EE143 F2010 Lecture 14 Etching - University of California, …ee143/fa10/lectures/Lec_… · ·...

Professor N Cheung, U.C. Berkeley

Lecture 14EE143 F2010

1

Etching

• Etching Terminology

• Etching Considerations for ICs

• Wet Etching

• Reactive Ion Etching (plasma etching)



2

Etch Process - Figures of Merit

• Etch rate

• Etch rate uniformity

• Selectivity

• Anisotropy



3

(1) Bias

substratedf

hf film

dm etching mask

df

substrate

dm

Bias B d dB can be or

f m

0 0.



4

Complete Isotropic Etching

B hf= ×2

Vertical Etching = Lateral Etching Rate

Lateral Etching rate = 0

B = 0

Complete Anisotropic Etching



5

10

21

f

ff

A

hB

A

02 BfhB

anisotropicisotropic

(2) Degree of Anisotropy



6

(3) Etching Selectivity S

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

)Bmateralofvelocityetchingvertical(v)Amateralofvelocityetchingvertical(vS

B

AAB



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



8

Etching of Steps with a Slope

rateetchlateralvetch rateverticalv

ttimeetchingLet

l

v

x v tx v t

x x xv v t

v

l

v l

1

2

1 2

cot

cot

substrate

start

finalfilm

v tv

x1 x2

vtv

cot = 1/ tan = b / a

* Etching velocity has vertical component vv and lateral component vl



9

o

o

0

90

vvt

vlt

start

final

sub

film

Slope etching examples

film

sub

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



10

Worst-Case Design Considerations for Etching

step

Substrate

Film thicker here dueto surface topography Variation of film

thickness acrosswafer due todeposition method

Mask materialcan be erodedduring filmetching

EtchingMask

Film

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



11

Nominal thickness

(a) Film thickness variation across wafer

1max ff hhThickness variation factor

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

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



12

(b) Film etching rate variation

ff

f

minf

maxf

ffminf

11

vh

vh

filmtheetchtorequiredtimeetchingcaseWorst

1vv

variation factor



13

(c) Over-etch around step

step

h2

h2h1

Fractional over-etch time tocompletely remove film= h1 / h2

111

ff

fT v

ht

Total worst-case etching time

hh

1

2

film

substrate



14

Example 1: Worst-case Consideration for substrate erosion

Film thickness hasvariation factor =

Film etching rate hasvariation factor = f

film

substrateWorst-casesubstrate erosion

hf (1+ f)hf (1- f)

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

ff

f1

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

ff

f2

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



15

Example 2: Worst-case Design With Mask Erosion

Let be the verticaland lateral etching rates ofthe mask.

Let be the verticaletching rate of the film.

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

v vm m , / /

v f

Before

film

W/2After

film

hf

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.



16

W v v t

vv

h vv

m m T

m

ff

f

m

m

21 1

1

cot

cot

/ /

/ /

To minimize W

vh small

o

f m

f

90



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

2

[Note] If varies from run-to-run,vm

m

m

factoruniformityfmU

f

mffm

mmm

vv

Wh

S

vv

//(min)

max

cot1

111

2

1



18

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



19

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



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



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



22

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)



23

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)



24

Effect of Slow {111} Etching with KOH or EDP

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



25

[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}



26

(3) Field-Emission Tips



27

Drawbacks of Wet Etching

• Lack of anisotropy

• Poor process control

• Excessive particulate contamination

=> Wet etching used for noncritical featuresizes

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