wet etching and bulk micromachining fundamentals of
TRANSCRIPT
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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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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