Laser trimming of thick film resistors on aluminum nitride substrates

Problems, and their countermeasures encountered in laser trimming of thick film resistors on aluminum nitride (AlN) substrates were studied. Trimming was done in air by the selection of a suitable laser power (up to 1 W of YAG laser at the interface between the resistor and AlN). Under these conditions, resistance values could be controlled to within ±1% of their target values. Resistance changes of trimmed resistors were less than ±0.4% after a thermal cycle test (-55 to 150°C, 1000 cycles) and a high temperature storage test (150°C, 1000 h). Under the conditions generally used for trimming of alumina hybrid integrated circuits (YAG, 2 W, in air), resistance control was possible, but the insulation characteristics at the irradiated regions were degraded. When AlN substrates were directly irradiated under these conditions, the surfaces were also damaged. Damaged regions were fully covered with a fluid substance consisting mainly of Al₂O₃. Insulation degradation was attributed to formation of free Al, along with the Al₂O₃. However, under irradiation at low power, no insulation degradation was found, because the formation of free Al and Al₂O₃ was suppressed