Innovative Annealing Apparatus for Mercury-Based, Compound Semiconductors

Infrared photovoltaic imagers based on HgCdTe are being developed to meet ever more stringent requirements. Anneals are used for reducing the Hg vacancy concentration by annealing at near Hg saturation pressure, for diffusions under a specific Hg vapor pressure, for annealing after ion implants to diffuse the implanted element and for activating implanted dopants. In the most commonly used anneal procedure several wafers are sealed (using a glass-blowing torch) into an evacuated fused quartz ampoule along with a small amount of Hg. The practical problems that occur for wafers over about 5 cm in diameter are first of all that larger quartz ampoules become more difficult to seal under vacuum and the extended sealing time may lead to heating and damaging the wafers. Also, the cost of the quartzware that is destroyed in the process becomes more significant in larger sizes. A new anneal process is required that avoids these problems. The difficulty at first appears to be that Hg vapor must be contained by a seal that is itself at the same temperature as the wafers, in order to prevent Hg vapor from condensing or escaping. This requirement can, however, be avoided by use of our new technology.