Effects of mask bias on the Mask Error Enhancement Factor (MEEF) for low k1 lithography process

Mask Error Enhancement Factor(MEEF) has recently become an important topic in determining requirements of process. MEEF is the ratio of the CD range on the wafer and the expected CD range due to the mask. It indicates that mask CD errors are in effect magnified during the optical transfer to the wafer. The resolution capability of a optical system is given by Rayleigh's criterions: Resolution=k1 * λ/NA, where λ is the wavelength of the light used and NA is defined as the sine of the maximum half angle(α) of diffracted light which can enter the lens. The k1 resolution-scaling factor (kl=CD*NA/λ) is a practical measure for expressing imaging feasibility of a given optical system. It is a important parameter and direct proportion to resolution requirement. For driving critical CD dimension contraction bellow 0.11 μ m, lower k1 factor is needed. In this work we use strong OAI (Quasar 90°) to push kl reach 0.29 by KrF exposure tool and analysis the MEEF value on 90nm generation. The simulation result shows the predicted MEEF value is close to 9 while using KrF to 90nm resolution and real MEEF value from exposuring Line/Space pattern on wafer data is 6.2. In such high MEEF process, it is very important to control mask CD accuracy. We bring up a test pattern of serial combinations with different Line/Space dimension with the same pitch size to reduce the mask array CD variation. Finally, we compare the process window (PW) between equal and nonequal Line/Space situation. The process window can be improved 18% while line width extends from 90nm to 95nm at fixed pitch 180nm.