Abstract:
Aligning metal fill shapes with corresponding holes of a metal shield is provided. The holes of the metal shield are laid out corresponding to a pre-selected grid referenced to a pre-selected origin. The metal fill shapes of the metal fill pattern, are arranged in accordance with the same pre-selected grid and referenced to the same pre-selected origin. Accordingly, regardless of the size or spacing of the metal fill holes, a metal fill shape will substantially align with a corresponding metal fill hole. Such alignment between metallization levels and the structure of the metal shield and metal fill shape pattern enhance the electric noise blocking properties of the metal shield in conjunction with the metal fill shape.

Description:
DESCRIPTION 
   1. Field of the Invention 
   The invention relates to electrical shielding, and more particularly to electrical shielding for integrated circuits and a method thereof. 
   2. Background Description 
   In integrated circuit (IC) circuit design, large expanses of metal areas may be necessary in certain areas of the circuit. Such large metal sheets or shields are frequently a fraction of a micron thick and tens of microns wide and long, resulting in a thin sheet of metal being fabricated in that particular area. Such large metal areas are frequently used to shield electronic noise produced by a nearby portion of the circuit. 
   Such large areas of metal are frequently fabricated using a damascene process such as, a copper damascene process. Additionally, the large metal areas are frequently designed to have holes in them so that the large area of metal is not continuous, but rather interrupted by isolated islands of substrate material which is level with the top of the metal. These isolated islands of dielectric are referred to as holes and are typically incorporated into the circuit design as an aid in the fabrication process, such as, for example, maintaining a uniform thickness of the metal. For example, in a copper damascene fabrication process, the holes allow the substrate contained therein to act as polish stops during the polishing step and help promote a uniform metal thickness. Accordingly, a large metal area or sheet may have multiple holes arranged throughout its area. 
   When a large metal area having holes is included in a particular layer, it is common for at least one of the adjacent layers, either above or below the metal sheet to include a pattern of metal fill. The metal fill includes metal fill shapes which are typically approximately the size of the holes on the adjoining layer. 
   Typical design processes include orienting the holes in the metal sheet relative to the borders of the metal sheet and relative to the area occupied by the metal sheet. Furthermore, typical design processes for the metal fill includes orienting the metal fill relative to the border and surrounding area of the metal fill area. Typically, each metal sheet and the metal fill is optimized for its own corresponding level and local environment. Accordingly, there is typically little design consideration given to the relationship between the holes pattern, spacing, and density as related to the pattern, spacing and density of the adjoining metal fill. 
   SUMMARY OF THE INVENTION 
   In a first aspect of the invention, a method of electrical shielding includes arranging an electrical shield on a first layer of an integrated circuit, and arranging a pattern of holes in the electrical shield, wherein the pattern of holes is positioned a prescribed distance from a reference point. The method also includes arranging a corresponding pattern of metal fill shapes on a second layer of the integrated circuit, wherein the pattern of metal fill shapes is positioned the prescribed distance from the reference point. 
   In another aspect of the invention, a method of forming a metal shield structure in an integrated circuit includes defining a hole grid having hole centers defining locations for holes on the hole grid, and defining a fill grid having fill centers defining a location for fills on the fill grid. The method also includes positioning the hole grid on a first level relative to a reference point, and positioning the fill grid on a second level relative to the reference point such that at least one fill center substantially aligns with a corresponding hole center. 
   In another aspect of the invention, an apparatus for shielding an integrated circuit includes a metal shield on a first level having holes arranged in a pattern, wherein centers of the holes on the pattern define a grid referenced to a reference point. The apparatus also includes metal fill shapes on a second layer arranged in a pattern, wherein centers of the metal fill shapes define a grid referenced to the reference point, and at least one center of a metal fill shape substantially aligns with a center of a corresponding hole. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of a metal shield with holes; 
       FIG. 2  is an illustration of an embodiment of metal fill shapes aligned to holes in accordance with the invention; 
       FIG. 3  is an illustration of a metal shield having different size holes; and 
       FIG. 4  is an illustration of an embodiment of metal fill shapes aligned with different sized holes in accordance with the invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
   Embodiments of the invention are directed to aligning a metal fill of a metal shield on a layer with corresponding holes in a metal shield of another layer, regardless of the chosen pattern, density and pitch of the holes in the metal shield. The alignment between the holes and the corresponding metal fill is achieved, for example, by using the same pattern for the metal fill as used for the corresponding holes, and referencing both patterns to the same point on the wafer. Thus, as the metal shield is specified to have a certain shape, pitch and density of holes as well as a pattern origin, the corresponding metal fill will also have the same geometric specifications assuring that each metal fill shape aligns with the corresponding hole. The alignment between metal fill and the holes of a metal shield provides improved RF shielding. Additionally, the alignment method simplifies the design process by allowing easy alignment between holes and corresponding metal fill regardless of location on a wafer. 
     FIGS. 1-4  represent respective metal shields, fill shapes and corresponding holes. However, in view of the below discussion it should be recognized that  FIGS. 1-4  may equally represent a method of electrical shielding in an integrated circuit. By way of example,  FIG. 3  may represent forming a metal shield having holes on a first level and  FIG. 4  represents forming a metal fill having metal fill shapes on a second level. The method includes forming the metal fill shapes to align with and have a corresponding shape to the holes. The method further includes shielding a circuit by forming the holes and metal fill shapes in alignment with one another. 
   Referring now to  FIG. 1 , a metal shield  10  is shown having holes  12  therein. The holes  12  in the metal shield  10  may aid in the fabrication of the metal shield, for example, functioning as polish stops during a polishing step. The metal shield  10  may be made from any conductor including copper. The metal shield  10  is typically a few tenths of microns in thickness, and tens or many tens of microns in length and width. Although the metal shield  10  is shown as a rectangle, any general shape of a metal shield may be used. Accordingly, the metal shield may be square, circular, triangular, elliptical, etc. The holes  12  in the metal shield  10  are typically a uniform pattern of repeating holes spaced across the metal shield  10 . Although in this example, holes  12  are shown as squares, the holes may generally have any shape in addition to the square holes  12  shown, including rectangle, triangle, circle and ellipse. 
   Generally, the holes  12  may range from a width of a tenth of a micron to many tenths of a micron, and even larger. Typically, the size of the holes  12  is a multiple of the minimum width achievable in the prescribed fabrication process. Thus, for example, where the minimum width achievable in the fabrication process is 0.14 microns (μm), a typical hole size in a metal shield will be about three times that size, i.e. 0.42 μm. Generally, the holes  12  are dimensioned to be as small as easily achievable in the fabrication process. 
   The placement of the holes, or “hole pattern” includes indicating a size of the hole  12 , the location of one hole  12  as referenced by an origin on the wafer, and the hole pattern, and hole density. Thus, the hole pattern can include virtually any desired spacing and orientation and is laid out with reference to an easily identifiable point on the wafer. 
   Referring to  FIG. 2 , the metal shield  10  including the holes  12  is shown with a corresponding metal fill pattern  14  overlaid thereupon. The metal fill pattern  14  includes individual metal fill shapes  16 . As can be seen, some of the metal fill shapes  16  align with a corresponding hole  12 . Accordingly, the center of each metal fill shape  16  substantially corresponds to the center of each corresponding hole  12 . Additionally, each shape  16  may be created to be slightly larger than the corresponding hole  12 . As such, there may be overlap between each metal fill shape  16  and a portion of the metal shield  10  surrounding the corresponding hole  12 . The metal shield  10  may be placed on a layer over an integrated circuit with a perimeter defining, for example, a square  11 . 
   The metal fill pattern  14  generally has the same pattern, distribution, and pitch as the holes  12 . Additionally, each metal fill shape  16  generally has the same shape as the corresponding hole  12 . The metal fill pattern  14  also is referenced to the same origin as the holes  12 . Accordingly, the metal fill shapes  16  will align with each corresponding holes  12 . 
   The metal fill shapes  16  are on the same center as the corresponding hole  14  and have the same shape. Additionally, the dimensions of the metal fill shapes  16  are typically about 50% larger in the x and y directions as the corresponding hole  14 . Thus, a metal fill shape  16  is typically about 4.5 times the size of the minimum feature which may be fabricated by the particular fabrication process. This typically is the minimum line width and minimum space between wires achievable by the process. 
   The pattern of the holes  12  and metal fill shapes  16  are each placed on a single grid referenced to the same point such as the center or lower left region of the chip or wafer, for example, and thus are forced to coincide with one another, regardless of which level each may be formed on. The holes  12  and metal fill shapes  14  may be placed by specifying the location of the first shape, and specifying the periodicity, i.e., pitch of the pattern, for example. Accordingly, as long as the holes  12  and metal fill shapes  16  have the same origin and the same periodicity, each will line up with the other. 
   Typically the metal fill shapes  16  are applied using the same process as forming the metal shield  10 . For example, the metal fill shapes  16  may be made of copper and formed by a copper damascene process. Thus, the metal fill shapes  16  may be formed by etching a trench in a substrate where the trench has the final desired shape of the metal fill shapes  16 . Copper is then deposited in the trench and on the surrounding top surface of the substrate. After deposition, the copper is removed with a polishing process such as, for example, chemical mechanical polishing (“CMP”). The resulting metal fill shape  16  will then be flush with the surrounding top surface of the supporting substrate and have the shape of the original trench. 
   Referring to  FIG. 3 , a first metal shield  18  and a second metal shield  22  are illustrated. The first metal shield  18  has first holes  20  therein. The second metal shield  22  has second holes  24  therein. The first holes  20  are smaller than the second holes  24 . It should be noted that, even though the two sets of holes are of different sizes, the centers for the first holes  20  and the centers for the second holes  22  are arranged on the same grid, both oriented to the same origin. Furthermore, the centers of the first holes  20  and the second holes  24  have the same pitch and spacing as one another. 
   Due to the overlap between the metal shield and the corresponding hole around the perimeter of the hole, some mis-alignment between the hole pattern and the metal fill pattern may be tolerated. Additionally, the overlap reduces electronic leakage from passing through the metal shield and metal fill shape structure. A wide range of overlap may be utilized between the two layers, and the overlap may, or may not, occur along all edges of the hole and metal fill shape combination. Additionally, such overlap is not a required feature. Thus, the metal fill shape pattern which does not provide overlap may be preferable under certain circumstances and is easily accommodated by embodiments of the invention. 
   Referring to  FIG. 4 , the first metal shield  18  and the second metal shield  22  of  FIG. 3  are illustrated being overlaid with a metal fill pattern  26 . The metal fill pattern  26  includes metal fill shapes  28 . In this embodiment of the invention, the metal fill shapes  28  are of a uniform size. The metal fill shapes  28  are a uniform size because the size of the metal fill shapes  28  are sufficient to cover both the first holes  20  and the second holes  24 . Accordingly, the metal fill shapes  28  covering the first holes  20  has a larger overlap with the first metal shield  18  along the edges of the metal fill shapes  28 . The metal fill shapes  28  covering the second holes  24  of the second metal shield  22  has a relatively smaller overlap with the second metal shield  22  around the perimeter of the second holes  24 . The metal shields  18  and  22  may be placed on a layer over an integrated circuit, with a perimeter defining, for example, a square  19 . 
   As can be seen, the metal fill shapes  28  of the metal fill pattern  26  are arranged on a grid where the metal fill shapes  28  have centers which correspond to the centers of both the first holes  20  and the second holes  24 . Additionally, the pattern of the first holes  20  and the second holes  24  are on the same grid. Thus, the single pattern of the metal fill shapes  28  of the metal fill pattern  26  corresponds to the pattern of both first and second holes  20  and  24 , of the metal shields  18  and  22 , respectively. 
   Although a portion of the metal fill shapes  28  which correspond to the region between the first metal shield  18  and the second metal shield  22  does not correspond to holes of either metal shield,  18  and  22 , metal fill shapes occupy that region nonetheless. The metal fill shapes  28  are passive components, and the lack of correspondence hole and metal fill between the first and second metal shield  18  and  22  has substantially no effect on the functionality of the metal fill pattern  26 . 
   The first holes  20 , the second holes  24 , and the metal fill shapes  28  are designed to each have their centers positioned on a grid having the same spacing and density, as well as each being referenced to the same reference point. Thus, regardless of whether a particular hole is located on the first metal shield  18  or the second metal shield  22 , the particular hole will align with a corresponding metal fill shape  28  of the metal fill pattern  26 . Additionally, because the first and second fill holes,  20  and  24 , are arranged on the same grid, the first and second holes  20  and  24  will align with a corresponding metal fill shape  28  regardless of the size of the particular hole. 
   While the invention has been described in terms of exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modifications and in the spirit and scope of the appended claims.