The scandium compounds were first evaluated, because the melting point of the scandium fluoride, 1552˚C, is the highest of the metal fluorides. As shown in Figure 3, the scandium oxide was exposed to the chlorine trifluoride gas at room temperature, 300˚C and 700˚C. Figure 3(a) shows the appearance of the scandium oxide before exposure to the chlorine trifluoride gas. It had a white powder appearance which did not change after exposure at every temperature, as shown in Figures 3(b)-(d). However, the decrease in its amount was considerable. At room temperature, 300˚C and 700˚C, the weight decrease was 39%, 7% and 21%, respectively. The percent decrease showed no obvious relationship to the temperature. However, the loss due to the reaction with the chlorine trifluoride gas was clearly recognized, because the change from scandium oxide (Sc₂O₃) to scandium fluoride (ScF₃) should cause a weight increase of about +47%. For clarifying the details of the materials change, the X-ray diffraction patterns should be studied.

Figure 4 is the XPS spectra showing the chemical bonding of the obtained material from the scandium oxide. As shown in Figure 4(a), fluorine was detected along with oxygen in the material obtained at room temperature. The contained oxygen was considered to be that not fully replaced with fluorine. After exposure at 300˚C, distinct fluorine peaks were detected; the oxygen peaks became significantly small, as shown in Figure 4(b). In Figure 4(c), the material obtained after exposure at 500˚C clearly showed the fluorine and scandium peaks, similar to those at 300˚C. This indicated that the percent of scandium fluoride in the material obtained from scandium oxide significantly increased at temperatures higher than 300˚C. The composition of the metal fluoride evaluated from the XPS is listed in Table 1. The percent of scandium oxide was less than 60% at room temperature; it increased to 99% at 300˚C and 500˚C.

Next, the scandium chloride was exposed to the chlorine trifluoride gas at various temperatures. The appearance of the scandium chloride is shown in Figure 5. Before exposure, the scandium chloride was a white powder, as shown Figure 5(a). The weight decreased after exposure to the chlorine trifluoride gas at room temperature, although the appearance seemed to have expanded, as shown in Figure 5(b). As shown in Figure 5(c) and Figure 5(d), the appearance and the weight at 500˚C and 700˚C changed similar to that at 300˚C. The weight decrease greater than 50% was significant, because the weight decrease from scandium chloride to its fluoride is theoretically only 32%.

Figure 6 shows the XPS spectra of the samples shown in Figure 5. After exposure at room temperature, the fluorine peaks were clearly observed, while there were no chlorine peaks, as shown in Figure 6(a). At 500˚C

and 700˚C, as shown in Figure 6(b) and Figure 6(c), respectively, chlorines contained in the source material did not remain, while distinct fluorine peaks were present. The concentration of the scandium fluoride was nearly 99% due to the process at temperatures higher than room temperature.

In order to compare the ease of fluorination, the fluorine to scandium ratio, F/Sc, is shown in Figure 7. When the scandium oxide was exposed to the chlorine trifluoride gas at room temperature, the F/Sc value was still near 1. At 300˚C, the F/Sc value increased to nearly 3, while that at high temperatures tended to be saturated at 3.

The scandium chloride showed an F/Sc value near 3 after exposure to the chlorine trifluoride gas even at room temperature. The exposure to chlorine trifluoride gas at 500˚C and 700˚C achieved the F/Sc value near 3. The F/Sc value of scandium chloride was entirely higher than that of scandium oxide. The fluorination of scandium chloride is concluded to be easier than that of scandium oxide. This difference might be due to the bonding energy. The bonding energy of scandium-oxygen is greater than that of scandium-chlorine.

Because the weight loss during the fluorination occurred for both the scandium oxide and scandium chloride, it might be due to the behavior of the intermediate compounds. Some of scandium fluorides [16] [17] produced during the fluorination might be volatile.

Taking into account the ease of fluorination for scandium chloride, various metal chlorides were studied. Magnesium chloride was exposed to the chlorine trifluoride gas at room temperature and 700˚C. Similar to the scandium chloride, the XPS results, shown in Figure 8, indicated that the fluorine peaks were distinct after exposure to the chlorine trifluoride gas at room temperature and 700˚C. As listed in Table 1, the concentration of magnesium fluoride in the obtained sample was 99% after the treatment both at room temperature and 700˚C.

As shown in Figure 9 and Figure 10, lanthanum chloride and barium chloride were easily fluorinated similar to the chlorides of scandium and magnesium. As listed in Table 1, the concentration of lanthanum fluoride and barium fluoride were both 99% after exposure to the chlorine trifluoride gas at room temperature and 700˚C.

As listed in Table 1, aluminum oxide and chloride were also exposed to the chlorine trifluoride gas at room temperature, 500˚C and 700˚C. The percent of aluminum fluoride was 99% even at room temperature for both the aluminum oxide and aluminum chloride. Strontium fluoride was not easily obtained from its oxide, because the concentration of fluoride was 73% even after exposure to the chlorine trifluoride gas at 700˚C.

Based on these results, various metal fluorides could be obtained even at room temperature using the chlorine trifluoride gas. As references, the oxides of strontium, lanthanum, barium, magnesium and aluminum were also fluorinated in this study. Similar to the scandium compounds, the fluorination of these oxides required higher temperatures than those for the chlorides.

The weight change from the source materials to the fluorides was reasonable for strontium chloride, lanthanum chloride, magnesium chloride, and aluminum chloride. Strontium oxide, lanthanum oxide and barium oxide might show small weight losses, while barium chloride, magnesium oxide and aluminum oxide showed significant weight losses.

Because the CVD process very often uses ambient hydrogen at high temperatures [1] , the obtained metal fluorides were heated in ambient hydrogen at various temperatures. The weight decrease after the heating in ambient hydrogen is listed in Table 2.

The fluorides of scandium, magnesium and aluminum had significantly decreased during heating in ambient hydrogen. In contrast, the fluorides of lanthanum and barium maintained their weights even at 1100˚C. This trend was totally independent of the source materials, oxide and chloride. Some fluorides directly obtained from the supplier showed exactly the same trend as that from the various fluorides produced in this study. Additionally, as listed in Table 2, the weight loss values measured after the second heating were the same as those after the first one.

Although the temperature of 1100˚C is lower than the melting points of scandium fluoride, magnesium fluoride and aluminum fluoride, their weight losses were considerable. The vapor pressure of scandium fluoride was reported in detail by Rinehart and Behrens [16] . Following their equation, scandium fluoride has about a 6 Pa vapor pressure at 1100˚C. The vapor pressure of aluminum fluoride is 133 Pa at 1238˚C [18] . Taking into account that the weight loss was similar to those measured in ambient argon in this study, the scandium and aluminum fluorides were considered to simply sublimate.

As shown in Table 2, the strontium fluoride produced from strontium oxide and the magnesium fluoride produced from magnesium oxide and chloride showed significant weight loss. However, these fluorides obtained from the suppliers showed negligible losses. Thus, some volatile intermediate species might be contained in the samples obtained using the chlorine trifluoride gas in this study. The weight loss of strontium fluoride and magnesium fluoride can be recognized to be similar to those of the fluorides of lanthanum and barium.