Section 3: Etching

1
Section 3 Etching

• Jaeger Chapter 2
• Reader

2
Etch Process - Figures of Merit

• Etch rate
• Etch rate uniformity
• Selectivity
• Anisotropy

3
Bias and anisotropy
Complete Isotropic Etching
Vertical Etching Lateral Etching Rate
Complete Anisotropic Etching
Lateral Etching rate 0 B 0
4
Degree of Anisotropy
rlat lateral etch rate rver vertical etch rate
Af degree of isotropy
anisotropic
isotropic
5
Etching Selectivity S
Wet Etching
S is controlled by chemicals, concentration,
temperature
RIE
S is controlled by plasma parameters, plasma
chemistry, gas pressure, flow rate temperature.
6
Selectivity Example
SiO2/Si etched by HF solution
SSiO2, Si
Selectivity is very large ( infinity)
SiO2/Si etched by RIE (e.g. CF4 plasma)
SSiO2, Si
Selectivity is finite ( 10 )
7
Uniformity
(a) Film thickness variation across wafer

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

(b) Film etching rate variation
variation factor
8
Wet Etching
1
3
2

• 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
9
Wet Etching (cont.)

• Wet etch processes are generally isotropic
• Etch rate is governed by temperature,
  concentration, chemicals, etc.
• Wet etch processes can be highly selective
• Acids are commonly used for etching
• HNO3 ltgt H NO3-
• HF ltgt H F-
• H is a strong oxidizing agent
• gt high reactivity of acids

10
Wet Etch Processes
Etch rate (A/min)

• (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). This HF buffer is called Buffered
  Oxide Etch (BOE)
• NH4F NH3 HF

61 BOE
1200
650
18
26
T (oC)
11
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
• 101 for nitride over oxide
• 301 for nitride over Si

12
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)

13
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) for
  single crystalline Si

14
Drawbacks of Wet Etching

• Lack of anisotropy
• Poor process control
• Excessive particulate contamination
• gt Wet etching used for noncritical feature sizes

15
Reactive Ion Etching (RIE)
Parallel-Plate Reactor
RF 13.56 MHz
plasma

wafers
Plasma generates (1) Ions
(2) Activated neutrals
Enhance chemical reaction
16
Remote Plasma Reactors
e.g. quartz
Plasma Sources (1) Transformer Coupled
Plasma (TCP) (2) Electron
Cyclotron Resonance (ECR)
coils
plasma
wafers
-bias
Pressure 1mTorr 10mTorr bias 1kV
pump
17
RIE Etching Sequence
gas flow
5
1
diffusion of by product desorption
diffusion of reactant
2
3
4
X
chemical reaction gaseous by products
absorption
Substrate
18
Volatility of Etching Product
Higher vapor pressure higher
volatility
(high vapor pressure)
Example Difficult to RIE Al-Cu alloy with high
Cu content
19
Examples
Use CF4 gas
For etching Si
F are Fluorine radicals (highly reactive, but
neutral)
Aluminum
Photoresist
20
How to Control Anisotropy ?
1) ionic bombardment to damage expose surface. 2)
sidewall coating by inhibitor prevents sidewall
etching.
21
How to Control Selectivity ?
E.g. SiO2 etching in CF4H2 plasma
S
Rates
P.R.
SiO2
Si
SiO2
H2
Si
H2 in (CF4H2)
Reason
22
Example Si etching in CF4O2 mixture
Reason
Rates
Si
1
2
O2 in CF4
Poly-Si
Oxide
23
Example RIE of Aluminum Lines
It is a three-step sequence 1) Remove native
oxide with BCl3 2) Etch Al with Cl-based
plasma 3) Protect fresh Al surface with thin
oxidation