Accurate and precise metrology and control for stepper or scanner objective lens focal-length is critical for sub- 0.25micrometers design-rule photolithography, specifically for CD, isolated-dense offset, and resist sidewall control of minimum feature geometries. As next generation processes result in reduction of the manufacturing process latitudes, contribution of the focus metrology error will continue to consume a larger portion of the useable depth of focus. In addition to improvement of the cross-field photoresists image fidelity, if the focal length measurement precision is improved, lithographic rework and device yield implications, due to mask and imaging linewidth error, are minimized in production.
Maintaining projection-aligners' stage grids-is critical for maximum overlay performance of production lithography. It seems that, particularly in the U.S., the industry has chosen the 'artifact-wafer' strategy as the standard technique to achieve this goal. This paper is intended to identify problems in overlay management using artifact wafers and to provide solutions to address the issues. One of the major sources that degrade accuracy in overlay management is the expansion/shrinkage of wafers and reticles. Both wafers and reticles expand during printing due to the heat delivered from the illumination source. The amount of the expansion tends to increase as the power of the illumination source increases per industry's demand on higher throughput. Wafers and reticles expand/shrink also due to environmental temperature change. The significance of wafer expansion/shrinkage in this mode has tended to be neglected. This is probably because, since it is measured and compensated by the 2nd print alignment, wafer scaling in the first print does not impact overlay performance evaluated at the shot center. Wafer expansion/shrinkage, however, does cause intra-shot scaling errors in overlay. And more importantly, since artifact wafers serve as absolute stage-grid-references, their expansion/shrinkage directly impact accuracy of overlay management. Reticle expansion/shrinkage due to temperature difference between where the reticles were created and where they are used along with reticle manufacturing errors causes inaccuracy in intra- shot performance evaluation. As product design rules continues to tighten, the intra-shot overlay performance can no longer be neglected. The impacts of reticle- and wafer-elasticity on total overlay management will be discussed. Multiple techniques to address the elasticity issues will be demonstrated. The discussion will conclude with recommendations for generation and usage of the artifact wafers.