High quality thick epitaxial films for power semiconductor devices

Abstract

High quality epitaxial layers with low defect levels are the key to fabrication of high voltage power devices. Growth of 100 μm thick epitaxial layers has been performed using a thermally driven chemical vapor deposition process. Three important parameters that have significant influence on epitaxy quality have been identified: substrate surface condition, reactor system cleanliness, and deposition process. The prerequisite p- n- p structure of a 6 kV thyristor was fabricated using 100 μm thick epitaxial layers to form the p-base. Defect density (hillocks and dislocations) in epitaxial layers has been correlated with the breakdown voltage of the p- n- p structure. It was found that an order of magnitude improvement of the defect level was obtained using well polished substrates instead of poorly polished substrates. Further improvement was achieved with the use of an etched reactor system before epitaxial growth, resulting in the reduction of defect density by another order of magnitude. A new cycled process, consisting of successive H 2 purges and deposition steps, is proposed that effectively reduces the defect level by an additional factor of 4, as compared with the conventional continuous deposition process. Specular epitaxial layers without spikes were obtained. Experimental results showed that for thick epitaxial films (100 μm), a susceptor with round-bottomed depressions provides higher dislocation density than one with flat-bottomed depressions, leading to a lower breakdown voltage of test devices.