PULSE CURRENT TRIMMING OF POLYSILICON RESISTORS

The practical application of pulse current trimming of polysilicon res istors has been investigated and successfully implemented in large sca le integrated circuit production. In-package pulse current trimming of heavily doped polysilicon resistors allows precise control of the fin al resistor value and can effectively compensate for process variation in polysilicon sheet resistance. The technique requires no additional process complexity, is layout efficient, remarkably accurate, and is quick and inexpensive from a test perspective. Resistance reduction oc curring during the trim process is shown to be reversible to a small, but usable, extent for n-type polysilicon. Thermal modeling of the res istor trim process shows that the peak temperature reached within the polysilicon film must exceed the highest temperature encountered durin g wafer fabrication before any permanent resistance change occurs. As the resistor is further trimmed, the film temperature approaches the m elting point of silicon.