Wet Etching of
Silicon, Silicon Dioxide and Silicon Nitride
Silicon etch
- Single crystal silicon etch is used for
forming the isolation blocks between neighboring transistors.
- Polysilicon etch forms the gates and local
interconnections.
- Single-crystal and polycrystalline silicon can
be etched isotropically with a mixture of nitric acid and hydrofluoric
acid.
- First nitric acid oxidizes the silicon on the
surface resulting in a thin layer of silicon dioxide, which can block the
oxidation process.
- Hydrofluoric acid then reacts with silicon
dioxide and dissolves it, exposing the silicon under it, which is oxidized
again by nitric acid.
- The overall reaction can be written as:
Si + HNO3 +6HF ® H2SiF6 + HNO2 +2H2O
Nitride Etch
- Nitride is widely used for isolation formation
processes.
- Hot phosphoric acid is most commonly used to
etch silicon nitride.
- This silicon nitride etch process has very
good selectivity to thermally grown silicon dioxide and to silicon.
- The chemical reaction of the silicon nitride
etch is:
Si3N4 + 4H3PO4
® Si3(PO4)4 +4NH3
- Both of the byproducts are water-soluble.
Metal Etch
- A mixture of phosphoric acid, acetic acid, and
nitric acid and water can etch aluminum.
- The etch mechanism of aluminum etch is very
similar to silicon etch: Nitric acid oxidizes the aluminum to form
aluminum oxide, and phosphoric acid dissolves Al2O3.
Both oxidization and oxide dissolution in phosphoric acid proceed
simultaneously.
- Another common metal wet etch process is
titanium strip after the titanium silicide formation.
- The 1:1 mixture of hydrogen peroxide and
sulfuric acid is commonly used to selectively etch away titanium while
keeping silicon oxide and titanium silicide intact. [2]