New sample preparation method for improved defect characterization yield on bare wafers

SEM-based defect characterization is a critical technology for wafer manufacturers and others using unpatterned wafers for process monitoring. One of the main drivers of this technology is the need to characterize increasingly smaller defects whose dimensions scale with the shrinking design rules of semiconductor devices. Light-scattering based inspection tools (e.g. KLA/Tencor 6200, SP1) are used to detect defects on the wafer surface and to output a file which contains the xy coordinates of defects relative to the wafer's alignment features. The wafer and defect file are then transferred to the SEM review tool. The defect file is transformed into the coordinate system of the SEM's xy stage in two steps: first an approximate transformation is performed based on the wafer's orientation on the SEM's stage, and then, after several defects have been located, a more accurate transformation is performed using two or more updated defect coordinates. Review of further defects then proceeds and may include high resolution imaging, cross sectioning, and chemical characterization by EDS. This above method can be tedious and somewhat unreliable. It depends largely on the accuracy of the defect file, which contains both systematic and random error. Searching is often required, and it is generally true that the smaller the defect, the more difficult it is to locate by SEM. In this paper, we will discuss the added value and drawbacks of employing a new sample preparation technique which uses precision surface marking and high accuracy defect mapping to minimize the difficulties of SEM-based defect review on unpatterned wafers.