Two complementary methods to characterize long range proximity effects due to develop loading

Abstract

Variations in critical dimension (CD) as a function of the proximity of an individual feature to other exposed areas are a continuing problem both in mask fabrication and in optical lithography. For example, the CD uniformity (CDU) may degrade significantly depending on the proximity to densely or sparsely exposed areas. These pattern density effects will continue to worsen as feature sizes decrease to 22 nm and below. Pattern density effects in electron beam lithography using chemically amplified resists are believed to arise from several sources. One such source, fogging, refers to the backscattering of secondary electrons onto the resist to cause deviations from the nominal pattern size. A second contributor is acid volatility, where photogenerated acid is presumed to redeposit on the wafer or mask during exposure or bake; here we refer to this effect as chemical flare. A third source of pattern density effects is develop loading, which results in local depletion of developer in highly exposed regions. All three of these may simultaneously contribute to a net observed CD variation. In this report we describe the application of two different techniques for evaluating these proximity effects. The first is based on electron-beam lithography patterning, and compares CD values of test patterns which are exposed under bright-field and dark-field conditions. The second uses a series of different test patterns formed by DUV (248nm) exposure and a custom liquid flow cell to separately characterize resist related density effects. © 2010 SPIE.