Editing Photomask (section)

== Mask error enhancement factor (MEEF) ==
Leading-edge photomasks (pre-corrected) images of the final chip patterns are magnified by four times. This magnification factor has been a key benefit in reducing pattern sensitivity to imaging errors. However, as features continue to shrink, two trends come into play: the first is that the mask error factor begins to exceed one, i.e., the dimension error on the wafer may be more than 1/4 the dimension error on the mask,<ref>E. Hendrickx ''et al.'', Proc. SPIE 7140, 714007 (2008).</ref> and the second is that the mask feature is becoming smaller, and the dimension tolerance is approaching a few nanometers. For example, a 25&nbsp;nm wafer pattern should correspond to a 100&nbsp;nm mask pattern, but the wafer tolerance could be 1.25&nbsp;nm (5% spec), which translates into 5&nbsp;nm on the photomask. The variation of electron beam scattering in directly writing the photomask pattern can easily well exceed this.<ref>C-J. Chen ''et al.'', Proc. SPIE 5256, 673 (2003).</ref><ref>W-H. Cheng and J. Farnsworth, Proc. SPIE 6607, 660724 (2007).</ref>