Abstract:
A method and system are presented for use in measuring/inspecting a patterned article. Optical measurements are applied to a measurement site on the article by illuminating the measurement site with a plurality of wavelengths at substantially normal incidence of the illuminating light, detecting light returned from the illuminated site, and generating measured data indicative thereof. The measurements are applied to the measurement site through a polarizer rotatable between its different orientations selected from a number of pre-calibrated orientations.

Description:
FIELD OF THE INVENTION 
   This invention relates in general to the field of optical measurements/inspection, and particularly relates to measurement/inspection of patterned articles. 
   BACKGROUND OF THE INVENTION 
   It is known that polarized normal incidence spectrometry may be used for measuring line profiles&#39; parameters being applied to one-dimensional (1D) line arrays (pattern) on an article by help of Rigorous Coupled-wave Theory (Further below RCWT) and the like modeling. The same approach can also be applied to two-dimensional (2D) line arrays or any other 2D periodic pattern (e.g. arrays of lines, pads, vias, etc.). 
   U.S. Pat. No. 6,665,070 discloses a metrology device with a rotatable polarizer that is calibrated to align the transmission axis of the polarizer with the axis of orientation of a sample, such as a diffraction grating. The axis of orientation of the diffraction grating can be either the TE or TM axis. The system offset angle between the transmission axis of the polarizer in its home position and an axis of motion of the stage, such as a polar coordinate stage, is determined. Whenever a new substrate is loaded onto the stage, the sample. offset angle between the axis of motion of the stage and the axis of orientation of a sample is measured. The polarizer offset angle, which is the angle between transmission axis of the polarizer and the axis of orientation of the sample, is the sum of the system offset angle and the sample offset angle. Thus, by rotating the polarizer by an amount equivalent to the sum of the system offset angle and the sample offset angle, the polarizer offset angle is reduced to zero. If desired, the polarizer may be rotated once to compensate for the system offset angle and then rotated to compensate for the sample offset angle for each newly loaded substrate or the polarizer may be rotated to compensate for both the system offset angle and the sample offset angle for each newly loaded substrate. 
   U.S. Pat. No. 6,657,736 discloses a method and system for determining a line profile in a patterned structure, aimed at controlling a process of manufacture of the structure. The patterned structure comprises a plurality of different layers, the pattern in the structure being formed by patterned regions and un-patterned regions. At least first and second measurements are carried out, each utilizing illumination of the structure with a broad wavelengths band of incident light directed on the structure at a certain angle of incidence, detection of spectral characteristics of light returned from the structure, and generation of measured data representative thereof. The measured data obtained with the first measurement is analyzed, and at least one parameter of the structure is thereby determined. Then, this determined parameter is utilized, while analyzing the measured data obtained with the second measurements enabling the determination of the profile of the structure. 
   According to the techniques of these patents, measurement of TE and TM polarization components is used that require a certain specific orientation of the polarizer&#39;s axis relative to the orientation of the line arrays on a patterned article. Usually, the polarizer&#39;s axis is orientated parallel to the line (TE polarization measurement) and/or perpendicular to the line (TM measurements). 
   SUMMARY OF THE INVENTION 
   There is a need in the art to facilitate polarization-based measurements on patterned structures, by providing a novel measurement system and method. 
   The main idea of the present invention is associated with the following. As indicated above, the prior art techniques utilize measurements of TE and TM polarization components requiring a certain specific orientation of the polarizer&#39;s axis relative to the orientation of the line arrays on a patterned article. If an article under measurements is positioned on a movable X,Y-stage, it is sufficient to align the article relative to coordinate system of the stage, so a pre-aligned polarizer can be used at two predefined positions relative to the stage coordinates. If R-θ stage is used, it is necessary to align the polarizer to the article on the stage in each orientation of the article. This technique suffers from drawbacks associated with the following: 
   Since an optical system induces some polarization and intensity change, it is necessary to calibrate the efficiency of the optical system for each orientation of the polarizer. Since the time acceptable for such calibration is never infinite, practically calibration is carried out at a limited number of angles of polarization. Based on such a sampling, the efficiency of the optical system for any angle is calculated by interpolating calibrated values at sampled angles, thus unavoidably inducing an additional source of measurement error. Another drawback of the above technique relates to the optical axis deviation caused by the polarizer rotation. If the optical axis of the polarizer (e.g., Glan-Thompson prisms based polarizer) does not coincide perfectly with the rotation axis (which is always the case), an optical beam that passes the polarizer is deflected and therefore impinges the measured article slightly aside the nominal coordinates. Such a deflection is a function of the angle of the polarizer&#39;s axis relative to its zero-position. It is thus clear that if the polarizer is to be aligned to an arbitrary positioned grating (the orientation of a patterned article), such a deflection should be pre-calibrated (with associated interpolation) and taken into account in positioning the article. This is another source of calibrating error that is caused by the polarizer rotation in the full range of possible orientations (±90°). 
   Yet another common drawback of the prior art measurement techniques is that they do not exploit all the information that may be obtained from the sample, but use only the amplitude of diffraction efficiency for TE and TM polarization states. Valuable information about a phase shift between these two polarization states in such case is ignored. 
   The present invention solves the above problems by measuring in a patterned article with at least three polarization states of incident light. This technique takes into account a phase difference between the TE and TM polarizations. It is known that this phase difference may be obtained by taking measurement in at least one additional orientation of a polarizer relative to a grating. Usually, from the symmetry point of view, a 45° angle is selected. Use of more angles does not provide independent measurements but may be useful for reducing measurement errors. So, by measuring with at least three polarization states, it is possible to measure both the amplitude of the diffraction efficiency for each polarization state and the phase shift between the polarization states. 
   There is thus provided according to one aspect of the invention, a method for use in measuring/inspecting a patterned article, the method comprising applying optical measurements to a measurement site on the article by illuminating the measurement site with a plurality of wavelengths at substantially normal incidence of the illuminating light, detecting light returned from the illuminated site, and generating measured data indicative thereof, wherein the measurements are applied to the measurement site through a polarizer rotatable between its different orientations selected from a number of pre-calibrated orientations. 
   The method utilizes calibration data indicative of an angular orientation φ of the pattern on the article under measurements and the selected polarization. 
   Preferably, these three selected polarization states include two mutually perpendicular polarizations corresponding to the angular orientations φ and (90+φ), and at least one intermediate polarization state corresponding to the polarization vector orientation in between these mutually perpendicular polarization states. The intermediate polarization may for example correspond to a 45° angular orientation. Considering four selected polarizations, the three polarization states are selected as those corresponding to the orientations closest to the given orientation of the pattern on the article under measurements. 
   The measured data is processed to determine diffraction efficiencies R TE  and R TM  for, respectively, linear polarization states of the illuminating light relative to the pattern on the article under measurements, and a phase shift between them, thereby enabling calculation of a diffraction efficiency R(φ) for any angular orientation φ of the polarizer relative to the pattern. 
   According to another aspect of the invention there is provided a method for use in measuring/inspecting a patterned article, the method comprising applying optical measurements to a measurement site on the article by illuminating the measurement site with a plurality of wavelengths at substantially normal incidence of the illuminating light, detecting light returned from the illuminated site, and generating measured data indicative thereof, wherein the measurements are applied to the measurement site through a polarizer, rotatable between its different orientations, with at least three different orientations of the polarizer selected from at least four pre-calibrated orientations of the polarizer. 
   According to yet another aspect of the invention, there is provided a method for use in measuring/inspecting a patterned article, the method comprising applying optical measurements to a measurement site on the article by illuminating the measurement site with a plurality of wavelengths at substantially normal incidence of the illuminating light, detecting light returned from the illuminated site, and generating measured data indicative thereof, wherein the measurements are applied to the measurement site through a polarizer rotatable between its different orientations selected from a number of pre-calibrated orientations, data being provided about an angular orientation a of the pattern on the article under measurements with respect to a selected zero position of the polarizer. 
   According to yet another aspect of the invention, there is provided a system for use in measuring/inspecting a patterned article, the system comprising:
         an illumination assembly configured and operated for producing illuminating light of a plurality of wavelengths;   a detection assembly comprising a spectrometer for receiving light of different wavelengths and generating measured data indicative thereof;   a light directing assembly configured for directing the illuminating light onto the article along an axis substantially perpendicular to the article and collecting light returned from the article along said axis and directing the collected light to the detection assembly, the light directing assembly comprising a polarizer assembly operated to be rotatable between its different orientations; and   a control unit operable for receiving and processing the measured data, the control unit utilizing calibrated data indicative of an angular orientation φ of the pattern on the article under measurements and at least three pre-calibrated orientations of the polarizer, to thereby select the different polarizer orientations for measurements on each measurement site.       

   
     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  is a schematic illustration of an optical system suitable to be used in the present invention; 
       FIG. 2  illustrates the principles of the present invention; 
       FIG. 3  exemplifies a calibration method of the invention for creating reference data indicative of an optimal set of angles from which angles for measurements are to be selected; 
       FIGS. 4A-4B  and  5 A- 5 B exemplify the technique of the present invention for selecting three polarizer angles for measurements from four optimal angles; 
       FIG. 6  graphically exemplifies the principles of the present invention for selecting three optimal angular orientations of a polarizer for measurements in each measurement site of the article; 
       FIG. 7  illustrates a flow diagram of the main operational steps in a method of the present invention; 
       FIGS. 8 and 9  illustrate flow diagrams of two examples, respectively, of the method according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , there is schematically illustrated an optical system  10  for inspecting/measuring in a patterned article such as semiconductor wafer W actually presenting a grating (e.g., array of lines) on a substrate. The wafer is located on a stage  11 , which is preferably configured for rotation and back and forward movement along at least one axis (the R-Theta stage). 
   The system  10  is configured as a normal incidence polarized reflectance spectrometer, and includes a light source assembly  12 ; a detector assembly  14  including a spectrometer; a polarizer assembly  16  associated with a drive  18  that is operated to vary the orientation of the preferred polarization of the polarizer with respect to the orientation of the grating; a light directing assembly  20 ; and a control unit  22  that receives and interprets measured data coming from the detector assembly. The light source assembly  12  includes at least one light emitting element  12 A and optionally includes a condenser lens arrangement  12 B, as well as a fiber  12 C through which light may be conveyed from the remote light emitter element(s). The light source assembly is configured for producing light of different wavelengths, which may be achieved by using different light emitting elements or a broadband illuminator. The light directing assembly  20  includes a beam splitter  24  accommodated in an optical path of light coming from the light source  12  to reflect it towards the wafer W and in the optical path of reflected light coming from the wafer to transmit the reflected light towards the detector  14 ; a focusing/collecting lens arrangement  26 ; a pinhole mirror  28  for transmitting a part of collected light to the detector  14  via a relay lens  29  and possibly another mirror  30  and for reflecting the other part of the collected light towards an imaging assembly  32  via an imaging lens  33 . The polarizer  16  is operated by the control unit  22  to provide predetermined orientation of the preferred plane of polarization. 
   It should be noted that the pinhole mirror  28  works as a beam separator between the measurement path propagating toward the detector  14  and an imaging path propagating toward the imaging assembly  32  (e.g. CCD camera); however other beam separator may be used instead of the pinhole mirror, for example a beam splitter, switching mirror, etc. 
   Reference is made to  FIG. 2  illustrating the principles of the present invention. Generally, during the measurements the polarizer is positioned with any angular orientation φ i  relative to the grating under measurements. Considering β 1  as the zero-angle polarization position (β 1 =0), φ=α, wherein α is an angle between the line array axis (grating) and the zero-angle polarizer position β 1 . In general case, when β 1 ≠0, the angular orientation φ i  relative to the grating under measurements could be defined as φ i =β i −α. Diffraction efficiency R(φ) for any angular orientation φ i  can be calculated by the following equation:
 
 R (φ)= R   TE ·cos 4    φ+R   TM ·sin 4  φ+2√{square root over ( R   TE   ·R   TM )}·cos δ·sin 2  φ·cos 2  φ  (1)
 
   In order to determine three independent parameters—R TE , R TM  and cos δ (phase shift between TE and TM polarizations), that should be compared to the theoretical calculated data, it is needed to measure diffraction efficiency on at least three different angles, φ 1 , φ 2  and φ 3 , corresponding to three fixed polarizer positions β 1 , β 2  and β 3  and to solve the following system of equations: 
                 {             R   ⁡     (     φ   1     )       =           R   TE     ·     cos   4       ⁢     φ   1       +         R   TM     ·     sin   4       ⁢     φ   1       +     2   ⁢           R   TE     ·     R   TM         ·   cos     ⁢           ⁢     δ   ·     sin   2       ⁢       φ   1     ·     cos   2       ⁢     φ   1                       R   ⁡     (     φ   2     )       =           R   TE     ·     cos   4       ⁢     φ   2       +         R   TM     ·     sin   4       ⁢     φ   2       +     2   ⁢           R   TE     ·     R   TM         ·   cos     ⁢           ⁢     δ   ·     sin   2       ⁢       φ   2     ·     cos   2       ⁢     φ   2                       R   ⁡     (     φ   3     )       =           R   TE     ·     cos   4       ⁢     φ   3       +         R   TM     ·     sin   4       ⁢     φ   3       +     2   ⁢           R   TE     ·     R   TM         ·   cos     ⁢           ⁢     δ   ·     sin   2       ⁢       φ   3     ·     cos   2       ⁢     φ   3                         (   2   )               
which may be rewritten in matrix form as:
 
                     (           R   ⁡     (     φ   1     )                 R   ⁡     (     φ   2     )                 R   ⁡     (     φ   3     )             )     =       (             cos   4     ⁢     φ   1               sin   4     ⁢     φ   1               sin   2     ⁢     φ   1               cos   2     ⁢     φ   1                   cos   4     ⁢     φ   2               sin   4     ⁢     φ   2               sin   2     ⁢     φ   2               cos   2     ⁢     φ   2                   cos   4     ⁢     φ   3               sin   4     ⁢     φ   3               sin   2     ⁢     φ   3               cos   2     ⁢     φ   3             )       ︸   A         ⁢     
     ⁢           ⁢     (           R   TE               R   TM               2   ⁢         R   TE     ⁢     R   TM         ⁢   cos   ⁢           ⁢   δ           )             (   3   )               
wherein the matrix of coefficients will be denoted by A.
 
   Here, A is a three-by-three matrix, which depends on three polarizer orientations selected for measurements. 
   In these linear equations, the unknowns are R TE , R TM  and cos δ, while the known parameters are φ 1,2,3  and R(φ 1,2,3 ). After determining the unknown parameters, they are compared to a theoretical model, as will be described further below with reference to  FIG. 8 . Alternatively, the output of the theoretical model is first transformed into R(φ 1,2,3 ) and then compared to the measurements, as will be described further below with reference to  FIG. 9 . 
   So, it is possible, in principle, to keep an actual measurement position β i  of the polarizer constant, e.g., 0°, 90° or 45°, for determining the diffraction efficiency R for each orientation φ i  of the patterned article and thus to allow calibration of the polarizer only at three measurement angles avoiding a need for the calibration data interpolation. 
   There is a special case when the grating is oriented at a 45°-angle relative to the polarizer&#39;s axis (φ=45°), and the diffraction efficiencies R 0  and R 90  are actually the same values. Hence, instead of three independent different measurement conditions, only two are unique. This problem can be overcome by calibrating the polarizer for at least one additional angle, e.g., (−45°), and thus four polarization states will be available for measurements: 0°, 90°, 45° and −45°. It is thus clear that even with four pre-selected angles, there might be grating angles in which the matrix of coefficients A has a large condition-number (defined as the ratio of its largest eigenvalue to its lowest eigenvalue, both in absolute values), indicating a high degree of non-uniqueness. The inventors have found that for the set of measurement angles β{−45°, 0°, 45°, 90°}, this occurs for angles of the form 22.5+45n where n=0, 1, 2, . . . . 
   From the standpoint of uniqueness of three selected measurement angles, the use of more than three pre-calibrated angles β i  is preferred, and thus the higher uniqueness of the selected three measurements (at β 1 , β 2  and β 3 ) can be achieved. For simplicity, let&#39;s consider that angles β are accurate multiples of 15°, e.g., 0°, 15°, 30°, 45° and so on, until 180°. A simple algorithm for selecting these three angles from the pre-calibrated orientations is as follows:
         β 1  is selected as the closest angle to the actual angle φ of the array;   β 2  is selected as β 2 =β 1 +45°;   β 3  is selected as β 3 =β 1 +90°.       

   If limiting the measurements to only four pre-selected angles is considered, it can be shown that the smallest condition number of the matrix A is achieved for the set of angles {0°,65.2°,90.4°,116.9°}. For simplicity, such a set may be rounded to the following angles: {0°,60°,90°,120°}. Since matrix coefficients do not depend on application, this set of angles is optimal for any application. 
   Referring to  FIG. 3 , there is exemplified a flowchart of a method of the invention showing the way of selecting the best set of N angles (for any number larger than 3) that can be found systematically. It makes use of the condition number CN (ratio of the largest eigenvalue and smallest eigenvalue) of the coefficients matrix A, defined above. Initially, a first, arbitrary set of N angles of polarizer orientation is selected, β 1 , . . . β N . All possible M combinations of 3 angles (triplets) from these N angles are defined, (triplet) 1 , . . . , (triplet) M . Then, for each grating angle α from a certain variety of K grating angles, α 1 , . . . , α K , the triplet that provides the best (minimal) CN value is found, namely CN (1)   best , . . . , CN (K)   best . Then, the worst value of CN best , namely, the maximal value, is found to present the merit function, CN (1)   set , for this first set of polarizer orientation angles. Then, a second set of N polarizer orientation angles is defined, β 1 , . . . , β N , by slightly varying the angles of the first set, and similarly, a merit function, CN (2)   set  for this second set of polarizer orientation angles is determined. This procedure is repeated Q times, and merit functions CN (1)   set , . . . , CN (Q)   set  are found, which process actually presents searching for an N-dimensional space and characterizing this space by the minimal value of CN (min)   set  that has its corresponding set of N polarizer orientation angles. The set of N angles defining this space of CN (min)   set  presents the best set of angles for actual measurements on the article. 
   After the best set of N angles is found, the selection rules are determined for selecting 3 angles out of N for each possible grating angle β i  which has been recorded during the above calibration procedure. Now, during the actual measurements with a given grating angle α, the corresponding best triplet of polarizer orientation angle is used, utilizing the above reference data. 
   Reference is made to  FIGS. 4A-4B  and  5 A- 5 B exemplifying the technique of the present invention for selecting three polarizer angles for measurements from four optimal angles.  FIG. 4A  shows four graphs that correspond to, respectively, the selected angular orientations β 1 , β 2 , β 3  and β 4 . Thus, each orientation of the grating (x-axis) has its three optimal corresponding orientations of the polarizer (y-axis), at which the measurements are to be taken.  FIG. 4B  is a table showing the best three angles out of the four angles for every possible grating angle: 
   For simplicity, such a set may be rounded to the following angles: {0°,60°,90°,120°}. This is illustrated in  FIGS. 5A-5B . 
   The result of selecting three polarizer angles for each angle α, according to the above algorithm, but for more than 4 selected polarizer angles is illustrated in  FIG. 6 . The figure shows an example of using three-angles selecting algorithm from pre-calibrated orientations: 0°, 15°, 30°, etc. (i.e., the polarizer rotation step of 15°). This figure shows the angular orientation β of the polarizer (vertical axis) vs. the angular orientation of the grating, namely, the angle α between the line array axis and the zero-angle polarizer position (horizontal axis). Thus, each orientation of the grating has its three optimal corresponding orientations of the polarizer, β 1 , β 2  and β 3 , at which the measurements are to be taken. 
   Referring to  FIG. 7 , there is illustrated a flow diagram of the main steps of a method of the present invention. Measurements are applied to a measurement site on the article using polarized broadband light (generally, a plurality of wavelengths) normal incident into the article. The measurements are taken with at least three different orientations of a polarizer at which the system was previously calibrated. A light response of the measurement site is detected. Measured data indicative of the detected light is processed to determine the diffraction efficiencies corresponding to the selected polarization states, thereby enabling determining the pattern parameters. 
   Examples of a method of the present invention for one measurement site will now be described with reference to  FIGS. 8 and 9 . The measurement system is calibrated for a pre-selected set of N polarizer orientations, which includes at least four polarizer angular orientations β. When the article (wafer) is loaded in the system, actual measurements are taken. In order to optimize the measurements, the following procedure is carried out: a grating-containing (patterned) site is selected for measurements; the grating orientation α (φ) is determined; and based on this information the optimal three polarizer orientations β 1 , β 2  and β 3  are selected from the calibration set β 1 , β 2 , . . . β N  (the set selected to be the best one during the above-described calibration procedure aimed at determining the reference data). Grating angular orientations φ 1 , φ 2  and φ 3  corresponding to the polarizer orientations β 1 , β 2  and β 3  are defined based on information on orientation of the line array axis of the pattern. Preferably, angle φ 1 =α being an angle between the line array axis and the zero-angle polarizer position β 1 . It should be noted that this optimization procedure may be done off-line, e.g., as a recipe design step for semiconductor wafers. Measurements are applied to the article with the three selected polarizer orientations and diffraction efficiencies R meas (φ 1 ), R meas (φ 2 ) and R meas (φ 3 ) are determined. 
   The further interpretation of the so-determined (measured) diffraction efficiencies R meas (φ 1 ), R meas (φ 2 ) and R meas (φ 3 ) is based on a suitable theoretical analysis of light diffraction on gratings, for example, based on RCWT. The standard RCWT equations are formulated so as to provide such output parameters as R TE , R TM  and δ (phase shift). The theoretical analysis utilizes a predefined model based on the parameters of pattern (e.g., profile, period, tilt, etc.). 
   It should be understood that a certain inconvenience is caused by that the measured parameters R(φ 1 ), R(φ 2 ) and R(φ 3 ) are different from the output parameters of the calculation which are usually R TE , R TM  and cos δ, because the mostly used data interpretation method is based on iterative calculation of theoretical parameters and comparing them with the measured parameters: when the measured and calculated parameters are equal within predefined measurement errors, the optical model used for calculating the theoretical output parameters provides the required information about the line profile. Such inconvenience can be avoided. The preferred way for this is to transform the calculated parameters R TE , R TM  and cos δ for each orientation β using the equation (1) above. Since the calculation accuracy is practically unlimited, such transformation will not induce any additional errors. After such transformation, the theoretical output parameters R sim (φ 1 ), R sim (φ 2 ) and R sim (φ 3 ) will be compared directly to the measured parameters R meas (φ 1 ), R meas (φ 2 ) and R meas (φ 3 ). 
   According to the example of  FIG. 8 , the data indicative of the optimized polarization orientations set is processed using coefficient matrix to transform R TE , R TM , and cos(δ) (determined using RCWT) to R sim (φ 1 ), R sim (φ 2 ) and R sim (φ 3 ) by the equation (1) for each φ. Then, measured values R meas (φ 1 ), R meas (φ 2 ) and R meas (φ 3 ) are compared to R sim (φ 1 ), R sim (φ 2 ) and R sim (φ 3 ). If these values satisfy a matching condition with a predefined accuracy, the measurements are accepted and are continued for a next site on the article. If no matching is detected, the model parameters are changed to minimize the difference, and new values of R TE , R TM , and cos(δ) are determined. 
   There is another way of matching the measured and calculated parameters. 
   In the example of  FIG. 9 , the measured values R meas (φ 1 ), R meas (φ 2 ) and R meas (φ 3 ) are processed using the matrix coefficients to determined the corresponding values of R meas   TE , R meas   TM , and cos(δ) meas  by solving the equation (3). Then, the so-measured values R meas   TE , R meas   TM , and cos(δ) meas  are compared to simulated (using RCWT) values R sim   TE , R sim   TM , and cos(δ) sim . If the matching exists (with predefined accuracy), the measurement is accepted; if not, the model parameters are changed and new values of R sim   TE , R sim   TM , and cos(δ) sim  are obtained. 
   Those skilled in the art will readily appreciate that various modifications and changes can be applied to the embodiments of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims.