Negative Bias Temperature Stress on PFETs within fast Wafer Level Reliability Monitoring

The challenges of measuring by means of fast WLR the Vt degradation caused by temperature bias stress are discussed in this work. Two methods, the fast two point measurement with smart intermediate stress (SIS) and the back-extrapolation based on measuring the recovery curve are compared. Considering the properties of the test equipment an adjusted SIS approach is implemented in order to get an equivalent method to the stress interruption free OTF-method. Nevertheless recovery causes a stronger degradation vs. time dependence at short times than at long times leading to the implementation of the back-extrapolation of the recovery curve. This method requires the implementation of a curve fitting algorithm like the Levenberg-Marquardt algorithm in the fWLR stress routine for data evaluation upon completion of the stress. The method was tested on three PFET types with different oxide thickness and in all cases shows reasonable results for various stress conditions.