Laser trimming of thick film metal resistors on aluminum nitride substrates

Problems and their countermeasures in laser trimming of thick-film metal resistors on aluminum nitride (AIN) substrates are studied. Trimming was made possible by selection and combination of suitable Q-switching frequency, laser power, and number of irradiations. For example, under the conditions of 1 kHz, 2.0 W, and double irradiation, resistance values could be controlled to within +or-0.3% of target values. Resistance changes of trimmed metal resistors were less than +5.5% and comparable to those on alumina after thermal cycle tests (-55-150 degrees C, 1000 times), high-temperature storage tests (150 degrees C, 1000 h), and high-temperature and humidity storage tests (80 degrees C, 90% RH, 1000 h). Since the insulation characteristics of an irradiated portion strongly depended on the irradiation conditions, suitable conditions were selected for practical applications. The degree of insulation degradation after the laser irradiation was different for the two types of AlN substrates studied. This is attributed to the different amounts of free Al formed on the AlN surfaces. Further, AlN has a higher absorption factor and smaller reflectivity than alumina for a YAG laser beam. Thermal and electrical damage of AlN are affected by these properties.<
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