Abstract:
A method for projecting an electron beam used notably in lithography by direct or indirect writing as well as in electron microscopy, is provided. Notably for critical dimensions or resolutions of less than 50 nm, the proximity effects created by the forward and backward scattering of the electrons of the beam in interaction with the target must be corrected. This is traditionally done using the convolution of a point spread function with the geometry of the target. In the prior art, said point spread function uses Gaussian distribution laws. At least one of the components of the point spread function is a linear combination of Voigt functions and/or of functions approximating Voigt functions, such as the Pearson VII functions. In certain embodiments, some of the functions are centered on the backward scattering peaks of the radiation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to foreign French patent application No. FR 1253389, filed on Apr. 12, 2012, the disclosure of which is incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention applies notably to the field of electron lithography for direct on-wafer etching or for fabricating masks. More generally, it applies to any field where it is required to model the interaction of a beam of electrons with a target, this also being the case in electron microscopy, notably for the inspection of wafers and masks. 
       BACKGROUND 
       [0003]    This interaction is notably affected by a scattering of the electrons around the initial trajectory (forward scattering effect) as well as by a backscattering (backward scattering effect). These effects, referred to as proximity effects, depend notably on the materials of the target and its geometry. Whatever the reason for desiring to perform this electron bombardment (etching, imaging or analysis), it is therefore necessary to take account of the proximity effects in order to obtain a result which is faithful to the objective sought. A correction of the proximity effects is therefore performed. Accordingly, it is known to predict them through a model so as to take account thereof in the calculation of the electron radiation doses used to bombard the target. It is accordingly known to use a so-called point spread or scattering function (or PSF) and the PSF is convolved with the geometry of the target. A commonly used PSF is a combination of Gaussians, a first Gaussian to model forward scattering (PSF of the forward scattering), and one or more additional Gaussians to model the backscattering (PSF of the backscattering). 
         [0004]    The equation of the PSF is thus traditionally represented by a function f(x,y) of the following form: 
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         [0000]    With the following notation:
       α is the width of the direct radiation;   β is the backscattering width;   η is the ratio of the intensities of the direct and backscattered radiations.   ξ is the radial position of a point
 
The values of the parameters α, β and η can be determined experimentally for a given method. These parameters are dependent on the acceleration voltage of the machine and the target. Typically for an acceleration voltage of the order of 50 KV and a silicon or glass target (SiO 2 ), α is of the order of 30 nm, β of the order of 10 μm and q of the order of 0.5.
       
 
         [0009]    However, if the energy distribution around the impact point, given by a PSF of this double Gaussian type, is compared with that produced by a simulation using a Monte-Carlo model, significant deviations are noted. The simulations of Monte-Carlo type, referred to in the subsequent description as the reference model, are closer to the experimental results but are difficult to use in production because of the long calculation times necessary for the simulation. An indicator of the quality of a model will therefore be the “fit” between the PSF used and the reference model, the fit being measured by the sum of the quadratic deviations between the two curves representing the two models on a normalized sample size of points. 
         [0010]    It appears particularly useful to find forms of a PSF that are closer to the simulation results and experimental results but that can be parametrized and therefore calculated in a much shorter time than Monte-Carlo simulations. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention solves the problem posed by the deviations of modeling of the proximity effects by using a class of PSF combining Lorentzian functions with Gaussian functions, for example a Voigt function, or approximations of these functions. 
         [0012]    For this purpose, the invention envisages a method for projecting an electron beam onto a wafer or a mask comprising a step of correcting the effects of scattering of said beam, said step comprising a sub-step of calculating a point spread function, wherein said point spread function is a linear combination of functions chosen from a group comprising at least one Voigt function ( 230 ) and a function approximating a Voigt function. 
         [0013]    Advantageously, said at least one function approximating a Voigt function is a Pearson VII function. 
         [0014]    Advantageously, said linear combination of functions also comprises at least one Gaussian function. 
         [0015]    Advantageously, the sub-step of calculating a point spread function comprises a sub-step of selection of the parameters of said functions which are included in the linear combination and of the coefficients of said linear combination, said selection sub-step comprising an execution of an optimization function of the fit of the point spread function with a set of experimental results or with a representation of a Monte-Carlo simulation of the scattering of the radiation on the target. 
         [0016]    Advantageously, said at least one function has an extremum which is not located at the center of the beam. 
         [0017]    Advantageously, said at least one function has an extremum which is located on a backward scattering peak of the electron beam. 
         [0018]    Advantageously, said point spread function is a linear combination of at least as many functions as the radiation comprises backward scattering peaks. 
         [0019]    To implement the method, the invention also envisages a computer program comprising program code instructions configured for the execution of a method for projecting an electron beam on a wafer or a mask of claim  1  when the program is executed on a computer, said program comprising a module for simulating and/or correcting the scattering effects in said beam, said module comprising a sub-module for calculating a point spread function, wherein said point spread function is a linear combination of functions chosen from a group comprising at least one Voigt function and a function approximating a Voigt function. 
         [0020]    The invention also covers an electron lithography system comprising a module for projecting an electron beam onto a wafer or a mask and a computer program having the characteristics defined hereinabove. 
         [0021]    The invention also covers a system for simulating at least one electron lithography step comprising a module for simulating a step of projecting an electron beam onto a wafer or a mask and a computer program having the characteristics defined hereinabove. 
         [0022]    The invention also covers an electron microscopy system comprising a module for projecting an electron beam onto a wafer or a mask and a computer program having the characteristics defined hereinabove. 
         [0023]    In one embodiment of the invention in which use is made of a PSF comprising a function having four Voigt functions, it has been possible to demonstrate a very significant improvement in the residual mean square error which is 88% lower than that noted with four Gaussians. 
         [0024]    Furthermore, the classes of function used in this embodiment have an analytical expression and they can easily be integrated into off-the-shelf tools without significant modification. The cumulative distribution function being analytical itself, the convolution calculations which are necessary for the correction of the proximity effects remain of the same order of computational complexity as the solutions of the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be better understood, its various characteristics and advantages will emerge from the description which follows of several exemplary embodiments and its appended figures wherein: 
           [0026]      FIG. 1  represents the distribution of energy absorbed by a resin according to a reference model; 
           [0027]      FIG. 2  represents a Gaussian function, a Lorentz function and a Voigt function; 
           [0028]      FIGS. 3   a  and  3   b  represent the distribution of energy absorbed by a resin or by a mask insulated under extreme UV according to one embodiment of the invention; 
           [0029]      FIGS. 4   a  to  4   f  represent the distribution of energy absorbed in various scenarios of use of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  represents the distribution of energy absorbed by a resin according to a reference model. 
         [0031]    The publication CASINO V2.42 (“A Fast and Easy-to-use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users”, Scanning, vol. 29, 92-101 (2007), D. Drouin et al) sets forth the operation of software for simulating the energy distribution of an electron beam by the Monte-Carlo procedure. The software makes it possible to best describe how the energy is distributed around the center of the beam of electrons by choosing the parameters of various possible physical models of interaction between the electrons and the target of the beam, nature of the materials and thicknesses of the layers mainly. 
         [0032]    In the example of the figure, one can see the distribution of energy absorbed in 100 nm of PMMA (polymethyl methacrylate) deposited on a quartz substrate as a function of the radial distance of a 50 keV (10 6  electrons) beam, the representation being on a double logarithmic scale (log (keV/nm2) as ordinate and log (nm) as abscissa). It is observed that this distribution does not exhibit any horizontal plateaus, plateaus necessarily being present when using Gaussian functions to describe this distribution. Thus, the use of Gaussian functions in the PSF is not sufficiently accurate to best describe an absorbed energy distribution of this type. As already indicated, the object of the invention is to replace at least one of the Gaussian functions of the PSF by another function so as to obtain a better fit with the distribution curves of of the type of those represented on  FIG. 1  which correspond to the reference model. 
         [0033]      FIG. 2  represents a Gaussian function, a Lorentz function and a Voigt function. 
         [0034]    The inventors have shown that Voigt type functions, which are the convolution product of a Gaussian function and of a Lorentzian function or sufficiently accurate approximations of these functions, improve the fit with the reference model. A Voigt function of a distance ξ to the center of the beam ξ c  calculated via the convolution formula: 
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         [0000]    in which a is the standard deviation of the Gauss function and b is the half-width at mid-height of the Lorenz function, the height being the value of said function at 
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         [0035]    The three functions are represented in the figure: a Gaussian function, by the curve  210 , a Lorentzian function by the curve  220  and a Voigt function by the curve  230 . 
         [0036]    The Voigt function not being easy to calculate analytically, use is often made of an approximation by a Pearson VII function which is expressed by the formula: 
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         [0000]    in which M is the shape parameter or Pearson width and w a distance giving the width at mid-height FWHM (Full Width at Half Maximum) of the Pearson peak. When M&lt;&lt;1, the shape of the curve approximates a Lorentz curve. When M&gt;&gt;10, the shape of the curve approximates a Gauss curve. 
         [0037]    According to the invention, for best possible approximation of a radiation distribution corresponding to layers of specific materials having a given thickness, it is possible to linearly combine Voigt functions, Pearson VII functions, as well as Gaussians with either Voigt functions or Pearson VII functions, the coefficients of the linear combination and the parameters of said functions being judiciously chosen to optimize the fit with the reference model. The fit is measured by the residual mean square error. To calculate the values of the parameters which optimize the fit with the reference model it is possible to use procedures for local optimization for example those based on the Levenberg-Marquardt algorithm or the simplex algorithm or procedures for more global optimization, for example kriging or procedures based on genetic algorithms. 
         [0038]    In the case where the share of the backscattering is significant, notably for the etching of masks under extreme ultraviolet or the etching of structured substrates (locally comprising differences of compositions, for example zones of oxides adjacent to semi-conductor zones), it will be conceivable to combine the method of the present invention with that described in French patent application No.11/57338 filed notably by one of the applicants of the present application. According to this invention, use is made of a PSF which is not centered at the center of the beam. 
         [0039]      FIGS. 3   a  and  3   b  represent the distribution of energy absorbed by a resin or by a mask insulated under extreme UV according to one embodiment of the invention. 
         [0040]    In  FIG. 3   a  is represented, purely by way of non-limiting illustration of the generality of the invention, an embodiment of the invention in which a mask adapted for this use (that is to say consisting for example of a glass support covered with a chromium or tantalum absorber) is subjected to a radiation. The reference model given by curve  310   a  is determined by the CASINO simulation software using the ESELPA database which accurately gives the collision cross sections for electrons colliding with matter in a wide energy range. Four Pearson VII functions are combined to produce the PSF  360   a:  
       A first Pearson1 function, represented by curve  320   a , is a Pearson VII function whose parameters M and w are fixed respectively at 3 and 2.2 nm;   A second Pearson2 function, represented by curve  330   a , is a Pearson VII function whose parameters M and w are fixed respectively at 1.7 and 8 nm;   A third Pearson3 function, represented by curve  340   a , is a Pearson VII function whose parameters M and w are fixed respectively at 1.5 and 1200 nm;   A fourth Pearson4 function, represented by curve  350   a , is a Pearson VII function whose parameters M and w are fixed respectively at 50 and 300 microns.
 
The PSF is the linear combination of the four Pearson VII functions of formula:
       
 
         [0000]      PSF=Pearson1 ×A 1+Pearson2 ×A 2+Pearson3 ×A 3 +Pearson4 ×A 4 
         [0000]    In which the coefficients of the combination have the following values:
       A1 such that log(A1)=−1.25;   A2 such that log(A2)=−3.7;   A3 such that log(A3)=−8.5;   A4 such that log(A4)=−10.       
 
         [0049]    In  FIG. 3   b  is represented, purely by way of non-limiting illustration of the generality of the invention, an embodiment of the invention in which a substrate covered with a resin, for example PMMA, receives a radiation. The representations and notation are the same as those of  FIG. 3   a . The Pearson VII functions which are used are identical to those of  FIG. 3   a . The only change is the coefficients Ai of the linear combination. They have in this case the following values:
       A1 such that log(A1)=−1.25;   A2 such that log(A2)=−5.4;   A3 such that log(A3)=−10;   A4 such that log(A4)=−11.2.
 
These values make it possible to obtain a better fit with the reference model.
       
 
         [0054]      FIGS. 4   a  to  4   f  represent the distribution of energy absorbed in various scenarios of use of the invention. 
         [0055]    In all of  FIGS. 4   a  to  4   f , the radiation curve for the reference model  410   a  is compared with the PSF calculated according to the method of the invention  420   a  for the sum of four Voigt functions and with those calculated according to the conventional methodology  430   a  consisting in using a sum of four Gaussian functions. 
         [0056]    In  FIGS. 4   a  to  4   e , the radiation is emitted by a small beam (of 2 nm in the illustrative example of these figures). 
         [0057]    In  FIG. 4   a  is illustrated a case in which the target is a 100-nm PMMA stack on silicon substrate, the radiation having an energy of 100 keV. 
         [0058]    In  FIG. 4   b  is illustrated a case in which the target is an EUV (extreme UV) stack, the radiation having an energy of 100 keV. 
         [0059]    In  FIG. 4   c  is illustrated a case in which the target is a 100-nm PMMA stack on silicon substrate, the radiation having an energy of 50 keV. 
         [0060]    In  FIG. 4   d  is illustrated a case in which the target is an EUV (extreme UV) stack, the radiation having an energy of 50 keV. 
         [0061]    In  FIG. 4   e  is illustrated a case in which the target is a 100-nm PMMA stack on silicon substrate, the radiation having an energy of 10 keV. 
         [0062]    In  FIG. 4   f  is illustrated a case in which the target is a 100-nm PMMA stack on silicon substrate, the radiation consisting of a broad beam (30 nm) having an energy of 100 keV. 
         [0063]    From the various curves we note that the fit between the PSF of the invention and the reference model is particularly good in all the configurations which are represented. The quality of the fit of the method of the invention does not allow to differentiate the reference curves  410   x  from the curves  420   x  representative of the invention whereas sharp deviations are observed with the curves  430   x  representative of the prior art and using Gaussian functions. 
         [0064]    The method of the invention can, however, also be implemented in other usage conditions. 
         [0065]    According to the invention, the Voigt functions or the Pearson VII functions are chosen in such a way that the resulting distribution is centered or non-centered with respect to the center of the electron beam. An off-centering of certain peaks is conceivable for conditions where the backscatter becomes predominant. In a favored manner, the center of the distribution is adjusted to the backward scattering peak (backscatter). This choice is particularly advantageous when the backward scattering effect is particularly significant, notably when layers of tantalum or of nitrogen-doped tantalum are implanted on the substrate as is the case in applications to the fabrication of extreme UV masks. Indeed, these layers of heavy materials cause very appreciable backward scattering. It is indeed noted both experimentally and by Monte-Carlo simulation that said peak is off-centered with respect to the center of the electron beam. There may be several scattering peaks. In this case, it will advantageously be chosen to combine as many Voigt or Pearson VII functions as scattering peaks. On the curve representing the absorbed energy distribution, the scattering peaks can be manifested by a change of slope at the level of this curve. It will thus be possible to choose advantageously to combine as many Voigt or Pearson VII functions as observable change of slope. It will if appropriate be possible to choose to center at least one function or each on each peak even if this is not always necessary to improve the fit. 
         [0066]    To implement the method of the invention in its applications to electron lithography by direct projection on wafers or by mask etching, it is possible to use for example a machine of the type SB 3054 from the company VISTEC™. Dose modulation may for example be performed, according to the invention, by modifying dose modulation software, for example the PROXECCO™ software distributed by the company Synopsis™ or the Inscal™ software from the company Aselta Nanographics™ to replace the PSF of the backscattering of the prior art by the PSF of the backscattering described hereinabove. For the forward scattering PSF, it will be possible to use as in the prior art a centered Gaussian or any other type of bell curve centered on the center of the beam. 
         [0067]    Dose modulation is performed by convolving the PSF (PSF of the forward scattering and PSF of the backscattering) with the geometry of the pattern to be etched. The software can also be advantageously modified to carry out combined optimization of the dose modulation and of the geometry of the pattern to be etched, according to a method such as that described in international application PCT/EP2011/05583 licensed to one of the applicants of the present patent application. 
         [0068]    The method of the invention and the computer program for implementing it can also be used to optimize the PSF of scanning, field-effect or tunnel-effect electron microscopy systems which can be used in imaging or to carry out the inspection of wafers or masks. They can also be adapted for carrying out a simulation of one or more steps of an electron lithography method. 
         [0069]    The examples described hereinabove are therefore given by way of illustration of certain embodiments of the invention. They do not in any way limit the field of the invention which is defined by the claims which follow.