APPENDIX A Table 1: Dimensional measurements of wire width and pinhole diameter using filar and image-shearing eyepieces on an optical microscope. Table 2: Comparison of dimensional measurements for a nominally 12-μm width chromium-on-glass line (486-nm illumination); 30 value for each measurement is approximately 0.25 μm. APPENDIX B EFFECTS OF ILLUMINATION COHERENCE ON APPARENT EDGE POSITION IN OPTICAL-IMAGING SYSTEMS By Richard E. Swing When viewing objects with discontinuities such as sharp edges and well-defined lines in the microscope, it is possible to make incorrect measurements of their dimensions because of the partial coherence of the illumination. Indeed, the observational device does not need to be a microscope; a microdensitometer, for example, employs microscope optics and displays the scanned image as a chart-trace or magnetictape record. If widths of lines or locations of edges are based on the location of the half-power point (the mean between maximum and minimum image transmittance), the degree of partial coherence of the illuminating system can cause serious errors in the results. These effects were reported by P. S. Considine [48]; his report is an experimental summary based on the theoretical considerations of a doctoral dissertation by T. J. Skinner [49]. In the summary by Considine, it is shown that if the edge discontinuity is located at the half-intensity point, the coherent image of the edge is shifted by where m is the magnification, N.A. is the numerical aperture of the objective lens, and is the illumination wavelength; it is assumed that an ocular is not used in combination with the objective (for an ocular, the value of m is increased). The significance of D is shown in figure 3. For example, let This 2-um shift of the image is equivalent to a 0.2-μm shift of the object, i.e., the object shift is the image shift divided by the magnification or D/m. Even if the optics are improved by going to a 0.65 numerical aperture (with a consequent increase in magnification to 20X), D is 1.6 μm or still about 2 μm. Thus, there is an error associated with measuring the image at the half-power point if the illumination is fully coherent. Since the illumination in a microscope system is usually partially coherent, the error is between zero and the value of D. Unless the degree of partial coherence is known, the error is only bracketed. It would be less ambiguous to illuminate with completely coherent light and calculate the edge location at the APPENDIX B quarter-intensity point (where the edge will be found in such illumination). The Considine summary considered only edges. These edges can be treated in isolation and do not interact with any other image structure. Skinner, in his dissertation, treated the problem of lines and found that when the line widths were greater than about ten times the width of the impulse response of the imaging optics, the line could be treated as the combination of two edges whose effects did not interact. As an example, consider an f/1.6 lens, such as these used in high-quality step-and-repeat cameras, to be diffraction limited for actinic radiation of 405 nm. The impulse response for this lens is 1.3 μm; therefore, the line width that images without interaction between its two edges is approximately 13 μm or greater. A detailed analysis for line widths below such values must be carried out before the assessment of the effects of partial coherence on line-width measurements can be made. Recent research [50] indicates that with an image-splitting eyepiece the measurement of line widths can be accurate to better than one tenth of the smallest resolvable dimension. For this accuracy, it is assumed that the object is incoherently illuminated. This accuracy may not be realized in practice because of the inability to incoherently illuminate the object as indicated in the study of tri-bar target imagery by D. N. Grimes [51]. The program needs an investigation with considerable attention given to microscope adjustments and to the effects of partial coherence relative to the object structure. Figure 3. Theoretical plot of the intensity distribution for a coherent and incoherent image of an edge. (Philip S. Considine, Journal of the Optical Society of America, Vol. 56, p. 1003, 1966.) Note - The abscissa is proportional to the horizontal distance from the edge. The horizontal distance between the two curves at the half-power point represents an edge image displacement of about 2 um for the examples of appendix B. APPENDIX C SELECTED BIBLIOGRAPHY OF PHOTOMASK PUBLICATIONS The literature sources cited in this bibliography contain information related to the optical and micrometrological aspects of the manufacture and use of photomasks in semiconductor processing. This listing contains book chapters and sections, articles from technical journals, contract reports, reports by U.S. Government agencies, company publications, and proceedings of technical conferences, seminars, and symposia. Publications that describe complete procedures for fabrication of a specific IC device and only briefly describe photomask procedures or publications that concentrate primarily on other aspect of photomask technology, such as artwork generation, photoresists, or defect inspection, have generally not been included It should also be noted that references to electron-beam and x-ray generation of masks are excluded since the present study was concerned primarily with masks made using visible and ultraviolet radiation (photolithography). The approximate period covered by the bibliography is from January 1966' through November 1974 and the arrangement is chronological. Some of these literature sources are also cited as references in the present report. The abbreviations used for technical journals follow the recommendations of (1) ACCESS Key to the Source Literature of the Chemical Sciences (The American Chemical Society, 1969) and (2) Publications Indexed for Engineering (Engineering Index, Inc., New York, 1974). Some of the bibliographic sources are also available directly from NTIS (National Technical Information Service, 5285 Port Road, Springfield, Va. 22151) and DDC (Defense Documentation Center, Cameron Station, Alexandria, Va. 22314). The ASTM documents referenced are available as separate reprints from the American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103. These documents appear in the 1974 edition of the Annual Book of ASTM Standards in essentially the same form as Tentatives. APPENDIX C 1966 Newman, P. A., and Rible, V. E., Pinhole Camera for Integrated Circuits, Appl. Opt. 5, No. 7, 1225-1228 (July 1966). Schuetze, H. J., and Hennings, K. E., Large-Area Masking with Patterns of Micron and Submicron Element Size, Semicond. Prod. and Solid State Technol. 9, No. 7, 31-35 (July 1966). Maple, T. G., Integrated Circuit Mask Fabrication, Semicond. Prod. and Solid State Technol. 2, No. 8, 23-34 (Aug. 1966). Stevens, G. W. W., Control of Line Width in Photographic Masks, Trans. Inst. Metal Finishing 44, Printed Circuit Supplement, 123-127 (1966). 1967 Gaudiano, S., Microcircuit Masking Techniques, Proceedings of Second NASA Microelectronics Symposium, NASA TM X-55834 (June 1967). Payne, P. D., Photomask Technology in Integrated Circuits, Semicond. Prod. and Integrated Circuit Technology - Instrumentation and Techniques for Measurement, Process and Failure Analysis, S. Schwartz, ed., pp. 63-79 (McGraw-Hill Book Co., Inc., New York, 1967). 1968. Stevens, G. W. W., Microphotography, Second edition, Chapter 11 (John Wiley and Sons, Inc., New York, 1968). Geikas, G. I., and Ables, B. D., Contact Printing Associated Problems, Proceedings of the 1968 Kodak Photoresist Seminar, Los Angeles, Ca., May 20-21, 1968; Eastman Kodak Co., Rochester, N. Y., Publication P-192-B, pp. 47-54. Levine, J. E., Process Analysis of Mask Making, Solid State Technol. 11, No. 7, Tong, J. B., Mask Manufacture for Integrated Circuits, Solid State Technol. 11, Lovering, H. B., Direct Exposure of Photorest by Projection, Solid State Technol. 11, Beeh, R. C. M., Automation and Motor Function Routines for Mask Making, Solid State Technol. 11, No.7, 27-34 (July 1968). Reams, Jr., R. B., and Klute, C. H., An Automatic Step-and-Repeat Camera, Harry Diamond Laboratories Report TR-1417 (Nov. 1968). (Available from NTIS as AD682068.) |