Page 42 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

24 ELECTRONIC MATERIALS AND PROCESSING

Table 2.5 Wet etchants used in etching some selected electronic materials

Material Etchant composition Etch rate (A/min)
Si 3 ml HF + 5 ml HNO 3 3.5 x 10 5
GaAs 8 ml H 2SO 4 + 1 ml H 2O 2 + 1 ml H 2O 0.8 x 10 5
28 ml HF + 170 ml H 2O + 113 g NH 4F 1000
SiO 2
or
15 ml HF -I- 10 ml HNO 3 + 300 ml H 2O 120
Buffered HF or 5 or 100
Si 3N 4 H 3PO 4
Al 1 ml HNO 3 + 4 ml CH 3COOH + 4 ml 350
+ 1 ml H 2O
H 3PO 4
Au 4 g KI + 1 g I 2 + 40 ml H 2O 1.0 x 10 5

molecular gas. The etching gas is chosen so as to produce species that react chemically
with the material to be etched to form a reaction product that is volatile. The etch
product then desorbs from the etched material into the gas phase and is removed by
the vacuum pumping system. The most common example of the application of plasma
etching is in the etching of carbonaceous materials, for example, resist polymers, in
oxygen plasma - a process referred to as plasma ashing or plasma stripping. In this case,
the etch species are oxygen atoms and the volatile etch products are CO, CO 2, and H2O
gases.
In etching silicon and silicon compounds, glow discharges of fluorine-containing
gases, such as CF4, are used. In this case, the volatile etch product is SiF 4 and the
etching species are mainly fluorine atoms. In principle, any material that reacts with
fluorine atoms to form a volatile product can be etched in this way (e.g. W, Ta, C,
Ge, Ti, Mo, B, U, etc.). Chlorine-containing gases have also been used to etch some
of the same materials, but the most important uses of chlorine-based gases have been
in the etching of aluminum and poly-Si. Both aluminum and silicon form volatile
chlorides. Aluminum is not etched in fluorine-containing plasmas because its fluoride
is nonvolatile.
The characteristic of etching processes, which is becoming more and more important
as the lateral dimensions of the lithography become smaller, is the so-called directionality
(anisotropy) of the etch process. This characteristic is illustrated in Figure 2.12 in
which the lithographic pattern is in the x-y plane and the z-direction is normal to this
plane. If the etch rates in the x and y directions are equal to the etch rate in the z-
direction, the etching process is said to be isotropic (or nondirectional) and the shape
of the sidewall of the etched feature will be as shown in Figure 2.12(a). Etch processes
that are anisotropic or directional have etch rates in the z-direction and are larger than
the lateral (x or y) etch rates. The extreme case of directional etching in which the
lateral etch rate is zero (to be referred to here as vertical etch process) is shown in
Figure 2. 12(b).
Plasma etching, as described in the preceding discussion, is predominantly an isotropic
process. However, anisotropy in dry etching can be achieved by means of the chemical
reaction preferentially enhanced in a given direction to the surface of the wafer by
some mechanism. The mechanism used in dry etching to achieve etch anisotropy is ion
bombardment. Under the influence of an RF field, the highly energised ions impinge
on the surface either to stimulate reaction in a direction perpendicular to the wafer