Spatial profiling of electron traps in silicon nitride thin films

Abstract

A method is described for determining the spatial profiles of electron traps in dielectric thin films. The method is an extension of avalanche injection and charge centroid measurements. By determining the change in the charge centroid and the injected charge after a sequential series of pulses, the densities of electron traps as a function of depth in both low-pressure-chemical vapor deposited (LPCVD) and plasma-enhanced-chemical vapor deposited (PECVD) silicon nitrides were determined. Contrary to previous assumptions of a uniform trap density in the nitride, both nitrides exhibit interface trap densities extending 10–15 nm into the film that is between 6 and 15 times larger than the bulk trap density of 0.5–2×1018 cm−3. The trap capture cross section was determined to be 6–10×10−13 cm2. The interface trap density of commercial LPCVD nitride deposited at higher temperature was higher than that found for PECVD nitride. The spatial resolution and limitations of the profiling technique, avalanche injection and charge trapping were modeled by numerically solving equations describing charge trapping, current continuity, and electric field. Important issues such as the effects of field, trap density and detrapping on the profiling analysis are examined both experimentally and through the use of simulation.