Hans R. Rottmann

Particle sizing from disk images by counting contiguous grid squares or vidicon pixels

Abstract The location of the light-scattering image of a particle with respect to the grid pattern of pixels on a vidicon screen used in particle surface monitoring can cause variation in the response of the vidicon for particles giving the same amount of light scattering. A simple disk-and-square model of the interaction is used to predict such geometrical interaction effects, which are shown to influence the mean number of pixels (squares) activated by particle images (disks) and the variation in the number of pixels activated. The same problem exists when trying to estimate disk area from squares covered or squares covered and touched. If one counts all squares touched by the disk, the mean number of squares, E ( n ), is shown to be exactly equal to the following equation, for image diameters ( D ) smaller than the edge length ( D L ) and approximately equal to it for larger images: E(n) = 1 + 2( D L ) + ( π 4 )( D L ) 2 . The variance of n is calculated for D L . The variation in n for D > L is demonstrated through geometrical modeling and square counting, distinguishing between the number of squares touched and the number completely covered. The geometrical interaction effects on the mean number of squares touched or mean number completely covered can be compensated for by calibration, but the effects on variation cannot. The variation of number of pixels activated compared to the mean number of pixels activated is predicted to be roughly 50% for situations where tens of pixels or less are activated by the particle image. Data that support this by showing substantially greater variability in the pixel distributions than the variability in the particle distributions are presented. The data show particles being missed in some readings and being counted in others, as the analysis predicts. The data also show relatively small error in the total counts, believed due to compensating errors.

SEM Measurements In Lithographic Metrology

As the dimensions of semiconductor devices on wafers approach the micron range, it is readily apparent that measurement of these types of geometries on the fab line will not be obtained using optical methods (1-4). In addition, non-destructive scanning electron microscopy methods must be found to make more representative measurements than those provided by fracturing of the specimen or use of conductive coatings. Further, measurements of semiconductor geometries on wafers must be made in-process. That is, the measurements must be made on uncoated specimens so that the specimen can be returned to the process quickly and intact to add layers for both product and monitoring wafers. The primary objective of this paper is to demonstrate procedures for using Scanning Electron Microscopes for small feature measurements in Lithography. To this purpose, test wafers were generated containing resist valleys with varying widths and slope angles (resist features present the most challenging measurement objects). A selected number of these features were measured on Scanning Electron Microscopes. Recorded were SEM micrographs, backscattered electron profiles and numerical data. In addition, the same features were also measured on optical measurement microscopes and the discrepancies between different systems were analyzed. A second objective of this study was to assess the usefulness of continued optical measurements in lithography because they are faster and considerably less expensive and, therefore, should continue to play an important part in lithography. It was also found that (uncontrollable) resist slope and adhesion variations are responsible for a major portion of optical measurement errors. Finally, a method will be proposed for the application of SEM systems to advanced lithographic process and equipment characterization and monitoring.

Some Remarks About Linewidth Variations

The aim of this work is to determine different types of contributions to dimensional errors so that appropriate and selective improvements can be undertaken toward optimization of the photolithographic process. Preliminary results are given on contributions by the resist, lens, and errors due to focus and exposure. Systematic variations below O.lμm were found in many instances.