ELECTRICAL TRIMMING OF ION-BEAM-SPUTTERED POLYSILICON RESISTORS BY HIGH-CURRENT PULSES

Phosphorus doped polysilicon resistors have been fabricated from microcrystalline silicon films which were deposited by ion beam sputtering using an argon ion beam of diameter 3 cm, energy 1 keV and current density 7mA/cm2, with a deposition rate of 100-120 angstrom/min. The resistor's, having a sheet resistance of 70 OMEGA/square and a carrier concentration of 7.5 x 10(19) cm-3, were stressed with current pulses of width 10 mus and duty cycle 0.6% for 5 min. There was a steady decrease of resistance with increasing pulse current density above a threshold value 5 x 10(5)A/cm2. A maximum fall of 27% was observed for a 95 mum long resistor. The current-voltage characteristics were also recorded during the trimming process. The trimming characteristics were simulated using a small-signal resistivity model of Lu et al. [11] and the I-V characteristics by a large-bias conduction model [12]. A close fitting of the experimental data with the theoretical values needed an adjustment of some grain boundary parameters for the different pulse current densities used for stressing. The nature of variation of the grain boundary parameters indicates that the rapid Joule heating of the grain boundaries due to current pulses passivates the grain boundary interfaces, at lower currents above the threshold, and then, at higher values of currents, causes zone melting and gradual recrystallization of the disordered boundary layers and subsequent dopant segregation. It confirms the mechanism suggested in the physical model of Kato et al. [7]. The role played by the field-enhanced diffusivity and electromigration of dopant ions, due to the high instantaneous temperature of the grain boundaries, has also been discussed. The pulse trimming technique is simple and does not cause damage to the adjacent components on a monolithic chip.