ee143 f2010 lecture 24 micro-electro-mechanical systems (mems) fabrication fabrication...

Professor N Cheung , U.C. Berkeley

Lecture 24EE143 F2010

1

• Fabrication Considerations– Stress-Strain, Thin-film Stress, Stiction

• Special Process Modules for MEMS– Bonding, Cavity Sealing, Deep RIE, Spatial

forming (Molding), Layer Transfer

• Principle of Sensing and Actuation– Beam and Thin-Plate Deflections

• Micromachining Process Flows– MEMS-IC Integration

– BioMEMS, PhotoMEMS

Micro-Electro-Mechanical Systems (MEMS)

Fabrication



2

Axial Stress and Strain

Stress s: force per unit area acting on a material

[unit: Newtons/m2 (pascal)]

s = F/A , A = area

s > 0 tensile

s < 0 compressive

Strain e: displacement per unit length (dimensionless)

e = L/ Lo

* Figure assumes there is no change in lateral dimensions



3

E = s / e [ in N/m2 (Pascal) ]

Poisson’s Ratio

= 0.5 volume conserved

E in GPa ( 1E9 N/m2)

Si 190

SiO2 73

Diamond 1035

Young’s Modulus of a material



4

Stress-Strain CharacteristicFor low stress:

• material responds in elastic fashion

• (Hooke’s Law) stress/strain = constant

sy = yield stress

Ultimate stress - material will break;

For Si (brittle) ultimate stress ~ yield stress



5

Mechanical Properties of Microelectronic Materials



6

Material Choices

(a) Stiffness (b) Strength



7

Poly-Si For MEMS Structure

• Effect of substrate:

single-crystal substrate

(clean surface)

epitaxial layer

amorphous substrate

polycrystalline film

• Average grain size

depends on

deposition &

annealing conditions



8

Stress in LPCVD Poly-Si Films

• Stress varies significantly with process conditions

– strong correlation between microstructure and stress

Str

ain

vs. t a

nn

ea

l:

Tdep~620oC



9

1) Begin with a bonded SOI wafer. Grow

and etch a thin thermal oxide layer to act

as a mask for the silicon etch.

2) Etch the silicon device layer to expose

the buried oxide layer.

3) Etch the buried oxide layer in buffered

HF to release free-standing structures.

Si device layer, 20 µm thick

buried oxide layer

Si handle wafer

oxide mask layer

silicon

Thermal oxide

Use of SOI for MEMS Process



10

Origins of Thin-film Stress

• Extrinsic– Applied stress

– Thermal expansion

– Plastic deformation

• Intrinsic– Growth

morphology

– Lattice misfit

– Phase transformation

stot = sth + sint + sext



11

substratesubstrate

Effect of Thin-film Stress Gradient on Cantilever Deflection

substrate

z

(1) No stress gradient along z-direction

(2) Higher tensile stress

near top surface of cantilever

before release from substarte

(3) Higher compressive stress

near top surface of cantilever

before release from substrate

Cantilever



12

Thin-films Stress Gradient Effects on MEMS Structures

Top of beam more tensile

Top of beam more compressive



13

Stressing along the x-direction, all layers take the same strain

Ex = fA EA+ fB EB

fA and fB are fractional volumes

* Material with larger E takes the larger stress

Stressing along the y-direction, all layers take the same stress

Ey =1/ [ fA / EA+ fB / EB ] * Material with smaller E takes the larger strain

AB

Effective Young’s Modulus of Composite Layers



14

PECVD silicon nitride using the SiH+ NH+ N chemistry.

Substrate RF bias is used to induce ion bombardment.

Because of the light mass, H+ ions can be assumed as the

dominant ion bombardment flux

H+ bombardment energy (eV)

Mechanical

Stress in nitride (in 1E8 Pa)

0

1000eV

Compressive

Tensile

-3

-6

+3

+6



15

Use of Stressed Composite layer to reduce bending



16

Thermal Strain



17

Biaxial Stress in Thin Film on Thick Substrate

No stress occurs in direction normal to substrate (sz=0)

Assume isotropic film (ex=ey=e so that sx=sy=s)

* See derivation in EE143 handout

(Tu et al, Electronic Thin Film Science)



18

t s

= substrate thickness

t f

= film thickness

E = Young’s modulus of substrate

n = Poisson’s ratio of substrate

Radius of Curvature of warpage

“Stoney Equation”

r = Es ts

2

( 1- )s 6 sf tf

See handout for derivation

Substrate Warpage



19

Typical Thin Film stress: 108 to 5x1010 dynes/cm2

(107 dynes/cm2 = 1 MPa)

• Compressive (e <0)

– film tends to expand upon release

--> buckling, blistering, delamination

• Tensile (e >0)

– film tends to contract upon release

--> cracking if forces > fracture limit



20



21

The oxide stress is compressive

since r changes from 300m to

200m (Si wafer more curved)

Calculate Film Stress from change of curvature



22

Deflection of Microstructures - Thin Plate approximation

Cantilever Beam with length L, width w, and thickness t

F in Newton

in N/meter

* Assumes L >> w and t, small deflection approximation

where

L = length of beam (in meter)

t =thickness of beam (in meter)

I = bending moment of inertia

= wt3/12 (in meter4) For reference only



23

Deflection of Circular thin membrane

r = radius, t=thickness, P= uniform pressure (in N/m2)

For small deflections, maximum deflection in center

A more accurate

relationship

For reference only



24

kHz



25

Stiction

Poly-Si beam released

without stiction after

sacrificial layer etching

Poly-Si beam

with two stiction points

after sacrificial layer etching



26

As the etching liquid is removed during a dehydration cycle, a liquid bridge is formed between the

suspended member and the substrate. An attractive capillary force which may be sufficiently strong

to collapse it. Even after drying, the inter-solid adhesion will not release the structure.

Solutions

• Dry etching (e.g. XeF2)

• Super-critical drying (e.g. rinse solution gradually

replaced by liquid CO2 under

high pressure)

• Hydrophobic Coatings

• Use textured surfaces

See C. H. Mastrangelo, “Adhesion-Related Failure Mechanisms in

Micromechanical Devices”, Tribology Letters, 1997



27

Super-critical drying

i) release by immersion in aqueous HF;

ii) substrate and structure hydrophilic

passivation by immersion in a sulfuric

peroxide or hydrogen peroxide solution

resulting in hydrophilic silicon surfaces;

iii) thorough deionized water rinses

followed by a methanol soak to displace

the water;

iv) methanol-soaked samples placed in

the supercritical drying chamber for

drying.



28

* Dry, isotropic, vapor-phase etch

XeF2 vapor pressure (~3.8 Torr at 25 °C)

2 XeF2 + Si 2 Xe (g) + SiF4

(g)

Advantages :

• Highly selective to silicon with respect to Al, photoresist, and SiO2.

• Isotropic, large structures can be undercut.

• Fast ( ~10mm per hour)

• Gas phase etching, no stiction between freed structure and substrate

Disadvantages:

• No known etch stops for Si substrate

XeF2 Etching of Si



29



30

Wafer Bonding

• Anodic bonding (E-field enhanced)

• Adhesive bonding (molten metal, epoxy)

• Direct wafer bonding

*can produce unique MEMS structures



31

Anodic Bonding

Anodic Bonding: Low to moderate Temp, Rapid Process

glass

silicon

-1kV

T=300oC

* works mainly with alkaline-containing glass



32

Example of Anodic Bonding

Pressure Transducer using membrane deflection

glass

glassglass

glass



33

Bump

Chiao and Lin, UCB

Vibrating resonator

Al width=125 mm

Nitride

Water outsideAir inside

• After RTP for

750oC/10secs

• Al sealing ring

width=125mm

• Water is blocked outside

• Al does not wet glass well.

Add Cr adhesion layer

between Al and glass

Liquid Phase Bonding

Top viewCross-section



34

Heat

Direct Bonding

Examples: Si-Si bonding and Si-SiO2 bonding



35

Plasma TreatmentGases Used:

O2, He, N2, Ar

Pressure

200mTorr

Power

50W-300W

Time

15-30 seconds

+

O2

Bond Strengthening Annealing

Room Temp Bonding

Chemical Cleaning:

Piranha + RCA

-

Plasma Assisted Direct Bonding



36

Requirements of Direct Bonding Surfaces

Surface micro-roughness ~ nm

No macroscopic wafer warpage

Minimal particle density and size: 1mm particle will give 1000mm void

Contamination free surface



37

Permanent Bond Formation

O

O

O

O

Si

Si Si

O

OSi

Si Si

O

Si

Si Si

Formation of

Covalent bondHydrogen Bonding



38

3-D circuit with a metal

interconnect (at top), followed by a

memory substrate, a bond

interface, then logic metal, logic

transistors and a logic substrate.

• Bond memory substrate and

logic substrate.

• Thin both substrates with

grinding and CMP.

• Etch vias and metallization to

connect the two die.

http://www.ziptronix.com

Logic substrate

Memory substrate

Metal interconnects



39

Uses high density plasma to alternatively

etch silicon and deposit an etch-resistant

polymer on side walls

Polymer deposition Silicon etch using

SF6 chemistry

Polymer

Deep Reactive Ion Etching



40

Lithography , electroplating, and molding processes

to produce microstructures.

Very

thick

resist

Metal platingPlastic molding

Final microstructure

MoldingExample:LIGA Process (Lithographie, Galvanoformung, Abformung)



41

1) Deep silicon mold etch. 2) Sacrificial layer deposition.

3) Structural layer deposition. 4) Chemical-mechanical polish (optional).

and Deposition and patterning of a second polysilicon layer forms cross

linkages between the high-aspect-ratio molded polysilicon structures.

5) Release and extract molded part. Source: Keller and Ferrari



42

Thermal

OxidationDeposition

Sealing of Cavities

ee143 f2010 lecture 24 micro-electro-mechanical systems (mems) fabrication fabrication...

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