nmems-7a [compatibility mode]- etching

7/29/2019 Nmems-7a [Compatibility Mode]- Etching

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INTRODUCTION TO MEMS

EA C415


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ETCHING WET

DRY

WET ETCHING:

substrate in a suitable liquid chemical (etchant) that

attacks the exposed region

DRY ETCHING:

Removal of material by chemically reactive vapors

& ionic species acts as etchant and removes material

from substrate


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ISOTROPIC ETCHING : Rate of material

removal does not depend on the orientation of thesubstrate

ANISOTROPIC ETCHING: Etching single

crystal substrate with certain etchants results in

orientation dependent etching.

ISOTROPIC ETCH

Masking Layer


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Curved Undercut

HEAVY ETCH OF SUBSTRATE/BULK MICROMACHINING

ISOTROPIC ANISOTROPIC COMPLETELY

ANISOTROPIC

direction


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ETCH SELECTIVITY: etch rate of layer to be etched should be fast

compared to etch rate of mask

x

y

xy >>

#EX. HF etches SiO2 100 nm/min. and mask Si3N4 0.04 nm/min.

x

1. CVD SiO2

2. Si3N4

3. Polysilicon

4. Aluminium

5. Copper

6. Gold

Material ETCHANT

1. Buffered HF (5:1 NH4F: Conc. HF)

2. Hot phosphoric acid

3. KOH/Ethyl diamine pyrochatel

4. PAN (phosphoric, acetic, nitric acid)

5. Ferric chloride

6. Ammonium Iodide


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More physical an etch process more directional and less selective

More chemical an etch process lessdirectional and more selective

Strong bases like KOH, EDP, Tetramethyl ammonium hydroxide (TmAH)

exhibit orientation dependent etchcharacteristics

* Suitable mask for strong base etchant are SiO2, Si3N4


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DRY ETCHING Vapor etching

#Ex: XeF2 etch Si very fast incomparison to other materials)

Plasma Assisted etching

Low pressure ionic species of gases (etchants) produced in plasma

removes material i) by direct bombardment and ii) chemical reaction by

converting surface atoms to volatile species

1. SiO2

2. Si3N4

3. Silicon/Polysilicon

4. Aluminum

5. Organic

Material ETCHANT

1. CF4, H2

2. CF4, O2

3. CF4, SF6

4. BCl3

5. O2, CF4, SF6


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Parameters affecting rate of etching and

shape of etch

1. Type of substrate

2. Specific chemistry of etchant

3. Masking layer

4. Temperature (controls rate of reaction, directionality)

5. Solution stirring

6. Pressure (in dry etching)High pressure Higher rate, more isotropic etch

Low pressure Low rate, more directional etch


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At limits of low pressure (high directionality),the process is called REACTIVE ION ETCHING

(RIE)

DEEP REACTIVE ION ETCHING

Used for creation of deep vertical wells on substrate

working principle is based on use of polymeric species

most plasma process are critical race between deposition of

polymeric material and material removal from surface

Plasma chemistries can be controlled and under right temp.

and pressure condition, removal dominates


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ETCH PASSIVATE ETCH

SF6 C4F8 SF6

Etch proceeds in alternating steps of reactive ion

etching in SF6 plasma and polymer deposition from C4F8

plasma

During etch, the polymer is rapidly removed from the

bottom but lingers on the sidewall

DEEP REACTIVE ION ETCHING www-samlab.unine.ch/activities/process.htm


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SEM image of a microscale tensile test specimen in theprocess of fabrication using SF6 and Ar plasma. Single-

crystal SiC has been etched to a depth of 80 m.

www.grc.nasa.gov/


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Time-multiplexed

etch-passivate

(TMEP) process is

widely used in the

DRIE of silicon informing structures

with aspect ratios

reater than 30.


SF6 is used as the etching gas, and octafluorocyclobutane(C4F8) is used to deposit a fluorocarbon polymer film that

protects the sidewalls from lateral etching.

www.grc.nasa.gov/


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LINEAR ETCH MODEL

fluxIonflux;Chemical

constantrate&t;coefficiensticking

==

==

+

==

ic

ifc

iicfc

e

FF

kkSN

FkFkSrrateetch

#Ex Silicon is plasma etched for 5 min. and the process follows the linear etchmodel. The flux of chemical species, Fc, on an unobstructed flat surface is

equal to . The sticking coefficient is 0.01 and kf is 0.2. The

flux of ionic species on flat surface is equal to and ki is 1. The

s/atom/cm105.2 218

s/atom/cm101 216322 .

trench is etched in silicon. How far is the trench etched in vertical and lateraldirection under these conditions

mrr 9.0300depthetchand105

1011105.22.001.022

1618

==

+

=

Solution: In vertical direction (both chemical & ionic fluxes are present)

In Lateral direction (virtually no ionic fluxes)

mrr 3.0300depthetchand105

0105.22.001.0

22

18

==

+=


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Synergistic Model/ Ion Enhanced Etching Model

11

+

=

cci FSF

Nr

1

k

1

i


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ETCH STOPS

NOTE: While it is always possible to use a rate and a specified

time to determine the depth of etched features, it is always

desirable to use a well defined structural feature to stop an etch

Diffused n-type region

p- type s con

Electrode EtchantFinal Structure

Working Principle: reverse bias p-n junction, no current flows

p-silicon etched

on reaching n-type (junction exposed), current flows

n-silicon oxidizes, gets passivated, no more etching


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MICROMACHINING

nmems-7a [compatibility mode]- etching

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