Photoexcitation Effects During Laser Trimming of Thin-Film Resistors on Silicon

Laser trimming of thin films on silicon is normally done with a Q-switched yttrium aluminum garnet (YAG) laser. Pulses from such a laser consist of photons which are sufficiently energetic to create electron-hole pairs in the silicon. The excess carriers created by the laser may then alter the electrical behavior of silicon devices which are in proximity to the thin films. An integrated circuit amplifier chip was used to investigate laserinduced photoexcitation effects on various silicon devices. For silicon resistors substantial conductivity modulation was observed under laser illumination. For active devices laser light alters the electrical characteristics for the duration of the effect. In all cases, excess carrier recombination occurred in several tens of microseconds--a period much shorter than the normal interval between laser pulses. Based on this investigation it is clear that in trimming, active devices tracking techniques (whereby parameters are continuously monitored) cannot be used. Measure and predict routines must be developed for trimming active thin film on silicon devices. However, when employing these routines, delay times necessary to eliminate photoexcitation effects are likely to be much shorter than the actual measurement times, so that the effect should not lengthen trimming times.