Carrier lifetime measurements using free carrier absorption transients. II. Lifetime mapping and effects of surface recombination

Abstract

A novel noncontact technique for semiconductor wafer mapping of the charge carrier lifetime is reviewed. The principle is based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. The technique is demonstrated here for Si wafer lifetime mapping, both for homogenous wafers as well as for wafers having pn junctions, and the performance and limitations are addressed. In a companion article J. Appl. Phys. 84, 275 (1998), (part I), the injection dependence is treated in more detail. We demonstrate the broad range of accessible injections, allowing on-wafer lifetime mapping both of the minority carrier lifetime and of the high injection lifetime as well as of Auger recombination effects. Furthermore, problems of surface recombination for bare Si surfaces are elucidated while pn junctions or other barriers are shown to suppress diffusion of carriers to surfaces. Also, a scheme for low temperature surface passivation is demonstrated and the resulting surface recombination velocity is derived. Finally, a few examples of characteristic lifetime maps from processed samples are discussed. We conclude that the technique is one of the few available for monitoring of the carrier lifetime during a full device process sequence.