Abstract:
Device and design structures for memory cells in a non-volatile random access memory (NVRAM) and methods for fabricating such device structures using complementary metal-oxide-semiconductor (CMOS) processes. The device structure, which is formed using a semiconductor-on-insulator (SOI) substrate, includes a floating gate electrode, a semiconductor body, and a control gate electrode separated from the semiconductor body by the floating gate electrode. The floating gate electrode, the control gate electrode, and the semiconductor body, which are both formed from the monocrystalline SOI layer of the SOI substrate, are respectively separated by dielectric layers. The dielectric layers may each be composed of thermal oxide layers grown on confronting sidewalls of the semiconductor body, the floating gate electrode, and the control gate electrode. An optional deposited dielectric material may fill any remaining gap between either pair of the thermal oxide layers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to application Ser. No. ______, filed as Attorney Docket No. BUR920070165US1 on even date herewith and entitled “Device Structures for a Metal-Oxide-Semiconductor Field Effect Transistor and Methods of Fabricating Such Device Structures”. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to semiconductor device fabrication and, in particular, to device structures and design structures for memory cells in a non-volatile random access memory (NVRAM) and methods for fabricating such device structures for memory cells using complementary metal-oxide-semiconductor (CMOS) processes. 
       BACKGROUND OF THE INVENTION 
       [0003]    Complementary metal-oxide-semiconductor (CMOS) technology is used in microprocessors, static random access memories, and other diverse types of digital logic integrated circuits and analog integrated circuits. Conventional device structures for a planar field effect transistor (FET) fabricated using CMOS technology include a semiconductor layer, a source and a drain defined in the semiconductor layer, a channel defined in the semiconductor layer between the source and drain, and a control gate electrode. The material constituting the gate electrode in such conventional planar device structures contains polycrystalline silicon (polysilicon) or a metal applied by an additive process that involves blanket deposition of the material and patterning with a conventional lithography and etching process. When a control voltage exceeding a characteristic threshold voltage is applied to the control gate electrode, an inversion or depletion layer is formed in the channel by the resultant electric field and carrier flow occurs in the depletion layer between the source and drain (i.e., the device output current). 
         [0004]    Non-volatile random access memory (NVRAM) refers generally any type of random access memory that retains the stored binary data even when not powered. A conventional device structure used as a memory cell in a NVRAM modifies the FET to add an electrically isolated (floating) gate electrode that affects conduction between the source and drain. In the vertical stack, a tunnel dielectric layer is interposed between the floating gate electrode and the channel. The control gate electrode, which has an overlying relationship in the vertical stack with the floating gate electrode, is separated from the floating gate electrode by an intergate dielectric layer. In such memory cells, binary data is represented by charge stored on the floating gate electrode. To provide one binary state, the floating gate electrode is charged during a write operation in which charge carriers tunnel or are injected from the biased control gate electrode through the tunnel dielectric layer to the floating gate electrode. Once the floating gate electrode has been charged, because the floating gate electrode is electrically isolated in the circuit, that charge remains intact without the requirement of being refreshed. To provide the other binary state, the charge stored by the floating gate electrode can be removed by reversing the bias on the control gate electrode, which drains charge carriers from the floating gate electrode. The memory cell is read by sensing the current flowing in the channel when the source and drain are properly biased, which is influenced by the charge stored by the floating gate electrode. 
         [0005]    Improved device structures and fabrication methods are needed for a NVRAM that permit the use of high operating voltages for the constituent memory cells and that simplify their fabrication using CMOS technology. 
       SUMMARY OF THE INVENTION 
       [0006]    In one embodiment, a device structure is provided for a non-volatile random access memory formed on an insulating layer. The device structure includes a semiconductor body, a control gate electrode, and a floating gate electrode in contact with the insulating layer. The semiconductor body includes a source, a drain, and a channel disposed between the source and the drain. The floating gate electrode is disposed between the control gate electrode and the channel of the semiconductor body. A first dielectric layer is disposed between the channel of the semiconductor body and the floating gate electrode. A second dielectric layer is disposed between the floating gate electrode and the control gate electrode. The first dielectric layer is composed of a first dielectric material that electrically isolates the floating gate electrode from the channel. The second dielectric layer is composed of a second dielectric material that electrically isolates the floating gate electrode from the control gate electrode. 
         [0007]    In another embodiment, the device structure may be included in a design structure embodied in a machine readable medium for designing, manufacturing, or testing an integrated circuit. The design structure may comprise a netlist and may reside on storage medium as a data format used for the exchange of layout data of integrated circuits. 
         [0008]    In another embodiment, a method is provided for fabricating a device structure for a non-volatile random access memory from a semiconductor layer carried on an insulating layer. The method includes forming first, second, and third semiconductor bodies from the semiconductor layer and with a juxtaposed relationship in which the second semiconductor body is disposed between the first and third semiconductor bodies. The first semiconductor body is doped to form a source and a drain. A first dielectric layer is formed between the first semiconductor body and the second semiconductor body and a second dielectric layer is formed between the second semiconductor body and the third semiconductor body. The method further includes partially removing the second semiconductor body and the third semiconductor body to respectively define a floating gate electrode and a control gate electrode that cooperate to control carrier flow in a channel between the source and the drain. The floating gate electrode may be self-aligned with the control gate electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  is a diagrammatic top plan view of a device structure built on a portion of a semiconductor-on-insulator substrate at an initial fabrication stage of a processing method in accordance with an embodiment of the invention. 
           [0010]      FIG. 1B  is a diagrammatic cross-sectional view taken generally along line  1 B- 1 B in  FIG. 1A . 
           [0011]      FIG. 2A  is a diagrammatic top plan view of the device structure of  FIG. 1A  at a subsequent fabrication stage. 
           [0012]      FIG. 2B  is a diagrammatic cross-sectional view taken generally along line  2 B- 2 B in  FIG. 2A . 
           [0013]      FIG. 3A  is a diagrammatic top plan view of the device structure of  FIG. 2A  at a subsequent fabrication stage. 
           [0014]      FIG. 3B  is a diagrammatic cross-sectional view taken generally along line  3 B- 3 B in  FIG. 3A . 
           [0015]      FIG. 4A  is a diagrammatic top plan view of the device structure of  FIG. 3A  at a subsequent fabrication stage. 
           [0016]      FIG. 4B  is a diagrammatic cross-sectional view taken generally along line  4 B- 4 B in  FIG. 4A . 
           [0017]      FIG. 5  is a flow diagram of a design process used in semiconductor design, manufacture, and/or test. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    With reference to FIGS.  1 A,B and in accordance with an embodiment of the invention, a semiconductor-on-insulator (SOI) substrate  10  includes a handle wafer  12 , a buried insulating layer  14  formed of an insulating material such as silicon dioxide (e.g., SiO 2 ), and an active semiconductor or SOI layer  16  separated from the handle wafer  12  by the intervening buried insulating layer  14 . The SOI layer  16  is constituted by a single crystal or monocrystalline semiconductor material, such as silicon or a material that primarily contains silicon. The handle wafer  12  may also be constituted by a single crystal or monocrystalline semiconductor material, such as silicon, or another type of material. The monocrystalline semiconductor material of the SOI layer  16  may contain a definite defect concentration and still be considered single crystal. The SOI layer  16 , which is considerably thinner than the handle wafer  12  and is in direct contact with a top surface  15  of the buried insulating layer  14  to define an interface, is electrically isolated from the handle wafer  12  by the buried insulating layer  14 . The SOI substrate  10  may be fabricated by any suitable conventional technique, such as a wafer bonding technique or a separation by implantation of oxygen (SIMOX) technique, which are techniques familiar to a person having ordinary skill in the art. 
         [0019]    A pad layer  18  is disposed on a top surface  20  of the SOI layer  16  across the SOI substrate  10 . The material forming pad layer  18  is selected to operate as a hardmask that etches selectively to the semiconductor material constituting the SOI layer  16 . The hardness and wear resistance of the material constituting pad layer  18  are also adequate to function as a polish stop layer and reactive ion etch mask during subsequent fabrication stages. The pad layer  18  may be SiO 2  deposited by a thermal CVD process or SiO 2  grown by oxidizing a surface thickness of the SOI layer  16 . Alternatively, the pad layer  18  may be composed of a different material, such as silicon-oxynitride (SiO x N y ). 
         [0020]    A plurality of juxtaposed bodies, of which three representative bodies  22 ,  24 ,  26  are visible in FIGS.  1 A,B, are defined from the material of the SOI layer  16  by a conventional lithography and anisotropic etching process. The lithography process applies a resist (not shown) on pad layer  18 , exposes the resist to a pattern of radiation, and develops the transferred pattern in the exposed resist. The pattern is transferred to the SOI layer  16  by a series of anisotropic dry etches, such as reactive-ion etching (RIE) or a plasma etching process, that patterns the pad layer  18  using the patterned resist as an etch mask and then patterns the SOI layer  16  using the patterned pad layer  18  as an etch mask. The etching process removes the material of the SOI layer  16  selective (i.e., at a significantly higher etch rate) to the material of the pad layer  18  and resist layer and stops on the buried insulating layer  14 . After etching is concluded, residual resist is stripped by, for example, plasma ashing or chemical stripping. 
         [0021]    The bodies  22 ,  24 ,  26  represent adjacent, parallel lines of the semiconductor material originating from the SOI layer  16  and are in direct contact with the top surface  15  of buried insulating layer  14 . The gap, G 1 , between the bodies  22 ,  24  and the gap, G 2 , between the bodies  24 ,  26  are determined by the line width and pitch of the lithography process or by other sub-lithographic definition processes, such as pitch doubling sidewall image transfer. Residual dielectric caps  28 ,  30 ,  32 , which represent residual portions of the material of pad layer  18  that remain intact after etching with the patterned resist in place, cover the respective bodies  22 ,  24 ,  26  in a stacked arrangement. Dielectric caps  28 ,  30 ,  32  have approximately the same footprint, in terms of width and length, as the bodies  22 ,  24 ,  26 . 
         [0022]    Sidewalls  34 ,  36  of body  22  extend from the top surface  20  toward the buried insulating layer  14  and intersect the buried insulating layer  14 . Body  24  includes sidewalls  38 ,  40  that extend from top surface  20  toward the buried insulating layer  14  and intersect the buried insulating layer  14 . Body  26  includes sidewalls  42 ,  44  that extend from top surface  20  toward the buried insulating layer  14  and intersect the buried insulating layer  14 . Sidewall  36  of body  22  is contained in a plane that is aligned substantially parallel to, but spaced from, a plane containing sidewall  38  of body  24 . Sidewalls  36 ,  38  are separated by the gap, G 1 . Sidewall  40  of body  24  is contained in a plane that is aligned substantially parallel to, but spaced from, a plane containing sidewall  42  of body  26 . Sidewalls  40 ,  42  are separated by the gap, G 2 . 
         [0023]    With reference to FIGS.  2 A,B in which like reference numerals refer to like features in FIGS.  1 A,B and at a subsequent fabrication stage, a dielectric layer  46  is then formed on opposite sidewalls  34 ,  36  of body  22 , a dielectric layer  48  is formed on the opposite sidewalls  38 ,  40  of body  24 , and a dielectric layer  50  is formed on the opposite sidewalls  42 ,  44  of body  26 . In one embodiment, the dielectric material constituting dielectric layers  46 ,  48 ,  50  may be SiO 2  grown by a thermal oxidation process. The oxidation process entails exposing the bodies  22 ,  24 ,  26  to a dry or wet oxygen-laden, heated ambient in, for example, an oxidation furnace or a rapid thermal anneal chamber. Oxidation conditions are selected to provide an appropriate thickness for the dielectric layers  46 ,  48 ,  50 , of which dielectric layers  46 ,  48  at least partially fill the gap, G 1 , and dielectric layers  48 ,  50  at least partially fill the gap, G 2 . In the representative embodiment, the dielectric layers  48 ,  50  completely fill the gap, G 2 , between sidewalls  40 ,  42 , and the dielectric layers  46 ,  48  only partially fill the gap, G 1 , between sidewalls  36 ,  38 , although the invention is not so limited. 
         [0024]    A blanket layer  52  of a dielectric material is deposited to fill the open spaces about the bodies  22 ,  24 ,  26 . In particular, a portion of the dielectric layer  52  fills the remainder of the narrowed gap, G 1 , between the sidewalls  36 ,  38  that is unfilled by dielectric layers  46 ,  48 . Another portion of the dielectric layer  52  fills the remainder of the narrowed gap, G 2 , between the sidewalls  40 ,  42  if not completely filled by dielectric layers  46 ,  48 . The dielectric layer  52  may be composed of stoichiometric or non-stoichiometric SiO 2  deposited by a chemical vapor deposition (CVD) process, or other dielectrics, including silicon oxy-nitride, hafnium oxide, or any other material with predominately dielectric properties. The dielectric layer  52  is planarized by a conventional planarization process, such as a chemical mechanical polishing (CMP) process, that removes the excess overburden of the blanket dielectric layer  52  and stops on the dielectric caps  28 ,  30 ,  32 . Typically if the dielectric material is SiO 2 , the quality of SiO 2  in the dielectric layers  46 ,  48 ,  50  is superior to the quality of the SiO 2  in dielectric layer  52  because of the different formation processes. 
         [0025]    A person having ordinary skill in the art will appreciate that the portion of the dielectric layer  52  in the gap, G 1 , between the sidewalls  36 ,  38  is optional and that the dielectric layers  46 ,  48  may have a thickness sufficient to completely fill and close the gap, G 1 , between the sidewalls  36 ,  38 . 
         [0026]    A photoresist mask  54  is then formed from a resist layer that is patterned by a conventional lithography and anisotropic etching process. Opposite side edges of the photoresist mask  54  have a roughly orthogonal alignment relative to the bodies  22 ,  24 ,  26 . An anisotropic etching process is applied to remove the material of the dielectric layers  46 ,  48 ,  50 ,  52  selective to the semiconductor material of the bodies  22 ,  24 ,  26 . Dielectric material in the dielectric caps  28 ,  30 ,  32  and portions of the dielectric layers  46 ,  48 ,  50 ,  52  located beneath the photoresist mask  54  are preserved during the etching process because of the masking. The etching process operates to remove the dielectric caps  28 ,  30 ,  32  and dielectric layers  46 ,  48 ,  50 ,  52  outside of the protective footprint of the photoresist mask  54  so that the unmasked semiconductor material of the bodies  22 ,  24 ,  26  is exposed. 
         [0027]    After etching, the photoresist mask  54  intersects body  22  along a central channel  56 , covers a portion  58  of body  24 , and covers a portion  60  of body  26 . Opposite end regions  62 ,  64  of body  22 , which flank the channel  56 , opposite end regions  66 ,  68  of body  24 , and opposite end regions  70 ,  72  of body  26  project or protrude outwardly from opposite sidewalls  74 ,  76  of the photoresist mask  54 . The end regions  62 ,  64  of body  22  are implanted with a suitable n-type or p-type dopant at a dose effective to define a source and a drain, respectively, for the device structure  81  (FIGS.  4 A,B). The photoresist mask  54  blocks the dopant from entering the channel  56  of body  22 , the covered portion  58  of body  24 , and the covered portion  60  of body  26 . End regions  66 ,  68  of body  24 , and end regions  70 ,  72  of body  26 , which are unmasked and receive dopant, are excised in a subsequent fabrication stage. Shallow source/drain extensions and halos (not shown) may be introduced into body  22  beneath the opposing sidewalls  74 ,  76  of the photoresist mask  54  by angled ion implantations. 
         [0028]    With reference to FIGS.  3 A,B in which like reference numerals refer to like features in FIGS.  2 A,B and at a subsequent fabrication stage, the photoresist mask  54  (FIGS.  2 A,B) is removed by, for example, plasma ashing or chemical stripping. Another photoresist mask  78  is then formed from a resist layer that is patterned by a conventional lithography and anisotropic etching process. The photoresist mask  78  has a parallel alignment with body  22  and extends laterally beyond the opposite sidewalls  34 ,  36  of body  22 . The dielectric cap  30  and dielectric layers  48 ,  52  operate as a hard mask for the covered portion  58  of body  24 . The dielectric cap  32  and dielectric layers  50 ,  52  operate as a hard mask for the covered portion  60  of body  26 . 
         [0029]    With reference to FIGS.  4 A,B in which like reference numerals refer to like features in FIGS.  3 A,B and at a subsequent fabrication stage, the unprotected semiconductor material in the end regions  66 ,  68  of the body  24  and the end regions  70 ,  72  of the body  26  is removed by an anisotropic etching process selective to the dielectric materials constituting dielectric caps  30 ,  32  and dielectric layers  48 ,  50 ,  52 . The covered portion  58  ( FIG. 3B ) of the semiconductor material of body  24  is preserved and serves as a floating gate electrode  80  for a device structure, which is generally indicated by reference numeral  81 . The etching process, which stops on the buried insulating layer  14 , truncates the body  24  so that floating gate electrode  80  has opposite end walls  82 ,  84  that extend to the buried insulating layer  14 . The covered portion  60  ( FIG. 3B ) of the semiconductor material of body  26  is preserved and serves as a control gate electrode  86  for device structure  81 . The etching process, which stops on the buried insulating layer  14 , truncates the body  26  so that control gate electrode  86  has opposite end walls  88 ,  90  that extend to the buried insulating layer  14 . The photoresist mask  78 , which protects the opposite end regions  62 ,  64  of body  22  during the etching process, is removed by, for example, plasma ashing or chemical stripping. 
         [0030]    The hard mask supplied by the dielectric cap  30  and the presence of the dielectric layers  48 ,  52 , as well as the presence of the photoresist mask  78 , operates to self-align the floating gate electrode  80  with the channel  56  of body  22 , as well as to self-align the floating gate electrode  80  with the source and drain in the doped opposite end regions  62 ,  64  of body  24 . Specifically, end wall  82  of the floating gate electrode  80  is generally aligned (i.e., coplanar) with the planar interfaces between the channel  56  in body  22  and the doped region in the end region  62  of body  22  representing the source for the device structure  81 . Similarly, end wall  84  of the floating gate electrode  80  is generally aligned (i.e., coplanar) with the planar interfaces between the channel  56  in body  22  and the doped region in the end region  64  of body  22  representing the drain for the device structure  81 . These planar interfaces generally underlie the opposite side edges  92 ,  94  of the dielectric cap  28  and extend through the body  22  from these positions underlying the side edges  92 ,  94  to the buried insulating layer  14 . 
         [0031]    The hard mask supplied by the dielectric cap  32  and the presence of the dielectric layers  50 ,  52  operates to self-align the control gate electrode  86  with floating gate electrode  80 . Specifically, end wall  88  of the control gate electrode  86  is generally aligned (i.e., coplanar) with end wall  82  of the floating gate electrode  80 . Similarly, end wall  90  of the control gate electrode  86  is generally aligned (i.e., coplanar) with end wall  84  of the floating gate electrode  80 . 
         [0032]    After etching, the floating gate electrode  80  represents the residual portion of the monocrystalline semiconductor material of body  24  and the control gate electrode  86  represents the residual portion of the monocrystalline semiconductor material of body  26 . The floating gate electrode  80  is physically separated from the channel  56  of body  22  by the thickness of the dielectric layer  46  on sidewall  36  of body  22 , the thickness of the dielectric layer  48  on sidewall  38  of body  24 , and the thickness of the dielectric layer  52  between dielectric layers  46 ,  48 , which collectively define a tunnel dielectric layer generally indicated by reference numeral  96 . The tunnel dielectric layer  96  physically separates the floating gate electrode  80  from the channel  56  of body  22  and electrically isolates the floating gate electrode  80  from the channel  56 . The thickness of the tunnel dielectric layer  96 , which is primarily determined when the SOI layer  16  is lithographically patterned, is selected to prevent excess charge leakage from the floating gate electrode  80  that, if not prevented, would reduce data retention time. 
         [0033]    The control gate electrode  86  is physically separated from the floating gate electrode  80  by the thickness of the dielectric layer  48  on sidewall  40  of body  24 , the thickness of the dielectric layer  50  on sidewall  42  of body  26 , and optionally any portion of dielectric layer  52  between dielectric layers  48 ,  50 , which collectively define an intergate dielectric layer generally indicated by reference numeral  95 . The intergate dielectric layer  95  physically separates the control gate electrode  86  from the floating gate electrode  80  and, because of the constituent dielectric material, also electrically isolates the control gate electrode  86  from the floating gate electrode  80 . The intergate dielectric layer  95  is thinner than the tunnel dielectric layer  96  to promote the operability of the device structure  81  as a memory cell in a NVRAM. 
         [0034]    The floating gate electrode  80 , the control gate electrode  86 , and the body  22  containing channel  56  have nominally equal thicknesses measured from their respective top surfaces to the buried insulating layer  14  and are composed of substantially identical portions of monocrystalline silicon that originated from the SOI layer  16 . 
         [0035]    A silicide layer  98  is formed on the exposed end regions  62 ,  64  of body  22  that are not covered by dielectric cap  28  and dielectric layers  46 ,  52  and, in particular, on the top surfaces of end regions  62 ,  64 . Silicidation processes are familiar to a person having ordinary skill in the art. In an exemplary silicidation process, the silicide layer  98  may be formed by depositing a layer of suitable metal, such as nickel, cobalt, tungsten, titanium, etc., and then annealing with, for example, a rapid thermal annealing process, to react the metal with the silicon-containing semiconductor material (e.g., silicon) of the end regions  62 ,  64  of body  22 . The silicidation process may be conducted in an inert gas atmosphere or in a nitrogen-rich gas atmosphere, and at a temperature of about 350° C. to about 600° C. contingent upon the type of silicide being considered for silicide layer  98 . Following the high temperature anneal, unreacted metal remains on areas of the device structure  81  where the deposited metal is not in contact with a silicon-containing material. The unreacted metal is selectively removed with an isotropic wet chemical etch process. The process self aligns the silicide to the exposed silicon-containing regions because of the selective reaction between the metal and silicon-containing semiconductor material. 
         [0036]    The device structure  81  includes the source and drain defined in the end regions  62 ,  64  of body  22  and channel  56  between end regions  62 ,  64 , the floating gate electrode  80  defined from the adjacent body  24  of single crystal semiconductor, and the control gate electrode  86  formed from the adjacent body  26  of single crystal semiconductor, as well as the tunnel dielectric layer  96  and the intergate dielectric layer  95 . The body thickness of the body  22 , the thickness of the intergate dielectric layer  95 , and the thickness of the tunnel dielectric layer  96  can be independently adjusted during fabrication. 
         [0037]    During the fabrication process, the device structure  81  is replicated across at least a portion of the surface area of the SOI layer  16  of the SOI substrate  10 . Standard processing follows, which includes formation of metallic contacts, metallization for the M1 level interconnect wiring, and interlayer dielectric layers, conductive vias, and metallization for upper level (M2-level, M3-level, etc.) interconnect wiring. Metallization in the contact level of the interconnect wiring establishes an electrical contact  93  with the control gate electrode  86  and independent electrical contacts  97 ,  99  mediated by the silicide layer  98  with the source and drain defined in the end regions  62 ,  64  of body  22 . The floating gate electrode  80  remains uncontacted. Other types of device structures may be fabricated on other surface areas of the SOI substrate  10  and entirely independent of the fabrication process forming device structures like device structure  81 , or some of the process steps may be shared with steps used to form the conventional devices. 
         [0038]      FIG. 5  shows a block diagram of an exemplary design flow  100  used for example, in semiconductor design, manufacturing, and/or test. Design flow  100  may vary depending on the type of integrated circuit (IC) being designed. For example, a design flow  100  for building an application specific IC (ASIC) may differ from a design flow  100  for designing a standard component. Design structure  102  is preferably an input to a design process  104  and may come from an IP provider, a core developer, or other design company or may be generated by the operator of the design flow, or from other sources. Design structure  102  comprises an embodiment of the invention as shown in FIGS.  4 A,B in the form of schematics or HDL, a hardware-description language (e.g., Verilog, VHDL, C, etc.). Design structure  102  may be contained on one or more machine readable medium. For example, design structure  102  may be a text file or a graphical representation of an embodiment of the invention as shown in FIGS.  4 A,B. Design process  104  preferably synthesizes (or translates) an embodiment of the invention as shown in FIGS.  4 A,B into a netlist  106 , where netlist  106  is, for example, a list of wires, transistors, logic gates, control circuits, I/O, models, etc. that describes the connections to other elements and circuits in an integrated circuit design and recorded on at least one of machine readable medium. For example, the medium may be a CD, a compact flash, other flash memory, a packet of data to be sent via the Internet, or other networking suitable means. The synthesis may be an iterative process in which netlist  106  is resynthesized one or more times depending on design specifications and parameters for the circuit. 
         [0039]    Design process  104  may include using a variety of inputs; for example, inputs from library elements  108  which may house a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications  110 , characterization data  112 , verification data  114 , design rules  116 , and test data files  118  (which may include test patterns and other testing information). Design process  104  may further include, for example, standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. One of ordinary skill in the art of integrated circuit design can appreciate the extent of possible electronic design automation tools and applications used in design process  104  without deviating from the scope and spirit of the invention. The design structure of the invention is not limited to any specific design flow. 
         [0040]    Design process  104  preferably translates an embodiment of the invention as shown in FIGS.  4 A,B, along with any additional integrated circuit design or data (if applicable), into a second design structure  120 . Design structure  120  resides on a storage medium in a data format used for the exchange of layout data of integrated circuits and/or symbolic data format (e.g. information stored in a GDSII (GDS2), GL1, OASIS, map files, or any other suitable format for storing such design structures). Design structure  120  may comprise information such as, for example, symbolic data, map files, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a semiconductor manufacturer to produce an embodiment of the invention as shown in FIGS.  4 A,B. Design structure  120  may then proceed to a stage  122  where, for example, design structure  120  proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc. 
         [0041]    References herein to terms such as “vertical”, “horizontal”, etc. are made by way of example, and not by way of limitation, to establish a frame of reference. The term “horizontal” as used herein is defined as a plane parallel to a conventional plane of a semiconductor substrate, regardless of its actual three-dimensional spatial orientation. The term “vertical” refers to a direction perpendicular to the horizontal, as just defined. Terms, such as “on”, “above”, “below”, “side” (as in “sidewall”), “upper”, “lower”, “over”, “beneath”, and “under”, are defined with respect to the horizontal plane. It is understood that various other frames of reference may be employed for describing the invention without departing from the spirit and scope of the invention. It is also understood that features of the invention are not necessarily shown to scale in the drawings. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” 
         [0042]    The fabrication of the semiconductor structure herein has been described by a specific order of fabrication stages and steps. However, it is understood that the order may differ from that described. For example, the order of two or more fabrication steps may be swapped relative to the order shown. Moreover, two or more fabrication steps may be conducted either concurrently or with partial concurrence. In addition, various fabrication steps may be omitted and other fabrication steps may be added. It is understood that all such variations are within the scope of the present invention. It is also understood that features of the present invention are not necessarily shown to scale in the drawings. 
         [0043]    While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.