Abstract:
A method is provided for forming an integrated circuit having junctions of n-channel transistors dissimilar to junctions of p-channel transistors. First and second gate conductors are formed upon a gate dielectric on a semiconductor substrate. Spacers are formed on sidewalls of the first and second gate conductors. Portions of the spacers are subsequently removed such that the lateral width of each spacer is reduced to form residual spacers. The residual spacers may subsequently be removed, exposing sidewalls of the first and second gate conductors. At various stages of the method, n-type impurities may be implanted into the substrate, masked by the first gate conductor and any adjacent spacers or residual spacers. P-type impurites may also be implanted into the substrate, masked by the second gate conductor and any adjacent spacers or residual spacers. More n-type than p-type impurity implants are preferably used, so that n-type junction regions formed on either side of the first gate conductor have a more graded profile than do the p-type junction regions formed on either side of the second gate conductor.

Description:
This is a Division of application Ser. No. 08/761,401, filed Dec. 6, 1996 now U.S. Pat. No. 5,869,866. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor processing and, more particularly, to a CMOS integrated circuit having dissimilarly placed graded junctions produced by successive removal of a gate conductor sidewall spacer. 
     2. Description of Relevant Art 
     Fabrication of a metal-oxide semiconductor (“MOS”) transistor is well known. The manufacture of an MOS transistor begins by defining active areas where the transistor will be formed. The active areas are isolated from other areas on the semiconductor substrate by various isolation structures formed upon and within the substrate. Isolation structures come in many forms. For example, the isolation structures can be formed by etching trenches into the substrate and then filling the trenches with a dielectric fill material. Isolation structures may also be formed by locally oxidizing the silicon substrate using the well recognized LOCOS technique. 
     Once the isolation structures are defined between transistor active areas, a gate dielectric is formed. Typically, the gate dielectric is formed by thermal oxidation of the silicon substrate. Thermal oxidation is achieved by subjecting the substrate to an oxygen-bearing, heated ambient in, for example, an oxidation furnace or a rapid thermal anneal (“RTA”) chamber. The conductor material is then deposited across the entire dielectric-covered substrate. The gate conductor material is preferably polycrystalline silicon, or polysilicon. The polysilicon layer is then patterned using a photolithography mask. The mask allows selective removal of a light-sensitive material deposited entirely across polysilicon. The material which is exposed can, according to one embodiment, be polymerized, and that which is not exposed removed. Selective polymerization is often referred to as the “develop” stage of lithography. The regions which are non-polymerized are removed using the etch stage of lithography. 
     An n-channel transistor, or NMOS transistor, must in most instances be fabricated differently from a p-channel transistor, or PMOS transistor. NMOS transistors employ n-type dopants on opposite sides of the NMOS gate conductor, whereas PMOS transistors employ p-type dopants on opposite sides of the PMOS transistor gate conductor. The regions of the substrate which receive dopants on opposite sides of the gate conductor are generally referred to as junction regions, and the distance between junction regions is typically referred to as the physical channel length. After implantation and subsequent diffusion of the junction regions, the distance between the junction regions become less than the physical channel length and is often referred to as the effective channel length (“Leff”). In high density designs, not only does the physical channel length become small so too must the Leff. As Leff decreases below approximately 1.0 μm, for example, a problem known as short channel effects (“SCE”) becomes predominant. 
     A problem related to SCE, and the subthreshold currents associated therewith, but altogether different is the problem of hot-carrier effects (“HCE”). HCE is a phenomenon by which hot-carriers (“holes and electrons”) arrive at or near an electric field gradient. 
     The electric field gradient, often referred to as the maximum electric field (“Em”), occurs near the drain during saturated operation. More specifically, the electric field is predominant at the lateral junction of the drain adjacent to the channel. The electric field at the drain causes primarily electrons in the channel to gain kinetic energy and become “hot”. These hot electrons traveling to the drain lose their energy by a process called impact ionization. Impact ionization serves to generate electron-hole pairs, wherein the pairs migrate to and become injected within the gate dielectric near the drain junction. 
     Traps within the gate dielectric generally become electron traps, even if they are partially filled with holes. As a result, there is a net negative charge density in the gate dielectric. 
     The trapped charge accumulates with time, resulting in a positive threshold shift in the NMOS transistor, or a negative threshold shift in a PMOS transistor. It is known that since hot electrons are more mobile than hot holes, HCE causes a greater threshold skew in NMOS transistors than PMOS transistors. Nonetheless, a PMOS transistor will undergo negative threshold skew if its Leff is less than, e.g., 0.8 μm. 
     Unless modifications are made to the fabrication sequence, problems resulting from HCE will remain. To minimize these problems, a mechanism must be derived that disperses and thereby reduces Em. That mechanism is often referred to as the double-diffised drain (“DDD”) and lightly doped drain (“LDD”) techniques. The purpose behind using DDDs and LDDs structures is to absorb some of the potential into the drain and away from the drain/channel interface. The popularity of DDD structures has given way somewhat to LDD structures since DDD may cause unacceptably deep junctions and deleterious junction capacitance. 
     A conventional LDD structure is one whereby a light concentration of dopant is self-aligned to the edge of the gate conductor. The light-dopant concentration is then followed by a heavier-dopant concentration which is self-aligned to a spacer formed on the sidewalls of the gate conductor. The purpose of the first implant dose is to produce a lightly doped section of both the source and drain junction areas at the gate edge near the channel. The second implant dose is spaced from the channel a distance dictated by the thickness of the sidewall spacer. Resulting from the first and second implants, a dopant gradient occurs across the junction from the source/drain area of the junction to the LDD area adjacent the channel. 
     The dopant gradient across the junction, henceforth referred to as a graded junction, is necessary for several reasons. First, the lightly doped region (LDD area) is used to assume a substantial portion of the entire voltage drop associated with Em. It has been reported that the LDD area may in some instances reduce Em at the drain juncture by approximately 30-40%. Secondly, the heavier dosage within the source/drain area forms a low resistivity region suitable for enhanced contact conductivity. Further, the source/drain dose is implanted at a higher energy necessary to produce deeper source/drain junctions and thereby provide better protection against junction spiking. 
     The benefits of using an LDD a in conjunction with a source/drain area are generally well documented. However, the benefits differ depending upon whether an NMOS device or a PMOS device is produced. For example, an NMOS device requires an LDD area more so than a PMOS device. However, an unduly large LDD area would hamper NMOS performance by increasing the source-drain resistance. On the other hand, the dopants used to form a PMOS device (e.g., boron) are more mobile than the dopants used to form an NMOS device. As such, the p-type dopants regularly segregate and migrate from their original implant position toward and into the channel area. This lessens Leff and produces deleterious SCE problems. 
     It would therefore be desirable to employ a CMOS fabrication process which can produce NMOS junctions dissimilar from PMOS junctions. If the NMOS junction is graded such that the LDD area is relatively small, in conjunction with other HCE-prevent dopant areas, then performance may be enhanced. Similarly, if the PMOS junction can be graded such that the highly mobile source/drain implants are drawn further from the channel then SCE can be more carefully controlled. 
     SUMMARY OF THE INVENTION 
     The problems outlined above are in large part solved by an improved fabrication process hereof. The process can be used to form a PMOS transistor, an NMOS transistor, or CMOS device (having both PMOS and NMOS transistors), wherein the transistors have junctions of possibly dissimilar dopant gradients. The NMOS transistor includes a junction having numerous implant areas of increasing concentration as they exist in successive areas spaced from the transistor channel. Thus, the NMOS transistor may include an LDD area, at least one medium doped drain (“MDD”) area and a source/drain area. The LDD serves to absorb and thereby lessen Em and the problems of HCE. The MDD serves not only to lessen Em but also to enhance contact conductivity and source-drain responsiveness. The PMOS transistor may be formed on a separate monolithic substrate or on the same substrate as the NMOS transistor in accordance with the present CMOS process. The PMOS transistor includes a less graded junction then that of the counterpart NMOS transistor. More specifically, the PMOS transistor may merely employ an LDD area and a source/drain area. The source/drain area is drawn away from the PMOS channel. This is due to the highly mobile nature of boron atoms which exist in the PMOS source/drain area. The mobile boron atoms readily migrate from the heavier concentration source/drain region toward the channel. By drawing the source/drain area further from the channel, a lessened opportunity exists to migrate into the channel and thereby change the designed PMOS operation. 
     The benefits of having dissimilar graded junctions, depending upon whether a PMOS transistor or an NMOS transistor is employed, are herein achieved using a spacer which can be gradually removed. The removable spacer is configured upon the outer surface of a dielectric-covered gate conductor. Preferably, the removable spacer exists exclusively on sidewall surfaces of an oxide-covered conductor. According to one embodiment, the gate conductor comprises polycrystalline silicon (“polysilicon”) having thermally grown oxide on its upper and sidewall surfaces. The removable spacer preferably comprises a material which has a dissimilar etch characteristic from that of the oxide-covered polysilicon. The spacer material must be chosen such that it can be removed partially in steps, and thereafter completely removed while not sufficiently attacking material underneath the oxide. The removable spacer preferably comprises any material which has a dissimilar etch characteristic from polycrystalline oxide, a suitable material being nitride (i.e., silicon nitride) or polysilicon. 
     Of prime importance is the selectivity by which the removable spacer can be etched in steps to reduce its thickness. After each step, an implant may be undertaken. Successive etch steps interspersed with decreasing implant concentrations causes a graded junction to be formed in the semiconductor substrate. Removal of the spacer can vary depending upon whether the spacer exists on a sidewall surface of an NMOS transistor or a PMOS transistor. If the NMOS transistor is to have a junction with a more gradual grading, then the number of etch steps followed by implant steps will be larger for the NMOS transistor than the PMOS transistor. Both transistor types can undergo partial removal of their spacers; however, the number of implants following spacer etch may be different depending upon whether the NMOS junction is implanted versus the PMOS junction. For example, only two implants may be needed to form a PMOS junction, whereas more than two may be required for an NMOS junction. 
     The present sequence forms source/drain regions prior to the lesser concentration MDD and LDD regions. This is beneficial in that a higher temperature anneal takes place before a lower temperature anneal. The higher temperature anneal is needed to activate the higher concentration source/drain dopants. However, lower temperatures are all that is required to activate the lower concentration LDD and MDD implants. The lower temperature anneals which occur subsequent to the higher temperature anneals help prevent substantial movement of highly mobile dopants, such as boron present near the channel region. Higher temperature anneals on the source/drain dopants will cause migration but, since the dopants exist a spaced region away from the channel, migration will not generally occur into the channel. It is the LDD area that must incur minimal temperature anneals since those areas pre-exist near the channel. 
     Broadly speaking, the present invention contemplates a CMOS integrated circuit having possibly numerous transistors. Each transistor comprises a gate conductor dielectrically spaced over the semiconductor substrate. The gate conductor has opposed sidewall surfaces on which and into which an oxide is configured. Extending laterally from the oxide-covered sidewall surfaces is a removable spacer. The spacer is not only partially removable in successive steps, but can be entirely removed if desired. The oxide which covers the gate conductor serves as an etch stop. The etch stop prevents the etchant species from disrupting and contaminating the underlying polysilicon gate material and/or single crystalline silicon substrate material. 
     According to one embodiment, the integrated circuit comprises both NMOS and PMOS transistors. Each transistor comprises a spacer which is at least partially reducible in response to a first etch. Thereafter, a first implant species is forwarded into the semiconductor substrate, wherein the thickness of the spacer serves to mask the first implant species. A second etch may be applied to further reduce the spacer thickness after the first implant species has been forwarded into the semiconductor substrate. A second implant species may then be forwarded into the semiconductor substrate after the second etch has been concluded. Successive removal and implantation steps preferably forms a junction area defined as having at least three regions of increasing dopant concentration extending respective incremental distances from the gate conductor or, more specifically, the channel region underlying the gate conductor. 
     The present invention further contemplates a method for forming an integrated circuit. The method includes the steps of providing a semiconductor topography and patterning first and second gate conductors upon the semiconductor topography. A spacer is then formed on sidewall surfaces of both the first and second gate conductors. 
     The second gate conductor is then covered and, using the first gate conductor and the spacer as a mask, a first n-type dopant is implanted into the semiconductor topography. Thereafter, the first gate conductor is covered and, using the second gate conductor and the spacer as a mask, a first p-type dopant is implanted into the semiconductor substrate. A portion of the spacer is then removed to form a first residual spacer. The steps of covering the first and second gate conductors may be repeated or, alternatively, only the second gate conductor need be covered. In the latter instance, the first gate conductor and the first residual spacer is used as a mask, and a second n-type dopant is implanted into the semiconductor substrate of lesser concentration than the first n-type concentration. Yet further, another portion of the spacer may be removed to form a second residual spacer, if desired. Again, the second gate conductor may be covered and, using the first gate conductor and the second residual spacer as a mask, a third n-type dopant may be implanted into the semiconductor topography. The third n-type dopant is preferably of lesser concentration than the second n-type dopant. Thereafter, the spacer may be entirely removed. After the spacer is entirely removed, fourth n-type dopant species may be implanted into the semiconductor topography during times when the second gate conductor is covered and only the first gate conductor serves as a mask. Conversely, the first gate conductor may be covered and, using only the second gate conductor as a mask, a second p-type dopant may be implanted into the semiconductor topography. The fourth n-type dopant is of lesser concentration than the third n-type dopant, and the second p-type dopant is of lesser concentration than the first p-type dopant. The above process describes formation of a graded junction within both an NMOS transistor and a PMOS transistor using successive, partial removal of a spacer and implantation into a semiconductor substrate. 
    
    
     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 is a partial cross-sectional view of a semiconductor topography showing a gate dielectric formed upon active areas of the topography; 
     FIG. 2 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 1, wherein gate conductors are formed having sidewall oxides; 
     FIG. 3 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 2, wherein a removable spacer is formed upon the sidewall oxides; 
     FIG. 4 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 3, wherein an n-type source/drain implant is forwarded into the semiconductor substrate; 
     FIG. 5 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 4, wherein a p-type source/drain implant is forwarded into the semiconductor substrate; 
     FIG. 6 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 5, wherein the removable spacers are partially removed; 
     FIG. 7 is a detailed view along region  7  of FIG. 6 illustrating the contours of the spacer as it is being removed (etched); 
     FIG. 8 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 6, wherein an n-type MDD implant is forwarded into the semiconductor substrate; 
     FIG. 9 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 8, wherein additional spacer material is removed; 
     FIG. 10 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 9, wherein another n-type MDD implant is forwarded into the semiconductor substrate; 
     FIG. 11 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 10, wherein the removable spacer is entirely removed; 
     FIG. 12 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 11, wherein an n-type LDD implant is forwarded into the semiconductor substrate; and 
     FIG. 13 is a partial cross-sectional view of a semiconductor topography according to a processing step subsequent to FIG. 12, wherein a p-type LDD implant is forwarded into the semiconductor substrate. 
    
    
     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 PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIG. 1 illustrates a partial cross-sectional view of a semiconductor topography  10 . Topography  10  includes a semiconductor substrate  12  into which and upon which various isolation structures  14  are formed. Isolation structure can be formed using either the shallow trench process, or the LOCOS process. In the embodiment shown, a shallow trench process is illustrated as an exemplary isolation structure. Isolation structure  14  comprises a dielectric which does not readily allow gate dielectric formation. Instead, a gate dielectric  16  is formed in regions absent structure  14 . Gate dielectric  16  is preferably formed according to a thermal oxidation process upon and within the silicon-based substrate  12 . The thickness of the grown oxide depends upon the processing parameters desired, and the needed performance of the ensuing transistor. 
     Referring to FIG. 2, gate conductors  18  are patterned from a layer of material  18   a . According to a preferred embodiment, material  18   a  comprises a layer of polycrystalline silicon. The polycrystalline silicon (“polysilicon”) is selectively removed using well-known lithography procedures. The remaining portions of the polysilicon layer form gate conductors  18 . Each gate conductor may thereby comprise polysilicon patterned upon gate dielectric  16 , and the combination of gate conductor  18  and gate dielectric  16  exist in select regions, defined as active regions of semiconductor topography  10 . Active regions are those regions which exist between isolation structures  14 . Preferably, gate conductor  18  includes a polysilicon oxide  19 . Polysilicon oxide  19  exists on the upper and sidewall surfaces of gate conductor  18 , and is grown according to well-known thermal oxidation procedures. 
     FIG. 3 depicts a removable material  20   a  deposited across the entire topography  10 . Material  20   a  can be deposited using various techniques, a suitable technique being chemical vapor deposition (“CVD”). Alternatively, material  20   a  may be spin-on deposited. In either instance, material  20   a  conforms to the upper topography over and between gate conductors  18 . Thereafter, an anisotropic etch is applied to material  20   a . The anisotropic etch is chosen as a dry etch or plasma etch, and serves to remove material from horizontal surfaces at a faster rate than material from vertical surfaces. Plasma etch techniques are often referred to as “ion-assisted etch”. The wafer generally resides on a chuck, and ions from an ion source are directed at a perpendicular angle upon the wafer. The ions dislodge material from the exposed topography, and the chemical (gaseous) etchant interacts with the bond sites to aid in the removal process. The etch duration is terminated, however, before the entirety of material  20   a  is removed. Accordingly, anisotropic etch duration is chosen to remove material from horizontal surfaces while retaining some material on vertical surfaces. Retainage occurs primarily on the sidewall surfaces of  18   c  and  18   d  of the oxide-covered gate conductor  18 . The retained portion is henceforth referred to as a removable spacer  20 . 
     The deposition thickness of material  20   a  is varied to account for a desired thickness which would preferably equal if not exceed the thickness of gate conductor  18 . Once material  20   a  is removed from horizontal surfaces to form spacer  20 , spacers  20  serve to mask, in combination with gate conductor  18 , subsequent implant species. 
     FIG. 4 illustrates the masking of an n-type source/drain implant  24 . Implant  24  is preferably arsenic which is implanted at a substantially heavy concentration and at significant energies to form NMOS source/drain regions  26 . Spacers  20 , gate conductor  18  and implant masking material  28  serve to mask implant species  24  from the channel area of NMOS transistor  30  as well as from the entire active region of PMOS transistor  32 . 
     Spacer  20  and, more specifically, the horizontal thickness of spacer  20  serves to mask implant species  24  away from the channel underneath NMOS transistor  30 . The process used in forming source/drain regions  26  is repeated to form PMOS source/drain regions  36 . This repeated process is shown in FIG.  5 . The implant species used in forming regions  36  are shown in FIG. 5 as p-type species which, according to one embodiment, comprise boron such as boron found in, for example, BF 2  or B 11 . Boron implant  38  is of relatively high concentrations, and is implanted at relatively high energies that form the corresponding source/drain regions. A masking material  40  is used to cover the active areas of NMOS transistor  30  while leaving only spacer  20  and gate conductor  18  to mask active areas of the PMOS transistor  32 . 
     Masking material  28  is formed as shown in FIG. 4 by depositing a light-sensitive material, such as photoresist across the entire topography. Thereafter, select regions of that material are exposed and then removed using conventional lithography techniques. As such, material  28  is patterned to cover select areas prior to n-type implant. Thereafter, material  28  is readily removed in readiness for repeating the deposition, select exposure and etch process to form material  40  in FIG.  5 . The mechanism for patterning a photoresist is generally known in the art. Moreover, the selective formation of a masking material upon specific active areas is also known. 
     FIG. 6 illustrates partial removal of spacer  20  on sidewall surfaces of gate conductors attributed to both PMOS and NMOS devices. According to a preferred embodiment, spacer  20  comprises nitride or polysilicon which can be etched according to various wet etch techniques. Accordingly, the etch process is preferably isotropic. If spacer  20  comprises nitride (i.e., silicon nitride—Si 3 N 4 ), then a suitable etch comprises reflux boiling phosphoric acid. Not shown but present is an oxide layer which forms across the semiconductor substrate at the same time in which oxide  19  is formed on the polysilicon sidewall surfaces. The oxide serves as a protectant against the spacer etchant and also serves to protect the substrate during the previous source/drain implantation. If spacer  20  comprises polysilicon, then the etchant is preferably a mixture of nitric acid and hydrofluoric acid. Regardless of the composition of spacer  20 , the aforesaid etchant substantially attacks spacer  20  yet does not attack the oxide etch stop/barrier  19  existing on the upper surface of the polysilicon gate conductor, the sidewall surfaces of the polysilicon gate conductor and the upper surfaces of the semiconductor substrate active regions. The mixtures of polysilicon etchant or nitride etchant can be varied to not only achieve the desired selectivity, but also to control the rate at which spacer  20  is removed. For example, a low HF and high HNO 3  ratio will remove polysilicon spacer  20  at a slower etch rate. The benefits of using a wet etchant are primarily attributed to its high selectivity to the spacer as opposed to adjacent oxide etch stop materials. However, this implies that an oxide  19  must exist across the substrate and across the gate conductor adjacent spacer  20 . Given the processing step shown in FIG. 2, this pre-existing condition is readily achieved and therefore having an oxide does not inconvenience the present fabrication sequence. 
     FIG. 7 depicts etchant profiles  40   a ,  40   b ,  40   c , etc. Profiles  40  indicate a steady decrease in both the horizontal and vertical directions  42   a  and  42   b , respectively, of residual spacer  20   b . Contours  40  change in accordance with the duration of the etch step. The duration is terminated after a pre-defined amount of time to produce a residual spacer  20   b  having a horizontal distance which is drawn inward toward the respective gate conductors of both the NMOS and PMOS transistors. The change in the spacer thickness proves beneficial in allowing a subsequent implant to occur as shown in FIG.  8 . 
     FIG. 8 depicts a masking material  46  formed in accordance with the formation of masking material  28  shown if FIG.  4 . Masking material  46 , in conjunction with gate conductor  18  and residual spacer  20   b  allows masking of n-type implant species  48 . Implant species  48  is denoted as a MDD implant which is placed into junction regions of NMOS transistor  30  to produce MDD areas  50 . The MDD (medium doped drain) regions receive a dopant concentration and dopant energies which are less than the source/drain regions  26 . 
     FIG. 9 depicts further removal of the spacer using the same etch technique used in producing residual spacer  20   b  from the original spacer  20 . Resulting from the additional removal step, a second residual spacer  20   c  occurs. The etch duration is timed such that a pre-defined horizontal thickness exists after etch completion. That pre-defined thickness proves beneficial in masking another n-type implant shown in FIG.  10 . 
     FIG. 10 depicts n-type implant (second MDD) implant  52 . Implant  52  is masked similar to that shown in FIG. 8, except that second residual spacer  20   c  is used instead of first residual spacer  20   b . This allows a second MDD area  54  to be formed nearer the channel region of NMOS transistor  30 . The masking material used to mask off PMOS device  32  is shown as reference numeral  56 . 
     FIG. 11 depicts removal of what remains of the spacer—in this case second residual spacer  20   c . The complete removal of spacer  20   c  is achieved using the same etch composition and procedure as that used in partially removing the spacer. Complete removal is performed without harming the underlying silicon-based material due to the oxide etch stop which exists between the spacer and either the polysilicon gate  18  or the silicon substrate  12 . 
     Etch stop oxide  19  remains to some degree on the silicon substrate as well as on the polysilicon even after overetch occurs. FIG. 12 depicts an n-type LDD implant  64  forwarded through the remaining oxide (not shown) and into substrate  12 . The resulting LDD area is shown as reference numeral  62 . Gate conductor  18 , which includes a residual sidewall oxide  19  serves to self-align implant  60  near the channel of the NMOS transistor  30 . A masking material  64  is used to cover the active regions of the PMOS transistor  32 . 
     FIG. 13 depicts a p-type LDD implant  66  forwarded into substrate  12  about the oxide-coated gate conductor of PMOS device  32 . A masking material  68  serves to block off p-type implant  66  from NMOS transistor active regions. 
     Active areas which are not masked receive implant  66 , and are denoted as p-type LDD areas  70 . LDD areas  70 , like LDD areas  62  are formed having a lower dopant concentration and are configured at a shallower depths than the MDD areas or the source/drain areas. Accordingly, the present process employs lower dopant concentrations and lower implant energies as the implant being produced is configured nearer the channel of either the NMOS or PMOS transistor. The NMOS transistor includes not only source/drain areas  26  and LDD areas  62 , but also include at least one MDD area. The present sequence illustrates two MDD areas  50  and  54 ; however, it is understood that possibly numerous other MDD areas may also be employed. The total number of MDD areas can therefore vary as demanded by the required doping profile of the ensuing NMOS transistor. There may be instances in which a doping profile must be as gradual and therefore as smooth as possible. In those instances, possibly more than two MDD are needed. Otherwise, two or fewer MDD areas are all that is required to achieve the aforementioned objectives. Conversely, the PMOS transistor typically only requires a source/drain area. However, in instances where the PMOS transistor gate length is quite small, then not only is a p-type LDD required, but the associative LDD must be fairly large to compensate for highly mobile boron atoms placed within the PMOS source/drain areas. The enlarged p-type LDD therefore draws the highly concentrated boron atoms associated with the source/drain away from the channel to minimize their effect on channel operation. While requiring an LDD in an NMOS device, the LDD need only be fairly small since the heavier concentration source/drain and MDD areas employ larger atomic structures which do not readily migrate. Having one or more MDD areas thereby enhances transistor operation by reducing parasitic resistance in the NMOS transistor source-drain path. 
     It is understood that, although not shown, a silicide is formed in the junctions of the ensuing NMOS and PMOS transistors. A silicide results from depositing a refractory metal on the exposed semiconductor topography. The metal undergoes a two-step anneal process. The first anneal cycle causes a first phase reaction. All non-reacted metal is removed except in regions where the silicon atoms are prevalent. Thus, the second anneal step causes a second phase reaction of refractory metal only in silicon-based junctions and possibly on the upper surfaces of the polysilicon gate conductor  18 . The silicide serves to enhance conductivity of subsequent metalization layers drawn to the junctions. For sake of brevity and clarity of the drawings, the silicide formation steps and subsequent metalization layers are not shown. However, it is to be understood that an integrated circuit which utilizes the present process will have subsequent fabrication steps involving interlevel dielectrics in one or more layers of metalization. 
     It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to be capable of forming MOS transistor junctions which are graded not only to minimize HCE, but also to enhance operation and ohmic connectivity thereto. It is to be understood that the form of the invention shown and described is to be taken as exemplary, presently preferred embodiments. Various modifications and changes may be made to each and every processing step without departing from the spirit and scope of the invention given the benefit of this disclosure as set forth in the claims. 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.