Thermal stability of polysilicon resistors

The stability of polysilicon sheet resistance (R_s ) is studied as a function of conventional furnace anneal (CFA) and rapid thermal anneal (RTA). Polysilicon films of 100-nm, 200-nm, and 300-nm thickness are doped with boron, borofluoride, arsenic, or phosphorous, and subjected to a CFA anneal matrix simulating the thermal cycles of a digital-base BiCMOS process. RTA is introduced at the beginning or the end of the CFA anneal cycle to study its effectiveness in stabilizing the sheet resistance. CFA temperatures are varied from 550°C to 900°C, and RTA temperatures from 900°C to 1100°C. R_s is not only sensitive to anneal temperatures and duration, but also to the sequence in which the different thermal steps are introduced. A furnace anneal as low as 550°C, following a furnace activation anneal of 900°C or RTA at 1000°C, results in a dramatic increase in R_s. Then, subjecting the film to RTA at 1000°C for five seconds recovers R_s to an extent which depends on dopant species and thermal history. The final thermal cycle in the sequence has the most dramatic effect on sheet resistance