Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 128

4.1.3 Polysilicon
4.1.3.1 Polysilicon surface micromachining
Polycrystalline silicon is referred to as polysilicon, which is deposited during an LPCVD process
with silane. The deposition temperatures range from 575 Cto650 C. At temperatures below
575 C, the silicon layer is amorphous. Above 650 C, polycrystalline has a columnar structure. The
grain size is typically between 0.03 and 0.3

1000 Cforseveralminutes,
crystallization and grain growth occur. The grain size is then on the order of 1

m m. After annealing at 900

e

m. Polysilicon can be
doped in situ with the same gases used for epitaxial silicon. The deposition rates range from 10 to
20 nm/min [4] .
Polysilicon layers are generally conformal. In surface micromachining, polysilicon is used directly
as mechanical material. For fabrication of micromixers, polysilicon can be used for making channel
walls and sealing etched channel structures. Devices made of polycrystalline silicon (or polysilicon)
are mostly fabricated with the traditional surface micromachining. The term “traditional” is used here
because the same fabrication concept with a sacrificial layer has also been used recently for polymer
and metallic structures. A surface micromachining process starts with the deposition of a sacrificial
layer ( Fig. 4.7 (a)). The sacrificial layer can be structured to generate anchor points for the later
functional structure. Subsequent deposition and patterning of the functional layer define the micro-
structures ( Fig. 4.7 (b)). Removing the underlying sacrificial layer releases the freestanding micro-
structures ( Fig. 4.7 (c)).
The main advantage of surface micromachining based on polysilicon is compatibility to CMOS
processes. The well-established CMOS techniques, such as reactive ion etching (RIE), allow the
fabrication of very small structures with sizes on the order of 1

m

m. There are a few challenges in
polysilicon surface micromachining. First, very long etching time is required for a flat and large area.
Thus, designing etch access into the MEMS structure is necessary for reducing the etching time. Since
the sacrificial layer (commonly, silicon oxide or phosphorous-doped silicon oxide) and the polysilicon
layers are deposited under different conditions, it is important to deposit a stress-free polysilicon layer.
The stress after deposition can be removed by subsequent annealing up to a temperature where
crystallization starts.

m

FIGURE 4.7
Polysilicon surface micromachining: (a) deposition and patterning of sacrificial layer; (b) deposition and
patterning of polysilicon; (c) etching of sacrificial layer.