Evaluation of Stress Effects on Electrical Characteristics of N-Type MOSFETs: Variations of DC Characteristics During the Resin-Molding Process

Stress-induced changes in the electrical characteristics of a semiconductor device become a major concern in the production of semiconductor packages because the electrical characteristics are adversely affected by packaging (residual) stresses. The objective of our project is to evaluate the effects of stress on semiconductor devices. In this study, the shift of the DC characteristics of nMOSFETs during the resin-molding process was investigated experimentally. After a silicon chip including the n-type metal oxide semiconductor field effect transistors (nMOSFETs) was encapsulated in a quad flat package, the drain current variations and the transconductance shifts were measured. The drain current decreased during the resin-molding process while no significant shift in threshold voltage was observed. The experimental results were estimated adequately from the residual stress predicted by numerical and experimental analyses and from the stress-sensitivity of the nMOSFETs measured by the four-point bending method. Also, we tested the validity of an electron-mobility model that included the effect of stress. The electron-mobility model takes into account the variation in the relative occupancy of the electrons in each conduction-band energy valley. It was found that the effect of biaxial stress on the variation in electron-mobility can be qualitatively evaluated by the electron-mobility model but are quantitatively different from the experimental results. Several needed improvements to the electron-mobility model are proposed in this article.