申論題

 10. You are the intern of TSMC and in charge of a CMOS fabrication line. One day you have a problem of threshold voltage shift in your transistors. Assume the threshold voltages of n-and p-channel MOSFETs, which are supposed to be +1 V and -1 V, respectively, are turning out to be +3 V and +1 V, instcad. You suspect that the interface between the silicon and the 20 nm thick oxide is contaminated with ions.

(b) What could be the sign of the ions, positive or negative? Explain your answers. (5%)