SEM Studies on Laser Trimmed Resistors

Abstract

This paper reports the studies conducted using scanning electron microscope (SEM) as a tool for optimizing laser trim parameters leading to reliable trimming. The samples drawn from the regular production lots use DUPONT 1400 BIROX series resistor pastes with compatible conductor and dielectric compositions printed on Alsimag 614 substrates. Samples of the typical size of 5 mm × 5 mm prepared by laser cutting were observed in the secondary electron mode of SEM. Teradyne model W311 Q-switched YAG laser was used in trimming the resistors. A substrate surface removal of 3–5 microns which is necessary for the good quality' cut was confirmed in the cross-sectionally observed samples. Also uniform and well fused kerf edges which are another important aspect of reliable trimming were checked. This dielectric protection of kerf edges ensures the non-exposure of disturbed resistor material. Microcracks were found to be present at the end of the cuts but they seemed to have insignificant effect on the thermal stability of the trimmed resistors; the storage at 150°C for 1000 hrs. has brought about the drift (ΔR/R × 100) of less than 0.2% in case of L cut and less than 0.35% in case of double plunge cut. Further efforts are being made to find out the depth of these microcracks using energy dispersive scan.