Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2000-06-20
2003-09-30
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06628381
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of optical inspection techniques and relates to a method and an apparatus for inspecting patterned articles, such as integrated circuits, printed circuit boards, photolithographic masks, liquid crystal displays, etc., utilizing a collection angle design.
BACKGROUND OF THE INVENTION
A typical structure of the patterned article, such as a semiconductor wafer, includes a basic cell element repeated numerous times in both lateral dimensions, creating an almost perfectly periodical two-dimensional pattern, Semiconductor wafers are inspected prior to and after a pattering procedure, since the timely detection of anomalies on the wafer's surface is a very important factor, subsequently leading to an increase in production yields.
The prior-to-patterning inspection of wafers relies on the fact that light is scattered mainly from anomalies present on the generally flat and smooth surface of the non-patterned wafer. Thus, any detection of scattered light may be indicative of a defect. When applying an optical inspection to a patterned wafer aimed at detecting defects (e.g., the existence of foreign particles), scattered light can be caused by the pattern. Therefore, the detection of scattered light is not necessarily indicative of a defect. Conventional techniques for detecting defects are the so-called “die-to-database” and “die-to-die” techniques, according to which light scattered from an individual die is compared to, respectively, the previously prepared database indicative of light scattered from the “non-defective” die and a “neighbor” of this individual die. Differences between the signals are indicative of light scattered from anomalies present on the surface of the article. Generally speaking, detected difference in light components scattered from the individual die is indicative of the absence or addition of some features in this die, as compared to the “non-defective” die or “neighbor” die, and is therefore considered to be a defect.
An optical inspection apparatus is typically composed of such main constructional parts as an illumination system and a light collection/detection system utilizing either a bright field or dark field mode. The bright field detection mode is based on the variability in the specular reflectance from the wafer under inspection, created by defects squandered on the wafer. The dark field detection mode uses the scattering from defects that does not orient in specular reflectance.
It is a common goal of the collection scheme to increase the signal-to-noise ratio of the detected signal as much as possible. Various forms of dark field detection schemes are found to be very effective for the purpose of defect detection. According to one known technique of this kind, disclosed in U.S. Pat. Nos. 4,898,471 and 5,604,585, to facilitate meaningful signal comparison, the light collection system collects light at one constant collection angle at the azimuth and elevation other than those where the specular reflection occurs. However, light scattered from the patterned article (e.g., wafer) always contains light components scattered from the pattern towards this collection angle, and, therefore, detection of these light components is indicated by increased “noise” in the detected signal.
SUMMARY OF THE INVENTION
The present invention is aimed at improving automatic optical inspection of patterned articles by providing a novel method and apparatus enabling the signal-to-noise ratio of detected signals to be significantly increased.
The present invention takes advantage of a technique utilizing a variable angle design of the light collection system. This technique is disclosed in a co-pending application assigned to the assignee of the present application. According to this technique, an article (e.g., wafer) under inspection is scanned region-by-region, light scattered from each of the scan regions is collected with a certain maximum collection angle constant for each scan region, and directed towards a detection means trough a filter. The latter is selected such as to pick up from the entire collected light only that part thereof propagating with a solid angle segment of the entire collection angle, where the intensity of light scattered from the pattern is minimal, as compared to other solid angle segments of the maximum collection angle. This enables to increase the signal-noise ratio of the detected signal. The detection means comprises at least one detection unit operating in a dark field imaging mode, i.e. collecting light components scattered from the article at the azimuth and elevation different from those where the most specular reflection occurs.
The main idea of the present invention consists of selecting the variable angle solution. This is implemented by acquiring a bright field or high resolution dark field image and analyzing the acquired image to determine a solid angle segment of the propagation of light returned from the pattern within the collection angular field. This enables customized light collection (CLC) to be applied to dark field scattering signals, so as to prevent that part of the scattered signals, which is associated with the pattern on the article and constitutes “noise”, from reaching the detector, thereby allowing the detection of only that part of the scattering signals which is associated with any feature in the illuminated region other than those of the pattern. In other words, a suitable mask is placed in the optical path of light scattered from the article and propagating towards a dark field detector, such that the mask cuts off the solid angle segment of propagation of light scattered from the pattern. This results in a significant increase in the signal-to-noise of the detected signals.
Thus, according to one aspect of the present invention, there is provided a method for optical inspection of a patterned article, the method comprising the steps of:
(i) illuminating a region on the article with incident light to produce light returned from the illuminated region;
(ii) acquiring an image of the illuminating region, and generating data representative thereof;
(iii) analyzing the generated data and determining intensity distribution of light components scattered from the pattern of the illuminated region within a certain collection angular field outside a solid angle of propagation of specularly reflected light;
(iv) based on the determined distribution, filtering light collected with said certain collection angular field, so as to collect light components scattered from the illuminated region and propagating with at least one predetermined solid angle segment of said certain collection angular field, and to direct the collected light components to a detection unit.
The term “collection angular field” used herein signifies a maximum solid angle of collection of light scattered from the article defined by an optical arrangement of the detection unit.
In step (ii) above, light detected for acquiring the image of the illuminated region may be that specularly reflected from the illuminated region (i.e., bright field detection mode) or the scattered light propagating with a solid angle outside the solid angle of propagation of specularly reflected light (i.e., dark field detection mode). To obtain data indicative of the acquired image of the pattern in the illuminated region, sufficiently high resolution should be provided. To his end, the light collecting optics utilizes a high numerical aperture objective lens.
Analysis of the generated data consists of the so-called “modeling” of the pattern structure, and obtaining a discrete two-dimensional (or three-dimensional) array indicative of the scattering pattern. This data is used to simulate the dark-field scattering pattern within the collection angular field. The simulation results enable to determine the “background” intensity rising from the periodic pattern as the intensity lobe in the simulation/imaging plot, and collect light components propagating with the solid angle segment(s) of the collection angular f
Komem Amir
Milshtein Erel
Applied Materials Inc.
Rosenberger Richard A.
Sughrue Mion LLP.
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