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The Lithographic Process
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In the photolithographic process (see Figure), a planar substrate (for example, a thin silicon wafer) is
coated with a photoresist (a photosensitive polymer film) , and then exposed to a pattern of light.
Photochemical reactions in those areas of the film exposed to light alter its solubility so that either
exposed or unexposed areas are selectively dissolved in a solvent to uncover portions of the substrate
surface. If the exposed areas of resist become insoluble, then the resist is considered a negative resist;
if the photochemical reaction increases resist solubility then the material is termed positive-acting. In
typical usage, the resist acts as a temporary template for one of several methods of image transfer; these
include plating and deposition (additive processing), chemical etching or physical milling (subtractive
processing) or bombardment with energetic ions that are implanted in the substrate surface. The remaining
resist is removed or stripped after the image transfer step.
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