Monitoring thickness, resistivity and grain structure of electroplated copper films with laser-based surface wave metrology

An optical technique based on the laser generation of surface acoustic waves (SAWs) allows fast non-contact monitoring of ECD copper thickness on product wafers. In this work, we extend the technique to monitoring the variations in copper resistivity and grain size by extracting additional information from the signal waveform. In addition to the SAW frequency, which is used to determine the film thickness, we measure the decay-time of the laser- induced thermal grating in order to monitor the electrical conductivity of Cu via its thermal conductivity. At the same time, we measure the attenuation of high-frequency (∼1 GHz) SA Ws in order to monitor variations in the grain size. Measurements conducted on a set of blanket ECD Cu wafers deposited at different plating current are found to be in good agreement with reference techniques such as 4-point probe and orientation imaging microscopy (OIM). The capability of the surface wave technique to detect both wafer-to-wafer and within wafer variations in thickness, resistivity and grain size is demonstrated.