Patent Publication Number: US-7910885-B2

Title: System and method for determining a cross sectional feature of a structural element using a reference structural element

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present patent application is a national phase application of International Application No. PCT/US2004/022347 filed Jul. 14, 2004, which is a claims priority to U.S. Provisional Application No. 60/486,566 filed Jul. 11, 2003. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to metrology systems and methods for determining features of sub-micron structural elements such as lines, contacts, trenches and the like, of measured objects such as, but not limited to, semiconductors wafers and reticles. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits are very complex devices that include multiple layers. Each layer may include conductive material and isolating material, while other layers may include semi-conductive materials. These various materials are arranged in patterns, usually in accordance with the expected functionality of the integrated circuit. The patterns also reflect the manufacturing process of the integrated circuits. 
     Integrated circuits are manufactured by complex multi-staged manufacturing processes. During this multi-staged process, resistive material is (i) deposited on a substrate layer, (ii) exposed by a photolithographic process, and (iii) developed to produce a pattern that defines some areas to be later etched. 
     Various metrology, inspection and failure analysis techniques have evolved for inspecting integrated circuits both during the fabrication stages, between consecutive manufacturing stages, either in combination with the manufacturing process (also termed “in line” inspection techniques) or not (also termed “off line” inspection techniques). Various optical as well as charged particle beam inspection tools and review tools are known in the art, such as the VeraSEM™, Compluss™ and SEM Vision™ of Applied Materials Inc. of Santa Clara, Calif. 
     Manufacturing failures may affect the electrical characteristics of the integrated circuits. Some of these failures result from unwanted deviations from the required dimensions of the patterns. A “critical dimension” is usually the width of a patterned line, the distance between two patterned lines, the width of a contact and the like. 
     One of the goals of metrology is to determine whether the inspected objects include deviations from these critical dimensions. This inspection is usually done by charged particle beam imaging that provides the high resolution required to measure said deviations. 
     A typical measured structural element is a line that has two opposing sidewalls. The measurement of the bottom width of the line involves measuring the top width of the line as well as measuring its sidewalls. 
     Measurement of a structural element line&#39;s critical dimensions using only a top view (in which the electron beam that scans the line is perpendicular to the substrate) may result in faulty results, especially when one of the sidewalls has a negative sidewall angle such that an upper end of the sidewall obscures a lower end of that sidewall. 
     In order to address said inaccuracies, CD-SEM tools that enable electronic tilt of an electron beam were introduced. NanoSem 3D, of Applied Materials from Santa Clara, is a fully automated CD-SEM that has a column that allows electronic tilting as well as mechanical tilting of the scanning electron beam to scan the wafer surface with various tilt angles from several directions. 
     Critical dimension measurement may involve illuminating a test object by multiple tilted beams and processing the detected waveforms to define critical dimensions. 
     Multiple measurements have some disadvantages. First, they reduce the throughput of the inspection system, especially when the measurement involves changing the tilt of the scanning electron beam. Such a change may require a de-Gauess stage, as well as an electron beam stabilization stage. A further disadvantage of multiple measurements results from degradation (for example shrinkage and carbonization) of the measured structural element, as well as unwanted charging of the measured structural element. 
     Due to various reasons, such as process variations, measurement inaccuracies and the like, the height and accordingly the measured height of structural elements varies across measured objects. In order to determine the structural height of a measured structural element, there is a need to perform at least two measurements of said structural element, at two different tilt angles. The height of the structural element may be estimated, usually in response to multiple height measurements of structural element across the measured object. The estimation may be associated with height measurement errors, as well as estimation errors, that can affect the critical dimension measurements. 
     SUMMARY OF THE INVENTION 
     The invention provides various scanning schemes that enable selectively reducing the amount of measurements required for determining cross sectional features of structural elements. 
     The invention provides methods and systems that allow accurate measurement of cross-dimensional features, with high accuracy, without knowing the height of the structural element. 
     The invention provides methods and systems that allow measurement of the whole cross-section of structural elements with a reduced number of scans and amount of measurements. 
     The invention provides a method for determining a cross sectional feature of a measured structural element having a sub-micron cross section, the cross section defined by an intermediate section that is located between a first and a second traverse sections. The method includes a first step of scanning a portion of at least one reference structural element and a portion of the measured structural element to determine one or more relationships between the at least one reference structural element and the measured structural element. The first scanning step may be followed by additional scanning steps. According to an embodiment of the invention, at least one additional scanning step is mandatory. According to other embodiments of the invention, one or more additional scanning steps are optional, and their execution may be responsive to a fulfillment of various conditions, such as an uncertainty associated with the results of the previous scan. The uncertainty may be related to a steepness or orientation of a portion of a section of a measured structural element, to a relationship between a width of a section of the measured element and to a width of an electron beam that scans that section. The conditions may also reflect the required signal to noise ratio of the measurement, the accuracy of the measurement, and, optionally or alternatively, to the topography of the measured object. 
     If additional scans are performed, additional relationships may be determined. Usually, the scans are performed at different scan states. 
     According to an aspect of the invention, one or more reference structural elements are provided. If multiple reference structural elements are provided, reference structural elements may be positioned on both sides of a measured structural element, but this is not necessarily so. 
     The invention provides a system for determining a cross sectional feature of a structural element having a sub-micron cross section, the cross section defined by an intermediate section that is located between first and second traverse sections, the system including means for directing an electron beam towards an inspected object, at least one detector that is positioned so as to detect electrons emitted from the beam; and a processor, coupled to the at least one detector. The system is capable of performing at least one measurement, at one or more tilt states, to determine one or more relationships between one or more structural elements and portions of a measured structural element. At least two relationships help to determine the cross sectional feature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1   a  is a schematic illustration of a critical dimension scanning electron microscope, in accordance with an embodiment of the invention; 
         FIG. 1   b  is a perspective view of an objective lens, according to another embodiment of the invention; 
         FIGS. 2   a - 2   b  illustrate various lines and reference structural elements; 
         FIGS. 3   a  - 3   c  are schematic illustrations of waveforms that represent a relatively wide positively oriented traverse section, a relatively narrow traverse section and a negative oriented traverse section; 
         FIG. 4  is a flow chart of a method for determining an edge of a structural element having a sub-micron cross section, in accordance with an embodiment of the invention; and 
         FIGS. 5-9  are flow charts of methods for determining a cross sectional feature of a measured structural element having a sub-micron cross section, the cross section defined by an intermediate section that is located between first and second traverse sections. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     A typical CD-SEM includes an electron gun, for generating an electron beam, deflection and tilt units as well as focusing lens, for enabling scanning of a specimen with an electron beam, that may be in a certain tilt condition, while reducing various aberrations and misalignments. Electrons, such as secondary electrons, emitted as result of an interaction between the specimen and the electron beam are attracted to a detector that provides detection signals which are subsequently processed by a processing unit. The detection signals may be used to determine various features of the specimen, as well as form images of the inspected specimen. 
     The invention may be implemented on CD-SEMs of various architectures that may differ from each other by the number of parts as well as the arrangement of said parts. For example the number of deflection units, as well as the exact structure of each unit may vary. The CD-SEM may include in-lens as well as out-of-lens detectors or a combination of both. 
     A block diagram of a critical dimension scanning electron microscope (CD-SEM)  100  is shown schematically in  FIG. 1   a . CD-SEM  100  includes an electron gun  103  emitting an electron beam  101 , which is extracted by the anode  104 . The objective lens  112  focuses the electron beam on the specimen surface  105   a . The beam is scanned over the specimen using the scanning deflection unit  102 . An alignment of the beam to the aperture  106  or a desired optical axis respectively can be achieved by the deflection units  108  to  111 . Instead of deflection unit coils, electrostatic modules in the form of charged plates or a combination of coils and electrostatic deflectors can be used. 
     In-lens detector  16  is able to detect secondary electrons that escape from the specimen  105  at a variety of angles with relatively low energy (3 to 50 eV). Measurements of scattered or secondary corpuscles from a specimen can be conducted with detectors in the form of scintillators connected to photomultiplier tubes or the like. Since the way of measuring the signals does not influence the inventive idea in general, this is not to be understood as limiting the invention. It is noted that the CD-SEM may include, additionally or alternatively, at least one out-of-lens detector. 
     Detection signals are processed by a processing unit (that may be a part of controller  33 , but this is not necessarily so) that may have image processing capabilities and is able to process the detection signals in various manners. A typical processing scheme includes generating a waveform that reflects the amplitude of the detection signal versus the scan direction. The waveform is further processed to determine locations of at least one edge, and other cross sectional features of inspected structural elements. 
     The different parts of the system are connected to corresponding supply units (such as high voltage supply unit  21 ) that are controlled by various control units, most of them omitted from the figure for simplifying the explanation. The control units may determine the current supplied to a certain part, as well as the voltage. 
     CD-SEM  100  includes a double deflection system that includes deflection units  110  and  111 . Thus, the beam tilt introduced in the first deflection unit  110 , can be corrected for in the second deflection unit  111 . Due to this double deflection system, the electron beam can be shifted in one direction without introducing a beam tilt of the electron beam with respect to the optical axis. 
       FIG. 1   b  is a perspective view of an objective lens  120 , according to another embodiment of the invention. In  FIG. 1   b , the tilt deflection is performed below (downstream direction) of the objective lens. Objective lens differs from objective lens  112  by having a pole-piece arranged in a quadrupole formation, positioned between the objective lens and specimen, for controlling the tilt condition of the electron beam. The polepieces are electrically connected to a ring and a core that bears additional coils (not shown) that are arranged so as to concentrate a magnetic flux in the space between the polepieces, through which the electron beam passes. 
     Modern CD-SEMs are able to measure structural elements that have cross sections that have sub-micron dimensions, with an accuracy of several nanometers. The size of these cross sections is expected to reduce in the future, as manufacturing and inspection processes continue to improve. 
     Various features of the cross section may be of interest. These features may include, for example, the shape of the cross section, the shape of one or more sections of the cross section, the width and/or height and/or angular orientation of the cross section sections, as well as the relationship between cross section sections. The feature can reflect typical values, as well as maximal and/or minimal values. Typically the width of the bottom of a line is of interest, but this is not necessarily so, and other features may be of interest. 
       FIG. 2   a  illustrates a perspective as well as a cross sectional view of line  210  and a reference structural element, such as bump  250 . Line  210  has a cross section  230  that includes a top section  224  and two substantially opposing traverse sections  222  and  226  (that correspond to the top section  214  as well as to two sidewalls  212  and  216  of line  210 ) that are both positively oriented at substantially opposing angles, such that the bottom of the line is not obscured by the top section  214 . 
     Bump  250  is much smaller than line  210  and has at least one point, such as certain point  251 , that is located substantially at the same plane as the lower points  233  and  237  of first and second traverse sections  222  and  226 . Bump  250  is located so as to be viewed by electron beams that are used to scan the first traverse section as well as electron beams used to scan the second traverse section. The location of bump  250 , and especially the relationship between its height (which is usually much smaller than the height of line  210 ) and the distance between bump  250  and line  210  are calculated to allow the bump  250  to be viewed by both types of electron beams. The distance can be responsive to the maximal tilt angle of the scanning electron beam. As a rule of thumb, the distance between bump  250  and line  210  has to be greater than H max  × tangent(A max ), in which H max is the maximal height of line  210  and A max  is the maximal tilt angle of a scanning electron beam. 
     The inventors found that a bump can be generated relatively quickly using a charged electron beam that interacts with an object to provide a bump based upon the carbonization phenomena. For example, the inventors generated a 0.5 micron long carbonization line in less than a minute. Very good results were achieved by generating the bump by scanning the object multiple times (such as, but not limited to 10-50 scans) with a relatively low current electron beam (such as, but not limited to 20, 50, 100pA and the like). The inventors also found that such bumps can be achieved by using various acceleration voltages, in particular using 200v provided better results than using 500v or 1000v. Longer bumps usually required to increase the number of scans. It is noted that inaccuracies of the bumps can reflect the accuracy of the critical dimension measurements. The inventors found that the added error is less than 1 nano-meter, and that using several bumps and averaging the results for all bumps may improve the precision of the measurement. 
       FIG. 2   b  illustrates a cross section  230 ′ of another line (not depicted) that has a top section  234 , a first traverse section  232  that is positively oriented and a negatively oriented second traverse section  236 .  FIG. 2   b  also illustrates the convention of positive angles, negative angles and zero angle. 
     In some cases, a line may be positioned between first and second reference structural elements (e.g., two bumps similar to bump  250 ). The distance between the reference structural elements may be measured by performing multiple scans of said reference structural elements. 
     The inventors found that using reference elements that are positioned at opposing sides of a measured structural element may be useful in various cases, such as when a single structural element cannot be viewed by electron beams that scan the measured structural element from both of its sides. This can occur when other structural elements are positioned in a very close proximity to the measured structural element, such as to obscure the reference structural element. 
     Thus, in a dense array of lines  210 , reference structural elements (e.g., bump  250 ) may be positioned between pairs of lines  210 . 
     In other embodiments of the invention, multiple reference structural elements are positioned in proximity to a measured structural element, so as to allow multiple measurements of the relationship between the measured structural element and the multiple reference structural elements and to allow statistical processing of the multiple relationships to provide better results. Said statistical processing may average out or otherwise reduce errors and/or inaccuracies associated with a single measurement. This may result from an increment of the overall signal to noise ratio. In some cases, an array of reference structural elements (e.g., similar to bump  250 ) may be positioned at a first side of a line (e.g., similar to line  210 ), while in other cases, an array of reference structural elements may be positioned on both sides of a measured structural element. The bottom CD of a line can be responsive to multiple distances measured with respect to these reference structural elements. 
     It is noted that although the above discussion refers to line shaped measured structural elements and reference structural elements, the method and system are applicable to determine cross sectional features (such as top CD, bottom CD, maximal CD, and the like) of various structural elements, such as contacts, recesses and the like. The reference structural elements may have other shapes. 
       FIGS. 3   a  - 3   c  are schematic illustrations of waveforms  260 - 262  that represent a relatively wide positively oriented traverse section, a relatively narrow traverse section and a negative oriented traverse section respectively. As can be seen from these figures, the waveform portion associated with steep sidewalls, as well as negative oriented sidewalls is relatively narrow and corresponds to the width of the scanning electron beam. 
       FIG. 4  illustrates a method  400  for determining a location of a traverse section. When a waveform is obtained from scanning a structural element, such as a line, by an electron beam, a portion of a derivative of this waveform may be obtained. For example, a waveform used to provide an estimate of the measure of a traverse section may include 3 points. An intermediate point has a maximal derivative value. The intermediate point is located between an upper point and a lower point, each associated with lower derivative values. The inventors have used an upper point that has a derivative value of about 80% of the maximal derivative value, and a lower point that has a derivative value of about 30% of the maximal derivative value, but other values can be used. A linear estimate of the traverse section is drawn between the upper and lower points. The intersection between said linear estimate and a height threshold provides a location point of the traverse section. The inventors used a height threshold of 35% of the maximal height of waveform  410 , but other values can be used. 
     Referring to  FIG. 4 , method  400  starts by step  470  of obtaining a waveform, said waveform representative of detection signals generated as a result of an interaction between a scanning electron beam and a structural element. 
     Step  470  is followed by step  472  of calculating at least three points—a first point that is characterized by a maximal derivative value, and a lower point and an upper point that are characterized by a predefined derivative values. 
     Step  472  is followed by step  474  of determining a location point in response to an intersection between a height threshold and a line that is drawn between the upper and lower points. 
     According to an embodiment of the invention, the relationship between a reference structural element and a traverse section is actually the relationship between a certain point of the reference structural element and a location point of that traverse section. The certain point of the reference structural element may also be a location point. 
     It is noted that other points, such as an upper point, an intermediate point, a lower point and/or other points of waveforms  260 - 262  may be used in determining a relationship between a traverse section and a reference structural element. 
       FIG. 5  is a flow chart of method  500  for determining a cross sectional feature of a measured structural element having a sub-micron cross section, the cross section defined by an intermediate section located between first and second traverse sections. 
     Method  500  measures a feature of a structural element. The measured structural element is a part of a measured object, such as a wafer, die or dice. If appropriate reference structural elements are not positioned at the vicinity of the measured element, they may be added by use of prior art methods for adding and/or removing material, such as Focused Ion Beam (FIB) based methods, electron beam or even laser based methods. These methods may also require the provision/injection of gas/plasma in a manner known in the art. A system that implements some of these techniques is the SEM Vision G2 FIB of Applied Materials Inc. of Santa Clara, Calif. 
     Method  500  starts at step  520  of scanning, at a first tilt state, a first portion of a reference structural element and at least the first traverse section of the measured structural element, to determine a first relationship between the reference structural element and the first traverse section. 
     Conveniently, the first relationship is a distance between a certain point of the reference structural element and a first edge of the first traverse section. As illustrated by  FIG. 2   a , method  500  can be responsive to the relationship between a measured structural element and a single reference structural element, but may be responsive to additional relationships between the measured structural element and multiple reference structural elements. 
     Conveniently, the bottom of the measured structural element and the certain point of the reference structural element are substantially located on the same plane. Usually the height of the reference structural element, and especially the height of a certain reference point, is much smaller than a height of the measured structural element. 
     It is noted that the invention can be adapted to handle other shaped and/or sized structural elements. The height of the reference structural element may be taken into account, for example by using elementary geometrical equation to compensate for a height difference between the bottom of the measured structural element and the reference structural element. 
     Step  520  is followed by step  530  of scanning, at a second tilt state, a second portion of a reference structural element and at least the second traverse section of the measured structural element, to determine a second relationship between the reference structural element and the second traverse section. 
     Step  530  is followed by step  540  of determining a cross sectional feature of the measured structural element in response to the first and second relationships. 
     According to another embodiment of the invention, if one or more additional relationships were determined during steps  520  and  530 , then the determination of step  540  is further responsive to at least one of the additional relationships. 
     It is noted that during a tilt stage the electron beam may be tilted in relation to an imaginary plane that is perpendicular to the measured object. The tilt angle can be positive or negative. According to an embodiment of the invention, during the first tilt state the electron beam is tilted at a positive angle, while during the second tilt state the electron beam is tilted at a negative angle. 
     According to another embodiment of the invention, during the first tilt state the electron beam is tilted at a negative angle, while during the second tilt state the electron beam is tilted at a positive angle. 
     According to an embodiment of the invention, during the first or second tilt state the electron beam is tilted at substantially zero degrees. 
     It is noted that large tilt angles, such as 15 degrees and even more, can be achieved by using prior art tools such as the NanoSem 3D or the VERASem of Applied Materials Inc. 
       FIG. 6  is a flow chart of method  600  for determining a cross sectional feature of a measured structural element having a sub-micron cross section, the cross section defined by an intermediate section located between first and second traverse sections. 
     Method  600  starts by step  620  of scanning, at a first tilt state, a first portion of a reference element and at least the first traverse section of the measured structural element, to determine a first relationship between the reference structural element and the first traverse section. 
     Step  620  is followed by query step  625  of determining whether to perform an additional scanning step. If the answer is yes, method  600  proceeds to step  630 , else the method proceeds to step  640 . 
     The determination of step  625  may be responsive to various parameters, such as, but not limited to, top, bottom, maximal and/or minimal width of the measured structural element, estimated or measured orientation of a traverse section and/or estimated or measured width of a traverse section. The estimation can involve processing of detection signals and comparison with detection signals originating from other structural elements. The comparison may involve die to die comparison, die to data base comparison and the like. 
     An additional scanning step may be required if one traverse section (or both) is (are) suspected to be either steep (for example above a steepness threshold) or to be negatively oriented. This orientation may be estimated from waveforms acquired during the detection of signals resulting from the scan of the measured structural element. Steep traverse sections, as well as negative oriented traverse sections, are associated with certain waveforms. The inventors found that such traverse sections are suspected if the width of the waveform substantially equals the width of the electron beam. The inventors also found that a comparison of waveforms to previously recorded waveforms of known elements may also be used. 
     Step  630  includes scanning, at a second tilt state, a second portion of a reference element and at least the second traverse section of the measured structural element, to determine a second relationship between the reference structural element and the second traverse section. 
     Step  630  is followed by step  640  of determining a cross sectional feature of the measured structural element in response to at least the first relationship. 
     According to another embodiment of the invention, if one or more additional relationships were determined during steps  620  and  630 , then the determination of step  640  is further responsive to at least one of the additional relationships. 
       FIG. 7  is a flow chart of method  700  for determining a cross sectional feature of a measured structural element having a sub-micron cross section, the cross section defined by an intermediate section located between a first and a second traverse sections. 
     Method  700  starts at step  720  of scanning, at a first tilt state, at least a point of a first reference structural element and at least the first traverse section of the measured structural element, to determine a first relationship between the first reference structural element and the first traverse section. 
     Step  720  is followed by step  730  of scanning, at a second tilt state, at least a point of a second reference structural element and at least the second traverse section of the measured structural element, to determine a second relationship between the second reference structural element and the second traverse section. 
     Step  730  is followed by step  740  of determining a cross sectional feature of the measured structural element in response to the first and second relationships. 
     According to an embodiment of the invention, the measured structural element is positioned between the first and second reference structural elements. 
     The relationship (usually distance and optionally height difference) between the first and second reference elements must be measured/ estimated. Inaccuracies in this measurement may affect the measurement of the cross sectional feature. 
     Step  700  may include a preliminary step of either estimating or receiving this reference relationship, but it may also include a step  710  of measuring it. The measurement may take place in locations other than the location in which the measured element is being measured, so that various contamination, carbonization or shrinkage effects will not affect the measurement of the measured structural element. 
     Step  710  may include one or more scans of an area that includes the first and second points of the first and second reference elements and the measured structural element. Step  710  may also include preventing the electron beam from illuminating the measured structural element. This can be implemented by blanking the beam while it is supposed to illuminate the measured structural element. 
       FIG. 8  is a flow chart of method  800  for determining a cross sectional feature of a measured structural element having a sub-micron cross section, the cross section defined by an intermediate section located between a first and a second traverse section. 
     Method  800  starts at step  820  of scanning, at a first tilt state, a portion of a reference element and at least the first and second traverse sections, to determine at least one relationship between the reference element and the at least one scanned traverse element. During this step the electron beam can be substantially perpendicular to the measured object that includes the measured structural element. 
     Step  820  is followed by query step  825  for determining whether an additional scanning step is required for determining an additional relationship between the reference structural element and the at least one traverse section. If the answer is positive, method  820  proceeds to step  830 , else it proceeds to step  840 . This determination may be responsive to an estimated width of a traverse section, to an estimated orientation of a traverse section, to an estimated cross sectional feature of the measured structural element, and the like. 
     Step  830  includes performing at least one additional scanning step. Conveniently, the tilt state differs from the first tilt state of step  820 . 
     Step  840  involves determining a cross sectional feature of the measured structural element in response to at least one relationship. 
       FIG. 9  is a flow chart of method  900  for determining a cross sectional feature of a measured structural element having a sub-micron cross section, the cross section defined by an intermediate section located between a first and a second traverse sections. 
     Method  900  starts by step  920  of scanning, at a first tilt state, first portions of a set of reference structural elements and at least the first traverse section of the measured structural element, to determine a first set of relationships between reference structural elements of the set of reference structural elements and the first traverse section. 
     Step  920  is followed by step  930  of scanning, at a second tilt state, second portions of the set of reference structural elements and at least the second traverse section of the measured structural element, to determine a second set of relationships between reference structural elements of the set of reference structural elements and the second traverse section. 
     Step  930  is followed by step  940  of determining a cross sectional feature of the measured structural element in response to the first and second sets of relationships. Step  940  may include statistical processing of the relationships of the first set to provide a first relationship. It may also include statistical processing of the relationships of the second set to provide a second relationship. 
     According to another embodiment of the invention, method  900  may include a query step for determining a necessity of additional scanning steps, in addition to step  920 , and to determine whether step  930  (or even additional scanning steps) is required. 
     Is noted that the intermediate section, which may be a top section in the case of an elevated structural element, may be determined from each of the scanning steps. It is further noted that given the first and second traverse section cross sectional features the cross section of the structural element as well as any feature (such as, but not limited to, top CD, bottom CD, maximal CD) of said cross section can be determined. A typical cross sectional feature is the horizontal projection of a traverse section. In cases where the tilt angle is relatively small, it is assumed that the tilt angle is approximately equal to the tangent of this angle. 
     It is noted that some of the measurements may be repeated, and that additional tilted scans of the structural element (with the same and/or differing tilt angles) may be performed for various reasons, such as averaging out statistical noise, and the like. Accordingly, methods  400  and  500  may include multiple measurements of one or more cross sectional features, even if the height of the structural elements were known or estimated and even if a certain cross sectional feature were measured. 
     The present invention can be practiced by employing conventional tools, methodology and components. Accordingly, the details of such tools, component and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as shapes of cross sections of typical lines, number of deflection units, etc., in order to provide a thorough understanding of the present invention. However, it should be recognized that the present invention might be practiced without resorting to the details specifically set forth. 
     Only exemplary embodiments of the present invention and but a few examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.