Page 165 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

6

Silicon Micromaching: Surface

6.1 INTRODUCTION

Since the beginning of the 1980s, much interest has been directed toward micromechanical
structures fabricated by a technique called surface micromachining. The resulting two-and-
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a-half-dimensional structures are mainly located on the surface of a silicon wafer and exist
as a thin film - hence the half dimension. The dimensions of these surface-micromachined
structures can be an order of magnitude smaller than the bulk-micromachined structures.
The main advantage of surface-micromachined structures is their easy integration with
integrated circuit (IC) components, as the same wafer surface can also be processed
for IC elements. However, as miniaturisation is immensely increased by silicon surface
micromachining, the small sizes or masses that are created are often insufficient for viable
sensors and, particularly, for actuators. The problem is most acute in capacitive mechanical
microsensors (Section 8.4) and especially in capacitively driven microactuators because
of the low coupling capacitances. Deep etching techniques, such as lithography, electro-
plating, and moulding process (LIGA), have been developed to address this problem but
are difficult to realise in silicon.
There are several common approaches to the making of microelectromechanical system
(MEMS) devices using surface micromachining. The first of these approaches is the
sacrificial layer technology for the realisation of mechanical microstructures. The second
approach incorporates IC technology and wet anisotropic etching and the third approach
uses plasma etching to fabricate microstructures at the silicon wafer surface. These
approaches are illustrated in this chapter by a set of ten worked examples that cover a
range of microsensor and MEMS devices from cantilever beams, resonant comb structures
through to micromotors.

6.2 SACRIFICIAL LAYER TECHNOLOGY

Sacrificial layer technology uses, in most situations, polycrystalline rather than single-
crystal silicon as the structural material for the fabrication of microstructures. Low-
pressure chemical vapour deposition (LPCVD) of polysilicon (poly-Si) is well known

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Full three-dimensional structures can now be made using microstereolithography; this important technology
is discussed in Chapter 7.