The subject invention relates to optical metrology methods for inspecting and evaluating semiconductor wafers. The preferred embodiment is particularly suited for eliminating artifacts arising in the interline CCD in a two-dimensional (2D) optical metrology applications
There is considerable interest in monitoring the properties of semiconductors at various stages during the fabrication process. Monitoring the properties during fabrication allows the manufacturer to spot and correct process problems prior to the completion of the wafer.
The inspection of actual product wafers during or between process steps usually require non-contact techniques. Accordingly, a number of tools have been developed for optically inspecting semiconductor wafers. Such tools include reflectometers and ellipsometers. To increase the robustness of the measurements, these tools can often obtain measurements at multiple wavelengths and/or multiple angles of incidents using one-dimensional or two-dimensional CCD optical detectors.
Noise mitigation in multi-element optical detectors (1D line, and 2D array) has primarily focused on reducing the noise contribution due to “dark current”, which is due to electron accumulation at the optical sensor element, and “fixed pattern noise”, which is primarily due to variations in the detector element responsivity. “Fixed pattern noise” is, in fact not noise, since once measured, it is predictable.
Still other 2D detector “noise mitigation” or “noise reduction” techniques rely on non-linear processing of the pixels. Examples of these techniques are described in U.S. Pat. No. 6,731,806. However, these methods cannot be used in many optical metrology applications as they confound the later stages of processing needed to extract the surface metrology information.
However, scientific image detectors (such as would be used for precision metrology applications) have very little “fixed pattern noise” due to careful fabrication and device testing/selection. Therefore, noise mitigation for these detectors is primarily concerned with “dark noise”, 1/f noise, “burst noise”, and readout electronics thermal noise.
Most of the prior art “noise mitigation” techniques are designed for general application to arbitrary images and cannot take advantage of the substantial dark areas (portions of readout lines) within a frame that are present in two-dimensional optical metrology images.
Another example of prior art noise correction technique is described in U.S. Pat. No. 6,885,397. This patent discusses the use of embedded “correction” pixels, where the “corrector” pixels are used to correct the values of the “light-sensitive” pixels. However, the “dark” or “reference pixels” discussed in this patent are specially configured to avoid illumination and the “image correction” employed uses a circuit for correction.
Yet another example of noise reduction method is described in the publication “A Temporal Noise Reduction Filter Based on Image Sensor Full-Frame Data” by A. Bosco, K. Findlater, S. Battiato, A. Castorina published in Proceedings of IEEE ICCE 03—International Conference on Consumer Electronics, June 2003, pp. 402-403. This paper describes the use of embedded “dark lines” in an image, but instead of subtracting the “local dark reference” pixels directly from neighboring pixels (pixels within the same line), it teaches the use of a much more complicated non-linear filter whose operation depends on multi-frame “dark line” statistics. Such a filter and method would be inappropriate for optical metrology applications, as it can produce erroneous outputs from the subsequent estimation algorithms.
Yet another example of noise reduction techniques is described in U.S. Pat. No. 4,032,975. This is one is directed to methods for “pattern noise” reduction. However, this method applies mainly to noise that is “fixed” across the field of the 2D detector (often referred to as “fixed pattern noise”). Therefore, this method cannot reduce low-frequency (1/f) noise that is found in the detector elements and in the “readout” electronics.
Another example of “dark noise” reduction, correction and mitigation techniques is described in U.S. Pat. No. 5,355,164. This patent discusses the use of “blind” pixels (light-shielded pixels on a 2D imaging array) to estimate the dark-current. This method relies on pixels that are at the outer edges of the 2D imaging detector and so, are not near to the image of interest. Because, upon readout, the “blind” pixels discussed in this patent are not temporally close to the imaging pixels, low-frequency noise is not reduced.
Yet another example of dark signal compensation in 2D arrays is disclosed in U.S. Pat. No. 4,933,543. This patent is typical of many of the prior art techniques used to reduce dark-noise, that is the use of “shielded” or “masked” pixels (the “dark” pixels) to obtain values for correction of the image pixels. As a result, the technique of this patent cannot compensate for “noise” introduced by stray reflected light, whose value may change with the illuminated image.
For at least the reasons discussed above, all prior art image improvement and artifacts eliminating techniques are not suitable for their application to image processing in two-dimensional semiconductor optical metrology. A need exists for a simple and reliable method for eliminating image artifacts and improving signal-to-noise (S/N) ratio suitable for commercial optical metrology applications.
The subject invention describes two methods for eliminating artifacts in two-dimensional optical metrology utilizing the interline CCD detectors. These methods are based on a dark-subtraction principle. Self-dark subtraction method takes advantage of strong correlation between the noise patterns in illuminated and dark regions within the same image. Image artifacts are removed and the S/N ratio is improved significantly by subtraction of selected dark region of the image from the illuminated one within the same frame. Dark-frame subtraction technique reduces a “smear” effect by applying a digital processing based on subtraction of the dark frame images from the normal light frame images. Both methods are suitable for application to images obtained using two-dimensional optical metrology systems such as spectrometers, ellipsometers, beam profile reflectometers/ellipsometers, scatterometers and spectroscopic scatterometers.
Self-Dark Subtraction Method.
In two-dimensional optical metrology, CCD detectors are used to capture images of the sample. These detectors are known to suffer from electronic fluctuations in the dark signal.
These coherent fluctuations set a noise floor and, therefore, limit the S/N ratio that can be achieved for an optical metrology system in this application.
However, since these coherent fluctuations have similar patterns and comparable intensities in both illuminated regions and dark regions within the same frame (
The self-dark subtraction algorithm can have a number of variants, depending on the degree and type of correlation within the frame and between the frames. In the preferred embodiment, the self-dark subtraction method subtracts the dark-region of the image (area 300 or 400 in
new_pixel(N+i,M+j)=pixel(N+i,M+j)−pixel(O+i,P+j), for i=0 to X−1,j=0 to Y−1 (1)
as shown schematically in
Dark-Frame Subtraction Method.
Interline CCD cameras can suffer from a smear effect due to light leakage into the nominally covered readout pixels. This spurious smear signal adds to the intended image captured by the camera. It has been found that, the “dark frame”, e.g. the frame taken when the electronic shutter is closed (active pixels being reset), carries the same smear information as the normal light frame image taken during a normal capture. Therefore, the dark frames can be used to remove the artifacts caused by the smear effect from the images, as illustrated in
It has been found that the dark-frame subtraction without the self-dark subtraction adds extra noise to the original signal. In
The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/721,602, filed Sep. 27, 2005, the disclosure of which is incorporated herein by reference.
Number | Date | Country | |
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60721602 | Sep 2005 | US |