ME 645: MEMS: Design Fabrication Design, Fabrication and - - PDF document

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ME 645: MEMS: Design Fabrication

Lecture 7 and 8: Etching 1 and 2

Design, Fabrication and Characterization

P.S. Gandhi Mechanical Engineering IIT Bombay

PRASANNA S GANDHI gandhi@me.iitb.ac.in

Recap: Last Class

E-beam lithography Direct laser writer X-ray lithography Ion beam lithography Photoresist fundamentals Soft lithography

PRASANNA S GANDHI gandhi@me.iitb.ac.in

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Today’s Class

Clean room fundamentals Si wafer preparation Chemical etching process Anisotropic Etching Silicon micromachining Silicon micromachining

Surface micromachining Bulk micromachining How to produce devices

PRASANNA S GANDHI gandhi@me.iitb.ac.in

Clean Room Fundamentals

Need

Need Class of a clean environment Class X clean room not more than X

particles (of size 0.5μm or larger) per cubic foot of air; ex class 10 100 1000 cubic foot of air; ex class 10, 100, 1000

How this cleanliness is produced and

maintained??

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Clean Room Fundamentals

Air conditioning plant

Air conditioning plant Air handling unit: Closed loop for air

through clean room

HEPA (High Efficiency Particle Arresting)

filters filters

Air recirculation through these filters

multiple times per hour

PRASANNA S GANDHI gandhi@me.iitb.ac.in

Ai conditioning plant

HEPA filter

Air conditioning plant Air handling unit: Closed loop for air

through clean room

HEPA (High Efficiency Particle Arresting)

filters filters

Air recirculation through these filters

multiple times per hour

Clean Room: Suman Mashruwala Advanced Microengineering Lab

Laminar flow benches

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Czochralski process

Si-wafer preparation

Silicon Chunks Molten Silicon Seed crystal Silicon Ingot Pull Pull

Cutting Chemical Mechanical Polishing Cleaning

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Secondary flat Primary flat Primary flat { 110} direction (100) n-type (100) p-type Secondary flat Secondary flat Primary flat Primary flat (111) n-type (111) p-type Secondary flat (45o)

Primary and Secondary wafer flats are used to identify orientation and type.

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Chemical Etching

Isotropic etching

Isotropic etching

– Etchant: HNA mixture. – HNA can dissolve 550μm thick silicon wafer in about 20 min. Without agitation (5) With it ti (20) – HNA mixture removes silicon equally in all directions.

SiO2 etch: 10-30nm/min

With agitation (20)

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Chemical Etching

Isot opic etching

Isotropic etching

Undercut Etch bias

Materials & etchants

Without agitation (5)

Materials & etchants

Refer to http://www.bsac.EECS.Berkeley.EDU/db for all details of etch rates and other data

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Chemical Etching

Choice of etchant: Choice of etchant:

Etch rate Topology of the surface to be etched Etch selectivity of mask material and

• ther materials
• ther materials

Toxicity Ease of handling

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Etch Stop Mechanisms

Time etch stop self e plainato

Time etch stop: self explainatory Dopant B+ (heavy dope) as etch stop Thin films Electrochemical etch stop Anisotropic Etching planes

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Etch Stop Mechanisms

Dopant B+ (heavy Dopant B+ (heavy

dope) as etch stop

Disadv.: creating

electronic piezoresistive sensors not easily possible because of

Dopant dose

possible because of high dose

Etching stops at point when it encounters high dopant dose region

PRASANNA S GANDHI gandhi@me.iitb.ac.in

Etch Stop Mechanisms

Thin film as etch stop Thin film as etch stop Excellent control over

thickness of say diaphragm or so.

Disadvantage: the

material may not be

Thin film deposition

material may not be very appropriate for the application!!!

Etching stops at point when it encounters thin film having relatively low etch rate

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Etch Stop Mechanisms

Electrochemical etch stop

Diffused n-type region

Electrochemical etch stop

Principle: When si is biased

with sufficient anodic potential relative to etchant it tends to oxidize (electrochemical passivation)

Create a reverse biased pn

Diffused n type region

V

+

• Electrolyte

p junction to implement this principle to achieve etch stop

As soon as the n-type region is hit electrochemical reaction starts the electrochemical passivation.

PRASANNA S GANDHI gandhi@me.iitb.ac.in

Chemical Etching

Anisotropic bulk etching

< 100> < 010> < 001> < 111>

Anisotropic bulk etching

– Etchant: KOH, EDP (ethylen diamine pyrocatechol), TMAH (Tetra methyl ammonium hydroxide) – < 111> direction has lower

110 f f < 100> surface wafer

etching rates than < 100>

Can produce grooves,

slanted/vertical walls

< 110> surface wafer

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Chemical Etching

Silicon crystal geometry

< 100> < 010> < 001> < 111>

Silicon crystal geometry Examples of use of the

crystal geometry in etching

Fundes regarding etch

shapes under different conditions*

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Anisotropic Etching

KOH EDP and TMAH

< 100> < 010> < 001> < 111>

KOH, EDP and TMAH

EDP etches oxide 100 times slower than KOH, KOH, TMAH dangerous to eye KOH less dangerous than EDP & TMAH Etch curves (etch rate vs concentration) 5hrs to

etch 300μm thick wafer etch 300μm thick wafer

H2 bubbles during KOH etching of Si EDP ages quickly in contact with oxygen

producing red brown color, vapor is harmful

HF dip is necessary for EDP: native oxide problem

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A [110] [110] 54.7o A’ Top View Cross Section A-A’

A square < 110> -oriented mask feature results in a pyramidal pit.

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A A’ A [ 100] Silicon [110] [100] Masking Layer Cross Section A-A’ A A’ A Convex corners are rapidly undercut [100] A’ 54.7o Cross Section A-A’

Convex corners where { 111} planes meet are not stable. They are rapidly undercut. This permits creation of suspended structures.

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A Boundary of rectangular pit [ 110] [100] Masking Layer A’ Undercut regions Cross Section A-A’ A

Any mask-layer feature, if etched long enough, will result in a rectangular V-groove pit beneath a rectangular that is tangent to the mask features, with edges oriented along < 110> directions.

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Boundary of rectangular pit [ 110] 5o misalignment

The effect of misalignment is to enlarge the etched region. This figure shows the effect of a 5o misalignment for a rectangular feature.

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Re-entrant resist profile Wafer with photoresist Directional evaporation (e-beam) Strip resist and lift off metal

Illustrating the lift-off method for patterning evaporated metals

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Wafer with double-layer undercut masking Directional evaporation evaporation gradually closes opening At closure, a sharp tip is formed

The use of a modified lift-off process to create sharp tips.

After lift-off

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Conclusions

Chemical etching

Isotropic Anisotropic

Bulk and Surface micromachining Etch stop mechanisms Liftoff process

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Thank You

PRASANNA S GANDHI gandhi@me.iitb.ac.in