Page 166 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

146 SILICON MICROMACHINING: SURFACE

in standard IC technologies (see Chapter 4) and has excellent mechanical properties that
2
are similar to those of single-crystalline silicon . When polycrystalline silicon is used as
the structural layer, sacrificial layer technology normally employs silicon dioxide (SiO 2)
as the sacrificial material, which is employed during the fabrication process to realise
some microstructure but does not constitute any part of the final miniature device.
In sacrificial layer technology, the key processing steps are as follows:

1. Deposition and patterning of a sacrificial SiO 2 layer on the substrate
2. Deposition and definition of a poly-Si film
3. Removal of the sacrificial oxide by lateral etching in hydrofluoric acid (HF), that is,
etching away of the oxide underneath the poly-Si structure

Here, we refer to poly-Si and SiO 2 as the structural and sacrificial materials, respectively.
The reason for doing this is that in almost all practical situations this is the preferred
choice of material combination. However, other material combinations are also being
used in surface micromachining, some of which are discussed in Section 6.3.

6.2.1 Simple Process

The simplest of surface-micromachining processes involves just one poly-Si layer and
one oxide layer. This process is a one-mask process and is illustrated in Figure 6.1 in
which it is designed to form a poly-Si cantilever anchored to a Si substrate by means of
an oxide layer. The oxide sacrificial layer is deposited first (Figure 6.1 (a)). The poly-Si
structural layer is then deposited on top of the oxide. Next, the poly-Si layer is patterned,
forming both the cantilever beam and the anchor region (Figure 6.1(b)). Following the
poly-Si patterning step, the cantilever beam is released by laterally etching the oxide in
an HF solution. The oxide etch needs to be timed so that the anchor region is not etched
away (Figure 6.1(c)).
To implement successfully the process described in the preceding paragraph, the release
etch must be very carefully controlled. If the release etch is continued for too long a period,
the anchor region will be completely cut, resulting in device failure. However, to avoid
such a failure, the process may be extended to a two-mask process in which the poly-
Si cantilever is directly anchored to the substrate. This two-mask process is shown in
Figure 6.2. In this process, the deposited oxide (Figure 6.2(a)) is patterned for an anchor
opening by the first mask (Figure 6.2(b)). This is followed by a conformal deposition of
poly-Si and subsequent patterning of the poly-Si cantilever beam using the second mask
(Figure 6.2(c)). The cantilever is then released by a lateral oxide etch in HF solution
(Figure 6.2(d)). Because the anchor region in this case is made out of poly-Si, the oxide
release etch poses no threat of device failure.
Modifications of these simple one-mask and two-mask processes are the addition of
bushings (or dimples) and/or an insulating layer between the cantilever and the substrate.
The process with an added bushing is shown in Figure 6.3. An additional mask is
needed to pattern the bushing mould in either the one-mask or in the two-mask process
(Figure 6.3(b)). Because of the difference in depth between the bushing mould and the

2
See Appendix G for tabulated properties of silicon.