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ELEC9703: MST&D

MICROMACHINING •Surface Micromachining

---- W4 •Bulk Silicon Micromachining

--- W3 [Please refer to UNSW

Moodle to download the relevant

references on silicon surface micromachining] ELEC9703: MST&D

from multi-layers of deposited/grown thin film materials that can be

patterned. basic requirements:

Substrate sacrificial layer

that can be removed by etching structural layer from which the micromechanical structure is to be

patterned what is the processing sequence? Issues:

choice of material for structural/sacrificial layers silicon based or non-silicon based materials mechanical properties of the thin films influence of thin film processing on the mechanical properties characterisation of mechanical properties Applications?

SURFACE

MICROMACHINING ELEC9703: MST&D

W3&4 © 2025 A/Prof. A Michael 3 Basic processing sequences substrate substrate substrate Sacrificial layerAnchor cut Structural layer patterned Sacrificial layer removedReleased Cantilever Released bridge •Deposit isolation layer on substrate •Deposit sacrificial layer •Pattern anchor cut •Deposit structural layer •Pattern structural layer •Etch sacrificial layer •Cantilever & bridge released • Rinsing and drying procedures Isolation layer SURFACE

MICROMACHINING ELEC9703: MST&D

versus

Surface micromachining Smaller structures Better dimensional control SURFACE

MICROMACHINING [4] SCS stress free • Isotropic etching of sacrificial layer •But anisotropic etching of the structural layer ELEC9703: MST&D

least have an etch rate for the structural layer several orders lower that

the sacrificial layer) Most common choice for silicon surface micromachining: Structural material:

polysilicon Sacrificial layer:

PSG/ SiO2 SURFACE

MICROMACHINING [4] ELEC9703: MST&D

(dry etching techniques also used) High selectivity Model for sacrificial layer etching process Example - HF removal of PSG/SiO2 (1) Mass transfer of reactant by diffusion from the

bulk to the external etch opening (2)

Diffusion of the reactant from the etch

opening through the etch channels to vicinity of

the internal catalytic surface (3) Adsorption of the reactant onto the catalyst (4) Reaction on the surface of the catalyst (5) Desorption of the products from the surface SURFACE

MICROMACHINING [6] ELEC9703: MST&D

MICROMACHINING Removal of Sacrificial Layer (cont.) (6) Diffusion of the products from the interior of the etch channels (7) Subsequent mass transfer of the products from the etch opening to

the bulk fluid Overall reaction: SiO2 + 6HF

→ H2SiF6 +

2H2O •Two elementary steps involved: • Protons break-up the siloxane bonds (Si-O-Si) to from silanol species (Si-OH) at the surface. • Attachment of F ions on the Si in the silanol. Leading to SiF4 formation. • Dissolve in aqueous solution as H2SiF6 ELEC9703: MST&D

MICROMACHINING Removal of Sacrificial Layer (cont.) polySi oxideC(x,t)Co bulk conc. x (t) position of etch front 0 J dt td HF  −= )(   2 2 6 1 SiO SiOm = 0 2 2 21 = +=−= C CkCkCDJ HF  +  =      )( )( 2 21 CkCk t •molecular weight •density •Fick’s 1st Law + empirical rate law •Fick’s 2nd Law, neglecting convective component and instantaneous rate change of concentration. •Solve for

(t)

and t() [WP Eaton, et.al, Transducer ’97, p249] ELEC9703: MST&D

solutions of different concentrations and for structures of varying

dimensions ( based on a combined first and second order reaction model

etc.) Diffusion limitations observed at about 200µm etch lengths. Etch rate of PSG is higher and increases with phosphorus content. SURFACE

MICROMACHINING [6] Removal of Sacrificial Layer (cont.) HF conc. ELEC9703: MST&D

removal of the sacrificial layer rendering the surface micromachined structure unusable Attempts to physically detach the structure from the substrate will be destructive. Mechanisms of stiction: • forces that pull the structure down to the surface is probably due to surface

tension between the liquid in which the wafer is rinsed. liquid substrate Released structure Opposing forces Surface Tension Forces • as liquid evaporates, structure and substrate are bounded by meniscus - cause of the

attractive force between structure and substrate • structure collapses and comes in

permanent contact with substrate : held together by either van der Waals forces,

hydrogen bridging etc. • Hydrophobic surfaces

-- van der Waals forces responsible for stiction • Hydrophilic surfaces -- hydrogen bridging responsible for stiction SURFACE

MICROMACHINING •Capillary action in wet release •Utotal=Ubending+Ustretching+Usurfacetension Van der Waals forces:

attractive and repulsive electrostatic dipole

interaction between molecules.

Hydrogen bridging: attraction

between a H atom of one molecule and a pair of unshared electrons

of another molecule. H H

O: H

O: H Two situations:

in-use

or during release During release ELEC9703: MST&D

W3&4 ©

11 How to minimise stiction after wet release? 1. Critical Point Drying ◼ under suitable conditions, the liquid and

vapour phases cease to exist as distinct states

in the supercritical region ◼ CO2, supercritical region exist above 31.1°C

and at 1073 psi  Structure is on LTO oxide etched in HF  DI (deionised) water rinse without

drying  Water exchanged by methanol, then

transfer to pressure vessel and methanol

exchanged for CO2 at 25ºC and 1200psi.  Heat up to 35°C and CO2 vented

- structures released  L → SF

→ G SOLID GAS Supercritical region Temperature P re ss u re SURFACE

MICROMACHINING Which methods are useful for in-use stiction free conditions? ❑ Freeze drying

- L → S → G ❑ Direct

2025 A/Prof A Michael evaporation L

→ G

serious stiction problem

ELEC9703: MST&D

MICROMACHINING • Hydrophilic:

A surface that invites wetting by water

Get stiction

Occurs when the contact angle θwater < 90o •Hydrophobic:

A surface that repels wetting by water

Avoids stiction

Occurs when the contact angle θ water > 90o Prof Clark Nguyen

http://www- inst.eecs.berkeley.edu/~ee245/fa11/modules/LecM5.SurfaceMicromachining. ee245.f11-1.pdf ELEC9703: MST&D

MICROMACHINING How to minimise stiction after wet release? (cont.) 2. Other phases-change release methods:

‘freeze-drying’

- eg. ▪ replace water by tertiary butyl alcohol and frozen ▪ sublimated at low vacuum ▪ others use multi-solvent drying process - methanol, ethyl ether and Flourinet

(low surface

tension

perflourinated hydrocarbon liquid) ▪ Rinse-freeze-sublimation procedure: (i) Rinse in DI water after HF(hydroflouric acid) etch to remove

etchant (maintain wet) (ii) Add IPA(iso-propyl alcohol) to keep maintain wafer surface

hydrophobic (iiii)Place wafer in beaker of IPA (iv) Final rinse in cyclohexane (v) Place on Peltier element already cooled to -10°C , passing N2 helps in the sublimation process as cyclohexane vapor is removed. ELEC9703: MST&D

stand-off bumps or increase surface

roughness 4. Geometry modification

: ‘anti-stiction structure

SURFACE

MICROMACHINING Sacrifical oxide •cantilever structure •bumps substrate short long cantilever modifier long cantilever ‘anti-stiction structure ELEC9703: MST&D

for release and in-use stiction SURFACE

MICROMACHINING How to minimise stiction after wet release? (Cont.) • good for stiction free release

& in –use stiction free •Chemical modification of surface

- reduce adhesive energy of in-use stiction by 4 orders of magnitude compare to SiO2 coated surfaces •Use self-assembled monolayer (SAM) from the precursor molecule of DDS, OTS (Octadecyltrichlorosilane), FDTS (Perflourodecyltrichlorosilane) •Good temperature independence •Deposits onto hydrophilic polysilicon DS: dicholorosilane

TS:

trichlorosilane DDS: R2SiCl2 -- dialkyl-dichlorosilane MTS:

RSiCl3 - monoalkyl-dichlorosilane DDMS: (CH3)2SiCl2 OTS: C18H37SiCl3 - octadecyltrichlorosilane MTS FDTS:

C10H4F17SiCl3 - 1H,1H,2H,-2H-perfluorodecyltrichlorosilane ELEC9703: MST&D

MICROMACHINING How to minimise stiction after wet release? (Cont.) 5. Surface modifiers:

for release and in-use stiction (cont.) Process flow for polySi release sacrificial etch

in 49% HF D I water rinse surface

oxidation H2O2 dip Dip1: 2m Dip2: 8min IPA rinse Dip: 1 min Iso-octane Dip: 1min DDS coating Dip:

15 min. Iso-octane – Remove pre-cursor Dip: 2 min. Dry 1 min. [ref: BK Kim, et.al.,Proc.

MEMS’99, p189]] ELEC9703: MST&D

surfaces so that they induce a water

contact angle > 90° • Self-Assembled Monolayers (SAM’s): • Monolayers of “stringy” molecules

covalently bonded to the surface that then raise

the contact angle •

Beneficial characteristics:

Conformal, ultrathin

Low surface energy

Covalent bonding makes them wear

resistant

Thermally stable (to a point) ELEC9703: MST&D

(annealing) ▪ Film stress in relation to deposition conditions and post-deposition

treatment ▪ Test structures &

characterisation

techniques (mechanical) Elwenp153 risticp106 Examples of film stress: SURFACE

MICROMACHINING [6] [4] ELEC9703: MST&D

Caused, e.g., by differences in film vs. substrate thermal expansion coefficients

If suspended above a substrate and anchored to it at two points, the film will be “stretched” by the substrate. •Under compressive stress, a film wants to expand with respect to its substrate.

If suspended above a substrate and anchored to it at two points, the film will buckle over the substrate Tensile and compressive stress ELEC9703: MST&D

(Low Pressure CVD)

- most commonly used - pyrolysis of SiH4 Typical conditions of deposition: • 530°C - 850°C

( 950°C -1000° for ‘epi-poly’) • Total pressure

10-3 –10-2 Torr • Process parameters: temperature, SiH4 partial pressure • Amorphous, pseudo amorphous (partially polycrystalline),

polycrystalline • Transition temp.:

575°C -600°C Below 575°C

- amorphous Above 600°C

- polycrystalline • Temp. spread due to incubation time for nucleation / crystallisation rate versus total deposition

time SURFACE

MICROMACHINING Deposition Methods and conditions [4] ELEC9703: MST&D

micromachined structures:

affects film

morphology, has strong bearing on ultimate

average residual stress and stress gradient

through thickness of the polySi film. eg.

Constrained structures like bridges and

diaphragms

- tensile stress films are desirable

to avoid buckling but excessive tensile stress can fracture the beam. As-deposited polySi films do not usually give the

desired stress requirements – need to conduct post-deposition thermal treatment on as

deposited amorphous films/polycrystalline film Grain size a strong function of deposition

condition (temp) and annealing temp.

- large grain size obtained from annealing of films

deposited at low temp. and high deposition rates. SURFACE

MICROMACHINING Polysilicon as a structural layer/member Deposition Methods and conditions (cont.) [4] Deposit above 600°C, grains have columnar

structure. ELEC9703: MST&D

W3&4 © 2025 A/Prof. A Michael 22 risticp107 • As deposited film stress: ➢ average residual stress peaks at around 630°C and rapidly drops off at higher deposition temperature ➢ Stress gradient over the film thickness results in a bending moment -- deflection of cantilever beam - type of structures ➢ Stress gradient can also be reduced at higher deposition temperatures. ➢Average residual stress and stress gradient appear to be a function of the

dominant orientation in fully polycrystalline films, with the <110> orientation the highest in magnitude and randomly oriented films, the lowest. ➢ At 700°C

(0.1Torr –1 Torr)

film texture is mainly <100> and stress is

considerably reduced.

SURFACE

MICROMACHINING Polysilicon as a structural layer/member [4] ELEC9703: MST&D

A/Prof. A Michael 23 • Post-deposition annealing ➢Annealing of as-deposited polycrystalline films below 1100°C

show little change in microstructure but above 1100°C re- crystallisation and grain growth occurs. ➢Annealing (650°C-950C) of as-deposited

amorphous/psuedo- amorphous (initially

compressive) to become tensile

-- could

due to contraction of volume due to crystallisation of the

amorphous Si layer

on the top surface. ➢Annealing at high temp. induces re-crystallisation which allows

intrinsic stresses in the Si film

to relax. SURFACE

MICROMACHINING Polysilicon as a structural layer/member ELEC9703: MST&D

MICROMACHINING Polysilicon as a structural layer/member • Post-deposition annealing • Annealing above 1000°C - almost complete relaxation of internal stress. [4] [4] • comparing doped and undoped film annealing behaviour • curvature of doped and undoped polySi cantilever as a function of annealing temp. • residual stress gradient tends to be significantly more sensitive to phos. predeposition

than ave. residual stress. merge Indicates increase in compressive stress at the polySi surface. Deposited at 580°C • compressive as deposited •Low temp. anneal will reduce strain over a long period. • mod. Temp anneal will cause film to become tensile [6] • high temp. reduces stress quickly to zero. ELEC9703: MST&D

- unannealed :

1.72 +/- 0.09% - annealed at 1000°C (1Hr) polySi:

0.93 +/- 0.04% ➢Young’s modulus:

nearly similar to SCS and not affect by annealing ➢ Fracture stress:

unannealed:

3.2 x 109 N/m2 annealed:

1.8 x 109 N/m2 SCS

7 x 109 N/m2 ➢ Conclusion:

unannealed polySi is stronger, BUT

not as strong as SCS SURFACE

MICROMACHINING ELEC9703: MST&D

W3&4 © 2025 A/Prof. A Michael 26 Test structures and characterisation •Surface micromachined structures tend to have large residual stress fields •Very sensitive to deposition conditions and post-deposition treatment •Residual stress affect load response behaviour,

frequency response etc. •Need test structures: In-situ characterisation • Distribute across wafer • Fabricate in parallel with devices. • Process monitor External load characterisation Two kinds of stress deform released micromachined structures: (1) Ave. compressive/tensile axial stress - important for bridges/diaphragms (2) Vertical stress gradients (across the film thickness) Specially designed test structures to monitor the forms of stress present SURFACE

MICROMACHINING ELEC9703: MST&D

Cantilever length changes: l ll 0 0 − = • measurement technique is problematic • change in length extremely difficult to measure over a long length –not practical 2.

Bridges: • fixed at both ends, under sufficient compressive axial stress, the structure will buckle • Euler buckling strain for a thin beam under an axial load

(Critical) KL t cr 2 22  = • where

t is film thickness, L is beam length and K is a constant (values ranging from 3-12, depending on the shape and type of beam support) • need to fabricate a array of bridges of varying length and observe the point at which the critical strain is exceeded.

Observe using optical intereference microscope. SURFACE

MICROMACHINING [Guckel: J Applied Phys., vol.57, No.5,1985, p1671] [4] [4] ELEC9703: MST&D

Ring structure: • for characterising tensile stress • relaxation of of the internal stress in the film results in application of tensile stress at the two opposing anchor points • ring contracts

radially at the point where the ring joins the internal crossbar, applying a compressive stress to the crossbar that

is related to the average residual strain it film critical =

G film • critical value of residual strain need to buckle crossbar:               = GR t cr cr film 1 12 2 22  • G is specific to geometry,

Rcr is the critical radius of ring • Need array of rings to determine upper and lower bound of residual tensile strain. G: Geometry dependant

ratio SURFACE

MICROMACHINING [6] [4] ELEC9703: MST&D

W3&4 © 2025 A/Prof. A Michael 29 Test structures and characterisation (cont.) 4. Wafer curvature: • simple, requires no patterning or etching • Measure radius of curvature of wafer covered on one side by thin film • Use profilometer •Stoney’s formula: ( ) Rt Et f s 61 2 0  − = Young’s Mod. Substrate thickness Film thickness Radius of curvature of substrate Poisson’s ratio SURFACE

MICROMACHINING • Poisson’s ratio,

ν:

defined as the ratio of transverse strain to

axial strain under condition of

uniform and uniaxial

longitudinal stress in the

elastic

region. F F e’ e L d  =

2e L ’ = d 2e’ =  ’>0 <0 >0 original final ELEC9703: MST&D

Cantilever curvature: • residual strain gradient introduces eccentricity to axial

loading – due to variation of stress along the direction of film

growth. • in plane stress varies across the thickness of the film, causing

a effective bending moment ( )ywdyyM t t x= − 2/ 2/  • Bending moment causes film to curl when released eg.

Released cantilever structure • Assuming

residual stress gradient does not change

along

the length of the cantilever, the vertical deflection

(x) at any point along the length

Mx EI K x x 2 )1()( 2 − += risticp132K = Constant determined by boundary condition E/(1-v2) = biaxial modulus of the film (compensating Young’s modulus

for stiffening of the beam due to stretching I = moment of inertia of cantilever about the z-axis

M = internal bonding moment Slope gives M(1-v2)/EI - process sensitive SURFACE

MICROMACHINING -t/2 t/2 x {see ref. [10]} dAz 2 [4] [4] y ELEC9703: MST&D

MICROMACHINING Test structures and characterisation (cont.) 6.

Archemedian spiral structure:

residual stress gradient measurement • spiral structure tends to expand or contract when released • end point rotation, endpoint height and lateral

contraction - related to residual strain gradient (tedious to measure) • only single data point, but occupies a

large area for good sensitivity Anchor

outside Anchor

inside [4] •+ve stress gradient (increasingly tensile towards film surface) – open bowl shape for center anchored spiral •-ve stress gradient (increasingly

compressive towards film surface – dome shape

for edge anchored spiral [6] ELEC9703: MST&D

W3&4 © 2025 A/Prof. A Michael 33 Test structures and characterisation (cont.) [10] 7. Response characteristics to external loading on structure: risticp134 risticp135 (i) Deflection under load •Mechanically deflect beam while

continuously monitoring the applied load Young’s modulus Yield strength Poisson’s ratio (x) =

4 P x3(1 –2) E w t3 Film thickness Film width SURFACE

MICROMACHINING in the elastic region •load controlled sub- micrometer [4] [4] ELEC9703: MST&D

Resonant frequency measurements risticp136 ristic137 risticp138         −= Q ff rcr 4 1 1 2 2/1               = LA IE f r  42 52.32 Dynamic properties used to evaluate mechanical

properties Av

voltage → freq. sweep Amplitude

vibration

measured

optically – ampl. vs

freq Resonant freq. vibrational amplitude goes through max. at fcr critical freq. - underdamped cantilever reflects the energy loss due to damping

→ Q extracted from the width of resonant

freq. •Function of geometry and mechanical property – Plot ƒr vs 1/L 2 [Ref: K.Petersen;

IEEE ED – 25 , 1978, p1241; JAP, vol.50,1979, P6761] SURFACE

MICROMACHINING [4] [4] fr = mechanical resonant frequency ELEC9703: MST&D

W3&4 © 2025 A/Prof. A Michael 35 Test structures and characterisation (cont.) risticp139 risticp140       +=− a t Bfd a t APP 4)(0201  (iii) Membrane deflection under pressure risticp139 measure height of diaphragm

bulge/ deflection of membrane Applied pressure Differential

pressure Residual

stress 2a = width of diaphragm Function of Poisson’s ratio A, B, constants

determined by

geometry Centre displacement E d3 1 – v2 SURFACE

MICROMACHINING thickness Biaxial modulus [4][4] • diaphragm is bowed out due to differential pressure • optical measurement of diaphragm centre displacement d ELEC9703: MST&D

A Michael 36 Applications of surface micromachining:

examples Electrostatic comb drive SURFACE

MICROMACHINING [4] ELEC9703: MST&D

MICROMACHINING [6] [6] [6] ELEC9703: MST&D