Biomedical Engineering Reference
In-Depth Information
(4) ZnO thin films can be deposited at low temperature (<200°C), which is compat-
ible with post-processing on CMOS circuitry. However, high-temperature deposi-
tion enhances the atom mobility, decreases the defects, promotes film adhesion to
the substrate, improves film texture, quality, and increases grain size, resulting in
compact and dense film structure.
(5) The sputtered film normally exhibits a good PE effect. Thus, post-deposition pol-
ing to obtain a good PE effect can be unnecessary.
Film Texture
The film texture of the ZnO film is crucial for the PE and acoustic wave properties of the
acoustic devices with the substrate having significant influence on nucleation, growth,
texture, acoustic wave velocity/frequency, and electromechanical coupling coefficient of
the ZnO acoustic wave devices.
Self-Texture and Wave Mode
ZnO normally crystallizes in a hexagonal or wurtzite type crystalline structure
(
Özgür
et
al
., 2005) (see Figure 8.8), which is dominated by three crystal planes: (0001), (
1010
), and
(
1120
), with surface energy density of 0.099, 0.123, and 0.209 eV/Å
2
, respectively (Fujimura et
al., 1993). The (0001) plane has the lowest surface-free energy. Therefore, under equilibrium,
if there is no epitaxy between the film and substrate, or without any external energy source,
the films exhibit self-texture and grow along the (0001) orientation on both crystalline and
amorphous substrates (Koch et al., 1997). However, as the film thickness increases, other
orientation peaks may appear and become stronger (Lee et al., 1998). Excess Zn during film
growth can cause the deterioration of the ZnO film crystallinity (Chiu and Liu, 2003). The
O
2
/Ar gas ratio and gas pressure also have significant effects on the film stress and texture.
ZnO SAW devices with a (0001) film texture can be used for sensing in air or gaseous
environments. However, many biosensors need to detect chemical reactions in a liquid
(0001)
Zinc
Oxygen
(1010)
(1120)
FIGURE 8.8
ZnO crystalline structure-Wurtzite structure with directions of (0001), (
1120
), and (
1010
) indicated. Lattice con-
stants are
a
= 3.25 Å and
c
= 5.2 Å (http://upload.wikimedia.org/wikipedia/commons/8/8e/Wurtzite_poly
hedra.png).
