Acoustic Monitoring of Nonuniformly Eroded PVD Targets

Material source disks (sputtering targets) used in the physical vapor deposition process exhibit nonuniform erosion profiles and thickness variations in the radial direction. Noninvasive prediction of the remaining life (end-of-life) on a sputtering target during its normal service is of practical interest in semiconductor manufacturing. Based on the piezoelectric generation/detection of acoustic waves propagating and signal processing techniques, a real-time process monitoring system for predicting the end-of-life of a sputtering target is proposed and implemented. The nonstationary signal processing approaches (the time-frequency (reassigned spectrogram) analysis, the cross-spectral density estimation method, and the cross-correlation technique) were employed in analyzing the acquired ultrasonic waveforms. The advantages and shortcomings of each approach in identifying the status of erosion profiles of the sputtering target are also discussed