Surface Micromachining

Dr. Marc Madou,

Fall 2012, UCI

Class 10

Blanket n+ diffusion of Si substrate (ground plane)

Passivation layer (e.g. SiO2 , Si3N4 , LPCVD Si3N4 on top of SiO2)

Opening up the passivation layer for contacts (observe color change or hydrophobic/hydrophilic behavior):

– wet (BHF)

– dry (SF6)

Strip resist in piranha (adds some oxide in the window)

Short BHF etch to remove thin oxide

n+

n

n+

ntSiO2

Lm

Lm + 2tSiO2Photoresist

Si3N4

Surface MicromachiningBasic Process Sequence (poly-Si)

Deposition of a base, spacer or sacrificial layer-phosphosilicate glass (PSG)-CVD

Densification at 950 °C for 30-60 min in wet oxygen

Base window etching in BHF for anchors

Structural material deposition e.g. poly-Si (doped or undoped) from (CVD at about 600°C , 73 Pa and 125 sccm (standard centimeter cube per minute); at about 100Å/min) e.g.

Anneal of the poly-Si at 1050°C for 1 hour to reduce stress in the structure

PSG

Structural layer

SiH4 → Si + 2H2

nitride

Surface Micromachining

Basic Process Sequence (poly-Si).

Basic Process Sequence (poly-Si)

Doping of poly-Si: in-situ, PSG sandwich and ion implantation

Patterning of structural material e.g. RIE in , say, CF4-O2

Release step, selective etching of spacer layer e.g. in diluted HF

RS(μm / )min >> Rm(μm / )min >> R i(μm )min

RS

Ri Rm

nitride

Surface Micromachining

Etchants-Spacer and Microstructural LayerEtchant Buffer/Isolation Spacer MicrostructureBuffered HF(5:1, NH4F:conc. HF)

LPCVD Si3N4/thermal

SiO2

PSG Poly-Si

RIE us ing CHF3BHF (6:1)

LPCVDSi3N4

LP CVDSiO2

CVD Tungsten

KOH LPCVDSi3N4/thermal SiO2

Poly-Si Si3N4

Ferri c Chloride Thermal SiO2 Cu Polyimide

HF LPCVDSi3N4/thermal SiO2

PSG Polyimide

Phosphoric/Aceti cAcid/Ni tri c Acid(PAN or5:8:1:1water:phosphoric:aceti c:ni tri c)

Thermal SiO2 Al PE CVD Si3N4Nickel

Ammonium Iodide/ Iodine Alcohol

Thermal SiO2 Au Ti

EDP Thermal SiO2 Poly-Si SiO2

Generic principle of surface micromachining

Si

Si

Al

Etch access

Si

Sacrificial layer definition

Releasing diaphragm:phosphoric/acetic acid/nitric acid (PAN)

Polyimide diaphragm deposition

Surface Micromachining

LPCVD of poly-Si Hot wall, horizontal reactor Reaction rate controlled--at lower

pressures and well controlled temperatures (100 to 200 wafers)

Poly-Si deposits everywhere requiring periodic cleaning (e.g. every 20 runs if each run deposits 0.5 µm)

Visit: http://mems.eeap.cwru.edu/shortcourse/partII_2.html and http://www-mtl.mit.edu:800/htdocs/tutorial.html

Surface Micromachining

ABCDESi Phosphosilicate glass PolysiliconStiction Stiction during release:– Surface tension during drying pulls movable

members together (See also room temperature bonding of Si to Si and glass to Si)

– Solutions:» Stand-off bumps» Sacrificial polymer » Sacrificial poly-Si links to stiffen the structures» HF vapor» Freeze-drying water/methanol mixtures» Super critical cleaning

Stiction after release:– Hydrophobic monolayers– Rough surfaces– Bumps

Surface Micromachining

Control of film stress With L=150 µm and W=t=2 µm, fo=10 to 100 kHz.

Annealing at high temperature (900-1150°C)

Fine-grained tensile vs large grained compressive

Doping elements Sandwich doping and annealing. Vary material composition e.g Si

rich Si3N4

In PECVD: change the RF power and frequency

In sputtering: gas pressure and substrate bias

fo ≈12π

4EtW3

ML3 +24σ rtW5ML

Polysilicon Drive combSense combAnchorFlexure ( length L, width W, thickness t)Drive combcontact padSense contact padyxSuspended shuttle (mass M)

Surface Micromachining

Control of film stress

MMMM

Folding flexures makes the resonant frequency independent of the residual stress but warpage becomes an issue Corrugated structural members (see above)

Y

X

ky >> kx( )force constant

fx =kx

M ⎛ ⎝

⎞ ⎠

12

Surface Micromachining

Sealing processes Microshells a wafer level

packaging strategy Thin gaps (e.g. 100 nm) are etched

out and then sealed:– Reactive sealing by oxidation

– LPCVD deposition

Surface Micromined Sealed ResonatorSiO2Si Polysilicon VacuumResonatorAEtched spacerCavityO2B

COMPOSITE SI3 N4 /POLYSILICONPOLYSILICON PIEZORESISTOR

Surface Micromachining

IC compatibility

CMOS SurfaceMicromachining

Common Features Silicon based processes Same materials Same etching principles

Process flow Standard Application specificVertical dimens ion ~ 1 µm ~1-5 µmLateral dimension <1 µm 2-10 µmComplexity (# masks) >10 2-6

Temperature (°C) MaterialLP CVD Depos ition 450 Low Temperature Oxide

(LTO)/PSG" " 610 Low stress poly Si" " 650 Doped poly Si" " 800 NitrideAnnealing 950

1050PSG densificationPoly Si stress annealing

Comparison of CMOS and Surface Micromachining

Critical Process Temperatures for Microstructures

- Junction migration at 800 to 950°C- Al interconnect suffers at 400-450 °C- Topography

Surface Micromachining

Poly-Si surface micromachining modifications: porous poly-Si

Just like we can make porous Si from single crystal Si we can do the same with poly-Si (low currents densities in highly concentrated HF)

Applications:– Channels parallel to a flat surface

(switch from porous to polishing and back--chambers with porous plugs)

– Enclosed chambers (blisters of free poly-Si)

– Hollow resonators (higher Q)

CVD Si3N4

CVD poly-Si

CVD Si3N4

Surface Micromachining

Poly-Si surface micromachining modifications: hinged poly-Si

Make structures horizontally and erect them on a poly-Si hinge (probe station)---rigid structures (Prof. Pister, UCB)

Polyimide hinges also have been made ( butterfly wing)---movable structures

Poly 1OxidePoly 2Poly 2Poly 1Poly 2 staple holdingpoly 1 plate

polyimide hinge (E= 3 GPa) poly-Si hinge (E= 140 GPa)

Surface Micromachining

Poly-Si surface micromachining modifications:hinged poly-Si

Pister et al., UCB

Micromachined integrated optics for free space interconnections

Surface Micromachining

Poly-Si surface micromachining modifications: thick poly-Si and HEXSIL

Thick poly-Si--10 µm in 20 ‘ with SiH2Cl2 at 1000°C has become possible (low tensile stress)

HEXSIL (Dr. Keller, UCB):– Deep dry etching of trenches in SCS

(e.g. 100 µm deep)-short isotropic etch to smooth the walls

– Deposition of sacrificial and structural materials (undoped, doped poly-Si and metal e.g. Ni)

– Demolding by etching away the sacrificial material

Surface Micromachining

Poly-Si surface micromachining modifications: HEXSIL

Dr. Keller, UCB

Membrane filter with stiffening rib HEXSIL tweezers

Surface Micromachining

Poly-Si surface micromachining modifications: SIMOX

Types of Silicon On Insulator (SOI) processes:

– SIMOX (Separated by IMplanted OXygen)

– Si fusion bonded wafers

– Zone-melt recrystallized polysilicon (ZMR)

SIMOX substrate

0.2 μ -m epi Si

0.4 μ m buried oxide

Thicken up epi Si to

4 μ m

Dry etch access hole

S acrificial layer etching Hermetic sealing of etching hole by plasma CVD of

non stressed dielectric plug

Metallization and diaphragm definition

PP

Surface Micromachining

Non-poly-Si surface micromaching.

Polyimide: e.g. SRI flat panel display

UV depth lithography– AZ-4000 (high viscosity, many

layers)

– SU-8 (IBM)

Surface Micromachining

Capp Spindt