Page 38 - MEMS Mechanical Sensors

2.3 Fabrication Techniques 27

Commercial software is available with which it is possible to simulate the
results of anisotropic etching [18]. This is useful in predicting the outcome from
employing various mask designs, and thereby it facilitates design of the layout.

2.3.3.3 Silicon Dry Isotropic Etching
Dry isotropic etches are not often used for bulk micromachining. However, there
are a few examples. Etching in an SF plasma has been used as an alternative to wet
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anisotropic etching. The advantage is that a resist mask can be used and the problem
of protecting other materials on the wafers is easily overcome. The etch rates are
comparable to wet etching, but it is considered to be slow because of the inability to
process large numbers of wafers at a time. In another application the high selectivity
of silicon dry isotropic etching in an SF /O plasma against etching aluminum and
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silicon dioxide is utilized. By undercutting the aluminum after completion of a
CMOS process, suspended structures can be made.

2.3.3.4 Silicon Dry Anisotropic Etching

Anisotropic etching of silicon has been used in the microelectronics industry for
many years. The main applications have been in forming deep trench capacitors for
memory devices and in constructing isolation trenches between active devices.
However, for these applications the etch depth and aspect ratio used have been at
most 10 µm and 10:1, respectively. For MEMS applications there is a requirement
for much deeper trenches, often through the thickness of the wafer, and in some
cases higher aspect ratios are needed. The dry etch process that can achieve this is
called DRIE and there are currently two different processes being used by equip-
ment manufacturers. In each case the deep anisotropy is achieved by passivation of
the sidewalls of the trench as it is etched. One process uses cryogenic cooling of the
wafer to liquid nitrogen temperatures, which, it is believed, causes condensation of
the reactant gases on to the silicon surface, thus passivating it. On horizontal sur-
faces, such as the bottom of trenches, this condensate is removed by ion bombard-
ment and these surfaces are therefore etched. SF is typically used because of the
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high etch rates that can be achieved. The passivation can be enhanced by the addi-
tion of oxygen to the plasma, which results in oxidation of the sidewalls. Possible
problems with the cryogenic approach are in maintaining the temperature of struc-
tures during the etch process. Some structures may become thermally isolated
resulting in adverse thermal stress. The other process is one patented by Bosch
which uses alternate etch and passivation steps [19]. The passivation is achieved by
deposition of a polymer using C F as a source gas. Concurrent with this deposition
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step is some ion bombardment, and this prevents the formation of polymer on the
bottom of the trench. The polymer on the bottom of the trench is, in any case,
removed by energetic ions during the following etch step done in SF . The cycle time
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for this deposition/etch process is typically about 5 seconds with etch rates of
between 1.5 and 4 µm/min. Aspect ratios of more than 40:1 can be obtained. A limi-
tation encountered with both DRIE processes is the etch rate dependence on trench
width. The etch process is diffusion limited and for trench widths less than 60 µm
the etch rate becomes progressively slower as the trenches become narrower. This