Biomedical Engineering Reference
In-Depth Information
MEMS Processing of AlN Films
There are several reports on surface micromachining AlN (Hara et al., 2005). Germanium
can be used as the sacrificial layer for the AlN films, instead of common amorphous sili-
con, SiO
2
, or a metal. AlN can be etched in aqueous solutions, such as KOH, NaOH, HF/
H
2
O, HF/HNO
3
, tetramethyl ammonium hydroxide (TMAH), and the etch rate is tempera-
ture- and crystal polarity-sensitive (Jasinki
et al., 2003; Sheng et al., 1988; Tan et al., 1995).
Cr can also be used to form both a good etch mask and electrode (Saravanan
et al., 2006).
AlN can be electrochemically etched in electrolytes, such as HPO
3
(60°C to 90°C) or KOH
solutions, and the etch rate is strongly dependent on the coating quality (from tens of nm/
min up to a few
μ
m/min). The reaction is (Zhang and Edgar, 2005):
AlN + 6KOH
→
Al(OH)
3
→
+ NH
3
→
+ 3K
2
O
→
(8.5)
For dry etching process, AlN is normally etched using a chlorine-based plasma, such
as chlorine and BCl
3
, rather than a fluorine plasma because aluminum fluoride is stable
and nonvolatile (Khan et al., 2002). Etching in a Cl-based plasma is normally isotropic,
and the volatile reaction product is AlCl
3
at high temperature (above 180°C) or Al
2
Cl
6
at
a room temperature (Engelmark, 2003). SF
6
/Ar plasma has also been used to etch AlN,
and the etching process is believed to be a combination of the reactions of F and Al
and sputtering of reactive a product (AlF
3
), with highly anisotropic and smooth side
walls.
Functionalization of AlN Surface for Biosensing
Recently, there have been some studies for the surface functionalization of AlN film for
biosensing applications (Chiu et al., 2008). For example, by using silane, a new chemical
layer can form on the AlN, and the functional groups on the silane surface can then be used
as anchor points for the antibodies. A
generic method for immobilization of gold nanopar-
ticle bioconjugates onto AlN surfaces using aminosilane molecules as cross-linkers has
been demonstrated
for SAW sensor applications (Chiu et al., 2008). Electrostatic interaction
between the positively charged surface amine groups and negatively charged DNA-Au
nanoparticle
conjugates allows the self-assembly of a probe nanoparticle monolayer onto
the functionalized AlN surfaces under physiological conditions. Results showed that Au
nanoparticles can play
multiple roles in SAW sensing for probe molecule immobilization,
signal amplification,
and labeling (Chiu et al., 2008).
In reference (Cao et al., 2008), antibody immobilized AlN/sapphire was prepared using
the process shown schematically in Figure 8.18. The AlN films were pretreated before
silanization using two methods. In the first method, they were treated by exposure to an
oxygen plasma. The second method treated the AlN surfaces by ultrasonication in 3:1 (in
vol%) piranha solution, followed by rinsing in DI water. Piranha treatment was chosen
because it is commonly used as a surface preparation method for silanization of other
types of inorganic surfaces. Improved silane surfaces should create a more stable and
ordered silane layer for the linkage of antibody, phages or other detecting ligands in the
biosensor under development (Cao et al., 2008). The treated AlN samples were silanized
with OTS. The ability to produce repeatable, homogeneous layers of selected chemical
groups by silane derivatization of the AlN surface is considered to be a promising step in
the development of a biosensor that uses surface immobilized phage or antibody ligands
for analyte detection (Cao et al., 2008).
