Abstract:
A normal incidence spectroscopic polarimeter is combined with an oblique incidence spectroscopic polarimeter to provide an accurate characterization of complex grating structures, e.g., structures with sloping sidewalls, with notches and with multiple underlying layers. The normal incidence spectroscopic polarimeter includes a polarizing element that is in the path of the normal incidence light beam such that the light beam is transmitted through the polarizing element before reaching the sample and after being reflected off the sample. The two systems may advantageously share a single light source and/or the spectrophotometer.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to metrology devices and in particular to metrology devices that may be used to measure diffracting and anisotropic structures. 
     BACKGROUND 
     The reduction on the size of microchip components requires new metrology technologies for monitoring critical dimensions (CDs). Optical metrology techniques are particularly advantageous because they are non-destructive, non-invasive, economical and compact. Certain structures are particularly difficult to accurately measure. For example, complex line profiles, such as sloping sidewalls, undercut sidewalls, and notches in the sidewalls can be difficult to measure accurately. 
     Thus, what is needed is an optical metrology tool to measure quickly and accurately diffraction gratings, including diffraction gratings having complex cross- sectional profiles. 
     SUMMARY 
     A normal incidence polarimeter is combined with an oblique incidence polarimeter, in accordance with an embodiment of the present invention, to provide an accurate characterization of complex grating structures, e.g., structures with sloping sidewalls, notches, and/or multiple underlying layers. In one embodiment, spectroscopic polarimeters are used. 
     The normal incidence polarimeter includes a polarizing element that is in the path of the normal incidence light beam. The normally incident light passes through the polarizing element before reaching the sample and after being reflected off the sample. The oblique incidence polarimeter includes a polarization stage generator in the light path before the sample and a polarization state detector in the light path after the sample. 
     The metrology device may use a single light source to produce the normally incident light beam and the obliquely incident light beam. Alternatively, multiple light sources may be used. In addition, the metrology device may use a single spectrophotometer to detect both the obliquely incident light beam and the normally incident light beam after being reflected off the sample. Alternatively, the normal incidence polarimeter and the oblique incidence polarimeter may use separate spectrophotometers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a normal incidence spectroscopic polarimeter that may be used for the characterization of profiles of lines on a grating sample. 
     FIGS. 2A to  2 D show various line profiles in a diffraction grating. 
     FIG. 3 shows a combined metrology system that includes a normal incidence spectroscopic polarized reflectometer and an oblique incidence polarimeter. 
     FIG. 4 shows another embodiment of a combined normal incidence spectroscopic polarized reflectometer and an oblique incidence polarimeter. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a normal incidence spectroscopic polarimeter  100 , which acts as a polarized reflectometer that may be used for the characterization of profiles of lines on a grating sample  101 . The system shown in FIG. 1 includes a broadband light source  102 , a beam splitter  104 , which deviates the light beam towards the microscope objective lens  106 . The light beam passes through objective lens  106  and is then polarized by a polarizer  108 , which is rotatable. The polarized light beam is incident on and reflected by the grating sample  101 . The reflected light is again transmitted through the polarizer  108  and the objective lens  106 . The light beam passes through the beam splitter  104  and is focused into the slit of a spectrograph  110 , which includes, e.g., a diffraction grating  112  and a multichannel array detector  114 . Normal incidence spectrocopic polarimeter  100  is described in more detail in U.S. Patent Application entitled “Apparatus And Method For The Measurement Of Diffracting Structures” by J. Holden et al., Ser. No. 09/670,000, filed Sep. 25, 2000, and in U.S. Patent Application entitled “Measurement Of Diffracting Structures Using One-Half Of The Non-Zero Diffracted Orders” by W. McGahan, Ser. No. 09/844,559, filed Apr. 27, 2001, both of which have the same assignee as the present disclosure and both of which are incorporated herein by reference. 
     One of the advantages of normal incidence polarimetry is the relatively simple calibration and alignment procedures, as well as the ease of integration with microelectronics processing tools. Further, the normal incidence spectroscopic polarimeter  100  can be used to measure reflectance spectra, i.e., R ET  and R TM , or ellipsometry data, i.e., ψ and Δ. Furthermore, the simulation of reflectance or ellipsometry spectra requires the use of Rigorous Coupled-Wave Analysis calculation, which is time consuming. Due to the symmetry of the normal-incidence configuration, however, the calculation time to generate a modeled spectrum at normal incidence can be orders of magnitude smaller than at oblique incidence. Consequently, normal incidence spectroscopic polarimetry is a useful technique for integrated metrology, where the measurement throughput is one of the most important issues. 
     As shown in FIG. 2A, line profiles are often approximated as rectangular grating  202 , e.g., with horizontal sidewalls. However, line profiles can become more complicated, as shown in FIGS. 2B,  2 C and  2 D. FIG. 2B shows a trapezoid grating  204  having sloped sidewalls. FIG. 2C shows a notched grating  206  with horizontal sidewalls with a groove at the base of each line. FIG. 2D shows a rectangular grating  208  with a stack of films  210 ,  212  underneath the grating  208 . Of course, many other complicated structures may exist, including combinations of the structures shown in FIGS. 2B,  2 C, and  2 D. 
     For complicated structures, such as those shown in FIGS. 2B,  2 C, or  2 D, it may be desirable to provide more data related to the structure than can be provided by normal incidence polarimeter  100  in order to accurately analyze the total structure. 
     FIG. 3 shows a combined metrology system  300  that includes a normal incidence spectroscopic polarimeter  100  and an oblique incidence polarimeter  302 . It should be understood that if desired, polarimeters  100  and  302  may use a single wavelength of light. The normal incidence spectroscopic polarimeter  100  operates as described above. The oblique incidence polarimeter  302  includes a light source  303  and a polarization state generator (PSG)  304 , a lens  305   a  (or series of lenses) before the sample  101 , another lens  305   b  (or series of lenses) after the sample  101 , a polarization state detector (PSD)  306 , a diffraction grating  307  and a multichannel array detector  308 , which analyze the polarization state after reflection, from which the ellipsometry angles (ψ, Δ) can be obtained. The PSG  304 , e.g., can be a linear polarizer with its transmission axis at an angle from the plane if incidence, whereas the PSD  306  may consist on either a rotating compensator and fixed analyzer, a rotating analyzer, a photoelastic modulator followed by an analyzer, or anything else that creates an intensity modulation as a function of a known system parameter such as position of the compensator fast axis or analyzer transmission axis in the case of a rotating compensator or rotating analyzer system, respectively, or effective phase retardance as a function of time in case of a photoelastic modulator. The ellipsometry angles (ψ, Δ) can then be extracted from the mathematical analysis of the modulated intensity using Jones matrix or Muller matrix formalisms, as can be understood by someone skilled in the art. 
     FIG. 4 shows another embodiment of a combined normal incidence spectroscopic polarimeter  100  and an oblique incidence polarimeter  402 . The normal incidence spectroscopic polarimeter  100  operates as described above. As shown in FIG. 4, the two systems may share the same light source  403  and same spectrograph, which includes grating  412  and multichannel detector  414 , similar to that described in the U.S. Patent Application entitled, “Discrete Polarization State Ellipsometer/Reflectometer Metrology System”, by B. Johs, et al., Ser. No. 09/598,000, filed 06/19/2000, which is assigned to the assignee of the present disclosure and is incorporated herein by reference. 
     If desired, the two systems may share only one of the light source  403  or spectrograph, or as shown in FIG. 3, the two systems may have independent light sources and spectrographs. Similar to the embodiment shown in FIG. 3, the oblique incidence polarimeter  402  includes a PSG  404 , and PSD  406  and lenses  405   a  and  405   b . An optical element, such as a mirror  408  is used to redirect the reflected light beam toward beam splitter  104 , which then redirects the beam towards the spectrograph. If desired other optical elements, such as fiber optic cables may be used in place of mirror  408 . 
     The combination of the normal and oblique incidence spectroscopies provides a powerful technique in the characterization of complex structures, such as those shown in FIGS. 2B to  2 C, including non-rectangular gratings and gratings over thin film stacks. Using the combined metrology system, such as that shown in FIG. 3, both normal incidence and oblique incidence data can be obtained. The experimental data collected at normal incidence (θ=0°) can be either the reflectance spectra [R ET (θ=0°) and R TM (θ=0°)] or the ellipsometry angles ψ(θ=0°) and Δ(θ=0°), whereas at oblique incidence, the experimental data can be reflectance spectra [R TE (θ=φ) and R TM (θ=φ)], or the ellipsometry angles ψ(θ=φ) and Δ(θ=φ), where 0&lt;φ&lt;90°. In one embodiment, the value of φ is 70°, but of course other angles may be used. 
     To obtain the useful parameters such as such as film thickness, side wall angles, linewidths, etc., an optical model is produced that is fit to the experimental data. As described before, an adequate optical model for critical dimension samples is RCWA, which is described in detail in Ser. No. 09/670,000 and Ser. No. 09/844,559. 
     To take advantage of the information gained by measuring both at oblique and normal incidence, the same physical model describing the sample structure is used to generate the optical models fitted to oblique and normal incidence sets of experimental data. The mathematical details to obtain the modeled spectra may vary for oblique incidence and normal incidence, i.e., the calculation of the modeled spectra uses the angle of incidence as a parameter. Furthermore, the speed of the normal incidence spectra calculation can be increased up by taking advantage of the symmetric spread of the diffraction orders, whereas at oblique incidence this symmetry is broken and therefore the calculation speed will be slower. Nevertheless, the basic theory (e.g., RCWA), and the basic optical model parameters (thickness, optical constants, side wall angles, etc.) must be the same for both data sets. 
     For example, the experimental data sets will be named y i , where y i  refers to any of the experimental data points measured at wavelength i, and y i  can be any reflectance or ellipsometry data point, measured at either normal or oblique incidence. The fitting of the optical model generated, e.g., by RCWA, to the experimental data sets y i  can be achieved by using Levenberg-Marquart regression analysis as described in W. H. Press, P. P. Flannery, S. A. Teukolsky, and W. T. Vetterling, “Numerical Recipes”, Cambridge University Press (1986), which is incorporated by reference. The Levenberg-Marquart regression analysis is used to adjust the values of the fitting parameters in the optical model to minimize the mean-squared error (MSE), which is defined as:                  MSE        (     x   →     )       =       1     N   -   M              ∑     i   =   1     N                       (         y   i     -       y   i          (     x   →     )           σ   i       )     2           ;           eq   .              1                                
     where N is the total number of data points (normal and oblique incidence); M is the total number of fitting parameters; y i  is the experimental data point i (normal or oblique incidence reflectance, or ellipsometry angles (φ and Δ)); y i  ({right arrow over (x)})) is the ith calculated data point; {right arrow over (x)} is the vector representing the variable fitting parameters [{right arrow over (x)}=(x 1 ,x 2 , . . . , x M )]; and σ i  is the standard deviation associated to the measured point y i , which is the sum of systematic and statistical system errors, coming from calibration inaccuracies and signal noise, respectively. As can be seen from equation  1 , the MSE will increase if σ i  decreases. Therefore, data points with more error will weight less in the fitting result, since the Levenberg-Marquardt analysis is a procedure to minimize the MSE. 
     The contribution to the MSE can be very different for reflectance and ellipsometry data because reflectance values vary from 0 to 1 and ellipsometry angles vary from −180° to +180° for Δ, and from 0° to 90° to ψ. In order to have a similar range of values for all the quantities y i  so that all the spectra are fit with same accuracy, usually the data points y i =cos(Δ) and y i =tan(ψ) are used so that all the data points y i  vary about the same range (−1 to 1). An other option is to fit Re(ρ) and Im(ρ), where ρ tanψ exp(iΔ). 
     The data analysis is performed by first, constructing an optical model for the sample under study. The optical model includes the substrate material, the number of films, the films configuration, i.e., grating structure or thin films, and the optical constants of each layer. The optical constants can be specified in a table as a function of wavelength or in a form of a dispersion model, such as, e.g., a Cauchy model. The optical model also includes initial guesses for variable parameters, such as thickness, linewidth, pitch, line profile for the films and gratings, and/or the parameters defining the dispersion model of the optical constants. The variable parameters are the parameters that are varied in order to fit the optical model to the experimental data. The Levenberg-Marquardt algorithm is then used to determine the values of the variable parameters that yield the calculated data that best matches the experimental values. 
     There is often a strong correlation between fitting parameters, i.e., a small change in a parameter x j1  can result in a change of a calculated spectral value y i  which may also be affected when adjusting another parameter X j2 . By simultaneously fitting normal and oblique incidence data collected from the same sample by the combined metrology system, the strong correlation of fitting parameters may advantageously be broken up. Breaking up the correlation of fitting parameters most commonly occurs in multilayered samples, since the optical path length differs for different angles of incidence. In addition, the typically strong correlation between thickness, sidewall angles and linewidths in diffraction grating structures can be broken when combining two different angles of incidence. 
     Unfortunately, the fitting speed of the combined normal and oblique incidence data is considerably reduced relative to a single angle incidence data, due to the time-consuming RCWA calculation. To increase the fitting speed in most applications, a hybrid methodology can be used that, when operating at fast mode, uses only normal incidence data. Then, the fitting parameters correlation matrix, whose elements quantitatively indicate how strongly correlated the fitting parameters are, can be tracked at real time. If the correlation becomes strong, the oblique incidence measurement can be used and the data analyzed as described above to break up the correlation and get more accurate parameters. Thus, for most instances, only normal incidence data is used to provide a relatively fast fitting speed, but where there is a strong correlation in fitting parameters, the additional incidence data is used to break up the correlation. 
     Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.