Microcrystalline Cubic Boron Nitride/Amorphous Hydrogenated Boron Nitride Mixed Phase Thin Films
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hydrogenated boron films were grown on the anode heated to 250C, using partial pressures of 3.5 mT of diborane and 396 mT of hydrogen, a self-bias voltage of 420V, and a rf power of 22W. The films on aluminum foil were immersed in dilute HCl, dissolving the Al; the free standing films were sent to Galbraith Laboratories for chemical analysis. The films grown on silicon were thinned and then mounted in a transmission electron microscope (TDE) for structural measurements. The films grown on silicon substrates were also used for infrared absorption measurements using a Perkin Elmer Model 1610 FTIR spectrophotometer. The films grown on quartz substrates were used for visible and ultraviolet absorption measurements on a Hitachi Model U3100 spectrophotometer. The thicknesses of our films were measured on a Alpha-step profilometer. And the adhesion of our films was crudely determined by the Scotch tape test and by observation of any peeling. RESULTS From the combustion analysis, the chemical composition for Samples 1 and 2 are listed in Table 1. Notice that both films had significantly more boron than nitrogen and that both had large hydrogen concentrations. The film with the higher boron to nitrogen ratio had a higher diborane to ammonia ratio in the feedstock. We attempted to grow a film with a concentration of boron closer to that of nitrogen by decreasing the diborane to ammonia ratio in the feedstock--Sample 3; unfortunately, with this feedstock, the film growth rate was zero. In contrast, at the higher diborane to ammonia ratios, the growth rates were about 100 nm/hour.
TABLE 1 Combustion Analysis Chemical Composition Sample 1 2
Boron (at.%)
Nitrogen (at.%)
49 44
28 33
Hydrogen (at.%) 23 23
From the TEM, we observed small regions of crystalline material, evidenced by electron diffraction rings. The integrated intensity of these rings as a function of radial distance is shown in Figure 1. The positions of the peaks in Figure 1 correspond to the lattice spacings of crystalline cubic boron nitride, as shown in Table 2. From the widths of the peaks in Figure 1, we can estimate that these cubic boron nitride crystallites are about 2.5 nm in size.
818
140 • 120 100 ,2
Cn
80
60 40 20 0
20
40
60
80
100
120
Distance (mm) Figure 1. Integrated intensity of the electron diffraction pattern of the boron nitride Sample 1. The horizontal axis is the radial distance from the center of the diffraction pattern on the photographic plate. The lattice spacings in Table 2 are calculated by dividing this radial distance into the camera length constant. TABLE 2 Diffraction Pattern Lattice Spacings Rmeasured (A)
2.175
1.90
1.33
1.135
0.85
R liturature (A)
2.09
1.81
1.28
1.09
0.83
The infrared absorption spectra of the boron nitride Sample 1 and the amorphous hydrogenated boron thin films are shown in Figure 2, curves a and b, respectively. The optical absorption spectra of the boron nitride Sample 1 and the amorphous hydrogenated boron thin films are shown in Figure 3. As for the crude adhesion tests, the bo
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