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3-42 MEMS: Design and Fabrication

After hole injection, OH groups attach to the oxidized Si species to form SiO , liberating hydrogen in
2
the process:

Si 4 4OH → SiO H 2 (Reaction 3.2)
2
Hydrofluoric acid (HF) dissolves the SiO by forming the water-soluble H SiF .The overall reaction of
2 2 6
HNA with Si looks like:

Si HNO 6HF → H SiF HNO H O H (bubbles) (Reaction 3.3)
3 2 6 2 2 2

The simplification in the above reaction scheme is that only holes are taken into account. In the actual Si
acidic corrosion reaction, both holes and electrons are involved. The question of hole and/or electron
participation in Si corrosion will be considered after we introduce the model for the Si/electrolyte inter-
facial energetics. We will learn from that model that the rate-determining step in acidic etching involves
hole injection in the valence band, whereas in alkaline anisotropic etching it involves electron injection
in the conduction band by surface states. The reactivity of a hole injected in the valence band is signifi-
cantly greater than that of an electron injected in the conduction band. The observation of isotropy in
acidic etchants and anisotropy in alkaline etchants centers on this difference in reactivity.

3.7.2.3 Iso-Etch Curves
By the early 1960s, the isotropic HNA silicon etch was well characterized. Schwartz and Robbins pub-
lished a series of four very detailed papers on the topic between 1959 and 1976 [Robbins and Schwartz,
1959, 1960; Schwartz and Robbins, 1961; 1976]. Most of the material presented below is based on their
work.
HNA etching results represented as iso-etch curves for various weight percentages of the constituents
are shown in Figure 3.29.For this work, normally available concentrated acids of 49.2 wt% HF and 69.5
wt% HNO are used. Water as diluent is indicated by dashed-line curves and acetic acid by solid-line
3
curves. Acetic acid is less polar than water and helps prevent the dissociation of HNO , thereby allowing
3
the formation of more of the species directly responsible for the oxidation of Si. A typical formulation for
HNA is 250mL HF, 500 mL HNO , and 800mL CH COOH. When used at room temperature, this for-
3 3
mulation results in an etch rate of about 4 to 20µm/min, increasing with agitation [Kovacs et al., 1998].
In Figure 3.29, as in Wong’s representation [Wong, 1990], we have recalculated the curves from Schwartz
et al. to express the etch rate in µm/min and divided the authors’ numbers by 2 because we are consider-
ing one-sided etching only. The highest etch rate is observed around a weight ratio HF-HNO of 2:1 and
3
is nearly 100 times faster than anisotropic etch rates. Adding a diluent slows the etching. From these
curves, the following characteristics of the HNA system can be summarized:
1. At high HF and low HNO concentrations, the iso-etch curves describe lines of constant HNO 3
3
concentrations (parallel to the HF-diluent axis); consequently, the HNO concentration controls
3
the etch rate. Etching at those concentrations tends to be difficult to initiate and exhibits an uncer-
tain induction period. In addition, it results in relatively unstable silicon surfaces proceeding to
slowly grow a layer of SiO over time. The etch is limited by the rate of oxidation so that it tends
2
to be orientation dependent and affected by dopant concentration, defects, and catalysts (sodium
nitrate often is used). In this regime, the temperature influence is more pronounced, and activa-
tion energies for the etching reaction of 10 to 20Kcal/mol have been measured.
2. At low HF and high HNO concentrations, iso-etch curves are lines parallel to the nitric-diluent
3
axis; that is, they are at constant HF composition. In this case, the etch rate is controlled by the abil-
ity of HF to remove the SiO as it is formed. Etches in this regime are isotropic and truly polish-
2
ing, producing a bright surface with anisotropies of 1% or less (favoring the 110 direction)
when used on 100 wafers [Wise et al., 1981]. An activation energy of 4Kcal/mol is indicative
of the diffusion-limited character of the process; consequently, in this regime, temperature
changes are less important.

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