The photolithography process consists of aligning and optically transferring the pattern from a reticle onto a partially processed wafer that has been coated with photoresist. The resist is then developed and the resist image transferred into the underlying material by a chemical and/or thermal step (i.e., dry/wet etch, sinter, implant, etc.). This process may be repeated 15 to 25 times in the coarse of building a complex integrated circuit. The process steps themselves may distort the wafers, causing differences between wafers and lots. In order for the devices to function properly, it is necessary to ensure that levels are precisely aligned to one another, i.e., overlay, and to control the critical dimensions, i.e., image size, within each patterned layer.
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