MODELING X-RAY PROXIMITY LITHOGRAPHY

Advanced semiconductor circuits, such as DRAMs, are based on very comp lex fabrication processes. Because of the cost and complexity involved , it is rapidly becoming impossible to adopt a ''trial-and-error'' app roach in the development stage of a new process. Fortunately, the adva nces in computer power spurred by the new semiconductor devices have m ade it possible to compute the response of complex systems in a reason able time on workstations. Thus, the study of a virtual representation of the process (that is, a model) can represent a solution to the hig h cost of process development-of course, after verification of the mod el accuracy through controlled experiments. A correct physical interpr etation of the process under study is necessary in order to implement a model that is both accurate and extendible. This is particularly tru e for new approaches, such as those involved in X-ray lithography. We have studied the process of image formation in X-ray lithography and h ave implemented several models to predict the intensity distribution a t the wafer plane. The models can be applied to the definition of an o ptimal exposure system that will provide the maximum exposure latitude , and to the study of new types of X-ray masks.