| Polycrystalline silicon films of 1500 A thickness and ion implanted with boron at various dosages above 10{dollar}sp{lcub}19{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar} have been characterized with respect to their conduction and stability properties. An experimental design of annealing temperatures and times was completed to compare the changes of the conductive properties with respect to the structural changes of the films. The carrier concentration was determined by Hall measurements and all grain sizes and distributions were determined by TEM. Strong correlations were found between the temperature coefficient of resistance (TCR) and the inverse grain size, the film resistance and the inverse grain size, and the device electrical stability to current exposure and the device power density. The film resistivities were well modeled as a composite of the grains and grain boundaries. The change of the TCR with respect to the process and grain growth was interpreted to be an indication of the negative TCR nature of the grain boundaries. The device stability was related to the Joule heating of the sample as current was passed through it. When a critical current density was approached, the increased film temperature permanently altered the films resistance. In all cases, the conductive nature of the doped polysilicon became more metallic suggesting a change in the grain boundary properties. Currents below the critical value did not change the resistance of the films but the measured Joule heating at nominal current densities of 1 ma/{dollar}mu{dollar}m of contact width was sufficient to cause early contact wearout due to an enhanced electromigration rate of the contact metal. |
