Etching in device fabrication

1. Etching
removal of material from the wafer surface
process is usually combined with lithography in
order to select specific areas on the wafer from
which material is to be removed
represents one way of permanently transferring
the mask pattern from the photoresist to the
wafer surface

2. Classification of Etching
Wet etching
Dry etching

3. Wet etching
wafers are immersed in a tank of the etchant
chemical reaction between the wafer surface and the
etchants that helps in material removal
time for etching depends on the amount and type of
material that needs to be removed
used for removal of material from large areas
process is anisotropic

4. Wet etching process

5. Disadvantages of wet etching
used only for large pattern sizes
wet etch has to be combined with subsequent rinse and dry
steps. This increases chances of defects or contamination
hazardous chemicals and conditions are used, so safety is
an issue. Safe disposal of chemicals is essential
undercutting and resist peel off can happen if time is not
controlled or etch conditions change during process

6. Dry etching
removal of material in the absence of solvent
used for removal of material from smaller areas
Dry etching is also called plasma etching
isotropic process

7. Dry etching process

8. Wet etching process is anisotropic i.e. the
etch rate depends on the plane of the wafer,
from which atoms are being removed
Dry etching is an isotropic process - sloped
sidewalls rather than straight walls
Isotropic and Anisotropic Etching

9. Non-Plasma Based Etching
Classification of Dry etching
Non-plasma based etching uses a spontaneous reaction
of an appropriate reactive gas mixture
Plasma Based Etching
Plasma-based etching uses radio frequency power
to drive the chemical reaction

10. Differences between Wet etching and Dry etching
Wet etching Dry etching
Etching done at liquid phase Etching done at plasma phase
Applicable for size greater than
2 µm
Applicable for size less than 2
µm
Use liquid phase chemicals Use gaseous phase chemicals
Disposing of hazardous
materials can cause
contamination
Comparatively safer than wet
etching
Less precise More precise

11. Differences between Wet etching and Dry etching
Wet etching Dry etching
Varieties of chemicals used Les chemicals used
Less expensive as only a
chemical bath is used
More expensive as specialized
equipment are used
Selectivity is greater than 100
times
Selectivity is less than 10 times
Throughput is very high Throughput is very low

12. Lightly doped
n-substrate
p-well
p-well
P-well process

13. Lightly doped
p-substrate
n-well
n-well
N-well process

14. n-substrate
p-well
p-well
n-well
n-well
Twin-tub process

15. n-substrate
p-well
p-well
n-well
n-well
Twin-tub process

16. n+
n+
p+ p+
n-substrate
p-well
Composite twin-tub process

17. separate optimization of the n-type and p-type
transistors will be provided
Composite twin-tub process
independent optimization of Vt, body effect and gain of
the P-devices, N-devices can be made possible with this
process