EE143 S06 Lecture 15 Etching - University of California ...ee243/sp10/EE143/lectures/Lec_15.pdf ·...

Professor N Cheung, U.C. Berkeley

Lecture 15EE143 S06

1

Etching

• Etching Terminology

• Etching Considerations for ICs

• Wet Etching

• Reactive Ion Etching (plasma etching)


Lecture 15EE143 S06

2

Etch Process - Figures of Merit

• Etch rate

• Etch rate uniformity

• Selectivity

• Anisotropy


Lecture 15EE143 S06

3

(1) Bias

substratedf

hf film

dm etching mask

df

substrate

dm

Bias B d d

B can be orf m≡ −

> <0 0.


Lecture 15EE143 S06

4

Complete Isotropic Etching

B hf= ×2

Vertical Etching = Lateral Etching Rate

Lateral Etching rate = 0

B = 0

Complete Anisotropic Etching


Lecture 15EE143 S06

5

10

21

≤≤

−≡

f

ff

A

hB

A

02 ==∴ BfhB

anisotropicisotropic

(2) Degree of Anisotropy


Lecture 15EE143 S06

6

(3) Etching Selectivity S

Wet EtchingS is controlled by: chemicals, concentration, temperature RIES is controlled by:plasma parameters, plasma chemistry,gas pressure, flow rate & temperature.

)Bmateralofvelocityetchingvertical(v)Amateralofvelocityetchingvertical(v

SB

AAB =


Lecture 15EE143 S06

7

SiO2

Si

SSiO2, Si Selectivity is very large ( ~ infinity)

SiO2/Si etched by HF solution

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

SSiO2, Si Selectivity is finite ( ~ 10 )

Selectivity Example


Lecture 15EE143 S06

8

Etching of Steps with a Slope

rateetchlateralvetch rateverticalv

ttimeetchingLet

l

v

==

=

( )

x v tx v t

x x x

v v t

v

l

v l

1

2

1 2

= ⋅= ⋅

= +

= + ⋅

cot

cot

θ

θ

substrate

start

finalfilmθ

v tv⋅

x1 x2

v tv⋅θ

cot θ = 1/ tan θ = b / a

* Etching velocity has vertical component vv and lateral component vl


Lecture 15EE143 S06

9

o

o

0

90

=θ

=θvvt

vlt

start

final

sub

film

Slope etching examples

film

sub

Film etched uniformlylaterally. Movement of“step” is “infinity”.


Lecture 15EE143 S06

10

Worst-Case Design Considerations for Etching

step

Substrate

Film thicker here due to surface topography

Variation of filmthickness across wafer due to deposition method

Mask materialcan be erodedduring film etching

EtchingMask

Film

Etching rate of film can vary from run-to-run


Lecture 15EE143 S06

11

Nominal thickness

(a) Film thickness variation across wafer

( ) ( )δ+⋅= 1max ff hh

Thickness variation factor

•The variation factor δ is dictated by the deposition method,deposition equipment, and manufacturing practice.

•Run-to-run variation of thickness data are recorded. Once the deposition process is under control, the maximum/minimumvalues will be used to define δ.


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12

(b) Film etching rate variation

( ) ( )

( )

( )

( )( )ff

f

minf

maxf

ffminf

11

vh

v

h

filmtheetchtorequiredtimeetchingcaseWorst

1vv

φ−δ+

⋅==

−

φ−=variation factor


Lecture 15EE143 S06

13

(c) Over-etch around step

step

h2

h2h1

Fractional over-etch time to completely remove film= h1 / h2

( )( ) ( )∆+⋅

−+

=∴ 111

ff

fT v

ht

φδ

Total worst-case etching time

∆ =

hh

1

2

film

substrate


Lecture 15EE143 S06

14

Example 1: Worst-case Consideration for substrate erosion

Film thickness has variation factor = δ

Film etching rate has variation factor = φf

film

substrateWorst-casesubstrate erosion

hf (1+φf)hf (1-φf)

Thinnest part of film with be completely removed in )1(v)1(h

tff

f1 φ+

δ−=

Thickest part of film with be completely removed in )1(v)1(h

tff

f2 φ−

δ+=

Worst-case substrate erosion = vsubstrate • (t2 - t1)


Lecture 15EE143 S06

15

Example 2: Worst-case Design With Mask Erosion

Let be the verticaland lateral etching rates of the mask.

Let be the vertical etching rate of the film.

(lateral film etch rate is ignoredIn this example for simplicity)

v vm m⊥ , / /

v f

Before

film

W/2After

film

θhf

State-of-problem:Mask material can be eroded during film etching.Top of film will be smaller than original mask size by an amount W/2.


Lecture 15EE143 S06

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( )

( )( )( )

Wv v t

vv

hvv

m m T

m

ff

f

m

m

2

1 11

= + ⋅

=

⋅ ⋅

+ +−

+

⊥

⊥

⊥

cot

cot

/ /

/ /

θ

δφ

θ∆

To minimize W

v ν

h small

o

f m

f

→>> ⊥

θ 90


Lecture 15EE143 S06

17

Question: For a given allowable W/2 , what is the minimumetching selectivity between film and mask 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 S06

18

Wet Etching

Reactant transport to surface

Selective and controlled reaction of etchant with the film to be etched

Transport of by-products away from surface

1

2

3

1

2

3


Lecture 15EE143 S06

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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 S06

20

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 S06

21

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


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(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)


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(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)


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Effect of Slow {111} Etching with KOH or EDP

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


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[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}


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(3) Field-Emission Tips


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Drawbacks of Wet Etching

• Lack of anisotropy

• Poor process control

• Excessive particulate contamination

=> Wet etching used for noncritical feature sizes

EE143 S06 Lecture 15 Etching - University of California ...ee243/sp10/EE143/lectures/Lec_15.pdf ·...

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