Photomask - Mask Error Enhancement Factor

Mask Error Enhancement Factor

Leading-edge photomasks (pre-corrected) images of the final chip patterns magnified by 4 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, and the second is that the mask feature is becoming smaller, and the dimension tolerance is approaching a few nanometers. For example, a 25 nm wafer pattern should correspond to a 100 nm mask pattern, but the wafer tolerance could be 1.25 nm (5% spec), which translates into 5 nm on the photomask. The variation of electron beam scattering in directly writing the photomask pattern can easily well exceed this.