Patent Application
20060159225
The present invention relates to an X-ray detection system.
Typically, an X-ray measurement device includes an X-ray source and an X-ray detector. The X-ray source may have X-ray optics to form an X-ray beam which is then directed towards a sample, which is the object of the measurements. The detector may be positioned to receive diffracted or specular X-rays depending on the measurement required. In most types of measurements, a physical slit or aperture is required before the X-ray detector to limit the angle or area of the X-ray beam that is scattered from the sample. There are two main reasons for having a slit or aperture:
An example of a prior art X-ray reflectivity measurement system 100 is shown in
The deflected X-ray beam 106 contains additional information from the sample 104 from that contained in the specular X-ray beam 112. This additional information is contained within the diffuse scatter around the specular X-ray beam 112 and requires the detector 110 and slit 108 to be repositioned to enable measurement by a separate scan.
Furthermore, it is often difficult to decide on the slit or aperture size before performing an X-ray measurement. Factors that must be taken into consideration when selecting an appropriate slit or aperture size include intensity, resolution and signal to noise ratio.
According to a first aspect of the present invention there is provided an X-ray detection system for detecting a deflected X-ray beam from a sample comprising:
Preferably, the sub-image is a stripe.
Typically, a stripe is a substantially rectangular portion of the sample image. For example, for a digital sample image made up of pixels, a stripe may be extracted by selecting a number of columns of adjacent pixels.
Alternatively, the sub-image is a curved stripe.
A curved stripe may be required with the use of certain microfocus sources when placed close to the sample.
Further alternatively, the sub-image is a substantially circular portion.
When the sub-image is a stripe the x-ray detection means is representing a “virtual” slit and, when the sub-image is a circular portion, the x-ray detection means is representing a “virtual” aperture.
Preferably, the x-ray detection system further comprises an image analysis means, wherein the sub-image is analysed according to one or more algorithms.
Preferably, the image analysis means is enabled to instruct the region of interest extraction means to extract a further sub-image of a second area from the sample image.
Preferably, the detector comprises an array or matrix of pixels.
Preferably, the detector is a Charge Coupled Device (CCD) image sensor suitable for X-rays.
According to a second aspect of the invention there is provided a method of detecting a deflected X-ray beam from a sample comprising the steps of:
According to a third aspect of the present invention there is provided a x-ray measurement system for measurement of a sample comprising:
Preferably, the X-ray measurement system is an X-ray topographic, reflectometer or diffractometer system.
Embodiments of the present invention will now be described with reference to the accompanying drawings, in which;
Referring to
The detector array 210 may be any suitable imaging device, such as an X-ray sensitive CCD (Charge Coupled Device) with suitable signal to noise ratio and dynamic range capability.
The sample image 300 is acquired by the image processing means of the combined data, which, in general, will contain both specular and diffuse data.
The sample image 300 is then processed digitally to extract information. The specular or elastic scattered data is found by integrating a vertical stripe between points A-A.
The positions of the points A are decided on processing the acquired data, not before the experiment is made, and may be altered as often as required in order to find the optimum dimensions of the stripe, without requiring additional X-ray measurements.
The diffuse scatter lies between points A and B on either side of the vertical slice A-A. It may be integrated totally to measure the total diffuse scatter (TDS) or integrated in narrow strips to obtain a plot of the diffuse scatter against angular position.
Consequently, we obtain a set of “virtual” slits that are independent of instrument settings. The “virtual slit” size may be altered after the sample image 300 has been acquired. Furthermore, the sample image 300 may be processed as often as required, using different slit sizes, to obtain the optimum information.
The above examples describe the use of slits and virtual slits to obtain stripes of data related to a sample. In certain X-ray measurements, a small area, such as a circle, is illuminated by the X-rays rather than a stripe through the use of an aperture. Identical principles apply to the construction of a “virtual” aperture from the sample image.
Furthermore, in the X-ray measurement system 200 the detector 210 is positioned to receive the deflected X-ray beam 206 specularly. The X-ray detection system of the present invention is equally applicable to X-ray measurements where the X-ray beam is diffracted and the detector is positioned on either the same or on the opposite side of the sample from the X-ray generator. That is, the detector 210 can equally employed for transmission measurements.
Referring to
In use, an X-ray beam 412 is received by the detector array 402 and a sample image, such as that shown in
A particular case arises when using a microfocus X-ray source, such as the microsource® X-ray source, close to the sample. The stripe obtained from the specimen is significantly curved when measured on a flat detector. The curvature may be calculated from geometric diffraction principles and is a conic section (the “Bragg conic”). If a physical slit is used, it also has to be curved and the curvature has to match that of the conic. Use of a virtual slit is far simpler, since the “curvature” is performed in the definition of the Region Of Interest (ROI) in the image integration. The virtual slit is thus significantly simpler than a variable-curvature mechanical slit.
In the digital X-ray topography system known as BedeScan™ (U.S. Pat. No. 6,782,076), data are acquired on an imaging detector and subsequently processed to form a full image. Use of the X-ray detection system according to the present invention means that a topographic scan may be reconstructed in different ways, to emphasize different aspects of the specimen.
Referring to
In high resolution X-Ray diffraction, the x-ray detection system of the present invention has application to improving the repeatability and throughput of a semiconductor metrology system, typically for measuring thickness and composition of epitaxial layers.
A typical measurement uses a slit-limited metrology detector scanning at twice the rate of the sample (an omega-2theta scan). A slit-limited detector is a detector whose sensitive region of data acquisition is defined by a mechanical slit, that is, a mechanical slit is placed in front of the X-ray detector. The slit has to be narrowed enough to obtain good enough signal to noise ratio for the scan, but this loses intensity and gives worse throughput. Throughput is the total time of data acquisition (plus overheads, e.g. change of specimen).
Use of the x-ray detection system of the present invention means that an algorithm can be employed that progressively reduces the slit setting in post processing of data, until the optimum repeatability is obtained. This can be gauged by the “Goodness of Fit” reported by an automatic fitting algorithm, such as the Genetic Algorithm software, U.S. Pat. No. 6,192,103.
A further example is in the use of an area detector to gather triple-axis reciprocal space data. Information on triple-axis reciprocal space data can be found in “High resolution X-Ray diffractometry and topography”, D. K. Bowen and B. K. Tanner, Taylor & Francis, London, 1998, chapter 7. This can usefully be post-processed to show reciprocal space maps at different resolutions, by use of a virtual aperture on the detector.
In X-ray powder diffraction, replacement of a mechanical slit by the X-ray detection means of the present invention allows an infinitely variable receiving slit to be synthesized. Data may be post-processed to give either high resolution or high intensity in the peaks.
In X-ray reflectivity, the specular and diffuse scatter may be separated but measured simultaneously. In this case the virtual slit width is set to match the incident beam properties.
Improvements and modifications may be incorporated without departing from the scope of the present invention.
