Fast detection of mobile ions for WLR monitoring

A new fast electrical technique for the detection of mobile ions in silicon processes using a simple field oxide test structure is presented. The method is ideal for WLR monitoring of mobile ions since its throughput is much higher than conventional slower methods such as the BTS (bias temperature sweep) method. The technique is based upon biasing a field oxide FET in a configuration resembling a source-follower, and observing the time varying output voltage. A simple model based upon ideal MOSFET characteristics is used to interpret the voltage waveforms produced by the new fast method. Data from about 30 wafer lots is presented, and a comparison made between the new method and a HTBS (high temperature bias sweep) measurement-a modified version of the BTS method. The correlation was found to be greatly improved by introducing a short bake to diffuse the mobile ions (suspected to be Na) into the region of the field oxide near the channel.