Effects of mask bias on the Mask Error Enhancement Factor (MEEF) of contact holes

The effects of mask bias on the Mask Error Enhancement Factor (MEEF) of 180 nm contact holes is studied through lithographic simulation using commercial software and a DUV (248 nm) ESCAP photoresist model. Dense contacts show higher MEEF than isolated or semi-dense contacts. However, dense features exhibit a minimum in MEEF at a single negative mask bias (CD on reticle > 180 nm). Aerial image simulations indicated that low MEEF correlates approximately with high normalized aerial image log-slope (NILS). Hence, factors that affect NILS, such as numerical aperture, partial coherence, and wavelength, also influence MEEF, although without altering the optimum mask bias for minimum dense MEEF. Numerical aperture and wavelength of exposure have the greatest influence on MEEF. For 180 nm contact holes worst case MEEF values below 2 can be achieved by increasing NA to 0.8 at 248 nm or by decreasing lambda to 193 nm at 0.6 NA. Resist identity has little influence on the magnitude of MEEF but was the only factor affecting the mask bias setting for minimum dense MEEF.