Innovative Annealing Apparatus for Mercury-Based, Compound Semiconductors

Infrared imaging systems using HgCdTe photodiode arrays are being developed to meet ever more stringent requirements. Thermal anneals are used for a number of purposes in HgCdTe technology: for reducing the Hg vacancy concentration by annealing at or near the Hg saturation pressure, for dopant diffusions under a specific Hg vapor pressure, for annealing after ion implants to diffuse the implanted element, and for activating implanted/diffused dopants. In the most commonly used anneal procedure, several HgCdTe wafers are sealed (using a glass-blowing torch) into an evacuated fused-quartz ampoule along with a small amount of Hg. Practical problems occur for HgCdTe wafers over about 5 cm in diameter. First, larger quartz ampoules become more difficult to seal under vacuum and the extended sealing time may lead to heating and damaging the HgCdTe wafers. Second, the cost of the quartzware destroyed in the process becomes more significant in larger sizes. Under an NVESD Phase I SBIR Contract we have developed and demonstrated a new open-tube anneal apparatus and process for HgCdTe wafers that avoids these problems, and which has a number of additional benefits, including the ability to rapidly switch anneal conditions for multi-step diffusion/activation anneals.