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Resolution enhancement techniques (RETs) are beneficial and necessary for successful low k 1 lithography. Continual critical dimension (CD) decrease and delay of next-generation microlithography technologies mean that optical lithography will be increasingly stressed. As shown in Fig. 8.1, the manufacture process window has been decreasing exponentially with time. Introduction of RETs provides relief but does not halt the decreasing trend. To keep pace with the ever more stringent CD control requirement, tightening of equipment and process control is inevitable. Sources of linewidth and placement errors such as aberrations, mask CD error, focus and dose variation, stage asynchronization, topography and substrate reflectivity fluctuation, photoresist development and hot plate temperature nonuniformity, and reactive ion etch rate variation must be reduced to a minimum. At the same time, further enhancement of image quality is essential. Although techniques discussed in the previous chapters provide much process latitude gain, they will not suffice. Additional improvement over these first-generation methods can only be achieved with more drastic second-generation resolution enhancement techniques. Of the many potential methods [178-€“189], a few promising ones are discussed in more detail as follows.
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