Biomedical Engineering Reference
In-Depth Information
development to the deposition parameters. Moreover, the polycrystalline morphology has
been related to Raman spectroscopy and x-ray diffraction measurements. An excellent
review has been given by Zhu et al. [52]. A morphology field map has been presented [53]
that depicts different zones of surface texture of diamond films as a function of methane
concentration in hydrogen and substrate temperature. Tang et al. [54] reported that the
film of best quality has very smooth crystalline facets free of second nucleation, and the
full width at half maximum of the diamond Raman peak is 2.2 cm
-1
, as narrow as that of
IIa natural diamond. Larson and Girshick [55] concluded increasing substrate temperature
causes the film morphology to shift from {100} toward {111} faceting. Although a number
of different chemical vapor deposition (CVD) techniques have been developed to deposit
diamond films, the study of the factors controlling the morphology, quality, and related
properties of the deposited films is far from complete [56,57]. Among the multitude of
diamond CVD processes that have been developed for about 20 years, the microwave
plasma-enhanced CVD process (MPECVD), together with the hot-filament CVD method
(HFCVD), has continued to be one of the most used diamond deposition processes. This
section compares the impedance properties of polycrystalline films deposited using both
MPECVD and HFCVD methods.
Free-standing diamond films grown by microwave plasma-enhanced CVD and hot-
filament CVD were used to investigate the influence of the film quality on the impedance
spectrum.
SEM photographs of investigated films are shown in Figure 4.13. MPECVD diamond
film shows a well-faceted and polycrystalline morphology with (111) triangle crystal ori-
entation dominating. However, HFCVD diamond shows a rather rougher surface than
MPECVD with the merging surface of different crystal orientations. Rounded and layered
morphology was also observed for HFCVD diamond. The corresponding Raman spec-
tra are shown in Figure 4.14. MPECVD diamond has a sharp peak at 1331 cm
-1
, which is
characteristic of high-purity diamond and a tiny shoulder at about 1580 cm
-1
, which is
the signal of graphite phases embedded in this sample. The HFCVD film shows a typi-
cal diamond peak at 1331 cm
-1
as well, but the intensity of this peak is much lower in the
HFCVD diamond than in the MPECVD diamond. In addition, there is a broader shoulder
(a)
(b)
~200 µm
~100 µm
FIGURE 4.13
SEM photographs of MPECVD diamond film (a) and HFCVD diamond film (b). (From Ye, H.T., PhD thesis,
University College London, 2004. With permission.)
