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MEMS Fabrication 3-41

exposed crystal plane. Typically, the pH stays above 12, while more elevated temperatures are used for
these slower-type etchants ( 50°C). The latter type of etchants surged in importance in the late 1960s for
the fabrication of dielectrically isolated structures in silicon [Stoller and Wolff, 1966; Stoller, 1970; Forster
and Singleton, 1966; Kenney, 1967; Lepselter, 1966, 1967; Waggener, 1970; Kragness and Waggener, 1973;
Waggener et al., 1967a; Waggener et al., 1967b; Bean and Runyan, 1977; Rodgers et al., 1977; Rodgers
et al., 1976; Ammar and Rodgers, 1980; Schnable and Schmidt, 1976]. Isotropic etchants typically show
diffusion limitation, while anisotropic etchants are reaction rate limited. In both cases, the two principal
reactions are oxidation of the silicon followed by dissolution of the hydrated silicate.
Preferential or selective etching (also structural etching) is usually isotropic but exhibits some
anisotropy [Kern and Deckert, 1978]. These etchants are used to produce a difference in etch rate between
different materials or between compositional or structural variations of the same material on the same
crystal plane. These types of etches are often the fastest and simplest techniques to delineate electrical
junctions and to evaluate the structural perfection of a single crystal in terms of, for example, slip and
stacking faults. The artifacts introduced by the defects etch into small pits of characteristic shape. Most
of the etchants used for this purpose are acids with an oxidizing additive [Yang, 1984; Chu and Gavaler,
1965; Archer, 1982; Schimmel and Elkind, 1973; d’Aragona, 1972].

3.7.2 Isotropic Etching

3.7.2.1 Usage of Isotropic Etchants

When etching silicon with aggressive acidic etchants, rounded isotropic patterns form. This method is
widely used for

1. Removal of work-damaged surfaces
2. Rounding of sharp anisotropically etched corners (to avoid stress concentration)
3. Removing of roughness after dry or anisotropic etching
4. Creating structures or planar surfaces in single-crystal slices (thinning)
5. Patterning single-crystal, polycrystalline, or amorphous films
6. Delineation of electrical junctions and defect evaluation (with preferential isotropic etchants)

For isotropic etching of silicon, the most commonly used etchants are mixtures of nitric acid (HNO ) and
3
hydrofluoric acids (HF). Water can be used as a diluent, but acetic acid (CH COOH) is preferred because
3
it prevents the dissociation of the nitric acid better and so preserves the oxidizing power of HNO , which
3
depends on the undissociated nitric acid species for a wide range of dilution [Robbins and Schwartz,
1960]. The etchant is called the HNA system; we will return to this etch system below.

3.7.2.2 Simplified Reaction Scheme

In acidic media, the Si etching process involves hole injection into the Si valence band by an oxidant, an
electrical field, or photons. Nitric acid in the HNA system acts as an oxidant; other oxidants such as H O 2
2
and Br also work [Tuck, 1975]. The holes attack the covalently bonded Si, oxidizing the material,
2

followed by a reaction of the oxidized Si fragments with OH and subsequent dissolution of the silicon
oxidation products in HF. The following reactions describe these processes.
The holes are, in the absence of photons and an applied field, produced by HNO , along with water
3
and trace impurities of HNO :
2

HNO H O HNO → 2HNO 2OH 2H (Reaction 3.1)
3 2 2 2
The holes in reaction (3.1) are generated in an autocatalytic process; HNO generated in the above reac-
2
tion re-enters into the further reaction with HNO to produce more holes. With a reaction of this type,
3
an induction period begins before the oxidation reaction takes off and continues until a steady-state con-
centration of HNO has been reached. This has been observed at low HNO concentrations [Tuck, 1975].
2 3

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