Biomedical Engineering Reference
In-Depth Information
(a)
(b)
N
0
107.7˚
Al
N
3
N
1
c
110.5˚
N
2
a
FIGURE 8.14
(a) Hexagonal structure of AlN and (b) tetrahedral structure, with one Al atom surrounded by four N atoms.
(From Chiu et al.,
Appl. Phys. Lett.
, 93, 163106, 2007. With permission.)
and high-sensitivity applications (Lee et al., 2004). AlN has a very large volume resistiv-
ity and is a hard material with a bulk hardness similar to quartz. Pure AlN is chemically
stable with atmospheric gases at temperatures less than about 700°C. The combination of
these physical and chemical properties is consequently promising in practical applications
of AlN both in bulk and thin film forms. Using AlN could lead to the development of
acoustic devices operating at higher frequencies, with improved sensitivity and perfor-
mance (insertion loss and resistance) in harsh environments (Wingqvist
et al., 2007). AlN
thin films have other good properties, such as high thermal conductivity, good electrical
isolation, a wide band gap (6.2 eV), and a thermal expansion coefficient, similar to that of
GaAs. Therefore, AlN thin films have been applied not only to surface passivation of semi-
conductors and insulators but also to optical devices in the ultraviolet spectral region and
acousto-optic devices.
The AlN crystal belongs to a hexagonal class (see Figure 8.14) or a distorted tetrahe-
dron, with each Al atom surrounded by four N atoms (Chiu et al., 2007). The four Al-N
bonds can be categorized into two types: three are equivalent Al-N
(
x
)
(
x
= 1, 2, 3) bonds,
B
1
, and one is a unique Al-N
0
bond, B
2
, in the
c
-axis direction or the (002) orientation.
Because B
2
is more ionic, it has a lower bonding energy than the other bonds. The highest
value of
k
t
2
and the PE stress constant are in the
c
-axis direction. Hence, when an acoustic
wave device is excited in the film thickness direction, the AlN film will grow with
c
-axis
orientation in the direction of film growth (normal to the bottom electrode) and enhance
piezoelectricity.
AlN Film Deposition
AlN film can be deposited using CVD or PECVD (Sanchez et al., 2008), PLD (Tabbal et
al., 2001; Tanosch et al.,
2006; Ishihara
et al., 2000; Liu et al., 2003), MBE (Kern et al., 1998),
sputtering (Mortet
et al., 2003, 2004; Auger et al., 2004; Clement
et al., 2003), and FCVA
(Ji et al., 2004). Of these technologies, MBE can grow a single-crystal epitaxial AlN film
but with limitations that include low growth rate, expensive instrument setup, and a high
process temperature. CVD, including MOCVD and PECVD, can be used to grow high-
quality crystalline AlN films, but its high process temperature (about 500 to 1000°C)
may be inappropriate for CMOS-compatible processes. Reactive sputtering methods can
deposit a good crystalline AlN thin film at a relatively low temperature (between 25°C and
