Abstract:
Methods and apparatuses for fully defining static random access memory (SRAM) using phase shifting layouts are described. The approach includes identifying that a layout includes SRAM cells and defining phase shifting regions in a mask description to fully define the SRAM cells. The phase conflicts between adjacent phase shifters are resolved by selecting cutting patterns designed for the SRAM shape and functional structure. Additionally, the transistor gates of the SRAM cells can be reduced in size relative to the original SRAM layout design. Thus, an SRAM cell can be lithographically printed with small, consistent critical dimensions including extremely small gate lengths resulting in higher yields and improved performance.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to manufacturing small dimension features of objects, such as integrated circuits, using photolithographic masks. More particularly, the present invention relates to phase shift masking of complex layouts for integrated circuits and similar objects.  
           [0003]    2. Description of Related Art  
           [0004]    Phase shift masking has been applied to create small dimension features in integrated circuits. Typically the features have been limited to selected elements of the design, which have a small, critical dimension. See, for example, U.S. Pat. No. 5,766,806.  
           [0005]    Although manufacturing of small dimension features in integrated circuits has resulted in improved speed and performance, it is desirable to apply phase shift masking more extensively in the manufacturing of such devices. However, the extension of phase shift masking to more complex designs results in a large increase in the complexity of the mask layout problem. For example, when laying out phase shift windows on dense designs, phase conflicts will occur. One type of phase conflict is a location in the layout at which two phase shift windows having the same phase are laid out in proximity to a feature to be exposed by the masks, such as by overlapping of the phase shift windows intended for implementation of adjacent lines in the exposure pattern. If the phase shift windows have the same phase, then they do not result in the optical interference necessary to create the desired feature. Thus, it is necessary to prevent inadvertent layout of phase shift windows in phase conflict near features to be formed in the layer defined by the mask.  
           [0006]    In the design of a single integrated circuit, millions of features may be laid out. The burden on data processing resources for iterative operations over such large numbers of features can be huge, and in some cases makes the iterative operation impractical. The layout of phase shift windows and the assignment of phase shift values to such windows, for circuits in which a significant amount of the layout is accomplished by phase shifting, is one such iterative operation which has been impractical using prior art techniques.  
           [0007]    Phase shifting layouts for memory cells have been developed that phase shift gate portions of the memory design for improved performance.  
           [0008]    Because of these and other complexities, implementation of a phase shift masking technology for complex designs will require improvements in the approach to the design of phase shift masks.  
         SUMMARY OF THE INVENTION  
         [0009]    Methods and apparatuses for fully defining static random access memory (SRAM) using phase shifting layouts are described. By producing the SRAM memory using a “full phase” mask, yield can be improved at smaller sizes (relative to using the same lithographic process with a non-phase shifting mask, particularly the wavelength of light, λ), integrated circuit density is improved by tighter packing of smaller memory cells, and also the performance of the memory can be improved.  
           [0010]    The approach includes identifying that a layout includes SRAM cells and defining phase shifting regions in a mask description to fully define the SRAM cells. The identification may include an automated detection of layout patterns that correspond to SRAM cells, parameterized shape detection, user identification of SRAM cells either interactively through a user interface and/or through input parameters, and/or other identification approaches.  
           [0011]    A region around the layout shapes for an SRAM cell can be identified where phase shifters will be placed in the mask definition. By placing shifters in this region, destructive interference of light of opposite phases will cause definition of the pattern. However, it is necessary to break, or cut, the phase windows in the region to fully permit definition of the feature using phase shifters of opposite phases on opposing edges of the layout shapes of the SRAM cell.  
           [0012]    The cuts can be light transmissive phase shifters as well at intermediate phase values (continuous,  90 ,  60 - 120 ) relative to the primary phase shifters ( 0  and  180 ).  
           [0013]    The portion of the SRAM memory cell layout that is more difficult to define using phase sifting looks generally like two T&#39;s with off-centered bars interlaced with one another. There are contacts at the base of the bars and four transistors on either end of the top of the T. There are two additional transistors disposed above the interlaced T portion.  
           [0014]    Several locations where cuts will be admitted are used by embodiments of the invention: contact to contact, inside corners of the T&#39;s to field, back of T&#39;s to back of adjacent T&#39;s, contacts to field, and corners of T&#39;s to contacts. By selecting one or more of these cutting locations a phase shifting layout of the SRAM memory cell is possible.  
           [0015]    Most mask layouts will select a single cutting pattern for all SRAM memory cells in a particular area. For example, the cutting pattern of using the inside corners of the T&#39;s to field together with the back of T&#39;s to back of adjacent T&#39;s for all SRAM memory cells could be used for all of the SRAM memory cells in a given integrated circuit.  
           [0016]    Additionally, attention may be given to ensuring that corresponding features from one SRAM memory cell to another are defined using the same phase ordering. For example if the phase shifter on the left a given transistor is phase 0 and the one on the right is 180, then it may be desirable to ensure that the phase shifter on the left of the corresponding transistor on another SRAM memory cell is 0 and the one on the right is 180. This ensures consistency in the SRAM memory cell layout even if there is a light intensity imbalance between 0 and 180 degree phase shifters.  
           [0017]    Embodiments of the invention can be viewed as methods of manufacturing an integrated circuit. Embodiments of the invention include phase shifting and/or complementary trim masks for use in defining a layer of material in a photolithographic process.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0018]    The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.  
         [0019]    [0019]FIG. 1 illustrates a combination T-L junction.  
         [0020]    [0020]FIG. 2 illustrates a phase shifting layout for the T-L junction of FIG. 1.  
         [0021]    [0021]FIG. 3 illustrates a squared-U layout with the cutting location indicated for the phase layout.  
         [0022]    [0022]FIG. 4 illustrates a U layout with the cutting location indicated for the phase layout.  
         [0023]    [0023]FIG. 5 illustrates a H shaped features with a variety of spacings and phase shift layout arrangements.  
         [0024]    [0024]FIG. 6 is a simulation of the layout of FIG. 5.  
         [0025]    [0025]FIG. 7 includes portions of the simulation of FIG. 6 at higher magnification.  
         [0026]    [0026]FIG. 8 illustrates a portion of a layout of an static random access memory (SRAM) cell being defined using phase shifting.  
         [0027]    FIGS.  9 - 12  illustrate different phase layout designs for the SRAM cell of FIG. 8  
     
    
     DETAILED DESCRIPTION  
       [0028]    Overview  
         [0029]    Layouts and arrangements for defining several types of patterns using phase shifting will be considered.  
         [0030]    T-L Junction  
         [0031]    [0031]FIG. 1 illustrates a combination T-L junction, specifically the layout of FIG. 1 includes the feature  100  that includes an L shape adjacent to a T shape. A preferred shifter area  102  is shown surrounding the feature  100 . The preferred shifter area  102  corresponds to the preferred phase shifter width for use in defining the feature  100  using phase shifting. Further, cutting locations where shifters defined in the preferred shifter area  102  may be placed. Specifically, the cut locations  104 ,  106 ,  108 , and  110  are identified.  
         [0032]    [0032]FIG. 2 illustrates a phase shifting layout for the T-L junction of FIG. 1. More specifically, the cut location  106  was selected—to minimize the number of cuts—and thus four shifters: the shifter  204 , the shifter  206 , the shifter  208 , and the shifter  210 , are used to define the feature  100 . The phase shifting layout (and corresponding mask) would include only the shifters. Additionally, a complimentary trim mask for use in conjunction with the layout of FIG. 2 can be developed. See, e.g., United States patent application Ser. No. 09/932,239, having inventors Christophe Pierrat, et. al., entitled “Phase Conflict Resolution for Photolithographic Masks”, filed Aug. 17, 2001, and assigned to the assignee of the present application, which is incorporated herein by reference.  
         [0033]    U Shapes  
         [0034]    Turning to FIGS.  3 - 4 , two U-shaped layouts are shown. FIG. 3 illustrates a squared-U layout with the cutting location indicated for the phase layout. FIG. 4 illustrates a U layout with the cutting location indicated for the phase layout. In each, a single cut on the inside of the U will be used to separate the phase shifters (a corresponding cut can be used on the outside of the U).  
         [0035]    Specifically, FIG. 3 includes the feature  300  and the feature  302 . A single cut  304  extends from interior of the corner of the feature  300  towards the feature  302  and then runs parallel along the endcap of the feature  302 . Similarly, in FIG. 4 the feature  400  and feature  402  comprise the layout pattern and a cut  404  is used in the interior. Additionally, with respect to FIG. 4, in some embodiments a slightly different cut shape is used in the interior bend of the U. Specifically, a corner  410  and a corner  412  are shown interior to the bended U. In some embodiments, the cut is centered about one of the two corners rather than the full length of the interior angled wall of the U.  
         [0036]    H-Shapes  
         [0037]    [0037]FIG. 5 includes a test pattern for H-shapes arranged in a 12 wide by 5 high grid. There are twelve different spacings between the vertical bars of the H shown across FIG. 5 and for each spacing, the column shows a possible shifter arrangement for that spacing. For clarity of reference, each H pattern can be referred to by its x-y position, e.g. ( 1 , 1 ) being the upper leftmost H and ( 12 , 5 ) being the bottom rightmost H. Thus, within a row the space between the bars of the H wider for the H (j′,k) as compared to the H (j,k) where j′&gt;j.  
         [0038]    Which option is ultimately selected will depend on the surrounding environment, e.g. the adjacent polygons, as well as the process latitude. For example, the H shapes in the first row ( 1 , 1 ). . . ( 12 , 1 ) are premised on the assumption that it will be possible to make a cut on both sides of the H. In contrast the H shapes in the second row assume only one cut is possible, e.g. H&#39;s ( 1 , 2 ). . . ( 12 , 2 ). The remaining rows make no use of cuts on the outside of the H. But, may result in difficult to manufacture masks, e.g. H ( 5 , 3 ) which has a small phase shift area. Other paterns may allow the phase conflict by not using any cuts, H&#39;s ( 1 , 5 ). . . ( 12 , 5 ). It should be noted that some of the H patterns in rows two through four lack adequate space to admit the cutting pattern used in the remainder of the row, c.f. H&#39;s ( 1 , 2 ). . . ( 4 , 2 ), ( 1 , 3 ). . . ( 4 , 3 ), and ( 1 , 4 )  5  . . . ( 4 , 4 ) with the remaining H&#39;s in those rows.  
         [0039]    In FIG. 6, a simulated aerial image of the test pattern of FIG. 5 is shown. The exposure conditions assumed that the phase shift mask was exposed with a 248 nm wavelength (λ) light, N.A.=0.75, and σ=0.5 and that the trim mask (not shown) was exposed using the same λ and N.A., but with σ=0.5 and three times the dosage. As can be seen from the simulation in most cases the layouts will be correctable with optical proximity correction. However, there will be a resulting impact on process latitude. For example, the H ( 6 , 2 ) and the H ( 7 , 2 ) can likely print however there will be a limited amount of process latitude for the vertical lines. Similar problem, e.g. with H ( 5 , 4 ), but note that other H&#39;s in row four are more likely to be correctable with OPC, e.g. H&#39;s ( 8 , 4 ). . . ( 12 , 4 ). In contrast note that in row five, the bars of the smaller H&#39;s are more easily corrected with OPC across the phase conflict, e.g. H&#39;s ( 1 , 5 ). . . ( 4 . 5 ), while in contrast as the length of the phase conflict area increases correction with OPC becomes more difficult or perhaps impossible, e.g. H&#39;s ( 5 , 5 ). . . ( 12 , 5 ).  
         [0040]    Turning to FIG. 7, a magnified view of the simulations for one of the columns, j, at separation 0.5 μm, is shown with the magnified images arranged sideways (e.g. top most row on the left, bottom most row on the right). As shown OPC correction can be applied in the OPC cut regions  710  and the phase conflict region  720 . In some instances there may be insufficient process latitude to reliably print the feature, e.g. (j, 5 ) may lack sufficient process latitude to reliably print on the wafer even with optical proximity correction.  
         [0041]    SRAM Cutting  
         [0042]    [0042]FIG. 8 illustrates a portion of a layout of an SRAM cell  800  being defined using phase shifting. The SRAM cell  800  is representative of common designs and includes a largely repeating pattern including a portion  810  which is surrounded by a heavy, dashed line. FIGS.  9 - 12  show several possible phase layouts for fully defining the layout of FIG. 8 using phase shifting.  
         [0043]    Turning to FIG. 9, the portion  810  is shown with a phase shifting layout. The orientation of the cross hatching indicates the relative phase. For example, the phase shifter  912  and the phase shifter  914  have opposite phase (X,X+180) as indicated by the different directions of the cross hatching. The features defined by the shifters, e.g. feature  916 , are shown for clarity but are not part of the layout itself.  
         [0044]    The cutting arrangement used in FIG. 9 can be described as having a cut in the phase shifters between the two contacts as well as cuts in the inside corners of the T&#39;s opposite the cuts between the contacts.  
         [0045]    As can be seen in FIG. 9, the particular cutting arrangement used cause corresponding features, e.g. the feature  916  and the feature  926 , to be defined by a different shifter ordering, e.g. (X, X+180) vs. (X+180, X), as seen by the cross hatching on the shifter  912 ,  914 ,  922 , and  924 . This may be undesirable because there can be a light intensity imbalance between for example 0 and 180 degree phase shifters.  
         [0046]    Thus, the same feature may print slightly differently depending on where it fell within the larger pattern of the cell. For example, if the 0 degree phase shifter is slightly more intense than the 180 degree shifter then the light imbalance will tend to move features slightly towards the 180 degree shifter. If the phase ordering is flipped from corresponding feature to corresponding feature then in some cases the features will print slightly to one side and on others slightly to the other side. As such it may be desirable to ensure that the cutting arrangement used to define the individual memory cells of the SRAM is such that the same feature is consistently defined using the same shifter ordering.  
         [0047]    Such an arrangement is shown in FIG. 10. Specifically, the region  810  is shown with a different shifter and cutting arrangement. Here, the corresponding features, e.g. the feature  1016  and the feature  1026 , are consistently defined using the same phase orderings, e.g. phase shifters  1012 ,  1014 ,  1022 , and  1024 .  
         [0048]    The cutting arrangement used in FIG. 10 can be described as having a cut in the phase shifters between the two contacts as well as cuts on the back of the T&#39;s to the adjacent T-back.  
         [0049]    [0049]FIG. 11 shows a variation on the cutting arrangement of FIG. 10 however, like with FIG. 9 the cutting arrangement causes corresponding features to be defined using alternating phase patterns.  
         [0050]    The cutting arrangement used in FIG. 11 can be described as having a cut in the phase shifters from the contact to the field as well as cuts on the back of the T&#39;s to the adjacent T-back.  
         [0051]    [0051]FIG. 12 shows a cutting arrangement that like FIG. 10 ensures that the corresponding features are defined using the same phase ordering.  
         [0052]    The cutting arrangement used in FIG. 12 can be described as having cuts in the opposing corners of the T to the contact.  
         [0053]    The particular cutting arrangement selected will depend on mask manufacturability concerns, the process design rules, and/or one or more simulations of the cutting arrangement for a given SRAM design and lithographic process.  
         [0054]    It should also be noted that in some memory designs the horizontally disposed  5  bars for contacts of adjacent memory cells are aligned. Specifically instead both a vertical and horizontal offset between bars  812  and bar  814 , the adjacent horizontal bars are in vertical alignment. This would impact the cutting patterns by facilitating the use of straight line cuts between adjacent memory cells, c.f. FIG. 10 and FIG. 11 where a diagonal cut is used.  
         [0055]    Representative Alternative Embodiments  
         [0056]    Additionally, although the description has primarily focused on examples of defining a polysilicon, or “poly”, layer within an IC, phase shifting can be used to define other layers of material, e.g. interconnects, metal, etc.  
         [0057]    Although in many instances, an angled cut is shown as a preferred cutting arrangement, from a mask manufacturing perspective 90 degree cuts are more easily manufactured. Accordingly, in some embodiments, to the extent practical cuts at 90 degrees to the feature are selected in preference to other cuts. This works well at outside corners where the angled cut can be modified to a straight line cut.  
         [0058]    Although the cut areas are shown as clear regions in fact a gradual, e.g. continuous, phase transition can be used as can a tri- or quad-tone mask, e.g. 0-90-180 or 0-60120-180, with the middle phase values used in the cut openings.  
         [0059]    Some embodiments of the invention include computer programs for performing the processes of defining the phase shifting layers and/or corresponding trim layers. In one embodiment, the process is implemented using the abraCAD(TM) software produced by Cadabra Design Automation, a Numerical Technologies company, San Jose, Calif. In some embodiments, the computer programs are stored in computer readable media, e.g. CD-ROM, DVD, etc. In other embodiments, the computer programs are embodied in an electromagnetic carrier wave. For example, the electromagnetic carrier wave may include the programs being accessed over a network.  
         [0060]    As used herein, the term optical lithography refers processes that include the use of visible, ultraviolet, deep ultraviolet, extreme ultraviolet, x-ray, e-beam, and other radiation sources for lithography purposes. The masks designs used should be appropriately adapted, e.g. reflective vs. transmissive, etc., to the particular lithographic process.  
         [0061]    Conclusion  
         [0062]    The foregoing description of embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations will be apparent. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.