The specifications of mask critical dimension (CD) have become much tighter for sub-100nm nodes to satisfy wafer CD uniformity requirements. The small patterns produced by aggressive optical proximity correction compound the difficulty, thereby necessitating the use of e-beam writers in combination with chemical amplified resists (CARs). Challenges of resists include post coating delay (PCD), post exposure delay (PED) in vacuum, and strong post exposure bake (PEB) sensitivity. CD errors are classified into localized area and global ones; machines causing each type of errors are then identified. Focus variation and fogging effect have to be emphasized for the 65-nm requirements. Although the DOF of e-beam systems is larger than that of the optical systems, high current density and/or plate-to-plate deviation may cause focus variation to result in poor CD uniformity. Therefore, dosage optimization is necessary for getting the best focus. The fogging effective level is around 3~10 nm at various pattern loadings. The paper presents, quantitative results and the methodology leading to them.