Backside-activation technique of power device IGBTs by a microsecond-pulsed green laser

Abstract

Rccently ion-implantation technique and activation technique are emphasized to enhance the performance of Insulated Gate Bipolar Transistor (IGBT) replacing epitaxial wafers with FZ wafers. In the IGBT's structure, an n-type layer and a p-type layer are introduced into the back side of the wafer at the different depth to form PN junction. The next generation IGBTs whose wafer thickness decreases to few tens of micrometers, require not only the high activation of these two layers without interdiffusion of boron (B) and phosphorous (P) but also the deep activation without damaging the metal circuit wired on the front side of the wafer. Laser annealing processes have been focused as low thermal budget heat treatment to fulfill these demands. In this paper, new activation technique is demonstrated to accomplish high activation in the deep area over a depth of 2um. B and P implant wafers were irradiated by a single microsecond-pulsed laser, i.e., a green DPSS laser (JenaRas ASAMA, controllable pulse duration of 250ns to 1200ns). The laser annealing process at a pulse duration of 1200ns enabled the deep activation over a depth of 2um and the sufficient activation ratios of 89% and 62% for B and P dopants, particularly on the thin wafer of thickness 150um. We conclude that the ADL annealing system can be applied to the backside activation process for the next generation IGBRs, adopting a thin FZ wafer of less than 100um.