Abstract:
A method for designing masks adapted to the forming of integrated circuits, including the steps of: (a) forming a first test file including a set of configurations of integrated circuit elements; (b) forming a second test file comprising the elements of the first test file, less the elements corresponding to configurations forbidden by design rule manuals; (c) trans-forming the second test file by means of a set of logical operations implemented by computing means to obtain a mask file; (d) testing the mask file and, if the test is negative, modifying the design rule manuals; and (e) repeating steps (a) to (d) until the test of step (d) is positive.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 10/54304, filed on Jun. 2, 2010, entitled “METHOD FOR DESIGNING MASKS USED TO FORM ELECTRONIC COMPONENTS,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the designing of masks to be used to manufacture integrated circuits. More specifically, the present invention relates to a method for improving the design of such masks. 
     2. Discussion of the Related Art 
     To manufacture integrated electronic circuits, a set of several masks comprising openings defining work areas on the circuit is used. For example, different masks may be successively used to define locations of dopant implantation, of etching, etc. 
     The manufacturing of the different masks necessary to obtain an integrated circuit implies a relatively high cost. Further, modern integrated circuits may require, for their production, several tens of masks. It is thus essential to properly test the mask generation files before launching the production of the masks, or even the production of the integrated circuits. Especially, the compatibility of the masks, for example, for their superposition, should be optimal. Having to redesign a set of masks, at the last minute before the launching of the production of integrated circuits or after the launching of the production, may imply very large manufacturing delays and costs. 
     For each electronic component technology, integrated circuit designers should comply with a number of design rules put together in a “Design Rules Manual”, or DRM. Such a manual gathers, among other things, sets of rules relative to the superposition or to the juxtaposition of the layers necessary for the forming of the electronic components. 
       FIG. 1  illustrates a few examples of rules that may be imposed on designers for the forming of integrated circuits. This drawing shows, in hatchings, different areas at the surface of a substrate intended to receive electronic components. Among the rules to be respected for electronic components, the following can be mentioned:
         respecting a minimum width W of some elements of the components, for example, the width of MOS transistor gates, the length of a transistor channel;   respecting a minimum space S between different elements, to avoid interferences between these elements, for example, between two metal tracks, or again leakage currents;   respecting a minimum surface area A for some elements.       

       FIG. 2  illustrates rules that may be imposed by DRMs when several layers are used to form electronic components, at close or superposed locations of a circuit. This drawing shows the design levels from which are formed masks which will subsequently be necessary to the manufacturing of electronic components, LAYER 1  for which the contour of the openings is shown in full lines, and LAYER 2  having its openings shown in hatched portions. 
     The examples of rules to be respected may be:
         respecting a minimum enclosure E between the edge of the openings of the second mask LAYER 2  and the edge of the openings of the first mask LAYER 1 . This, for example, corresponds to the case of electronic components formed in a well of a specific conductivity type. In this case, for their proper operation, the components should not be formed too close to the edge of the well. This may also correspond to elements which should be formed in superposed fashion: for example, a transistor gate above a well of a specific conductivity type.   respecting a minimum distance D between elements formed by means of second mask LAYER 2  with respect to elements formed by means of first mask LAYER 1 . This, for example, corresponds to the case where the first mask defines a well of a given conductivity type and where the components formed at the level of the openings of the second mask should not be formed too close to this well due to a risk of interactions.       

     It should be noted that the rules imposed by DRMs may also integrate an alignment error margin to take into account inaccuracies in the mask alignment on manufacturing of the circuits. The rules imposed by DRMs thus eliminate a number of situations with critical sizings, which could not operate properly and which are thus not accessible to designers. 
     Once the integrated circuits have been designed by the designers, the obtained CAD files should be turned into image files of the masks which will be necessary to form the integrated circuits. To achieve this, logical operations are defined by technologists to be applied to the integrated circuit files. 
     The logical operations also define all the elements missing for the proper operation of the circuit, which are not available to designers. Indeed, for an easy design of integrated circuits, designers only define some of the elements necessary to the forming of the integrated circuit. For example, in the case of a MOS transistor, designers may define the location of a well of a given conductivity type, and a second well necessary to the proper operation of the transistor may be automatically generated by the logical operations. 
     The logical operations finally define an optimized shape of the different masks. They may in particular provide to slightly widen the openings formed in the masks to compensate for a possible subsequent narrowing when the mask is being used. 
       FIG. 3  shows a conventional flowchart of the steps carried out to design integrated circuits, until the manufacturing of the masks used for manufacturing the integrated circuits. 
     As described hereabove, a first step  10  comprises forming a computer file which is an image of the desired integrated circuit (DESIGN FILE). This file is formed by designers  12  (DESIGNER), in compliance with the rules imposed by integrated circuit DRMs  14  associated with the technology used. 
     The integrated circuit file is then transformed, by a computer system, at a step  16  (LOP, Logical Operation Processing) and by means of a set of logical operations  18  (LO), to obtain an image file of the masks necessary to manufacture integrated circuits  20  (MASK FILES). The logical operations are especially provided to gather, within a same mask, the regions of the electronic components of the integrated circuits requiring a same processing. 
     As an example, low-voltage MOS transistors, high-voltage MOS transistors, dual-gate transistors, etc. may be provided on a same integrated circuit. Each of these transistors requires, to be formed, a specific processing, often obtained by a mask differentiation, for example, to form the wells of these different transistors. The logical operations of step  18  are used to generate the right masks according to the different steps to be carried out. 
     Step  16  of transformation of integrated circuit file  10  into a mask file may return errors, for example, in the case where the density of electronic components on the circuit would be too high, or in the case where there would be an incompatibility with the integrated circuit design rule manual. In this case, it is necessary to revise the transformation formulas  18  (LO) applied in transformation step  16  to validate or invalidate certain configurations provided by the designers. 
     Once step  18  has been carried out, all masks  20  are visually verified by a technologist (to spot evident errors, for example, of superposition of elements which should not be superposed), then is tested statistically again, at a step  22  (PLC) before the mask production. This last test, performed by a computer system, is a dimensional verification of the generated masks, for example, in comparison with dimensional criteria imposed by mask manufacturers (MRC, Manufacturing Rule Check), or with criteria imposed by integrated circuit manufacturers (PLC, Post Logical Check). 
     If test step  22  generates errors, a step  24  (ERROR) is provided, to modify logical operations  18  of transformation of step  20  so that the masks fulfill the conditions imposed by the mask manufacturers. This modification step is carried out manually by technologists and may be relatively long. Indeed, among all logical operations, the one having caused the incompatibility with the dimensional criteria imposed by the mask manufacturers should be targeted, and the required operation(s) should then be eliminated. 
     Once logical operations  18  have been modified, the operation of transformation of the integrated circuit file into a mask file is applied again to the integrated circuit file provided by the designers. If an error still occurs after test step  22 , logical operations  18  are modified again and the transformation operation of step  16  is repeated as many times as necessary. 
     When test step  22  is validated, the masks are sent to production at a step  26  (MASK FAB) and the integrated circuit production may start. 
     A problem may arise in specific cases where the designers desire to integrate new components in the integrated circuits. “New component” here means an entirely new component or a new adaptation of a known component, for example, the adding of a doped region at a new location of a transistor, the modification of the dimensions of an insulated gate, etc. 
     When a new component is designed, the integrated circuit in which this component is provided may be transformed according to the method described in relation with  FIG. 3  to obtain the set of masks corresponding to this circuit, if this set of masks can be generated with no error. It is generally provided, before performing this transformation, to form a test file in which many configurations of the new component, in interaction with other components, are gathered. This test file is then tested to see if it complies with the rules imposed by the DRMs, after which it is transformed by means of the logical operations. This enables to verify that this new component poses no problem, related to the DRMs, of integration into the desired integrated circuit, but also into other future configurations that may be given thereto. 
     However, it is possible to have a test circuit comprising new components complying with the conditions imposed by the DRMs, where the transformations of the logical operations pose no problem, with a good post-transformation test regarding the criteria of mask manufacturers, but with finally produced masks which do not provide high-quality components. 
     This may be due to the fact that the logical operations transforming the integrated circuit file into a mask file may incorrectly process the design of the new component, or may introduce errors during the transformation. 
     If an erroneous set of masks is used to produce integrated circuits, this may have significant consequences in terms of time and cost, especially if an entire new set of masks has to be designed and manufactured. 
     It thus cannot be envisaged to detect errors at the end of the mask manufacturing process. A method for limiting as much as possible the need to redesign integrated circuit masks is thus needed. 
     SUMMARY OF THE INVENTION 
     An embodiment provides a method for designing integrated circuit manufacturing masks overcoming all or part of the disadvantages of usual methods. 
     More specifically, an embodiment is a method for designing integrated circuit manufacturing masks implementing particularly efficient test steps. 
     Thus, an embodiment provides a method for designing masks adapted to the forming of integrated circuits in a considered technology, comprising the steps of: (a) forming a first test file comprising a set of randomly-generated configurations of integrated circuit elements arranged according to layouts that may exceed the cases authorized by design rule manuals; (b) forming a second test file comprising all the elements of the first test file, less the elements corresponding to configurations forbidden by design rule manuals; (c) transforming the second test file by means of a set of logical operations implemented by computing means to obtain a mask file comprising the configuration of the set of masks necessary to obtain the integrated circuit associated with the second test file; (d) testing the mask file and, if the test is negative, modifying and adapting the design rule manuals according to the test result; and (e) reiterating steps (a) to (d) as many times as necessary until the test of step (d) is positive. 
     An embodiment provides a method for designing masks adapted to the forming of integrated circuits in a considered technology, comprising the steps of: (a) forming a first test file comprising a set of configurations of integrated circuit elements arranged according to layouts that may exceed the cases authorized by design rule manuals, generated by using mathematical models of interaction of segments or of polygons; (b) forming a second test file comprising all the elements of the first test file, less the elements corresponding to configurations forbidden by design rule manuals; (c) transforming the second test file by means of a set of logical operations implemented by computing means to obtain a mask file comprising the configuration of the set of masks necessary to obtain the integrated circuit associated with the second test file; (d) testing the mask file and, if the test is negative, modifying and adapting the design rule manuals according to the test result; and (e) reiterating steps (a) to (d) as many times as necessary until the test of step (d) is positive. 
     According to an embodiment, step (d) further comprises the step of modifying and adapting the logical operations of transformation of the second test file. 
     According to an embodiment, step (d) further comprises the step of modifying and adapting the test of the mask file. 
     According to an embodiment, step (e) is followed by the steps of: (f) forming an integrated circuit file respecting the rules imposed by the modified and adapted design rule manuals obtained by the last reiteration of step (d); and (g) transforming the integrated circuit file into a mask file by applying the logical transformation operations modified and adapted at step (d). 
     According to an embodiment, step (g) is followed by a step of manufacturing of a set of integrated circuit masks based on the mask file obtained at step (g). 
     According to an embodiment, the first test file comprises a set of randomly generated configurations of integrated circuit elements. 
     According to an embodiment, the first test file comprises a set of configurations of integrated circuit elements generated by using mathematical models of interaction of segments or of polygons. 
     The foregoing and other objects, features, and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates examples of rules that may be imposed to integrated circuit designers to form different elements of electronic components on a same level; 
         FIG. 2  illustrates examples of rules that may be imposed to integrated circuit designers for the forming of different elements of electronic components which require the use of several mask levels for their manufacturing; 
         FIG. 3  is a flowchart illustrating steps currently carried out between the design of integrated circuit files and the manufacturing of the masks for manufacturing the integrated circuits; 
         FIG. 4  is a flowchart illustrating steps according to an embodiment of the present invention; and 
         FIG. 5  is a flowchart illustrating steps of the manufacturing of integrated circuit masks according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  is a flowchart of a method provided to improve the design of integrated circuit masks, and especially when new components are provided by integrated circuit designers. More specifically, this drawing illustrates steps of a method for performing efficient and robust integrated circuit file tests, to avoid that erroneous masks are formed based on new components which comply with conventional tests. 
     The method described in relation with the flowchart of  FIG. 4  will preferably be followed for each new electronic component technology. This method enables defining optimized integrated circuit design rules which may, later on, be adapted to any new component in the considered technology. 
     The method of  FIG. 4  starts with a step  30  (ALL CONF) of forming of a complete test file. This complete test file comprises all the possible configurations of the various elements of a given technology (doped wells, metal tracks, etc.), be they known and existing configurations or configurations which do not appear as achievable. 
     It should be noted that “all the possible configurations of the various available elements” means that the complete test file for example comprises a set of random configurations of all the elements available to the designer, or also a set of configurations generated by using mathematical models of interactions of segments or of polygons. Any other method of formation of such a complete test file, comprising many configurations of the elements available to the designers, may also be used. 
     The complete test file is then tested (step  32 ) by means of the design rules imposed in the DRMs associated with this technology (step  34 ) to form an improved test file (step  36 , IMPROVED TEST CASE) comprising all the configurations of complete test file  30  less all the configurations which do not comply with the rules imposed by DRMs  34 . Thus, the improved test file comprises all the configurations authorized by the DRMs of the considered technology, be they conventional configurations (currently used by designers) or unusual configurations which are not filtered by the DRMs. 
     The obtained improved test file  36  is then transformed, at a step  38  (LOP), by a set of logical operations  40  (LO) implemented by computing means, for example, a computer, and defined by technologists, to form a mask set (MASK SET) at a step  42 . 
     The mask set obtained at step  42  is then submitted to a first visual test performed by a technologist, then to a mask post-generation test at a step  44  (PLC). The mask post-generation test  44  enables verifying that the obtained masks comply not only with the dimensional requirements of mask manufacturers, but also with the requirements of integrated circuit manufacturers. 
     If an error is detected during one of the tests of step  44  (FAIL), and this error corresponds to a case which should not be reproduced afterwards in the considered technology, it is provided to report the specific case having caused the error directly at the level of design rules  34 , at a step  46  (MOD DRM). Thus, such a situation can not be allowed afterwards by the test performed by means of the DRMs, for new integrated circuits. 
     If the detected error does not correspond to a case to be forbidden afterwards for new circuits, in the considered technology, or corresponds to a case to be refined and/or verified differently, it may be provided, at a step  48  (MOD LO), to modify the logical operations  40  performed at step  38 . 
     The set of steps  32 ,  36 ,  38 ,  42 , and  44  is then repeated as often as necessary until a mask post-generation test step  44  providing no error is obtained (PASS). In this case, it is proceeded to a step  50  (ROBUST DRM/LO) where it can be said that the subsequent integrated circuit configurations authorized by the modified DRMs will provide high-quality and error-free mask files. The mask files thus obtained will be adapted to the manufacturing of high-quality and error-free integrated circuits. 
     It should be noted that the tests of mask post-generation test step  44  may also be modified and adapted when steps  32  to  42  are repeated. For example, some test steps may be modified and made less demanding if such a requirement appears not to be necessary. 
     Indeed, if a new integrated circuit is provided by the designers, this circuit will follow the conventional methods of transformation of the integrated circuit into a corresponding mask file for the integrated circuit manufacturing. 
       FIG. 5  illustrates the timing diagram of such a method. A new integrated circuit design file (NEW DESIGN FILE) is created at a step  60  by the integrated circuit designers. This integrated circuit design file complies with the integrated circuit design rules imposed by the improved design rule manuals formed by the method of  FIG. 4  (IMPROVED DRM, step  62 ). 
     The integrated circuit file is then transformed, in a step  64  (IMPROVED LOP), by the improved logical operations formed by the method of  FIG. 4  (step  66 , IMPROVED LO). The mask file thus obtained may be tested again with the mask post-generation steps, at a step  70  (PLC). If such a test is provided, the modified test will preferably be used on repeating of the method of  FIG. 4 , if it has been modified. A mask set is thus obtained at a step  72  (MASK FAB), the masks being of good quality. 
     Thus, once the method described in relation with  FIG. 4  has been carried out, any new integrated circuit file can be transformed into a mask file without fearing errors in the masks since the method described in relation with  FIG. 4  has eliminated all forbidden design configurations. 
     Specific embodiments of the present invention have been described. Various alterations, modifications and improvements will occur to those skilled in the art. In particular, any other test step generally provided in addition to the steps disclosed herein for the manufacturing of integrated circuit masks may be carried out in combination with the method steps disclosed herein. 
     It should be noted that, conventionally, the complete test file, the improved test file, the mask files, and the integrated circuit design files may be formed in the GDSII file format, currently used for such files. It should however be noted that any other integrated circuit file format (for example, OASIS) may be used to form these files. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.