In addition to exacting requirements regarding uniformity, precision and matching, thin-film polysilicon resistors for analog telecommunication circuits must be extremely stable. In this paper a new method for improving the stability of polysilicon resistors is presented. It utilizes the strength of the Si-F bond to prevent hydrogen and other impurity atoms from attaching to the dangling bonds in the grain boundaries and is based on individual control of implantation dose and annealing conditions for dopant and fluorine, respectively.
Polysilicon films, 500 nm thick, were implanted with boron at a dose of 51014 cm-2 and annealed at 1000 °C. This was followed by an implantation of fluorine and another anneal. The fluorine dose ranged up to 1·1016 cm-2 and the annealing temperature was varied between 600 °C and 1000 °C.
Resistivity measurements showed that the fluorine annealing temperature has to be higher than 700 °C to reduce implantation damage. The sensitivity of the polysilicon films to hydrogen, an important indication of their electrical stability, was investigated by annealing both in a hydrogen plasma and in a molecular hydrogen ambient. For low hydrogen sensitivity, the fluorine annealing temperature has to be less than 750 °C. The temperature was therefore set to 750 °C so as to strike a balance between a low hydrogen sensitivity and the need for a reduced implantation damage. SIMS measurements showed the amount of fluorine in the film to decrease by two orders of magnitude if the fluorine annealing temperature was 1000 °C, and to remain essentially constant if the temperature was 750 °C. Stress tests were made in order to see the effect of fluorine on the resistivity during actual operating conditions. The stability of the resistors during the stress tests was found to improve by at least a factor of two when fluorine doping was used.