A study of SRAM NBTI by OTF measurement

A mechanism of DeltaVth variation of NBTI for SRAM load transistor is examined. The variation data is in good agreement with our proposed model. A large fluctuation of NBTI for small size pMOS is not observed in HCI measurement for nMOS, which may be due to the difference of recovery. Two types of equations to get DeltaVth from DeltaIdL by OTF measurement are compared and we have concluded which equation is appropriate by using pulse IV method. We have also proposed a new method to predict precise median value of NBTI for SRAM by OTF measurement with parallel TEG. This method is very important to design initial Vth region of SRAM.