Abstract:
In this invention, by offering specific array-end structures and their fabrication method, the three resistive layers of diffusion bit line, control gate and word gate polysilicons, where control gate polysilicon can run on top of the diffusion bit line, are most effectively stitched with only three layers of metal lines keeping minimum metal pitches. The stitching method can also incorporate a bit diffusion select transistor and/or a control gate line select transistor. The purpose of the select transistors may be to reduce the overall capacitance of the bit line or control gate line, or to limit the disturb conditions that a grouped sub-array of cells may be subjected to during program and/or erase.

Description:
RELATED PATENT APPLICATION  
       [0001]    This application claims priority to U.S. Provisional Patent Application serial No. 60/278,622, filed on Mar. 26, 2001, which is herein incorporated by reference.  
         [0002]    U.S. patent application Ser. No. 09/810,122 filed on Mar. 19, 2001, assigned to the same assignee as the present invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1) Field of Invention  
           [0004]    The invention relates to stitching (strapping) methods of forming high-density Metal/polysilicon Oxide Nitride Oxide Silicon (MONOS) memory arrays with reduced bit line resistance, reduced control gate resistance and reduced word gate resistance using three-level metal lines, resulting in high density MONOS memory arrays with high performance.  
           [0005]    2) Description of Prior Art and Background  
           [0006]    Twin MONOS structures were introduced in the U.S. Pat. No. 6,255,166, and U.S. patent application Ser. Nos. 09/861,489 and 09/595,059 by Seiki Ogura et al. and also various array fabrication methods of the twin MONOS memory array were introduced in U.S. Pat. Nos. 6,177,318 and 6,248,633 B1 and U.S. patent application Ser. No. 09/994,084 filed on Nov. 21, 2001.  
           [0007]    The twin ballistic MONOS memory cell, illustrated in FIG. 1A, may be arranged into a bit diffusion array as follows: each memory cell contains two nitride regions  031  which comprise storage elements for one word gate  040 , and half a source diffusion and half a bit diffusion ( 003 ). The diffusion junctions are shared by two adjacent storage elements. Control gates can be defined separately ( 042 ) or shared together ( 043 ) over the same diffusion ( 003 ). The control gate is electrically isolated from the underlying diffusion junctions. Diffusions are shared between cells and run in parallel to the sidewall control gates ( 042 ), and perpendicular to the word line ( 041 ). The diffusion lines become bit lines.  
           [0008]    In a conventional MOSFET memory, a transistor structure consisting of one polysilicon gate between source and drain diffusions is used and word gate polysilicon line and diffusion bit lines are orthogonally placed. When the memory array gets large, the bit line (BL) and word gate line (WG) become long. The word line resistance due to the series of word gates is high in large memory devices. In order to reduce word line resistance, it is necessary to connect the word line periodically to a metal line that runs in parallel to the poly word lines. This is referred to as a “stitched” or “strapped” word line. Also the bit diffusion line can be sub-arrayed and the bit line can be “stitched” by a conductive metal line. In a typical memory, each polysilicon word line is stitched to a metal word line which runs on top of each poly word line, and each diffusion line, which runs orthogonally to the word lines is stitched by another layer of metal line.  
           [0009]    However, in the high-density twin MONOS cell shown in FIG. 1A, the transistor consists of three gates between source and drain diffusions. Three resistive layers of control gate and word gate and bit diffusion may need to be stitched to reduce resistance and to achieve the target performance. For higher density, the polysilicon control gate lines and diffusion bit lines may run in parallel to and on top of each other. If the cell is metal-pitch limited and requires stitching, that means that two additional layers, of metal lines have to run on top of and contact to the two resistive layers. This is a layout and process challenge, as it is not possible to stitch two resistive layers to two respective metal layers when the set of the composite four lines are running on top of each other within the minimum metal pitch.  
         SUMMARY OF THE INVENTION  
         [0010]    In the memory cell described hereinabove, however, another third resistive layer is added and stitched by the third level metal. Then a clever three-dimensional solution makes it possible to stitch three resistive layers by three metal lines.  
           [0011]    An objective of the present invention is to provide a new method of stitching between high resistance lines and low resistance metal lines in a memory cell having three types of high resistance lines.  
           [0012]    Another objective of the present invention is to provide a new method of stitching such that the three high resistance lines can be stitched by three low resistance metal lines within a cell size that is limited by the minimum metal pitch.  
           [0013]    Yet another objective of the present invention is to provide a method of forming the stitch contact areas for the high resistance line.  
           [0014]    A further objective of the present invention is to provide a method of stitching three high resistance lines to low resistance metal lines while providing bit line select transistors.  
           [0015]    A still further objective of the present invention is to provide a method of stitching three high resistance lines to low resistance metal lines while providing bit line and control gate select transistors.  
           [0016]    In this invention, by offering specific array-end structures and their fabrication method, the three resistive layers of diffusion bit line, control gate and word gate polysilicons, where control gate polysilicon can run on top of the diffusion bit line, are most effectively stitched with only three layers of metal lines keeping minimum metal pitches.  
           [0017]    When the memory becomes too large, the total capacitance of the bit line also becomes too large and the RC time constraint becomes too large for a specific application speed. Therefore, the bit line needs to be subdivided into several sections. Each section is selected by placing a select transistor at each end of the subdivided section. Thus, the total bit line capacitance is reduced to the sum of the global metal line capacitance and the selected section of devices. Also the above stitching invention is extended to the case of placing select transistors on the bit line. Also another stitching method for the deviated array structure provided in U.S. patent application Ser. No. 09/994,084 is also presented using a similar method. FIG. 2 provides a conceptual illustration of a memory cell array having control gate lines  142  and bit lines  103  running in parallel to each other, and the word gate line  140  perpendicular to both the control gate and bit lines. Word gate polysilicon lines are stitched to metal. The diffusion bit line is further divided into a sub-array by a bit line select transistor  196 , which connects to a main bit line. The control gate polysilicon line is also divided into a sub-array by a control gate line select transistor  195 , which connects to a main control gate.  
           [0018]    The first embodiment of the invention provides a stitching method of three resistive layers to three conductive layers where two resistive layers ( 003 , 042 ) run on top of, and in parallel to each other, and the third resistive layer ( 040 ) runs orthogonally to the first two resistive layers (FIG. 3). The cell width and height allows for one conductive metal in both the vertical and horizontal directions. Each resistive layer is periodically contacted (stitched) by a respective upper conductive layer to reduce the total resistive layer resistance. In order to reduce resistance, the middle resistive layer  2  ( 042 ) is periodically connected to the conductive layer  061  (M 1 ), which is above it. In order to make a connection between the bottom resistive layer  1  ( 003 ) and the uppermost conductive layer M 3  ( 081 ), the second resistive layer  2  ( 042 ) is cut and separated in order to expose the bottom resistive layer  1  ( 003 ). Then a contact/via stack is built up from the bottom resistive layer  1  ( 003 ) to the top conductive layer  3  (M 3 )  081 . The two ends of the second resistive layer  2  ( 042 ) are connected together by contacting to the second conductive layer M 2  ( 071 ). This second conductive layer M 2  ( 071 ) wire bypasses the contact/via stack by using the open space of the adjacent cell. This bypass path will hereafter be referred to as a “loop”. Since this bypass loop of second conductive layer M 2  ( 071 ) blocks contact to the bottom resistive layer  1  ( 061 ), the stitch is placed on every other set of composite lines. The unstitched lines may be stitched at another location, a short or far distance away. Thus by utilizing one extra conductive metal layer, two resistive layers can be stitched to two conductive layers, when all four layers run in parallel to and on top of one another. The extra second conductive layer M 2  ( 071 ) is used only in the stitch area, and may otherwise be used in the other areas to stitch between the third resistive layer  3  ( 040 ), which runs orthogonally to the first and second resistive layers  1  ( 003 ) and  2  ( 042 ). For this explanation, in order to reduce the resistance of the resistive layers, conductive layer  1  ( 061 ) stitches to resistive layer  2  ( 042 ); conductive layer  2  ( 071 ) stitches to resistive layer  3  ( 040 ) and conductive layer  3  ( 081 ) stitches to resistive layer  1  ( 003 ). In the loop, conductive layer  2  ( 071 ) is used to bypass the contact stack and connects the cut edges of resistive layer  2  ( 042 ) together. However, it is also possible to exchange the function of conductive layer  1  ( 061 ) and conductive layer  2  ( 071 ), and stitch them to resistive layer  3  ( 040 ) and resistive layer  2  ( 042 ), respectively. Thus three resistive layers may be stitched by three conductive metal layers within minimum cell/metal pitch.  
           [0019]    In the second embodiment, in a diffusion bit array of the twin MONOS memory where the memory device structure is fabricated as described in U.S. Pat. No. 6,248,633 B1, the bit diffusion contact for stitch  151  is formed. Then by utilizing the resistive to conductive layer stitching method which was described in the first embodiment scheme, the control gate polysilicon  143  is stitched with Metal  1  ( 161 ) and at the line edge, as shown in FIG. 5B. In the array, metal  2  (M 2 )  171  is used to lower the resistance of the polysilicon word gate line. However, in the stitch area, as demonstrated in FIG. 5C, M 2   172  is also used to connect the edges of the severed CG lines that are contacted to Metal  1  (M 1 )  161 . The M 2  line loops around the contact/via stack  151 , which connects the diffusion bit line  103  to the parallel running Metal  3   181  in FIG. 5D. Since the loop of M 2   171  blocks the bit line contact in the adjacent cell, the stitch region contacts alternate bit lines and alternate CG lines. The uncontacted set of lines may be stitched immediately below in a separate stitch area or at the other end of the sub array. It is also possible to interchange the functions of metal  1  and metal  2  for this array, such that metal  1  is used to contact to the word line and for the stitch loop, and metal  2  is used to stitch to and reduce the resistance of the control gate line.  
           [0020]    In the third embodiment of this invention, the stitching method also incorporates a bit diffusion select transistor and/or a control gate line select transistor. The purpose of the select transistors may be to reduce the overall capacitance of the bit line or control gate line, or to limit the disturb conditions that a grouped sub-array of cells may be subjected to during program and/or erase. These select transistors are added into the stitch areas between memory cell sub-arrays. FIG. 8A and FIG. 9C show an implementation example of a bit line select gate  211  and control gate select gate  212  in the stitch area. Referring to FIGS.  7 A-E and  8 A, the stitch areas on both sides of a sub-array are shown. Bit line select gates  211  are placed closest to the array and the control gate select gates  212  are placed outside of the bit line select gates from the array. At the end of the sub-array, the bit diffusion is extended past the edge of the control gate by implanting N+ species such as As prior to formation of control gate sidewalls (FIG. 9A). The bit diffusion extension  204  and bit select transistors  211  are provided alternatively on both sides of the sub-array. Select transistors are isolated from each other by shallow trench isolation. (FIG. 7E and FIG. 8A) The bit select gate  211  is placed horizontally across the extended bit diffusion and the horizontal gate becomes the bit select gate. The diffusion on the other side of the bit select transistor gate is connected to the main bit line by a contact stack  251  between the diffusion to second level metal  2  ( 271 ) as shown in FIG. 9A. When control gate select transistors  212  are also needed, a pair of control gate select transistors  212  are placed out of phase, and between the two bit line select transistors  211  inside the two edges of two sub-arrays. The pair of control gate select lines run parallel to the word gate and perpendicular to the bit line and control gate lines (FIG. 8A). The center contact  254  between two control gates  212  becomes the control gate connection point to the main control gate line which runs vertically in metal M 3  ( 281 ), as illustrated in FIGS. 8A and 8D. The other diffusion region of the control gate select transistor is locally connected by metal M 1  ( 261 ) to the other end of the polysilicon control gate stitch  252 . (FIG. 8B) Main bit lines run in metal  2  ( 271 ), but near the main CG contact, they are cut and connected down to metal  1  ( 261 ), in order to loop around the main control gate contact  254  to complete the bit stitch (FIG. 8C). Thus at the one edge of sub-array space, alternative bit select gate/stitch via M 2  line and control gate select/stitch via M 3  may be completed using a M 1 -local connection and loop. Metal  1  may also be used in the array region to stitch the word gate lines at intervals to reduce the polysilicon word gate resistance. This example shows a bit select transistor and control gate select transistor. Using the same contact and metal wiring approach, it is also possible to implement the stitch and select area with transistors for only bit line selection or with transistors for only control gate line selection.  
           [0021]    The fourth embodiment shows a stitching method in another type of array arrangement called “metal bit” where the diffusion of each cell is connected to first level metal (Ml)  361  by a contact  351  (see FIG. 10C, FIG. 11B, and FIG. 12B). The polysilicon control gate lines  342  and the polysilicon word gate lines  340  run parallel to each other and orthogonally to the bit metal line  361  FIG. 12. A polysilicon pad is prepared in order to contact between the control gate polysilicon and metal (FIG. 10A-C). This polysilicon pad  343  is formed by using the self-aligned methods described in the previous embodiment. Metal M 2   371  is used to stitch control gate  342  (FIG. 11C) and the metal M 3   381  is used to stitch the word gate  340  (FIG. 11 D). The word gate contact  355  is placed in the open space which is created by cutting the control gate M 2  line and looping around in metal  1  in order to avoid the word gate contact area (FIG. 11 C). By shifting the metal  2  and metal  3  lines half a metal pitch and looping with M 2 , as well as M 1 , every control gate line  342  and every other word gate line  340  can be contacted within the same region (FIG. 12A). Since the control gate line is a narrow sidewall polysilicon with higher resistance than the word gate line, the ability to stitch to every CG line on both ends of the sub-array is beneficial for high performance applications. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    In the accompanying drawings forming a material part of this description, there is shown:  
         [0023]    [0023]FIG. 1A is a cross sectional presentation of a twin MONOS memory device of the prior art.  
         [0024]    [0024]FIG. 1B is an equivalent circuit diagram of FIG. 1C  
         [0025]    [0025]FIG. 1 C is a top view of a twin MONOS array before formation of contact and metal wiring.  
         [0026]    [0026]FIG. 2 is a conceptual illustration of stitching resistive word line by metal line  2  and placing bit and control gate select transistors to improve RC time constant as well as achieving stitching objective, according to the third embodiment of the present invention.  
         [0027]    [0027]FIG. 3 is a three dimensional view of stitching three resistive layers by three conductive lines in a minimum pitch, according to the first embodiment of the present invention.  
         [0028]    [0028]FIGS. 4A through 4C are cross sectional representations of unifying two sidewall gate devices into a single control gate and forming a bit contact area by etching out the unified control gate polysilicon, according to the second embodiment of the present invention.  
         [0029]    [0029]FIG. 5A is a top view of the memory array obtained by the process in FIGS.  4 A-C.  
         [0030]    [0030]FIG. 5B is a top view after processing metal  1  and via in FIG. 5A.  
         [0031]    [0031]FIG. 5C is a top view after metal  2  formation in FIG. 5A.  
         [0032]    [0032]FIG. 5D is a top view after metal  3  formation in FIG. 5A.  
         [0033]    [0033]FIG. 6A is a magnified top view of the stitch area in FIG. 5A.  
         [0034]    [0034]FIG. 6B is a magnified top view of both Control gate and Bit line connection at the stitch area of FIG. 6A.  
         [0035]    [0035]FIG. 6C is a cross sectional representation of the control gate stitch area after metal  3  formation showing cross section A-A′ of FIG. 6B.  
         [0036]    [0036]FIG. 6D is a cross sectional representation of the control gate stitch area after metal  3  formation showing cross section B-B′ of FIG. 6B.  
         [0037]    [0037]FIG. 6E is an equivalent circuit diagram of a sub-array with three level metal stitch.  
         [0038]    [0038]FIG. 7A through 7C are cross sectional representations of forming sidewall control gates and landing pads for control gate contacts, according to the third embodiment of the present invention.  
         [0039]    [0039]FIG. 7D is a top view of FIG. 7B.  
         [0040]    [0040]FIG. 7E is a top view after control gate completion in FIG. 7B and bit select gate formation. Bit diffusion N+ is extended passing under the control gate to select transistor.  
         [0041]    [0041]FIG. 8A is a top view of the stitching area with bit select and control select transistors prior to metal placement.  
         [0042]    [0042]FIG. 8B is a top view after metal  1  wiring in FIG. 8A.  
         [0043]    [0043]FIG. 8C is a top view after metal  2  Wiring in FIG. 8A.  
         [0044]    [0044]FIG. 8D is a bird&#39;s view after metal  3  wiring in FIG. 8A.  
         [0045]    [0045]FIG. 9A is a cross-sectional representation of view B-B′ of FIGS. 7E and 8A.  
         [0046]    [0046]FIG. 9B is a top view of an alternative method of placing select devices.  
         [0047]    [0047]FIG. 9C is an equivalent circuit diagram according to the third embodiment of the present invention.  
         [0048]    [0048]FIGS. 10A through 10C are cross sectional representations at various stages in forming twin MONOS device with a bit contact on each memory cell, according to the fourth embodiment of the present invention.  
         [0049]    [0049]FIG. 11A is a top view just before Metal  1  wiring in FIG. 10C.  
         [0050]    [0050]FIG. 11B is a top view just after Metal  1  wiring in FIG. 11C.  
         [0051]    [0051]FIG. 11C is a top view just after Metal  2  wiring in FIG. 10C.  
         [0052]    [0052]FIG. 11D is a top view just after Metal  3  wiring in FIG. 10C.  
         [0053]    [0053]FIG. 12A is a magnified top view in the stitch area of FIG. 10C.  
         [0054]    [0054]FIG. 12B is a cross sectional representation or the control gate contact area in FIG. 12A (A-A′).  
         [0055]    [0055]FIG. 12C is a cross sectional representation of the word gate contact area in FIG. 12A (B-B′).  
         [0056]    [0056]FIG. 12D is an equivalent circuit diagram of a sub array with a contact on each bit diffusion and connected by a first metal line. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]    The first embodiment of the present invention provides a stitching method of three resistive layers to three conductive layers where two resistive layers ( 003 , 042 ) run on top of, and in parallel to each other, and the third resistive layer ( 040 ) runs orthogonally to the first two resistive layers (FIG. 3). The cell width and height allows for one conductive metal in both the vertical and horizontal directions. Each resistive layer is periodically contacted (stitched) by a respective upper conductive layer to reduce the total resistive layer resistance. in order to reduce resistance, the middle resistive layer  2  ( 042 ) is periodically connected to the conductive layer  061  (M 1 ), which is above it. In order to make a connection between the bottom resistive layer  1  ( 003 ) and the uppermost conductive layer M 3  ( 081 ), the second resistive layer  2  ( 042 ) is cut and separated in order to expose the bottom resistive layer  1  ( 003 ). Then a contact/via stack is built up from the bottom resistive layer  1  ( 003 ) to the top conductive layer  3  (M 3 )  081 . The two ends of the second resistive layer  2  ( 042 ) are connected together by contacting to the second conductive layer M 2  ( 071 ). This second conductive layer M 2  ( 071 ) wire bypasses the contact/via stack by using the open space of the adjacent cell. This bypass path will hereafter be referred to as a “loop”. Since this bypass loop of second conductive layer M 2  ( 071 ) blocks contact to the bottom resistive layer  1  ( 003 ), the stitch is placed on every other set of composite lines. The unstitched lines may be stitched at another location, a short or far distance away. Thus by utilizing one extra conductive metal layer, two resistive layers can be stitched to two conductive layers, when all four layers run in parallel to and on top of one another. The extra second conductive layer M 2  ( 071 ) is used only in the stitch area, and may otherwise be used in the other areas to stitch between the third resistive layer  3  ( 040 ), which is runs orthogonally to the first and second resistive layers  1  ( 003 ) and  2  ( 042 ). For this explanation, in order to reduce the resistance of the resistive layers, conductive layer  1  ( 061 ) stitches to resistive layer  2  ( 042 ); conductive layer  2  ( 071 ) stitches to resistive layer  3  ( 040 ) and conductive layer  3  ( 081 ) stitches to. resistive layer  1  ( 003 ). In the loop, conductive layer  2  ( 071 ) is used to bypass the contact stack and connects the cut edges of resistive layer  2  ( 042 ) together. However, it is also possible to exchange the function of conductive layer  1  ( 061 ) and conductive layer  2  ( 071 ), and stitch them to resistive layer  3  ( 040 ) and resistive layer  2  ( 042 ), respectively. Thus three resistive layers may be stitched by three conductive metal layers within minimum cell/metal pitch.  
         [0058]    The second preferred embodiment of the present invention will be discussed with reference to FIGS.  4 - 6 .  
         [0059]    The ballistic twin MONOS memory cell is fabricated as taught in U.S. Pat. No. 6,248,633 B1. As illustrated in FIG. 4A, each. memory cell contains two nitride regions  131  which comprise storage elements for one word gate  140 , and half a source diffusion and half a bit diffusion ( 103 ). The diffusion junctions are shared by two adjacent storage elements. Control gates  142  are defined by vertical reactive ion etching on both word gate sidewalls. A pair of control gates  142  sharing the bit diffusion  103  may be connected together by plugging polysilicon  143  as shown FIG. 4B to reduce the resistance. After defining the sidewall gates  142 , a silicon oxide film  124 -A is grown or deposited over the control gates and the diffusion junctions  103  to form an isolation layer  124  in FIG. 4B. Using a recess mask  190  in FIG. 4A, the oxide  124 -A is etched to leave the isolation oxide  124  in FIG. 4B. Then polysilicon in the canyon between separate control gates  142  is deposited and planarized. Unnecessary oxide  124 -A over the polysilicon control gate is removed by wet or dry etching. A polysilicon is deposited to fill the space between the word lines and planarized by CMP to remove the polysilicon over the word gates as shown in FIG. 4B. The control gate  143  is electrically isolated from the underlying diffusion junctions  103 . Diffusions run under the sidewall control gate  142 , and perpendicular to the word line to be formed later overlying the control gate.  
         [0060]    The bit diffusion area is exposed. using photoresist mask  193 , shown in FIG. 4B. The polysilicon  142  and  143  is selectively etched using, for example, a reactive ion etching (RIE) process having a chlorine base, followed by a bit contact ion implantation  104 , such as As, as shown in FIG. 4C to form all n+ region profile under the. ONO and make the contact area borderless with n+ dopant.  
         [0061]    A conventional metal contact process is followed; for example, oxide deposition within the opening, chemical mechanical polish (CMP) of the oxide, opening of a contact hole, tungsten fill, and CMP of the tungsten. FIG. 4C illustrates the completed bit diffusion stitch contact  151  and control gate contact  152 . FIG. 5A shows a top view of the MONOS device at this point. FIG. 4C shows cross section B-B′ of FIG. 6B. The stitch contact holes  152  and the bit contact hole  151  are placed on alternate bit lines and control gate lines. Control gate contacts placed on the both edges, as shown in FIG. 5A, connect together with 1 st  metal  161  as shown in FIG. 5B, where the 1 st  metal pads are placed on bit contacts to stack up. The via process follows to stack the bit contacts and control gate contacts. The 1 st  metal lines are opened at both sides of a bit contact  151  to bypass with 2 nd  metal loop  171  as shown in FIG. 5C. The word lines are also stitched by 2 nd  metal, not shown. The bit contacts are stacked by 2 nd  metal pads and 2 nd  via holes and stitched by 3 rd  metal as shown in FIG. 5D.  
         [0062]    [0062]FIG. 6A is a magnified top view of the stitch area in FIG. 5A. FIG. 6B is a magnified top view of both control gate and bit line connection at the stitch area of FIG. 6A. FIG. 6C is a cross sectional representation of the control gate stitch area after metal  3  formation showing cross section A-A′ of FIG. 6B. FIG. 6D is a cross sectional representation of the control gate stitch area after metal  3  formation showing cross section B-B′ of FIG. 6B. FIG. 6E is an equivalent circuit diagram of a sub-array with three level metal stitch.  
         [0063]    In a minimum line space, a metal stitch is thus formed to the control gate using 1 st  metal and 2 nd  metal and bit line running immediately underneath the control gate  143  using 3 rd  metal. A metal stitch to the word line is formed using the 2 nd  metal.  
         [0064]    The third embodiment of the present invention will be described with reference to FIGS.  7 - 9 . The third embodiment completes the stitching method having select devices. The bit line and control gate capacitance reduction of the present invention is achieved by placing bit select gates and control select gates in a tight sub-array space in conjunction with the previous metal stitches. FIG. 8A illustrates a top view after defining the contact. The control gate contacts  252  are placed at the end of a sub-array. The bit diffusion select gates  211  are placed on both sides of the sub-array (FIG. 7E). The bit diffusion contacts  251  are placed on alternate side of the sub-array as shown in FIG. 8A. In the area of a pair of control gate select devices  212 , three contacts  253 ,  254 ,  253  are placed as shown in FIG. 8A. The select gates define which sub-array is selected. The center contacts  254  are connected to the main control line. Both sides of contacts  253  are connected to the sub-array control gate. These bit line and control gate contacts are wired using 3 metal layers. Loop  262  and local connection  261  as 1 st  metal, shown in FIG. 8B, main bit line  271  as 2 nd  metal, shown in FIG. 8C, and main control line  281  as 3 rd  metal, shown in FIG. 8D.  
         [0065]    [0065]FIGS. 7A, 7B and  7 C illustrate cross-sections at various process steps to form control gates and their contacts. The conformal polysilicon layer  242 / 243  is deposited over the word gates  240 , as shown in FIG. 7A. In the process of the present invention, the control gate contact area  243  is placed over the shallow trench isolation (STI) region  202 , where it is covered with recessed photoresist mask or hard mask  290 . The mask is applied to expose the control gate polysilicon except in the control pad area. Then the vertical etch of the sidewall polysilicon is performed to get the sidewall control gates  242 . Polysilicon overlying the bit diffusion junctions  203  is etched out. However, the polysilicon  243  over the STI-region is covered with the recessed mask  290  and the filled poly silicon for the control gate contact pad remains as shown in FIG. 7B, whose top view is provided in FIG. 7D.  
         [0066]    After defining the peripheral area including the select gates, oxide  245  is deposited to fill the areas between the word gates and planarized until the cap nitride  230  is exposed. The word line (wire) polysilicon  246  is deposited, following the cap nitride strip to allow self-alignment. The word wire is defined by the conventional lithography and subsequent RIE etching the deposited polysilicon  246  and the word gate polysilicon  240  all the way down to the word gate oxide. This is followed by the conventional contact process with a sequence of oxide fill, oxide CMP, contact open, tungsten deposition and Tungsten CMP to form control gate contact  252 . FIG. 7E is a top view after the contact process. FIG. 7C is a cross section cutting along A-A′ running on control gate contacts  252  in FIG. 7E.  
         [0067]    The extended diffusion  204  is defined in the N+ diffusion under the enclosing memory gate  210  by Arsenic ion implantation with a dose of between about 1E15 to 2E15 ions/cm 3  and an energy of between about 40 and 60 KeV immediately after STI formation to keep electrical continuity between the diffusion bit  204  and bit line select diffusion  206  (FIG. 9A)  
         [0068]    This control select device  212  can be N-channel with isolated P-well from P-substrate or it can be a P-channel device with an independent N-well. When a P-channel device is used, the voltage application on the select gate  212  has to be chosen carefully to discharge near the low-ground level. The input voltage on the P-channel device is at least below the threshold voltage (Vt). If the p-threshold is −1.0V, then the select gate voltage for ON must be at least −1.0V instead of the normal 0V. However, this extra complexity with the negative voltage pays off in comparison with an N-channel select gate. In an N-channel select gate device, in order to pass high voltage Vcg(5˜6V), the control select gate requires at least Vcg+Vt (at Vsub=Vcg), which means about 7˜8 v on the select gate is required to pass 5.5V. Therefore, this extra high voltage requirement (almost 40% higher) forces the utilization of at least 40% thicker oxide for high voltage support devices. This extra oxide thickness can be avoided if the P-channel select device is chosen instead of the N-channel device.  
         [0069]    The pair of control gate select lines run parallel to the word gate and perpendicular to the bit line and control gate lines in FIG. 8A. The center contact  254  between two control gates  212  becomes the control gate connection point to the main control gate line that runs vertically in metal M 3  ( 281 ) FIG. 8A &amp; 8D. The other diffusion region of the control gate select transistor is locally connected by metal M 1  ( 261 ) to the other end the polysilicon control gate stitch  252 . (FIG. 8B) Main bit lines run in metal  2  ( 271 ), but near the main CG contact, they are cut and connected down to metal  1  ( 261 ), in order to loop around the main control gate contact  254  to complete the bit stitch FIG. 8C. Thus at the one edge of the sub-array space, alternative bit select gate/stitch via M 2  line and control gate select/stitch via M 3  may be completed using a M 1 -local connection and loop. Metal  1  may also be used In the array region to stitch the word gate lines at intervals to reduce the polysilicon word gate resistance.  
         [0070]    The application of the special wiring technique of the present invention in the twin MONOS cell device of U.S. Pat. No. 6,248,633 B1 is illustrated in FIG. 9C. The bit line signal BL[ 1 ] through the global Metal  2  is connected to one side of the bit select transistor (Bit Select  1 ) and the output is connected to the other side of the bit diffusion line which is connected to the drain of another bit select transistor (Bit Select  0 ). The source junction is connected to the BL[ 0 ] Metal  2  line. When one of the word lines in the block between two bit select transistors is selected, the bit signal from the BL[ 1 ] goes through the twin cells and reaches BL[ 0 ]. On the other hand, the control gate signal from CG[ 1 ] is dropped to the drain of the control gate transistor CG[ 1 ] and passes through the select transistor. Then the control gate signal is delivered to the control gate between two bit select transistors.  
         [0071]    [0071]FIG. 9B shows an alternative method to place select devices with buried contacts  256  directly to the control gate. This reduces the stitch area since the direct contact eliminates 1 st  metal local wire connecting a sub-array control gate and a select device source diffusion. The control gate is extended to the select device source diffusion. The contact  256  is connected to the underlying diffusion after removing the ONO. The control gate signal from the main control gate line  281  with 3 rd  metal comes in through the stacked via  256  and is delivered to the source diffusion  256  when the select gate  213  is selected.  
         [0072]    In the fourth preferred embodiment of the present invention, the stitching method of the invention can be used in the nonvolatile memory disclosed in the co-pending U.S. patent applications Ser. Nos. 09/810,122 and 09/994,084. This embodiment is discussed with reference to FIGS.  10 - 12 .  
         [0073]    In another array arrangement of twin MONOS memory device, a stitching method of word gate and control gate to two metal lines running parallel is disclosed. In the process of the present invention, the control gate contact area  343  over the shallow trench isolation (STI) region  302  is covered with recessed photoresist or hard mask such as TEOS oxide. Then the vertical etch of the sidewall polysilicon is performed. Sidewall control gates  342  are left on the sidewalls of the word gate  340 . However, the polysilicon  343  is protected by the recessed mask  391  as shown in FIG. 10A. A conventional CMOS gate definition follows to form the logic gate structure while protecting the memory area Then the sidewall dielectric spacer for logic gate follows as shown in FIG. 10B. The sidewall dielectric spacer on the logic gate can be a thin oxide and nitride spacer. Since the sidewall control gate can be thinner than 40 nm and the top of the control polysilicon gate can be reasonably lower than the top of the word gate polysilicon, the nitride spacer can cover the entire control gate polysilicon. During opening of the bit contact  351 , the slight overlap of contact hole over the nitride layer is tolerable since the nitride has a lower etch rate during oxide RIE. A control gate contact  352  is made over the control gate contact polysilicon  343 . Word line contacts  355  and Bit contacts  351  are also formed. The bit line is wired by 1 st  metal  361  in FIGS. 10C and 11B, the control gate is stitched by 2 nd  metal  371  in FIG. 11C, and the word line is stitched by 3 rd  metal  381  in FIG. 11D.  
         [0074]    The diffusion of each cell is connected to first level metal (M 1 )  361  by a contact  351  FIG. 10C, FIG. 11B, and FIG. 12B. The polysilicon control gate lines  342  and the polysilicon word gate lines  340  run parallel to each other and orthogonally to the bit metal line  361  FIG. 12A. Metal M 2   371  is used to stitch control gate  342  (FIG. 11C) and the metal M 3   381  is used to stitch the word gate  340  (FIG. 1D). The word gate contact  355  is placed in the open space which is created by cutting the control gate M 2  line and looping around in metal  1  in order to avoid the word gate contact area (FIG. 11C). By shifting the metal  2  and metal  3  lines half a metal pitch and looping with M 2  as well as M 1 , every control gate line  342  and every other word gate line  340  can be contacted within the same region (FIG. 12A).  
         [0075]    The equivalent circuit diagram to this embodiment is shown in FIG. 12D. The stitch area is placed on both sides of the memory array block, defined as upper and lower stitch areas. The control gate contacts to stitch the control gate line are placed on both upper and lower areas. Word contacts are placed alternately on upper and lower areas.  
         [0076]    The present invention provides methods stitching three high resistance lines to three low resistance metal lines within a cell size that is limited by the minimum metal pitch. Methods have been provided to form stitch contact areas for the high resistance line. Bit line select transistors and/or control gate select transistors may be provided in addition to the stitching.  
         [0077]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.