DOWNWARD LASER TRIMMING OF THICK-FILM RESISTORS

In this paper a method of laser trimming thick film resistors to lower resistance, that is downwards, is suggested. It involves heating of certain regions of the resistor film in the form of tracks (to a temperature where certain transformation in the structure of the film takes place, like segregation of the constituents of the resistor material, but not high enough to evaporate the material) by the use of a laser beam of appropriate power density. The heated regions are found to become regions of very high electrical conductivity. It has been found that trimming tracks made perpendicular to the current flow direction are not effective in reducing the resistance. On the other hand, trimming tracks made along the direction of the current flow are found to be effective in decreasing the value of the resistance. It has also been found that the resistance decreases with an increase in length and width of the track. This paper provides data from computer simulations and measurements made on analog resistor networks that enable one to design trimming tracks.