Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1999-04-16
2001-06-12
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
C250S234000
Reexamination Certificate
active
06246477
ABSTRACT:
This invention pertains to optical microscopes, particularly to scanning optical microscopes.
During the last few years several techniques have been developed to enhance the resolution of optical microscopes. These techniques include confocal microscopes, which are now commercially available, and which improve resolution by a factor of about 2½, and near field microscopes, which have a resolution of about &lgr;/10, but which have a low light level and a working distance much less than 1 &mgr;m. Although such microscopes have applications in biology, to a large extent these recent efforts have been driven by the integrated circuit industry, which has a need to measure small features on masks and wafers. A device commonly used to locate the edges and hence to measure the widths and locations of features on masks and wafers is the linearly scanned microdensitometer. Such microdensitometers are frequently designed to have reduced resolution in the direction perpendicular to the scan, for example by passing the image through a slit or by averaging several consecutive video scan lines. The edges may be located by comparing the video signal to a predetermined threshold, or by more extensive image processing. As minimum feature sizes, especially on wafers, have dropped into the submicron range, accurate optical measurements have become increasingly challenging. Features near or below the limits of optical resolution do not produce fully modulated video signals, significantly compromising the accurate determination of edge locations. This problem is particularly serious on wafers where, in addition to small feature size, uncontrolled variations in feature contrast may preclude knowledge of the absolute signal levels. Moreover, the height of the features being measured may exceed the depth of focus of the optical system used to generate the video signal.
M. Levenson et al., “Improving Resolution in Photolithography with a Phase-Shifting Mask,”
IEEE Trans. Electron. Dev.,
vol. ED-29, pp. 1828-1836 (1982) is representative of prior papers disclosing a phase shift plate to alter the phase of transmitted light. There was no suggestion to use such a device as part of a microscope.
I have discovered a scanning optical microscope that uses an optical phase shift technique to improve optical resolution by a factor of about two compared to conventional light microscopes, without reducing either the depth of focus or the working distance. The novel technique reduces, for example, error in the location of edges of features on lithographic masks or wafers. The technique also permits the accurate determination of edges on features that would otherwise be too narrow to be measured. The novel microscope is useful in any field where optical microscopes are employed, including for example both semiconductor and biological applications. It is also useful in optical data storage applications, where it could be used to increase the amount of data stored per unit area by a factor between about 2 and about 4. In addition, by reducing the numerical aperture of the optical system, the depth of focus may be extended by a factor of four in comparison to a conventional optical system, while maintaining the resolution. The new microscopes use standard components to produce high intensity images, with large working distances and large depths of focus, without substantial mechanical complexity.
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Feldman, M. et al., “High-resolution Scanning Microdensitometer,” Rev. Sci. Instrum., vol. 68, pp. 3112-3115 (1997).
Levenson, M. et al., “Improving Resolution in Photolithography with a Phase-Shifting Mask,” IEEE Trans. Electron Dev., vol. ED-29, pp. 1828-1836 (1982).
Board of Supervisors of Louisiana State University and Agricultu
Pham Hoa Q.
Runnels John H.
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