Patent Publication Number: US-9425280-B2

Title: Semiconductor device with low-K spacers

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of co-pending application Ser. No. 13/656,794, filed Oct. 22, 2012. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Generally, the present disclosure relates to the manufacture of sophisticated semiconductor devices, and, more specifically, to various methods of forming a semiconductor device with low-k spacers and various semiconductor devices incorporating such low-k spacers. 
     2. Description of the Related Art 
     The fabrication of advanced integrated circuits, such as CPU&#39;s, storage devices, ASIC&#39;s (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements in a given chip area according to a specified circuit layout. Field effect transistors (NMOS and PMOS transistors) represent one important type of circuit element that substantially determines performance of such integrated circuits. During the fabrication of complex integrated circuits using, for instance, MOS technology, millions of transistors, e.g., NFET transistors and/or PFET transistors, are formed on a substrate including a crystalline semiconductor layer. A field effect transistor, whether an NFET or a PFET device, is a planar device that typically includes a source region, a drain region, a channel region that is positioned between the source region and the drain region, a gate insulation layer and a gate electrode positioned above the gate insulation layer over the channel region. By applying an appropriate voltage to the gate electrode, the channel region becomes conductive and current is allowed to flow from the source region to the drain region. 
     For many early device technology generations, the gate structures of most transistor elements has been comprised of a plurality of silicon-based materials, such as a silicon dioxide and/or silicon oxynitride gate insulation layer, in combination with a polysilicon gate electrode. However, as the channel length of aggressively scaled transistor elements has become increasingly smaller, many newer generation devices employ gate structures that contain alternative materials in an effort to avoid the short channel effects which may be associated with the use of traditional silicon-based materials in reduced channel length transistors. For example, in some aggressively scaled transistor elements, which may have channel lengths on the order of approximately 10-20 nm or less, gate structures that include a so-called high-k dielectric gate insulation layer and one or metal layers that function as the gate electrode (HK/MG) have been implemented. Such alternative gate structures have been shown to provide significantly enhanced operational characteristics over the heretofore more traditional silicon dioxide/polysilicon gate structure configurations. 
     Depending on the specific overall device requirements, several different high-k materials—i.e., materials having a dielectric constant, or k-value, of approximately 10 or greater—have been used with varying degrees of success for the gate insulation layer in an HK/MG gate electrode structure. For example, in some transistor element designs, a high-k gate insulation layer may include tantalum oxide (Ta 2 O 5 ), hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), titanium oxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), hafnium silicates (HfSiO x ) and the like. Furthermore, one or more non-polysilicon metal gate electrode materials—i.e., a metal gate stack—may be used in HK/MG configurations so as to control the work function of the transistor. These metal gate electrode materials may include, for example, one or more layers of titanium (Ti), titanium nitride (TiN), titanium-aluminum (TiAl), aluminum (Al), aluminum nitride (AlN), tantalum (Ta), tantalum nitride (TaN), tantalum carbide (TaC), tantalum carbonitride (TaCN), tantalum silicon nitride (TaSiN), tantalum silicide (TaSi) and the like. 
     One well-known processing method that has been used for forming a transistor with a high-k/metal gate structure is the so-called “gate last” or “replacement gate” technique.  FIGS. 1A-1D  depict one illustrative prior art method for forming an HK/MG replacement gate structure using a gate last technique. As shown in  FIG. 1A , the process includes the formation of a basic transistor structure  200  above a semiconducting substrate  210  in an active area defined by a shallow trench isolation structure  211 . At the point of fabrication depicted in  FIG. 1A , the device  200  includes a sacrificial gate insulation layer  212 , a dummy or sacrificial gate electrode  214 , sidewall spacers  216 , a layer of insulating material  217  and source/drain regions  218  formed in the substrate  210 . The various components and structures of the device  200  may be formed using a variety of different materials and by performing a variety of known techniques. For example, the sacrificial gate insulation layer  212  may be comprised of silicon dioxide, the sacrificial gate electrode  214  may be comprised of polysilicon, the sidewall spacers  216  may be comprised of silicon nitride and the layer of insulating material  217  may be comprised of silicon dioxide. The source/drain regions  218  may be comprised of implanted dopant materials (N-type dopants for NFET devices and P-type dopants for PFET devices) that are implanted into the substrate  210  using known masking and ion implantation techniques. Of course, those skilled in the art will recognize that there are other features of the transistor  200  that are not depicted in the drawings for purposes of clarity. For example, so-called halo implant regions are not depicted in the drawings, as well as various layers or regions of silicon/germanium that are typically found in high performance PFET transistors. At the point of fabrication depicted in  FIG. 1A , the various structures of the device  200  have been formed and a chemical mechanical polishing process (CMP) has been performed to remove any materials above the sacrificial gate electrode  214  (such as a protective cap layer (not shown) comprised of silicon nitride) so that at least the sacrificial gate electrode  214  may be removed. 
     As shown in  FIG. 1B , one or more etching processes are performed to remove the sacrificial gate electrode  214  and the sacrificial gate insulation layer  212  to thereby define a gate cavity  220  where a replacement gate structure will subsequently be formed. A masking layer that is typically used in such etching processes is not depicted for purposes of clarity. Typically, the sacrificial gate insulation layer  212  is removed as part of the replacement gate technique, as depicted herein. However, the sacrificial gate insulation layer  212  may not be removed in all applications. 
     Next, as shown in  FIG. 1C , various layers of material that will constitute a replacement gate structure  230  are formed in the gate cavity  220 . The materials used for such replacement gate structures  230  may vary depending upon the particular application. Even in cases where the sacrificial gate insulation layer  212  is intentionally removed, there will typically be a very thin native oxide layer (not shown) that forms on the substrate  210  within the gate cavity  220 . In one illustrative example, the replacement gate structure  230  is comprised of a high-k gate insulation layer  230 A, such as hafnium oxide, having a thickness of approximately 2 nm, a first metal layer  230 B (e.g., a layer of titanium nitride with a thickness of about 1-2 nm), a second metal layer  230 C (e.g., a layer of tantalum nitride with a thickness of about 1-2 nm), a third metal layer  230 D (e.g., a layer of titanium nitride with a thickness of about 5 nm) and a bulk metal layer  230 E, such as aluminum. Ultimately, as shown in  FIG. 1D , one or more CMP processes are performed to remove excess portions of the gate insulation layer  230 A, the first metal layer  230 B, the second metal layer  230 C, the third metal layer  230 D and the bulk metal layer  230 E positioned outside of the gate cavity  220  to thereby define the replacement gate structure  230 . 
     As device dimensions have decreased and packing densities have increased, parasitic capacitance is becoming more important as a factor to consider to improve the operating speed of transistor devices. Typically, as noted above, the gate structure of a transistor will include at least one sidewall spacer positioned adjacent the gate structure. Typically, the sidewall spacers are made of silicon nitride and they are normally formed very soon after the final gate structure is formed using a “gate-first” manufacturing technique or after the sacrificial gate structure is formed for devices manufactured using the replacement gate technique. One of the primary purposes of the silicon nitride spacers for “gate-first” devices is to protect the gate materials during subsequent processing operations. For “replacement gate” devices, the spacers also serve to protect the replacement gate structure and to define the gate cavity in the replacement gate manufacturing process. 
     Unfortunately, the k-value of silicon nitride is relatively high, e.g., about 7-8. The presence of the silicon nitride spacer material (with a relatively high k-value) tends to increase the parasitic capacitance between the conductive gate electrode and conductive contacts (like trench silicide regions) that are formed in close proximity to the gate structure of the transistor. This problem has become even more problematic as packing densities have increased which causes the gate structures of adjacent transistor to be positioned ever closer to one another. The use of so-called self-aligned contacts has also exacerbated this problem. 
     The use of alternative materials for the sidewall spacers, such as materials having k values less than about 6 or so, has been problematic. Most of such low-k materials are based upon carbon or boron doped silicon nitride. The low-k material, when used as a traditional spacer material, is subjected to a reactive ion etching (RIE) process to define the spacer from such a low-k material. The RIE process tends to deplete the carbon and boron, thereby effectively increasing the k-value of the low-k material. Such low-k materials also tend to be weaker mechanically than silicon nitride, which makes them less capable of standing up to the rigors of processing after they are formed. Moreover, such spacers are typically subjected to relatively high temperature source/drain anneal processes, which also tends to deplete the carbon and boron from such low-k materials. 
     The present disclosure is directed to various methods of forming a semiconductor device with low-k spacers and various semiconductor devices incorporating such low-k spacers that may solve or reduce one or more of the problems identified above. 
     SUMMARY OF THE INVENTION 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
     Generally, the present disclosure is directed to various methods of forming a semiconductor device with low-k spacers and various semiconductor devices incorporating such low-k spacers. One illustrative method disclosed herein includes forming a sacrificial gate structure above a semiconducting substrate, forming at least one sacrificial sidewall spacer adjacent the sacrificial gate structure, removing at least a portion of the sacrificial gate structure to thereby define a gate cavity that is laterally defined by the sacrificial sidewall spacer, forming a replacement gate structure in the gate cavity, after forming the replacement gate structure, removing the sacrificial sidewall spacer to thereby define a spacer cavity adjacent the replacement gate structure, and forming a low-k spacer in the spacer cavity. 
     Another illustrative method includes forming a sacrificial gate structure above a semiconducting substrate, forming a first sacrificial sidewall spacer adjacent the sacrificial gate structure, forming a second sacrificial sidewall spacer adjacent the first sacrificial sidewall spacer and performing a first etching process to remove the second sacrificial sidewall spacer relative to the first sacrificial sidewall spacer to thereby define a first spacer cavity. In this embodiment, the method further includes the steps of forming a third sacrificial sidewall spacer in the first spacer cavity adjacent the first sacrificial sidewall spacer, after forming the third sacrificial sidewall spacer, removing at least a portion of the sacrificial gate structure to thereby define a gate cavity that is laterally defined by the first sacrificial sidewall spacer, forming a replacement gate structure in the gate cavity, after forming the replacement gate structure, removing at least the first and third sacrificial sidewall spacers to thereby define a second spacer cavity adjacent the replacement gate structure, and forming a low-k spacer in the second spacer cavity. 
     One example of a novel device disclosed herein includes a gate structure positioned above a semiconducting substrate, wherein the gate structure includes a gate insulation layer and a gate electrode. In this example, the gate insulation layer has two upstanding portions that are substantially vertically oriented relative to an upper surface of the substrate. The device further includes a low-k sidewall spacer positioned adjacent each of the vertically oriented upstanding portions of the gate insulation layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
         FIGS. 1A-1D  depict one illustrative prior art process flow for forming a semiconductor device using a so-called “gate last” or “replacement gate” approach; and 
         FIGS. 2A-2Q  depict one illustrative method disclosed herein for forming an illustrative semiconductor device with low-k spacers. 
     
    
    
     While the subject matter disclosed herein is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Various illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The present subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
     The present disclosure is directed to various methods of forming a semiconductor device with low-k spacers and various semiconductor devices incorporating such low-k spacers. As will be readily apparent to those skilled in the art upon a complete reading of the present application, the methods disclosed herein may be employed with a variety of different technologies, e.g., NMOS, PMOS, CMOS, etc., and in manufacturing a variety of different devices, including, but not limited to, logic devices, memory devices, etc. With reference to the attached figures, various illustrative embodiments of the methods and devices disclosed herein will now be described in more detail. 
     In general, the present disclosure is directed to a novel method of forming a semiconductor device with low-k spacers and various semiconductor devices incorporating such low-k spacers. As will be appreciated by those skilled in the art after a complete reading of the present application, the inventions disclosed herein may be employed in forming planar transistor devices (NFET or PFET devices), as well as so-called 3-D devices, such as FinFETs. For purposes of disclosure, reference will be made to an illustrative process flow for forming a planar transistor device. However, the inventions disclosed herein should not be considered to be limited to such an illustrative example. 
       FIG. 2A  is a simplified view of an illustrative semiconductor device  100  at an early stage of manufacturing. An isolation region  12  has been formed in a semiconducting substrate  10  which thereby defines an active region  13  where the device  100  will be formed. The substrate  10  may have a variety of configurations, such as the depicted bulk silicon configuration. The substrate  10  may also have a silicon-on-insulator (SOI) configuration that includes a bulk silicon layer, a buried insulation layer and an active layer, wherein semiconductor devices are formed in and above the active layer. The substrate  10  may be made of silicon or it may be made of materials other than silicon. Thus, the terms “substrate” or “semiconducting substrate” should be understood to cover all semiconducting materials and all forms of such materials. 
     With continuing reference to  FIG. 2A , the process begins with the formation of several layers of material above the substrate  10 : a sacrificial gate insulation layer  14 , a sacrificial gate electrode layer  16  and a mask layer  18 . Such layers may be formed using a variety of different materials and by performing a variety of known techniques. For example, the sacrificial gate insulation layer  14  may be comprised of silicon dioxide, the sacrificial gate electrode layer  16  may be comprised of polysilicon or amorphous silicon and the mask layer  18  may be comprised of silicon nitride. The sacrificial gate insulation layer  14 , sacrificial gate electrode layer  16  and the mask layer  18  may be of any desired thickness or configuration. Moreover, the mask layer  18  could be comprised of multiple layers of material, such as, for example, a combination of a silicon nitride layer and a layer of silicon dioxide. Thus, the particular form and composition of the mask layer  18  and the manner in which it is made should not be considered a limitation of the present invention. Of course, those skilled in the art will recognize that there are other features of the transistor  100  that are not depicted in the drawings so as not to obscure the present invention. For example, so-called halo implant regions and various layers or regions of silicon/germanium that are typically found in high performance PFET transistors are not depicted in the drawings. The layers of material depicted in  FIG. 2A  may be formed by any of a variety of different known techniques, e.g., a chemical vapor deposition (CVD) process, an atomic layer deposition (ALD) process, a thermal growth process, etc. 
     As shown in  FIG. 2B , one or more patterning and etching processes are performed to define the sacrificial gate electrode  16 A and the gate cap layer  18 A. Typically, the mask layer  18  is patterned using known photolithography and etching techniques to thereby define a patterned mask layer. Alternatively, the mask layer  18  may be patterned using known sidewall image transfer techniques. Thereafter one or more etching processes are performed through the patterned mask layer to remove exposed portions of the sacrificial gate electrode layer  16  to thereby define the depicted sacrificial gate electrode  16 A. 
       FIG. 2C  depicts the device  100  after several process operations have been performed. First, a first sacrificial sidewall spacer  20  was formed adjacent the sacrificial gate electrode  16 A. Thereafter, a second sacrificial  22  was formed adjacent the first sidewall spacer  20 . Then, illustrative raised source/drain regions  24  were formed for the device  100 . However, such raised source/drain regions  24  need not be formed in all applications, and the disclosed inventions should not be considered to be limited to such an illustrative configuration. The first and second sacrificial sidewall spacers  20 ,  22  may be formed by depositing a layer of spacer material and thereafter performing an anisotropic etching process. The first sacrificial sidewall spacer  20  may be comprised of carbon or other conformably deposited films that are made of a material that exhibits etch selectivity relative to silicon nitride and silicon dioxide, i.e., silicon nitride or silicon dioxide may be selectively removed from the first sacrificial sidewall spacer  20 , and the spacer  20  may have a thickness at its base of about 1-2 nm. The second sacrificial sidewall spacer  22  may be comprised of silicon nitride and the spacer  22  may have a thickness at its base of about 5-10 nm. The raised source/drain region  24  may be formed by forming cavities in the substrate  10  and thereafter performing an epitaxial deposition process to thereby form doped semiconductor material in the cavities in the substrate  10 . Although not depicted in the drawings, an extension implantation process may be performed to form extension implant regions (not shown) in the substrate  10  prior to or after the formation of the first and second sacrificial sidewall spacers  20 ,  22 . 
     Next, as shown in  FIG. 2D , a conformably deposited layer of insulating material  26 , e.g., a conformably deposited layer of silicon nitride, having a thickness of about 2-5 nm is formed on the device  100 . Thereafter, a layer of insulating material  28  is blanket-deposited over the device  100 . In one illustrative example, the layer of insulating material  28  may be a silicon dioxide material, such as a flowable oxide material, and it may be formed using traditional techniques, e.g., CVD, spin-on/cure, etc. 
       FIG. 2E  depicts the device  100  after one or more CMP processes have been performed on the insulating layer  28 . The CMP process stops on the upper surface  18 S of the gate cap layer  18 A and thereby results in the layer of insulating material  28  having a polished upper surface  28 S that is substantially planar with the surface  18 S. 
     Then, as shown in  FIG. 2F , a material removal process is performed on the layer of insulating material  28  to reduce its overall thickness and thereby define a reduced thickness layer of insulating material  28 R having an upper surface  28 E. In one illustrative embodiment, the material removal process is a highly-controllable chemical oxide atomic layer removal process that may be performed using Applied Materials&#39; SiCoNi process or the well-known COR process from Tokyo Electron. In one example, this removal process should be controlled such that the upper surface  28 E of the reduced thickness layer of insulating material  28 R is positioned slightly below (e.g., about 3-10 nm) the upper surface  16 S of the sacrificial gate electrode  16 A. 
     Next, as shown in  FIG. 2G , a layer of insulating material  30  is formed above the reduced thickness layer of insulating material  28 R and a CMP process is performed on the reduced thickness layer of insulating material  28 R using the gate cap  18 A as a polish stop. This results in the structure depicted in  FIG. 2G . In one illustrative embodiment, the layer of insulating material  30  is an HDP oxide, a better quality oxide material, that is more etch resistant than the flowable oxide layer of insulating material  28 R. In one illustrative example, the layer of insulating material  30  may have a thickness that falls within the range of about 5-20 nm. 
     Next, as shown in  FIG. 2H , another CMP process is performed on the layer of insulating material  30 , the gate cap layer  18 A, and the upper portions of the first and second sacrificial sidewall spacers  20 ,  22  using the sacrificial gate electrode  16 A as a polish stop. This CMP process exposes the upper surface  16 S of the sacrificial gate electrode  16 A. 
     Then, as shown in  FIG. 2I , a wet etching process  34  is performed to remove the residual portions of the second sacrificial sidewall spacers  22  and the layer of insulating material  26 , i.e., the etching process  34  removes exposed silicon nitride material while leaving the first sacrificial sidewall spacers  20  intact. The etching process  34  results in the formation of a plurality of first spacer cavities or gaps  34 G adjacent the first sacrificial sidewall spacers  20 . During the etching process  34 , there may be some undercutting of the layer of insulating material  26  in the area  36 . However, in this embodiment, the parameters of the etching process are controlled so as to limit or eliminate the amount of undercutting in the area  36 . 
       FIG. 2J  depicts the device  100  after a plurality of third sacrificial sidewall spacers  38  have been formed in the first spacer cavities  34 G adjacent the first sacrificial sidewall spacers  20 . The structure depicted in  FIG. 2J  is the result of several process operations. Initially, in one embodiment, a layer of third spacer material (not shown) used for the third sacrificial sidewall spacers  38  is blanket-deposited across the device  100  above the layer of insulating material  30 , above the upper surface  16 S of the sacrificial gate structure  16 A and in the first spacer cavities  34 G. Thereafter, a dry etching process is performed on the layer of third spacer material  38  so as to clear the third spacer material from above the upper surface of the layer of insulating material  30  and from above the upper surface  16 S of the sacrificial gate structure  16 A. As the third spacer material layer is etched away, upper portions of the first sacrificial sidewall spacers  20  are also removed during this etching process. Alternatively, a CMP process (that stops on the layer  30 ) could be performed to remove the excess material of the third spacer material layer to thereby form the third sacrificial spacers  38  in the first spacer cavities  34 G adjacent the first sacrificial sidewall spacers  20 . The third sacrificial sidewall spacers  38  may be made of the same material as the first sacrificial sidewall spacers  20 , as discussed above. 
       FIGS. 2K and 2L  depict several process operations that are directed to the removal of the sacrificial gate electrode  16 A and the sacrificial gate insulation layer  14 , and the formation of a replacement gate structure  50  for the device  100 . As shown in  FIG. 2K , one or more etching processes are performed to remove the sacrificial gate electrode  16 A and portions of the sacrificial gate insulation layer  14  exposed by the removal of the sacrificial gate electrode  16 A to thereby define a gate cavity  40  where the replacement gate structure  50  will subsequently be formed. Note that the gate cavity  40  is laterally defined by the first sacrificial sidewall spacers  20  at this point in the process flow. A masking layer (not shown) that is typically used in such an etching process is not depicted for purposes of clarity. Typically, the sacrificial gate insulation layer  14  is removed as part of the replacement gate technique, as depicted herein. However, the sacrificial gate insulation layer  14  may not be removed in all applications. Even in cases where the sacrificial gate insulation layer  14  is intentionally removed, there will typically be a very thin native oxide layer (not shown) that forms on the substrate  10  within the gate cavity  40 . 
     As shown in  FIG. 2L , a schematically depicted replacement gate structure  50  for the device  100  is formed in the gate cavity  40 . The schematically depicted gate structure  50  includes an illustrative gate insulation layer  50 A and an illustrative gate electrode  50 B. As will be recognized by those skilled in the art after a complete reading of the present application, the gate structure  50  of the device  100  depicted in the drawings, i.e., the gate insulation layer  50 A and the gate electrode  50 B, is intended to be representative in nature. For example, the gate insulation layer  50 A may be comprised of a variety of different materials, such as, for example, silicon dioxide, a so-called high-k (k greater than 10) insulation material (where k is the relative dielectric constant), etc. The gate electrode  50 B may be comprised of one or more layers of conductive material, e.g., polysilicon, one or more layers of metal, etc. As noted above, in some cases, if desired, a metal layer (not shown), such as a very thin work function adjusting metal (e.g., a layer of titanium nitride), may be formed on the high-k gate insulation layer  50 A. As will be recognized by those skilled in the art after a complete reading of the present application, the insulating materials and the metal layer(s) that are part of the replacement gate structure  50  may be of any desired construction and comprised of any of a variety of different materials. Additionally, the replacement gate structure  50  for an NFET device may have different material combinations as compared to a replacement gate structure  50  for a PFET device. Thus, the particular details of construction of replacement gate structure  50 , and the manner in which such replacement gate electrode structure  50  is formed, should not be considered a limitation of the present invention unless such limitations are expressly recited in the attached claims. 
     In one illustrative example, the replacement gate formation process begins with performing a conformal deposition process to form the high-k gate insulation layer  50 A in the gate cavity  40  and above the layer of insulating material  30 . Thereafter, the conductive materials that will be used for the gate electrode  50 B, e.g., one or more metal layers, will be deposited across the devices by performing one or more conformal deposition processes and/or one or more blanket-deposition processes so as to substantially overfill the gate cavity  40  with conductive gate electrode material(s). At that point, one or more CMP processes are performed to remove excess portions of the gate insulation layer  50 A and the layers of conductive material that will be used to form the gate electrode  50 B are positioned above the layer of insulating material  30 . This CMP process essentially planarizes the upper surface of the materials of the gate structure  50  with the upper surface of the layer of insulating material  30 . Thereafter, an etching process is performed to reduce the height of the replacement gate structure  50  such that the upper surface  50 S of the replacement gate electrode  50 B is positioned below the upper surface  20 S of the first sacrificial sidewall spacers  20 . 
     Next, as shown in  FIG. 2M , an illustrative gate cap layer  52 , comprised of, for example, silicon nitride, has been formed above the recessed gate structure  50 . The gate cap layer  52  may be formed by depositing a layer of the cap material and thereafter performing a CMP process to remove excess portions of the cap material positioned on top of the layer of insulating material  30 . 
     Then, as shown in  FIG. 2N , an etching process  54  is performed to remove the residual portions of the first and third sacrificial sidewall spacers  20 ,  38 . The etching process  54  results in the formation of second spacer cavities or low-k spacer cavities  54 A adjacent the replacement gate structure  50 . More specifically, the etching process  54  exposes the sidewall  50 W of the replacement gate structure  50 , i.e., it exposes the gate insulation layer  50 A. The etching process  54  may be a dry or wet etching process. In the illustrative case where the first and third sacrificial sidewall spacers  20 ,  38  are made of carbon, the etching process may be performed using a mild plasma chemistry, such as, for example, oxygen-based ashing with mild power, or a wet H 2 O 2 /H 2 SO 4  based chemistry. 
       FIG. 2O  depicts the device  100  after a plurality of low-k sidewall spacers  60  have been formed in the second spacer cavities  54 A adjacent the replacement gate structure  50 . The structure depicted in  FIG. 2O  is the result of several process operations. Initially, in one embodiment, a layer of low-k insulating material (not shown) is blanket-deposited across the device  100  above the layer of insulating material  30 . Thereafter, excess portions of the low-k insulating material positioned outside of the second spacer cavities  54 A may be removed by performing a dry etch-back process or by performing a CMP process. As used herein and in the claims, as it relates to the formation of the low-k spacers  60 , the term “low-k material” or “low-k spacer” should be understood to mean any material having a dielectric constant less than that of traditional spacer material—silicon nitride, i.e., “low-k spacer” or “low-k material” means a material with a k value less than 7. Some illustrative materials that may be used for the low-k spacers  60  include, for example, SiCN, SiBN, SiOCN and SiBCN. The low-k material for the low-k spacers  60  may be formed by performing a CVD process, an ALD process, etc. Note that, in this example, the low-k spacers  60  are formed after the source/drain anneal processes have been performed on the device  100  so the low-k spacers  60  will not be degraded by being subjected to such an anneal process. Additionally, in the depicted embodiment, the low-k spacers  60  actually engage the gate insulation layer  50 A of the replacement gate structure  50 . More specifically, the gate insulation layer  50 A depicted herein has a generally “U” shaped configuration with a substantially horizontal portion  51 H (that contacts the substrate  10 ) and two upstanding vertically oriented (relative to the surface of the substrate) portions  51 V. In the depicted example, the inside surface  61  of the low-k spacers  60  engages the vertically oriented portions  51 V of the gate insulation layer  50 A along substantially the entire length (in the vertical direction normal to the surface of the substrate  10 ) of the vertical portions  51 V of the gate insulation layer  50 A. 
     At the point of fabrication depicted in  FIG. 2O , traditional manufacturing operations may be performed to complete the formation of the device  100 . For example, contact openings (not shown) may be formed through the layers of insulating material  30 ,  28 R,  26  to expose the underlying source/drain regions  24 . Thereafter metal silicide regions (not shown) may be formed on the exposed portions of the source/drain regions  24  and conductive contacts (not shown) may be formed in the contact openings to provide electrical connection to the source/drain regions  24 . Various metallization layers may then be formed above the device  100  using known processing techniques. 
       FIGS. 2P-2Q  depict another illustrative embodiment disclosed herein.  FIG. 2P  corresponds to the point of fabrication depicted in  FIG. 2I  wherein the wet etching process  34  is performed to remove the residual portions of the second sacrificial sidewall spacers  22  and the layer of insulating material  26 , i.e., the etching process  34  removes exposed silicon nitride material while leaving the first sacrificial sidewall spacers  20  intact. The etching process  34  results in the formation of a plurality of first spacer cavities or gaps  34 G adjacent the first sacrificial sidewall spacers  20 . However, in this embodiment, the parameters of the etching process  34  are controlled so as to intentionally recess the layer of insulating material  26 , i.e., to intentionally form recesses  26 A in the area  36 , as shown in  FIG. 2P . Thereafter, as shown in  FIG. 2Q , processing is continued as described above until it is time to form the low-k sidewall spacers  60  in the second spacer cavities  54 A adjacent the replacement gate structure  50 . Due to the formation of the recesses  26 A in the layer of insulating material  26 , illustrative voids  70  are formed within the sidewall spacers  60 . That is, the low-k material tends to fill the recesses  26 A first and then “pinch-off” as the remainder of the cavity  54 A is filled with low-k material, thereby resulting in the formation of the illustrative voids  70 . The size and configuration of the voids  70  may vary, but their presence within the low-k sidewall spacers  60  effectively reduces the overall k value of the low-k sidewall spacers. The size of the voids  70  may be determined, at least in part, based upon the size of the recesses  26 A formed in the layer of insulating material  26 . 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.