Mask error factor in proximity X-ray lithography

We discuss the mask error factor (MEF) in proximity X-ray lithography. The MEFs for dense-line, isolated-line, and isolated-hole patterns were estimated by using aerial image simulations and exposure experiments. When the minimum feature size was 100 nm, MEFs in the patterns were kept within unity by setting the proximity gap to 15 μm. For feature sizes of 70 nm, the proximity gap should be reduced to as small as 10 μm to maintain the MEF at around unity for dense- and isolated-line patterns. For isolated-hole patterns, a high-resolution resist with a shorter acid-diffusion length (<20 nm) should be used to keep the MEF at around unity. The MEF changes depending on geometric conditions, such as the pattern size, proximity gap, X-ray wavelength, and absorber thickness. The MEF is determined mainly by the interference between the diffracted X-rays, which have different phases. Thus, the MEF can be controlled by choosing the appropriate exposure conditions. In most cases, the MEF in proximity X-ray lithography is much smaller than that in optical lithography, which is advantageous for the proximity lithography because it allows a larger margin in the critical dimension control for 1X X-ray masks.