Optical lithography will continue to be the dominating tool for the production of integrated circuits for the foreseeable future. The evolution of phase shift masks, in particular, gives increased importance to optical lithography developments. In the near future, 0.3 micron structures within field diameters of around 30 mm will be needed for microchip production (eg 64 Mbit DRAM). This cannot be obtained with existing microlithography lenses, and a new generation is needed, preferably operating in the deep UV (DUV) because of the increased depth of focus obtained. A drastic increase in the weight and dimensions of the reduction lenses is required in order to meet these demands. The properties of this new generation of lenses will significantly influence the design of future wafer steppers. Project 5002 provided the research and design approach needed to address the developments required.
Optical lithography will continue to be the dominating tool for the production of integrated circuits for the foreseeable future. This project provides the research and design approach needed to address the developments required.
For the prototype lens, Suprasil 311 was used as optical material. Results of ongoing investigations show that laser damage resistance is influenced by materials stoichiometry, purity and thermal history. Due to room temperature recovery effects of laser induced defect centres, a transient short time measurement technique has been developed. A theorectical model describing defect creation and recovery has been established in order to calculate the optical performance of the material under different illumination parameters. This model has to be validated for long term exposures. Further investigations are necessary, bearing in mind the large dimensions (250 mm diameter) of the fused silica pieces required for the next generation of lenses. The production of the prototype lens is finished and a resolution of 0.25 micron structures has already been shown. Optical design studies concerning deep ultraviolet (DUV) lenses concentrated on investigations of the tracklength of the lens, chromatic problems and UDOF behaviour. Mechanical studies dealt with the stability problems that occur when fitting lens elements with diameters up to 250 mm into their mechanical mountings. Results of mechanical studies have been used for the mechanical design of the prototype lens. Work on measurement techniques for lens qualification concentrated on the design of new optical test masks and on the preparation of an improved method of characterizing lens distortion.
The design of the prototype illumination system has been completed. Studies on the principles of DUV illumination systems concentrated on problems of field illumination, coherence and efficiency.
The feasibility studies on the lens mounting and mechanical layout have been finished. Satisfactory solutio ns have been found to deal with future lenses with lengths up to 1000 mm and weights up to 150 kg. In a test setup the mechanical design and vibration sensitivity of the stepper have been tested. The results are in good agreement with the design values, resulting in an accuracy of the wafer stage better than 30 nm.
5503 HN Veldhoven