Disclosed is a method of operating a manufacturing facility. A processor detects whether abnormal first image data exists at a first common location for each of the last N wafers of the first set of wafers to be placed on a platform, excluding any abnormal image data at a location for wafers that previously had abnormal image data at the location; and detects whether abnormal second image data exists at as 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. A decision is made whether to clean the platform depending on results from the processor.