Theory and application of polysilicon resistor trimming

Abstract

The resistivity of polysilicon films are dramatically altered when subjected to high current densities via a technique known as pulse current trimming[1,2,8,10]. A new physical model for the mechanisms governing the observed resistance reduction and recovery is presented. The model views the grain boundary as a defect-laden transition region between grains of different crystal orientation. Severe temperatures induced in the polysilicon film during pulse current trimming lead to non-reversible restructuring of the grain boundary. This produces a more coherent transition between grains and a reduction of scattering centers leading to increased carrier mobility without a significant increase in free carrier concentration. The effect is aided by “liquid phase” segregation of dopant atoms at the grain boundaries which provides a smaller, reversible component to the resistance change. Results of the first comprehensive reliability study show the trimmed resistors to be exceptionally robust. Finally, circuit design considerations for manufacturability and ESD tolerance are presented.