No abstract

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.

Oxygen, boron, and phosphorus additions to polysilicon films were followed up to 23% oxygen. Above 500 ⍀/ᮀ, oxygen resulted in temperature coefficients of resistivity ͑TCR͒ magnitudes a factor of two smaller than without oxygen. For p-type films, the TCR saturated at ϳ0.05% for low resistivities. Surface energy considerations show that the oxygen atoms are likely to attach to the surface of the growing grains. This explains the dependence on oxygen concentration of grain size, boron segregation, and tunneling barrier. A model for polysilicon resistivity was used to study changes with added oxygen and dopants. The potential barrier was followed down to a saturation region. The latter was found to be independent of oxygen, but to depend on carrier concentration and type according to the U-shaped trap distribution in oxygen-free films. This is also responsible for the saturation in the TCR. The f factor showed temperature-independent tunneling to gradually outweigh the temperature-dependent contribution from the potential barrier. A dopant-dependent f factor showed that dopant scattering begins to dominate over tunneling. Boron segregation in oxygen-rich films was 30-35%. Boron gave rise to a slight increase in grain size and phosphorus gave rise to a large increase. There were significantly more charged traps in films containing oxygen, where they exceeded 10 13 cm Ϫ2 .The temperature coefficient of resistivity ͑TCR͒ in polysilicon is negative and quite large for resistivity values characteristic of, for example, bipolar circuits. This is not desirable since it puts an extra burden on the design work. In extreme situations, it can also cause thermal runway of the resistor. A zero TCR is possible, but only in the resistivity range of 100 to 300 ⍀/ᮀ. 1,2 The design of resistors in bipolar circuits is often based on resistivities around 1000 ⍀/ᮀ. For such resistors, a zero TCR can only be achieved with low resistivity polysilicon and an increased length of the resistor, which wastes valuable silicon area. Kim and Choi 3 investigated ways of overcoming this limitation by means of compensation doping. However, they found that compensation-doped resistors have the same TCR as single-doped resistors. Laser recrystallization has also been tried 4 based on the assumption that the large negative contribution to the TCR from the grain boundaries is reduced in films with larger grains. Unfortunately, this method does not lend itself easily to volume production.TCRs lower than those in polysilicon can be obtained by means of semi-insulating polycrystalline silicon ͑SIPOS͒ films. 5,6 These semi-insulating, or oxygen-rich, polysilicon films can be deposited in the same way as polysilicon simply by adding N 2 O to the SiH 4 gas flow. The amount of oxygen in the film is varied by using different N 2 O/SiH 4 ratios. Lai et al. 7 were able to show that in undoped, high resistivity films, the temperature sensitivity could be lowered by more than an order of magnitude compared to polysilicon. Ong et al. 8 studied the temperature depen...