Analysis of critical dimension uniformity for LEEPL

Low-energy electron-beam proximity projection lithography (LEEPL) has been developed for sub-65 nm lithography. Critical dimension (CD) control of resist patterns is critical to be a production-worthy lithography technique. In this study, the LEEPL mass-production tool was used to print 180-nm-pitch contact holes in a tri-layer resist and the CD uniformity of the contact holes was analyzed to know primary issues degrading process maturity. The intra-wafer CD uniformity in an 8" wafer was 15.8 nm. Temperature fluctuation during a resist-baking process had little impact on the inter-shot CD uniformity of 3.5 nm because the CD variation was less than 0.4 nm when the baking temperature increased by 1 degree C. The CD uniformity of the 8" silicon stencil mask used in this study was 4.7 nm, which was a primary factor of the intra-shot CD uniformity of 8.8 nm. The impacts of causes of a mask error enhancement factor (MEEF) on the intra-shot CD uniformity were calculated based on the quantitative analysis of the blur of a latent image profile. The electron-optical blur caused by lens aberrations and the Coulomb effect accounted for 4.5 nm of the total uniformity, and it would be improved by 4.0 nm if there was no blur by scattering of 2 keV electrons in a 70-nm-thick resist. Although causes of residual 12.6 nm were attributed to pattern edge roughness (10.1 nm), statistical fluctuation of exposure dose (3.2 nm), and traceability of a scanning electron microscope (1.6 nm), the origin for 6.7 nm remained unknown. This unknown CD variation jumped from 2.6 nm to 6.7 nm when the CD shrank from 150 nm to 90 nm. Since the pattern edge roughness accounts for the largest portion of the CD uniformity, making the contact holes perfectly round by optimizing process conditions is most effective in improving the CD uniformity for the current LEEPL process.