Abstract:
An integrated circuit fabrication process is provided for forming, a mutual implant region within a well which is shared by a source region of a transistor residing within the well and a well-tie region coupled to the well, thereby providing a single electrical link to the well and the source region. Contacts may be coupled to the mutual implant region, and a conductor may be connected to the contacts. In the instance that the well is a p-type well in which NMOS transistors are formed, a ground voltage may be applied to the conductor to bias both the source region and the well. On the other hand, if the well is an n-type well in which PMOS transistors are formed, a power voltage, VCC, may be applied to the conductor to bias both the source region and the well. Absent the need to form contacts to both the source region and the well-tie region and conductors to such contacts, less space is required to bias the well and the source region. Also, merging a portion of the well-tie region with a portion of the source region affords increased packing density of an integrated circuit. The higher packing density is achieved without resorting to decreasing the dimensions of the well-tie region, and thus without detrimentally increasing the resistance of the well-tie region.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to integrated circuit fabrication and, more particularly, to a mutual implant region formed within a well which is shared by a source region of a transistor and a well-tie region coupled to the well, thereby providing high integration density for the integrated circuit. 
     2. Description of the Related Art 
     The structure and the various components, or features, of a metal oxide semiconductor (“MOS”) device are generally well known. A MOS transistor typically comprises a date conductor spaced above a semiconductor substrate by a gate dielectric. The gate conductor is typically patterned from a layer of polysilicon using various lithography techniques. The substrate generally comprises a lightly doped monolithic silicon-based wafer. The gate conductor serves to self-align impurities forwarded into the substrate on opposite sides of the gate conductor. The impurities placed into the substrate define junctions which are also known as source/drain regions. A “well” which is doped opposite the bulk substrate may exist within a portion of the substrate to accommodate junctions of an impurity type opposite that of the junctions formed in the non-well areas. A typical n-channel MOS (“NMOS”) transistor employs n-type junctions placed into a p-type substrate or a p-type well of an n-type substrate. Conversely, a typical p-channel MOS (“PMOS”) transistor comprises p-type junctions placed into an n-type substrate or an n-type well of a p-type substrate. Wells are often employed when both n-type and p-type transistors are needed to form a complementary MOS (“CMOS”) circuit. 
     Fabrication of an integrated circuit involves placing numerous multiple-input logic devices above and within a semiconductor substrate. Different logic devices employ different configurations of MOS transistors. FIG. 1 depicts an exemplary logic device known as the NAND gate in symbolic form. Although FIG. 1 shows the NAND gate as having only two inputs, A IN  and B IN , a NAND gate may have several inputs. FIG. 2 depicts the circuit diagram of the NAND gate in FIG.  1 . As shown, the NAND gate includes a pair of PMOS transistors  10  connected in parallel and a pair of NMOS transistors  12  connected in series. The source of the lower-most transistor  12  is connected to ground, and the source of each transistor  10  is connected to a VCC voltage, i.e., a power source. 
     FIG. 3 illustrates a cross-sectional view of a portion of a NAND gate which embodies NMOS series-connected transistors  12 . A p-type well  16  resides within an n-type substrate  14 . Well  16  is bounded between trench isolation structures  17 . A p-type implant region  18  is arranged within well  16 . Implant region  18  has been implanted with a higher concentration of p-type species (often referred to as a p +  implant) than has well  16 . Implant region  18  thusly formed is often referred to as a “well-tie” implant region which serves as a low resistive path from a contact  28  to well  16 . A source region  20  of one transistor  12  is laterally spaced from implant region  18  and from a common source/drain region  22  shared by both transistors  12 . Source/drain region  22  functions as a drain for one transistor and as a source for another transistor of the series-connected transistors  12 . A drain region  24  of the other transistor  12  is laterally spaced from source/drain region  22  within well  16 . 
     An interlevel dielectric  26  which serves to isolate transistors  12  extends across the transistors and substrate  14 . Contacts  28 ,  30 , and  32  which comprise a conductive material extend vertically through a portion of interlevel dielectric  26  to implant region  18 , source region  20 , and drain region  24 , respectively. Ground conductors  34  and  35  extend horizontally across interlevel dielectric  26 , electrically linking contacts  28  and  30  to ground. Coupling well-tie implant region  18  to ground conductor  34  affords biasing p-well  16  to ground, and thereby inhibits forward biasing the p-well, and thereby prevents current from flowing from well  16  to the bulk of substrate  14 . Otherwise, current might inadvertently flow from well  16  to other devices residing in substrate  14 , rendering the integrated circuit inoperable. Applying ground voltage to source region  20  biases the source region relative to source/drain region  22 . Assuming that the gate-to-source voltages of n-channel transistors  12  exceed the transistor threshold voltage for each respective transistor, biasing source  20  will allow adequate gate voltages to cause drive current to flow from drain region  24  to source region  20 . An output conductor  36  into which the drive current (i.e., load sink current) of the NAND gate may be measured is connected to drain region  24  through contact  32 . 
     A pervasive trend in modern integrated circuit manufacturing is to produce more complex integrated circuits which operate at higher frequencies (i.e., quickly transition between logic states). Unfortunately, the packing density of an integrated circuit limits the amount of complexity that can be achieved for an integrated circuit. While the well-tie implant region is a critical feature of an integrated circuit which employs wells, it undesirably occupies valuable space within a substrate and/or well of limited lateral area. Moreover, the contact and conductor coupled to the well-tie region increase the amount of space required to bias a well. While reducing the sizes of the contact and the conductor would increase the packing density of the integrated circuit, this is not possible because of the limitations of optical lithography. It is well known that the dimensions of features, e.g., contacts and conductors, patterned using lithography cannot be reduced beyond a lower limit. Also, decreasing the lateral area occupied by a well-tie region is not a viable option because doing so would lead to an undesirable increase in the resistance of the well-tie region. 
     It would therefore be of benefit to develop a technique for reducing the amount of space required to bias a well residing within a semiconductor substrate. That is, the lateral area occupied by only the well-tie region within the well needs to be reduced. However, to avoid an unwanted increase in the resistance to the pathway of current flowing from a contact coupled to the well-tie region to the well, the dimensions of the well-tie region itself cannot be reduced significantly. Moreover, it would also be desirable to reduce the amount of space occupied by the contact and the conductor connected to the well-tie region. Absent the ability to reduce the dimensions of the contact and the cconductor, other measures must be taken to increase the packing density of those elements within the integrated circuit. Therefore, it would be beneficial to improve the layout scheme of conventional well and source contacts by merging the well-tie region, the contact coupled to the well-tie region, and the conductor connected to the contact with other elements of an integrated circuit. Decreasing the space occupied by only those elements used to bias the well would advantageously improve the packing density of the integrated circuit, providing for higher circuit complexity. 
     SUMMARY OF THE INVENTION 
     The problems outlined above are in large part solved by the technique hereof for forming a mutual implant region within a well which is shared by a source region of a transistor residing within the well and a well-tie region coupled to the well. According to a preferred embodiment, a single electrical link is provided to the well and the source region. Several contacts may be coupled to the mutual implant region, and a conductor may be connected to the contacts. In the instance that the well is a p-type well in which NMOS transistors are formed, the conductor may be, rounded to bias both the source region and the well. On the other hand, if the well is an n-type well in which PMOS transistors are formed, a power voltage, VCC, may be applied to the conductor to bias both the source region and the well. Absent the need to form contacts to both the source region and the well-tie region and conductors to such contacts, less space is required to bias the well and the source region. Also, merging a portion of the well-tie region with a portion of the source region affords increased packing density of the ensuing integrated circuit. Advantageously, the high packing density is achieved without resorting to minimizing the dimensions of the well-tie region, and thus without detrimentally increasing the resistance of the well-tie region. 
     In an embodiment of the present invention, a well residing in a semiconductor substrate doped opposite the bulk substrate is provided. A conductive layer comprising, e.g., polycrystalline silicon (“polysilicon”), is then formed across the semiconductor substrate. The conductive layer is patterned to form a gate conductor upon the well. A source/drain implant which is self-aligned to the opposed sidewall surfaces of the gate conductor may be forwarded into the substrate. In this manner, source and drain regions for a PMOS or NMOS transistor are formed within the well. A masking layer is formed across the gate conductors, the drain region, and a portion of the source region. A dopant species of the same type as the dopant species positioned within the well is then implanted into a select region of the well to form a well-tie region coupled to the well. The concentration of dopant species implanted into the well-tie region is greater than that of the dopant species arranged within the well so as to produce a low resistance pathway to the well. The source region overlaps a portion of the well-tie region, creating a mutual implant region. In one embodiment, another transistor configured in parallel or in series with the transistor having the shared source region may also be formed within the well. 
     Subsequent to forming the well-tie region and the transistors within the well, a low resistivity suicide may be formed upon the well-tie region, the mutual implant region, and the source and drain regions. An interlevel dielectric is then deposited across the semiconductor topography to a level spaced above the gate conductors to isolate the transistors from each other and from overlying levels of the ensuing integrated circuit. Conductive contacts are formed through the interlevel dielectric to the mutual implant region. Advantageously, the contacts formed to the mutual implant region are coupled to both the well-tie region and the source region. Contacts may also be formed through the interlevel dielectric to the source and drain regions of the transistors other than the shared source region. Horizontally extending conductors which abut the contacts may be formed within the interlevel dielectric. In one embodiment in which a p-type well and NMOS transistors residing in the well are fabricated, a ground conductor may be coupled to the mutual region. The ground conductor may be grounded to simultaneously bias the shared source region and the well-tie region which is coupled to the well. In another embodiment in which an n-type well and PMOS transistors are formed, a power conductor may be coupled to the mutual region. A power source, VCC, may be supplied to the power conductor to bias both the shared source region and the well-tie region, and hence the well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
     FIG. 1 depicts a logic level diagram of a NAND gate according to conventional design; 
     FIG. 2 depicts a circuit level diagram corresponding to the logic level diagram of FIG. 1 in which the NAND gate includes a pair of parallel-connected PMOS transistors and a pair of series-connected NMOS transistors; 
     FIG. 3 is a cross-sectional view of a semiconductor topography embodying the pair of series-connected transistors depicted in FIG. 2; 
     FIG. 4 a  is a top layout view of a semiconductor topography comprising a pair of series-connected transistors, wherein a mutual implant region is shared by a well-tie region coupled to a well and a source region of one transistor residing within the well; 
     FIG. 4 b  is a cross-sectional view along plane  4   b  of FIG. 4 a , wherein a ground or power source is applied to a conductor electrically linked to the mutual implant region; 
     FIG. 5 depicts a processing step used to form the semiconductor topography of FIG. 4 b , wherein a first type of dopant species is implanted into a semiconductor substrate to form a well; 
     FIG. 6 depicts a processing step used to form the semiconductor topography of FIG. 4 b , wherein a gate dielectric is formed across the substrate and a gate conductor material is deposited across the gate dielectric, subsequent to the step in FIG. 5; 
     FIG. 7 depicts a processing step used to form the semiconductor topography of FIG. 4 b , wherein a pair of gate conductors are patterned from the gate conductor material a lateral spaced distance apart upon the semiconductor substrate, subsequent to the step in FIG. 6; 
     FIG. 8 depicts a processing step used to form the semiconductor topography of FIG. 4 b , wherein a masking layer is formed across a select portion of the substrate and a source/drain implant which is self-aligned to the opposed sidewall surfaces of the gate conductors is forwarded into the well to form source/drain regions; subsequent to the step in FIG. 9; 
     FIG. 9 depicts a processing step used to form the semiconductor topography of FIG. 8, wherein a masking layer is formed across the gate conductors, the drain region, and a portion of the source region, followed by implanting a higher concentration of the first type of dopant species into an exposed region of the well to form a well-tie region and a mutual implant region shared by the well-tie region and the source region, subsequent to the step in FIG. 5; 
     FIG. 10 depicts a processing step used to form the semiconductor topography of FIG. 4 b , wherein dielectric sidewall spacers are formed which extend laterally from the opposed sidewall surfaces of the gate conductors, subsequent to the step in FIG. 9; 
     FIG. 11 depicts a processing step used to form the semiconductor topography of FIG. 4 b , wherein a refractory metal is deposited across the topography and subjected to a heat cycle to initiate reaction between silicon-based surfaces and the metal to form metal suicide, subsequent to the step in FIG. 10; 
     FIG. 12 depicts a processing step used to form the semiconductor topogaraphy of FIG. 4 b , wherein unreacted metal has been removed from the topography to form metal silicide exclusively upon silicon-based surfaces, subsequent to the step in FIG. 11; 
     FIG. 13 depicts a processing step used to form the semiconductor topography of FIG. 4 b , wherein an interlevel dielectric is formed across the topography, subsequent to the step in FIG. 12; 
     FIG. 14 a  is a top layout view of a semiconductor comprising a pair of parallel-connected transistors, wherein a mutual implant region of each transistor is shared by a well-tie region coupled to a well and source regions of transistors residing within the well; and 
     FIG. 14 b  is a cross-sectional view along plane  14   b  of FIG. 4 a , wherein a ground or power source is supplied to a conductor electrically linked to the mutual implant region. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4 a  depicts a top layout view of a semiconductor topography according to an embodiment of the present invention. A well  40  arranged within a semiconductor substrate is shown. A pair of series-connected transistors  42  and  44  are formed upon and within well  40 . A source/drain region  48  implanted into well  40  is shared by both transistors  42  and transistor  44 . Source/drain region  48  functions as the drain of transistor  44  and the source of transistor  42 . Transistor  42  also includes a drain region  50  implanted into well  40  and a gate conductor  52  arranged above a region of well  40  interposed between source/drain region  48  and drain region  50 . Transistor  44  comprises a source region  46  which partially overlaps a well-tie region  56  residing within well  40 . Thus, a mutual implant region  58  which is shared by source region  46  and well-tie region  56  resides within well  40 . Since source region  46  overlaps well-tie region  56 , less lateral area within well  40  is required for the two regions than if they were separated from each other. Transistor  44  also includes a gate conductor  54  arranged above well  44  a spaced distance from well-tie region  58 . Contacts  62  extend vertically between mutual implant region  58  and an overlying horizontally extending conductor  66 . As such, a voltage may be applied to conductor  66  which is coupled to both source region  46  and well-tie region  56  by mutual implant region  58 . Contacts  60  are also formed between drain region  50  and an overlying conductor  64 . 
     FIG. 4 b  is a cross-sectional view along plane  4   b  of FIG. 4 a . Well  40  is arranged within a semiconductor substrate  68 . Trench isolation structures  69  are positioned at the lateral boundaries of well  40 . Alternately, trench isolation structures  69  may be replaced with well-known LOCOS isolation structures. Well  40  is oppositely doped relative to substrate  68 . Well-tie region  56  is doped with the same type of dopant as well  40 . However, the dopant concentration within well-tie region  56  is greater than that within well  40 . Source region  46 , source/drain region  48 , and drain region  50  comprise a relatively hi,h concentration of dopant species opposite in type to the dopant species in well  40  and well-tie region  56 . Accordingly, mutual implant region  58  is heavily doped with both types of dopant species, i.e., n-type species and p-type species. In one embodiment, well  40  may be a p-type well having NMOS transistors  42  and  44  residing therein. In this instance, FIGS. 4 a  and  4   b  depict a portion of, e.g., a NAND gate. In another embodiment, well  40  may be an n-type well having PMOS transistors  42  and  44  residing, therein. In this case, FIGS. 4 a  and  4   b  may depict a portion of, e.g., a NOR gate. 
     Silicide structures  71  are formed upon well-tie region  56 , mutual implant region  58 , source region  46 , source/drain region  48 , and drain region  50 . Silicide structures  71  provide low resistivity contact regions at the interfaces between contact  62  and mutual implant region  58  and between contact  60  and drain region  50 . Silicide structures  73  may also reside upon the upper surfaces of gate conductors  54  and  52 . Conductor  66  may effectuate either a ground conductor or a power conductor. That is, during operation of an ensuing, integrated circuit, a ground supply or a power supply, VCC, may be supplied to conductor  66  to bias both source region  46  and well-tie region  56  which is coupled to well  40 . If well  40  is a p-type well, ground is applied, and if well  40  is an n-type well, VCC is applied. Conductor  64  may function as an output conductor in that any current flowing from source region  46  to drain  50  may be measured from conductor  64 . An interlevel dielectric  70  which serves to isolate transistors  44  and  42  is arranged across the semiconductor topography. 
     FIGS. 5-13 illustrate a sequence of steps which may be performed to form the semiconductor topography depicted in FIGS. 4 a  and  4   b . Turning to FIG. 5, a single crystalline silicon substrate  68  is depicted which is slightly doped with p-type or n-type dopant species. Dopant species opposite in type to those residing within bulk substrate  68  are implanted into a portion of substrate  68  to form a well  40  therein. Trench isolation structures  69  comprising a dielectric, e.g., silicon dioxide (“oxide”), may be formed within substrate  68  proximate the lateral boundaries of well  40 . Alternately, trench isolation structures  69  may be substituted with LOCOS isolation structures. As shown in FIG. 6, a gate dielectric  72  is formed across the surface of substrate  68 . Gate dielectric  72  may, e.g., comprise a thermally grown oxide which is formed by heating substrate  68  while in an oxygen-bearing ambient. A gate conductor material  74  is deposited across gate dielectric  72 . Gate conductor material  74  is preferably chemically-vapor deposited (“CVD”) from, e.g., a silane source, and thus preferably comprises polysilicon. The composition of gate conductor material  74  is not limited to polysilicon and may also be composed of other semiconductive or conductive materials, such as tungsten and aluminum. As depicted in FIG. 7, select portions of gate conductor material  74  may be removed to define a pair of gate conductors  52  and  54  which are laterally spaced apart from each other. Those portions of gate conductor material  74  may be removed by first patterning photoresist across the gate conductor material exclusive of upon those select portions. Those portions of gate conductor material  74  not covered by the photoresist may then be etched away using, e.g., a dry, plasma etch technique. 
     As shown in FIG. 8, a masking layer  76  comprising, e.g., photoresist may then be patterned above a select portion of substrate  68  a lateral spaced distance from gate conductor  54 . Subsequently, a source/drain implant of dopant species which are opposite in type to those previously implanted into well  40  are forwarded into unmasked areas of well  40 . That is, if NMOS transistors are being formed, n-type species are implanted, and if PMOS transistors are being formed, p-type species are implanted. Some commonly used p-type species are boron and boron difluoride, and some commonly used n-type dopants are arsenic and phosphorus. Gate dielectric  72  provides adequate distribution of the implanted species. Gate conductors  52  and  54  serve as masks during the implantation step, thereby inhibiting dopant species from passing into channel regions of well  40  residing beneath the gate conductors. The dopant species that become positioned within gate conductors  52  and  54  may render the gate conductors conductive if they comprise polysilicon. Further, masking layer  76  prevents dopant species from passing into the underlying portion of well  68 . As a result of the source/drain implant, a source region  46  is formed for one transistor, a drain region  50  is formed for another transistor, and a source/drain region  48  shared by the two series-connected transistors is formed within well  40 . Masking layer  76  may be stripped from the semiconductor topography after the source/drain implant. 
     Turning to FIG. 9, a masking layer  70  comprising, e.g., photoresist, may be patterned across gate conductors  52  and  54 , drain region  50 , source/drain region  48 , and a portion of source region  46  using optical lithography. Masking layer  70  is not limited to photoresist and may include any material since the masking layer is sacrificial in that it will be removed. If masking layer  70  is not photoresist, it may be patterned using both lithography and an etch technique. The same type of dopant species as those previously implanted into substrate  68  may then be forwarded into an exposed portion of well  40  to form a well-tie region  56 . The implant used to form well-tie region  56  is preferably performed at a higher dose and lower energy than the implant used to form well  40 . As such, well-tie region  56  contains a higher concentration of dopant and is shallower than well  40 . Well-tie region  56  partially overlaps source region  46 . A mutual implant region  58  is thusly formed which is common to both well-tie region  56  and a source region  46 . Concurrent with implanting dopant species into well-tie region  56 , the dopant species are also implanted into other well regions of substrate  68  which are doped opposite to well  40 . As a result, source and drain regions for other transistors of the integrated circuit are formed to effectuate a CMOS circuit which includes both NMOS transistors residing in p-type wells and PMOS transistors residing in n-type wells. 
     Turning to FIG. 10, subsequent to removing masking layer  70 , dielectric sidewall spacers  57  are formed which extend laterally from the opposed sidewall surfaces of gate conductors  52  and  54 . Sidewall spacers  57  may be formed by first CVD depositing a dielectric, e.g., silicon dioxide, silicon nitride, or silicon oxynitride, across the semiconductor topography. The dielectric is then subjected to an anisotropic etch which occurs at a faster rate in a vertical direction than in a horizontal direction. The etch duration is chosen to terminate after the dielectric has been removed from horizontally oriented surfaces such that the dielectric is only retained upon the vertically oriented sidewall surfaces of gate conductors  52  and  54 . Preferably, the etch duration is chosen to terminate before substantial portions of substrate  68  can be removed such that gate dielectric  72  is removed from the substrate exclusive of gate conductors  52  and  54 . In this manner, a portion  55  of the dielectric is removed while dielectric sidewall spacers  57  are retained upon the opposed sidewall surfaces. Alternately, dielectric sidewall spacers  57  may be formed prior to performing the source/drain implant depicted in FIG.  9 . As such, a lightly doped drain (“LDD”) implant which is self-aligned to the opposed sidewall surfaces of gate conductors  52  and  54  may be forwarded into well  40  subsequent to forming masking layer  76  and before forming sidewall spacers  57 . The LDD implant is preferably formed at a lower dose and energy than the source/drain implant. If the source/drain implant is performed after the formation of sidewall spacers  57 , the source/drain implant will be self-aligned to the exposed lateral surfaces of the spacers. As such, junctions  46 ,  48 , and  50  will be graded such that the dopant concentration decreases in a lateral direction toward the gate conductors. 
     FIGS. 11-12 illustrate the formation of silicide structures  71  upon well-tie region  56 , mutual implant region  58 , source region  46 , source/drain region  48 , and drain region  50  and of polycide structures  73  upon the upper surfaces of gate conductors  52  and  54 . As shown in FIG. 11, a refractory metal  79 , eg., titanium or cobalt, is deposited across the semiconductor topography using either sputter deposition from a metal target or metal organic chemical vapor deposition (“MOCVD”) from a gas comprising a metal organic-containing compound. Sidewall spacers  57  are strategically placed laterally adjacent the sidewall surfaces of gate conductors  52  and  54  to inhibit refractory metal  79  form contacting the gate conductors. Metal  79  is then exposed to a form of radiation  78  supplied from either an annealing furnace or a Rapid Thermal Anneal (“RTA”) chamber. As a result of being subjected to a heat cycle, metal  79  reacts with underlying silicon of substrate well  40  and polysilicon gate conductors  52  and  54  to form a metal silicide. Unreacted portions of refractory metal  79  are then removed using an etch technique which is highly selective to the metal. Consequently, self-aligned silicide (i.e., salicide) structures  71  are formed exclusively well  40  and polycide structures  73  are formed exclusively upon the upper surfaces of gate conductors  52  and  54 , as shown in FIG.  12 . 
     Subsequently, an interlevel dielectric  70  may be formed across the semiconductor topography, as shown in FIG.  13 . Interlevel dielectric  70  may comprise a CVD deposited dielectric. For example, interlevel dielectric  70  may be LPCVD deposited from a TEOS source across the semiconductor topography. Alternately, interlevel dielectric  70  may comprise a spin-on deposited dielectric, e.g., spin-on-glass. The upper surface of interlevel dielectric  70  is substantially planarized using, e.g., chemical-mechanical polish or sacrificial etchback. Openings may be etched entirely through select portions of interlevel dielectric  70  using, e.g., a plasma (anisotropic) etch technique. A conductive material, e.g., tungsten or titanium, may be deposited into the opening,s to form the contacts  62  and  60  depicted in FIG. 4 b . Trenches may also be etched horizontally across interlevel dielectric  70  and above contacts  62  and  60 . Those trenches may be filled with a conductive material, e.g., copper or aluminum, to form conductors  66  and  64 . 
     FIG. 14 a  depicts a top layout view of a semiconductor topography according to another embodiment of the present invention. A well  80  residing within a semiconductor substrate is depicted. A pair of parallel-connected transistors  82  and  84  are formed upon and within well  80 . Transistor  82  includes a source region  92  which partially overlaps a well-tie region  98  residing within well  80 . Therefore, a mutual implant region  102  which is shared by source region  92  and well-tie region  98  is arranged within well  80 . Transistor  82  also includes a drain region  94  implanted into well  80  and a gate conductor  96  arranged above a region of well  40  interposed between source region  92  and drain region  94 . Moreover, transistor  84  also includes a source region  86  which partially overlaps well-tie region  98 . As such, another mutual implant region  100  common to both source region  86  and well-tie region  98  is arranged within well  80 . 
     A drain region  88  of transistor  84  is arranged within well  80  a lateral spaced distance from source region  86 . Transistor  84  also includes a gate conductor arranged above the region of well  80  interposed between drain region  88  and source region  86 . Although not shown, an isolation structure arranged within the substrate is interposed between transistors  84  and  86  to isolate the transistors. Contacts  104  extend vertically between mutual implant region  100  and an overlying horizontally extending conductor  114 . Also, conductor  144  is coupled to mutual implant region  102  via contacts  106 . Thus, conductor  114  may be grounded or subjected to a power source to bias source regions  100  and  102  and well-tie region  98  through mutual implant regions  100  and  102 . Contacts  108  and  110  couple respective drain regions  88  and  94  to an overlying horizontally extending conductor  112 . 
     FIG. 14 b  is a cross-sectional view along plane  14   b  of FIG. 14 a . Well  80  is arranged within a semiconductor substrate  1   16  comprising slightly doped single crystalline silicon. Trench isolation structures  118  are positioned at the lateral boundaries of well  80 . Alternately, trench isolation structures  118  may be replaced with well-known LOCOS isolation structures. Well  80  is oppositely doped relative to substrate  116 . Well-tie region  98  is doped with the same type of dopant as well  80 . However, the dopant concentration within well-tie region  98  is greater than that within well  80 . Source region  86  and drain region  50  comprise a relatively high concentration of dopant species opposite in type to the dopant species in well  80  and well-tie region  98 . Accordingly, mutual implant region  100  is heavily doped with both types of dopant species, i.e., n-type species and p-type species. In one embodiment, well  80  may be an n-type well having PMOS transistors  82  and  84  residing therein. In this instance, FIGS. 14 a  and  14   b  depict a portion of, e.g., a NAND gate. In another embodiment, well  80  may be a p-type well having NMOS transistors  82  and  84  residing therein. In this case, FIGS. 4 a  and  4   b  may depict a portion of, e.g., a NOR gate. 
     Silicide structures  120  are formed upon well-tie region  98 , mutual implant region  100 , source region  86 , and drain region  88 . Silicide structures  120  provide low resistivity contact regions at the interfaces between contact  104  and mutual implant region  100  as well as between contact  108  and drain region  88 . Gate conductor  90  is spaced above well  80  by a gate dielectric  119 . A silicide (i.e., polycide) structure  122  may also reside upon the upper surface of gate conductor  90 . Dielectric sidewall spacers  91  may extend from the opposed sidewall surfaces of ate conductor  90 . The sidewall spacers  91  serve to inhibit the formation of silicide laterally adjacent gate conductor  90 , and thus prevent a conductive path from forming between gate conductor  90  and source and drain region  86  and  88 . In an alternate embodiment, LDD areas may be arranged within well  80  directly underneath spacers  91  such that source and drain regions  86  and  88  are laterally spaced from gate conductor  90 . Conductor  114  may effectuate either a ground conductor or a power conductor. That is, during operation of an ensuing integrated circuit, a ground or a a power source, VCC, may be suppled to conductor  114  to bias both source region  86  and well-tie region  98  which is coupled to well  80 . If well  80  is a p-type well, conductor  114  is connected to ground, and if well  40  is an n-type well, conductor  14  is connected to a power supply. Conductor  112  may function as an output conductor in that any current flowing from source region  86  to drain  88  may be measured from conductor  112 . An interlevel dielectric  124  which serves to isolate transistor  94  is arranged across the semiconductor topography. 
     It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide a method for forming a transistor having an ultra short channel length dictated by the width of a gate conductor patterned upon a gate dielectric having a relatively high dielectric constant. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. It is intended that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.