Patent Number: 
Section: description

An exemplary system 10 for obtaining EBSD patterns from polycrystalline samples, for correcting distortions in the EBSD patterns resulting from magnetic fields produced by a scanning electron microscope (SEM), and for analyzing the corrected EBSD patterns to obtain crystallographic information for the samples, is illustrated in FIG. 1, and will be described in detail with reference thereto. It should be noted that any of various conventional hardware configurations for collecting EBSD patterns using an SEM may be employed to implement the system 10, and conventional methods may be employed for obtaining EBSD pattern images using the system 10 and for analyzing EBSD patterns which have been corrected to remove magnetic field distortions therefrom using a method in accordance with the present invention. The system 10 includes a conventional SEM 12. Those skilled in the art will recognize, however, that the system 10 may utilize other types of microscopes for investigating and characterizing the features within a sample of interest using electron or other energy beams, depending upon the specific application. For example, a transmission electron microscope may be used. The SEM includes an electron beam generator 16, which discharges a focused electron beam 18 into a vacuum chamber 20. A holding stage 22 is mounted in the vacuum chamber 20 such that a material specimen 24 mounted thereon is bombarded or illuminated by the electron beam 18. An image collection system 26 is utilized to collect images of backscattered electrons diffracted from specimen 24. The image collection system 26 includes a detector 28, e.g., a screen that is coated with a scintillating material, such as phosphorous, for detecting the electrons backscattered from the sample 24. The detector 28 is coupled to a video camera 30 in a conventional manner. The detector 28 luminesces in accordance with the pattern of the diffracted electrons falling thereon. The resulting electron backscatter diffraction (EBSD) patterns are captured by the video camera 30, where they are converted into electronic signals, which are converted into digital data in an image digitizer 36. The digitized EBSD pattern image is provided from the digitizer 36 to a computer system 38 whereby the EBSD pattern is displayed and wherein the EBSD pattern is corrected for magnetic field distortion in accordance with the present invention and analyzed to obtain crystallographic information for the sample 24 in a conventional manner. The computer system 38 may be implemented as a conventional computer system. The computer 38 includes conventional computer components and peripheral devices, including computer output devices, such as a system or operator display 40, e.g., a computer monitor, and input devices 42, such as a keyboard, mouse, trackball, etc. The computer 38 also includes computer memory 44, such as disk storage memory, which stores programming instructions which define various processes carried out by the computer system 38, including programming instructions implementing a method for correcting magnetic field distortions in EBSD patterns in accordance with the present invention, and correction parameters used in such a method. Conventional computer programs for analyzing EBSD patterns to obtain crystallographic information may also be stored in memory 44, along with saved EBSD pattern images and analysis results. The computer 38 also controls the SEM beam generator 16, movement of the sample holding stage 22, and the image collection system 26, in a conventional manner, e.g., via control lines 46, 48, and 50, respectively. As is known in the art, the system 10 may be controlled to collect EBSD patterns from a series of points on a specimen 24 to provide an analysis of the entire structure of the specimen 24. An exemplary (undistorted) EBSD pattern 60, which may be obtained by a conventional EBSD pattern collection system 10, and displayed on the operator display 40, is shown in FIG. 2. The EBSD pattern 60 includes a plurality of bands 62 (typically referred to as Kikuchi bands) formed by the diffraction of the electron beam 18 by the crystalline structure of the sample 24. The Kikuchi bands 62 intersect each other and are generally bordered by thin lines of lower intensity. Conventional analysis programs and techniques are available for analyzing the Kikuchi bands 62 to obtain crystallographic information for the sample 24 from the EBSD pattern. Note how the Kikuchi bands 62 in the EBSD pattern 60 run straight. Certain SEMs (immersion-lens SEMs) employ a final (objective) electron lens 64 (FIG. 1) for directing and focusing the electron beam 18. Such a lens 64 produces magnetic fields which may extend into the vacuum chamber 20 near the sample 24. Although these magnetic fields are required for superior image resolution, they also distort the near-linear trajectory of the electrons diffracted from the sample 24 into curved paths, thereby distorting the resulting EBSD pattern. An exemplary distorted EBSD pattern which may be obtained using such an SEM system is illustrated at 66 in the left side of FIG. 3. (This EBSD pattern 66 was obtained from the same sample as was used to obtain the EBSD pattern 60 as shown in FIG. 2.) Note how the Kikuchi band 68, which should run straight, is bent in the distorted EBSD pattern image 66 (compare EBSD pattern 66 of FIG. 3 with EBSD pattern 60 of FIG. 2). Accurate analysis of such a distorted EBSD pattern using conventional EBSD pattern analysis systems is impossible. The present invention, however, provides a system and method for correcting automatically the magnetic field distortions in EBSD patterns, thereby allowing conventional analysis of EBSD patterns to obtain crystallographic information therefrom. Since the magnetic field strengths and distributions employed in SEMs are not generally available, the present invention provides EBSD pattern correction based only upon available empirical information. An exemplary method 70 in accordance with the present invention for correcting an EBSD pattern which has been distorted by the magnetic field produced by an SEM will now be described in detail with reference to FIG. 3 and to the flowchart diagram of FIG. 4. The method 70 of the present invention to be described below may be implemented in a conventional computer 38 in a conventional manner, using conventional computer programming techniques in, for example, a Windows-based operating system or any other conventional operating system and using any conventional programming language. The method 70 for correcting the magnetic field distortions in EBSD patterns in accordance with the present invention may be started 74 each time an EBSD pattern is collected, e.g., using conventional methods and the system 10 as described above, and may be applied to each EBSD pattern collected to correct any distortion created by the magnetic field produced by the SEM 16. As described above, an EBSD pattern is acquired 76, digitized, and provided to the system computer 38 in a conventional manner. Conventional image processing 78 may be applied to the EBSD pattern image thus obtained in the normal manner. For instance, such conventional processing may include image brightness and contrast adjustment, background removal, etc. A determination 80 then is made whether this is the first EBSD pattern obtained using this particular SEM geometry. If this is the first EBSD pattern obtained using the SEM geometry, the distortion correction method of the present invention runs a calibration procedure, whereby parameters used for correcting the magnetic field distortion in the EBSD pattern are obtained. As will be discussed in more detail below, these parameters may be used for correcting the magnetic field distortion in all subsequent EBSD patterns obtained using the SEM geometry for which the calibration procedure was run. In the calibration procedure, the first distorted EBSD pattern image obtained using a particular SEM geometry is displayed 82 to an operator on the operator display 40. The calibration procedure is preferably performed using an EBSD pattern obtained from a polycrystalline sample having a known crystal structure. For example, a silicon  less than 100 greater than  calibration sample wafer may be mounted in the SEM 12 with the low-index direction mounted vertically. The resulting EBSD pattern includes a Kikuchi band 68 which runs vertically across the pattern but which is curved due to magnetic field distortion. Another type of calibration sample material may also be used. The distorted EBSD pattern thus obtained is displayed on the operator display 40 to provide an operator/user interface which allows an operator of the computer 38 to interact with the displayed EBSD pattern using an input device 42, such as a mouse, trackball, etc. An exemplary user interface 84 is illustrated in FIG. 3. Such a user interface may be generated using conventional programming techniques. It should be understood that other user interface designs and layouts may also be employed. Using the user input device 42, e.g., a mouse, the operator defines 86 the endpoints 88 of line segments 89 along the length of a feature in the distorted EBSD pattern which is curved (distorted) but should be straight. In the example shown in FIG. 3, six segment endpoints 88 are defined in this manner along curved Kikuchi band 68 by the operator. It should be understood, however, that more or fewer endpoints 88 may be defined and used in this manner, and the number of segment endpoints to be used may be made user selectable. Having defined the segment endpoints 88, an operator may select the Correct Image button 94 on the user interface 84 to initiate the next step in the calibration procedure. From the user-defined line segment endpoints 88, a single or series of mathematical curves fitting the points, and, therefore, the distorted linear feature 68, is calculated in a conventional manner. For example, a cubic spline or other polynomial which defines a curved line or lines running through each of the endpoints 88 defined by the operator using the user interface 84 may be calculated automatically in a conventional manner. The mathematical curve thus calculated defines the curvature of, e.g., the Kikuchi band 68 in the distorted EBSD pattern 66. As discussed above, in an undistorted EBSD pattern, the Kikuchi band 68 would run straight. Therefore, in accordance with the present invention, the mathematical curve or curves define correction parameters which define the distance of points along the Kikuchi band 68 from a straight line 90 which represents the direction that the Kikuchi band 68 should run in an undistorted pattern. For example, the correction parameters may be calculated based on a cubic spline calculated relative to a straight line 90, which may be user defined and selected. For example, the straight line 90 may preferably be defined by an operator using the input device 42 to extend from a segment endpoint 92 located at one end of the Kikuchi band 68 vertically across the displayed EBSD pattern. If an EBSD pattern image from a known calibration sample is used, it may not be necessary for the user to define a straight line 90 on the operator display 84. In such a case, the undistorted Kikuchi band may be assumed to run vertically across the EBSD pattern beginning at the segment endpoint 92 defined at the end of the Kikuchi band. The cubic spline may be calculated in a conventional manner using the X and Y data position segment endpoints 88 defined by the user along the Kikuchi band 68 to calculate the spline. (From six defined segment endpoints, a six-point cubic spline fit may be calculated, but, as discussed above, more or fewer segment endpoints could be used.) The segment relative X and Y endpoints are input into a routine and four cubic spline coefficients are calculated for each endpoint. The cubic spline effectively defines the amount of bend in the distorted Kikuchi band 68 along the length of the band. In other words, the cubic spline defines the distance of points along the Kikuchi band 68 in the distorted EBSD pattern from a corresponding point, in the same vertical position, along the straight line 90. The cubic spline thus defines correction parameters which define the amount by which the picture elements (pixels) in each vertical row in the distorted EBSD pattern 66 must be shifted, in a horizontal direction, to correct for the magnetic field distortion in the EBSD pattern 66. Because the correction is relative in the horizontal direction, the base position is subtracted from each data point. This results in no shift for the bottom point of the bottom segment for, e.g., the example shown in FIG. 3. The correction parameters thus calculated by the calibration procedure are saved at step 98. The parameters that may be saved are, for example, the number of cubic spline segments and the four cubic spline parameters for each segment endpoint and the vertical position of each endpoint. The calibration procedure is now complete 100. The correction parameters determined by the calibration procedure may now be applied to the EBSD pattern 66 used during the calibration procedure to correct for magnetic field distortion of the EBSD pattern 66 in a manner to be described in more detail below. The saved correction parameters are also used to correct for magnetic field distortion in subsequent EBSD patterns obtained using the same SEM geometry. Thus, it is not necessary to perform the calibration procedure each time an EBSD pattern is obtained. The correction parameters which are established using the calibration procedure may be retrieved from memory each time an EBSD pattern is acquired as the first step 102 in an EBSD pattern correction procedure. The correction parameters are applied to the distorted EBSD pattern at step 104 to adjust the EBSD pattern image to correct for magnetic field distortions thereof. This is performed, for example, row-by-row by shifting the intensities of each line of pixels in the distorted EBSD pattern image by the amount determined by the mathematical curve (e.g., cubic spline) calculation for that vertical position in the EBSD pattern. For example, for the exemplary distorted EBSD pattern image 66 illustrated in FIG. 3, the intensities of the pixels in the top row of pixels in the image are shifted to the left by distance D. The next row of pixels in the image would be shifted by a slightly smaller amount, and so on down the entire vertical length of the EBSD pattern image 66. The resulting corrected EBSD pattern image 108 may then be displayed 106 to an operator on the operator display 40. For example, EBSD pattern 108 displayed in the right side of operator display 84 of FIG. 3 shows the distorted EBSD pattern 66 after being corrected by application of the pattern correction method of the present invention. Note that in the corrected EBSD pattern 108 the low index Kikuchi band 68 runs straight across the EBSD pattern 108. Shifting of the rows of pixels in the distorted EBSD pattern in accordance with the present invention will create a portion of the image space which is xe2x80x9cunfilledxe2x80x9d by the obtained corrected EBSD pattern image. The result will typically be a curved xe2x80x9cwedgexe2x80x9d feature 110 along one side of the corrected EBSD pattern image 108. This xe2x80x9cunfilledxe2x80x9d area may be filled with pixels of a selected intensity, such as pixels of an intensity equal to the average intensity of the whole corrected EBSD pattern image 108. EBSD pattern images corrected in accordance with the present invention may be saved 112 before the correction procedure ends 114. Such corrected EBSD pattern images may be analyzed using conventional EBSD pattern analysis systems and software to determine crystallographic parameters of a sample from the corrected EBSD pattern images. For analysis purposes, an EBSD pattern image corrected in accordance with the present invention is indistinguishable from an undistorted EBSD pattern image which did not require correction in the first place. It should be noted that, once correction parameters are established using the correction procedure, the process of obtaining, correcting, displaying, and analyzing EBSD patterns may proceed automatically without further user intervention in the correction procedure. It should be understood that the present invention is not limited to the particular exemplary applications and embodiments illustrated and described herein, but embraces all such modified forms thereof as come with in the scope of the following claims. In particular, the present invention is not limited to the particular steps or order of steps for the calibration and correction procedures as illustrated in FIG. 4 and described herein. Furthermore, although a calibration procedure requiring an operator manually to define line segment end points along a Kikuchi band in a distorted EBSD pattern is described herein, an entirely automated calibration procedure, which uses, e.g., a pattern matching technique, to automatically determine the required correction parameters from a distorted EBSD pattern image, without operator intervention, may also be employed.