Abstract:
Design methods and a computer-readable medium having computer-executable instructions thereon for sidelobe suppression in a radiation-patterning tool or mask. Sidelobe artifacts are mitigated by identifying elements as a function of the radiation wavelength for forming desired profiles on a semiconductor wafer. A diffraction ring is calculated around each of the elements to identify sidelobe interference zones and intesections of diffraction rings are located. When a guard ring around one of the intersections would otherwise overlap with a guard ring around another one of the intersections, a common sidelobe inhibitor is located at the common overlap region of the guard rings. A method for forming a mask with the addition of sidelobe inhibitors as well as a method for determining the location of placement of sidelobe inhibitors is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of semiconductor fabrication and, more particularly, to a mask for use in a photolithography process employed during semiconductor fabrication. 
     2. State of the Art 
     Photolithography is commonly used during the fabrication of integrated circuits on semiconductor wafers and other bulk substrates comprising a layer of semiconductor material. During photolithography, a form of radiant energy is passed through a radiation-patterning tool onto a radiation-sensitive material, commonly referred to as photoresist, which is placed upon a surface of a semiconductor wafer. The radiation-patterning tool is commonly known as a photomask or reticle. The term “photomask” is used to reference a structure that performs a function of masking or defining a pattern over an entire semiconductor wafer while the term “reticle” is used to reference a structure that functions to define a pattern over a portion of a wafer. 
     Radiation-patterning tools contain light-restrictive regions and light-transmissive regions. Light-restrictive regions may be, for example, opaque or partially light transmissive. The light-transmissive regions or portions of a radiation-patterning tool, in conjunction with the light-restrictive regions, cooperatively facilitate the formation of a desired pattern on a semiconductor wafer. For the formation of patterns on a semiconductor wafer, the wafer is coated with a layer of radiation-sensitive material (e.g., photoresist). Subsequently, radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers onto the photoresist a pattern defined by the radiation-patterning tool. Using a form of a photographic process, the photoresist is then developed to remove either the portions exposed to the radiant energy in the case of a “positive” photoresist or the unexposed portions when a “negative” photoresist is utilized. The residual photoresist pattern thereafter serves as a mask for a subsequent semiconductor fabrication process. 
     With advances in semiconductor integrated circuit processes, the dimensions associated with integrated circuit device features have decreased. Furthermore, the demand for smaller and faster-performing semiconductor devices requires increasing precision and accuracy in photolithographic processes. 
       FIG. 1  illustrates a flow chart of a conventional process used for creating a pattern for a radiation-patterning tool. Initially, a preliminary design is created and verified  10  with the desired pattern identified to form the desired pattern on the target photoresist. Subsequently, profiles are developed for the radiation-patterning tool to roughly produce the desired pattern when radiation is passed through the radiation-patterning tool. The profiles or elements form a rough correspondence to the desired pattern as the profiles or elements initially disregard the effects of interference of radiation passing through the radiation-patterning tool. 
     Following creation and verification of the design, optical proximity correction  12  accounts for various interference factors that influence radiation passing through the radiation-patterning tool. Such interference factors may include constructive and destructive interference effects resulting as the radiation wavelength approximates the dimensions of portions of the profiles or elements of the radiation-patterning tool. Optical proximity correction modifies the profile or element dimensions to shapes such that a resultant patterned photoresist more closely approximates the desired pattern. The processes of designing, verifying and optically correcting a design are typically accomplished primarily through the use of software, such as is available from Synopsys Corporation of Mountain View, Calif. 
     As a result of the optical proximity correction process  12 , a data set that corresponds to a pattern capable of generation by a radiation-patterning tool is typically generated. The data set is subsequently “taped out” to a radiation-patterning tool through the use of, for example, laser writing and/or electron-beam writing methodologies. Following the formation of the pattern on the radiation-patterning tool, the tool is capable of being utilized for semiconductor fabrication. 
       FIG. 2  illustrates an exemplary apparatus  14  in which a radiation-patterning tool is utilized for a patterning process. Apparatus  14  comprises a radiation source  16  that generates radiation  18  and further includes a radiation-patterning tool  20  through which radiation  18  is passed. A semiconductor wafer or substrate  22  includes a radiation-sensitive layer  24  thereon. As illustrated, radiation  18  passes through radiation-patterning tool  20  and impacts radiation-sensitive layer  24  to form a pattern. This process of forming a pattern on a radiation-sensitive material with a radiation-patterning tool is commonly referred to as a printing process. 
     A radiation-patterning tool  20  typically includes an obscuring material that may either be an opaque (e.g., chrome) or a semi-opaque material placed over a transparent material (e.g., glass). Radiation-patterning tool  20  is illustrated in  FIG. 2  as having a front side  28  for forming features or windows and an opposing back side  26 . Some radiation-patterning tools further utilize both the front side and back side for the formation of windows. 
     Radiation-patterning tool  20  typically has a pattern with dimensions on the order of, or smaller than, the wavelength of radiation passing through the radiation-patterning tool. Therefore, interference effects may occur when radiation passes through the radiation-patterning tool and exits onto the radiation-sensitive material. Accordingly, the pattern of the radiation-patterning tool must be modified to compensate for such interference effects.  FIG. 3  illustrates an exemplary pattern  30  desired to be formed on the radiation-sensitive material by subsequent semiconductor processes. Due to the interference effects, pattern  30  cannot be directly utilized but must undergo the optical proximity correction  12  of  FIG. 1 . Pattern  32  illustrates a corrected pattern that accommodates the interference effects resulting from near-wavelength dimension patterns. 
       FIG. 4  illustrates a radiation-patterning tool  34  and further illustrates elements utilized to create the targeted or printed images. In the exemplary printing process, a radiation-sensitive material  38  is illustrated as having formed therein a plurality of repeating patterns  40 , illustrated as circular in dimension, which may be used, for example, in the formation of contact openings. One of the patterns  40  is illustrated as being centered around a location  42  while another one of the repeating patterns is illustrated as being centered around a location  44 . Still referring to  FIG. 4 , radiation-patterning tool  34  includes a plurality of repeating elements  36  that are in a one-to-one correspondence with patterns  40  formed on the radiation-sensitive material  38 . As shown, each of elements  36  is approximately square in shape that, when passing radiation therethrough, forms the circular patterns  40  on radiation-sensitive material  38 . Elements  36  on the radiation-patterning tool  34  may be either more transparent to radiation than surrounding regions or less transparent, depending on whether the radiation-sensitive material  38  is implemented as a positive or negative photoresist material. When elements  36  are more transmissive to radiation than surrounding regions, elements  36  effectively act as windows that allow radiation to pass through onto the radiation-sensitive material  38 . 
     The printed patterns  40  correspond to regions where light has passed through elements  36  of the radiation-patterning tool  34 . As described above, when elements  36  exhibit dimensions approximating the wavelength of the radiation, interference effects may occur.  FIG. 4  illustrates interference effects in the form of sidelobes  46  extending around each of the patterns  40 . Exposure of radiation-sensitive material  38  to a single sidelobe  46  generally does not result in a printed feature within the radiation-sensitive material  38 . However, when two or more sidelobes  46  overlap, it is possible to form a printed feature. Regions  48  and  50  illustrate locations where, specifically, four sidelobes converge, and accordingly illustrate locations where printed features may be undesirably manifested. 
       FIGS. 5-7  further describe sidelobe convergence in additional detail with respect to the interference effects of radiation having a wavelength that is on the order of the dimensions of the desired pattern. In  FIG. 5 , the electric field strength of radiation passing through an element  36  ( FIG. 4 ) is diagrammatically illustrated for forming a pattern  40  ( FIG. 4 ) centered around a location  42 . As illustrated, a large, positive field strength occurs at location  42 , which creates undesirable sidelobes  46 . A large, positive field strength centered around location  42  may be referred to as a primary lobe while the lobes or concentrations of energy centered away from the primary lobe are referred to as sidelobes  46 . 
     The exposure of radiation-sensitive material  38  is proportional to the intensity of the radiation rather than the field strength, as the intensity is a function of the square of the field strength.  FIG. 6  illustrates the intensity of the radiation utilized to pattern the feature centered around location  42 . Accordingly, since the intensity is the square of the field strength, the sidelobes  46  have a positive value as does the primary lobe centered around location  42 . Therefore, if the magnitudes of the sidelobes  46  are sufficient, the sidelobes can induce printed features in the radiation-sensitive material  38 .  FIG. 7  illustrates the additive effect of a sidelobe formed from radiation centered around location  42  and a sidelobe formed from radiation centered around location  44 . The radiation formed around location  44  is consistent in magnitude with that formed around location  42 . Accordingly, since the adjacent sidelobes overlap, the two sidelobes combine to form a resultant overlapping lobe  52 . Lobe  52  results from the combination of overlapping sidelobes of adjacent patterns of radiation. Such an overlapping combination, as illustrated in  FIG. 7 , can be extended to combinations of  3 ,  4  or more proximate patterns of radiation. Accordingly, the energy combination of adjacent overlapping sidelobes can grow significantly in intensity relative to the main lobes, which can eventually result in a generated printed feature. 
     Identification of locations where sidelobes may combine to form a printed feature has been undertaken and, when such locations are identified, a radiation-patterning tool can be modified to prevent the undesired combination of sidelobes.  FIG. 8  illustrates an arrangement  54  that identifies design elements illustrated as design features  56  corresponding to elements  36  ( FIG. 4 ) of a radiation-patterning tool. One prior approach for identifying locations includes a mathematical calculation performed on the spatial characteristics of design features  56  to create a common region  58  extending between design features  56 . From the region  58 , a sidelobe inhibitor  60  may be calculated. The sidelobe inhibitor is utilized to prevent formation of an undesired printed feature from occurring at the location where sidelobes converge. 
       FIG. 9  illustrates a portion of a prior art radiation-patterning tool  34  similar to the tool described with reference to  FIG. 4 , except that  FIG. 9  further illustrates a sidelobe inhibitor  62  for preventing sidelobes of radiation from combining when passing through elements  36 . Sidelobe inhibitor  62  typically includes dimensions of approximately one-half of the wavelength of the radiation passed through radiation-patterning tool  34 . Sidelobe inhibitor  62  may be formed, for example, by etching onto an opaque material associated with radiation-patterning tool  34  to form a region where radiation will be in phase with the main energy lobe and thus out of phase relative to other portions of the sidelobe radiation. Such a combination is typically known as destructive interference, which results in a cancellation of a significant amount of intensity from the combined sidelobes. 
     Accordingly, there is a need and desire to minimize and even eliminate sidelobe effects on a radiation-patterning process. 
     BRIEF SUMMARY OF THE INVENTION 
     A method and system for sidelobe suppression in a radiation-patterning tool is provided. In one embodiment of the present invention, a method for mitigating sidelobe artifacts in a radiation-patterning process is described. Elements to be formed in a radiation-patterning tool are defined as a function of the radiation wavelength, which creates a desired pattern along with resultant sidelobes. Diffraction rings are calculated about each of the elements to identify where sidelobe interference zones and intersections of diffraction rings are located. Sidelobe inhibitors are located at the identified locations. 
     In another embodiment of the present invention, a method of generating sidelobe inhibitors on a radiation-patterning tool is described. Elements for forming in the radiation-patterning tool are defined with a mathematical definition of the spatial orientations formed. Diffraction rings about each of the elements are further defined with intersections of adjacent diffraction rings identified as prospective locations for the placement of sidelobe inhibitors. 
     In yet another embodiment of the present invention, a method for designing a mask for illuminating a pattern defines elements to be formed in a mask. Diffraction rings about each of the elements that coincide with the locations of sidelobes about the elements are calculated. Sidelobe inhibitors are formed at intersections of the diffraction rings of adjacent elements. 
     In yet a further embodiment of the present invention, a mask for exposing a resist-covered wafer in a radiation-patterning process includes transmissive elements corresponding to features on the wafer that are to be exposed and one or more sidelobe inhibitors to suppress radiation sidelobes with the sidelobe inhibitors arranged at intersections of the diffraction rings. 
     A computer-readable media embodiment of the present invention is also provided for determining the placement of sidelobe inhibitors relative to elements to be formed on a radiation-patterning tool. Diffraction rings are calculated to coincide with an approximate location of radiation sidelobes, with the intersection of overlapping diffraction rings identified as locations for placement of sidelobe inhibitors. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention: 
         FIG. 1  is a flow chart of a prior art method of forming a radiation-patterning tool; 
         FIG. 2  is a cross-sectional view of a prior art apparatus utilized in printing a pattern on a radiation-sensitive material utilizing a radiation-patterning tool; 
         FIG. 3  is a view of a desired prior art pattern and a prior art element utilized for producing the pattern; 
         FIG. 4  is a top view of a prior art pattern in a radiation-sensitive material together with a top view of a prior art radiation-patterning tool utilized for forming the pattern; 
         FIG. 5  is a graphical illustration of radiation field strength across a substrate of an electrical field associated with radiation passing through a prior art radiation-patterning tool; 
         FIG. 6  is a graphical illustration of radiation intensity across a substrate of an electrical field associated with passing through a prior art radiation-patterning tool; 
         FIG. 7  is a graphical illustration of radiation intensity across a substrate and further illustrates a combination of intensities from two combining fields passing through a prior art radiation-patterning tool; 
         FIG. 8  is an illustration of a prior art process for determining a location of sidelobe inhibitors; 
         FIG. 9  is a top view of a prior art radiation-patterning tool comprising a sidelobe inhibitor; 
         FIG. 10  is a top view of a mathematical construct illustrating a process for determining locations of sidelobe inhibitors, in accordance with an embodiment of the present invention; mathematical construct for calculating the sidelobe convergences. Once diffraction rings  70  have been identified, locations that may be susceptible to sidelobe convergence or overlap are identified. Sidelobe convergence or overlap occurs at locations where one diffraction ring from one design feature intersects a second diffraction ring of a second design feature. Locations  72 ,  74  and  76  identify intersects of the respective diffraction rings  70 . These intersecting locations identify regions where electric-field energy of radiation sidelobes may become additive or converge, resulting in corresponding areas that may be susceptible to inadvertent patterning. 
         FIG. 11  is a diagrammatic top view of a radiation-patterning tool comprising sidelobe inhibitors, in accordance with an embodiment of the present invention; 
         FIG. 12A  is an enlarged view of a sidelobe convergence location illustrated in  FIG. 12 ; and 
         FIG. 13  is a diagrammatic top view of a radiation-patterning tool comprising sidelobe inhibitors, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method and algorithm for enabling the placement of sidelobe inhibitors using a geometrical approach for identifying the interaction or convergence of sidelobes of two or more desired patterns. The methodology of the present invention may be used to account for two or more interactions between contacts in a radiation-patterning tool design. Prior approaches could not address two or more contact interactions but rather placed an inhibitor at a midpoint between the windows or elements rather than at each of the overlapping locations. The present approach enables the placement of sidelobe inhibitors at multiple interaction areas of interacting contacts. The methodology of the present invention identifies interactions between two or more patterning elements and identifies locations for the addition of sidelobe inhibitors for suppressing undesired sidelobe interactions. When multiple sidelobe inhibitors occur in an undesirable proximity, then a common sidelobe inhibitor may be identified and placed in a common location. 
       FIG. 10  illustrates a geometric method for mitigating sidelobe artifacts in a radiation-patterning process, in accordance with an embodiment of the present invention. The present method generates a mathematical method or construct  64  that includes design features  66 . One methodology of the present invention determines which features  66  are adjacent to other features  66  and within a threshold spatial distance of one another. Design features  66  ultimately correspond to elements of a radiation-patterning tool that may be used for printing one or more patterns on a radiation-sensitive material. As interference is a function of the additive effects of radiation, a zone or ring is defined by a radius around each feature where other adjacent radiation could pose an additive effect. The threshold spatial distance utilized in identifying design features in the present methodology is about eight-tenths of the wavelength divided by the numerical aperture. 
     An algorithm used to generate a radius circumscribing the design feature is computed by calculating a radius  68  and forming corresponding diffraction rings  70  around each design element. It should be appreciated that the diffraction rings do not extend between actual elements of a radiation-patterning tool but rather circumscribe design features corresponding to a mathematical construct for calculating the sidelobe convergences. Once diffraction rings  70  have been identified, locations that may be susceptible to sidelobe convergence or overlap are identified. Sidelobe convergence or overlap occurs at locations where one diffraction ring from one design feature intersects a second diffraction ring of a second design feature. Locations  72 ,  74  and  76  identify intersections of the respective diffraction rings  70 . These intersecting locations identify regions where electric-field energy of radiation sidelobes may become additive or converge resulting in corresponding areas that may be susceptible to inadvertent patterning. 
     The mathematical construct  64  illustrates design features  66  that may be closer to one another than others of the design features, resulting in multiple potential sidelobe interaction locations, namely locations  72  and  74 , which would not typically be discerned through other prior art techniques. After the sidelobe overlap regions are located within mathematical construct  64 , the construct is utilized to form a radiation-patterning tool. Such radiation-patterning tool comprises elements corresponding to design features  66  and also comprises the mathematical sidelobe inhibitors  77  (e.g., phasing regions) formed across at least some of the regions of the tool corresponding to the identified sidelobe overlap locations.  FIG. 11  illustrates the conversion of the design features and corresponding sidelobe inhibitors from a modeling or construct domain into a real or radiation-patterning tool domain illustrated as the radiation-patterning tool  78 . In  FIG. 11 , radiation passing through windows or elements  80  may result in undesirably exposed locations that were calculated and are protected by the addition of sidelobe inhibitors  82 . The dimensions of sidelobe inhibitors  82  will typically be about one half of the wavelength of radiation passed through radiation-patterning tool  78 . Sidelobe inhibitors  82  may be formed by etching an opaque material associated with radiation-patterning tool  78  to form regions where radiation will be in phase with the main lobe and, thus, out of phase relative to other portions of the sidelobe radiation. Such destructive interference assists in the cancellation of a significant amount of intensity from the combined sidelobes. 
       FIGS. 12 and 13  illustrate further methods for determining locations of sidelobe interaction and, ultimately, locations for placement for sidelobe inhibitors on a radiation-patterning tool.  FIG. 12  illustrates a mathematical construct  84  having design features  86 . Respective diffraction rings  88  are generated about each of the design features  86  for a determination of convergence of sidelobe energies. As illustrated, intersections of diffraction rings  88  identify locations  90 ,  92 ,  94  and  96  that represent potential sidelobe convergence locations. In  FIG. 12 , convergence locations  90  and  96  are located an appreciable distance from any others of the sidelobe convergence locations. However, sidelobe convergence locations  92  and  94  are relatively adjacent to one another and, according to the previously disclosed embodiment with regard to  FIGS. 10 and 11 , sidelobe inhibitors would be placed at both regions about locations  92  and  94 , resulting in sidelobe inhibitors that encroach or nearly touch one another. Placement of sidelobe inhibitors in close proximity to one another may present design verification issues as well as mitigate the benefits associated with sidelobe inhibitors. Accordingly, an exemplary embodiment of the present invention contemplates a methodology and algorithm for determining when a plurality of sidelobe convergence locations is preferably substituted with a lesser number of locations for better facilitating any verification and/or sidelobe convergence suppression benefits. 
       FIG. 12A  illustrates an enlarged view of the sidelobe convergence locations  90 ,  92 ,  94  and  96  of  FIG. 12 . As described above, placement of two or more sidelobe inhibitors in close proximity with one another may present undesirable results from a design verification approach or from actual minimization of the sidelobe inhibiting effects. One method for determining the existence of these proximity concerns is illustrated in  FIG. 12A , wherein features  98  and  100  are centered about locations  92  and  94 , respectively. When a feature overlaps with another feature, then a common feature may be identified and placed either central or in another intermediate location as a common sidelobe inhibitor. Additionally as illustrated in  FIG. 12A , extensions or guard rings  102 ,  104  may be placed around features  98 ,  100  for use in analyzing any overlapping or common areas as a result of proximity of the features. In the present illustration, a common or overlapping area  106  is identified and, in the present embodiment, results in the definition of a resulting common sidelobe inhibitor. The placement of the common sidelobe inhibitor may be calculated, in one embodiment, by forming a line  108  between locations  92 ,  94  and placing the common feature either centered or in another weighted manner along the resulting line  108 . 
       FIG. 13  illustrates the formation of a radiation-patterning tool from the mathematical construct of  FIGS. 12 and 1   2 A. After the sidelobe overlap regions are located within construct  84  ( FIGS. 12 and 12A ), the construct is utilized to form a radiation-patterning tool  110 . Such radiation-patterning tool  110  comprises elements corresponding to design features  86  and also comprises the sidelobe inhibitors (e.g., phasing regions) formed across at least some of the regions of the tool corresponding to the identified sidelobe overlap locations. In the present embodiment, it is desirable to identify adjacent sidelobe inhibitors that are proximately undesirable.  FIG. 13  illustrates the conversion of the design features and corresponding sidelobe inhibitors from a modeling or construct domain into a real or radiation-patterning tool domain illustrated as the radiation-patterning tool  110 . Tool  110  includes windows or elements  114  with  sidelobe inhibitors  116  identified according to the overlap procedures previously described. The calculation of a common sidelobe inhibitor  112  results from the proximate location procedure of  FIGs. 12 and 12A . The dimensions of sidelobe inhibitors  112  and  116  are typically about one-half of the wavelength of radiation passed through radiation-patterning tool  110 . Sidelobe inhibitors  112  and  116  can be formed by etching an opaque material associated with radiation-patterning tool  110  to form regions where radiation will be in phase with the main lobe and thus out of phase relative to other portions of the sidelobe radiation. Such destructive interference assists in the cancellation of a significant amount of intensity from the combined sidelobes. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.