EE143 F05 Lecture 15 EtchingProfessor N Cheung, U.C. Berkeley 6 EE143 F05 Lecture 15 Etching of...

1Professor N Cheung, U.C. Berkeley

Lecture 15EE143 F05

Etching

• Etching Terminology

• Etching Considerations for ICs

• Wet Etching

• Reactive Ion Etching (plasma etching)


Lecture 15EE143 F05

Etch Process - Figures of Merit

• Etch rate

• Etch rate uniformity

• Selectivity

• Anisotropy


Lecture 15EE143 F05

(1) Bias

substratedf

hf film

dm etching mask

df

substrate

dm

Bias B d dB can be or

f m≡ −

> <0 0.


Lecture 15EE143 F05

Complete Isotropic Etching

B hf= ×2

Vertical Etching = Lateral Etching Rate

Lateral Etching rate = 0

B = 0

Complete Anisotropic Etching


Lecture 15EE143 F05

10

21

≤≤

−≡

f

ff

A

hB

A

02 ==∴ BfhB

anisotropicisotropic

(2) Degree of Anisotropy


Lecture 15EE143 F05

Etching of Steps with a Slope

substrate

startfinal

film

largeθmakexminimizeTorateetchlateralv

rateetchverticalvttimeetchingLet

l

v

⇒∴==

= ( )

x v tx v t

x x xv v t

v

l

v l

1

2

1 2

= ⋅= ⋅

= +

= + ⋅

cot

cot

θ

θ

θ

v tv ⋅

x1 x2

v tv⋅θ

cot θ = 1/ tan θ = b / a


Lecture 15EE143 F05

o

o

0

90

=

=

θ

θvvt

vlt

start

final

film

sub

sub

film

Example


Lecture 15EE143 F05

Worst-Case Design Considerations for Etching

step

Substrate

step heightvariation

variationof filmthicknessacross wafer

etching maskcan be eroded

duringfilm etchingMask

film

target thickness value

(a) Film thickness variation:

( ) ( )δ+⋅= 1max ff hhvariation

factor


Lecture 15EE143 F05

(b) Film etching rate variation

(c) Overetching around step

( ) ( )

( )

( )

( )( )

v v

Worst case etching time required to etch the film

filmhv

hv

f f f

f

f

f

f f

min

max

min

= −

∴ −

= = ⋅+

−

1

11

φ

δφ

step

h2

h2h1

Overetch time (fraction)

=+h hh1 2

2

variation


Lecture 15EE143 F05

( )( ) ( )∆+⋅

−+

=∴ 111

ff

fT v

ht

φδ

(d) With Mask ErosionLet be the verticaland lateral etching rates of the mask.

Let be the vertical etching rate of the film.

(ignoring lateral film rate forsimplicity)

v vm m⊥ , / /

v f

total

∆ =

hh1

2

before

film

W/2after

film

θhf


Lecture 15EE143 F05

( )

( )( )( )

W v v t

vv

h vv

m m T

m

ff

f

m

m

21 11

= + ⋅

=

⋅ ⋅

+ +−

+

⊥

⊥

⊥

cot

cot

/ /

/ /

θ

δφ

θ∆

Goal : Minimize W

Small Wv νh small

o

f m

f

→>> ⊥

θ 90


Lecture 15EE143 F05

Etching Selectivity Sfm between film material and mask material

( )onlycomponentsverticalVV

Sm

ffm

⊥

⊥≡

Wet EtchingSfm is controlled by: chemicals, concentration, temp.

RIE

Sfm is controlled by:plasma parameters, plasma chemistry,gas pressure, flow rate & temperature.


Lecture 15EE143 F05

SiO2

Si

mask

SSiO2, Si

HF solution

Selectivity is very large ( ~ infinity)

SiO2/Si etched by HFsolution

SiO2/Si etched by RIE (e.g. CF4)

SSiO2, Si Selectivity is finite ( ~ 10 )

Examples


Lecture 15EE143 F05

For a given allowable W/2 , what is the minimumselectivity required?

( ) ( )( )( )

( )

+

−∆++

=⊥m

m

f

ffm v

vWh

S //min cot

111

2θ

φδ

[Note] If varies from run-to-run,vm⊥

( ) ( )

( )( )( )

+

−+∆++

=∴

+=

⊥

=

⊥⊥

m

m

factoruniformityfmU

f

mffm

mmm

vv

Wh

S

vv

//(min)

max

cot1

111

2

1

θφ

φδ

φ


Lecture 15EE143 F05

Wet Etching

Reactant transport to surface

Selective and controlled reaction of etchantwith the film to be etched

Transport of by-products away from surface

1

2

3

1

2

3


Lecture 15EE143 F05

Wet Etching (cont.)

• Wet etch processes are generally isotropic

• Wet etch processes can be highly selective

• Acids are commonly used for etching:

HNO3 <=> H+ + NO3-

HF <=> H+ + F-

H+ is a strong oxidizing agent

=> high reactivity of acids


Lecture 15EE143 F05

Si wet etching mechanisms


Lecture 15EE143 F05

Wet Etch Processes

(1) Silicon Dioxide

To etch SiO2 film on Si, use

HF + H2O

SiO2 + 6HF → H2 + SiF6 + 2H2O

Note: HF is usually buffered with NH4F to maintain [H+] at a constant level (for constant etch rate)

NH4F → NH3 + HF

Etch rate (A/min)

T (oC)

6:1 BOE

18 26

6501200


Lecture 15EE143 F05

Wet Etch Processes (cont.)

(2) Silicon Nitride

To etch Si3N4 film on SiO2, use

H3PO4(phosphoric acid)

(180oC: ~100 A/min etch rate)

Typical selectivities:– 10:1 for nitride over oxide– 30:1 for nitride over Si


Lecture 15EE143 F05

Wet Etch Processes (cont.)(3) Aluminum

To etch Al film on Si or SiO2, use

H3PO4 + CH3COOH + HNO3 + H2O(phosphoric acid) (acetic acid) (nitric acid)

(~30oC)

6H+ + 2Al → 3H2 + 2Al3+

(Al3+ is water-soluble)


Lecture 15EE143 F05

Wet Etch Processes (cont.)

(4) Silicon

(i) Isotropic etching

Use HF + HNO3 + H2O

3Si + 4HNO3 → 3SiO2 + 4NO + 2H2O

3SiO2 + 18HF → 3H2SiF6 + 6H2O

(ii) Anisotropic etching (e.g. KOH, EDP)


Lecture 15EE143 F05

Effect of Slow {111} Etching with KOH or EDP

Mask opening aligned in <110> direction => {111} sidewalls


Lecture 15EE143 F05

[110]-Oriented Silicon

{111} planes oriented perpendicular to the (110) surface

=> possible to etch pits with vertical sidewalls!

Bottom of pits are• flat ({110} plane) if KOH is used

{100} etches slower than {110}

• V-shaped ({100} planes) if EDP is used{110} etches slower than {100}


Lecture 15EE143 F05

(3) Field-Emission Tips


Lecture 15EE143 F05

Drawbacks of Wet Etching

• Lack of anisotropy

• Poor process control

• Excessive particulate contamination

=> Wet etching used for noncritical feature sizes

EE143 F05 Lecture 15 EtchingProfessor N Cheung, U.C. Berkeley 6 EE143 F05 Lecture 15 Etching of...

Documents

Transcript of EE143 F05 Lecture 15 EtchingProfessor N Cheung, U.C. Berkeley 6 EE143 F05 Lecture 15 Etching of...