In the preceding chapters the focus was primarily on some of the mathematical tools available for controlling processes. Statistical process control is a methodology that enables people to identify abnormal levels of variation. Once it has been determined that a process is out of control, the sources of the excessive variation or process drift need to be identified and corrected. Moreover, it is preferred to have processes that normally have low levels of variation. In order to design processes which are intrinsically stable or to take corrective action when processes go out of control, it is necessary to understand and apply the science of lithography. For example, when linewidths drift, one needs to understand what parameters can potentially cause changes in the process.
Lithographers need to control linewidths, overlay and defects. Aspects of lithographic science relevant to linewidth control will be discussed in this chapter. Similar discussions for overlay will follow in Chapter 5, and for yield in Chapter 6.
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