Preventing Defects from Occuring: Mask Inspection
The photomask is one of the most important tools of photolithography. The mask contains the original image of the structures, which are transferred several thousand times to the individual chips during the exposure of the wafer. To create the structures of a microchip, the patterns of up to 40 different photomasks are projected step by step on the wafer.
Even minute defects in the layout of a single mask used in this process can render all chips in the exposed wafers unusable resulting in enormous losses. That is why it is critical to inspect the mask for defects and eliminate them before the mask is used in a scanner/stepper. The best way to reliably control quality at this stage is to optically emulate the photolithographic process with aerial imaging.
