244-nm imaging interferometric lithography test bed

The minimum half-pitch feature accessible by optical lithography is CD = λ/(4NA). For an imaging system, the ultimate resolution does not reach this level as a result of the need to capture the information in the pattern-dependent sidebands around the fundamental frequency; roughly a system with these capabilities should be able to print ~ λ/(3NA) to λ/(3.5NA) half-pitch patterns. Imaging interferometric lithography (IIL), combining low-σ, off-axis illumination (OAI) at the limits of the pupil with pupil-plane filters (PPF), to ensure a uniform frequency-space coverage, and multiple exposures covering different regions of spatial frequency-space, provides an approach to accessing these fundamental, linear-systemsresolution limits with simple binary chrome-on-glass masks without any requirement for either optical proximity correction (OPC) or phase shift masks (PSM). Polarization control is an advantage of IIL, as with other off-axis approaches, that is becoming more important as hyper-NA tools are introduced. Additionally, the PPFs allow for intensity adjustment in the pupil plane, which can further improve the image contrast. A test bed has been assembled at a 244-nm wavelength using a 0.9 NA optical system. For this system, the minimum half-pitch is 68 nm, and the limiting half-pitch for arbitrary features is ~ 70- to 60- nm. Demonstrations of printing arbitrary 113-, 100-, 90- and 86-nm half-pitch patterns are reported. Good imaging was achieved with simple binary chrome-on-glass masks without the use of any additional OPC or PSM. The image improvement by adjusting the contrast of the image with a simple intensity PPF mask is demonstrated. These results scale to the 45-nm half-pitch node for a 193 nm source and a 1.3 NA. The essential point is that robust imaging for a 45-nm half-pitch is available using IIL without any mask-based resolution enhancement techniques.