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2-14 MEMS: Design and Fabrication

certainly be a satisfactory candidate as a sacrificial layer, as it can be dissolved in acidic-based Al etchants
that do not etch polysilicon. However, the thermal properties tell a different story. LPCVD polysilicon is
often deposited at temperatures between 580 and 630°C, which are excessively close to the Al melting
temperature at the deposition pressure. Independent of the temperature incompatibility, polysilicon is
often used as the gate material in MOS processes. As a result, for MEMS and IC processes that share the
same LPCVD polysilicon furnace, as might be the case for an integrated MEMS process, putting Al-coated
wafers in a polysilicon furnace would be inadvisable due to cross-contamination considerations.
The release process associated with polysilicon surface micromachining is simple in principle but can
be complicated in practice. The objective is to completely dissolve the sacrificial oxide from beneath the
freestanding components without etching the polysilicon structural components. The wafers or dies are
simply immersed in the appropriate solution for a period of time sufficient to release all desired parts.
This is done with various concentrations of electronic-grade HF, including BOE, as the etch rates of SiO 2
and polysilicon are significantly different. It has been observed, however, that during the HF release step,
the mechanical properties of polysilicon including residual stress, Young’s modulus, and fracture strain
can be affected [Walker et al., 1991]. In general, the modulus and fracture strain of polysilicon decreases
with increasing time of exposure to HF and with increasing HF concentration. This decrease in the mod-
ulus and fracture strain indicates a degradation of the film mechanical integrity. To minimize the HF
release time, structures are designed with access holes and cuts of sufficient size to facilitate the flow of
HF to the sacrificial oxide. In this manner, polysilicon structures can be released without appreciable
degradation to film properties and hence device performance.
Following the HF release step, the devices must be rinsed and dried. A simple process includes rinses
in deionized (DI) water and then in methanol, followed by a drying step using N . The primary difficulty
2
with the wet release process is that surface tension forces, which are related to the surface properties of
the material, tend to pull the micromechanical parts toward the substrate as the devices are immersed and
pulled out of the solutions. Release processes that avoid the surface tension problem by using frozen alco-
hols that are sublimated at the final rinse step have been developed [Guckel et al., 1990]. Processes based
on the use of supercritical fluids [Mulhern et al., 1993], such as CO at 35°C and 1100 psi, to extinguish
2
surface tension effects vanish are now commonplace in many MEMS facilities.

2.6 Germanium-Based Materials

Germanium (Ge) has a long history in the development of semiconducting materials, dating to the devel-
opment of the earliest transistors. The same is true in the development of micromachined transducers
and the early work on the piezoresistive effect in semiconducting materials [Smith, 1954]. Development
of Ge for microelectronic devices might have continued if only a water-insoluble oxide could be formed
on Ge surfaces. Nonetheless, there is a renewed interest in Ge for micromachined devices, especially for
devices that require use of low-temperature processes.
Thin polycrystalline Ge (poly-Ge) films can be deposited by LPCVD at temperatures much lower than
polysilicon, namely 325°C at a pressure of 300 mtorr on Si, Ge, or SiGe substrates [Li et al., 1999]. Ge does
not nucleate on SiO surfaces, which prohibits use of thermal oxides and LTO films as sacrificial substrate
2
layers but does enable use of these films as sacrificial molds, as selective growth using SiO masking films
2
is possible. Residual stress in poly-Ge films deposited on Si substrates is about 125 MPa compressive,
which can be reduced to nearly zero after a 30s anneal at 600°C. Poly-Ge is essentially impervious to
KOH, tetramethyl ammonium hydroxide (TMAH), and BOE, making it an ideal masking and etch-stop
material in Si micromachining. In fact, the combination of low residual stress and inertness to Si
anisotropic etches enables the fabrication of Ge membranes on Si substrates [Li et al., 1999]. The
mechanical properties of poly-Ge are comparable with polysilicon, with a Young’s modulus measured at
132GPa and a fracture stress ranging between 1.5GPa and 3.0 GPa [Franke et al., 1999]. Poly-Ge can also
be used as a sacrificial layer. Typical wet etchants are based on mixtures of HNO , H O, and HCl and of
3
2
H O, H O and HCl as well as the RCA SC-1 cleaning solution. These mixtures do not etch Si, SiO , Si N ,
2 2 2 2 3 4

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