Electrical properties of polycrystalline silicon

Sammanfattning: Polysilicon is used, among other materials, in today's integrated circuits. In this thesis, the structural and electrical properties' of polysilicon thin film resistors have been studied. The aim has been to improve the long-term stability and increase the knowledge concerning grain-boundary related phenomena. Several methods, which improve the long-term stability at least by a factor of two, have been proposed and investigated. A new model for the time dependence of the resistivity in the presence of electrical and thermal stress has been developed. From this, valuable information on the trap density and f-factor have been obtained. It was found that a small group of hydrogen atoms at the grain-boundary, with a concentration of about 1010 cm-2 and activation energies not exceeding 0.3 - 0.7 eV, was responsible for the resistivity drift. Heat treatments in the interval 150 °C to 350 °C also established the presence of weakly bound hydrogen. Blocking some of the dangling bonds with atoms having higher bond strength to silicon than hydrogen, such as phosphorus and fluorine, reduces the amount of weakly bound hydrogen and thereby improves the long-term stability. When using fluorine, the annealing temperature must not exceed 750 °C. A formation of BF complexes are shown to decrease the active carrier concentration. In the case of phosphorus doping, boron has to be added to obtain the desired resistivity. In such compensation doped polysilicon, the presence of BP complexes leads to an increase in the number of hole traps. Trace amounts of Ti and W have been found to improve the long-term stability. This was due to a retardation of the diffusion of hydrogen through the oxide, caused by an ability of Ti and W to change the relative amounts of atomic and molecular hydrogen. Semi-insulating polysilicon (SIPOS) has a lower temperature dependence (TCR) than polysilicon. A sub-oxide at the grain boundaries was shown to explain the experimental data. It not only influences the grain size, it also causes a shift from thermionic emission towards tunneling, with a reduced TCR as a result.