Biomedical Engineering Reference
In-Depth Information
ickness = 3.10 µm
RMS = 11.12 nm
ickness = 0.8 µm
RMS = 0.37 nm
84 nm
4.4 nm
0 nm
0.0 nm
y
: 5.0 µm
x
: 5.0 µm
y
: 5.0 µm
x
: 5.0 µm
ickness = 5.5 µm
RMS = 34.50 nm
ickness = 6.6 µm
RMS = 44.98 nm
0.21 µm
0.28 µm
0.00 µm
0.00 µm
y
: 5.0 µm
x
: 5.0 µm
y
: 5.0 µm
x
: 5.0 µm
FIGURE 8.6
AFM images of surface morphologies for different thicknesses. (From Du, X.Y., PhD thesis, University of
Cambridge, 2008. With permission.)
post-deposition annealing that reduces the number of defects by providing the thermal acti-
vation energy required for the defects states to move to a lower-energy crystalline phase.
The effects of the processing parameters, such as the sputtering gas pressure, RF power,
total flux density, bias voltage, and substrate temperature, have been successfully described
using modified Thornton models (Kluth et al., 2003). Figure 8.7a and b shows two examples
of the modified Thornton zone model using a total energy flux density,
E
Φ
, instead of the
commonly used gas pressure (Kluth et al., 2003; Tvarozek et al., 2007). The total energy flux
density
E
Φ
(W/m
2
) can be expressed by the parameters of RF power, deposition rate, volt-
age on target, gas pressure, and substrate bias voltages. The ratio of the total energy flux
density
E
Φ
and its minimum value
E
Φ
min
, specified by the sputtering mode and the geo-
metrical arrangement of the sputtering ZnO, can clearly describe the effects of RF power,
gas pressure, and substrate voltages (see Figure 8.7a).
Several general conclusions about the effects of sputtering parameters on the growth of
ZnO thin films are summarized as follows:
(1) Higher plasma or bias powers result in a higher deposition rate because the depos-
ited particles have higher kinetic energies. However, the film surface roughness
can increase significantly at a higher power, due to ion bombardment.
(2) Low gas pressure generally results in a dense and fine grain film. Higher gas pres-
sure can result in porous, columnar films with rough surfaces (Zhu et al., 2000).
(3) The O
2
/Ar ratio is a critical parameter, and a sufficient oxygen partial pressure is
needed to maintain the stoichiometry of the ZnO films.
