A new test structure for in-depth lifetime profiling of thin Si epitaxial layers

A test structure is used to measure the recombination lifetime along very thin Si epitaxial layers used for bipolar technology. The measurement technique makes use of AC signals induced in the p/n diode by means of a bias modulation through a third (control) terminal. The experimental results show the ability of the technique to give detailed information on the quality of epi layers. Results show that the lifetime profile is not strictly correlated with the doping profile, and it can be largely altered by processes that do not change the doping profile significantly. The lifetime values detected in these layers are lower than the values reported on thicker epi layers of the same doping range, suggesting that degradation due to the proximity of the lower interface may take place in very thin layers.