The Mask Error Factor: Causes and implications for process latitude

In this paper, a broader understanding of the so called Mask Error Factor (MEF) will be described. MEF is defined as the ratio of the measured CD range on the wafer and the expected CD range due to the reticle. As a result, the MEF plays a very important role in the final CD range as observed on the wafer. It will be shown that the MEF can be controlled by Numerical Aperture (NA), illuminator settings, process conditions and resist type. Since the optimum conditions for Depth of Focus (DoF) will usually be found at different settings than the optimum for MEF, DoF has to be balanced with MEF in order to achieve a minimized CD uniformity over a required focus range. We will show experimental results for different illumination settings (e.g. quadrupole versus annular illumination). Results for contact holes, isolated lines and dense lines and spaces will be presented. Smaller resolutions, especially line widths below the exposure wavelength, will yield a higher MEF. In turn, decreasing reticle CD's yield reticles where CD uniformity is judged to have worse reticle quality. This stresses the importance of understanding and being able to control the MEF.