New approaches in optical lithography technology for subwavelength resolution

Hoyoung Kang

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works. Physical copy available through RIT's The Wallace Library at: TR940 .K36 2005


Advances in the semiconductor industry are mainly driven by improvements in optical lithography technology, which have enabled the continual shrinking of integrated circuit devices. However, optical lithography technology is approaching its limit, and within ten years, it may be substituted by new non-optical approaches. These may include Extreme Ultra Violet (EUV) lithography and charged particle beam projection lithography. While these technologies may have potentially better resolution, they can be very difficult to implement into manufacturing. During the course of the research presented here, the extension of optical lithography to sub 70nm resolution has been investigated. Since optical lithography is mature and well understood, extending it to allow for higher resolution can dramatically reduce manufacturing difficulties, compared to EUV or charged particle beam projection lithography. A majority of the existing infrastructure, such as photoresist materials, sources, optics, and photo-masks, remain applicable with the optical methods explored here. The avenues investigated in this research have concentrated on spatial frequency filtering in alternative Fourier Transform planes, vacuum UV wavelength lithography, and achieving ultra high numerical aperture imaging through the use of liquid immersion imaging. More specifically, novel spatial frequency filtering using angular transmission filters was developed and demonstrated. Multiple filter designs were proposed, one of which was successfully fabricated and implemented for lithographic imaging. Spatial filtering, using angular transmission filtering, proved to enhance the resolution of contact hole images by approximately 20%. Vacuum UV imaging at the 126nm wavelength was carried out but deemed likely to be less practical for commercial viability due to source, optics, and materials issues. Immersion lithography, using the 193nm wavelength ArF excimer laser, was investigated and demonstrated for very high numerical aperture imaging. Requirements for immersion lithography were established, including the necessary design of imaging fluids, optics, sources, and photoresist materials. As a development tool, an interference lithography system was built using the 193nm ArF excimer laser and water as an immersion fluid. Patterns below 70nm were printed using the process developed, which has established the potential to extend optical lithography further than was believed at the onset of this project. This research provides proof of the concept of extending optical lithography to the 70nm generation and below.