Thermal stability of polysilicon resistors

The stability of polysilicon sheet resistance was studied as a function of conventional furnace anneal (CFA) and rapid thermal anneal (RTA). Polysilicon films of 100, 200, and 300 nm thickness were doped with boron, borofluoride, arsenic, or phosphorus and subjected to a CFA anneal matrix simulating the thermal cycles of a digital base BiCMOS process. RTA was introduced at the beginning or the end of the CFA anneal cycle to study its effectiveness in stabilizing the sheet resistance. CFA temperature was varied from 550 to 900°C, and RTA temperature from 900 to 1100°C. Throughout this range, it was found that the resistance depends primarily on the final anneal temperature and duration. The resistance increases substantially when the samples are subjected to a final low temperature anneal cycle, and decreases dramatically when followed by high temperature RTA. This was observed for arsenic- phosphorus-, and boron-doped films. It is suggested that the duration of cooling plays an important role in determining the nominal film resistance, thus explaining the efficiency of RTA in reducing and stabilizing the resistance. The results are compared with available models on dopant redistribution and carrier transport in polysilicon films