Laser-Induced Resistance Fine Tuning of Integrated Polysilicon Thin-Film Resistors

In this brief, we present a novel polysilicon resistor trimming technique using a pulsed focused nanosecond laser at a fluence slightly lower than the melting threshold for polysilicon. Using this technique, we were able to trim a 4 μm ×40 μm Taiwan Semiconductor Manufacturing Company 180-nm n-doped polysilicon resistors with a 200-ppm precision. Much better precision is possible by using larger structures. The method can be applied to any CMOS process without any extra layer deposition or specific design restriction beside the fact that the laser beam must be able to reach the polysilicon structure. The high repeatability of the process allows an open-loop calibration. A complete characterization of the trimmed devices, including transverse electromagnetic and atomic force microscopy imaging as well as Raman spectroscopy, has been conducted, leading to the conclusion that a material restructuration in the grain boundaries of polysilicon, following laser irradiation, is responsible for the thin-film resistivity lowering. The stability of the polysilicon thin film, as tested by heating the device at 150°C during 1000 h, is about 1.3%, which is slightly higher than the 0.7% resistance variation for untrimmed thin films.