Fig. 6 Temperature Effects
Top down and cross-sectional SEM views of bulk etched Si at substrate temperatures of 227oC, 441oC, and 560oC. Note the anisotropy in the [100] and [010] directions in the top down views. Also, note how the sidewall taper angle gets smaller as the temperature increases in the cross-sectional views.

One important point to note about the trench etched using Cr (Figure 5) is the sidewall taper of about 10o. We did not observe this in our original results using CaF2 masked trenches etched at a substrate temperature of 500oC. This led us to believe that temperature might be an important factor in the taper angle of these trenches. To investigate this in more detail, we etched bulk Si at different temperatures, and looked at them top down and cross-sectionally with the SEM. Figure 6 shows the results. The top three SEM pictures show the etched Si top down at temperature of 227oC, 441oC, and 560oC. The three bottom SEM pictures show the corresponding cross-sectional view. From looking at the cross-sectional views of the bulk etched Si, it appears that substrate temperature does play an important role in the sort of taper angle formed. Qualitatively, the higher the temperature, the smaller the taper angle. Also, note the anisotropy in the [010] and [100] directions in the top down images. We are still in the process of researching the effect that temperature has on the formation of these trenches in greater detail.