Lecture 10 Wet Etching

Wet

Etc

hing

and

Bul

k M

icro

mac

hini

ng

Fu

ndam

enta

ls of

Mic

rom

achi

ning

D

r. B

ruce

K. G

ale

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�–

Sam

e et

ch r

ate

in a

ll di

rect

ions

–L

ater

al e

tch

rate

is a

bout

the

sam

e as

ver

tical

etc

h ra

te–

Etc

h ra

te d

oes

not d

epen

d up

on th

e or

ient

atio

n of

the

mas

k ed

ge

��

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�–

Etc

h ra

te d

epen

ds u

pon

orie

ntat

ion

to c

ryst

allin

e pl

anes

–L

ater

alet

ch r

ate

can

be m

uch

larg

er o

r sm

alle

r th

an v

ertic

al e

tch

rate

, dep

endi

ng u

pon

orie

ntat

ion

of m

ask

edge

to c

ryst

allin

e ax

es–

Ori

enta

tion

of m

ask

edge

and

the

deta

ils o

f th

e m

ask

patte

rnde

term

ine

the

fina

l etc

hed

shap

e•

Can

be

very

use

ful f

or m

akin

g co

mpl

ex s

hape

s•

Can

be

very

sur

pris

ing

if n

ot c

aref

ully

thou

ght o

ut•

Onl

y ce

rtai

n “s

tand

ard”

sha

pes

are

rout

inel

y us

ed

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(

KO

H E

tchi

ng

Etc

h R

ate

(1

10) >

(100

) > (1

11)

(100

) > (1

10) >

(111

) w/ I

PA

V

arie

s with

Tem

pera

ture

and

Con

cent

ratio

n (s

ee

appe

ndix

C in

Mad

ou)

54.7

° °°°

<100

> W

afer

Cro

ss-s

ectio

n To

p V

iew

[][

]kTE a

eKOH

OH

kR

−=

414

20

��

��

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��

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*�� 0��1

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�%

(1��0�� "))�

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+%$�� 3��

4��

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*

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�� 6��

�%

(

�7��6��

�'5��6��&)8

�6��

�� 6��

R. B

. D

arl

ing

/ E

E-5

27

Hy

dro

xid

e E

tch

ing

of

Sil

ico

n

•S

ev

era

l h

yd

rox

ides a

re u

sefu

l:

–K

OH

, N

aO

H,

CeO

H,

Rb

OH

, N

H4O

H, T

MA

H:

(C

H3) 4

NO

H

•O

xid

ati

on

of

sil

ico

n b

y h

yd

rox

yls

to f

orm

a s

ilic

ate

:

–S

i +

2O

H−

+ 4

h+ →

Si(

OH

) 2+

+

•R

ed

ucti

on

of

wate

r:

–4

H2O

→ 4

OH

− +

2H

2 +

4h

+

•S

ilic

ate

fu

rth

er

reacts

wit

h h

yd

rox

yls

to

fo

rm a

wate

r-

solu

ble

co

mp

lex

:

–S

i(O

H) 2

++ +

4O

H−

→ S

iO2(O

H) 2

2−

+ 2

H2O

•O

vera

ll red

ox r

eacti

on

is:

–S

i +

2O

H−

+ 4

H2O

→ S

i(O

H) 2

++ +

2H

2 +

4O

H−

R. B

. D

arl

ing

/ E

E-5

27

KO

H E

tch

ing

of

Sil

ico

n -

1

•T

yp

ical

an

d m

ost

used

of t

he h

yd

rox

ide e

tch

es.

•A

ty

pic

al

recip

e i

s:

–2

50

g K

OH

–2

00

g n

orm

al p

rop

an

ol

–8

00

g H

2O

–U

se a

t 8

0°C

wit

h a

git

ati

on

•E

tch r

ate

s:

–~

1 µ

m/m

in f

or

(100) S

i p

lan

es;

sto

ps a

t p

++ l

ay

ers

–~

14

An

gstr

om

s/h

r fo

r S

i 3N

4

–~

20

An

gstr

om

s/m

in f

or

SiO

2

•A

nis

otr

opy:

(111):

(110):

(100)

~ 1

:600:4

00

R. B

. D

arl

ing

/ E

E-5

27

KO

H E

tch

ing

of

Sil

ico

n -

2

•S

imp

le h

ard

ware

:

–H

ot

pla

te &

sti

rrer.

–K

eep

co

vere

d o

r u

se r

efl

ux

co

nd

en

ser

to k

eep

pro

pan

ol

fro

m

ev

ap

ora

tin

g.

•P

resen

ce o

f alk

ali

meta

l (p

ota

ssiu

m,

K)

mak

es t

his

com

ple

tely

in

co

mp

ati

ble

wit

h M

OS

or

CM

OS

pro

cessin

g!

•C

om

para

tiv

ely

safe

an

d n

on

-to

xic

.

R. B

. D

arl

ing

/ E

E-5

27

ED

P E

tchin

g o

f S

ilic

on

- 1

•E

thy

len

e D

iam

ine P

yro

cate

ch

ol

•A

lso

kn

ow

n a

s E

thy

len

e d

iam

ine - P

yro

cate

ch

ol -

Wate

r

(EP

W)

•E

DP

etc

hin

g i

s r

ead

ily

mask

ed

by

SiO

2, S

i 3N

4, A

u,

Cr,

Ag,

Cu

, an

d T

a. B

ut

ED

P c

an

etc

h A

l!

•A

nis

otr

opy

: (

11

1):

(10

0)

~ 1

:35

•E

DP

is v

ery

co

rro

siv

e, v

ery

carc

ino

gen

ic, an

d n

ev

er

all

ow

ed

near

main

str

eam

ele

ctr

on

ic m

icro

fab

ricati

on.

•T

yp

ical

etc

h r

ate

s f

or

(10

0)

sil

ico

n:

70°C

14 µ

m/h

r

80°C

20 µ

m/h

r

90°C

30 µ

m/h

r =

0.5

µm

/min

97°C

36 µ

m/h

r

R. B

. D

arl

ing

/ E

E-5

27

ED

P E

tchin

g o

f S

ilic

on

- 2

•T

yp

ical

form

ula

tio

n:

–1

L e

thy

len

e d

iam

ine,

NH

2-C

H2-C

H2-N

H2

–1

60

g p

yro

cate

ch

ol, C

6H

4(O

H) 2

–6

g p

yra

zin

e, C

4H

4N

2

–1

33

mL

H2O

•Io

niz

ati

on

of

eth

yle

ne d

iam

ine:

–N

H2(C

H2) 2

NH

2 +

H2O

→ N

H2(C

H2) 2

NH

3+ +

OH

−

•O

xid

ati

on

of

Si

an

d r

ed

ucti

on

of

wate

r:

–S

i +

2O

H−

+ 4

H2O

→ S

i(O

H) 6

2−

+ 2

H2

•C

hela

tio

n o

f h

yd

rou

s s

ilic

a:

–S

i(O

H) 6

2−

+ 3

C6H

4(O

H) 2

→ S

i(C

6H

4O

2) 3

2−

+ 6

H2O

OH

OH

cate

ch

ol

N N

pyra

zin

e

H2N

H2

CC H

2

NH

2

eth

yle

ne d

iam

ine

R. B

. D

arl

ing

/ E

E-5

27

ED

P E

tchin

g o

f S

ilic

on

- 3

•R

eq

uir

es r

efl

ux

co

nd

en

ser

to k

eep

vo

lati

le i

ng

red

ien

ts f

rom

ev

ap

ora

tin

g.

•C

om

ple

tely

in

co

mp

ati

ble

wit

h M

OS

or

CM

OS

pro

cessin

g!

–It

mu

st

be u

sed

in

a f

um

e c

oll

ecti

ng

ben

ch

by

its

elf

.

–It

wil

l ru

st

an

y m

eta

l in

th

e n

earb

y v

icin

ity

.

–It

leav

es b

row

n s

tain

s o

n s

urf

aces t

hat

are

dif

ficu

lt t

o r

em

ov

e.

•E

DP

has a

faste

r etc

h r

ate

on

co

nv

ex

co

rners

th

an

oth

er

an

iso

tro

pic

etc

hes:

–It

is g

en

era

lly

pre

ferr

ed

fo

r u

nd

erc

utt

ing

can

tile

vers

.

–It

ten

ds t

o l

eav

e a

sm

oo

ther

fin

ish

th

an

oth

er

etc

hes, sin

ce f

aste

r

etc

hin

g o

f co

nv

ex

co

rners

pro

du

ces a

po

lish

ing

acti

on

.

R. B

. D

arl

ing

/ E

E-5

27

ED

P E

tchin

g o

f S

ilic

on

- 4

•E

DP

etc

hin

g c

an

resu

lt i

n d

ep

osit

s o

f p

oly

meri

zed

Si(

OH

) 4

on

th

e e

tch

ed

su

rfaces a

nd

dep

osit

s o

f A

l(O

H) 3

on

Al

pad

s.

•M

oser’s post

ED

P p

roto

col

to e

lim

inate

th

is:

–2

0 s

ec. D

I w

ate

r ri

nse

–1

20

sec.

dip

in

5%

asco

rbic

acid

(v

itam

in C

) an

d H

2O

–1

20

sec. ri

nse i

n D

I w

ate

r

–6

0 s

ec.

dip

in

hex

an

e,

C6H

14

R. B

. D

arl

ing

/ E

E-5

27

Am

ine G

all

ate

Etc

hin

g o

f S

ilic

on

•M

uch

safe

r th

an

ED

P

•T

yp

ical

recip

e:

–1

00

g g

all

ic a

cid

–3

05

mL

eth

an

ola

min

e

–1

40

mL

H2O

–1.3

g p

yra

zin

e

–0

.26

mL

FC

-12

9 s

urf

acta

nt

•A

nis

otr

opy:

(111):

(100):

1:5

0 t

o 1

:100

•E

tch r

ate

: ~

1.7

µm

/min

at

11

8°C

R. B

. D

arl

ing

/ E

E-5

27

TM

AH

Etc

hin

g o

f S

ilic

on

- 1

•T

etr

a M

eth

yl A

mm

on

ium

Hy

dro

xid

e

•M

OS

/CM

OS

co

mp

ati

ble

:

–N

o a

lkali

meta

ls {

Li, N

a, K

, …

}.

–U

sed

in

po

sit

ive p

ho

tore

sis

t d

ev

elo

pers

wh

ich

do

no

t u

se c

ho

lin

e.

–D

oes n

ot

sig

nif

ican

tly

etc

h S

iO2 o

r A

l! (B

on

d w

ire s

afe

!)

•A

nis

otr

opy:

(111):

(100)

~ 1

:10 t

o 1

:35

•T

yp

ical

recip

e:

–2

50

mL

TM

AH

(2

5%

fro

m A

ldri

ch)

–3

75

mL

H2O

–2

2 g

Si

du

st

dis

so

lved

in

to s

olu

tio

n

–U

se a

t 9

0°C

–G

ives a

bo

ut

1 µ

m/m

in e

tch

rate

CH

3

N CH

3

H3C

H3C

OH

tetr

am

eth

yl

am

mo

niu

m h

yd

roxid

e

(TM

AH

)

R. B

. D

arl

ing

/ E

E-5

27

TM

AH

Etc

hin

g o

f S

ilic

on

- 2

•H

yd

rox

ide e

tch

es a

re g

en

era

lly

safe

an

d p

red

icta

ble

, b

ut

they

usu

all

y i

nv

olv

e a

n a

lkali

meta

l w

hic

h m

ak

es t

hem

inco

mp

ati

ble

wit

h M

OS

or

CM

OS

pro

cessin

g.

•A

mm

on

ium

hy

dro

xid

e (

NH

4O

H)

is o

ne h

yd

rox

ide w

hic

h

is f

ree o

f alk

ali

meta

l, b

ut it

is r

eall

y a

mm

on

ia w

hic

h i

s

dis

so

lved

in

to w

ate

r. H

eati

ng

to

90

°C

fo

r etc

hin

g w

ill

rap

idly

ev

ap

ora

te t

he a

mm

on

ia f

rom

so

luti

on

.

•B

all

asti

ng

th

e a

mm

on

ium

hy

dro

xid

e w

ith

a l

ess v

ola

tile

org

an

ic s

olv

es t

he p

rob

lem

:

–T

etr

am

eth

yl am

mo

niu

m h

yd

rox

ide:

(C

H3) 4

NO

H

–T

etr

aeth

yl

am

mo

niu

m h

yd

rox

ide:

(C

2H

5) 4

NO

H

R. B

. D

arl

ing

/ E

E-5

27

Hy

dra

zin

e a

nd

Wate

r E

tch

ing

of

Sil

ico

n

•P

rod

uces a

nis

otr

op

ic e

tch

ing

of

sil

ico

n, als

o.

•T

yp

ical

recip

e:

–1

00

mL

N2H

4

–1

00

mL

H2O

–~

2 µ

m/m

in a

t 1

00°C

•H

yd

razin

e i

s v

ery

dan

gero

us!

–A

very

po

werf

ul

red

ucin

g a

gen

t (u

sed

fo

r ro

ck

et

fuel)

–F

lam

mab

le l

iqu

id

–T

LV

= 1

pp

m b

y s

kin

co

nta

ct

–H

yp

erg

oli

c: N

2H

4 +

2H

2O

2 →

N2 +

4H

2O

(ex

plo

siv

ely

)

–P

yro

ph

ori

c:

N2H

4 +

O2 →

N2 +

2H

2O

(ex

plo

siv

ely

)

–F

lash

po

int

= 5

2°C

= 1

26

°F

in

air

.

R. B

. D

arl

ing

/ E

E-5

27

An

isotr

opic

Etc

h S

top L

ay

ers

- 1

•C

on

tro

llin

g t

he a

bso

lute

dep

th o

f an

etc

h i

s o

ften

dif

ficu

lt,

part

icu

larl

y i

f th

e e

tch

is g

oin

g m

ost

of

the w

ay

th

rou

gh

a

wafe

r.

•E

tch s

top l

ay

ers

can

be u

sed

to

dra

sti

call

y s

low

th

e e

tch

rate

, p

rov

idin

g a

sto

pp

ing

po

int

of

hig

h a

bso

lute

accu

racy

.

•B

oro

n d

opin

g i

s m

ost

com

mo

nly

used

fo

r sil

ico

n e

tch

ing

.

•R

eq

uir

em

en

ts f

or

sp

ecif

ic e

tch

es:

–H

NA

etc

h a

ctu

all

y s

peed

s u

p f

or

heav

ier

do

pin

g

–K

OH

etc

h r

ate

red

uces b

y 2

0×

for

bo

ron

do

pin

g >

10

20 c

m-3

–N

aO

H e

tch

rate

red

uces b

y 1

0×

for

bo

ron

do

pin

g >

3 ×

10

20 c

m-3

–E

DP

etc

h r

ate

red

uces b

y 5

0×

for

bo

ron

do

pin

g >

7 ×

10

19 c

m-3

–T

MA

H e

tch

rate

red

uces b

y 1

0×

for

bo

ron

do

pin

g >

10

20 c

m-3

R. B

. D

arl

ing

/ E

E-5

27

An

isotr

opic

Etc

h S

top L

ay

ers

- 2

2-5

µm th

ick

mem

bran

e

400

- 50

0 µm

thic

k w

afer

heav

ily b

oron

dop

ed e

tch

stop

laye

r

R. B

. D

arl

ing

/ E

E-5

27

Ele

ctr

och

em

ical

Etc

h E

ffects

- 1

Si w

afer

Pt r

efer

ence

ele

ctro

deH

F /

H2O

sol

utio

n

Si +

4h+

+ 2

OH

-

→ S

i(OH

) 22+

VI

R. B

. D

arl

ing

/ E

E-5

27

Ele

ctr

och

em

ical

Etc

h E

ffects

- 2

•H

F n

orm

all

y e

tch

es S

iO2 a

nd t

erm

inate

s o

n S

i.

•B

y b

iasin

g t

he S

i posit

ively

, h

ole

s c

an

be i

nje

cte

d b

y a

n

ex

tern

al

cir

cu

it w

hic

h w

ill

ox

idiz

e t

he Si

an

d f

orm

hy

dro

xid

es w

hic

h t

he H

F c

an

th

en

dis

so

lve.

•T

his

pro

du

ces a

n e

xcell

en

t p

oli

sh

ing

etc

h t

hat

can

be v

ery

well

mask

ed

by

LP

CV

D f

ilm

s o

f S

i 3N4.

•If

th

e e

tch

ing

is p

erf

orm

ed

in

very

co

ncen

trate

d H

F (

48

%

HF

, 98%

EtO

H),

then t

he S

i d

oes n

ot

full

y o

xid

ize w

hen

etc

hed, and p

oro

us s

ilic

on

is f

orm

ed

, w

hic

h a

pp

ears

bro

wn

ish

.

R. B

. D

arl

ing

/ E

E-5

27

Ele

ctr

och

em

ical

Etc

h E

ffects

- 3

0.0

-1.5

-1.0

-0.5

+0.

5+

1.0

-2.0

I, m

A/c

m2

V, V

olts

(100

) S

i in

40%

KO

H a

t 60°

C

pote

ntia

l of P

tre

fere

nce

elec

trod

eP

P:

pass

ivat

ion

pote

ntia

ln-ty

pe S

i

p-ty

pe S

i

OC

P:

open

-circ

uit

pote

ntia

l

R. B

. D

arl

ing

/ E

E-5

27

Ele

ctr

och

em

ical

Etc

h E

ffects

- 4

•In

cre

asin

g t

he w

afe

r b

ias a

bo

ve t

he O

CP

wil

l in

cre

ase t

he

etc

h r

ate

by

su

pp

lyin

g h

ole

s w

hic

h w

ill

ox

idiz

e t

he Si.

•In

cre

asin

g t

he w

afe

r b

ias f

urt

her

wil

l re

ach

th

e passiv

ati

on

pote

nti

al

(PP

) w

here

SiO

2 f

orm

s.

–T

his

passiv

ate

s th

e s

urf

ace a

nd

term

inate

s t

he e

tch

.

–T

he H

F /

H2O

so

luti

on

do

es n

ot

ex

hib

it a

PP

, sin

ce t

he S

iO2 i

s

dis

so

lved

by

th

e H

F.

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Lecture 10 Wet Etching

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