dioxide323 GPaSilicon nitride160 GPaSilicon (ave.)

3

3

0

421

MLtWE

f y

π≈

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• Fracture testing by beam bending• Test structure shuttle pushed by probe tip so test beams hit and

push against bumpers• Fracture limit is 1-3 GPa (2.8 GPa)• Fracture surface examined using SEM

MEMS Test Structure: Fracture

P.T.Jones PhDfolded flexure structure

shuttlevernier

test beams

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• Fatigue testing • Microdevice with notched flexure

resonated until stiffness change measured

MEMS Test Structure: Fatigue

C. Muhlstein et al.

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Variations in Microstructure Dimensions

• Sources of variation: • Lithography-to-etch variation • Non-vertical sidewalls; trapezoidal cross sections• Tolerance ± 5% (±0.1 µm for t = W = 2 µm)

• Resulting Resonant Frequency Variation• f ∝ (W/L)3/2 , σ negligible → ∆f = 15% for W = 2 µm• f ∝ (W/L)1/2 , σ dominant → ∆f = 5%

• Compensation• Laser trimming• Isotropic etch• Electrical tuning

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5Lecture Outline

• Today’s Lecture• Lateral Resonator Process Flow• MEMS Test Structures• Foundries and Design Rules• TI’s Digital Micromirror Display Process Flow• Microstructure Release and Surface Passivation

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Microstructure Release and Stiction• Stiction ~ the unintended

sticking of MEMS surfaces• Release stiction ~ While

drying after release etch, capillary forces of droplets pull surfaces into contact leading to permanent sticking

• In-use stiction ~ During device use, surfaces may come into contact and adhere due to

• Capillary condensation• Electrostatic forces• Hydrogen bonding• van der Waals forces

CJ Kim et al.

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• Reducing droplet area with mechanical approaches ~ standoff bumps, meniscus-shaping features and tethers

• Avoiding liquid-vapor meniscus formation completely • Supercritical CO2, sublimated solvents• Vapor-phase sacrificial layer etch

Avoiding Stiction

T

P

Supercritical drying

Critical point

STP

solid liquid

vaporSublimation

Evaporation• Surface modification to

change meniscus shape from concave to convex • Teflon-like films • Hydrophobic self-assembled

monolayers (SAMs)

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Dry Release

• Dry sacrificial layer etches• Etch sacrificial oxide with HF vapor• Etch sacrificial polymer layer using O2 plasma• Spin-on polymer spacer, etch with plasma

Kobayashi et al.CJ Kim et al.

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5CO2 Supercritical Drying

• Release with supercritical CO2• Supercritical phase avoids liquid-

vapor meniscus

• Procedure• HF etching of oxide• Thorough water rinses• Methanol rinses and soaks, then

put wafer into chamber• Liquid CO2 displaces methanol• CO2 goes from liquid to

supercritical to gas

T

P

Supercritical drying

Critical point

STP

solid liquid

vapor

Mulhern et al.

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Hydrophilic, Hydrophobic• Hydrophilic, θwater < 90°• Hydrophobic, θwater > 90°

1 2

3θ

contact angle

θ

Hydrophilic case P2

dP1

P2

P1

Hydrophobic case

Lotu

s su

rface

, U

niv.

Mai

nz

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Substrate Substrate

Self-Assembled Monolayers• SAMs as nonstick coatings

• Conformal, ultrathin• Low surface energy• Covalently bound→ wear

resistant• Thermally stable

θwater

ODT SAM 112 ± 0.7°SiO2 <10°

OTS

CH3(CH2)17SiCl3

1 2

3θ

contact angle

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Adhesion Test Structures

Si substrate

anchor actuation pad beam landing pad

ground-plane polysilicon

2 µm

voltage on

• Cantilever beam array• Electrostatically actuated• Beam length that remains

stuck after voltage turned off determines adhesion energy between surfaces

• Clamped-clamped beams

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• Friction between MEMS surfaces• Consumes significant portion of

motive force • Dominant failure mode is intermittent

sticking followed by seizure• Results in wear at contacting

surfaces

• Friction test structures

Friction in MEMS

equilibriumposition

displaced and clamped

. .

. . . .

Srinivasan, Howe, Maboudian et al.

post

beam y

beam x

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• Stiction results• With OTS self-assembled monolayer or Teflon coating…• Can release extremely compliant beams (up to 2 mm long, 2 µm thick,

10 µm wide for SAM)

• Coefficient of friction results from MEMS test structures• friction-testing microstructures and rotating gears

• plain polySi (oxide-coated) µs = 4.9 ± 1.2, µk ≈ 0.26 - 0.5

• OTS self-assembled monolayer µs = 0.09 ± 0.01, µk = 0.07 ± 0.01

• Teflon-coated polysilicon µk ≈ 0.035 - 0.12

• Sandia friction tester: 350× longer until device seizure

• Texas Instruments’ DMD: mean time to failure 100,000 h

Stiction, Friction Reduction