Abstract:
Reflectometry apparatus includes a radiation source, adapted to irradiate a sample with radiation over a range of angles relative to a surface of the sample, and a detector assembly, positioned to receive the radiation reflected from the sample over the range of angles and to generate a signal responsive thereto. A shutter is adjustably positionable to intercept the radiation, the shutter having a blocking position, in which it blocks the radiation in a lower portion of the range of angles, thereby allowing the reflected radiation to reach the array substantially only in a higher portion of the range, and a clear position, in which the radiation in the lower portion of the range reaches the array substantially without blockage.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.  
         BACKGROUND OF THE INVENTION  
         [0002]    X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a position-sensitive detector, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).  
           [0003]    A method for analyzing the X-ray data to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.  
           [0004]    U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.  
           [0005]    U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.  
           [0006]    Various types of position-sensitive X-ray detectors are known in the art of reflectometry. Solid-state arrays typically comprise multiple detector elements, which are read out by a CCD or other scanning mechanism. Typically, each element accumulates photoelectric charge over a period of time before being read out and therefore cannot resolve the energy or number of incident X-ray photons. XRR systems known in the art that are based on such arrays simply record the total integrated radiation flux that is incident on each element. The signals at low angles, below the total external reflection angle, are usually much stronger than the signals above this angle. A ratio of 10 5  to 10 7  in photon flux between 0° and 3° reflections is typical. The dynamic range of array detection systems known in the art is substantially smaller than this ratio. Consequently, high-order fringes at higher incidence angles cannot generally be detected. Photon counting sensitivity is needed in order to measure the weak signals at these angles.  
           [0007]    A further drawback of X-ray thin film measurement systems known in the art is their lack of spatial resolution. X-ray optics, such as the above-mentioned curved monochromators, are capable of focusing an X-ray beam to a spot diameter below 100 μm. When the beam is incident on a surface at a low angle, below 1°, for example, the spot on the surface is elongated by more than 50 times this diameter. A measurement that is made under these circumstances provides only an average of surface properties over the entire elongated area. For many applications, such as evaluating thin film microstructures on integrated circuit wafers, better spatial resolution is required.  
           [0008]    Although the present patent application is concerned mainly with systems in which a sample is irradiated by a monochromatic beam, other methods for X-ray reflectometry are also known in the art. One such method is described, for example, in an article by Chihab et al., entitled “New Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,” in  Journal of Applied Crystallography  22 (1989), p. 460, which is incorporated herein by reference. A narrow beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface in order to cut off the primary X-ray beam, so that only reflected X-rays reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U.S. Pat. No. 5,619,548) selects the wavelength of the reflected X-ray beam that is to reach the detector.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide improved methods and systems for X-ray analytical measurements, and particularly for measurements of thin film properties.  
           [0010]    It is a further object of some aspects of the present invention to provide systems for X-ray reflectometry with enhanced dynamic range.  
           [0011]    It is still a further object of some aspects of the present invention to provide systems for X-ray microanalysis with enhanced spatial resolution.  
           [0012]    In preferred embodiments of the present invention, a system for X-ray reflectometry is used to determine properties of thin films on the surface of a sample, typically a semiconductor wafer. The sample is irradiated by a monochromatic beam of X-rays, which is focused to a small spot size on the surface of the sample. X-rays reflected from the surface are incident on a detector array, preferably a CCD array, each detector element in the array corresponding to a different angle of reflection from the surface. Charge stored by the detector elements is clocked out of the array to a processor, which analyzes the charges to derive a fringe pattern, corresponding to the intensity of X-ray reflection from the surface as a function of angle. The X-ray optics and processing circuitry in the system are arranged to achieve a high dynamic range, whereby high-order fringes are plainly apparent in the reflected signal. The processor analyzes the fringe pattern based on a physical model of thin film properties, including density, thickness and surface roughness. The high dynamic range enables the system to determine these properties accurately not only for the upper thin film layer, but also for one or more underlying layers on the surface of the sample.  
           [0013]    As noted in the Background of the Invention, one of the major problems in X-ray reflectometry systems is that surface reflectivity is much greater at low angles (near 0°) than at higher angles. The strong flux of low-angle X-rays that is incident on the detector array tends to increase the overall background level, making it difficult to detect the weaker high-angle flux. To address this problem, in some preferred embodiments of the present invention, the system comprises a dynamic shutter, having low-angle and high-angle detection positions. In the low-angle position, the shutter allows the entire beam of X-rays from the X-ray source to impinge on the sample. In the high-angle position, however, the shutter blocks low-angle X-rays. As a result, the intense low-angle reflections are eliminated, so that the background level at the detector is reduced, and useful signals can be derived at high angles. The processor “stitches together” the signals received by the detector in the low- and high-angle positions of the shutter in order to derive a single fringe spectrum with enhanced dynamic range.  
           [0014]    Preferably, the housing and readout circuits associated with the detector array are also designed to reduce the background level in the high-angle elements. Most preferably, the CCD array is coupled to the readout circuits so that the signal output from the array to the circuits is adjacent to the end of the array that receives the high-angle reflections. In this configuration, the high-angle array elements are read out first, giving accurate readings of the weak signals received by these elements before the readings are contaminated by background charge transferred from the low-angle elements. Additionally or alternatively, the detector array is protected by an evacuated enclosure, closed off by an X-ray transparent window spaced a substantial distance in front of the array, between the array and the sample. As a result, scattering of low-angle X-rays from the window and from the air in the space immediately in front of the array is substantially reduced, thus further reducing the background level at the high-angle elements.  
           [0015]    As a further means for reducing both the size of the incident X-ray spot on the surface of the sample and the excessive signal at low incidence angles, in some preferred embodiments of the present invention, a dynamic knife edge is positioned over the surface. Preferably, the dynamic knife edge is operated in conjunction with the above-mentioned dynamic shutter. For measurements at low incidence angles, the knife edge is lowered very near to the surface, intercepting the incident X-ray beam and thus shortening the lateral dimension of the spot on the surface (i.e., the dimension in the direction along the surface that is roughly parallel to the beam axis). For high-angle measurements, at which the dynamic shutter is used, the knife edge is preferably raised out of the way, to allow the full intensity of the X-ray beam to be used. Such operation of the knife edge allows measurements to be made with high spatial resolution, particularly at low angles at which the lateral dimension of the spot is most greatly elongated, while maintaining high sensitivity even at high angles.  
           [0016]    In some preferred embodiments of the present invention, the processor analyzes the detector array output so as to determine a total, lumped X-ray flux for each detector element at high intensities (typically at low angles), while effectively counting individual X-ray photons per element at low intensities (high angles). The inventors have found that an X-ray photon of a known energy will generate a certain average charge in a detector element when the photon is incident as part of a high photon flux, and a lower average charge, which may be spread over two adjacent detector elements, at low flux. Preferably, to analyze the array output, the processor first determines the number of photons incident on each detector element that encountered a high X-ray flux, by dividing the total charge accumulated in these elements by the high-flux average charge. Then, over the remaining detector elements, the processor seeks individual elements or pairs of elements having charge levels comparable to the low-flux charge. The processor records a single photon count for each such element or pair of elements. This technique allows a single detector array, such as a CCD array, to be used simultaneously for both lumped flux and photon counting measurements, thus further enhancing the dynamic range with which the system can measure the pattern of reflection fringes.  
           [0017]    Although preferred embodiments of the present invention are directly mainly toward enhancing X-ray reflectometric measurements on thin films, and particularly on semiconductor wafers, the principles of the present invention can similarly be used in other applications of X-ray reflectometry, as well as in other types of radiation-based analysis.  
           [0018]    There is therefore provided, in accordance with a preferred embodiment of the present invention, reflectometry apparatus, including:  
           [0019]    a radiation source, adapted to irradiate a sample with radiation over a range of angles relative to a surface of the sample;  
           [0020]    a detector assembly, positioned to receive the radiation reflected from the sample over the range of angles and to generate a signal responsive thereto; and  
           [0021]    a shutter, adjustably positionable to intercept the radiation, the shutter having a blocking position, in which it blocks the radiation in a lower portion of the range of angles, thereby allowing the reflected radiation to reach the array substantially only in a higher portion of the range, and a clear position, in which the radiation in the lower portion of the range reaches the array substantially without blockage.  
           [0022]    Preferably, the radiation includes X-ray radiation, and the lower portion of the range includes angles below a critical angle for total external reflection of the radiation from the surface.  
           [0023]    Further preferably, the reflected radiation is characterized by a variation of intensity as a function of the angle due to the thin film layers, and when the shutter is in the blocking position, the signal generated by the detector assembly responsive to the reflected radiation in the higher portion of the range of angles has a reduced background level relative to the background level when the shutter is in the clear position. In a preferred embodiment, the sample includes one or more thin film layers, and the variation of intensity includes an oscillatory pattern. Most preferably, the apparatus includes a processor, which is coupled to receive the signal, from the detector assembly and to analyze the oscillatory pattern to determine one or more properties of the one or more thin film layers.  
           [0024]    Additionally or alternatively, the oscillatory pattern includes an initial shoulder occurring near a critical angle for total external reflection of the radiation from an outer one of the thin film layers at the surface of the sample, and the one or more properties includes a density of the outer thin film layer, such that the processor is adapted to estimate the density of the outer thin film layer responsive to the shoulder, irrespective of any other one of the properties.  
           [0025]    Further additionally or alternatively, the detector assembly includes an array of detector elements, and the signal is indicative of respective charges accumulated by the detector elements due to photons of the radiation that are incident on the elements, and the processor is adapted to estimate, responsive to the respective charges, a number of the photons that was incident on each of the elements.  
           [0026]    Preferably, the detector assembly is adapted to receive the radiation over a first integration period while the shutter is in the clear position and over a second integration period, substantially longer than the first integration period, while the shutter is in the blocking position, and including a processor, which is coupled to receive the signal from the detector assembly and to combine the signal generated by the detector assembly during the first integration period with the signal generated by the detector assembly during the second integration period so as to reconstruct the variation of the intensity over the entire range of angles.  
           [0027]    Preferably, the detector assembly includes an array of detector elements, including a first element positioned to receive the radiation reflected from the sample in the lower portion of the range of angles and a last element positioned to receive the radiation reflected from the sample in the higher portion of the range of angles. Most preferably, the detector assembly includes a readout circuit and a charge coupled device (CCD), which has an output connected to the readout circuit and is coupled to transfer charges generated by the detector elements responsive to the radiation from the detector elements to the output in sequence along the array beginning with the last element.  
           [0028]    Preferably, the radiation source is adapted to irradiate a spot on the sample, and the apparatus includes a knife edge, adjustably positionable to block a portion of the radiation while the shutter is in the clear position, so as to reduce a dimension of the spot. Most preferably, the radiation includes X-ray radiation, and the range includes angles in a vicinity of a critical angle for total external reflection of the radiation from the surface, and the knife edge is positionable so as to reduce the dimension of the spot to no more than 1 mm.  
           [0029]    There is also provided, in accordance with a preferred embodiment of the present invention, radiation sensing apparatus, including:  
           [0030]    a detector assembly, including an array of detector elements, positioned to receive X-ray photons emitted over a range of angles and to generate a signal indicative of respective charges accumulated by the detector elements due to the photons that are incident on the elements; and  
           [0031]    a processor, which is coupled to receive the signal from the detector assembly and to determine, responsive to the signal, whether a high flux of the photons or a low flux of the photons was incident on each of the elements, and to estimate the number of photons incident on each of the elements by dividing the charges accumulated by the elements on which the high flux was incident by a high-flux average charge, and dividing the charges accumulated by the elements on which the low flux was incident by a low-flux average charge, substantially different from the high-flux average charge.  
           [0032]    Preferably, the low flux is considered to be incident on one of the elements when no more than a single one of the photons is incident on the element over a period during which the charges are accumulated. Most preferably, for the elements on which the low flux was incident, the processor is adapted to divide the charges accumulated by a mutually-adjacent pair of the elements by the low-flux average charge, so as to determine whether one of the photons was incident on one of the elements in the pair.  
           [0033]    Additionally or alternatively, the detector assembly is adapted to receive the X-ray photons reflected by a sample over the range of angles, characterized by a variation of flux of the reflected photons as a function of angle, such that the high flux is incident on the elements in a low-angle portion of the range, and the low flux is incident on the elements in a high-angle portion of the range.  
           [0034]    There is additionally provided, in accordance with a preferred embodiment of the present invention, a detector assembly, including:  
           [0035]    an array of detector elements, positioned to receive radiation and to generate a signal responsive thereto, the array including a first element and a last element and having a length defined by a distance between the first and last elements; and  
           [0036]    an evacuable enclosure having a front side and a rear side separated by a distance that is at least equal to the length of the array, wherein the array is positioned at the rear side of the enclosure, and the enclosure includes a window at a front side thereof, which is adapted to allow the radiation to pass therethrough so as to impinge on the array.  
           [0037]    Preferably, the front and rear sides of the enclosure are separated by a distance of at least twice the length of the array.  
           [0038]    Further preferably, the radiation is emitted from a sample outside the enclosure, wherein the radiation includes X-rays reflected from the sample over a range of angles, such that the first element receives the radiation reflected from the sample in a lower portion of the range of angles and the last element receives the radiation reflected from the sample in a higher portion of the range of angles. Most preferably, the detector assembly includes a readout circuit and a charge coupled device (CCD), which has an output connected to the readout circuit and is coupled to transfer charges generated by the detector elements responsive to the radiation from the detector elements to the output in sequence along the array beginning with the last element.  
           [0039]    There is further provided, in accordance with a preferred embodiment of the present invention, a method for reflectometry, including:  
           [0040]    irradiating a sample with radiation over a range of angles relative to a surface of the sample;  
           [0041]    receiving the radiation reflected from the sample over the range of angles so as to generate a low-range signal responsive to the radiation reflected in a lower portion of the range;  
           [0042]    blocking a lower part of the range of angles, thereby allowing the reflected radiation to reach the array substantially only in a higher portion of the range;  
           [0043]    receiving the radiation reflected from the sample over the range of angles while the lower portion of the range is blocked, so as to generate a high-range signal responsive to the radiation reflected in a higher portion of the range; and  
           [0044]    combining the high-range and low-range signals to determine a pattern of the reflected radiation over the range of angles, including both the lower and higher portions.  
           [0045]    There is moreover provided, in accordance with a preferred embodiment of the present invention, a method for sensing radiation, including:  
           [0046]    receiving X-ray photons emitted over a range of angles at an array of detector elements, so as to generate a signal indicative of respective charges accumulated by the detector elements due to the photons that are incident on the elements; and  
           [0047]    determining, responsive to the signal, whether a high flux of the photons or a low flux of the photons was incident-on each of the elements;  
           [0048]    estimating the number of photons incident on each of the elements on which the high flux was incident by dividing the charges accumulated by the elements by a high-flux average charge; and  
           [0049]    estimating the number of photons incident on each of the elements on which the low flux was incident by dividing the charges accumulated by the elements by a low-flux average charge, substantially different from the high-flux average charge.  
           [0050]    Preferably, determining whether the high flux or the low flux was incident includes determining that the high flux was incident on one of the elements when the charge accumulated by the element, not including a background charge, is at least three times the high-flux average charge. Additionally or alternatively, determining whether the high flux or the low flux was incident includes determining that the low flux was incident on one of the elements when no more than a single one of the photons was incident on the element over a period during which the charges were accumulated.  
           [0051]    There is furthermore provided, in accordance with a preferred embodiment of the present invention, a method for detecting radiation, including:  
           [0052]    enclosing an array of detector elements in an enclosure, the array including a first element and a last element defining a length of the array therebetween, the enclosure having a window at a front side thereof, which is adapted to allow radiation to pass therethrough, and which is positioned at a distance from the array that is at least equal to the length of the array;  
           [0053]    evacuating the enclosure containing the array; and  
           [0054]    receiving the radiation at the array and generating a signal responsive thereto.  
           [0055]    There is also provided, in accordance with a preferred embodiment of the present invention, a method for reflectometry, including:  
           [0056]    irradiating a sample including one or more thin film layers with radiation over a range of angles relative to a surface of the sample;  
           [0057]    receiving the radiation reflected from the sample over the range of angles and generating a signal responsive to the reflected radiation, the signal having an oscillatory pattern as a function of the angle, the pattern including an initial shoulder occurring near a critical angle for total external reflection of the radiation from an outer one of the thin film layers at the surface of the sample; and  
           [0058]    estimating a density of the outer thin film layer responsive to the shoulder, irrespective of any other properties of the one or more thin film layers.  
           [0059]    Preferably, the method includes determining one or more of the other properties using the estimated density, wherein determining the one or more of the other properties includes estimating a thickness of at least one of the layers and/or a surface roughness of at least one of the layers.  
           [0060]    The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0061]    [0061]FIG. 1 is a schematic illustration of a system for X-ray reflectometry, in accordance with a preferred embodiment of the present invention;  
         [0062]    [0062]FIG. 2 is a schematic block diagram illustrating an X-ray detection assembly used in the system of FIG. 1, in accordance with a preferred embodiment of the present invention;  
         [0063]    [0063]FIG. 3 is a schematic plot of charge amplitude collected by an detector on which X-ray photons of a given energy are incident, in accordance with a preferred embodiment of the present invention;  
         [0064]    [0064]FIG. 4 is a flow chart that schematically illustrates a method for processing signals generated by an X-ray detector array, in accordance with a preferred embodiment of the present invention;  
         [0065]    [0065]FIGS. 5A and 5B are schematic detail views of the system of FIG. 1, illustrating the operation of a dynamic knife edge and shutter used in the system, in accordance with a preferred embodiment of the present invention;  
         [0066]    [0066]FIG. 6 is a schematic plot of X-ray reflectance signals as a function of reflection angle, under two different sets of detection conditions, in accordance with a preferred embodiment of the present invention;  
         [0067]    [0067]FIG. 7 is a schematic plot illustrating scaling of the signals of FIG. 6, in accordance with a preferred embodiment of the present invention;  
         [0068]    [0068]FIG. 8 is a schematic plot illustrating an X-ray reflectance spectrum, formed by combining the signals of FIG. 7, in accordance with a preferred embodiment of the present invention; and  
         [0069]    [0069]FIG. 9 is a flow chart that schematically illustrates a method for extracting thin film data from an X-ray reflectance spectrum, in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0070]    [0070]FIG. 1 is a schematic illustration of a system  20  for X-ray reflectometry of a sample  22 , in accordance with a preferred embodiment of the present invention. The sample is preferably mounted on a motion stage  24 , allowing accurate adjustment of its position and orientation. An X-ray source  26 , typically an X-ray tube with suitable monochromatizing optics (not shown), irradiates a small area  28  on sample  22 . A preferred X-ray tube for this purpose is the XTF5011 tube, produced by Oxford Instruments of Scotts Valley, Calif. A number of different optical configurations that may be used in source  26  are described in U.S. patent application Ser. No. 09/408,894, which is assigned to the assignee of the present patent application and is incorporated herein by reference. The optics preferably comprise a curved crystal monochromator, such as the Doubly-Bent Focusing Crystal Optic, produced by XOS Inc., of Albany, N.Y. Other suitable optics are described in patent application Ser. No. 09/408,894 and in the above-mentioned U.S. Pat. Nos. 5,619,548 and 5,923,720. Further possible optical configurations will be apparent to those skilled in the art. A typical X-ray energy for reflectometric measurements in system  20  is about 8.05 keV (CuKa1). Alternatively, other energies may be used, such as 5.4 keV (CrKa1). A dynamic knife edge  36  and shutter  38  are preferably used to limit an incident beam  27  of the X-rays, as described further hereinbelow.  
         [0071]    A reflected beam  29  of X-rays from sample  22  is collected by a detector assembly  30 . Preferably, assembly  30  collects reflected X-rays over a range of reflection angles between about 0° and 3°, both below and above the critical angle of the sample for total external reflection. Assembly  30  comprises a detector array  32 , preferably a CCD array, as described hereinbelow. Although for simplicity of illustration, only a single row of detectors elements is shown in FIG. 1, with a relatively small number of detector elements, in preferred embodiments of the present invention, array  32  generally includes a greater number of elements, arranged in either a linear or a matrix (two-dimensional) array. Assembly  30  further comprises a window  34  made of a suitable X-ray transparent material, such as beryllium, spaced in front of the detector array, between the array and the sample.  
         [0072]    A reflectometry processor  40  analyzes the output of assembly  30 , so as to determine a distribution  42  of the flux of X-ray photons reflected from sample  22  as a function of angle at a given energy or over a range of energies. Typically, sample  22  has one or more thin surface layers, such as thin films, at area  28 , so that distribution  42  exhibits an oscillatory structure due to interference effects among reflected X-ray waves from the interfaces between the layers. The processor analyzes characteristics of the oscillatory structure in order to determine the thickness, density and surface quality of one or more of the surface layers, using methods of analysis described hereinbelow.  
         [0073]    Although in the preferred embodiment shown in FIG. 1, system  20  is described with reference to X-ray reflectometry, it will be appreciated that the system may similarly be used, mutatis mutandis, in other fields of X-ray analysis. Possible fields of application include X-ray fluorescence (XRF) analysis, including particularly grazing emission XRF, as well as other XRF techniques known in the art. Grazing emission XRF is described, for example, in an article by Wiener et al., entitled “Characterization of Titanium Nitride Layers by Grazing-Emission X-ray Fluorescence Spectrometry,” in  Applied Surface Science  125 (1998), p. 129, which is incorporated herein by reference. Furthermore, the principles of system  20  may be implemented in position-sensitive detection systems for other energy ranges, such as for detection of gamma rays and other nuclear radiation.  
         [0074]    [0074]FIG. 2 is a block diagram that schematically shows details of detector assembly  30 , in accordance with a preferred embodiment of the present invention. As noted above, detector array  32  preferably comprises a CCD array, such as the model S7032-0908N array produced by Hamamatsu, of Hamamatsu City, Japan. This array comprises 536×256 pixels, with an overall size of 12.6×6 mm, and is preferably operated in a line-binning mode, using special hardware supplied for this purpose by Hamamatsu. Alternatively, the detector array may comprise an array of PIN diodes with suitable readout circuits, possibly including integrated processing electronics, as described in U.S. patent application Ser. No. 09/409,046, which is assigned to the assignee of the present patent application and is incorporated herein by reference. application Ser. No. 09/409,046 also describes alternative features of the array, including various geometrical configurations of the array (both one- and two-dimensional) and masking that may be applied to enhance the array&#39;s detection properties. These features are applicable to assembly  30  of the present patent application, as well. In any event, it will be understood that these detector types are described here by way of example, and detectors of any suitable type, dimension and number can be used.  
         [0075]    Detector assembly  30  comprises an evacuable enclosure  44  adjacent to detector array  32 . The front side of enclosure  44 , between array  32  and sample  22 , is closed off by window  34 , and the enclosure is evacuated during operation. Preferably, the distance from array  32  to window  34  is at least equal to the length of the array, measured from a first detector element  46  to a last detector element  48 , and is most preferably at least two to three times the length of the array. (First detector element  46  is positioned to capture the lowest-angle reflected photons, around 0°, while last element  48  captures the highest-angle photons, typically near 3°.) The inventors have found that removal of the air from the region immediately in front of the array, along with distancing the window from the array, substantially reduces the number of scattered X-ray photons that reach the array. When array  32  operates in air, or when window  34  is positioned close to the array, scatter of photons reflected from sample  22  at low angles ordinarily makes a substantial contribution to the signal background at high angles. Because the low-angle reflections are generally so intense by comparison with the high-angle reflections, this background significantly degrades or even masks the high-angle signal. The use of window  34  and evacuated enclosure  44 , as shown in FIG. 2, eliminates most of this scatter background, without the difficulty and expenses of having to evacuate the entire system.  
         [0076]    A further source of background in assembly  30  is residual charge in the CCD shift register of array  32 . CCDs operate by transferring charge in a “bucket brigade” mode, from one element to the next down the array. The charge is thus transferred, one pixel at a time, to readout circuits  50 , which are coupled to an output of the array at last element  48 . Although CCDs are highly efficient in transferring charge from element to element, there is still a small amount of residual charge left behind in each transfer, which is roughly proportional to the amount of charge transferred. In the configuration shown in FIG. 3, after each X-ray exposure period, last element  48  is read out first, while first element  46  is read out last, after its charge has been transferred down the entire array. By positioning array  32  so that last element  48 , which typically receives the weakest X-ray signal, is read out first, the background level due to residual charge in the weak signal elements near element  48  is minimized. The background added to the strong signals from detector elements near first element  46 , due to reading these signals out last, is not significant by comparison with the strength of the signals themselves.  
         [0077]    [0077]FIG. 3 is a schematic plot showing the response of the elements of detector array  32  to incident photons, in accordance with a preferred embodiment of the present invention. In this experiment, the detector array was irradiated with monochromatic X-ray radiation, and signals were gathered from the array in two different read-out modes. The amplitude units of the horizontal axis are arbitrary, but indicate the estimated number of electrons generated for each X-ray photon that is incident on an element of the array, based on the output signal from the array. Each incident photon constitutes an “event,” and the vertical axis shows, for each value of the amplitude, how many times a charge of that amplitude was generated by an incident photon.  
         [0078]    As can be seen in the figure, the distribution of events is substantially different for the different read-out modes. The charge generated due to an incident photon may typically be distributed between two adjacent pixels. The curve having a sharp peak near amplitude  100  is accordingly generated by combining the charge counted in adjacent pixels. This read-out mode, however, can be applied only when the flux is low enough so that during any given read-out frame, there is generally no more than one incident photon per pixel, with most pixels receiving no photons. At high flux, with many photons incident on each element, this “charge combining” approach cannot be used. In this case, the event distribution has the form of the second curve shown in FIG. 3, with a peak near amplitude  60 . Such behavior was observed for both Cu Kα (8.05 keV) and Cr Kα (5.41 keV) irradiation. The inventors have empirically found that the combination of these two read-out modes for high- and low-flux conditions can be used effectively in converting the electrical signal levels received from array  32  into units of photon counts over a very large dynamic range, as is commonly encountered in XRR measurements.  
         [0079]    [0079]FIG. 4 is a flow chart that schematically illustrates a method for analyzing signals received by detector array  32 , in accordance with a preferred embodiment of the present invention. The method is based on the principles described above with reference to FIG. 3. It is actuated by processor each time charge is read out of the elements of the array, with respect to each of the elements in turn, in order to translate the charge amplitude measured in each element into units of incident X-ray photons.  
         [0080]    The method includes four operations, which are performed over all of the pixels in the array:  
         [0081]    1. In a background subtraction step  52 , a general background level is subtracted from the digitized signal level measured in each of the pixels, thus generating a background-subtracted level y(j) for each pixel. The general background level is found by measuring the dark current output of the detector array in the absence of incident radiation.  
         [0082]    2. In a strong flux counting step  54 , the signal levels after background subtraction are compared to a high signal threshold, which is determined based on the high-flux mode of the distribution shown in FIG. 3. Specifically, the processor finds pixels (i.e., detector array elements) whose signal levels, y(j), are greater than three times a signal amplitude parameter, Av1, which is determined empirically based on the location of the peak in the high-flux single-photon signal (such as the high-amplitude peak shown in FIG. 3). For each such pixel j, the number of photon counts n(j) for the pixel is determined to be n(j)=int{[y(j)−BL1]/Av1}, wherein BL1 is an empirical background threshold. Processor  40  then sets the signal level for the pixel to zero, so that it is not counted again in subsequent steps.  
         [0083]    3. In a pixel pair counting step  56 , the processor searches the remaining pixels, not counted in step  54 , to find pairs of adjacent pixels whose total signal level (summed over the pair) is roughly equal to a low-flux single-photon signal amplitude parameter Av2. This parameter, as well as a second background threshold BL2, is determined empirically. Specifically, the inventors have found that setting Av2=Av1 gives good results. BL1 and BL2 are set so that the high- and low-intensity portions of the photon energy spectrum, found at steps  54  and  56  respectively, will match up. Based on these parameters, at step  56 , the processor finds pairs of pixels that satisfy |y(j)+y(j+1)−Av2|&lt;BL2. For each such pair, the processor records a single photon count, which is arbitrarily assigned to the photon count n(j) for the first of the two pixels. The signal levels in these pixels are then zeroed, as well.  
         [0084]    4. In a remainder counting step  58 , any other pixels with significant signal levels that were not counted at step  54  or  56  are evaluated. To carry out this step, any pixel signal values y(j) that are below the background level, so that y(j)&lt;0 after background subtraction at step  52 , are set to y(j)=0. Then, for each pixel that is a local maximum (i.e., greater signal value y(j) than its immediate neighbors), the pixel photon count n(j) receives the value n(j)=int{[y(j)+y(k)−BL1]/Av1}, wherein y(k) is the higher of the signal levels of the two pixels, j+1 and j−1, neighboring on pixel j. The values y(j) and y(k) are then set to zero. For any remaining pixels (not local maximum or their higher neighbors), the photon count receives the value n(j)=int{[y(j)−BL1]/Av1}, and y(j) is zeroed.  
         [0085]    After all four of the steps listed above are completed, the array is reset, and the processor is ready to receive the next signal readout from array  32 . The number of counts determined for each of the pixels is accumulated in a respective register, in a count accumulation step  60 . The steps of signal readout and processing, as described above, are preferably repeated enough times to determine a count spectrum over the entire array. Using this technique, fringe structure can be seen not only at the low-angle, high-flux pixels, where many pixels are incident in each signal readout cycle, but also at high angles, where only one count or less may arrive at each pixel in a given cycle.  
         [0086]    [0086]FIGS. 5A and 5B are detail views of system  20 , illustrating the use of knife edge  36  and shutter  38 , in accordance with a preferred embodiment of the present invention. In FIG. 5A, the knife edge and shutter are positioned to allow optimal detection of low-angle reflections, near 0°. Shutter  38  is withdrawn outside the extent of incident beam  27 . Knife edge  36  is positioned to cut the upper portion of the incident beam. As a result, most of the incident beam is cut off, and the lateral dimension of the X-ray spot incident on area  28  is reduced. Preferably, the knife edge is lowered to within less than 10 μm of the surface of sample  22 , and most preferably to as little as 1 μm from the surface. The lateral dimension of the spot is thus reduced to 1 mm or less, instead of the typical dimension of 5 mm or more when the knife-edge is not used. The reduced spot size on the sample means that low-angle reflection measurements made by system  20  have enhanced spatial resolution, providing more detailed information about thin film microstructures on sample  22 . Alternatively or additionally, when a certain area of the sample, such as a patterned semiconductor wafer, must be set aside for testing, the small spot size enables a smaller portion of sample “real estate” to be used for this purpose.  
         [0087]    Moreover, the inventors have found that wafers are prone to warping, particularly when held by a vacuum chuck, as is commonly practiced in test and fabrication equipment. When the X-ray spot is spread over a long lateral dimension, this warping can cause different parts of the spot to be incident on the wafer at slightly different angles. As a result, the fringe structure in the measured distribution of the reflected radiation is blurred. Thus, an additional benefit of the use of knife edge  36  is the reduction of this blurring due to warping of the wafer, since the range of angles of incidence of the X-rays within the spot is accordingly narrowed.  
         [0088]    In FIG. 5B, knife edge  36  and shutter  38  are positioned to enable effective detection of weaker, high-angle reflection. In this case, the knife edge is withdrawn from the beam, while the shutter is positioned to cut off the low-angle portion of incident beam  27 . Alternatively, the shutter may be positioned to cut off the low-angle portion of reflected beam  29 . Only the high-angle reflections from sample  22  reach the detector array, and not the strong low-angle reflections. As a result, the background level at the high-angle elements of the detector array is reduced, and X-ray photons can be collected by the array over a substantially longer integration period without saturation. Thus, the weak, high-angle signals are detected with enhanced signal/noise ratio.  
         [0089]    [0089]FIG. 6 is a schematic plot showing reflectometric signals gathered by processor  40 , in accordance with a preferred embodiment of the present invention, using dynamic shutter  38  as shown in FIGS. 5A and 5B and the signal processing method of FIG. 4. The plot shows, on a logarithmic scale, the number of counts n(j) accumulated at each pixel as a function of reflection angle. A high-intensity trace  70  is generated in the configuration of FIG. 5A (with or without the use of knife edge  36 ), using a relatively short exposure. A second, low-intensity trace  72  is generated with shutter  38  positioned to block the low-angle beam, as shown in FIG. 5B, using a long exposure. Trace  70  shows the low-angle fringe structure, while trace  72  shows the high-angle structure. Fringes in an intermediate region (around 1°) can be seen in both traces.  
         [0090]    [0090]FIG. 7 is a schematic plot showing traces  70  and  72  after trace  70  has been scaled to match the amplitude of trace  70  in the intermediate region.  
         [0091]    [0091]FIG. 8 shows a composite trace  74 , generated by combining the scaled and superimposed traces  70  and  72  of FIG. 7, in accordance with a preferred embodiment of the present invention. For each pixel, the value in composite trace  74  is a weighted sum of the corresponding values in traces  70  and  72 , with weighting factors that vary appropriately as a function of angle. Trace  74  shows a well-defined fringe pattern extending from near 0° out to 2.5°. The high-angle fringes are particularly important in determining properties of inner layers at the surface of sample  22 , when a multi-layer thin film structure is to be analyzed. The spikes seen at high angles are experimental artifacts, which are ignored in the analysis described below.  
         [0092]    [0092]FIG. 9 is a flow chart that schematically illustrates a method for analyzing trace  74  to determine the properties of thin films on sample  22 , in accordance with a preferred embodiment of the present invention. The method is based on a physical model of the reflected fringe pattern. According to this model, the angular position of an initial shoulder  78  (FIG. 8) in the fringe pattern is determined mainly by the density of the uppermost layer on the sample. The spatial frequency or frequencies of the fringes are indicative of the thickness of the film layers. The intensity of the higher-order fringes relative to the low-order ones, indicated by a decay curve  76  fitted to trace  74 , is determined mainly by the roughness of the outer surface of the sample and, secondarily, of the interfaces between the film layers on the sample.  
         [0093]    Based on this model, at a density fitting step  80 , an initial, theoretical fringe pattern is fitted to trace  74 , by adjusting the density in the model so that the theoretical pattern fits shoulder  78 . For the purpose of this step, the film is considered to be infinitely thick, and only the part of the fringe pattern in the immediate vicinity of the shoulder is considered. Next, at a roughness fitting step  82 , a parameter in the model corresponding to the roughness of the outer surface of the sample is adjusted so that curve  76  fits the actual, average decay of trace  74  as a function of angle. The fit is performed so that the difference between trace  74  and curve  76 , integrated over the entire angular range (or a substantial, selected portion of the range), is close to zero.  
         [0094]    The fitted decay curve is subtracted out of trace  74 , in order to isolate the oscillatory portion of the reflected signal, at an oscillation extraction step  84 . The oscillation frequency or frequencies in the subtracted signal are determined, at a frequency determination step  86 , preferably using a Fast Fourier Transform (FFT) analysis of the signal. The frequency spectrum is preferably filtered to eliminate spurious high-frequency components. The filtered spectrum is transformed back to the spatial domain, and a least squares fit is performed to determine the thicknesses of the detected layers on the sample surface, at a thickness measurement step  88 . Typically, when the sample has a multi-layer structure, the outer layer will give the strongest frequency component in the spectrum, at a relatively low frequency corresponding to the thickness of this layer. The next frequency component will be at a higher frequency, corresponding to the combined thickness of the outer layer and the next layer below it. The thickness of the next layer is determined by subtracting the outer layer thickness from the combined thickness. Additional layer thicknesses may be determined in like manner if the spectrum is sufficiently well resolved.  
         [0095]    Upon completion of step  88 , the physical properties of the outer layer on the sample—density, thickness, and outer surface roughness—are all known. Assuming that more than a single frequency was found at step  86 , corresponding to a multi-layer structure, trace  74  can be further analyzed to determine the properties of one or more inner layers. At an inner density fitting step  90 , the density of the second layer (below the outer layer) is introduced into the theoretical model and is adjusted to produce an optimal fit. A roughness parameter for the surface between the outermost and second layers is adjusted to improve the fit of the model curve to the amplitude of the oscillations in trace  74 , at an inner roughness fitting step  92 . Thickness parameters, corresponding to possible errors in the thickness of the outermost and second layers, are adjusted at a fine tuning step  94 , in order to correct any mismatch between the positions of the fringes and the derived model. To the extent that trace  74  provides sufficient resolution of fine fringe detail, as noted above, steps  90 ,  92  and  94  may be repeated for further, underlying layers on the sample.  
         [0096]    Although the features of system  20  have been described here in combination, it will be appreciated that individual ones or subgroups of these features can also be used independently of the other features. Furthermore, although these features are described in the context of X-ray reflectometry, at least some of them are also applicable in other fields of analysis, such as diffractometry, using X-rays and other radiation bands.  
         [0097]    It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.