SCALPEL Differs from optical lithography in its use of high- energy electron imaging, electron-scattering contrast, a segmented mask, and a sub-field stitching architecture. Imaging performance of a high-throughput SCALPEL tool is primarily determined by geometric and space-charge aberrations, which both have unique dependencies on key optics parameters. A complete understanding of the imaging point-spread function (PSF) is vital to the optimization of a high throughput SCALPEL optical subsystem and the optimum use of mask pattern data bias for proximity effect correction (PEC). Previous image modeling with an assumed Gaussian PSF model was updated to use the PSF result of Monte-Carlo analysis of electron landing distributions. We find substantially different behaviors as a result of this more rigorous analysis. A proper evaluation of the true nature of the SCALPEL electron optics PSF and its dependencies in a scanning exposure mode allows for meaningful optimization and performance prediction in the area of critical dimension (CD) control.
The high-energy (100KeV) electron imaging process used by SCALPEL causes a dynamic heat load and wafer expansion response. Despite good thermal contact between the wafer and chuck, the dynamic distortion on a die length scale is too large to allow in the overlay error budget, and is fundamentally difficult to prevent or dissipate. However, the sub-field scanning strategy of SCALPEL allows us to implement a dynamic distortion-correction algorithm, by making a real-time position adjustment of the imaging sub-field. Analytical tools developed for understanding these phenomena could be applied in a purely predictive correction algorithm, in principle. However, these predictions are limited by the variability of chuck thermal contact, as well as by difficulty in handling the frictional chuck-attachment boundary condition. Hence, a means of improving upon a basic prediction is necessary to support a robust correction strategy. In this paper, I will explore the practical real-time implementation of a correction algorithm based on Adaptive Kalman Filter techniques, using dynamic alignment updates. Simulations demonstrate the feasibility of this approach to make a very close estimate of the ideal correction.
The implementation of SCALPEL for post-optical production lithography generations, including mix-and-match options, involves unique issues in alignment and overlay. SCALPEL's use of stitching modifies the familiar analysis of overlay errors. Stitching may produce a small, localized image- placement error, but it creates negligible fixed image distortion. It also allows sub-field placement adjustments to correct some of the distortion errors in mix-and-match optimization. SCALPEL can use existing off-axis alignment sensor technologies, but a preferred electron back-scatter technique offers robustness and versatility. For high- throughput operation, a form of global alignment similar to that of full-field tools is likely, but implemented with the dynamic alignment mark scanning capabilities available in the writing strategy. Finally, it is expected that wafer- heating correction issues will factor into the coupled development of optimum writing and alignment strategies, possibly introducing novel mixed operating modes of fine alignment. We shall discus our present overlay error budgets, representing these unique challenges and opportunities for developing a high-throughput SCALPEL tool.
Successful deployment of SCALPEL for several post-optical production lithography generations requires a unique optimum writing-strategy. Since the electron optics sub-field and the strutted mask patten segment are both smaller than the final device image area, SCALPEL utilizes a stitching approach to image-formation. A dynamic sub-field placement scheme, or 'writing strategy', must provide precise 2D stitching at high speed, and eliminate mask strut images on the wafer. It should also provide the extended dynamic lens field necessary for good throughput, while minimizing all non-exposure times per wafer and maintaining the time- averaged current near the instantaneous space-charge limit. The preferred writing-strategy replaces mechanical stage acceleration events with beam deflection wherever possible. The unique writing-strategy presented here also generates the required 2D seam-blending dose-profiles, which are vital to robust CD control with stitching.
The potential for SCALPEL to provide economically viable production lithography capabilities for post-optical generations depends largely on achieving adequate wafer throughput. We have analyzed throughput-limiting performance attributes of the SCALPEL approach in order to identify critical design issues and develop a process for evaluating its unique parameter space. An important feature of the SCALPEL approach is that small image sub-fields are assembled to form complete device patterns. Further, electron-electron interactions result in a throughput- dependent image blur, which is a governing parameter for many inter-related performance areas of SCALPEL. Error budgets for key issues affecting critical dimension (CD) have been developed to analyze this unique design space, using models of the image-forming process including stitching on sub-field seams. These budgets assist in identifying the most critical design issues and demonstrating their inter-relationships and tradeoffs.
KEYWORDS: Visualization, Video, Photography, Lenses, Human vision and color perception, Electronic imaging, Visual process modeling, Spherical lenses, Monochromatic aberrations
Performance was measured on an editing task which required counting the number
of occurrences of an assigned letter in a paragraph of random letters. The task
was presented in three different display modes: (a) a video display (VDT) with
white characters on a black background, (b) a white-on-black photograph of the
VDT display, and (c) a black-on--white photograph of the VDT task display. The
viewing conditions for the three display modes were matched. Defocus was
introduced by cylindrical lenses (simulated astigmatism) and by plus lenses.
Performance was measured by time and accuracy in completing the counting task.
There were 19 normally-sighted young adult subjects tested with the task in the
three display modes under 6 levels of defocus.
For the hard copy displays, performance was significantly faster (on average by
6.6%) for black characters on a white background. Performance with the black
background photographs was consistently, but marginally (0.9%), faster than with
the VDT displays. Cylindrical defocus of 1.50 diopters substantially impaired
efficiency, but low-power plus lenses did not affect performance.
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