Identification and sizing of particle defects in semiconductor-wafer processing

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An experimental study of the sizing of submicron particles on semiconductor wafers is presented. The objective of the study was to determine the accuracy of a state-of-the-art optical wafer scanner, by comparing its size response with that of two well established techniques, namely, differential mobility analysis (DMA) and secondary electron microscopy (SEM). Test particles used include polystyrene–latex spheres and SiO2, Si3N4, W, and Cu particles of 64, 107, and 202 nm nominal (DMA) sizes. The scanner-indicated sizes using the oblique and normal operational modes were compared to the DMA sizes and to average sizes determined from the SEM images. Results show that the scanner-indicated size is comparable to the SEM-indicated size. However, both the scanner size and SEM size exhibit large distributions for a given nominal (DMA) size. The discrepancies are due to a combination of factors such as the effects of particle shape and material, and the presence of larger, doubly charged particles among those analyzed. A comparison of the normal and oblique scanner operational mode results for the scanner further show that the normal mode undersizes particles somewhat. These results collectively indicate that accurate sizing of complex and unknown submicron sized particles on semiconductor wafers is not an easy task. A thorough understanding of the scanner response to a variety of process particles is essential for the meaningful interpretation of scanning results.