Pushing the Limits of Hyper-Na Optical Lithography
The evolution of optical lithography to pattern smaller geometries was witnessed the shrinkage of source wavelength as a way to increase optical resolution. Shrinking of source wavelength into vacuum ultra-violet (VUV) faces a number of technical barriers with respect to imaging materials. Instead of source wavelength shrinking, the optical resolution can also be enhanced by increasing numerical aperture (NA) with immersion techniques. This dissertation is devoted to experimentally studying the imaging behaviors of hyper-NA optics in the context of liquid immersion and solid immersion lithography.
In this dissertation, the full-vector interference theory is described for two-beam and multi-beam interference. Polarization effects, resist absorption effects and BARC optimization are analyzed respectively. The experimental setup is analyzed in consideration of vibration, source temporal coherence and spatial coherence. Imaging with TE and TM polarization is studied respectively. A solid immersion technique is investigated experimentally to push the NA beyond what are available using fluids and imaging at NA values up to the index of the photoresist has been investigated. Moreover, NA values have been pushed higher than the refractive index of the resist, exposing resist using "evanescent wave imaging". This approach removes the index of a photoresist as a physical resolution limit, opening up new possibilities of resolution enhancement.
The ultimate resolution limits of optical lithography have been discussed for a long time but the limits have not been met yet. The achievements in this dissertation have shed light on this long-sought curiosity in the lithography community. Our experimental results proved the feasibility of 25 nm regime optical lithography. However, resolution beyond that would require innovations on higher index imaging materials, which are believed to be very limited.
Air bubble induced light scattering effects on lithograrhic imaging have also been studied using geometrical optics and Mie scattering model. Lithographic imaging of "bubbles" in an immersion water gap was studied by mimicking air bubbles with polystyrene spheres. By counting the number of "bubbles" which are actually imaged and evaluating the number which are present in the optical path, the distance beyond which bubbles will not print can be estimated.