Abstract:
Methods for fabricating facetless semiconductor structures using commercially available chemical vapor deposition systems are disclosed herein. A key aspect of the invention includes selectively depositing an epitaxial layer of at least one semiconductor material on the semiconductor substrate while in situ doping the epitaxial layer to suppress facet formation. Suppression of faceting during selective epitaxial growth by in situ doping of the epitaxial layer at a predetermined level rather than by manipulating spacer composition and geometry alleviates the stringent requirements on the device design and increases tolerance to variability during the spacer fabrication.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 60/387,867 filed Jun. 10, 2002, the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to semiconductor fabrication methods and, more specifically, to methods for fabricating semiconductor devices having substantially facetless raised source and drain elements.  
       BACKGROUND  
       [0003]     In order to improve performance and packing density of modern microelectronic devices, it is often desirable to reduce channel lengths of metal-oxide-semiconductor-field-effect transistors (“MOSFETs”) during device design. As the MOSFET channel length decreases, however, short channel effects and parasitic resistance become of increasing concern. To minimize short channel effects in bulk silicon devices, for example, the source/drain doping junction depths are decreased during scaling. But shallower junctions require the use of thinner silicides to minimize leakage current from the silicon/silicide interface to the junction depletion region, which, at the same time, may increase parasitic contact resistances. As another example, to improve performance of a silicon-on-insulator (“SOI”) device by reducing short channel effects, its silicon region should preferably be less than about 20 nm thick. Conventional silicide formation processes, however, may consume substantially the entire silicon layer of such thickness, which may, in turn, result in undesirably large leakage currents and parasitic contact resistance because the silicide/silicon interface area is small.  
         [0004]     The increasing operating speeds and computing power of microelectronic devices have recently given rise to the need for an increase in the complexity and functionality of the semiconductor substrates that are used as the starting substrate in these microelectronic devices. Such “virtual substrates” based on silicon and germanium provide a platform for new generations of VLSI devices that exhibit enhanced performance when compared to devices fabricated on bulk Si substrates. Specifically, new technological advances enable formation of heterostructures using silicon-germanium alloys (hereinafter referred to as “SiGe” or “Si 1-x Ge x ”) to further increase performance of the semiconductor devices by changing the atomic structure of Si to increase electron mobility. These substrates are called strained Si substrates.  
         [0005]     A strained Si substrate is generally formed by a first epitaxial growth of a relaxed SiGe layer on bulk Si, and then a second epitaxial growth of a thin (less than about 500 Å) Si layer on the relaxed SiGe layer. Because the lattice constant of relaxed SiGe heterostructure is different from Si, the thin Si layer becomes “strained,” resulting in enhanced mobilities (and hence improved device speeds) over bulk Si. The percentage of Ge in SiGe, and the method of deposition can have a dramatic effect on the characteristics of the strained Si layer. U.S. Pat. No. 5,442,205, “Semiconductor Heterostructure Devices with Strained Semiconductor Layers,” incorporated herein by reference, demonstrates one such method of producing a strained Si device structure.  
         [0006]     A method of epitaxially growing a relaxed SiGe layer on bulk Si is discussed in international application WO 01/22482, “Method of Producing Relaxed Silicon Germanium Layers,” incorporated herein by reference. The method includes providing a monocrystalline Si substrate, and then epitaxially growing a graded Si 1-x Ge x  layer with increasing Ge concentration at a gradient of less than 25% Ge per micron to a final composition in the range of 0.1&lt;x&lt;1, using a source gas of Ge x H y Cl z  for the Ge component, on the Si substrate at a temperature in excess of 850° C., and then epitaxially growing a semiconductor material on the graded layer.  
         [0007]     Another method of epitaxially growing a relaxed SiGe layer on bulk Si is discussed in a paper entitled, “Low Energy plasma enhanced chemical vapor deposition,” by M. Kummer et. al. (Mat. Sci. &amp; Eng. B89, 2002, pp. 288-95), incorporated herein by reference, in which a method of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) is shown, which allows the formation of a SiGe layer on bulk Si at high growth rates (0.6 micron per minute) and low temperatures (500-750° C.).  
         [0008]     To grow a high-quality, thin, epitaxial strained Si layer on a graded SiGe layer, the SiGe layer is, preferably, planarized to reduce the surface roughness in the final strained Si substrate. Current methods of chemical mechanical polishing (CMP) are typically used to improve the planarity of surfaces in semiconductor fabrication processes. U.S. Pat. No. 6,107,653, “Controlling Threading Dislocations in Ge on Si Using Graded GeSi Layers and Planarization,” incorporated herein by reference, describes how planarization can be used to improve the quality of SiGe graded layers.  
         [0009]     Although the resulting biaxial tensile or compressive strain alters the carrier mobilities in the layers of the “virtual substrate” enabling the fabrication of high-speed and/or low-power devices, short channel effects and parasitic resistance remain of concern for strained-Si-based devices as well.  
         [0010]     Raised source/drain structures have been proposed as a technique for forming high-quality contacts to improve performance of bulk silicon, silicon-on-insulator (SOI), and strained silicon devices. Raised source/drains are generally fabricated by raising the level of the source and drain regions by selective semiconductor, e.g., silicon, deposition. The extra silicon increases the process margin for the silicide process and extends the latitude for contact junction design. To maintain a similar crystalline structure, the extra silicon is “grown” by silicon epitaxy.  
         [0011]     For example, in bulk and strained Si devices, raised source/drain contacts have been proposed as a means of forming shallow dopant junctions to minimize contact resistances and short channel effects. In SOI devices, the raised source and drain regions provide sacrificial silicon useful for silicide consumption to form low resistance contacts on thin Si films.  
         [0012]     Typically, source and drain regions are formed by selective epitaxial silicon growth after formation of a sidewall dielectric spacer of silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), or a combination of both materials. In this process, silicon is epitaxially grown on exposed windows in a dielectric mask while nucleation of polysilicon on the masking material is suppressed during the incubation time by, for example, etching of spurious nuclei on the dielectric material by hydrogen chloride, the mediation of saturation by formation of a number of intermediate chlorine-containing silicon precursors, and passivation of surface defect sites which serve as heterogeneous nucleation centers. Also, selectivity is facilitated by growing for a period of time that is generally shorter than the incubation period needed for polysilicon nucleation on the dielectric mask.  
         [0013]     The dielectric spacer electrically isolates a gate made of, for example, polysilicon, from the source and drain regions. After the selective epitaxial growth step, a heavy low-energy implant forms a doped region, and is followed by a silicidation process for, e.g., low resistance complementary metal-oxide-semiconductor (CMOS) contacts.  
         [0014]     During selective epitaxial growth, however, decreased thickness of the epitaxial layer typically occurs at interfaces between the epitaxial layer and the dielectric spacer or field oxide, where facets form by growth of low-energy crystal planes that minimize the energetically less favorable epitaxial Si/dielectric material interface. Because the raised source/drain regions grow thinner near the dielectric spacer edge, faceting leads to non-planar device layers. This phenomenon raises processing concerns in bulk Si, SOI, and strained Si devices by complicating implementation of selective epitaxial growth in device applications. Specifically, thinner faceted regions cause localized deeper dopant penetration during ion implantation, which increases short channel effects and leakage currents. Also, thinner faceted regions typically result in deeper silicide penetration and greater Si consumption during silicidation. Thus, faceting may result in poor doping profile control and poor silicide depth control, leading to degraded device characteristics and more complex device designs.  
         [0015]     For example, referring to  FIG. 1 , a cross-sectional transmission electron microscope (XTEM) image illustrates undoped Si layers  10  grown adjacent to reactive ion etched &lt;110&gt;-oriented SiO 2  sidewalls  12 . The SiGe layers  14  were included as markers to track facet development and evolution. The selective Si epitaxy has noticeable (311)-oriented facets  15  and (110)-oriented facets  17  next to SiO 2  sidewalls. Such faceting would be detrimental during implant and silicidation of source and drain regions. Referring to  FIG. 2 , a silicided source region  20  is formed by selective epitaxial growth on a thin film SOI substrate  22  according to any method known in the art. Such faceted growth leads to deep penetration of a silicide  24  to a buried SiO 2  layer  26 . This penetration may dramatically increase the contact resistance because the silicide/silicon channel interfacial area is reduced, which leads to poor device characteristics. The electrical contact to the device channel is made through the area adjacent to the spacer  28 , which is only the width of the Si channel. Furthermore, the contact quality may degrade because of incomplete silicide formation if the thickness of consumed Si is insufficient.  
         [0016]     Similarly, in strained-Si-based devices, thinner faceted raised source and drain regions may cause the silicide to penetrate into a relaxed silicon-germanium (SiGe) layer underneath a strained Si layer. This may result in formation of a poor quality silicide because of the rejection of Ge during alloying of the SiGe layer with the silicide metal. This rejection may create high-resistivity regions that compromise the contact quality.  
         [0017]     Facetless selective epitaxial growth is, therefore, desirable for fully realizing the performance advantages of the raised source/drain scheme for fabrication of low-resistance contacts and shallow junctions in advanced devices that have wide ranging application to Si, SOI, and strained Si technologies.  
         [0018]     Recently, facetless epitaxy has received much attention in the design of raised source/drain contacts for CMOS applications. For example, selective growth in inverse patterns, i.e. using SiO 2  pillars with a Si field as opposed to Si windows in a SiO 2  field, has been proposed. Another approach involves fabrication of facetless raised source/drain structures by selective epitaxial growth in commercially available low-pressure and atmospheric-pressure chemical vapor deposition systems by carefully controlling the geometry of the multilayer spacer structure and sidewall spacer profile. For example, it has been suggested that undercutting the liner oxide and silicon growth under the silicon nitride spacer recesses the silicon-liner oxide interaction away from the source/drain region. This gives extra thickness in the source/drain region before the increase in silicon-dielectric interface energy causes the epitaxial layer to facet.  
         [0019]     In yet another approach, discussed in a thesis entitled “Selective SiGe Nanostructures” by Tom Langdo (Ph.D. Thesis, MIT, 2001)(“Langdo thesis”), incorporated herein by reference, facetless growth was demonstrated by in situ n-type doping of a silicon layer being deposited by selective epitaxy adjacent to a sidewall of a reactive ion etched SiO 2  spacer in a ultra-high vacuum chemical vapor deposition (UHVCVD) system. It was demonstrated that a combination of moderate n-type doping and relatively vertical SiO 2  sidewalls results in facet-free epitaxy along &lt;110&gt;-oriented SiO 2  sidewalls. Referring to  FIG. 3 , an XTEM image illustrates growth of 1×10 18 /cm 3  n-type doped Si with SiO 0.9 Ge 0.1  marker layers on &lt;110&gt;-oriented SiO 2  sidewalls. As shown in  FIG. 3 , all Si 0.9 Ge 0.1  marker layers follow (100) planes, and thus, facet formation is suppressed during the growth. SiO 2  was chosen as the sidewall material instead of more commonly used Si 3 N 4  because selective growth generally cannot be achieved with either SiH 4  or SiH 2 Cl 2  source gases, common Si precursors, on Si 3 N 4  spacers without the addition of hydrogen chloride. Because of the deleterious effects of chloride on a UHVCVD system, however, addition of hydrogen chloride to the source gases is impractical.  
         [0020]     Obstacles hinder the commercialization of this UHVCVD growth approach. First, UHVCVD is generally not feasible for large-scale commercial applications. Second, UHVCVD selective growth is not possible with Si 3 N 4  dielectric materials. Finally, the UHVCVD process described above is generally not compatible with multi-layered commercial spacer structures that include, e.g., Si 3 N 4  materials.  
         [0021]     Thus, there remains a need in the art for a process of fabricating semiconductor devices having facetless raised source/drain structures that is compatible with existing semiconductor manufacturing processes, systems, and materials and that also possesses improved process tolerances.  
       SUMMARY OF THE INVENTION  
       [0022]     It is an object of the present invention to provide a process for fabricating facetless semiconductor structures that overcomes the limitations of known processes. Specifically, it is an object of the invention to provide a process for fabricating semiconductor devices having facetless raised source/drain structures that complements existing epitaxial processes and does not require stringent control of the geometry of the dielectric regions. It is also desirable that such process be less expensive in comparison to the manufacturing processes known in the art and not limited to a particular dielectric or semiconductor material.  
         [0023]     Accordingly, methods for fabricating facetless semiconductor structures using commercially available chemical vapor deposition systems are disclosed herein. A key aspect of the invention includes selectively depositing an epitaxial layer of at least one semiconductor material on the semiconductor substrate while in situ doping the epitaxial layer to suppress facet formation. Suppression of facet formation during selective epitaxial growth by in situ doping of the epitaxial layer at a predetermined level rather than by manipulating spacer composition and geometry alleviates the stringent requirements on the device design. For example, in various embodiments of the fabrication methods according to the invention, it is not necessary to precisely control the liner oxide undercut beneath the silicon nitride spacer prior to epitaxial growth to minimize faceting, thus making the process more robust and compatible with very thin spacer dimensions. Accordingly, in situ doping during epitaxial growth to suppress faceting increases tolerance to variability during the spacer fabrication, providing an improved robust process suitable for volume manufacturing.  
         [0024]     Throughout the following description, the term “facet” is used to refer generally to a slanted low-energy crystal plane formed in a semiconductor material at its interface with a dielectric material. Further, the term “epitaxy” is used to refer generally to methods for growing thin layers of single crystal materials on a single crystal substrate whereby crystallographic structure of the substrate is reproduced in the deposited material. Also, the term “MOS” is used to refer generally to semiconductor devices that include a conductive gate spaced at least by an insulating layer from a semiconducting channel layer. Conversely, the terms “MOSFET” or “MOS transistor” refer to a field-effect transistor having a conductive gate spaced at least by an insulating layer from a semiconducting channel layer. The terms “SiGe” and “Si 1-x Ge x ” are used interchangeably to refer to silicon-germanium alloys. The term “silicide” is here used to refer to a reaction product of a metal, silicon, and optionally other components, such as germanium. The term “silicide” is also used, less formally, to refer to the reaction product of a metal with an elemental semiconductor, a compound semiconductor or an alloy semiconductor. The term “doping” is used to refer generally to the process of adding impurities to a semiconductor material to achieve desired properties.  
         [0025]     In general, in one aspect, the invention is directed to a method of fabricating a semiconductor structure including the steps of providing a chamber and providing a semiconductor substrate in the chamber. The semiconductor substrate has a surface including a first portion and a second portion proximal to the first portion. The method of the invention also includes forming a dielectric region on the first portion of the substrate and selectively depositing an epitaxial layer of at least one semiconductor material on the second portion of the substrate while in situ doping the epitaxial layer. The in situ doping occurs at a first predetermined level to substantially suppress facet formation thereby forming a substantially facetless semiconductor region. The pressure in the chamber during selective deposition of the epitaxial layer is greater than about 5 Torr.  
         [0026]     In one embodiment of the invention, the epitaxial layer is deposited in a chemical vapor deposition system, such as, for example, a reduced-pressure chemical vapor deposition system, atmospheric-pressure chemical vapor deposition system, or plasma-enhanced chemical vapor deposition system.  
         [0027]     In another embodiment of the invention, the step of selectively depositing an epitaxial layer includes introducing a source gas into the chamber. The source gas may include at least one precursor gas and a carrier gas, such as, for example hydrogen. According to one feature of this embodiment, the at least one precursor gas includes a silicon precursor gas, such as, for example, silane, disilane, trisilane, or dichlorosilane. According to another feature of this embodiment, the at least one precursor gas includes a germanium precursor gas, such as, for example, germane, digermane, germanium tetrachloride, or germanium dichloride. According to yet another feature of this embodiment, in addition to the at least one precursor gas and a carrier gas, the source gas also includes an etchant for suppressing nucleation of the at least one semiconductor material over the dielectric region during deposition. The etchant may include hydrogen chloride or chlorine.  
         [0028]     In yet another embodiment of the invention, the epitaxial layer is doped by adding a dopant to the epitaxial layer during deposition of the epitaxial layer. Examples of suitable dopants are phosphorus, arsenic, antimony, and boron. The dopant may be added to the epitaxial layer by introducing a dopant gas, such as phosphine, arsine, stibine, and diborane, into the chamber. According to one feature of this embodiment, the first predetermined level of doping ranges from about 10 17  to about 10 19  cm −3 .  
         [0029]     The epitaxial layer may include at least one of silicon and germanium. Also, the dielectric region may include at least one of silicon oxide and silicon nitride. Optionally, the dielectric region may have a two-layered spacer structure including a silicon oxide layer and a silicon nitride layer disposed thereon.  
         [0030]     In some embodiments of the invention, the semiconductor substrate includes silicon. In other embodiments, the semiconductor substrate may include a silicon wafer; an insulating layer disposed thereon; and a strained semiconductor layer, for example, silicon or germanium, disposed on the insulating layer. Alternatively, the semiconductor substrate may include a silicon wafer; a compositionally uniform relaxed Si 1-x Ge x  layer deposited thereon; and a strained silicon layer deposited on the relaxed Si 1-x Ge x  layer. In this embodiment, the semiconductor substrate may also include a compositionally graded Si 1-x Ge x  layer disposed between the compositionally uniform relaxed Si 1-x Ge x  layer and the silicon wafer, or an insulating layer disposed between the compositionally uniform relaxed Si 1-x Ge x  layer and the silicon wafer.  
         [0031]     In still another embodiment of the invention, the temperature in the chamber during selective deposition of the epitaxial layer ranges from about 300° C. to about 900° C., for example, from about 500° C. to about 700° C. Also, the epitaxial layer may be deposited at a rate greater than about 1 nm/min.  
         [0032]     In one embodiment of the invention, the method includes the steps of fabricating an n-channel MOSFET in a first portion of the semiconductor region; and fabricating a p-channel MOSFET in a second portion of the semiconductor region. According to one feature of this embodiment, the method includes counter-doping the first portion or the second portion at a second predetermined level. Optionally, the first predetermined level of doping does not exceed the second predetermined level of doping.  
         [0033]     In some embodiments, the method of the invention also includes forming a metal silicide layer over the semiconductor region. The surface of the semiconductor substrate may have a substantially (100) crystallographic orientation. Also, the dielectric region may include a sidewall having an angle relative to the semiconductor substrate that ranges from about 60° to about 90°. The sidewall may be substantially aligned with either the &lt;110&gt; crystallographic plane or the &lt;100&gt; crystallographic plane of the semiconductor substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:  
         [0035]      FIG. 1  depicts an XTEM image illustrating faceted selective epitaxial growth of alternating Si and SiGe layers proximate a sidewall of a SiO 2  spacer according to methods known in the art  
         [0036]      FIG. 2  depicts a cross-sectional schematic view of a semiconductor device having a faceted raised source region according to methods known in the art;  
         [0037]      FIG. 3  depicts an XTEM image illustrating facetless selective epitaxial growth according to one method known in the art;  
         [0038]      FIG. 4  depicts a cross-sectional view of a semiconductor substrate suitable for use with various embodiments of the invention;  
         [0039]      FIG. 5  depicts a cross-sectional view of a semiconductor device having selectively grown and in situ doped facetless Si source/drain epitaxial layers according to the invention; and  
         [0040]      FIG. 6  depicts a cross-sectional view of the device of  FIG. 5  after silicidation of the source/drain epitaxial layers. 
     
    
     DETAILED DESCRIPTION  
       [0041]     In various embodiments of the invention, facetless semiconductor structures, for example, raised source and drain elements of a MOSFET, are fabricated by selective epitaxial growth proximate commonly used spacer structures, such as those including Si 3 N 4 , SiO 2 , or both, using commercially available chemical vapor deposition systems. Facet formation in the epitaxial layer is suppressed by doping the epitaxial layer at a predetermined level in situ during epitaxial growth, which increases tolerance to variability during the spacer fabrication process.  
         [0042]     Referring to  FIG. 4 , a substrate  40  suitable for use with the invention, comprises a semiconductor, such as silicon or silicon deposited over an insulator, such as, for example, SiO 2 . In one embodiment, several layers collectively indicated as  42  are formed on the substrate  40 . The layers  42  may be grown, for example, in a CVD system, including a reduced-pressure chemical vapor deposition system (LPCVD), atmospheric-pressure chemical vapor deposition system (APCVD), and plasma-enhanced chemical vapor deposition system (PECVD). In this embodiment, the layers  42  and the substrate  40 -may be referred to together as a “semiconductor substrate  44 .” 
         [0043]     The layers  42  include a graded layer  46  disposed over the substrate  40 . The graded layer  46  may include SiGe with a grading rate of, for example, 10% Ge/μm of thickness, with a thickness T 1  of, for example, 2-9 μm, and grown, for example, at 600-1100° C. A relaxed layer  48  is disposed over the graded layer  46 . The relaxed layer  48  may include, for example, Si 1-x Ge x  with a uniform composition containing, for example, 20-90% Ge, (i.e., 0.2≦x≦0.9) having a thickness T 2  ranging from, e.g., about 0.2 μm to about 2 μm. In an alternative embodiment, the relaxed layer  48  may be formed directly on the substrate  40 , without the graded layer  46 .  
         [0044]     A tensilely strained layer  50  is disposed over relaxed layer  48 , sharing an interface therewith. In one embodiment, the tensilely strained layer  50  is formed of silicon. In other embodiments, the tensilely strained layer  50  may be formed of SiGe, or at least one of a group II, a group III, a group V, and a group VI element. The tensilely strained layer  50  may have a starting thickness T 3  ranging, for example, from about 50 angstroms to about 300 angstroms (Å).  
         [0045]     In some embodiments, a compressively strained layer (not shown) may be disposed between the relaxed layer  48  and the tensilely strained layer  50 . In an embodiment, the compressively strained layer includes Si 1-y Ge y  with a Ge content (y) higher than the Ge content (x) of the relaxed Si 1-x Ge x  layer  48 . The compressively strained layer may contain, for example 40-100% Ge and have a thickness ranging, e.g., from about 10 angstroms to about 200 angstroms (Å).  
         [0046]     Referring to  FIG. 5 , in one embodiment, a semiconductor device, such as, for example, a transistor  60  having facetless raised source/drain regions is fabricated in a chamber  90  of a chemical vapor deposition system, such as, for example, LPCVD, APCVD, or PECVD system. The transistor  60  is formed on a semiconductor substrate  62  including, for example, silicon, silicon-on-insulator, or strained silicon, as described in detail above. The surface of the semiconductor substrate may have a substantially (100) crystallographic orientation.  
         [0047]     In a particular variation of this embodiment of the invention, prior to MOSFET fabrication, the substrate  62  is cleaned using, for example, a dilute RCA process known in the art, in order to remove organic contaminants, particles, and any ionic or heavy metal contaminants from the substrate surface. Alternatively, or in addition to the RCA cleaning, the surface of the substrate may be passivated with hydrogen using a dilute hydrofluoric acid treatment. In yet another variation, a sacrificial SiO 2  layer may be grown on the substrate thermally or by an oxygen plasma and then completely removed by hydrofluoric acid. In still another variation, a non-selective dry-etch process can be used. The clean surface must be achieved while minimizing substrate consumption to ensure compatibility with thin film materials heterostructures and SOI wafers. Prior to epitaxial growth the substrate may be subjected to a high temperature bake step to remove any residual oxide on the surface. For example, after hydrogen passivation with a dilute HF treatment a suitable bake could include heating the substrate for 5 minutes at 800° C.  
         [0048]     The transistor  60  includes a gate electrode  64 , made of, for example, polycrystalline silicon, and a gate insulator  66  made of, for example, SiO 2  or a high-k material, patterned using, for example, reactive ion etching (“RIE”) whereby SiO 2  gate insulator is defined by selective reactive ion etching using, e.g. CHF 3 /O 2  gas mixture with subsequent post-RIE cleaning to remove the growth-inhibiting fluoropolymer layer, as described in the above-referenced Langdo thesis.  
         [0049]     Isolation regions  86 ,  88 , made of, e.g., SiO 2 , are introduced to isolate transistor  60  from other devices. Isolation regions  86 , 88  can be, for example, shallow trench isolation regions produced early in the transistor fabrication process by methods known in the art.  
         [0050]     The transistor  60  also includes a source region  68  and a drain region  70  defined in the substrate  60  proximate the gate electrode  64 . In a particular embodiment, the transistor  60  is an n-type MOSFET with the source and drain regions formed by n-type doping via, e.g. implantation of arsenic ions, as will be described in detail below. Shallow extension regions  68   a ,  70   a  of the source region  68  and the drain region  70 , respectively, are formed by, e.g., ion implantation after the gate electrode  64  and the gate insulator  66  are defined. The shallow extension regions  68   a ,  70   a  preferably extend to and, in one variation of this embodiment, slightly below the gate insulator  66 . The depth of the extension regions  68   a ,  70   a  may range from about 5 nm to about 50 nm. A first liner  72  and a second liner  74  are deposited proximate the gate electrode  64  and the gate insulator  66 . In a particular embodiment, the thickness of the liners  72 ,  74  is approximately 250 angstroms. The liners  72 ,  74  may be formed of, e.g., a low temperature oxide (LTO). During the formation of liners  72 ,  74 , a hard mask  76  is formed on a top surface  78  of the gate electrode  64 . Hard mask  76  is formed of, e.g., LTO.  
         [0051]     Subsequently, spacers  80 ,  82  are formed proximate liners  72 ,  74  to electrically isolate the gate and source/drain regions during the device operation. The spacers  80 ,  82  are formed of a dielectric such as, for example, Si 3 N 4 , by chemical vapor deposition followed by an etchback step, such as reactive ion etch. Alternatively, spacers can be formed from SiO 2 . In one embodiment, the height of the spacers  80 ,  82  roughly approximates or exceeds the height of the gate electrode  64  and ranges from about 80 nm to about 250 nm. The widths of the spacers  80 ,  82  range from about 30 nm to about 100 nm. Spacer sidewalls  81 ,  83  proximate to the source/drain regions  68 ,  70  may have at least partially concave profile. Sidewalls  81 ,  83  intersect the surface of the substrate  62  at angles α, β that range from about 60° to about 90°. In a particular embodiment, the angles α, β substantially, but not necessarily precisely, equal 90°. Furthermore, in various embodiments of the fabrication methods according to the invention, it is not necessary to precisely control the liner oxide undercut beneath the silicon nitride spacer prior to epitaxial growth to minimize faceting. The spacers  80 ,  82  may be fabricated so that the sidewalls  81 ,  83  are substantially aligned with a particular crystallographic plane of the semiconductor substrate  62 , such as, for example, the &lt;100&gt; or &lt;110&gt; crystallographic plane.  
         [0052]     The raised source/drain regions  68 ,  70  are formed by selective epitaxial growth coupled with ion implantation after formation of sidewall spacers  80 ,  82 , as described below. The height of the source/drain regions  68 ,  70  may range from about 10 nm to about 100 nm.  
         [0053]     Referring to  FIG. 6 , a contact material  100  is subsequently formed on raised source and drain regions  68 ,  70 . In a particular embodiment, the contact material  100  is a metal compound that is thermally stable and has low electrical resistivity at the silicon/refractory metal interface, such as a metal silicide including, for example, cobalt, titanium, tungsten, molybdenum, platinum, nickel, or tantalum. Preferably, the contact material  90  is formed by a self-aligned silicide process, in which the contacts are formed only in the areas where the deposited metal is in direct contact with the source/drain regions  68 ,  70 . Because of the facet-free formation, the metal uniformly penetrates the raised source  68  and drain  70  regions thereby maintaining the advantages of the raised source and drain approach.  
         [0054]     In various embodiments of the invention, the raised source/drain elements  68 ,  70  include, for example, Si, Ge, or SiGe alloys, and are formed by selective epitaxial growth in a CVD system, such as LPCVD, APCVD, or PECVD reactor. Suitable CVD systems commonly used for volume epitaxy in manufacturing applications include, for example, EPI CENTURA™ single-wafer multi-chamber systems available from Applied Materials of Santa Clara, Calif., or EPSILON® single-wafer epitaxial reactors available from ASM International based in Bilthoven, The Netherlands.  
         [0055]     In the CVD process, selective epitaxial growth typically includes introducing a source gas into the chamber. The source gas may include at least one precursor gas and a carrier gas, such as, for example hydrogen. In those embodiments of the invention where the raised regions  68 ,  70  are formed from Si, silicon precursor gases such as, for example, silane, disilane, trisilane, or dichlorosilane (DCS) are used. Conversely, in those embodiments of the invention where the raised regions  68 ,  70  are formed from Ge, germanium precursor gases, such as, for example, germane (GeH 4 ), digermane, germanium tetrachloride, or germanium dichloride are used. Finally, in the embodiments where the raised regions  68 ,  70  are formed from SiGe alloy, a combination of silicon and germanium precursor gases in various proportions is used. In a particular embodiment of the invention for selective deposition of Si 0.80 Ge 0.20  layers, 100 standard cubic centimeters (sccm) of DCS, 25 sccm 10% GeH 4 /H 2 , and 150 sccm of HCl in a hydrogen carrier gas at a growth temperature of 750° C. and pressure of 20 Torr may be used. In another embodiment of the invention for selective deposition of Si layers, 100 sccm of DCS and 100 sccm of HCl in a hydrogen carrier gas at a growth temperature of 850° C. and pressure of 10 Torr may be used.  
         [0056]     The LPCVD, APCVD, or PECVD system chamber is heated, such as, for example, by RF-heating. The growth temperature in the chamber ranges from about 300° C. to about 900° C. depending on the composition of the raised regions  68 ,  70 . Specifically, if the source gas predominantly contains silicon precursor, the temperature preferably ranges from about 500 to about 900° C., and if the source gas predominantly contains germanium precursor, the temperature ranges from about 300° C. to about 700° C. The chamber pressure during formation of raised regions  68 ,  70  is greater than about 5 Torr and the growth rate is greater than 1 nanometer/minute (nm/min).  
         [0057]     Referring again to  FIG. 5 , during selective epitaxial growth, the material composing raised regions  68 ,  70  forms only on the semiconductor substrate, such as the silicon substrate  62 . The top surface  78  of gate electrode  64  is protected from epitaxial growth by the hard mask  76 . Alternatively, hard mask  76  is absent from top surface  78  of gate electrode  64 , and epitaxial growth additionally occurs on top surface  78 . Epitaxy parameters are chosen such that substantially no epitaxial layer is formed on sidewall spacers  80 ,  82 , and, as described below, substantially no facets are formed at the interface between raised regions  68 ,  70  and spacers  80 ,  82 .  
         [0058]     Selective growth of epitaxial layers substantially only on silicon or other semiconductors, and not on dielectrics such as silicon nitride or silicon dioxide, is facilitated by the introduction of an etchant, such as, for example, hydrogen chloride or chlorine, to the source gas mixture. Specifically, epitaxial layers are grown on exposed windows in a dielectric mask while nucleation of polysilicon on the masking material is suppressed during the incubation time by, for example, etching of spurious nuclei on the dielectric material, the mediation of saturation by formation of a number of intermediate chlorine-containing silicon precursors, and passivation of surface defect sites which serve as heterogeneous nucleation centers. Also, selectivity is facilitated by growing for a period of time that is generally shorter than the incubation period needed for polysilicon nucleation on the dielectric mask.  
         [0059]     In various embodiments of the invention, the epitaxial layers of the source/drain regions  68 ,  70  are doped in situ by adding a dopant during deposition of the epitaxial layer to suppress facet formation at the interface with the spacers  80 ,  82 . Examples of suitable dopants are n-type dopant such as phosphorus, arsenic, and antimony, or p-type dopant, such as boron. The dopant may be added to the epitaxial layer by introducing a dopant gas, such as phosphine, arsine, stibine, and diborane, into the chamber. The dopant gas is diluted in a carrier gas of the source gas mixture to, for example, approximately 1% concentration.  
         [0060]     In a particular embodiment, facetless selective epitaxial growth according to the invention is compatible with CMOS fabrication. CMOS fabrication entails formation of a n-channel MOSFET (“NMOS”) in a first portion of the semiconductor region; and a p-channel MOSFET (“PMOS”) in a second portion of the semiconductor region so that both MOSFETs are disposed on the same substrate. Accordingly, in this embodiment, in situ doping that is used during the epitaxial growth to suppress facet formation is sufficiently low so that it will not interfere with introduction of additional dopants of opposite type (“counterdoping”) that is necessary in order to manufacture both n-channel and p-channel MOSFETs on the same substrate. This counterdoping may be performed with suitable masking in place for either NMOS (if the in situ doping to suppress faceting was p-type) or PMOS (if the in situ doping to suppress faceting was n-type), thus allowing CMOS fabrication. Accordingly, in this embodiment of the invention, a level of in situ doping that is used during the epitaxial growth to suppress facet formation does not exceed the level of counterdoping (for example, ion implantation) that is necessary for CMOS fabrication. For example, in one variation of this embodiment, the level of in situ doping ranges from about 10 17  to about 10 19  cm −3 , which does not interfere with a typical doping level used during CMOS fabrication that usually exceeds about 10 20  cm −3 .  
         [0061]     Other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. The described embodiments are to be considered in all respects as only illustrative and not restrictive. Therefore, it is intended that the scope of the invention be only limited by the following claims.