Page 31 - Principles and Applications of NanoMEMS Physics

1. NANOELECTROMECHANICAL SYSTEMS 17

1.2.2.1 Surface Micromachining

In surface micromachining, 3-D mechanical structures are constructed in
a layered fashion. Two types of layers, based on their material
composition/etching properties, are employed, namely, sacrificial and
structural layers. The former are ultimately dissolved via a process step
named release, and the latter remain, becoming part of the free-standing
movable structure proper. The simplest element illustrating the surface
micromachining technique is, perhaps, the cantilever beam. Figure 1.13
sketches its formation. Typical combinations of sacrificial and structural
materials, and corresponding etchant are shown in Table 1.5 [27].

TO P V IEW
S IDE VIE
S IDE VIE W W TO P V IEW
S a crificial la yer er
S a crificial la y
Wa fe
Wa fer r
S tru ctu ral la ye
S tru ctu ral la yer r

Bea m
Bea m

P rocess seq u en
P rocess seq u ence ce
Figure 1-13. Sketch of the formation of a cantilever beam by surface micromachining. From
top to bottom of the figure, the sacrificial material is deposited and patterned (top), then the
structural material is deposited and patterned (middle), the sacrificial layer is released
(bottom).
Table 1-5. Structural/Sacrificial/Etchant Material Systems [27].
Structural Material Sacrificial Material Etchant
Aluminum Single-crystal silicon EDP, TMAH, XeF 2
Aluminum Photoresist Oxygen plasma
Copper or Nickel Chrome HF
Polyimide Aluminum Al etch (Phosphoric, Acetic,
Nitric Acid)
Polysilicon Silicon dioxide HF
Photoresist Aluminum Al etch (Phosphoric, Acetic,
Nitric Acid)
Silicon dioxide Polysilicon XeF 2
Silicon nitride pr Boron- Undoped polysilicon KOH or TMAH
doped polysilicon