Abstract:
A method of electron-beam lithography is provided, notably for technologies of critical dimension of the order of 22 nm. In such methods applied notably to networks of lines, the methods of the prior art do not offer precise and efficient correction of the shortenings of line ends. The method provided solves this problem by carrying out the insertion of contrast intensification structures of types which are optimized for the structure of the lines to be corrected. The method allows the semi-automatic or automatic calculation of the dimensions and locations of said structures. Advantageously, these calculations may be modeled to produce a target design, derived from libraries of components. They may be supplemented with a joint optimization of the size of the etchings and of the radiated doses, as a function of the process energy latitude.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/EP2011/055843, filed on Apr. 13, 2011, which claims priority to foreign French patent application No. FR 1052863, filed on Apr. 15, 2010, the disclosures of which are incorporated by reference in their entirety. 
     BACKGROUND 
     The present invention applies to the domain of electron-beam lithography. To allow the etching of patterns whose critical dimension is below 50 nm, it is necessary to integrate into the methods of optical photolithography schemes for correcting the optical distortions that are increasingly complex both at the mask design and production stage and at the exposure stage. The costs of the equipment and developments for a new generation of technology consequently increase in very high proportions. Today, the critical dimensions accessible in photolithography are greater than or equal to 65 nm. The 32-45 nm generation is undergoing development and there is no viable solution envisaged for technological nodes below 22 nm. For its part, electron-beam lithography already allows the etching of 22-nm patterns; it does not require any mask and offers a fairly short development time, thus allowing better reactivity and flexibility in the realization of improvements to the technologies and to the designs. On the other hand, the production times are structurally substantially higher than in photolithography since it is necessary to carry out step-wise exposure (with a “stepper”), whereas photolithography requires only layer-wise exposure. 
     The electron beam or beams used to perform the etching of a pattern scatter notably over short distance (forward scattering or blur) in the resin and the substrate on the edges of the center of the beam, thus increasing the size of the beam and reducing its contrast. Moreover, the electrons are completely backscattered over a long distance (backward scattering). In particular, the line ends within dense networks will be shortened (“line end shortening” or LES), thus affecting the functionality of the component and therefore decreasing production efficiency. 
     A certain number of prior art methods, in the domains of photolithography and electron-beam lithography, have attempted to afford corrections to this LES phenomenon. Such is notably the case in photolithography of the methods providing for the modifications of the patterns to be etched, notably by extending their surface at the line end, such as those disclosed by K. Tsudaka et al., Japanese Journal of Applied Physics, Vol. 36 (1997), pp. 7477-7481, K. Kim et al., Japanese Journal of Applied Physics, Vol. 37 (1998), pp. 6681-6685 as well as by U.S. Pat. No. 7,494,751. 
     Similar methods have been applied in electron-beam lithography, such as those disclosed notably by S. Manakli et al., Japanese Journal of Applied Physics, Vol. 45, No. 8A, 2006, pp. 6462-6467, in which the dimensions of the lines are modified as a function of rules, and the international patent application published under the number WO2007/129135, in which a line is replaced with several lines of chosen widths and spacings. However, a solution remains to be found allowing the correction of LESs while affording sufficient resolution for technological nodes of less than or equal to 22 nm. 
     SUMMARY OF THE INVENTION 
     The present invention solves this problem by allowing the calculation and then the insertion of contrast improvement patterns (“Contrast Improvement Features” or CIFs) into the networks of lines to be etched. 
     For this purpose, the invention provides for a method of electron-beam lithography of networks of lines on a resin coated substrate comprising a step of selecting at least two opposite lines of the network each having a critical line end in opposition, a step of determining corrections to be applied to the geometry of the end of said lines, said method being characterized in that it furthermore comprises a step of determining a type of at least one contrast intensification structure chosen from among several, said contrast intensification structure comprising at least two elements, at least one of which being located in-between said two opposite lines, a step of calculating the characteristic dimensions and the location in the gap between the two opposite critical line ends of said contrast intensification structure, a step of generating said structure and a step of placing said structure in relation to the ends of said opposite critical line ends as a function of the parameters determined. 
     Advantageously, said contrast intensification structure is an H-like structure, whose branches each have inherent dimensions of between about 5 and 50 nm, whose branch perpendicular to the direction of said critical line is placed in the middle of the gap at a distance S 2  from the end of said critical line along the direction parallel to said critical line of between about 5 and 50 nm and whose branches parallel to the direction of said critical line are placed in a symmetric manner on either side of said critical line at a distance S 3  perpendicular to the direction of said critical line of between about 5 and 50 nm. 
     Advantageously, said contrast intensification structure consists of two parallel bars placed in a symmetric manner in the gap along a direction perpendicular to said critical line, each of the bars having inherent dimensions of between about 5 and 50 nm, spaced apart by a distance S 2  of between about 5 and 100 nm and each spaced from the end of said critical line by a distance S 4  perpendicular to said critical line of between about 5 and 50 nm. 
     Advantageously, said two parallel bars furthermore comprise at each of their ends four regions of inherent dimensions of between about 5 and 50 nm. 
     Advantageously, said contrast intensification structure consists of at least two CIFs (CIF 1 , . . . CIF i  . . . ) having inherent dimensions (L i , I i ) of substantially between 5 and 50 nm, said patterns being separated from one another by a distance S 5  perpendicular to the critical lines of substantially between 5 and 20 nm, the number of patterns being chosen in such a way that the exterior regions are placed in a symmetric manner on either side of the edges of the critical lines at a distance S 3  perpendicular to the direction of said critical lines of between about 5 and 50 nm. 
     Advantageously, said contrast intensification structure consists of two rows of substantially square regions passing through the interior of the gap, of inherent dimension of substantially between 5 and 20 nm, the two rows being separated by a distance S 5  in the direction of the critical line of substantially between 5 and 50 nm, the number of regions being chosen in such a way that the exterior regions are placed in a symmetric manner on either side of the edges of the critical lines at a distance S 3  perpendicular to the direction of said critical lines of between about 5 and 50 nm. 
     Advantageously, said types of contrast intensification structures, their dimensions and their locations are determined by a model for automatically determining a target design comprising the critical lines and the optimal contrast intensification structures. 
     Advantageously, the method of the invention furthermore comprises at least one step of calculating at least one dimension of said critical line and at least one dimension of a contrast intensification structure and a step of calculating modulation of doses on said critical line and contrast intensification structure, said calculations being related by an optimization criterion chosen from the group comprising the process energy latitude, the shape of the contrast intensification structure, the position of said contrast intensification structure, a combination of the latter with the process energy latitude. 
     To implement the method, the invention also provides for a computer program comprising program code instructions configured for the execution of a method of electron-beam lithography of networks of lines on a resin-coated substrate when the program is executed on a computer, said program comprising a module for selecting at least two opposite lines of the network each having a critical line end in opposition, a module for determining corrections to be applied to the geometry of the end of said lines, said computer program being characterized in that it furthermore comprises a module for determining a type of at least one contrast intensification structure chosen from among several, said contrast intensification structure comprising at least two elements, at least one of which being located in-between said two opposite lines, a module for calculating the characteristic dimensions and the location in the gap between the two opposite critical line ends of said contrast intensification structure, a module for generating said structure and a module for placing said structure in relation to the ends of said opposite critical line ends as a function of the parameters determined. 
     Advantageously, the program of the invention furthermore comprises a module for calculating at least one dimension of said critical line and at least one dimension of a contrast intensification structure and a module for calculating modulation of doses on said critical line and contrast intensification structure, said calculations being related by an optimization criterion chosen from the group comprising the process energy latitude, the shape of the contrast intensification structure, the position of said contrast intensification structure, a combination of the latter with the process energy latitude. 
     The invention uses a small number of CIF type geometries whose calculation and placement may be automated, thereby permitting the modification of the libraries of standard components by integrating suitable programs into computer-aided design tools. 
     Although there are a reduced number thereof so as to facilitate this automation of the conversion of the libraries of components, these CIFs have a large number of parameters whose variation, combined if appropriate with that of the parameters of the base patterns of the network allows very fine correction of the proximity effects and very faithful reproduction of said base patterns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood, its various characteristics and advantages will emerge from the description which follows of several exemplary embodiments and of its appended figures wherein: 
         FIG. 1  represents a view of a pattern to be reproduced and of a reproduced pattern, without correction of the LESs and with correction of the LESs in a photolithography method of the prior art; 
         FIG. 2  represents a view of a pattern to be reproduced and of a reproduced pattern, without correction of the LESs and with correction of the LESs in an electron-beam lithography method of the prior art; 
         FIGS. 3 and 3   a  represent several exemplary embodiments of corrections of the LESs by a lithography method according to the invention; 
         FIG. 4  is a flowchart of the processing implemented to carry out a correction of the LESs according to one embodiment of the invention; 
         FIG. 5  is a detail of a step of the flowchart of  FIG. 4  according to two embodiments of the invention; 
         FIG. 6  illustrates the method of resizing the geometries to be etched according to a variant of the invention; 
         FIG. 7  illustrates the calculation of the parameters of a CIF in one of the embodiments of the invention; 
         FIG. 8  illustrates the calculation of the parameters of a CIF in another of the embodiments of the invention; 
         FIGS. 9 ,  9   a ,  9   b  and  9   c  illustrate the calculation of the parameters of a CIF in yet several other embodiments of the invention; 
         FIGS. 10   a ,  10   b  and  10   c  illustrate the results of an LES correction method of the prior art; 
         FIGS. 11   a ,  11   b ,  11   c  and  11   d  illustrate the results of an LES correction method in an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  represents a view of a pattern to be reproduced and of a reproduced pattern, without correction of the LESs and with correction of the LESs in a photolithography method of the prior art. 
     The pattern  110  to be reproduced is transferred into the resin in the form of a deformed image  120 . In particular, this image comprises a line end shortening (LES),  121  and corner roundings such as  122 , on account of the contrast reduction effect at the extremities of the pattern. 
     The pattern  130  to be reproduced is modified by adding sub-resolved patterns such as a “hammerhead”,  131 , a “serif”,  133 , or one or more scattering bars,  134 . The pattern can also be a subtraction of material such as a “mousebite”  132 . The impact of an electron beam modulated in a particular manner with these sub-resolved patterns creates an energy distribution over the sample which improves the contrast and transfers the pattern modified by these additions and subtractions into the resin with an image,  140 , identical to the initial pattern before additions/subtractions. 
       FIG. 2  represents a view of a pattern to be reproduced and of a reproduced pattern, without correction of the LESs and with correction of the LESs in an electron-beam lithography method of the prior art. 
     In a method of electron-beam lithography by direct writing, a line end shortening LES effect,  210 , is also apparent. This LES effect may be corrected by modifying the geometry of the line end, by lengthening it, as at  220  or  230 . 
       FIGS. 3 and 3   a  represent several exemplary embodiments of corrections of the LESs by a lithography method according to the invention. 
     Essentially, networks are considered in which two line ends situated in opposition constitute a gap between them. 
     The idea of the invention is to insert into the network to be etched contrast improvement patterns,  310 ,  320 ,  330 ,  340 ,  350 ,  370 ,  380 ,  390  of appropriate geometry and location. The way of calculating the geometry and the location of these patterns will be detailed as a commentary to  FIGS. 4 to 9 . Two types of patterns are mainly used:
         the patterns of type  310  and  320  no part of which encroaches into a zone situated between two line ends; as a general rule, patterns of these types are chosen when the characteristic dimension of this zone,  360 , is less than about 10 nm; more specifically, in an empirical manner, patterns of type  310  will be chosen when the dimension  360  is less than or equal to 5 nm and patterns of type  320  when the dimension  360  is greater than 5 nm but less than or equal to 10 nm;   the patterns of types  330 ,  340  or  350  in which there exists at least one element perpendicular to the lines and situated in the zone between two line ends;   the patterns of type  370 , in which a single pattern is inserted into the gap;   the patterns of type  380 , in which the largest dimension of each of the patterns is parallel to the lines of the network;   the patterns of type  390 , in which several regions constituting two rows aligned perpendicularly to the lines of the network are inserted into the gap.       

     Other types of pattern are possible, without departing from the generality of the invention. 
     The line end constrictions increase in proportion for the pattern to be etched for the finest technologies. For the 32-45 nm technologies, the constriction attains 30 nm per line edge. If the spacing is sufficient (typically greater than 10 nm), one of the configurations  330 ,  340 ,  350  will be chosen by preference, in which the overdosages applied to the CIF pattern parts situated in the spacing allow an increase in the energy latitude (EL) of the method (rectification of the slope of the dose curve) and therefore an increase in the contrast. When the space between line ends is too restricted to allow the insertion of a pattern having a part in this gap, one of the configurations  310 ,  320  is chosen. 
       FIG. 4  is a flowchart of the processing implemented to carry out a correction of the LESs according to one embodiment of the invention. 
     In a manner conventional for a person skilled in the art, the method of the invention begins with a step,  410 , of reading the design. The layout of the patterns to be etched is conventionally encoded in files in the GDS II (Graphic Data System version2) or OASIS (Open Artwork System Interchange Standard) format. 
     The layers where the patterns must be etched are selected thereafter in the course of a step  420 . 
     The zones, lines and critical line ends, that is to say those having to form the subject of a specific processing, are identified in the course of a step  430 . The threshold below which the line ends must form the subject of a specific processing depends on the technology; thus for a 22-nm technology, the critical line ends will be those having a gap of less than or equal to about 30 nm. Generally, all the line ends for which the constrictions are greater than 10% will be processed. The value of the threshold therefore depends on the technology. 
     In the course of a step  440 , one chooses the type of the CIF pattern to be implanted as a function of the configuration of the line end to be processed. For line end spaces of less than 5 nm, only a CIF of the type  310  of  FIG. 3  is possible. For line end spaces of between 5 and 10 nm, a CIF of the type  320  is favored, although a CIF of type  310  is also possible, with less good performance in terms of contrast increase. 
     In the course of a step  450 , the shrinkage of the lines is calculated and carried out in the case where, on output from step  440 , it has been decided to implant a CIF of type  330 ,  340 ,  350 ,  370 ,  380  or  390 . The calculation is performed as indicated further on as a commentary to  FIGS. 8 ,  9 ,  9   a ,  9   b  and  9   c.    
     Step  460  of adding the CIFs is commented on in detail in connection with  FIG. 5 . 
     The modified layout is thereafter verified in the course of a step  470 . In particular, the line end spaces, possible CD errors, fracturation defects and the dose calculations are verified. 
     In the case where a method of lithography based on shape projection is used, a step  480  of fracturing the design to be reproduced into elementary shapes to be projected is carried out thereafter. 
     In the course of a step  490  the proximity effects of different nature of the LESs, notably those which occur at a dense network edge, are corrected. A prior art method can be used for this purpose, such as that calling upon modulation of the radiation doses as a function of the position of the shot in the network. But it is also possible to use the method of the invention forming the subject of patent application Ser. No. 10/52,862 filed by the applicant of the present application. In this case, the correction involves resizing the lines making it possible to reduce the doses radiated by these shots. 
     In the course of steps  4 A 0 ,  4 B 0  and  4 C 0 , the final design data are respectively converted into machine format, transferred to the exposure machine and used by said machine to carry out the exposure. 
     In the method of the invention, it is possible to use any electron-beam lithography device, for example a machine with the VISTEC™ or ADVANTEST™ brand name. Its drive software is adapted so as to be able to implement the various steps described hereinabove. 
       FIG. 5  is a detail of a step of the flowchart of  FIG. 4  according to two embodiments of the invention. 
     Step  460  of  FIG. 4  is now detailed. This step consists in calculating the dimensions of the CIFs to be implanted as a function of the contrast of the line ends. This step  450  may be carried out in a semi-automatic manner or in a completely automatic manner. 
     In the first case, in the course of this step,  510   a , we define a new pattern to be etched in the same manner as the base patterns to be overlaid onto the substrate. In the course of a step  511   a , the dimensions of the CIFs are calculated and in the course of a step  512   a  the location of the CIFs is calculated with respect to the line ends according to procedures which are explained further on in the description. Next, in a conventional manner, the placement of the CIFs, whose parameters were entirely determined in the course of the previous steps, is performed in an automatic manner in the course of a step  520 . 
     In the second case, in the course of a step  510   b , the dimensions and the locations of the CIFs are defined in an entirely automatic manner using an approach based on a modeling of the CIF structures to be implanted as a function of the characteristics of the network of lines to be etched. In the course of a step  511   b , the model to be applied is defined and, in the course of a step  512   b , the target design is defined. This modeling is explained as a commentary to  FIGS. 8 and 9  hereinbelow. 
     By way of variant embodiment, it is possible to perform, in the course of a step  513   b , a resizing of the target design using the method of combined optimization of the geometry of the CIF and of the lines and of the modulation of dose radiated according to the method disclosed by the patent application filed by the applicant of the present application under the Ser. No. 10/52,862. 
     According to the method described by this patent application, which is aimed at a correction of the proximity effects inherent in lithography methods for geometries of less than 50 nm, the doses to be applied in the shots corresponding to the patterns of the CIF and of the critical lines are calculated either by convolving the dose radiated with the pattern to be etched or by using a table of parameters. The combined calculation of the modulation of dose to be applied and of the size of the new patterns is performed so as to preserve the process energy latitude according to a calculation an example of which is given hereinbelow as a commentary to  FIG. 6 . 
       FIG. 6  illustrates the method of resizing the geometries to be etched according to a variant of the invention. 
     The geometry of the patterns is modified in at least one dimension so as to optimize the process energy latitude. More precisely, the displacement  650  to be performed along this dimension is calculated by seeking the intersection of the straight line  640  (tangent to the curve of dose received  620  at the point where the dose received is equal to the sensitivity threshold of the resin at 0.5) with the straight line  630  which demarcates said sensitivity threshold, and then by performing the displacement toward the point of intersection of the latter straight line with the profile of the radiated dose  610 . 
     The geometry and/or the dose of the patterns is/are modified in at least one of the dimensions so as to optimize the energy latitude or the shape or the position of the critical lines. It is possible to calculate the displacement  650  by performing progressive increments of said geometry and by adjusting the direction and value of these increments after having compared the quantity to be optimized (geometry or latitude or combination of the two or any other cost function) with an objective. The displacement  650  can be done using any other approach used in optimization or servocontrol problems. 
     The dose/patterns combined calculation may be iterated two or three times. 
     The modulation of the dose to be applied to the patterns can also be calculated on the basis of a table of parameters without any convolution calculation, notably when the modulation is applied only to the shots outside of the patterns, the other shots being applied at the normalized value of the method, or at a value of the order of 30% below the latter. 
     The following figures illustrate the rules for calculating the variations to be applied to the parameters for dimensioning the “main line” (or line of the pattern to be etched) to and of the CIFs which are defined hereinafter:
         L (main line) is the width of the main line to be etched;   I (main line) is the length of the main line to be etched;   L (CIF) is the common width of each CIF to be implanted;   I i  (CIF) are the lengths characteristic of each CIF to be implanted;   S 1  is the dimension of the gap between two opposite main lines;   S 2  is the dimension of the gap between two CIFs along the direction of the main line;   S 3  is the dimension of the gap between a CIF and the main line along the direction perpendicular to the main line;   S 4  is the dimension of the gap between a CIF and the main line along the direction of the main line;   S 5  is the dimension of the gap between two CIFs along the direction perpendicular to the main line.       

     By reference to  FIG. 7  which illustrates the calculation of the parameters of the method of the invention in one of the types  310  and  320  of  FIG. 3 , the rules for calculating the parameters of the main line and the CIFs are indicated:
         The parameters L (main line) and I (main line) are not modified;   The parameter S 1  is fixed by the minimum of the Design Rule Manual (DRM); in the case of the 22-nm process and of a suitable resin, it is equal to about 30 nm;   L (CIF) and I i  (CIF) are between about 5 and 20 nm;   S 2  and S 3  are fixed at values of between about 5 and 20 nm.       

     Advantageously, it is possible to carry out a pullback in the case of the dense structures in 16 nm technology. This pullback will achieve a gain in contrast with respect to a solution without pullback which is the reference solution for the CIFs of these two types for the less fine technologies. 
     By reference to  FIG. 8  which illustrates the calculation of the parameters of the method of the invention in one of the types  340  and  350  of  FIG. 3 , the rules for calculating the parameters of the main line and the CIFs are indicated:
         The parameter L (main line) is corrected of a shrinkage calculated as a function of the CIF;   The parameter I (main line) is not modified;   The parameter S 1  is fixed by the minimum of the Design Rule Manual (DRM); in the case of the 22-nm process and of a suitable resin, it is equal to about 30 nm;   L (CIF) and I i  (CIF) are between about 5 and 50 nm;   S 2  is fixed at a value of between about 5 and 100 nm as a function of the shrinkage of the main line;   S 4  is fixed at a value of between about 5 and 50 nm.       

     By reference to  FIG. 9  which illustrates the calculation of the parameters of the method of the invention in the type  330  of  FIG. 3 , the rules for calculating the parameters of the main line and the CIFs are indicated:
         The parameter L (main line) is corrected of a shrinkage calculated as a function of the CIF;   The parameter I (main line) is not modified;   The parameter S 1  is fixed by the minimum of the Design Rule Manual (DRM); in the case of the 22-nm process and of a suitable resin, it is equal to about 30 nm;   L (CIF) and I (CIF) are between about 5 and 50 nm;   S 2  is fixed at a value of between about 5 and 100 nm as a function of the shrinkage of the main line;   S 3  is fixed at a value of between about 5 and 50 nm.       

       FIGS. 9   a ,  9   b  and  9   c  illustrate the calculation of the parameters of the method of the invention respectively in the types  370 ,  380  and  390  of  FIG. 3   a ; the rules for calculating the parameters of the main line and the CIFs are indicated hereinbelow:
         The parameter L (main line) is corrected of a shrinkage calculated as a function of the CIF;   The parameter I (main line) is not modified;   The parameter S 1  is fixed by the minimum of the Design Rule Manual (DRM); in the case of the 22-nm process and of a suitable resin, it is equal to about 30 nm;   L (CIF) and I (CIF) are between about 5 and 50 nm;   S 2  is fixed at a value of between about 5 and 100 nm as a function of the shrinkage of the main line;   S 3 , S 4  and S 5  are fixed at values of between about 5 and 50 nm.       

     The parameters of  FIGS. 8 ,  9 ,  9   a ,  9   b  and  9   c  hereinabove are optimized by trial and error, the optimal values being thereafter integrated into charts called by the models for generating corrections. 
       FIGS. 10   a ,  10   b  and  10   c  illustrate the results of an LES correction method of the prior art. 
       FIG. 10   a  illustrates a simulation of two lines etched by using a method of electron-beam lithography applying LES corrections of the prior art, such as that shown diagrammatically in  FIG. 2 : it is clearly seen that the line ends are not controlled and that in particular the line end shrinkages are too considerable, and in a large number of cases this will necessarily cause short-circuit problems fatal to the integrated circuit etched according to this method.  FIG. 10   b  shows in a graphical manner a result of simulating the dose received as a function of distance from the starting point of the line.  FIG. 10   c  is a photograph of the etched circuit. 
       FIGS. 11   a ,  11   b ,  11   c  and  11   d  illustrate the results of an LES correction method in an embodiment of the invention. 
       FIG. 11  a illustrates a simulation of two lines etched by using a method of electron-beam lithography applying LES corrections according to one embodiment of the invention (that of the type  320  of  FIG. 3 , illustrated in this example in  FIG. 11   d ): it is seen in  FIG. 11   a  and in  FIG. 11   c  that the line ends are much better controlled than in the application of the method of the prior art illustrated by  FIGS. 10   a ,  10   b  and  10   c , and that exactly what is desired in the design is obtained. This result is confirmed by the simulation graph of  FIG. 11   b  where it is noted that the effect of the CIFs is to bring the line ends closer together. 
     The exemplary embodiments of the present description have been given in the case of a method of electron-beam lithography by direct writing. The invention is, however, applicable to mask etching, which will be used thereafter for writing by electron-beam radiation. 
     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.