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2-12 MEMS: Design and Fabrication

degree of thickness uniformity at the required deposition temperature and it allows the wafers to be spaced
close together, thus increasing the number of wafers per furnace load. To produce stoichiometric Si N , a
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NH –to–SiH Cl ratio of 10:1 is commonly used. The standard furnace configuration uses door injection
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of the source gases with a temperature gradient along the tube axis to accommodate for the gas depletion
effects. LPCVD Si N films deposited between 700 and 900°C are amorphous; therefore, the material
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properties do not vary significantly along the length of tube despite the temperature gradient. As with
polysilicon deposition, atypical furnace can accommodate over 100 wafers. Because Si N is deposited in
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the reaction-limited regime, film is deposited on both sides of each wafer with equal thickness.
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The residual stress in stochiometric Si N is large and tensile, with a magnitude of about 10 dyne/cm .
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Such a large residual stress limits the practical thickness of a deposited Si N film to a few thousand
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angstroms because thicker films tend to crack. Nevertheless, stoichiometric Si N films have been used as
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mechanical support structures and electrical insulating layers in piezoresistive pressure sensors [Folkmer
et al., 1995]. To reduce the residual stress, thus enabling the use of thick Si N films for applications that
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require durable, chemically resistant membranes, nonstoichiometric silicon nitride (SixNy) films can be
deposited by LPCVD. These films, often referred to as Si-rich or low-stress nitride, are intentionally
deposited with an excess of Si by simply decreasing the NH –to–SiH Cl ratio in the reaction furnace. For
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a NH –to–SiH Cl ratio of 1:6 at a deposition temperature of 850°C and pressure of 500 mtorr, the
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as-deposited films are nearly stress free [Sekimoto et al., 1982]. The increase in Si content not only leads
to a reduction in tensile stress but also decreases the etch rate of the film in HF. As a result, low-stress sil-
icon nitride films have replaced stoichiometric Si N in many MEMS applications and even have enabled
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the development of fabrication techniques that otherwise would not be feasible with stoichiometric Si N .
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For example, low-stress silicon nitride has been successfully used as a structural material in a surface
micromachining process that uses polysilicon as the sacrificial material [Monk et al., 1993]. In this case,
Si anisotropic etchants such as KOH and EDP were used for dissolving the sacrificial polysilicon. A sec-
ond low-stress nitride surface micromachining process used PSG as a sacrificial layer, which was removed
using a HF-based solution [French et al., 1997]. Of course, wide use of Si N as a MEMS material is
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restricted by its dielectric properties; however, its Young’s modulus (146GPa) is on par with Si ( 190GPa),
making it an attractive material for mechanical components.
The essential interactions among substrate, electrical isolation layer, sacrificial layers, and structural
layers are best illustratedbyexamining the critical steps in a multilevel surface micromachining process.
The example used here (shown in Figure 2.4) is the fabrication of aSi micromotor using a technique
called the rapid prototyping process. The rapid prototyping process utilizes three deposition and three
photolithography steps to implement flange-bearing side-drive micromotors such as in the SEM of
Figure 2.5. The device consists of heavily P-doped LPCVD polysilicon structural components deposited
on a Si wafer using LTO both as a sacrificial layer and as an electrical isolation layer. Initially, a 2.4µm
thick LTO film is deposited on the Si substrate. A 2µm thick doped polysilicon layer is then deposited on
the LTO film. Photolithography and RIE steps are then performed to define the rotor, stator, and
rotor–stator gap. To fabricate the flange, a sacrificial mold is created by etching into the LTO film with an
isotropic etchant and then partially oxidizing the polysilicon rotor and stator structures to form what is
called the bearing clearance oxide. This oxidation step also forms the bottom of the bearing flange mold.
A 1 to 2µm thick, heavily doped polysilicon film is then deposited and patterned by photolithography
and RIE to form the bearing. At this point, the structural components of the micromotor are completely
formed, and all that remains is to release the rotor by etching the sacrificial oxide in HF and performing
an appropriate drying procedure (detailed later in this chapter). In this example, the LTO film serves three
purposes: it is the sacrificial underlayer for the free-spinning rotor; it comprises part of the flange mold;
and it serves as an insulating anchor for the stators and bearing post. Likewise, the thermal oxide serves
as a mold and electrical isolation layer. The material properties of LTO and thermal oxide allow for these
films to be used as they are in the rapid prototyping process, thus enabling the fabrication of multilayer
structures with a minimum of processing steps.
Without question, SiO is an excellent sacrificial material for polysilicon surface micromachining;
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however, other materials could also be used. In terms of chemical properties, aluminum (Al) would

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