Biomedical Engineering Reference
In-Depth Information
(4) Microfluidics and biosensing:
• High sensitivity and selectivity
• Stability of performance
• High pumping/mixing efficiency
• Easy functionalization of surfaces for immobilization of antibodies
• Biocompatibility
The following sections will focus on the recent progress covering the above issues for
both the ZnO and AlN films.
ZnOFilm
ZnO Film Deposition
Many different methods have been used to prepare ZnO films. These include the sol-gel
process (Wang et al., 2006; Shinagawa
et al., 2007; Kamalasanan and Chandra, 1996), molec-
ular beam deposition (Nakamura
et al., 2000), chemical vapor deposition (CVD) (Smith et
al., 2003; Minami
et al., 1994), organometallic chemical vapor deposition (MOCVD) (Gorla
et al., 1999; Zhang et al., 2004; Park et al., 2001), and sputtering (Maniv and Zangvil, 1978;
Wu et al., 1998; Hachigo
et al., 1994; Sundaram and Khan, 1997), molecular beam epitaxy
(MBE) (Look et al., 2002; Chen et al., 1998), pulsed laser deposition (PLD) (Narayan
et
al., 1998; Liu et al., 2003), and filtered vacuum arc deposition (FCVA) (Wang et al., 2003;
Goldsmith, 2006). The main advantage of MBE is its precise control over the deposition
parameters and in situ diagnostic capabilities. For ZnO thin film deposition
by MBE, Zn
metal and O
2
are usually used as
the source materials, and the deposition is performed
at a high temperature ranging from 800 to 1000°C. In the PLD method, high-power laser
pulses are used to evaporate material
from a target surface such that the stoichiometry of
the
material is preserved in the process. As a result, a
supersonic jet of particles (plume) is
directed normal to the
target surface, and the ablated
species condense on the substrate,
which is located opposite to the target.
The main advantages of PLD are its
ability to create
high-energy source particles, permitting high-quality film growth
at potentially low sub-
strate temperatures (typically ranging from 200 to 800°C) in high ambient gas pressures in
the 10
−5
to 10
−1
Torr range. CVD technology is also important for ZnO film growth because
it not only gives rise to high-quality films but
also is applicable to large-scale production.
However, the high temperature is a potential issue for the CVD method, although plasma-
enhanced CVD (PECVD) and MOCVD do enable lower temperatures to be used. One of the
most popular deposition techniques for
the ZnO film is sputtering (DC sputtering, radio
frequency, RF magnetron sputtering,
and reactive sputtering). When compared with sol-
gel and chemical vapor
deposition, magnetron sputtering is a preferred method because
of
its low cost, simplicity, and low operating temperature. ZnO films can be deposited at
a controlled substrate temperature either by sputtering from a
high-purity ZnO target or
by sputtering a Zn target in an
Ar and O
2
gas mixture. Table 8.3 compares the different
deposition methods for the ZnO films.
From an MEMS fabrication point of view, RF magnetron reactive sputtering is one of the
best methods, with good reproducibility and compatibility with planar device fabrication
