Multiple wavelength or multiple shear distance quantitative...

Details Multiple wavelength or multiple shear distance quantitative... Multiple wavelength or multiple shear distance quantitative...

2000-05-26

2002-08-13

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

For dimensional measurement

C356S520000, C356S495000

Reexamination Certificate

active

06433876

ABSTRACT:

TECHNICAL FIELD
The present invention is directed generally to optical microscopy and in particular addresses how to increase the effective wavelength of a DIC measurement and thereby increase the slope of a surface that can be measured without ambiguity by combining data taken at two or more wavelengths or two or more shear distances. This measurement range is still limited by the depth-of-focus of the objective.
BACKGROUND ART
Many manufacturing processes require the measurement of microscopic surface topography. Applications include, but are not limited to, the control of surface finish in machined parts and the inspection of semiconductor wafers. Phase shifting interference, and more recently, white-light interference (WLI) microscopes are widely used to measure surface topography with lateral resolutions from less than one micron to tens of microns and height resolutions to less than a nanometer.
Phase shifting and WLI microscopes use optical interference between a reference surface and a sample to produce an interference image with fringes of constant height. Typical microscope types are based upon Mirau or Michelson interference objectives. A phase shifting interference microscope modulates the phase of the interference pattern by changing the relative length of the sample and reference arms of the interferometer. The acquisition and processing of a set of phase shifted interference images permits the calculation of the height of the sample at each pixel. The methods of phase shifting are well known.
WLI microscopes have superceded the use of phase shifting interference microscopes in almost all applications. One difference between a WLI microscope and a phase shifting microscope is the requirement for a short coherence length source in the former and its optional use in the latter. A major hardware difference between the two devices is the substantially larger vertical scan range of a WLI microscope—as much as hundreds of microns rather than less than one micron. The practical difference between the two types of microscopes is that a WLI microscope generates large amounts of data and uses substantially different processing algorithms to obtain a much greater measurement range than phase-shifting microscopes, with only a modest loss in precision. However, both microscopes share a significant problem—extreme sensitivity to environmental effects, especially vibration and air turbulence.
The environmental sensitivity of phase shifting and WLI interference microscopes is due to the non-common path nature of the test and reference arms of the instrument and the inherent sensitivity of optical interference. A Nomarski or differential interference contrast (DIC) microscope maintains the inherent measurement sensitivity of optical interference while removing the extreme environmental sensitivity because the two interfering light paths are nearly identical or common-path.
One of the earliest publications describing a DIC microscope is “Application á la métallographie des méthodes interférentielles á deux ondes polarisées”, by G. No-marski and A. R. Weill,
Revue de Matallurgie
, LII, #2, 1955, pp. 121-134. However, a DIC microscope is typically considered to be for qualitative use only. The reason for this is somewhat understandable. A WLI microscope produces fringes that are contour lines of constant height, just like a topographic map—except the contour spacing is on the order of 0.3 microns rather than meters or more. A DIC microscope produces fringes that are contours of constant slope in one direction. There are two difficulties with using an interferometer that produces slope fringes. First, slope fringes are difficult to visually interpret and second, slope must be measured in two directions to fully reconstruct a surface.
When personal computers became widely available, the natural course of action was to automate the processing of what people were used to looking at—fringes of constant height. In modern manufacturing processes, visual inspection of images is avoided not only because it is slower but also because it is less reliable, precise and accurate than automated image processing. Modern computers are quite capable of performing the necessary computations on slope data from a DIC microscope to obtain surface topography data making it possible to take advantage of the environmental insensitivity of a DIC microscope.
In qualitative DIC microscopy, a rotating polarizer in conjunction with a quarter-wave plate is used to modify the image produced by the microscope so that features of interest are clearly visible. Rotating the polarizer changes the relative phase of the two interfering beams providing the ability to phase shift (phase modulate) the interference image present on the detector. Devices based upon liquid-crystal technology may be used in place of the rotating polarizer and may optionally incorporate the quarter-wave plate as part of the liquid-crystal device. Multiple, phase shifted DIC images can be acquired and then processed using standard techniques from phase shifting interferometry to produce a quantitative measure of sample surface slope, where the slope measured is in the shear direction of the DIC prism. Quantitative DIC measurements were first presented in a paper by Hong et al in July 1993 at the annual SPIE conference; see, Gao Hong et al, “Three-dimensional optical profiler using No-marski interferometry,” in
SPIE,
Vol. 1994, pp. 150-153,
Advanced Optical Manufacturing and Testing IV
, published Febuary 1994, presented Jul. 11-16 1993 in San Diego, Calif. by Robert E. Parks (ISBN 0-8194-1243-0).
Delbert L. Lessor et al, in “Quantitative surface topography determination by Nomarski reflection microscopy. 1. Theory,”
Journal of the Optical Society of America
, Vol. 69, No.2, pp. 357-366 (Febuary 1979) presented early theoretical work in 1979. Lessor et al; provide basic theory and propose rotating the sample to obtain slope data in two orthogonal directions; notably, this paper was not referenced by Hong et al.
A major limitation of a DIC microscope as compared to a WLI microscope is the need, in general, for rapid, robust measurement of slope in two directions. Another broadly applicable constraint is that a phase shifted interference and a DIC microscope are both limited to measurements within the depth of focus (DOF) of the objective, while WLI can go far beyond the DOF.
The present invention addresses how to obtain, rapidly and robustly, the measurement of surfaces with a greater slope than is possible with a single wavelength or shear distance.
The detailed discussion begins with a disclosure about the use of wavelength multiplexing to obtain slope data in two directions in a DIC microscope.
DISCLOSURE OF INVENTION
In accordance with the present invention, a differential interference contrast (DIC) microscope system is provided comprising:
(a) an illumination source for illuminating a sample;
(b) a lens system for viewing the illuminated sample, including an objective, defining an optical axis;
(c) a DIC prism having a shear angle for separating an input light beam into two, orthogonally polarized output beams that are separated by the shear angle;
(d) at least one detector system for receiving a sample image;
(e) a mechanism for modulating the phase of the DIC image;
(f) a mechanism for selecting the wavelength of light used in acquiring at least two sets of DIC data taken in a single shear direction; and
(g) a mechanism for manipulating the multiple DIC data sets to synthesize data that appear to have been taken at a longer synthetic wavelength.
The various approaches disclosed and claimed herein permit the measurement of surfaces with a greater slope than is possible with a single wavelength or shear distance. The methods disclosed herein are applicable to measurement of slope in a single direction or in two directions. The disclosed methods are also compatible with the methods and devices of wavelength multiplexed quantitative DIC microscopy disclosed in above-referenced application Ser. No. 09/579,733 for obtaining slope data i

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