Page 138 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

118 SILICON MICROMACHINING: BULK

Some of these etch processes have already been used as a standard technology in the
microelectronics industry, for example, RIE (Chapter 2).
In addition to an etch process, bulk micromachining often utilises wafer bonding and
buried oxide-layer technologies. However, the use of the latter in bulk micromachining
is still in its infancy.
This chapter describes the commonly used bulk-micromachining processes and gives
2
a set of worked examples that illustrate the applications of each one, or a combination,
of these important processes. The discussion includes the important topics of etch-stops
and wafer-to-wafer bonding.

5.2 ISOTROPIC AND ORIENTATION-DEPENDENT WET
ETCHING

Wet chemical etching is widely used in semiconductor processing. It is used for lapping
and polishing to give an optically flat and damage-free surface and to remove contami-
nation that results from wafer handling and storing. Most importantly, it is used in the
fabrication of discrete devices and integrated circuits (ICs) of relatively large dimensions
to delineate patterns and to open windows in insulating materials. The basic mechanisms
for wet chemical etching of electronic materials were described in Section 2.4. It was
also mentioned that most of the wet-etching processes are isotropic, that is, unaffected by
crystallographic orientation.
However, some wet etchants are orientation-dependent, that is, they have the property
of dissolving a given crystal plane of a semiconductor much faster than other planes (see
Table 5.1). In diamond and zinc-blende lattices, the (111) plane is more closely packed
than the (100) plane and, hence, for any given etchant, the etch-rate is expected to be
slower.
A commonly used orientation-dependent etch for silicon consists of a mixture of potas-
sium hydroxide (KOH) in water and isopropyl alcohol. The etch-rate is about 2.1 um/min
for the (110) plane, 1.4 urn/min for the (100) plane, and only 0.003 um/min for the (111)
plane at 80 °C; therefore, the ratio of the etch rates for the (100) and (110) planes to the
(111) plane are very high at 400:1 and 600:1, respectively.

Table 5.1 Anisotropic etching characteristics of different
wet etchants for single-crystalline silicon
Etchant Temperature Etch-rate (jim/hour) of
(°C) Si(100) Si(110) Si(111)
KOH:H 2O 80 84 126 0.21
KOH 75 25-42 39-66 0.5
EDP 110 51 57 1.25
N 2H 4H 2O 118 176 99 11
NH 4OH 75 24 8 1

' Appendix M provides a list of all the worked examples provided in this book.