Patent ID: 7519216

Claim:
A method comprising: receiving a first set of wafers, the first set of wafers being part of a first lot; generating first image data by placing each wafer of the first set on a platform and receiving radiation from the wafer on the platform; receiving a second set of wafers, the second set of wafers being part of a second lot; generating second image data by placing each wafer of the second set on the platform and receiving radiation from the wafer on the platform; detecting, using a processor, whether abnormal first image data exists at a first common location for each of the last N wafers of the first set to be placed on the platform, excluding any abnormal image data at a location for wafers that previously had abnormal image data at the location; detecting, using the processor, whether abnormal second image data exists at a second common location for each of the last N wafers of the second set to be placed on the platform, excluding any abnormal image data at a location for wafers that previously had abnormal image data at the location; and conditionally determining whether to clean the platform, depending on results of the first and second detecting steps.