Page 176 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

156 SILICON MICROMACHINING: SURFACE

must also have good mechanical properties to avoid device failure during the fabrication
process. These properties include good adhesion and a low residual stress to eliminate
device failure by delamination and/or cracking. The etchants must have excellent etch
selectivity and must be able to etch off the sacrificial materials without affecting the
structural ones. In addition, the etchants must also have appropriate viscosity and surface
tension characteristics.
Some of the common material systems that are used in surface micromachining are
discussed in the following three sections, followed by one worked example (a silicon
condenser microphone).

6.3.1 Polycrystalline Silicon and Silicon Dioxide

This material system has already been mentioned in Section 6.1. The poly-Si/oxide mate-
rial system is the most common one and uses the poly-Si deposited by LPCVD as the
structural material and the thermally grown (or LPCVD) oxide as the sacrificial mate-
rial. The oxide is readily dissolved in HF solution without the poly-Si being affected.
Silicon nitride is often used together with this material system for electrical insulation.
The advantages of this material system include the following:

1. Both poly-Si and SiO 2 are used in 1C processing; therefore, their deposition technolo-
gies are readily available.
2. Poly-Si has excellent mechanical properties and can be doped for various electrical
applications. Doping not only modifies the electrical properties but can also modify
the mechanical properties of poly-Si. For example, the maximum mechanically sound
length of a freestanding beam is significantly larger for phosphorus-doped as compared
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with undoped poly-Si . However, in most cases, the maximum length attainable is
limited by the tendency of the beam to stick to the substrate.
3. The oxide can be thermally grown and deposited by CVD over a wide range of
temperatures (from about 200 to 1200°C), which is very useful for various processing
requirements. However, the quality of oxide will vary with deposition temperature.
4. The material system is compatible with 1C processing. Both poly-Si and SiO 2 are
standard materials for IC devices. This commonality makes them highly desirable in
sacrificial layer technology applications that demand integrated electronics.

6.3.2 Polyimide and Aluminum

In this second material system, the polymer 'polyimide' is used for the structural material,
whereas aluminum is used for the sacrificial material. Acid-based aluminum etchants are
used to dissolve the aluminum sacrificial layer. The three main advantages of this material
system are as follows:

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Caution is needed when doping because it can induce a significant amount of stress.