Keywords lithography, DUV steppers, microlithography lenses
Start Date: 01-JAN-90 / Duration: 36 months
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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.
The main acheivements of this project were the development of a new high-quality quartz material, the design and fabrication of new large field lenses for DUV lithography, and the mounting of this lens by ASM-L into a new-generation DUV stepper with a resolution down to 0.25 micron, a variable numerical aperture (NA = 0.40 - 0.50), and a distortion and overlay performance allowing matching of i-line and DUV lithography. R&D was carried out in the following areas:
Optical materials (Heraeus Quarzglas)
The subject of this work was the optimisation of materials properties with respect to fluorescence and laser damage without degrading the optical homogeneity at the large sizes demanded.
For the prototype lens, Suprasil 311 was used as optical material. Suprasil is the most homogeneous synthetic fused silica available at Heraeus Quarzglas at present. It meets all bubble and homogeneity requirements for the prototype lenses which were developed in the course of the project.
Results of 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 theoretical model describing defect creation and recovery has been established which allows the calculation of the optical performance of the material under different illumination parameters. This model has to be validated for long-term exposures. Heraeus improved the fabrication process of the quartz material, yielding fused silica without fast damage. The slow-damage mechanism could be reduced to a negligible level.
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.
Reduction Optics (Carl Zeiss)
The design for the prototype lenses was extended to meet advanced specifications, namely NA = 0.5, field diameter = 29.7 mm, wavelength = 248 nm, resolution (k = 0.7) = 0.35 micron, and tracklength = 800 mm.
A prototype lens was shipped to ASM-L, where a resolution of 0.25 micron structures was shown.
Optical design studies concerning 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 gained through cooperation with the subcontractor Philips CFT were used for the mechanical design of the prototype lens. A new mounting technique has been developed.
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 characterising lens distortion (Moire technique).
Illumination System (Carl Zeiss)
The design of the prototype illumination system has been completed, and two prototypes delivered to ASM-L.
Studies on the principles of DUV illumination systems concentrated on problems of field illumination, coherence and efficiency. Further work has to be done to improve reliability and uniformity of illumination.
Stepper Adaptations (ASM-L)
The feasibility studies on the lens mounting and mechanical lay-out have been carried out. Satisfactory solutions have been found to deal with future lenses with lengths up to 1000 mm and weights up to 150 kg.
Within the Subsystem Adaptations task, various designs have been studied for both the reticle table and the focus sensor. The experience and results of project 2048 (DEEP-UV) were useful as a basis for studies into the development of an image sensor and an improved through-the-lens alignment system.
Accuracy budgets estimates have been made for different stepper subsystems, supported by vibration studies.
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 of better than 30 nm.
The advanced synthetic fused silica material developed by Heraeus is now commercially available. The know-how gained in project 5002 will also be exploited in other areas of excimer laser application.
The developed lens mounting techniques have been introduced into production at Carl Zeiss.
The stepper and lens development in project 5002 have initiated the development at ASM-L of a new generation of production wafer steppers, the PAS 5500 family. These steppers are now successfully used worldwide.
Mr G. Oswald
D - 7082 OBERKOCHEN
tel: + 49/ 7364-203141
fax: + 49/ 7364-6808
CARL ZEISS - D - C
ASM-LITHOGRAPHY BV - NL - P
HERAEUS QUARZSCHMELZE GMBH - D - P
Project 5002, December 1993
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html version of synopsis by Nick Cook