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2-4 MEMS: Design and Fabrication

material, and Si N for electrical isolation of device structures. Heavy reliance on this material system
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stems in part from the fact these three materials find uses in the fabrication of ICs, and as a result, film
deposition and etching technologies are readily and widely available. Like single-crystal Si, polysilicon can
be doped during or after film deposition using standard IC processing techniques. SiO can be grown or
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deposited over a broad temperature range (e.g., 200 to 1150°C) to meet various process and material
requirements. SiO is readily dissolvable in hydrofluoric acid (HF), an IC-compatible chemical, without
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etching the polysilicon structural material [Adams, 1988]. HF does not wet bare Si surfaces; as a result, it
is automatically rejected from microscopic cavities between polysilicon layers after a SiO sacrificial layer
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is completely dissolved.
For surface micromachined structures, polysilicon is an attractive material because it has mechanical
properties comparable to single-crystal Si, because the required processing technology has been devel-
oped for IC applications, and because it is resistant to SiO etchants. In other words, polysilicon surface
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micromachining leverages on the significant capital investment made by the IC industry in the impor-
tant areas of film deposition, patterning, and material characterization.
For MEMS and IC applications, polysilicon thin films are commonly deposited by a process known as
low-pressure chemical vapor deposition (LPCVD). This deposition technique was first commercialized
in the mid-1970s [Rosler, 1977] and has since been a standard process in the microelectronics industry.
The typical polysilicon LPCVD reactor (or furnace) is based on a hot-wall resistance-heated horizontal
fused-silica tube design. The temperature of the wafers in the furnace is maintained by heating the tube
using resistive heating elements. The furnaces are equipped with quartz boats that have closely spaced
vertically oriented slots that hold the wafers. The close spacing requires that the deposition process be
performed in the reaction-limited regime to obtain uniform deposition across each wafer surface. In the
reaction-limited deposition regime, the deposition rate is determined by the reaction rate of the reacting
species on the substrate surface, as opposed to the arrival rate of the reacting species to the surface (which
is the diffusion-controlled regime). The relationship between the deposition rate and the substrate tem-
perature in the reaction-limited regime is exponential; therefore, precise temperature control of the reac-
tion chamber is required. Operating in the reaction-limited regime facilitates conformal deposition of the
film over the substrate topography, an important aspect of multilayer surface micromachining. Commercial
equipment is available to accommodate furnace loads exceeding 100 wafers.
Typical deposition conditions utilize temperatures from 580 to 650°C and pressures ranging from 100
to 400 mtorr. The most commonly used source gas is silane (SiH ), which readily decomposes into Si on
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substrates heated to these temperatures. Gas flow rates depend on the tube diameter and other condi-
tions. For processes performed at 630°C, the polysilicon deposition rate is about 100Å/min. The gas inlets
are typically at the load door end of the tube, with the outlet to the vacuum pump located at the oppo-
site end. For door injection systems, depletion of the source gas occurs along the length of the tube. To
keep the deposition rate uniform, a temperature gradient is maintained along the tube so that the
increased deposition rate associated with higher substrate temperatures offsets the reduction due to gas
depletion. Typical temperature gradients range from 5 to 15°C along the tube length. Some systems incor-
porate an injector inside the tube to allow for the additional supply of source gas to offset depletion
effects. In this case, the temperature gradient along the tube is zero. This is an important modification, as
the microstructure and physical properties of the deposited polysilicon are a function of the deposition
temperature.
Polysilicon is made up of small single-crystal domains called grains,whose orientations and/or align-
ment vary with respect to each other. The roughness often observed on polysilicon surfaces is due to the
granular nature of polysilicon. The microstructure of the as-deposited polysilicon is a function of the
deposition conditions [Kamins, 1998]. For typical LPCVD processes (e.g., 100% SiH source gas, 200
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mtorr deposition pressure), the amorphous-to-polycrystalline transition temperature is about 570°C,
with amorphous films deposited below this temperature (Figure 2.1) and polycrystalline films above this
temperature (Figure 2.2). As the deposition temperature increases significantly above 570°C, the grain
structure of the as-deposited polysilicon films changes dramatically. For example, at 600°C, the grains are
very fine and equiaxed, while at 625°C, the grains are larger and have a columnar structure that is aligned

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