Page 41 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

ETCHING ELECTRONIC MATERIALS 23

Let us consider, as an example, etching of silicon. For silicon, the most commonly used
etchants are mixtures of nitric acid (HNOs) and hydrofluoric acid (HF) in water or acetic
acid (CH3COOH). Wet chemical etching usually proceeds by oxidation. Initially, silicon
is oxidised in the presence of holes as follows:
Si + 2H+ > Si 2+ + H 2 (2.9)

Water dissociates according to the reaction

H 2O > (OH) - +H + (2.10)
The hydroxyl ions (OH) - recombine with positively charged silicon ions to form SiO2 in
two steps:
Si 2+ + 2(OH) > Si(OH) 2 (2.11)

and
> SiO 2 + (2.12)
Si(OH) 2 H 2
dissolves in HF acid according to the reaction
SiO 2
SiO 2 + 6HF » H 2SiF 6 + 2H 2O (2.13)

where H 2SiFe is soluble in water. The reactions of (2.9) to (2.13) may be represented
with HNO 3 by the following overall reaction:

Si + HNO 3 + 6HF * H 2SiF 6 + HNO 2 + H 2O + H 2 (2.14)
The chemical solution used for gallium arsenide (GaAs) etching is a combination of
hydrogen peroxide (H 2O 2) and sulfuric acid (H 2SO 4) dissolved in water. Dielectrics and
metals are etched using the same chemicals that dissolve these materials in bulk form and
involve their conversion into soluble salts or complexes. Generally, film materials will
etch more rapidly than their bulk counterparts.
Etching processes are characterised by three parameters:

1. Etch rate
2. Etch selectivity
3. Etch uniformity

The etch rate is defined as the material thickness etched per unit time. Etch selectivity is a
measure of how effective the etch process is in removing the material to be etched without
affecting other materials or films present in the wafer. Quantitatively, etch selectivity can
be expressed as the ratio between the etch rate of the material to be etched and etch-mask
materials on the wafer. Table 2.5 lists the properties of different wet etchants for different
materials.

2.4.2 Dry Etching

The basic concept of dry or plasma etching is very simple. A glow discharge is used
to generate chemically reactive species (atoms, radicals, and ions) from a relatively inert