Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
2002-01-15
2004-02-03
Gutierrez, Diego (Department: 2859)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S559400, C250S559460, C356S237100, C356S239300, C356S239700, C359S214100
Reexamination Certificate
active
06686602
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical inspection systems, and specifically to methods and systems for detecting defects on patterned substrates.
BACKGROUND OF THE INVENTION
The use of spatial filtering is well known in systems for optical inspection of patterned substrates, which contain regularly-repeating structures. When the repetitive patterns on such substrates are illuminated with coherent light, they generate constructive interference lobes along well-defined directions. A suitable lens can be used to collect the light reflected from the surface into an image that constitutes a spatial Fourier transform of the substrate pattern. (The lens used for this purpose is referred to as a Fourier transform lens, and the plane in which the Fourier transform image is formed is referred to as the Fourier plane.) The position and extent of the interference lobes in the Fourier plane depend on the period of the pattern and on a scaling factor determined by the wavelength of the incident radiation and the focal length of the lens.
It is known in the art that blocking the interference lobes in the Fourier plane facilitates the detection of defects and pattern irregularities on the substrate. For example, U.S. Pat. No. 3,614,232, to Mathisen, whose disclosure is incorporated herein by reference, describes a spatial filter for detecting defects in photomasks, using a transmission geometry and a simple filter consisting substantially of the negative of the Fourier transform of a defect-free specimen of the microcircuit. U.S. Pat. No. 5,177,559, to Batchelder et al., whose disclosure is likewise incorporated herein by reference, describes a dark field imaging system for inspecting repetitively patterned integrated circuits on a semiconductor wafer. The light scattered from the pattern is filtered with an opaque spatial filter, which attenuates spatial frequency components corresponding to the wafer pattern, and is then converted to an image on an imaging sensor. The substrate is illuminated at a grazing angle to the substrate plane, whereas the scattered light is collected in a direction essentially normal to the surface.
U.S. Pat. No. 4,370,024, to Task et al., whose disclosure is incorporated herein by reference, describes a dynamic binary Fourier filtered imaging system (not specifically for wafer inspection.) The spatial filter used in this case consists of a liquid crystal array, which can be programmed to dynamically produce different opaque filter patterns. Further in this vein, U.S. Pat. No. 5,276,498, to Galbraith et al., whose disclosure is also incorporated herein by reference, describes a system for performing dark field surface inspection of substrates, such as repetitively patterned semiconductor wafers, employing a scanned, focused laser beam and an adaptive spatial filter consisting of a liquid crystal light valve array. The laser beam is incident at a grazing angle on the wafer surface, and the scattered light is also collected at a grazing angle (away from the specularly reflected optical axis), and is measured by a detector. The proper configuration of the spatial filter is determined by illuminating the repetitive pattern with the laser, successively turning each liquid crystal element on and off, and measuring the level of the optical signal at the detector. The pattern is then stored in a computer memory for subsequent use.
U.S. Pat. No. 5,659,390, to Danko, whose disclosure is incorporated herein by reference, describes a system for performing dark field surface inspection of substrates, such as repetitively patterned semiconductor wafers, employing a scanned laser beam and an adaptive spatial filter. Here the spatial filter consists of an optically-addressable liquid crystal spatial light modulator. The laser beam is incident at a grazing angle to the wafer surface, and the scattered light is collected by a detector in a direction normal to the surface. A write beam derived directly from the Fourier-transformed scattered light automatically determines the proper configuration of the spatial filter.
Solutions based on adaptive spatial filters, such as those described in U.S. Pat. Nos. 5,276,498 and 5,659,390, are advantageous in their ability to match the spatial filter to different substrate patterns without the need to replace an optical element. Such filters are capable of generating arbitrary filter patterns, within the constraints of liquid crystal technology. This technology suffers from several shortcomings, however:
1) The transmission range of liquid crystals known in the art is limited to wavelengths in the visible spectrum, roughly 400-700 nm.
2) Liquid crystal modulators have limited contrast ratios, and they are sensitive to polarization and to incidence angle. Therefore, the optical signal transmission of the desired light may be reduced, while the transmission of the undesired light is increased, hence reducing the contrast of the spatial filter itself.
3) When the optically-addressed mechanism is used (as in U.S. Pat. No. 5,659,390), the spatial filter is determined completely automatically. There is no possibility of user intervention to adjust the filter configuration to compensate for unusual substrate features and for different measurement conditions.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved methods and devices for spatial filtering, and particularly for adaptively blocking undesired spatial frequency components in the Fourier plane.
It is a further object of some aspects of the present invention to provide improved methods and systems for inspection of patterned substrates.
In some preferred embodiments of the present invention, a sample is scanned by a beam of coherent light, and light scattered from the substrate is collected by a Fourier lens. An image sensor acquires an image of the Fourier plane and inputs the image to a filter controller, typically a computer, which analyzes the image to determine an optimal spatial filtering pattern. The computer accordingly controls a programmable spatial filter in the Fourier plane to filter the scattered light using the desired filtering pattern. The use of the image sensor in this manner allows the filtering pattern to be calculated adaptively for the particular substrate and inspection conditions, while simplifying the determination of the filtering pattern, relative to methods known in the art, since the spatial filter itself is not involved in the determination. The filtered light is received by a detector, and the detector signal is analyzed to detect defects in the substrate.
Typically, the sample comprises a patterned substrate, and the optimal filtering pattern is determined, based on the acquired image of the Fourier plane, so as to block spatial frequencies that correspond to the interference lobes generated by the substrate pattern. Additionally or alternatively, the computer maintains a database of optimal filter configurations that have been determined empirically and/or theoretically for different substrate types and measurement conditions. Information from the database is preferably combined with the acquired image to determine the optimal filter pattern to use in each case, either automatically or under the control of an operator. Alternatively, when appropriate, the filter pattern stored in the database may be used, while the acquired image is disregarded. In any case, the filter pattern is preferably determined not only by the acquired image, but also by other considerations, which may be addressed in the database and/or controlled by the user.
In some preferred embodiments of the present invention, the spatial filter comprises an array of micro-optical-electro-mechanical (MOEM) elements. The computer controls the “on” or “off” status of each pixel in the filter by varying the tilt angle of the particular micro-element. In some of these embodiments, the micro-elements comprise mirrors, and the filter operates in a reflective mode. Only the light that is reflected from the “on” pixels reach
Applied Materials Inc.
Blakely , Sokoloff, Taylor & Zafman LLP
Guadalupe Yaritza
Gutierrez Diego
LandOfFree
Patterned wafer inspection using spatial filtering does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Patterned wafer inspection using spatial filtering, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Patterned wafer inspection using spatial filtering will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3353009