Abstract:
A method of fabricating a transistor comprises forming a gate structure outwardly of a semiconductor substrate, wherein the gate structure comprises a gate, a gate insulator and sidewalls and forming source region and a drain region in the substrate using the gate structure as a mask, wherein a channel is defined in the substrate between the source region and the drain region. A bottomwall/sidewall junction capacitance reduction region extending within and between the source region and the drain region is formed, wherein the bottomwall/sidewall junction capacitance reduction region extends at least partially through the bottomwall junction or the sidewall junction.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of integrated circuits, and more particularly to transistors with bottomwall and sidewall junction capacitance reduction regions and a method for forming the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    Modern electronic equipment such as televisions, telephones, radios and computers are generally constructed of solid state devices. Solid state devices are preferred in electronic equipment because they are extremely small and relatively inexpensive. Additionally, solid state devices are very reliable because they have no moving parts, but are based on the movement of charge carriers.  
           [0003]    Solid state devices may be transistors, capacitors, resistors, and other semiconductor devices. Typically, such devices are formed in and on a substrate and are interconnected to form an integrated circuit. One type of transistor is the metal oxide semiconductor field effect transistor (MOSFET) in which current flows through a narrow conductive channel between a source and drain and is modulated by an electric field applied at the gate electrode.  
           [0004]    A problem with MOSFET transistors is bottomwall and sidewall capacitance which degrades device performance and can reduce the speed of a circuit. Efforts to bottomwall and sidewall junction capacitance have included tailoring of pocket implants, channel stop, and threshold adjust source/drain implants. All these implants serve other primary purposes. For example, pockets are used to minimize short channel effects. Threshold adjust is used for controlling device threshold. Channel stop is used for achieving isolation. Very deep source/drain implants result in increased short channel effects. For minimizing the bottomwall and sidewall junction capacitance, these implants require complex co-optimization and the reduction in bottomwall/sidewall capacitance may thus be limited.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a transistor with a bottomwall/sidewall junction capacitance reduction region that substantially eliminates or reduces the disadvantages and problems associated with prior systems and methods.  
           [0006]    In accordance with one embodiment of the present invention, a method of fabricating a transistor comprises forming a gate structure outwardly of a semiconductor substrate, wherein the gate structure comprises a gate, a gate insulator and sidewalls and forming source region and a drain region in the substrate using the gate structure as a mask, wherein a channel is defined in the substrate between the source region and the drain region. A bottomwall/sidewall junction capacitance reduction region extending within and between the source region and the drain region is formed, wherein the bottomwall/sidewall junction capacitance reduction region extends at least partially through the bottomwall junction or the sidewall junction.  
           [0007]    Technical advantages of the present invention include that the bottomwall/sidewall junction capacitance reduction can be adjusted relatively independently of, and reduce the dependence of the bottomwall and sidewall junction capacitance on, other implants and aspects of transistor fabrication (pocket implants, channel stop, threshold adjust, deep source/drains).  
           [0008]    Another technical advantage of the present invention is the achievement of the ultra-low bottomwall and sidewall capacitance reduction (&lt;0.7 fF/um2) needed for high-performance logic design.  
           [0009]    Yet another technical advantage is that the same masking configuration may be used during the implantation of the source and drain regions and the bottomwall/sidewall junction capacitance reduction region, and no additional masking or etching step is required for formation of the bottomwall/sidewall junction capacitance reduction region.  
           [0010]    Certain embodiments may possess none, one, some, or all of these technical features and advantages and/or additional technical features and advantages.  
           [0011]    Other technical advantages will be readily apparent to one skilled in the art from the following figures, description, and claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    For a more complete understanding of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:  
         [0013]    FIGS.  1 A-F are a series of schematic cross-sectional diagrams illustrating fabrication of a transistor with a bottomwall/sidewall junction capacitance reduction region in accordance with one embodiment of the present invention.  
         [0014]    [0014]FIGS. 2A and 2B are exemplary, generalized (not to scale) plots of the concentration of bottomwall/sidewall junction capacitance reduction region dopants as a function of depth in accordance with one embodiment of the present invention.  
         [0015]    [0015]FIGS. 3A and 3B are exemplary, generalized (not to scale) plots of various dopant concentrations in accordance with one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    FIGS.  1 A- 1 F are a series of schematic cross-sectional diagrams illustrating fabrication of a transistor with a bottomwall/sidewall junction capacitance reduction region in accordance with one embodiment of the present invention. In this embodiment, the transistor may be one of a complementary set of metal oxide semiconductor field effect transistors (MOSFETs) of a sub-micron regime. It will be understood that the type and size of the transistor may be varied within the scope of the present invention.  
         [0017]    Referring to FIG. 1A, an initial semiconductor structure  10  may comprise a semiconductor layer  12 . The semiconductor layer  12  may be a substrate such as a wafer. In this embodiment, the semiconductor layer  12  may comprise a single-crystalline silicon material. It will be understood that the semiconductor layer  12  may also be a layer of semiconductor material formed on a substrate, a semiconductor on insulator (SOI) layer and the like. For example, the semiconductor layer  12  may be an epitaxial layer grown on a wafer.  
         [0018]    A first isolation member  16  and a second isolation member  18  may be formed by Shallow Trench Isolation (STI) or Local oxidation (LOCOS) in the semiconductor layer  12 . The isolation members  16  and  18  may be independent structures or part of a unitary structure. For sub-micron applications, the isolation members  16  and  18  may comprise shallow isolation trenches. It will be understood that other types of isolation members and/or structures may be used within the scope of the present invention. For example, the isolation members  16  and  18  may comprise a field oxide.  
         [0019]    The isolation members  16  and  18  may define an active area  20  in the semiconductor layer  12 . As described in more detail below, source, drain and channel regions and/or structures, may be defined in the active area  20 . A gate electrode may control the flow of current from the source region to the drain region through the channel region to operate the transistor. It will be understood that the active area  20  may comprise other suitable regions and structures.  
         [0020]    A gate electrode  22  may be disposed over and insulated from the active area  20 . The gate electrode may have a width  21  of about 0.05 to 10 microns, or may have a different width. In one embodiment, the gate electrode  22  may be separated from an outer surface  24  of the active area  20  by a gate insulator  26 . In this embodiment, the gate electrode  22  may comprise polycrystalline silicon or other suitable semiconductor material. The gate insulator  26  may comprise silicon dioxide or other suitable insulating material. It will be understood that the gate electrode  22  may be otherwise suitably operationally associated with regions and structures in the active area  20 . The width of the active area  20  may be from 0.05 microns to 10&#39;s of microns.  
         [0021]    In this embodiment, the active area  20  may comprise a well  28  formed in the semiconductor layer  12 . The well  28  may comprise the single-crystalline silicon material of the semiconductor layer  12  implanted with well dopants  25 . In a particular embodiment, the transistor may comprise an n-MOS transistor and the well dopants  25  may comprise a p-type dopant such as boron. It will be understood that the semiconductor layer  12  may comprise other materials, may be suitably otherwise doped within the scope of the present invention, and that the well  28  may be omitted. For example, the semiconductor layer  12  may itself be slightly doped eliminating the need for the well  28 . In another embodiment, the transistor may comprise a p-MOS transistor, in which case the semiconductor layer  12  may be doped with well dopants  25  of an n-type such as arsenic.  
         [0022]    Referring to FIG. 1B, a masking layer  30  may be formed outwardly the semiconductor layer  12  and expose a first section  32  and a second section  34  of the active area  20 . In one embodiment, the exposed first section  32  may be proximate to a first side  33  of the gate electrode  22  facing the first isolation member  16 . The exposed second section  34  may be proximate to a second side  34  of the gate electrode  22  facing the second isolation member  18 . It will be understood that the sections  32  and  34  exposed by the masking layer  30  may be suitably varied within the scope of the present invention.  
         [0023]    The masking layer  30  may comprise photoresist material. In this embodiment, the masking layer  30  may be conventionally coated, patterned and etched to expose the first and second sections  32  and  34  of the active area  20 . It will be understood that the masking layer  30  may comprise other suitable materials and/or be otherwise suitably formed within the scope of the present invention.  
         [0024]    Referring to FIG. 1C, dopants  60  are implanted into the exposed first section  32  to form at least part of a source extension region and into the exposed second section  34  to form at least part of a drain extension region. The source/drain extension regions may be conventionally doped at an energy of &lt;100 keV and a concentration of &gt;1E14 cm −2 . In one embodiment, the doped exposed first section  32  may comprise a source extension  36 . The doped exposed second section  34  may comprise a drain extension  37 . It will be understood that the exposed first and second sections  32  and  34  of the active area  20  may comprise other suitable elements of the source and drain regions.  
         [0025]    The source extension  36  is localized in that it is spaced apart from the first isolation member  16  and thus does not extend the distance between the gate electrode  22  and the first isolation member  16 . Similarly, the drain extension  37  is localized in that it is spaced apart from the second isolation member  18  and thus does not extend the full distance between the gate electrode  22  and the second isolation member  18 . Accordingly, the localized source and drain extensions  36  and  37  reduce implant damage to the source and drain regions. Accordingly, the main body and contacts of the source and drain regions may be formed with minimal interference from the extensions.  
         [0026]    The localized source and drain extensions  36  and  37  may each vertically overlap the gate electrode  22  by &lt;300 angstroms. This overlap may be induced by thermal treatment or other migration of the implanted dopants. It will be understood that the localized source and drain extensions  36  and  37  may be otherwise disposed with respect to the gate electrode  22 . As used herein, the term each means every one of at least a subset of the identified items.  
         [0027]    Pocket dopants may be implanted into the exposed sections  32  and  34  inwardly of the extensions  36  and  37  to form a source pocket  70  and a drain pocket  72 . The pockets  70  and  72  may be used in connection with the extensions  36  and  37  to reduce gate length sensitivity of drive current. In one embodiment, the pocket dopants may be the dopants of the opposite type used to form the extensions  36  and  37 , but be implanted in the semiconductor layer  12  at a higher energy. It will be understood that the pockets  70  and  72  may comprise dopants otherwise introduced within the scope of the present invention. For example, the pocket dopants may be implanted at the same or other energy.  
         [0028]    For the embodiment where the transistor shown in FIGS.  1 A- 1 F is an n-MOS transistor, the localized source and drain extensions  36  and  37  may each comprise n-type dopants such as arsenic implanted at an energy of &lt;20 keV and a concentration of &gt;1E14 cm −2 . In this embodiment, the localized source and drain pockets  70  and  72  may comprise p-type dopants such as boron or indium implanted at an energy of &lt;50 keV (B) or &lt;200 keV (In) and a concentration of &gt;1E12 cm −2 . It will be understood that the localized source and drain extensions  36  and  37  and pockets  70  and  72  may be otherwise doped within the scope of the present invention. In another embodiment, the transistor may comprise a p-MOS transistor, in which case the localize source and drain extensions  36  and  37  may each comprise p-type dopants such as boron implanted at an energy of &lt;10 keV (B) or &lt;50 keV (BF2) and a concentration of &gt;1E14 cm −2  and the localized source and drain pockets  70  and  72  may comprise n-type dopants such as arsenic or phosphorous implanted at an energy of &lt;200 keV (As) or &lt;100 keV (Ph) and a concentration of &gt;1E12 cm −2 .  
         [0029]    After the localized source and drain extensions  36  and  37  and pockets  70  and  72  have been formed, the masking layer  30  may be conventionally removed.  
         [0030]    Referring to FIG. 1D, an insulating layer  40  is deposited outwardly of the semiconductor layer  12  and the gate electrode  22 . In one embodiment, the insulating layer  40  may be deposited directly onto the semiconductor layer  12  and the gate electrode  22 . In this embodiment, the insulating layer  40  may comprise an oxide and/or nitride layer. It will be understood that the insulating layer  40  may comprise other materials capable of insulating semiconductor elements.  
         [0031]    Referring to FIG. 1E, the insulating layer  40  is anisotropically etched to form a first sidewall  42  adjacent the first side  33  of the gate electrode  22  and a second sidewall  43  adjacent the second side  34  of the gate electrode  22 . The anisotropic etch may be a conventional reactive ion etch (RIE) or other suitable etch. The sidewalls  42  and  43  may electrically isolate sides  33  and  34  of the gate electrode  22  from other elements of the transistor. The sidewalls  42  and  43  in this embodiment have a width  44  of approximately &lt;2000 angstroms.  
         [0032]    Dopants  62  are implanted into the exposed portions of the active area  20  between the first sidewall  42  and isolation member  16  to form a source region  46  and between the second sidewall  43  and isolation member  18  to form a drain region  47 . A channel  50  is thus defined in the substrate between the source region and the drain region. For the embodiment where the transistor shown in FIGS.  1 A- 1 F is an n-MOS transistor, the dopants  62  may comprise n-type dopants such as arsenic. For an n-MOS transistor, the dopants  62  may be implanted to a concentration of greater than about 1E14 cm −2 , at an energy of &lt;200 keV. In another embodiment where the transistor shown in FIGS.  1 A- 1 F is a p-MOS transistor, the dopants  62  may comprise p-type dopants such as boron. For a p-MOS transistor, the dopants  62  may be implanted to a concentration of greater than about 1E14 cm −2 , at an energy of &lt;10 keV. A bottomwall junction  64  is defined at base of the source region  46  and the drain region  47 , where the concentration of dopants  62  equals the concentration of well dopants  25  (n-dopant concentration=p-dopant concentration). Near the channel  50 , where the bottomwall junction  64  curves upward towards the gate  22 , the junction may be referred to as the sidewall junction.  
         [0033]    In the illustrated embodiment wherein the transistor shown in FIGS.  1 A- 1 F is an n-MOS transistor and if transistor elements of both n-MOS and p-MOS types are present in the same circuit, the pMOS type transistor element may be masked or otherwise covered during implantation of the dopants  62  in an n-MOS transistor element. Likewise, in another embodiment where the transistor shown in FIGS.  1 A- 1 F is a p-MOS transistor and both types of elements are present in the same circuit, the n-MOS type transistor element may be masked or otherwise covered during implantation of the dopants  62  in a p-MOS transistor element.  
         [0034]    With reference to FIG. 1F, dopants  80  are implanted at a high energy into the active area  20  to form a bottomwall/sidewall junction capacitance reduction region  82 . If the transistor is an n-MOS type, the dopants  80  would be n-type. Likewise, if the transistor is an p-MOS type, the dopants  80  would be p-type. High-energy ion implantation of the dopants  80  (about 20-200 kV if the transistor is an n-MOS type transistor and about 30-100 kV if the transistor is a p-MOS type transistor) is sufficient to implant the dopants through the gate  22  and into the area of the channel  50 .  
         [0035]    Given dopant concentrations in the source region  46  and the drain region  47  as described in reference to FIG. 1E, concentrations of dopants comprising the bottomwall/sidewall junction capacitance reduction region  82  may be about 1E12 cm −2  to 1E14 cm −2  for an n-MOS type transistor, and about 1E12 cm −2  to 1E14 cm −2  for a p-MOS type transistor. Such concentrations in the bottomwall/sidewall junction capacitance reduction region  82  provide a smoothing and grading of the net dopant concentrations near the bottomwall and sidewall junctions, and reduce the net dopant concentration in the immediate area of the bottomwall and sidewall junctions. This effect is described further in reference to FIGS. 3A and 3B.  
         [0036]    If p-MOS type transistors are present in the same circuit as the n-MOS transistor element and the p-MOS type transistors are masked or otherwise covered during implantation of the dopants  62  in the n-MOS transistor element, the mask may remain in the same configuration during the implantation of dopants  82  in the n-MOS transistor elements. Likewise, if n-MOS type transistors are present in the same circuit as the p-MOS transistor element and the n-MOS type transistors are masked or otherwise covered during implantation of the dopants  62  in the p-MOS transistor element, the mask may remain in the same configuration during the implantation of dopants  82  in the p-MOS transistor elements. Thus, the same masking configuration may be used during the implantation of the source and drain regions  46  and  47  and the bottomwall/sidewall junction capacitance reduction region  82 , and no additional masking or etching step is required for formation of the bottomwall/sidewall junction capacitance reduction region  82 .  
         [0037]    The range point  84  (shown as a dotted line) is the point of the highest concentration of the dopants  80  in the bottomwall/sidewall junction capacitance reduction region  82  at a given point along the active area  20 . Further details concerning the concentration of the dopants  80  in the bottomwall/sidewall junction capacitance region  82  at points “X” and “Y” are described in reference to FIGS. 2A and 2B and FIGS. 3A and 3B, below.  
         [0038]    [0038]FIG. 2A is an exemplary, generalized (not to scale) plot of the concentration of dopants  80  as a function of depth at point “X” on FIG. 1F in accordance with one embodiment of the present invention. The range  100  is the depth from the outer surface  20  to the range point  84  at X. Range  100  may measure about 2000 angstroms. An upper straggle  102  and a lower straggle  104  are defined by the distance from the range point  84  to the point where the concentration of dopants  80  in the bottomwall/sidewall junction capacitance reduction region is approximately equal to ((peak concentration)/({square root}2)). The peak concentration of dopants  80  may be about 2E17 cm −3  to 3E17 cm −3 . The straggles may measure approximately 300 angstroms for upper straggle  102  and approximately 300 angstroms for lower straggle  104 .  
         [0039]    [0039]FIG. 2B is an exemplary, generalized plot of the concentration of dopants  80  as a function of depth at point “Y” on FIG. 1F in accordance with one embodiment of the present invention. The range  106  is the depth from the surface of the gate  20  to the range point  84  at Y. An upper straggle  108  and a lower straggle  110  are defined by the distance from the range point  84  to the point where the concentration of dopants  80  in the bottomwall/sidewall junction capacitance reduction region is approximately equal to ((peak concentration)/({square root}2)). The peak concentration of dopants  80  may be about 2E17 cm −3  to 3E17 cm −3 . The straggles may measure approximately 300 angstroms for upper straggle  108  and approximately 300 angstroms for lower straggle  110 . A non-encroachment distance  112  is defined as the shortest distance between the base of the gate  20  (or of the gate insulator  26 , if present) and the top of the upper straggle  108 . In order to minimize encroachment by the bottomwall/sidewall capacitance reduction region  82  into the inversion layer of the channel  50 , the non-encroachment distance  112  should be at least about 150 angstroms.  
         [0040]    [0040]FIG. 3A is an exemplary, generalized (not to scale) plot of the concentration at point “X” on FIG. 1E, as a function of depth, of source/drain dopants  62  and well dopants  25  in accordance with one embodiment of the present invention. As described above, well dopants  25  are of the opposite conductivity type as dopants  62 . Also shown in FIG. 3A is the net concentration of dopants, which equals the difference in concentration of dopants  62  and well dopants  25 .  
         [0041]    [0041]FIG. 3B is an exemplary, generalized (not to scale) plot of the concentration at point “X” on FIG. 1F, as a function of depth, of source/drain dopants  62 , well dopants  25 , and dopants  80  in accordance with one embodiment of the present invention. As above, dopants  80  form the bottomwall/sidewall junction capacitance reduction region in accordance with one embodiment of the present invention and are of the same conductivity type as dopants  62 . The net concentration of dopants equals the sum of the concentrations of dopants  62  and dopants  80 , minus the concentration of well dopants  25 .  
         [0042]    Capacitance varies in part as a function of the concentration of dopants at the junction, and also as a function of the shape or “grading” of the profile of the net concentration of dopants in the region of the bottomwall junction  64 . It should be noted that the profile of the net concentration in FIG. 3B is somewhat smoother and more graded as opposed to the net concentration profile from FIG. 3A. Furthermore, the net concentration of dopants is lower in FIG. 3B than in FIG. 3A at the region just below the bottomwall junction  64 . These features are an indication of the effect of the bottomwall/sidewall junction capacitance reduction region of reducing, relatively independently of other parameters, the bottomwall/sidewall junction capacitance. The use of the bottomwall/sidewall junction capacitance reduction region of the present method may allow for the achievement of ultra-low levels (&lt;0.7 fF/um2) of bottomwall/sidewall junction capacitance, such low levels being favorable for high-performance logic design.  
         [0043]    Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims.