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Proceedings Paper

Challenges and solutions in the calibration of projection lens pupil-image metrology tools
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Paper Abstract

As imaging requirements and limits continue to be pushed tighter and lower, it has become imperative that accurate and repeatable measurement of the projection lens (PL) pupil be readily available. These are important for setup and adjustment of the illumination distribution, measurement and optimization of the lens aberrations, and verification of lens NA and transmission. Accurate testing of these items is critical during initial installation and setup of a photolithography tool, but it continues to prove useful each time any projection lens pupil-image measurement is made. The basic raw data from any such measurement is in the form of a pixelized 'image' captured by a projection lens pupil microscope. Such images have typically been referred to as pupilgrams1, and many prior works have reported on their application and analysis1,2,3. Each of these measurements can be affected by errors in the measurement tool used. The error modes can be broadly divided into two distinct groups: uncompensated transmission loss, and uncompensated distortion (or remapping) error. For instance, in illuminator measurements, the first will yield intensity error and the second will yield image shape mapping error. These errors may or may not lie exclusively in the optics of the measurement tool. But, regardless of their source, they will propagate through the analysis of the pupilgram images. For this reason, at minimum they must be measured and judged for relative impact, if only to confirm that the errors do not change the conclusions or results. In this paper, we will discuss and present methods for measuring the image distortion present in a PL pupil-image microscope. These data are used to build a 'map' of errors vs. position in the lens pupil. The maps then serve as the basis for image-processing-based compensation that can be applied to all subsequent microscope images. A novel vitally important feature of the technique presented is that it calibrates the distortion of the microscope image field using fundamental optical physics. This is achieved by imaging, capturing, and analyzing the diffraction pattern of reticle gratings. Several different reticle pattern pitches are imaged and their diffraction patterns are captured by the microscope. The image field of the microscope is then calibrated to the spacing of the diffraction orders, meaning that the calibration is traceable through fundamental optics to the very precise reticle patterns. Details of this procedure, including an example, will be presented.

Paper Details

Date Published: 16 March 2009
PDF: 10 pages
Proc. SPIE 7274, Optical Microlithography XXII, 72740V (16 March 2009); doi: 10.1117/12.814333
Show Author Affiliations
Steve Slonaker, Nikon Precision, Inc. (United States)
Bryan Riffel, Nikon Precision, Inc. (United States)
Hisashi Nishinaga, Nikon Corp. (Japan)

Published in SPIE Proceedings Vol. 7274:
Optical Microlithography XXII
Harry J. Levinson; Mircea V. Dusa, Editor(s)

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