Determination of the maximum voltage for product screening stress based on TDDB and HC measurements

Sometimes it is desired to do a short voltage stress test on an amount of product chips in order to screen weak devices prior to shipment to the customers. The stress usually involves an increased supply voltage > Vddmax and an elevated temperature. Such a screening stress has the potential to seriously deteriorate the final product lifetime and must be conducted very carefully. A simple and fast method is required to determine the critical maximum voltage, rsp. field that may be applied without jeopardizing the product lifetime at the customer. In this work the impact of such a screening voltage on main reliability numbers as a) the gate oxide TDDB lifetime and b) on the hot carrier (HC) lifetime is investigated and expressions are derived to calculate the lifetime of the stress survivors. For both degradation mechanisms the maximum screening-stress field is calculated from TDDB and HC lifetime models fitted to the measurement data. Finally a comparison is made between the measured impacts of a short stress on TDDB lifetimes with the calculated one based on the according lifetime model.