Improvement of silicon wafer minority carrier lifetime through the implementation of a pre-thermal donor anneal cleaning process

In order to accomodate ever smaller device geometries, Si wafer quality requirements have become increasingly stringent. Si wafer minority carrier lifetime or diffusion length has become a routinely required parameter. It is well known that, in addition to crystal growth, metal contamination is a major limiting factor for Si wafer minority carrier lifetime. Optimization of the Si wafer manufacturing process flow is critical for minimization of metal contamination sources during processing. In the past, we have learned from device manufacturers that low device yields have been directly related to poor minority carrier lifetimes, or low diffusion lengths. In this paper, we show that, without a pre-thermal donor anneal cleaning process, the minority carrier lifetimes in CZ Si wafers could be degraded due to Fe incorporation during the thermal donor anneal (TDA) process. The TDA process eliminates the impact of oxygen-related thermal donor defects on wafer resistivity. Typically, the TDA process is carried out at relatively low temperatures, typically ~650°C for 20-30 minutes. Due to the low Fe solubility in Si at this temperature, less attention was paid to the Si wafer surface conditions, mainly metal contamination levels, prior to the TDA process. However, in this paper, we show that the TDA process, even at 650°C, has significant impact on minority carrier lifetime. In order to maintain the lifetime value at the “crystal” level, it is critical to implement an effective cleaning process to remove metal contamination from the Si wafer surface prior to TDA processing