Abstract
A study has been done on hydrogen diffusion into poly-Si load resistors in high density static random access memories (SRAMs) from plasma-enhanced chemical vapor deposition (CVD) silicon nitride film. For the first time, it was found that the resistance of load resistors in 1 Mbit SRAMs increases in proportion to the time during high temperature duration testing, resulting in time dependent retention current decrease. This phenomenon is due to the decrease in trap density at the poly-Si grain boundaries, which in turn is caused by continual hydrogen diffusion from plasma-CVD nitride film. It was also confirmed that hydrogen diffuses through not only upper-side dielectrics but also back-side dielectrics and/or the poly-Si/dielectric interface. Furthermore, from these experimental results, a empirical equation is proposed for the resistance change, and the increase in resistance over 10 years is predicted to be 20% at 125° C.
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