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The three-dimensional (3-D) photoresist imaging process simulator TRIPS-I has been improved to cope with the strong standing-wave effect in photoresists on flat substrate surfaces. To allow insertion of development vectors, which is necessary to advance photoresist-developer interface under the strong standing-wave effect, development vectors are calculated using the information of neighboring vectors. This information is recorded in units of triangles which are defined by tips of the three nearest development vectors. The triangular elements have also the advantage that precise expression is possible for complicated 3-D photoresist images resulting from a serious standing-wave effect. A photoresist image profile with a strong standing-wave effect showing good agreement with the actual photoresist image has been successfully simulated. In applications of TRIPS-I, photoresist patterns using lenses of different numerical apertures (NA's), 0.42 and 0.6, are compared. As a result, it is predicted that 0.3 X 0.3-μm hole patterns can be attained with the 0.6-NA lens.