Page 18 - MEMS Mechanical Sensors

CHAPTER 2
Materials and Fabrication Techniques

2.1 Introduction

MEMS devices and structures are fabricated using conventional integrated circuit
process techniques, such as lithography, deposition, and etching, together with a
broad range of specially developed micromachining techniques. Those techniques
borrowed from the integrated circuit processing industry are essentially two dimen-
sional, and control over parameters in the third dimension is only achieved by stack-
ing a series of two-dimensional layers on the workpiece, which is usually a silicon
wafer. There are practical and economic limits, however, to the number of layers
that can be managed in such a serial process, and therefore, the expansion of devices
into the third dimension is restricted. Micromachining techniques enable structures
to be extended further into the third dimension; however, it has to be understood
that these structures are simply either extruded two-dimensional shapes or are gov-
erned by the crystalline properties of the material. True three-dimensional process-
ing would allow any arbitrary curved surface to be formed, and this is clearly not
possible with the current equipment and techniques. An important aspect of MEMS
is to understand the limitations of the micromachining techniques currently avail-
able. Although the range of these techniques is continually being expanded, there
are some core techniques that have been part of the MEMS toolkit for many years.
This chapter deals mainly with these core techniques, but also with those process
techniques borrowed from integrated circuit manufacturing.

2.2 Materials

2.2.1 Substrates
2.2.1.1 Silicon
Just as silicon has dominated the integrated circuit industry, so too is it predominant
in MEMS. There are a number of reasons for this: (1) pure, cheap, and well-
characterized material readily available; (2) a large number and variety of mature,
easily accessible processing techniques; and (3) the potential for integration with
control and signal processing circuitry. In addition to these reasons, the mechanical
and physical properties of silicon give it a powerful advantage for its use in mechani-
cal sensors, and therefore, this book deals mainly with devices fabricated in bulk
silicon and silicon on insulator (SOI).
Crystalline silicon has a diamond structure. This is a face-centered cubic lattice
with two atoms (one at the lattice point and one at the coordinates ¼, ¼, ¼

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