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In-line microscopy must keep up with the flow of manufacturing. At an inspection point, of which there are several in the process stream, a fraction of the wafers will be inspected, typically on the order of 20%. The fraction of wafers inspected depends on the maturity of the process, the complexity of the product, and the particular process step being monitored. State-of-the-art inline imaging systems require from two to twenty minutes to scan an 8 inch wafer. At a defect sensitivity of 0.25-µm, a throughput is required of approximately 2-GHz, ie, 2x109 image pixels must be captured, moved through the image processing system, and reduced to a small set of descriptive features, eg, defect location and size, every second. These systems must therefore rely on high-speed and reliable imaging technology. Optical microscopy using charge coupled devices (CCD) are the systems of choice for in-line defect detection today with CCDs configured in line-scan or time-delay integration (TDI) format. Off-line review typically involves resampling a small fraction of the wafers inspected in-line. Off-line review tools therefore do not require the same high throughput but do depend on higher image fidelity so that human-level decisions can be made regarding defect type and source. Higher quality images for failure analysis are obtained by using, for example, color CCD sensors, confocal microscopy, and scanning electron microscopy (SEM). The following section will consider these most common methods of optical and electron microscopy used in semiconductor manufacturing today for both in-line detection and off-line review.