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2-10 MEMS: Design and Fabrication

films at temperatures much lower than thermal oxidation. Not only are LTO films deposited at low tem-
peratures, but the films also have a higher etch rate in HF than thermal oxides, which results in signifi-
cantly faster releases of polysilicon surface-micromachined devices. An advantage of the LPCVD
processes is that dopant gases can be included in the flow of source gases in order to dope the as-deposited
SiO films. One such example is the incorporation of P to form phosphosilicate glass (PSG). PSG is
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formed using the same deposition process as LTO, with PH added to dope the glass with a P content rang-
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ing from 2 to 8 wt%. PSG has an even higher etch rate in HF than LTO, further facilitating the release of
polysilicon surface-micromachined components. PSG flows at high temperatures (e.g., 1000 to 1100°C),
which can be exploited to create a smooth surface topography. Additionally, PSG layers sandwiching a
polysilicon film can be used as a P-doping source, improving the uniformity of diffusion-based doping.
Phosphosilicate glass and LTO films are deposited in hot-wall low-pressure fused-silica reactors in a
manner similar to the systems for polysilicon. Typical deposition rates are about 100Å/min. Precursor
gases include SiH as a Si source, O as an oxygen source, and in the case of PSG, PH as a source of phos-
4 2 3
phorus. Because SiH is pyrophoric (i.e., spontaneously combusts in the presence of O ), door injection
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of the deposition gases would result in a large depletion of the gases at deposition temperatures of 400 to
500°C and nonuniform deposition along the tube. Therefore, the gases are introduced in the furnace
through injectors distributed along the length of the tube. The wafers are placed vertically in caged boats;
this is to ensure uniform gas transport to the wafers. In the caged boats, two wafers are placed back to
back in each slot, thus minimizing the deposition of SiO on the wafers’ backs. The typical load of an LTO
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system is over 100 wafers.
Low-temperature oxidation and PSG films are typically deposited at temperatures of 425 to 450°C and
pressures ranging from 200 to 400 mtorr. The low deposition temperatures result in LTO and PSG films
that are slightly less dense than thermal oxides due to incorporation of hydrogen in the films. LTO films
can, however, be densified by an annealing step at high temperature (1000°C). The low density of LTO
and PSG films is partially responsible for the increased etch rate in HF, which makes them attractive sac-
rificial materials for polysilicon surface micromachining. LTO and PSG deposition processes are not typ-
ically conformal to nonplanar surfaces because the low substrate temperatures result in low surface
migration of reacting species. Step coverage is, however, sufficient for many polysilicon surface-
micromachining applications, although deposited films tend to thin at the bottom surfaces of deep
trenches and therefore must be thoroughly characterized for each application.
The dissolution of the sacrificial SiO to release free-standing structures is a critical step in polysilicon
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surface micromachining. Typically, 49% (by weight) HF is used for the release process. To pattern oxide
films using wet chemistries, etching in buffered HF (28 ml 49% HF, 170 ml H O, 113 g NH4F), also
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known as buffered oxide etch (BOE), is common for large structures. A third wet etchant, known as
P-etch, is traditionally used to selectively remove PSG over undoped oxide (e.g., to deglaze a wafer straight
from a diffusion furnace).
Thermal SiO , LTO, and PSG are electrical insulators suitable for many MEMS applications. The
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dielectric constants of thermal oxide and LTO are 3.9 and 4.3, respectively. The dielectric strength of ther-
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mal SiO is 1.1 10 V/cm, and for LTO it is about 80% that of thermal SiO [Ghandhi, 1983]. Thermal
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SiO is in compression with a stress level of about 3 10 dyne/cm [Ghandhi, 1983]. For LTO, however,
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the as-deposited residual stress is tensile, with a magnitude of about 1 to 4 10 dyne/cm [Ghandhi,
1983]. The addition of phosphorous to LTO (i.e., PSG) decreases the tensile residual stress to about
8
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10 dyne/cm for a phosphorus concentration of 8% [Pilskin, 1977]. These data are representative of oxide
films deposited directly on Si substrates under typical conditions; however, the final value of the stress in
an oxide film can be a strong function of the process parameters as well as any postprocessing steps.
One report documents the development of another low-pressure process, known as plasma-enhanced
chemical vapor deposition (PECVD), for MEMS applications. The objective was to deposit low-stress,
very thick (10 to 20µm) SiO films for insulating layers in micromachined gas turbine engines [Zhang
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et al., 2000]. PECVD was selected in part because it offers the possibility to deposit films of the desired
thickness at a reasonable deposition rate. The process used a conventional parallel plate reactor with
tetraethylorthosilicate (TEOS), a commonly used precursor in LPCVD processes, as the source gas.

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