A comparative electron spin resonance (ESR) study was performed on thermal (111) Si/SiO2 and (100) Si/SiO2 of the vacuum postoxidation-induced interface degradation in terms of interfacial trivalent Si dangling bond creation (ESR-active P-b, P-b0, and P-b1 defects). In (111) Si/SiO2, the degradation mechanism was isolated as pronounced permanent P-b (circle Si=Si-3) creation from similar to 640 degrees C onward in densities N-c monotonically increasing with anneal temperature; at similar to 1100 degrees C, about 1.1 x 10(13) P(b)s cm(-2) are created in addition to the as-oxidized state value N-o similar to 4.9 x 10(12) cm(-2). The (100) Si/SiO2 interface is found to be much less vulnerable. Only electrically harmless P-b1's are additionally created, the density reaching N-c(P-b1) similar to 4.4 x 10(12) cm(-2) at similar to 1100 degrees C. By contrast, the density of the electrically adverse P-b0 trap tends to decrease. Together with the recently established electrical irrelevance of P-b1, the results add to provide a fundamental reason for the preference of the (100) Si face in devicing.