Abstract:
Cross-shaped sub-resolution assist features may be utilized to print lithographic patterns in semiconductor fabrication processes. The crosses may be isolated structures or may be part of a grid arrangement. The main features, such as contacts, may be positioned on the mask so as to be intersected by the cross-shaped sub-resolution assist features. In some embodiments, the cross-shaped sub-resolution assist features may intersect the main feature at its center point in both the x and y directions.

Description:
BACKGROUND 
     This invention relates generally to semiconductor photolithograph and, in particular, to applying sub-resolution assist features. 
     Various systems may be used to print features, such as patterns, that define integrated circuits on semiconductor wafers. In general, electromagnetic wave emissions from a source are directed to expose selected locations on a substrate, while leaving other locations unexposed. Examples of emissions include visible light, ultraviolet (UV), and extreme ultraviolet (EUV) radiation, and x-rays. These emissions all have characteristic wavelengths in various media. 
     Example approaches to directing these emissions towards selected locations include selective attenuation (e.g., using binary photomasks), interference (e.g., using phase shifting masks), reflection (e.g., using EUV reflective optical elements), and beam steering. Regardless of the approach used, systems for printing features have a resolution limit below which certain features are not resolved during printing. In particular, it is desirable to manufacture features that are smaller than the lithographic wavelength. However, attempting to form features at these sizes, while achieving adequate resolution, may be complex. As mask features get closer together, a number of issues may arise, including diffraction issues. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic depiction of a semiconductor fabrication device suitable for application to embodiments of the present invention; 
         FIG. 2  is a depiction of a portion of a mask showing main features and sub-resolution assist features in accordance with one embodiment of the present invention; 
         FIG. 3  is a diagram of a portion of a mask showing main features and sub-resolution assist features in accordance with another embodiment of the present invention; and 
         FIG. 4  is a diagram of a portion of a mask showing main features and sub-resolution assist features in accordance with still another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A patterning system  100 , shown in  FIG. 1 , includes an illumination source  101 , an illumination system  105 , a mask  109 , an imaging system  113 , and a substrate  115 . 
     Illumination source  101  may generate electromagnetic radiation. The radiation may be adapted for use in lithographic patterning of microelectronic devices to expose photosensitive materials. The radiation can be fully or partially coherent in that the admitted optical waves maintain a fixed and predictable phase relationship with each other for a period of time. The radiation may have a characteristic wavelength in various portions of the system  100 , including in the vicinity of the substrate  125 . 
     Illumination system  105  can include an aperture, a condenser, as well as additional devices for collecting, shaping, collimating, directing, filtering, and focusing radiation emitted from the source  101 . 
     The mask  109  is positioned in the system  100  by a mask stage  107  to influence the incidence of radiation upon substrate  115 . The mask  109  can include different regions that transmit electromagnetic radiation with different transmissivities and/or phases. The transmission of electromagnetic radiation by a mask  109  can be engineered to image a pattern of desired features on the substrate  115 . For example, the mask  109  may be a binary mask or the mask  109  may be a phase shifting mask, such as an alternating phase shifting mask or an imbedded phase shifting mask. The mask  109  can transmit radiation (as shown) or a mask  109  can reflect radiation. 
     The imaging system  113  can include an aperture, an objective, as well as additional devices for collecting, filtering, and focusing the portion of radiation that passes through the mask  109  onto the substrate  115 . For example, the imaging system  113  can include a filtering projection lens and/or reduction optics. 
     The substrate  115  is a work piece to be patterned by the system  100 . The substrate  115  may include a working surface that includes a photosensitive material. The substrate  115  can be presented to the system  100  by a vacuum chuck  117  or other support such that radiation is focused in the vicinity of the working surface to image desired features in photosensitive material. 
     Given the optical characteristics and wavelength of radiation defining the resolution limit of the system  100 , the dimension and arrangement of features that can be imaged in the photosensitive material is limited. For example, certain features may have too small a pitch to be imaged using radiation and, thus, may fall below the resolution limit of the system  100 . As another example, adjacent features may be so closely spaced that radiation diffracts and/or constructively or destructively interferes to make the printing of such features more difficult. 
     Referring to  FIG. 2 , a mask, in accordance with one embodiment, may include main features D (e.g., D 1 , D 2 , D 3 , etc.), which may be contacts in one embodiment, and a cross-shaped sub-resolution assist feature  140  in the form of a grid pattern. The sub-resolution assist feature  140  is an element in a mask layout that enhances the printing of main features using the mask. 
     As used herein, “cross-shaped” simply means having at least two crossed segments. Those segments may be straight or curved. They may intersect perpendicularly or non-perpendicularly. The segments may have the same or different thicknesses. The term “cross-shaped” is not intended to refer to the religious symbol. 
     Of course, the main features D can be anything in other embodiments, including long rectangles and line/space patterns. The mask may be glass or chrome on the mask, resulting in a hole or island of photoresist. 
     Sub-resolution assist features are generally dimensioned below the resolution limit of the system  100  used to print the mask. Sub-resolution assist features can be adapted to a variety of masks and a variety of patterning systems. For example, sub-resolution assist features can direct electromagnetic radiation with different transmissivities and/or phases. Sub-resolution assist features can transmit or reflect the electromagnetic radiation used to print or the sub-resolution assist features can block or fail to reflect the electromagnetic radiation used to print (depending, e.g., on whether positive or negative photoresist materials are used). Thus, depending on the nature of the mask, the sub-resolution assist feature  140  can transmit/reflect/block/failure to do the same with different transmissivities and/or phases, as appropriate. 
     In the embodiment shown in  FIG. 2 , the contacts or main features D are joined physically by the intervening portions of the cross-shaped sub-resolution assist feature  140 . Thus, adjacent main features D 1  and D 2  may be physically joined by an intervening sub-resolution assist feature  140  horizontal segment S 1 . Similarly, main features D 1  and D 3  are joined by vertical sub-resolution assist feature  140  and vertical segment S 2 . In this example, the segments S 1  and S 2  are perpendicular to one another. 
     In some embodiments, the use of the sub-resolution assist feature that connects main features contacts may improve the aerial image contrast. The sub-resolution assist feature connects to the desired main features, such as the contacts D, so sub-resolution assist feature connects to the desired feature in both dimensions and is placed in open areas surrounding the desired feature. As a result, a regular diffraction pattern at a constant pitch is created. 
     The placement of sub-resolution assist features in both the x and y directions through the center of the desired feature, connecting to the main feature to adjacent features, may improve resolution in some embodiments. A patterning process window enhancement may be accomplished, in some embodiments, by optimizing the optical illumination conditions to the pitch of the sub-resolution assist feature. 
     Referring to  FIG. 3 , in accordance with still another embodiment of the present invention, the main features, such as contacts A, B, and C, are joined by sub-resolution assist strips  12  in the vertical direction and sub-resolution assist strips  14  in the horizontal direction. Note that the spacing between the vertical strips  12 , indicated as x in  FIG. 3 , may be less than the vertical spacing, indicated by y, in  FIG. 3 . 
     Referring to  FIG. 4 , each sub-resolution assist feature  16  has an isolated cross-shape uncoupled to other sub-resolution or assist features  16 . Thus, each of the sub-resolution assist features  16  may be represented by a plus or cross-shape which is centered on each main feature A, B, or C. As a result, the sub-resolution assist feature  16  extends from the center of the main feature A, B, or C and extends in the form of an upward vertical straight segment  12  and a horizontal straight segment  14  which intersect centrally on the feature A, B, or C. 
     The use of sub-resolution assist features in a cross-shape may enhance the patterning of isolated, semi-dense, and dense arrays of main features, such as contacts, by providing a regular array of sub-resolution assist features. 
     In some embodiments, no differential sizing need be applied to any of the main features in the array based on their proximity. Thus, the cross-shaped sub-resolution assist features are able to create a pseudo nested environment, allowing for process optimization at one pitch only, rather than trying to balance nested, semi-dense, and isolated process windows, in some embodiments. 
     References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.