Ryuichi Ebinuma

Evaluating next-generation reticle demands on lithography equipment

Industry trends indicate that the next generation of exposure tool will be scanning steppers. Scanning steppers have a 25 mm by 33 mm field using 6 inch reticles. For device manufacturing, first generation 256M DRAM chips are roughly 12.5 mm by 25 mm, while 1G DRAM are expected to be 30 mm by 15 mm. As a result, a 25 mm by 33 mm field does not allow the exposure of two or more chips per shot. Therefore introduction of a larger mask is expected. To determine next generation reticle size standard, many factors have been investigated. First, an assumption was made that DRAM chip size trends will remain constant. Then, the throughput of scanning stepper was calculated with 6, 7, 8 and 9 inch reticles. The advantages were dependent on chip generation and device (memory or logic). Finally, a 9 inch reticle standard was chosen. Making the leap to a 9 inch reticle standard avoids the development time and costs of incremental changes in the standard. The thickness of a reticle is dependent upon reticle distortions, projection lens focus error. The deformation of the reticle has been simulated for the 0.25 to 0.5 inch thickness range. The final outcome of these simulations was that 9 mm (approximately 0.35 inch) was selected as the best thickness.

Analysis of reticle deformation, reduction ratio, and MEEF of future optical lithography

As a result of aggressive line width shrinking of semiconductor devices in the recent years, the requirements for advanced reticles are getting more and more stringent. Therefore, it is beneficial to consider increasing the reduction ratio of projection optics in order to relax the reticle tolerances. This paper discusses quantitatively the reticle, CD, DOF and overlay accuracy requirement listed in the 1999 International Technology Roadmap for Semiconductor (ITRS) roadmap. Our simulation suggests mask drawing accuracy needs to be further improved for better CD control accuracy. Increasing reduction ratio to 6x is also another way to meet the line width requirement. Productively enhancement with 6x reduction in comparison to 4x reduction ratio is also shown.

Imaging performance of scanning exposure systems

Relative position between the projected image on the wafer and the wafer itself changes during exposure. Factors of change are, for example, stage control error, difference of scanning direction between wafer stage and reticle stage (skew) and distortion of projection optics. We can define a kind of probability density function (PDF) concerning these changes of relative position. Fourier transform of this PDF is the transfer function of image transformation by relative motion. In this paper, we call this transfer function MoTF. The modulation of MoTF becomes a barometer of image contrast and the phase of MoTF gives position deviation (distortion). By analytical study of MoTF, standard deviation and expected value of said PDF are found to be the key parameters. Derived approximate equation in this paper agree with a computer simulation result of image contrast deterioration by vibration. With these studies, we can establish adequate specifications of scanning stage control demanded by imaging performance. Canon has developed a new stage structure for scanning exposures. By this structure the wafer stage is separated from main body on which projection optics and measurement systems are mounted so that reaction forces of stage acceleration can not be transferred directly to the maim body. With this structure we achieved excellent stage performance which has achieved imaging performance below 0.18 micrometer with high speed scanning.