Page 91 - An Introduction to Microelectromechanical Systems Engineering

70 Processes for Micromachining

Mask Oxide

Resist

Substrate

Oxide
2. Resist development
1. Resist exposure
and oxide etch

Mask
Polysilicon
Resist

Oxide

3. Deposition of 4. Resist exposure
polysilicon
Suspended
beam

Anchor

5. Resist development 6. Sacrificial etching
and polysilicon etch of oxide
Figure 3.29 Schematic illustration of the basic process steps in surface micromachining.

structural layer, sacrificial layer, and etchant have been used, as shown in Table 3.5.
The etchant must etch the sacrificial layer at a useful rate, while having little or no
impact on the structural layer. Reasons for selecting materials other than polysilicon
include the need for higher electrical conductivity, higher optical reflectivity, and
lower deposition temperature for compatibility with CMOS circuitry that is already
on the wafer. For example, Texas Instruments’ Digital Mirror Device™ (DMD™)
display technology uses a surface-micromachined device with aluminum as its

Table 3.5 Some Systems of Materials for Surface Micromachining
Structural Material Sacrificial Material Etchant
Polysilicon Silicon dioxide/PSG Hydrofluoric acid
Silicon nitride Silicon dioxide/PSG Hydrofluoric acid
Silicon nitride Polysilicon Potassium hydroxide; xenon difluoride
Gold, tungsten, molybdenum, other metals Silicon dioxide/PSG Hydrofluoric acid
Aluminum Photoresist/organic Oxygen plasma
Nickel Copper Ammonium persulfate
Silicon-germanium Germanium Hydrogen peroxide
Silicon carbide Silicon dioxide Hydrofluoric acid