Patent Publication Number: US-11657202-B2

Title: Aware variable fill pattern generator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Under 35 U.S.C. § 120, this continuation application claims benefits of and priority to U.S. patent application Ser. No. 14/934,130, filed Nov. 5, 2015 and issued as U.S. Pat. No. 9,953,123, the entirety of which is hereby incorporated herein by reference. This application is related to U.S. patent application Ser. No. 15/949,460, issued as U.S. Pat. No. 10,210,300. This application is a continuation of U.S. patent application Ser. No. 16/264,719, issued as U.S. Pat. No. 10,713,408, the entirety of which is hereby incorporated herein by reference. 
    
    
     FIELD 
     This disclosure relates to the field of integrated circuits. More particularly, this disclosure relates to pattern fill structures in integrated circuits. 
     BACKGROUND 
     Integrated circuits contain several physical layers of circuit structures. The physical layers may include, for example, a layer of metal interconnects formed by a copper damascene process, which uses a copper chemical mechanical polish (CMP) process. The circuit structures include the metal interconnects which provide electrical connections between components of the integrated circuit. In other examples, the physical layers may include an active area layer laterally surrounded by a layer of field oxide formed by a shallow trench isolation (STI) process which uses an oxide CMP process, a layer of gates for metal oxide semiconductor (MOS) transistors formed by a polycrystalline silicon reactive ion etch (RIE) process, or a layer of metal interconnects formed by an aluminum ME process. The physical layers are formed using etch masks on the integrated circuit, which are patterned using photolithographic processes. CMP processes, ME processes, photolithographic processes, and other fabrication processes, have higher process latitude when the physical layer has increased uniformity across the integrated circuit. Functional structures are understood to be structures of the physical layers which are included in the functional design of the integrated circuit and may be related to operation of the integrated circuit. Other structures of the physical layers may include alignment marks, die marks, and test structures. 
     The process masks to form the physical layers are formed using photomasks which have mask elements corresponding to the structures of the physical layers. The photomasks are formed using layout files which include geometries corresponding to the mask elements of the photomasks. It is common to add fill geometries for fill structures to the layout files for making photomasks used in fabricating the integrated circuits. Fill geometries are sometimes referred to as fill pattern, dummy geometries or dummy pattern. The fill structures are not directly electrically connected to components of the integrated circuit. The fill geometries are added to improve pattern uniformity. Attaining uniform densities among various devices fabricated using a common process sequence has been problematic, thus undesirably reducing process latitude for the relevant operations. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the disclosure. This summary is not an extensive overview of the disclosure, and is neither intended to identify key or critical elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the disclosure in a simplified form as a prelude to a more detailed description that is presented later. 
     An integrated circuit is formed by computing local densities of drawn geometries of a layout file for a physical layer of the integrated circuit in windows in an area for the integrated circuit, generating a pluralities of variable fill geometries for the layout file in local areas associated with the windows, so that the variable fill geometries do not have the same density in every local area, and storing the layout file in a computer-readable medium. Densities of the fill geometries are higher in local areas associated with windows having lower densities of the drawn geometries. 
    
    
     
       DESCRIPTION OF THE VIEWS OF THE DRAWING 
         FIG.  1    is a flowchart of an example method of forming an integrated circuit. 
         FIG.  2    is a depiction of a layout file, showing an example arrangement of windows and local areas. 
         FIG.  3    is a depiction of a layout file, showing an example of variable fill geometries. 
         FIG.  4    depicts the layout file being stored in a computer-readable medium. 
         FIG.  5    depicts a photomask formed for formation of the physical layer of the integrated circuit. 
         FIG.  6    depicts the integrated circuit during formation, showing the physical layer. 
         FIG.  7    depicts variable fill geometries in two different layout files for two different integrated circuits. 
         FIG.  8    depicts variable fill geometries in two different layout files for two different integrated circuits. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The present disclosure is described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the disclosure. Several aspects of the disclosure are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide an understanding of the disclosure. One skilled in the relevant art, however, will readily recognize that the disclosed material can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the disclosure. The present disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present disclosure. 
     A layout file, such as a Graphic Database System II (GDSII) file or Open Artwork System Interchange Standard (OASIS) file, for an integrated circuit includes circuit geometries for a physical layer of an integrated circuit. The layout file may include other geometries for the physical layer, such as alignment marks, die marks, and test structures. The circuit geometries, alignment marks, die marks, and test structures and other geometries are referred to as drawn geometries, even though these geometries may be generated by a computer program rather than being drawn manually. A global density of the drawn geometries for the physical layer is computed; the global density is an area of the drawn geometries for the physical layer, at a 1:1 scale with the integrated circuit, divided by an area of the integrated circuit. Some layout files may scale the geometries to account for a magnification factor, such as 4 or 5, in a photomask formed using the layout file. Other layout files may scale the geometries by other factors. For the purposes of this disclosure, scaling is ignored and all dimensional relationships are expressed in units at a 1:1 scale with the integrated circuit. The global density is less than a target density for the physical layer. 
     A plurality of windows and corresponding local areas are defined in the area for the integrated circuit. Each window at least partially overlaps the corresponding local area. In one example, the windows may be larger than, and completely overlap, the corresponding local areas. In another example, the windows may be a same size as, and coincident with, the corresponding local areas. In one example, the plurality of local areas may be defined to completely cover the area for the integrated circuit. In another example, the local areas may be defined in certain portions of the integrated circuit and not in other portions. In each window, a local density of the drawn geometries for the physical layer is computed; the local density is defined as an area of the drawn geometries within the window divided by an area of the window. Variable fill geometries may be added to the layout file in a fill portion of the local area. A fill density of the variable fill geometries is dependent on the global density, the local density and the target density. The fill density, which is an area of the variable fill geometries in the fill portion of the local area, divided by an area of the fill portion, increases as a difference between the global density and the target density increases, and as a difference between the local density and the target density increases, when the local density is less than the target density. The fill density varies among the local areas. The variable fill geometries may be selected from a finite set of patterns, or may be selected from a continuous distribution of patterns. 
     The physical layer in the integrated circuit is formed using the layout file with the variable fill geometries. In one example, a Manufacturing Electron Beam Exposure System (MEBES) file is generated using the layout file. A photomask for the physical layer may be formed using the MEBES file. The photomask includes mask elements for the drawn geometries and the variable fill geometries. The photomask is used in a photolithographic process to form a process mask on the integrated circuit for the physical layer. In another example, the process mask may be formed using a maskless photolithographic using the layout file, possibly through an intermediate file such as a MEBES file. The physical layer in the integrated circuit includes fill structures corresponding to the variable fill geometries added to the layout file. 
     Adding the variable fill geometries to the layout file as described herein may provide physical layers of integrated circuit with global densities closer to the target density compared to other methods of adding fill geometries. Fabrication processes have higher process latitude when the global density of the physical layer is closer to the target density. The disclosed method may thus advantageously provide increased process latitude for a plurality of different integrated circuit designs. 
       FIG.  1    is a flowchart of an example method of forming an integrated circuit. The method begins with step  100 , which is to compute a first local density of drawn geometries of a layout file for a physical layer of the integrated circuit, in a first window of a plurality of windows in an area for the integrated circuit. The drawn geometries include circuit geometries, alignment marks, die marks, and test structures. The first window may have lateral dimensions of, for example, 50 microns to 1000 microns, and may have a square shape, a round shape, a rectangular shape, or other shape. The windows may each be a same size and/or shape, or the windows may have different sizes and/or shapes depending on a location in the integrated circuit or a type of underlying circuitry. For example, the windows may be larger over logic circuitry than over linear circuitry. Adjacent windows may abut each other or may partially overlap. In one version of the instant example, the plurality of windows may cover the area for the integrated circuit. In another version, the windows may not cover some portions of the area for the integrated circuit, such as memory cells or a periphery of the integrated circuit. To compute the first local density, an area of the drawn geometries within the first window, that is, a sum of the areas of those portions of the drawn geometries within the boundaries of the first window, is obtained. Overlapping portions of separate drawn geometries are not double counted, so that the sum of the areas of the drawn geometries in the first window corresponds to an area of elements of the physical layer in the first window. The sum of the areas of the drawn geometries in the first window is divided by an area of the first window to obtain the first local density. 
     The method continues with step  102 , which is to generate a first plurality of variable fill geometries for the layout file in a first fill portion of a first local area associated with the first window. The first local area is one local area of a plurality of local areas; each local area is associated with a corresponding window of the plurality of windows. In one version of the instant example, the first local area may be smaller than, and be completely overlapped by, the first window. In another version, the first local area may be a same size as, and coincident with, the first window. In one version of the instant example, adjacent local areas may abut. In another version, adjacent local areas may be separated by a space. The first local area may have lateral dimensions of, for example, 50 microns to 1000 microns, and may have a square shape, a round shape, a rectangular shape, or other shape. The first fill portion of the first local area is a portion of the first local area which is free of drawn geometries and is available for fill patterns, compatible with design rules for the integrated circuit. 
     The geometries of the first plurality of variable fill geometries are selected to have a first fill density which is a function of the first local density, a global density for the physical layer of the integrated circuit, and a target density for the physical layer. The first fill density is an area of the first plurality of variable fill geometries, that is, a first fill structure area, divided by an area of the first fill portion. The global density for the physical layer of the integrated circuit is an area of the drawn geometries for the physical layer divided by an area of the integrated circuit. The target density is determined by capabilities of the process operations used to form the physical layer. For example, a target density for a copper damascene process may be 45 percent. The first local density and the global density are both lower than the target density. Variable fill geometries with higher fill densities are selected for instances in which the global density is lower and the first local density is lower. In one example, the target density may be 45 percent, the global density may be 33 percent, and the first local density may be 33 percent, in which case variable fill geometries with a fill density of 23 percent to 27 percent may be added to the first fill portion. In another example, the target density may be 45 percent, the global density may be 33 percent, and the first local density may be 42 percent, in which case variable fill geometries with a fill density of 13 percent to 17 percent may be added to the first fill portion. In further example, the target density may be 45 percent, the global density may be 38 percent, and the first local density may be 35 percent, in which case variable fill geometries with a fill density of 18 percent to 22 percent may be added to the first fill portion. The variable fill geometries may be selected from a plurality of configurations, each having a different fill density, by using a lookup table which has selection choices based on pluralities of ranges for the target density, the global density and the first local density. The first plurality of variable fill geometries is added to the layout file in the first fill portion of the first local area. 
     The method continues with step  104  which is to compute a second local density of the drawn geometries of the layout file, in a second window of the plurality of windows. The second window may have a same size and shape as the first window, or may have a different size and/or shape. The second local density is computed similarly to the first local density: areas of portions of the drawn geometries within the boundaries of the second window are summed without double counting, and the sum of the areas of the drawn geometries in the second window is divided by an area of the second window to obtain the second local density. In the instant example, the second local density is less than the first local density. 
     The method continues with step  106 , which is to generate a second plurality of variable fill geometries for the layout file in a second fill portion of a second local area associated with the second window. The second local area is another local area of the plurality of local areas. The second local area may be smaller than, and be completely overlapped by, the second window, or may be a same size as, and coincident with, the second window, independently of the relationship between the first window and the first local area. The second window may have lateral dimensions of, for example, 50 microns to 1000 microns, and may have a square shape, a round shape, a rectangular shape, or other shape. The second fill portion of the second local area is a portion of the second local area which is free of drawn geometries and is available for fill patterns compatible with design rules for the integrated circuit. 
     The geometries of the second plurality of variable fill geometries are selected to have a second fill density which is a function of the second local density, the global density, and the target density. The second fill density is an area of the second plurality of variable fill geometries, that is, a second fill structure area, divided by an area of the second fill portion. The relationship between the second fill density and the second local density, the global density, and the target density is as described for the first fill density in reference to step  102 . In the instant example, the second fill density is greater than the first fill density. The second plurality of variable fill geometries is added to the layout file in the second fill portion of the second local area. 
     The method continues with step  108  which is to store the layout file with the added variable fill geometries, including the first plurality of variable fill geometries and the second plurality of variable fill geometries, in a computer-readable medium. The computer-readable medium may be, for example, a magnetic hard disk, an optical memory disk such as a CD-ROM, a magnetic tape, a flash memory, a dynamic random access memory (DRAM), or a static random access memory (SRAM). 
     After step  108  is completed, the stored layout file from step  108  may subsequently be read by a computer, a photomask generating tool, or a semiconductor processing tool. In one version of the instant example, the stored layout file from step  108  be read by a computer, and the computer may generate a MEBES file using the information from the layout file, including the added variable fill geometries. The MEBES file may be subsequently used to form a photomask for the physical layer, so that the photomask contains mask elements corresponding to the added variable fill geometries. The photomask may be used to form a process mask, such as an etch mask, for the physical layer on the integrated circuit by a photolithographic process. In another version of the instant example, the stored layout file from step  108  be read by a computer, and the computer may generate a MEBES file, which may be used in a direct-write lithography tool such as an electron beam lithography tool, to form a process mask for the physical layer on the integrated circuit. 
     The method continues with step  110 , which is to fabricate the integrated circuit using the process mask formed with the layout file with the added variable fill geometries. The process mask includes elements corresponding to the added variable fill geometries. The process mask may be used for an etch process in forming the integrated circuit, and may be removed after the etch process is completed. For some physical layers, for example copper damascene interconnects or STI field oxide, the etch process may form trenches which are subsequently filled by copper or silicon dioxide, respectively. Excess copper or silicon dioxide is removed by a CMP process. Forming the process mask with the elements corresponding to the variable fill geometries may provide a more uniform pattern to be polished which is closer to a target density, advantageously improving latitude for the CMP process, resulting in more uniform removal of the copper or silicon dioxide across the integrated circuit. For other physical layers, a layer of material, such as polycrystalline silicon or aluminum, may be formed on the integrated circuit and the process mask formed over the layer of material. The etch process removes material exposed by the process mask, leaving the physical layer under the mask. Forming the process mask with the elements corresponding to the variable fill geometries may provide a more uniform pattern to be etched which is closer to a target density, advantageously improving latitude for the etch process, resulting in more uniform undercut and overetch of the layer of material across the integrated circuit. Alternately, the process mask may be used for an ion implant process, and may be removed after the implant process is completed. Forming the process mask with the elements corresponding to the added variable fill geometries may provide a more uniform pattern for the process mask to be formed which is closer to a target density, advantageously improving latitude for the photolithographic or direct-write lithographic process, resulting in more accurate dimensions of the process mask across the integrated circuit. 
       FIG.  2    is a depiction of a layout file, showing an example arrangement of windows and local areas. The layout file  200  for an integrated circuit, indicated schematically in  FIG.  2    by a boundary around an area  202  for the integrated circuit, includes drawn geometries  204  for a physical layer of an integrated circuit. The drawn geometries  204  may be, for example, for metal interconnects formed by a copper damascene process. A plurality of windows  206  is defined in the area  202  covered by the layout file  200 . The windows  206  may have square or rectangular shapes, as depicted in  FIG.  2   , or may have other shapes such as rounded shapes. The windows  206  may have widths  208  and lengths  210  of 50 microns to 1000 microns. Adjacent windows  206  may be defined to abut each other without overlapping, as indicated in  FIG.  2   , or may be defined so as to overlap each other. Alternatively, adjacent windows  206  may be separated by a space. The windows  206  may be defined over all of the area  202  for the integrated circuit, or may be defined over a portion of the area  202 . For example, a portion of the area  202  for memory cells or logic gates may be free of the windows  206 . The windows  206  may be equal in size and shape, or may vary in size and/or shape across the area  202  for the integrated circuit. 
     A plurality of local areas  212  are defined, a separate local area  212  for each window  206 . The local areas  212  may be smaller than the associated windows  206  as depicted in  FIG.  2   , or may be substantially the same size. The local areas  212  may have widths  214  and lengths  216  of 50 microns to 1000 microns. In one version of the instant example, adjacent local areas  212  may abut each other without overlapping. The local areas  212  may be equal in size and shape, or may vary in size and/or shape across the area  202  for the integrated circuit. The drawn geometries  204  may extend into the local areas  212 , as shown in  FIG.  2   . 
       FIG.  3    is a depiction of a layout file, showing an example of variable fill geometries. The layout file  300  for an integrated circuit includes drawn geometries  304  for a physical layer of an integrated circuit. A plurality of windows, including a first window  306 , a second window  308  and a third window  310  are defined in the area covered by the layout file  300 , for example as described in reference to  FIG.  2   . A plurality of local areas are defined, with a separate local area for each window, including a first local area  312  associated with the first window  306 , a second local area  314  associated with the second window  308  and a third local area  316  associated with the third window  310 . The local areas may be defined as described in reference to  FIG.  2   . The drawn geometries  304  may extend into the local areas. 
     A global density for the physical layer of the integrated circuit is computed by dividing the areas of the drawn geometries  304  by an area of the integrated circuit. The global density is less than a target density. A plurality of local densities, one for each window, is computed by dividing the areas of the drawn geometries within the window by an area of the window. The plurality of local densities includes a first local density which is the areas of the drawn geometries  304  within the first window  306 , a second local density which is the areas of the drawn geometries  304  within the second window  308 , and a third local density which is the areas of the drawn geometries  304  within the third window  310 . In the instant example, the second local density is greater than the first local density which is greater than the third local density. 
     A fill portion is defined in each of at least a portion of the local areas, including a first fill portion  318  in the first local area  312 , a second fill portion  320  in the second local area  314  and a third fill portion  322  in the third local area  316 . Each fill portion is defined as a portion of the corresponding local area that is free of the drawn geometries  304  and separated from the drawn geometries  304  by a desired space. The desired space may be related to design rules for the physical layer of the integrated circuit. In some instances of the local areas, possibly no portion of the local area may meet the criteria for the fill portion, so that those local areas are free of fill portions. 
     A plurality of variable fill geometries are defined in the fill portions of the local areas, including a first plurality of variable fill geometries  324  in the first fill portion  318 , a second plurality of variable fill geometries  326  in the second fill portion  320 , and a third plurality of variable fill geometries  328  in the third fill portion  322 . The geometries of the first plurality of variable fill geometries  324  are selected to have a first fill density which is a function of the first local density, the global density, and the target density. Analogously, the geometries of the second plurality of variable fill geometries  326  are selected to have a second fill density which is similar function of the second local density, the global density, and the target density, and the geometries of the third plurality of variable fill geometries  328  are selected to have a third fill density which is similar function of the third local density, the global density, and the target density. Variable fill geometries with higher fill densities are selected for instances in which the global density is lower and the local density is lower. In the instant example, the third fill density of the third plurality of variable fill geometries  328  is higher than the first fill density of the first plurality of variable fill geometries  324 , which is higher than the second fill density of the second plurality of variable fill geometries  326 . 
     In the instant example, the first plurality of variable fill geometries  324 , the second plurality of variable fill geometries  326  and the third plurality of variable fill geometries  328  are defined to have a common horizontal pitch distance  330  and a common vertical pitch distance  332 . The horizontal pitch distance  330  is a horizontal distance  330  between centers of adjacent fill geometries of the first plurality of variable fill geometries  324 , the second plurality of variable fill geometries  326  and the third plurality of variable fill geometries  328 . In the instant example, the individual geometries of the first plurality of variable fill geometries  324  have equal sizes to each other, the individual geometries of the second plurality of variable fill geometries  326  have equal sizes to each other, and the individual geometries of the third plurality of variable fill geometries  328  have equal sizes to each other. The size of the individual geometries of the third plurality of variable fill geometries  328  is larger than the size of the individual geometries of the first plurality of variable fill geometries  324 , which is larger than the size of the individual geometries of the second plurality of variable fill geometries  326 , thus providing the relationship between the third fill density, the second fill density and the first fill density described above for the instant example. The variable fill geometries, including the first plurality of variable fill geometries  324 , the second plurality of variable fill geometries  326  and the third plurality of variable fill geometries  328 , are added to the layout file  300 . 
     Other configurations for the first plurality of variable fill geometries  324 , the second plurality of variable fill geometries  326  and the third plurality of variable fill geometries  328  which provide the relationship between the third fill density, the second fill density and the first fill density described above are within the scope of the instant example. In another version of the instant example, the individual geometries of the first plurality of variable fill geometries  324 , the second plurality of variable fill geometries  326  and the third plurality of variable fill geometries  328  may have a common size, with varying horizontal  330  and vertical  332  pitch distances. In one version of the instant example, the first fill portion  318  may be free of any other fill geometries which have a different size from the first plurality of variable fill geometries  324 , the second fill portion  320  may be free of any other fill geometries which have a different size from the second plurality of variable fill geometries  326 , and the third fill portion  322  may be free of any other fill geometries which have a different size from the third plurality of variable fill geometries  328 . 
     Referring to  FIG.  4   , the layout file  300  including the drawn geometries  304  and the variable fill geometries  334  are stored in a computer-readable medium  336 . The variable fill geometries  334  include the first plurality of variable fill geometries  324 , the second plurality of variable fill geometries  326  and the third plurality of variable fill geometries  328 , not shown in  FIG.  4   , described in reference to  FIG.  3   . The drawn geometries  304  and the variable fill geometries  334  may be stored as ASCII characters or binary sequences, corresponding to coordinates of vertices, or corresponding to coordinates of reference points and associated lateral dimensions, or other formats. The computer-readable medium  336  may be a memory unit such as a fixed hard disk in a computer  338  used for generating layout files such as the layout file  300 . The computer-readable medium  336  may be a removable memory unit such as a removable hard disk, a tape unit or a solid state memory card disposed in the computer  338  for the purpose of storing the layout file  300 . The computer-readable medium  336  may be a memory unit connected to a computer network, shared by several computers used for designing layout files. Other manifestations of the computer-readable medium  336  are within the scope of the instant example. 
     Referring to  FIG.  5   , a photomask  500  may be formed for formation of the physical layer of the integrated circuit. The photomask  500  is formed using the layout file  300  stored in the computer-readable medium  336  of  FIG.  4   . The photomask  500  includes a substrate  502  which transmits ultraviolet, deep ultraviolet, extreme ultraviolet and/or X-rays, depending on the photolithography process used to form the physical layer. The substrate  502  may include, for example, low thermal expansion glass, quartz, or sapphire. The photomask  500  includes optical elements  504  corresponding to the geometries of the layout file  300 . The optical elements  504  include a first plurality of optical elements  506  corresponding to the drawn geometries  304 , and a second plurality of optical elements  508  corresponding to the variable fill geometries  334 , of  FIG.  3   . The optical elements  504  may be, for example, optically dense or opaque elements of chromium, as depicted in  FIG.  5   . Alternatively, the optical elements  504  may be optically transmissive elements surrounded by an optically dense or opaque layer. The optical elements  504  may include phase shift layers. A pellicle, not shown in  FIG.  5   , may be added to the photomask  500  to protect the optical elements  504 . 
       FIG.  6    depicts the integrated circuit during formation, showing the physical layer. The integrated circuit  600  is formed in and on a substrate  602  including a semiconductor material. The substrate  602  may be, for example, a silicon wafer, possibly with an epitaxial layer of silicon at a top surface. Alternately, the substrate  602  may be a silicon-on-insulator (SOI) wafer. The semiconductor material may include silicon, or may include III-V material such as gallium arsenide or gallium nitride. The substrate  602  may include one or more dielectric layers over the semiconductor material. Metal interconnects may be formed in the dielectric layers to provide electrical connections to components in the integrated circuit  600 . The physical layer  604  is formed in and/or on the substrate  602 . The physical layer  604  may be, for example, metal interconnects formed by a copper damascene process and having a copper damascene structure, or active areas laterally surrounded by field oxide formed by an STI process and having an STI structure. An example copper damascene process includes forming interconnect trenches in a dielectric layer of the substrate  602 , forming a metal liner of tantalum nitride in the interconnect trenches and over a top surface of the dielectric layer, forming a layer of copper on the metal liner, and removing the copper and the metal liner from over the top surface of the dielectric layer using a copper CMP process. An example copper damascene structure has the metal interconnects disposed in interconnect trenches in a dielectric layer, the metal interconnects including a metal liner on bottoms and sidewalls of the interconnect trenches and copper on the metal liner in the interconnect trenches. An example STI process includes forming a silicon nitride active area mask over the semiconductor material of the substrate  602 , forming isolation trenches in the semiconductor material exposed by the active area mask, forming a layer of thermal oxide on bottoms and sidewalls of the isolation trenches, forming a layer of silicon dioxide-based dielectric material on the layer of thermal oxide in the trenches and over the active area mask, removing the dioxide-based dielectric material from over the active area mask using an oxide CMP process, and removing the active area mask. An example STI structure has the field oxide disposed in isolation trenches with a top surface of the field oxide within 100 nanometers of a top surface of the semiconductor material of the substrate  602 . Other physical layers of the integrated circuit  600  are within the scope of the instant example. The physical layer  604  includes a plurality of functional structures  606  corresponding to the drawn geometries  304  of  FIG.  3   . The plurality of functional structures  606  are directly electrically coupled, possibly through metal interconnects or other electrically conductive structure, to active components such as transistors of the integrated circuit  600 . The physical layer  604  also includes a plurality of fill structures  608  corresponding to the variable fill geometries  334  of  FIG.  3   . The plurality of fill structures  608  are not directly electrically connected to the active components of the integrated circuit  600 . The plurality of fill structures  608  have different densities in different regions of the integrated circuit  600 . The different densities reflect the different fill densities of the variable fill geometries of the layout file  300 . Forming the integrated circuit  600  with the plurality of fill structures  608  may advantageously provide more process latitude for forming the physical layer  604  compared to a comparable integrated circuit with no structures corresponding to variable fill geometries. 
       FIG.  7    depicts variable fill geometries in two different layout files for two different integrated circuits. A first layout file  700  for a first physical layer of a first integrated circuit includes drawn geometries  702 . The first physical layer may be, for example, a metal one interconnect layer formed by a copper damascene process. A plurality of windows are defined in an area for the first integrated circuit, including a first window  704 . A plurality of local areas are defined in the area for the first integrated circuit, one local area per window, including a first local area  706  for the first window  704 . In the instant example, the first window  704  is coincident with the first local area  706 ;  FIG.  7    shows the first window  704  slightly larger than the first local area  706  to depict their separate boundaries, even though the boundaries are coincident. A first fill portion  708  of the first local area  706  is defined as a portion of the first local area  706  that is free of the drawn geometries  702  and is available for fill geometries, consistent with design rules for the physical layer of the first integrated circuit. A first global density for the physical layer of the first integrated circuit is an area of the drawn geometries  702  for the physical layer divided by an area of the first integrated circuit. The first global density is less than a target density for the physical layer. A first local density is computed as an area of the drawn geometries  702  within the first window  704  divided by an area of the first window  704 . The first local density is less than the target density. A first plurality of variable fill geometries  710  is generated for the first fill portion  708 . The first plurality of variable fill geometries  710  have a first fill density which is a function of the first local density, the first global density, and the target density. In the instant example, the first plurality of variable fill geometries  710  have rectangular shapes. Other shapes for the first plurality of variable fill geometries  710  are within the scope of the instant example. 
     A second layout file  712  for a second physical layer of a second integrated circuit includes drawn geometries  714 . The second physical layer provides the same function for the second integrated circuit that the first physical layer provides for the first integrated circuit. For example, in the case that the first physical layer is a metal one interconnect layer formed by a copper damascene process, as mentioned above, the second physical layer is also a metal one interconnect layer formed by a copper damascene process. The target density for the second physical layer of the second integrated circuit is the same target density for the first physical layer of the first integrated circuit. The second integrated circuit has a different design than the first integrated circuit. The instant example may occur, for example, when the first integrated circuit and the second integrated circuit are designed using a same set of design rules and formed in a same fabrication facility using a same process sequence. A second global density for the physical layer of the second integrated circuit is an area of the drawn geometries  714  for the physical layer divided by an area of the second integrated circuit. The second global density is less than the target density for the physical layer. In the instant example, the second global density is greater than the first global density. A plurality of windows are defined in an area for the second integrated circuit, including a second window  716 . In the instant example, the second window  716  is the same size and shape as the first window  704 , and a configuration of the drawn geometries  714  in the second window  716  is the same as a configuration of the drawn geometries  702  in the first window  704 , so that a second local density, computed as an area of the drawn geometries  714  within the second window  716  divided by an area of the second window  716 , is the same as the first local density. The first window  704  and the second window  716  may be located in similar circuits of the first integrated circuit and the second integrated circuit, respectively. A plurality of local areas are defined in the area for the second integrated circuit, one local area per window, including a second local area  718  for the second window  716 . In the instant example, the second window  716  is coincident with the second local area  718 ;  FIG.  7    shows the second window  716  slightly larger than the second local area  718  to depict their separate boundaries, even though the boundaries are coincident. A second fill portion  720  of the second local area  718  is defined as a portion of the second local area  718  that is free of the drawn geometries  714  and is available for fill geometries, consistent with design rules for the physical layer of the second integrated circuit. A configuration of the drawn geometries  714  in the second local area  718  is the same as a configuration of the drawn geometries  702  in the first local area  706 , so that the second fill portion  720  has a same size and shape as the first fill portion  708 . A second plurality of variable fill geometries  722  is generated for the second fill portion  720 . In the instant example, the second plurality of variable fill geometries  722  have the same rectangular shapes and size as the first plurality of variable fill geometries  710 ; the second plurality of variable fill geometries  722  are separated from each other by larger distances than the first plurality of variable fill geometries  710 . Other shapes for the second plurality of variable fill geometries  722 , including shapes different from the first plurality of variable fill geometries  710 , are within the scope of the instant example. The second plurality of variable fill geometries  722  have a second fill density which is a function of the second local density, the second global density, and the target density. In the instant example, the second fill density is less than the first fill density because the second global density is higher than the first global density, even though the second local density is the same as the first local density, which may advantageously provide more process latitude for forming the first physical layer of the first integrated circuit and the second physical layer of the second integrated circuit, compared to generating substantially equal fill densities in both the first fill portion and the second fill portion. 
     The first layout file  700  is stored in a first computer-readable medium, for example as described in reference to  FIG.  4   . The second layout file  712  is stored in a second computer-readable medium, which may possibly be the first computer-readable medium. The first integrated circuit is formed using the first layout file  700 , and the second integrated circuit is formed using the second layout file  712 , for example as described in reference to  FIG.  5    and  FIG.  6   . The first physical layer of the first integrated circuit includes a first plurality of structures corresponding to the drawn geometries  702  of the first layout file  700 , and a second plurality of structures corresponding to the variable fill geometries  710  of the first layout file  700 . Similarly, the second physical layer of the second integrated circuit includes a third plurality of structures corresponding to the drawn geometries  714  of the second layout file  712 , and a fourth plurality of structures corresponding to the variable fill geometries  722  of the second layout file  712 . 
       FIG.  8    depicts variable fill geometries in two different layout files for two different integrated circuits. A first layout file  800  for a first physical layer of a first integrated circuit includes drawn geometries  802 . The first physical layer may be, for example, an active area layer with field oxide formed by an STI process. A plurality of windows are defined in an area for the first integrated circuit, including a first window  804 . A plurality of local areas are defined in the area for the first integrated circuit, one local area per window, including a first local area  806  for the first window  804 . In the instant example, the first window  804  is larger than the first local area  806 , and the first local area  806  is not centered in the first window  804 . A first fill portion  808  of the first local area  806  is defined as a portion of the first local area  806  that is free of the drawn geometries  802  and is available for fill geometries, consistent with design rules for the physical layer of the first integrated circuit. A first global density for the physical layer of the first integrated circuit is an area of the drawn geometries  802  for the physical layer divided by an area of the first integrated circuit. The first global density is less than a first target density for the physical layer. A first local density is computed as an area of the drawn geometries  802  within the first window  804  divided by an area of the first window  804 . The first local density is less than the first target density. A first plurality of variable fill geometries  810  is generated for the first fill portion  808 . In the instant example, the first plurality of variable fill geometries  810  have chevron shapes, other shapes for the first plurality of variable fill geometries  810  are within the scope of the instant example. The first plurality of variable fill geometries  810  have a first fill density which is a function of the first local density, the first global density, and the first target density. In the instant example, the variable fill geometries  810  are added to the first fill portion  808  until the first fill density is attained, a region of the first fill portion  808  is free of the variable fill geometries  810 . 
     A second layout file  812  for a second physical layer of a second integrated circuit includes drawn geometries  814 . The second physical layer provides the same function for the second integrated circuit that the first physical layer provides for the first integrated circuit. For example, in the case that the first physical layer is an active area layer with field oxide formed by an STI process, as mentioned above, the second physical layer is also an active area layer with field oxide formed by an STI process. The second integrated circuit has the same design as the first integrated circuit, so that the drawn geometries  814  are substantially the same as the drawn geometries  702  of the first layout file. A second target density for the second physical layer of the second integrated circuit is lower than the first target density for the first physical layer of the first integrated circuit. The instant example may occur, for example, when the first integrated circuit and the second integrated circuit are formed using different process tools for the first physical layer and the second physical layer. For example, the first physical layer may be formed using a first oxide CMP tool optimized for the first target density, and the second physical layer may be formed using a second oxide CMP tool optimized for the second target density. A second global density for the physical layer of the second integrated circuit is an area of the drawn geometries  814  for the physical layer divided by an area of the second integrated circuit. In the instant example, the second global density is equal to the first global density. The second global density is less than the second target density for the physical layer. A plurality of windows are defined in an area for the second integrated circuit, including a second window  816 . In the instant example, the second window  816  is the same size and shape as the first window  804 , and is located at a same position as the first window  804 , so that a configuration of the drawn geometries  814  in the second window  816  is the same as a configuration of the drawn geometries  802  in the first window  804 , and so that a second local density, computed as an area of the drawn geometries  814  within the second window  816  divided by an area of the second window  816 , is the same as the first local density. A plurality of local areas are defined in the area for the second integrated circuit, one local area per window, including a second local area  818  for the second window  816 . In the instant example, the second local area  818  has the same spatial relationship to the second window  816  that the first local area  806  has to the first window  806 . A second fill portion  820  of the second local area  818  is defined as a portion of the second local area  818  that is free of the drawn geometries  814  and is available for fill geometries, consistent with design rules for the physical layer of the second integrated circuit. A configuration of the drawn geometries  814  in the second local area  818  is the same as a configuration of the drawn geometries  802  in the first local area  806 , so that the second fill portion  820  has a same size and shape as the first fill portion  808 . A second plurality of variable fill geometries  822  is generated for the second fill portion  820 . In the instant example, the second plurality of variable fill geometries  822  have chevron shapes similar to the first plurality of variable fill geometries  810 . The second plurality of variable fill geometries  822  have a second fill density which is a function of the second local density, the second global density, and the second target density. In the instant example, the variable fill geometries  822  are added to the second fill portion  820  until the second fill density is attained, a region of the second fill portion  820  is free of the variable fill geometries  822 . In the instant example, the second fill density is less than the first fill density because the second target density is lower than the first target density, even though the second local density is the same as the first local density and the second global density is the same as the first global density, which may advantageously provide more process latitude for forming the first physical layer of the first integrated circuit and the second physical layer of the second integrated circuit, compared to generating substantially equal fill densities in both the first fill portion and the second fill portion. 
     The first layout file  800  is stored in a first computer-readable medium, for example as described in reference to  FIG.  4   . The second layout file  812  is stored in a second computer-readable medium, possibly the first computer-readable medium. The first integrated circuit is formed using the first layout file  800 , and the second integrated circuit is formed using the second layout file  812 , for example as described in reference to  FIG.  5    and  FIG.  6   . The first physical layer of the first integrated circuit includes a first plurality of structures corresponding to the drawn geometries  802  of the first layout file  800 , and a second plurality of structures corresponding to the variable fill geometries  810  of the first layout file  800 . Similarly, the second physical layer of the second integrated circuit includes a third plurality of structures corresponding to the drawn geometries  814  of the second layout file  812 , and a fourth plurality of structures corresponding to the variable fill geometries  822  of the second layout file  812 . 
     While various examples of the present disclosure have been described above, it should be understood that they have been presented by way of example only and not limitation. Numerous changes to the disclosed examples can be made in accordance with the disclosure herein without departing from the spirit or scope of the disclosure. Thus, the breadth and scope of the present disclosure should not be limited by any of the above described examples. Rather, the scope of the disclosure should be defined in accordance with the following claims and their equivalents.