Abstract:
Semiconductor structures and devices including strained material layers having impurity-free zones, and methods for fabricating same. Certain regions of the strained material layers are kept free of impurities that can interdiffuse from adjacent portions of the semiconductor. When impurities are present in certain regions of the strained material layers, there is degradation in device performance. By employing semiconductor structures and devices (e.g., field effect transistors or “FETs”) that have the features described, or are fabricated in accordance with the steps described, device operation is enhanced.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 10/972,578, filed Oct. 25, 2004, entitled “Semiconductor Structures Employing Strained Material Layers with Defined Impurity Gradients and Methods for Fabricating Same,” which is a continuation of U.S. patent application Ser. No. 10/251,424, filed Sep. 20, 2002, entitled “Semiconductor Structures Employing Strained Material Layers with Defined Impurity Gradients and Methods for Fabricating Same,” which claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 60/324,325, filed Sep. 21, 2001; each of these applications are hereby incorporated by reference herein in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to semiconductor structures and devices and, more specifically, to semiconductor structures and field effect transistors (hereinafter, “FETs”) incorporating strained material layers and controlled impurity diffusion gradients. 
       BACKGROUND OF THE INVENTION 
       [0003]    “Virtual substrates” based on silicon (Si) and germanium (Ge) provide a platform for new generations of VLSI devices that exhibit enhanced performance when compared to devices fabricated on bulk Si substrates. The important component of a SiGe virtual substrate is a layer of SiGe that has been relaxed to its equilibrium lattice constant (i.e., one that is larger than that of Si). This relaxed SiGe layer can be directly applied to a Si substrate (e.g., by wafer bonding or direct epitaxy) or atop a relaxed graded SiGe layer, in which the lattice constant of the SiGe material has been increased gradually over the thickness of the layer. The SiGe virtual substrate can also incorporate buried insulating layers, in the manner of a silicon-on-insulator (“SOI”) wafer. To fabricate high-performance devices on these platforms, thin strained layers of Si, Ge, or SiGe are grown on the relaxed SiGe virtual substrates. The resulting biaxial tensile or compressive strain alters the carrier mobilities in the layers, enabling the fabrication of high-speed devices, or low-power devices, or both. 
         [0004]    A technique for fabricating strained Si wafers includes the following steps:
       1. Providing a silicon substrate that has been edge polished;   2. Epitaxially depositing a relaxed graded SiGe buffer layer to a final Ge composition on the silicon substrate;   3. Epitaxially depositing a relaxed SiGe cap layer having a uniform composition on the graded SiGe buffer layer;   4. Planarizing the SiGe cap layer by, e.g., chemical mechanical polishing (“CMP”);   5. Epitaxially depositing a relaxed SiGe regrowth layer having a uniform composition on the planarized surface of the SiGe cap layer; and   6. Epitaxially depositing a strained silicon layer on the SiGe regrowth layer.       
 
         [0011]    The deposition of the relaxed graded SiGe buffer layer enables engineering of the lattice constant of the SiGe cap layer (and therefore the amount of strain in the strained silicon layer), while reducing the introduction of dislocations. The lattice constant of SiGe is larger than that of Si, and is a direct function of the amount of Ge in the SiGe alloy. Because the SiGe graded buffer layer is epitaxially deposited, it will initially be strained to match the in-plane lattice constant of the underlying silicon substrate. However, above a certain critical thickness, the SiGe graded buffer layer will relax to its inherently larger lattice constant. 
         [0012]    The process of relaxation occurs through the formation of misfit dislocations at the interface between two lattice-mismatched layers, e.g., a Si substrate and a SiGe epitaxial layer (epilayer). Because dislocations cannot terminate inside a crystal, misfit dislocations have vertical dislocation segments at each end, i.e., threading dislocations, that may rise through the crystal to reach a top surface of the wafer. Both misfit and threading dislocations have stress fields associated with them. As explained by Eugene Fitzgerald et al.,  Journal of Vacuum Science and Technology B , Vol. 10, No. 4, 1992, incorporated herein by reference, the stress field associated with the network of misfit dislocations affects the localized epitaxial growth rate at the surface of the crystal. This variation in growth rates may result in a surface cross-hatch on lattice-mismatched, relaxed SiGe buffer layers grown on Si. 
         [0013]    The stress field associated with misfit dislocations may also cause dislocation pile-ups under certain conditions. Dislocation pile-ups are a linear agglomeration of threading dislocations. Because pile-ups represent a high localized density of threading dislocations, they may render devices formed in that region unusable. Inhibiting the formation of dislocation pile-ups is, therefore, desirable. 
         [0014]    Dislocation pile-ups are formed as follows. (See, e.g., Srikanth Samavedam et al.,  Journal of Applied Physics , Vol. 81, No. 7, 1997, incorporated herein by reference.) A high density of misfit dislocations in a particular region of a crystal will result in that region having a high localized stress field. This stress field may have two effects. First, the stress field may present a barrier to the motion of other threading dislocations attempting to glide past the misfits. This pinning or trapping of threading dislocations due to the high stress field of other misfit dislocations is known as work hardening. Second, the high stress field may strongly reduce the local epitaxial growth rate in that region, resulting in a deeper trough in the surface morphology in comparison to the rest of the surface cross-hatch. This deep trough may also pin threading dislocations attempting to glide past the region of high misfit dislocations. This cycle may perpetuate itself and result in a linear region with a high density of trapped threading dislocations, i.e., dislocation pile-up. 
         [0015]    The term “MOS” (meaning “metal-oxide-semiconductor”) is here used generally to refer to semiconductor devices, such as FETs, that include a conductive gate spaced at least by an insulting layer from a semiconducting channel layer. The terms “SiGe” and “Si 1-x Ge x ” are here 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. 
         [0016]    One challenge to the manufacturability of MOS devices with strained layers is that one or more high temperature processing steps are typically employed after the addition of the strained material. This can cause intermixing of the strained layer and underlying material. This intermixing is generally referred to as interdiffusion, and it can be described by well-known diffusion theory (e.g., Fick&#39;s laws). One example of interdiffusion is found in a FET where a strained layer is used as the channel. In this example, one or more impurities (e.g., dopants) are implanted after addition of the strained layer. If implantation is followed by a moderately high temperature step (e.g., a drive-in or anneal step), there can be rampant interdiffusion of the channel by the implant impurity due to the presence of implant damage and excess point defects in the strained layer. A result is that the impurity is present in the strained layer. Stated differently, the impurity profile (i.e., a gradient describing the impurity concentration as a function of location in the overall semiconductor or device) has a non-zero value in the strained layer. Presence of one or more impurities in the strained layer can, at certain concentrations, degrade overall device performance. 
         [0017]    From the foregoing, it is apparent that there is still a need for a way to produce semiconductor structures and devices that include one or more strained layers that are not subject to the incursion of one or more impurity species during structure or device fabrication. 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention provides semiconductor structures and devices (e.g., FETs) that include one or more strained material layers that not only improve operational performance, but also are relatively free of interdiffused impurities. Consequently, the resulting semiconductor structures and devices do not exhibit the degraded performance that results from the presence of such impurities in the strained layers. 
         [0019]    The invention features a semiconductor structure where at least one strained layer is disposed on a semiconductor substrate, forming an interface between the two. This structure is characterized by an impurity gradient that describes the concentration of one or more impurities (i.e., dopants) as a function of location in the structure. At the furthest part of the strained layer (i.e., a “distal zone” of the layer away from the interface), this impurity gradient has a value that is substantially equal to zero. 
         [0020]    In one version of this embodiment, the invention provides a method for fabricating a semiconductor structure in a substrate. The method includes the step of disposing at least one strained layer on the substrate, forming an interface between the two. Performing at least one subsequent processing step on the substrate, after which the impurity gradient has a value substantially equal to zero in the distal zone, follows this. The subsequent processing step is generally performed within a predetermined temperature range, which affects the value of the impurity gradient, particularly in the distal zone. 
         [0021]    In certain embodiments, the semiconductor substrate can include Si, SiGe, or any combination of these materials. It can also be multi-layered. In this latter case, the layers can include relaxed SiGe disposed on compositionally graded SiGe. The layers can also include relaxed SiGe disposed on Si. One or more buried insulating layers may be included as well. 
         [0022]    In other embodiments, the strained layer can include Si, Ge, SiGe, or any combination of these materials. At least about fifty Angstroms of the furthest part of the strained layer defines a distal zone where the impurity gradient has a value that is substantially equal to zero. 
         [0023]    Various features of the invention are well suited to applications utilizing MOS transistors (e.g., FETs) that include, for example, one or more of Si, Si 1-x Ge x  or Ge layers in or on a substrate. 
         [0024]    In another embodiment, the invention includes a FET fabricated in a semiconductor substrate. The FET has a channel region that includes at least one strained channel layer. The strained channel layer has a distal zone away from the substrate. The impurity gradient that characterizes the substrate and the channel region has a value substantially equal to zero in the distal zone. 
         [0025]    In one version of this embodiment, the invention provides a method for fabricating a FET in a semiconductor substrate. The method includes the step of disposing at least one strained channel layer in at least the channel region of the FET. (The strained channel layer has a distal zone away from the substrate.) Performing at least one subsequent processing step on the substrate, after which the impurity gradient has a value substantially equal to zero in the distal zone, follows this. The subsequent processing step is generally performed within a predetermined temperature range, which affects the value of the impurity gradient, particularly in the distal zone. 
         [0026]    Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating the principles of the invention by way of example only. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The foregoing and other objects, features, and advantages of the present invention, as well as the invention itself, will be more fully understood from the following description of various embodiments, when read together with the accompanying drawings, in which: 
           [0028]      FIG. 1  is a schematic (unscaled) cross-sectional view that depicts a semiconductor structure in accordance with an embodiment of the invention; 
           [0029]      FIG. 2  is a schematic (unscaled) cross-sectional view that depicts a FET in accordance with an embodiment of the invention; 
           [0030]      FIG. 3  is a flowchart depicting the steps of fabricating a FET in accordance with an embodiment of the invention; and 
           [0031]      FIG. 4  is a flowchart depicting the steps of fabricating a FET in accordance with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    As shown in the drawings for the purposes of illustration, the invention may be embodied in a semiconductor structure or device, such as, for example, a FET, with specific structural features. A semiconductor structure or FET according to the invention includes one or more strained material layers that are relatively free of interdiffused impurities. These strained material layers are characterized by at least one diffusion impurity gradient that has a value that is substantially equal to zero in a particular area of the strained layer. Consequently, the semiconductor structure or FET does not exhibit the degraded performance that results from the presence of such impurities in certain parts of the strained layers. 
         [0033]    In brief overview,  FIG. 1  depicts a schematic (unsealed) cross-sectional view of a semiconductor structure  100  in accordance with an embodiment of the invention. The semiconductor structure  100  includes a substrate  102 . The substrate  102  may be Si, SiGe, or other compounds such as, for example, GaAs or InP. The substrate  102  may also include multiple layers  122 ,  124 ,  126 ,  128 , typically of different materials. (Although  FIG. 1  depicts four layers  122 ,  124 ,  126 ,  128 , this is for illustration only. A single, two, or more layers are all within the scope of the invention.) 
         [0034]    In one embodiment, the multiple layers  122 ,  124 ,  126 ,  128  include relaxed SiGe disposed on compositionally graded SiGe. In another embodiment, the multiple layers  122 ,  124 ,  126 ,  128  include relaxed SiGe disposed on Si. One or more of the multiple layers  122 ,  124 ,  126 ,  128  may also include a buried insulating layer, such as SiO 2  or Si 3 N 4 . The buried insulating layer may also be doped. 
         [0035]    In another embodiment, a relaxed, compositionally graded SiGe layer  124  is disposed on a Si layer  122  (typically part of an Si wafer that may be edge polished), using any conventional deposition method (e.g., chemical vapor deposition (“CVD”) or molecular beam epitaxy (“MBE”)), and the method may be plasma-assisted. A further relaxed SiGe layer  126 , but having a uniform composition, is disposed on the relaxed, compositionally graded SiGe layer  124 . The relaxed, uniform SiGe layer  126  is then planarized, typically by CMP. A relaxed SiGe regrowth layer  128  is then disposed on the relaxed, uniform SiGe layer  126 . 
         [0036]    One or more strained layers  104  are disposed on the substrate  102 . Between the substrate  102  and the strained layer  104  is an interface  106 . Located away from the interface  106  is the distal zone  108  of the strained layer  104 . 
         [0037]    In various embodiments, the strained layer  104  includes one or more layers of Si, Ge, or SiGe. The “strain” in the strained layer  104  may be compressive or tensile, and it may be induced by lattice mismatch with respect to an adjacent layer, as described above, or mechanically. For example, strain may be induced by the deposition of overlayers, such as Si 3 N 4 . Another way is to create underlying voids by, for example, implantation of one or more gases followed by annealing. Both of these approaches induce strain in the underlying substrate  102 , in turn causing strain in the strained layer  104 . 
         [0038]    The substrate  102 , strained layer  104 , and interface  106  are characterized, at least in part, by an impurity gradient  110 A,  110 B (collectively,  110 ). The impurity gradient  110  describes the concentration of the impurity species as a function of location across the substrate  102 , strained layer  104 , and interface  106 . The impurity gradient  110  may be determined by solving Fick&#39;s differential equations, which describe the transport of matter: 
         [0000]    
       
         
           
             
               
                 
                   J 
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                   ( 
                   
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         [0039]    In equations (1) and (2), “J” is the impurity flux, “D” is the diffusion coefficient, and “N” is the impurity concentration. Equation (1) describes the rate of the permeation of the diffusing species through unit cross sectional area of the medium under conditions of steady state flow. Equation (2) specifies the rate of accumulation of the diffusing species at different points in the medium as a function of time, and applies to transient processes. In the general case, equations (1) and (2) are vector-tensor relationships that describe these phenomena in three dimensions. In some cases, equations (1) and (2) may be simplified to one dimension. 
         [0040]    The steady state solution to equation (1), which is not detailed herein, is a function of the Gaussian error function: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0041]    An example solution is shown in  FIG. 1  as the impurity gradient  110 . Axis  112  represents the impurity concentration, typically in units of cm −3 . Axis  114  corresponds to the location in the semiconductor structure  100 . Axis  114  is aligned with the semiconductor structure  100  to illustrate a typical impurity profile, meaning that the impurity concentration at any point in the semiconductor structure  100  can be ascertained as a function of location. Except as described below, the depicted shape of the impurity gradient  110  is not intended to be limiting. For example, impurity gradient  110 A may describe a profile of a p-type (e.g., boron) or n-type (e.g., phosphorous or arsenic) dopant introduced in the substrate  102 . On the other hand, impurity gradient  110 B may, for example, describe a substantially constant concentration of Ge, or Si, or both, in the substrate  102  that takes on a desired value (e.g., a reduced value) in the strained layer  104 . Stated differently, the impurity gradient  110  may describe the concentration of any species in the substrate  102 , including the substrate species itself, at any point in the semiconductor structure  100 . 
         [0042]    Boundary  116  represents the interface between the substrate  102  and the strained layer  104 . Boundary  118  depicts the start of the distal zone  108  of the strained layer  104 . Boundary  120  corresponds to the edge of the strained layer  104 . Of note are the locations where the boundaries  116 ,  118 ,  120  intersect the axis  114  and the impurity gradient  110 . In particular, the impurity gradient  110  has a value substantially equal to zero in the distal zone  108 . This is depicted by the impurity gradient  110  approaching the axis  114  at boundary  118 , and remaining there, or at zero, or at another value substantially equal to zero, between boundary  118  and  120 . Of course, the impurity gradient  110  can also have a value substantially equal to zero before reaching the boundary  118 . In any case, one embodiment of the invention features a distal zone  108  that includes at least about fifty Angstroms of the furthest part of the strained layer  104 . That is, the distal zone  108  is at least about fifty Angstroms thick. 
         [0043]    In another embodiment depicted schematically (i.e., unscaled) in  FIG. 2 , a FET  200  is fabricated in a similar semiconductor structure. The FET  200  includes a semiconductor substrate  202 , which may be Si, SiGe, or other compounds such as, for example, GaAs or InP. The substrate  202  can be multi-layered, and it can include relaxed SiGe disposed on compositionally graded SiGe, or relaxed SiGe disposed on Si. The substrate  202  may also include a buried insulating layer, such as SiO 2  or Si 3 N 4 . The buried insulating layer may also be doped. 
         [0044]    Disposed on the substrate  202  is an isolation well  204 , typically including an oxide. Within the isolation well  204  are isolation trenches  206 . A source region  208  and a drain region  212  are typically formed by ion implantation. A FET channel  210  is formed from one or more strained layers. The strained layers can include one or more layers of Si, Ge, or SiGe. The “strain” in the strained layers may be compressive or tensile, and it may be induced as described above. The furthest part of the channel  210  is located away from the substrate  202 . This furthest part forms the distal zone of the channel  210 . 
         [0045]    Disposed on at least part of the channel  210  is a gate dielectric  214 , such as, for example, SiO 2 , Si 3 N 4 , or any other material with a dielectric constant greater than that of SiO 2  (e.g., HfO 2 , HfSiON). The gate dielectric  214  is typically twelve to one hundred Angstroms thick, and it can include a stacked structure (e.g., thin SiO 2  capped with another material having a high dielectric constant). 
         [0046]    Disposed on the gate dielectric  214  is the gate electrode  216 . The gate electrode  216  material can include doped or undoped polysilicon, doped or undoped poly-SiGe, or metal. Disposed about the gate electrode  216  are the transistor spacers  218 . The transistor spacers  218  are typically formed by depositing a dielectric material, which may be the same material as the gate dielectric  214 , followed by anisotropic etching. 
         [0047]    The impurity gradient  110  also characterizes the channel  210  and the substrate  202 , as well as the isolation well  204 . This is shown in  FIG. 2  in an expanded view that, for clarity, differs in scale compared to the remainder of (unscaled)  FIG. 2 . The distal zone of the channel  210  corresponds to that portion of the impurity gradient  110  between boundaries  118 ,  120  (expanded for clarity). Within the distal zone of the channel  210 , the impurity gradient  110  has a value substantially equal to zero. As discussed above, the depicted shape of the impurity gradient  110  is not intended to be limiting, and the impurity gradient  110  can also have a value substantially equal to zero before reaching the boundary  118 . One embodiment of the invention features a distal zone  108  that includes at least about fifty Angstroms of the furthest part of the channel  210 . That is, the distal zone is at least about fifty Angstroms thick. 
         [0048]    One version of an embodiment of the invention provides a method for fabricating a FET in a semiconductor substrate. The method includes the step of disposing one or more strained channel layers in the FET channel region. The channel layer has a distal zone away from the substrate. The distal zone includes at least about fifty Angstroms of the furthest part of the channel region. An impurity gradient characterizes at least the substrate and the strained layers. 
         [0049]    Next, one or more subsequent processing steps are performed on the substrate. After these subsequent processing steps are performed, the impurity gradient has a value that is substantially equal to zero in the distal zone. Since the impurity gradient can be influenced by temperature, the subsequent processing steps are typically performed within a predetermined temperature range that is chosen to ensure that the impurity gradient has a desired value, particularly in the distal zone. 
         [0050]      FIG. 3  depicts a method  300  for fabricating the FET in accordance with an embodiment of the invention. This method includes the step of providing a substrate, typically planarized, and without strained layers (step  302 ). The substrate can include relaxed SiGe on a compositionally graded SiGe layer, relaxed SiGe on a Si substrate, relaxed SiGe on Si, or other compounds such as GaAs or InP. The substrate can also contain a buried insulating layer. 
         [0051]    Next, initial VLSI processing steps are performed such as, for example, surface cleaning, sacrificial oxidation, deep well drive-in, and isolation processes like shallow trench isolation with liner oxidation or LOCOS (step  304 ). Any number of these steps may include high temperatures or surface material consumption. Features defined during step  304  can include deep isolation wells and trench etch-refill isolation structures. Typically, these isolation trenches will be refilled with SiO 2  or another insulating material, examples of which are described above. 
         [0052]    Next, the channel region is doped by techniques such as shallow ion implantation or outdiffusion from a solid source (step  306 ). For example, a dopant source from glass such as BSG or PSG may be deposited (step  308 ), followed by a high temperature step to outdiffuse dopants from the glass (step  310 ). The glass can then be etched away, leaving a sharp dopant spike in the near-surface region of the wafer (step  312 ). This dopant spike may be used to prevent short-channel effects in deeply scaled surface channel FETs, or as a supply layer for a buried channel FET that would typically operate in depletion mode. The subsequently deposited channel layers can then be undoped, leading to less mobility-limiting scattering in the channel of the device and improving its performance. Likewise, this shallow doping may be accomplished via diffusion from a gas source (e.g., rapid vapor phase doping or gas immersion laser doping) (step  314 ) or from a plasma source as in plasma immersion ion implantation doping (step  316 ). 
         [0053]    Next, deposit one or more strained channel layers, preferably by a CVD process (step  318 ). The channel may be Si, Ge, SiGe, or a combination of multiple layers of Si, Ge, or SiGe. Above the device isolation trenches or regions, the deposited channel material typically will be polycrystalline. Alternatively, the device channels may be deposited selectively, i.e., only in the device active area and not on top of the isolation regions. Typically, the remaining steps in the transistor fabrication sequence will involve lower thermal budgets and little or no surface material consumption. 
         [0054]    Next, the transistor fabrication sequence is continued with the growth or deposition of a gate dielectric (step  320 ) and the deposition of a gate electrode material (step  322 ). Examples of gate electrode material include doped or undoped polysilicon, doped or undoped poly-SiGe, or metal. This material stack is then etched (step  324 ), forming the gate of the transistor. Typically, this etch removes the gate electrode material by a process such as reactive ion etching (“RIE”) and stops on the gate dielectric, which is then generally removed by wet etching. After this, the deposited channel material typically is still present. 
         [0055]    Next, the transistor spacers are formed by the traditional process of dielectric material deposition and anisotropic etching (step  326 ). Step  326  may be preceded by extension implantation, or removal of the channel material in the regions not below the gate, or both. If the channel material is not removed before spacer material deposition, the spacer etch may be tailored to remove the excess channel material in the regions not below the gate. Failure to remove the excess channel material above the isolation regions can result in device leakage paths. 
         [0056]    Next, the source and drain regions are fabricated, typically by ion implantation (step  328 ). Further steps to complete the device fabrication can include salicidation (step  330 ) and metallization (step  332 ). 
         [0057]      FIG. 4  depicts another method  400  for fabricating the FET in accordance with an embodiment of the invention. This method includes creating the channel at a different point in the fabrication process, and starts with performing the traditional front-end VLSI processing steps, such as, for example, well formation, isolation, gate stack deposition and definition, spacer formation, source-drain implant, silicidation (step  402 ). In place of a gate electrode, fabricate a “dummy gate” (step  404 ). This dummy gate is etched and replaced in subsequent processing steps. The dummy gate may include an insulating material such as Si 3 N 4  (or any of the other dielectric materials discussed), or a conducting material such as polysilicon, poly-Ge, or metal. In contrast to a typical MOSFET process where the gate is separated from the semiconductor substrate by a gate dielectric, the dummy gate is separated from the substrate by an etch-stop layer. The etch-stop layer can be of SiO 2 , either thermally grown or deposited. 
         [0058]    Next, a dielectric layer is deposited (e.g., by a CVD process) (step  406 ) and planarized (step  408 ) by, for example, CMP. This “planarization layer” is typically a different material then the dummy gate. 
         [0059]    Next, the dummy gate is removed by a selective etching process (step  410 ). The etch-stop layer protects the substrate from this etching process. A wet or dry etch then removes the etch-stop layer. 
         [0060]    An example configuration includes a polysilicon dummy gate, an SiO 2  etch-stop layer, Si 3 N 4  spacers, and an SiO 2  planarization layer. This configuration allows selective removal of the dummy gate with an etchant such as heated tetramethylammonium hydroxide (“TMAH”), thereby leaving the SiO 2  and Si 3 N 4  intact. The etch-stop is subsequently removed by a wet or dry etch (e.g., by HF). 
         [0061]    Next, one or more strained channel layers is deposited, typically by a CVD process (step  412 ). The channel layers may be Si, Ge, SiGe, or a combination of multiple layers of Si, Ge, or SiGe. The gate dielectric is then thermally grown or deposited (by CVD or sputtering, for example) (step  416 ). This is followed by deposition of the gate electrode material (step  418 ), which can include doped or undoped polysilicon, doped or undoped poly-SiGe, or metal. 
         [0062]    Next, the gate electrode is defined (step  420 ). This can be by photomasking and etching (step  422 ) of the gate electrode material. This may also be done by a CMP step (step  424 ), where the gate electrode material above the planarization layer is removed. 
         [0063]    Using this method, a silicide is generally formed on the source and drain regions before the deposition of the planarization layer. In this case, all subsequent processing steps are typically limited to a temperature that the silicide can withstand without degradation. One alternative is to form the silicide at the end of the process. In this case, the planarization layer may be removed by a selective wet or dry etch which leaves the gate electrode material and the spacers intact. This is followed by a traditional silicide process, e.g., metal deposition and thermally activated silicide formation on the source and drain regions (and also on the gate electrode material, if the latter is polysilicon), followed by a wet etch strip of unreacted metal. Further steps to complete the device fabrication can include inter-layer dielectric deposition and metallization. Note that if the step of forming the gate dielectric is omitted, a metal gate electrode may be deposited directly on the channel, resulting in the fabrication of a self-aligned HEMT (or MESFET) structure. 
         [0064]    From the foregoing, it will be appreciated that the semiconductor structures and devices provided by the invention afford a simple and effective way to minimize or eliminate the impurities in certain parts of strained material layers used therein. The problem of degraded device performance that results from the presence of such impurities is largely eliminated. 
         [0065]    One skilled in the art will realize the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting of the invention described herein. Scope of the invention is thus indicated be the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.