Page 15 - MEMS Mechanical Sensors

4 Introduction

states that the voltage at which sparking occurs (the breakdown voltage) is depend-
ent on the product of air pressure and the separation between the electrodes. As the
gap between two electrodes is reduced, a plot of breakdown voltage against the gap
separation and gas pressure product (Paschen curve) reveals a minimum in the char-
acteristic, as shown in Figure 1.1. The consequence is that for air gaps of less than
several microns, the breakdown voltage increases.

1.4 Why Silicon?

Micromachining has been demonstrated in a variety of materials including glasses,
ceramics, polymers, metals, and various other alloys. Why, then, is silicon so
strongly associated with MEMS? The main reasons are given here:

• Its wide use within the microelectronic integrated circuit industry;
• Well understood and controllable electrical properties;
• Availability of existing design tools;
• Economical to produce single crystal substrates;
• Vast knowledge of the material exists;
• Its desirable mechanical properties.

The final point is, of course, particularly desirable for mechanical microsensors.
Single crystal silicon is elastic (up to its fracture point), is lighter than aluminum, and
has a modulus of elasticity similar to stainless steel. Its mechanical properties are
anisotropic and hence are dependent on the orientation to the crystal axis. Table 1.1
illustrates some of the main properties of silicon in relation to other materials. Typi-
cal values are given and variations in these figures may be found in the literature as
some of the listed properties are dependent upon the measurement conditions used
to determine the values. Stainless steel is used as a convenient reference as it is widely
used in the manufacture of traditional mechanical transducers. It must be noted,
however, that there are many different types of stainless steel exhibiting a broad
variation to those values listed here.
Silicon itself exists in three forms: crystalline, amorphous, and polycrystalline
(polysilicon). High purity, crystalline silicon substrates are readily available as

The Paschen curve
10,000
(V)
voltage 1,000

Breakdown Air

100
1 10 100 1,000 10,000
Gap separation x gas pressure (microns*atm)
Figure 1.1 A plot of breakdown voltage against electrode separation (in air at 1 atmosphere of
pressure).