1. Section 3: Etching
Jaeger Chapter 2
Reader

2. Etch Process - Figures of Merit
Etch rate
Etch rate uniformity
Selectivity
Anisotropy
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3. Complete Isotropic Etching
Bias and anisotropy
substrate df hf
film dm
etching mask
Bias B d f m -
Complete Isotropic Etching
Vertical Etching = Lateral Etching Rate
Complete Anisotropic Etching
Lateral Etching rate = 0
B = 0
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4. Degree of Anisotropy rlat: lateral etch rate rver: vertical etch rate
Af: degree of isotropy
isotropic
anisotropic
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5. Etching Selectivity S Wet Etching RIE S is controlled by:
chemicals, concentration, temperature
RIE
S is controlled by:
plasma parameters, plasma chemistry,
gas pressure, flow rate & temperature.
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6. Selectivity Example SiO2 Si 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 )
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7. Uniformity variation factor (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
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8. Reactant transport to surface
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
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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 <=> H+ + NO3-
HF <=> H+ + F-
H+ is a strong oxidizing agent
=> high reactivity of acids
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10. Wet Etch Processes (1) Silicon Dioxide To etch SiO2 film on Si, use
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
6:1 BOE
1200
650 18 26
T (oC)
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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:
10:1 for nitride over oxide
30:1 for nitride over Si
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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)
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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
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14. Drawbacks of Wet Etching
Lack of anisotropy
Poor process control
Excessive particulate contamination
=> Wet etching used for noncritical feature sizes
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15. Reactive Ion Etching (RIE)
Parallel-Plate
Reactor RF
13.56
MHz
plasma ~
wafers
Plasma generates (1) Ions
(2) Activated neutrals
Enhance chemical reaction
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16. Remote Plasma Reactors
Plasma Sources
(1) Transformer
Coupled
Plasma
(TCP)
(2) Electron
Cyclotron
Resonance
(ECR)
e.g. quartz
plasma
coils
wafers
-bias
Pressure
1mTorr mTorr
bias~ 1kV
pump
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17. RIE Etching Sequence Substrate gas flow 5 1 diffusion of by product
desorption
diffusion of
reactant 2 3 4 X
chemical
reaction gaseous by products
absorption
Substrate
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18. Volatility of Etching Product
* Higher vapor pressure higher volatility
(high vapor pressure)
Example
Difficult to RIE Al-Cu
alloy with high Cu content
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19. Examples For etching Si Use CF4 gas
F* are Fluorine radicals (highly reactive, but neutral)
Aluminum
Photoresist
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20. How to Control Anisotropy ?
1) ionic bombardment to damage expose surface.
2) sidewall coating by inhibitor prevents sidewall etching.
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21. How to Control Selectivity ?
E.g. SiO2 etching in CF4+H2 plasma S
Rates
P.R.
SiO2 Si
SiO2 Si
H2%
%H2 in (CF4+H2)
Reason:
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22. Example: Si etching in CF4+O2 mixture
Reason:
Rates Si 1 2
%O2 in CF4
Poly-Si
Oxide
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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
P.R. Al
BCl3 1 2
Cl2-based RIE
native Al2O3 3
Form oxide again (gently)
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