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Chapter 2:
Theory of the Lithographic Process
The object of semiconductor lithography is to transfer patterns of integrated circuits drawn on the mask or reticle to the semiconductor wafer substrate. The transfer is carried out by projecting the image of the reticle with the aid of appropriate optical elements of an exposure tool onto a radiation-sensitive resist material coated on the semiconductor wafer, typically made of silicon, then stepping or scanning the imaging field across the entire wafer to complete a layer. Following baking, if necessary, the resist film is at this point developed in an appropriate developer, with the standard being 0.26 N (2.38 wt%) tetramethyl ammonium hydroxide aqueous solution. The molecular interactions between the molecules of the exposed, unexposed, and baked resist film with the developer medium (discussed in Chapter 3) are responsible for the contrast between the exposed and unexposed areas of the film. Absent these interactions, it would not be possible to discriminate between the exposed and unexposed part of the resist, which, in turn, would make it impossible to transfer the mask patterns of the integrated circuit to the semiconductor wafer substrate. The steps in the semiconductor lithographic process are outlined in Fig. 2.1 (and illustrated in Fig. 1.1 of Chapter 1) for a negative and a positive resist. The chemical and physical principles underlying each step are discussed at length in the following sections. Where appropriate, lithographic models involving most of these steps are also provided, with a view to providing a framework for predicting lithographic outcomes given a defined set of input resist materials and process variables, as well as exposure conditions.
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