Patent Publication Number: US-9431507-B2

Title: Replacement gate structure with low-K sidewall spacer for semiconductor devices

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of co-pending application Ser. No. 14/172,058, filed Feb. 4, 2014. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure generally relates to the fabrication of integrated circuits, and, more particularly, to various methods of forming gate structures for semiconductor devices, such as transistors, using a replacement gate technique and the resulting semiconductor devices. 
     2. Description of the Related Art 
     In modern integrated circuits, such as microprocessors, storage devices and the like, a very large number of circuit elements, especially transistors, are provided and operated on a restricted chip area. Immense progress has been made over recent decades with respect to increased performance and reduced feature sizes of circuit elements, such as transistors. However, the ongoing demand for enhanced functionality of electronic devices forces semiconductor manufacturers to steadily reduce the dimensions of the circuit elements and to increase the operating speed of the circuit elements. The continuing scaling of feature sizes, however, involves great efforts in redesigning process techniques and developing new process strategies and tools so as to comply with new design rules. Generally, in complex circuitry including complex logic portions, MOS technology is presently a preferred manufacturing technique in view of device performance and/or power consumption and/or cost efficiency. In integrated circuits including logic portions fabricated by MOS technology, field effect transistors (FETs) are provided that are typically operated in a switched mode, that is, these devices exhibit a highly conductive state (on-state) and a high impedance state (off-state). The state of the field effect transistor is controlled by a gate electrode, which controls, upon application of an appropriate control voltage, the conductivity of a channel region formed between a drain region and a source region. 
     To improve the operating speed of FETs, and to increase the density of FETs on an integrated circuit device, device designers have greatly reduced the physical size of FETs over the years. More specifically, the channel length of FETs has been significantly decreased, which has resulted in improving the switching speed of FETs. However, decreasing the channel length of a FET also decreases the distance between the source region and the drain region. In some cases, this decrease in the separation between the source and the drain makes it difficult to efficiently inhibit the electrical potential of the source region and the channel from being adversely affected by the electrical potential of the drain. This is sometimes referred to as a so-called short channel effect, wherein the characteristic of the FET as an active switch is degraded. 
     In contrast to a FET, which has a planar structure, a so-called FinFET device has a three-dimensional (3D) structure.  FIG. 1A  is a perspective view of an illustrative prior art FinFET semiconductor device “A” that is formed above a semiconductor substrate B that will be referenced so as to explain, at a very high level, some basic features of a FinFET device. In this example, the FinFET device A includes three illustrative fins C, a gate structure D, sidewall spacers E and a gate cap layer F. The gate structure D is typically comprised of a layer of gate insulating material (not separately shown), e.g., a layer of high-k insulating material (k-value of 10 or greater) or silicon dioxide, and one or more conductive material layers (e.g., metal and/or polysilicon) that serve as the gate electrode for the device A. The fins C have a three-dimensional configuration: a height H, a width W and an axial length L. The axial length L corresponds to the direction of current travel in the device A when it is operational. The portions of the fins C covered by the gate structure D are the channel regions of the FinFET device A. In a conventional process flow, the portions of the fins C that are positioned outside of the spacers E, i.e., in the source/drain regions of the device A, may be increased in size or even merged together (a situation not shown in  FIG. 1A ) by performing one or more epitaxial growth processes. The process of increasing the size of or merging the fins C in the source/drain regions of the device A is performed to reduce the resistance of source/drain regions and/or make it easier to establish electrical contact to the source/drain regions. Even if an epi “merger” process is not performed, an epi growth process will typically be performed on the fins C to increase their physical size. 
     In the FinFET device, the gate structure D may enclose both the sides and the upper surface of all or a portion of the fins C to form a tri-gate structure so as to use a channel having a three-dimensional structure instead of a planar structure. In some cases, an insulating cap layer (not shown), e.g., silicon nitride, is positioned at the top of the fins C and the FinFET device only has a dual-gate structure (sidewalls only). Unlike a planar FET, in a FinFET device, a channel is formed perpendicular to a surface of the semiconducting substrate so as to reduce the physical size of the semiconductor device. Also, in a FinFET, the junction capacitance at the drain region of the device is greatly reduced, which tends to significantly reduce short channel effects. When an appropriate voltage is applied to the gate electrode of a FinFET device, the surfaces (and the inner portion near the surface) of the fins C, i.e., the vertically oriented sidewalls and the top upper surface of the fin, form a surface inversion layer or a volume inversion layer that contributes to current conduction. In a FinFET device, the “channel-width” is estimated to be about two times (2×) the vertical fin-height plus the width of the top surface of the fin, i.e., the fin width. Multiple fins can be formed in the same foot-print as that of a planar transistor device. Accordingly, for a given plot space (or foot-print), FinFETs tend to be able to generate significantly higher drive current density than planar transistor devices. Additionally, the leakage current of FinFET devices after the device is turned “OFF” is significantly reduced as compared to the leakage current of planar FETs, due to the superior gate electrostatic control of the “fin” channel on FinFET devices. In short, the 3D structure of a FinFET device is a superior MOSFET structure as compared to that of a planar FET, especially in the 20 nm CMOS technology node and beyond. The gate structures D for such FinFET devices may be manufactured using so-called “gate-first” or “replacement gate” (gate-last) manufacturing techniques. 
     For many early device technology generations, the gate structures of most transistor elements (planar and FinFET devices) were 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 10-32 nm or less, gate structures that include a so-called high-k dielectric gate insulation layer and one or more 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 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), titanium-aluminum-carbon (TiALC), 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. The replacement gate process may be used when forming planar devices or 3D devices.  FIGS. 1B-1E  simplistically depict one illustrative prior art method for forming an HK/MG replacement gate structure using a replacement gate technique on a planar transistor device. As shown in  FIG. 1B , the process includes the formation of a basic transistor structure above a semiconducting substrate  12  in an active area defined by a shallow trench isolation structure  13 . At the point of fabrication depicted in  FIG. 1A , the device  10  includes a sacrificial gate insulation layer  14 , a dummy or sacrificial gate electrode  15 , sidewall spacers  16 , a layer of insulating material  17  and source/drain regions  18  formed in the substrate  12 . The various components and structures of the device  10  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  15  may be comprised of polysilicon, the sidewall spacers  16  may be comprised of silicon nitride and the layer of insulating material  17  may be comprised of silicon dioxide. The source/drain regions  18  may be comprised of implanted dopant materials (N-type dopants for NMOS devices and P-type dopants for PMOS devices) that are implanted into the substrate  12  using known masking and ion implantation techniques. Of course, those skilled in the art will recognize that there are other features of the transistor  10  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 PMOS transistors. At the point of fabrication depicted in  FIG. 1B , the various structures of the device  10  have been formed and a chemical mechanical polishing (CMP) process has been performed to remove any materials above the sacrificial gate electrode  15  (such as a protective cap layer (not shown) comprised of silicon nitride) so that at least the sacrificial gate electrode  15  may be removed. 
     As shown in  FIG. 1C , one or more etching processes are performed to remove the sacrificial gate electrode  15  and the sacrificial gate insulation layer  14  to thereby define a replacement gate cavity  20  where a replacement gate structure will subsequently be formed. 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  12  within the gate cavity  20 . 
     Next, as shown in  FIG. 1D , various layers of material that will constitute a replacement gate structure  30  are formed in the gate cavity  20 . The materials used for the replacement gate structures  30  for NMOS and PMOS devices are typically different. For example, the replacement gate structure  30  for an NMOS device may be comprised of a high-k gate insulation layer  30 A, such as hafnium oxide, having a thickness of approximately 2 nm, a first metal layer  30 B (e.g., a layer of titanium nitride with a thickness of about 1-2 nm), a second metal layer  30 C—a so-called work function adjusting metal layer for the NMOS device—(e.g., a layer of titanium-aluminum or titanium-aluminum-carbon with a thickness of about 5 nm), a third metal layer  30 D (e.g., a layer of titanium nitride with a thickness of about 1-2 nm) and a bulk metal layer  30 E, such as aluminum or tungsten. 
     Ultimately, as shown in  FIG. 1E , one or more CMP processes are performed to remove excess portions of the gate insulation layer  30 A, the first metal layer  30 B, the second metal layer  30 C, the third metal layer  30 D and the bulk metal layer  30 E positioned outside of the gate cavity  20  to thereby define the replacement gate structure  30  for an illustrative NMOS device. Typically, the replacement gate structure  30  for a PMOS device does not include as many metal layers as does an NMOS device. For example, the gate structure  30  for a PMOS device may only include the high-k gate insulation layer  30 A, a single layer of titanium nitride—the work function adjusting metal for the PMOS device—having a thickness of about 3-4 nm, and the bulk metal layer  30 E. 
       FIG. 1F  depicts the device  10  after several process operations were performed. First, one or more etching processes were performed to remove upper portions of the various materials within the cavity  20  so as to form a recess within the gate cavity  20 . Then, a gate cap layer  31  was formed in the recess above the recessed gate materials. The gate cap layer  31  is typically comprised of silicon nitride and it may be formed by depositing a layer of gate cap material so as to over-fill the recess formed in the gate cavity and, thereafter, performing a CMP process to remove excess portions of the gate cap material layer positioned above the surface of the layer of insulating material  17 . The gate cap layer  31  is formed so as to protect the underlying gate materials during subsequent processing operations. 
     As the gate length of transistor devices has decreased, the physical size of the gate cavity  20  has also decreased. Thus, it is becoming physically difficult to fit all of the layers of material needed for the replacement gate structure  30  within such reduced-size gate cavities, particularly for NMOS devices, due to the greater number of layers of material that are typically used to form the gate structures for the NMOS devices. For example, as gate lengths continue to decrease, voids or seams may be formed as the various layers of material are deposited into the gate cavity  20 .  FIG. 1G  is a somewhat enlarged view of an illustrative NMOS device that is provided in an attempt to provide the reader with some idea of just how limited the lateral space  20 S is within the gate cavity  20  of an NMOS device as the various metal layers  30 A- 30 D are formed in the gate cavity  20 . In  FIG. 1G , the internal sidewall surfaces of the spacers  16  define a gate cavity  20  having a substantially uniform width  20 S throughout the height or depth of the gate cavity  20 . As the layers of material in the gate stack for the device are formed in the cavity  20 , the remaining space  39  within the gate cavity  20  becomes very small. As the latter metal layers are formed, the lateral space  39  may be about 1-2 nm in width or even smaller. In some cases, the space  39  may be essentially non-existent. This may lead to so-called “pinch-off” of metal layers such that voids or seams may be formed in the overall gate stack, which may result in devices that perform at levels less than anticipated or, in some cases, the formation of devices that are simply not acceptable and have to be discarded. 
     When manufacturing advanced integrated circuit products using replacement gate structures, particularly in situations where the products also include very tight spacing between source/drain contact structures, such as products using self-aligned source/drain contacts, some amount of the work function metals in the gate cavity  20  must be removed from the gate cavity  20  to make room for additional materials, i.e., to make room within the upper portion of the gate cavity  20  for a bulk conductor material, such as tungsten and aluminum, and a gate cap layer. This process operation is sometimes referred to as work-function chamfering. In such a work-function chamfering process, some form of a protective material must be formed in the gate cavity  20  above the metal layer  30 D to protect desired portions of the underlying metal layers during the recess etching process. If the lateral space  39  (to the extent it exists) cannot be reliably filled with such a protective material, such as a flowable oxide material, then the recessing etching process cannot be performed for fear of removing undesired portions of the metal layers during the course of performing the recess etching process. 
     Over recent years, due to the reduced dimensions of the transistor devices, the operating speed of the circuit components has been increased with every new device generation, and the “packing density,” i.e., the number of transistor devices per unit area, in such products has also increased during that time. Such improvements in the performance of transistor devices has reached the point where one limiting factor relating to the operating speed of the final integrated circuit product is no longer the individual transistor element but the electrical performance of the complex wiring system that is formed above the device level that includes the actual semiconductor-based circuit elements. 
     As device dimensions have decreased, the conductive contact elements in the contact level, e.g., source/drain contacts, have to be provided with critical dimensions in the same order of magnitude. For this reason, contact technologies have been developed in which contact openings are formed in a self-aligned manner by removing dielectric material, such as silicon dioxide, selectively from the spaces between closely spaced gate electrode structures. That is, after completing the transistor structure, the gate electrode structures are used as etch masks for selectively removing the silicon dioxide material in order to expose the source/drain regions of the transistors, thereby providing self-aligned trenches which are substantially laterally delineated by the spacer structures of the gate electrode structures. Additionally, due to the continued reduction in gate pitch, and the nature of how a self-aligned contact is formed, the contact structure is positioned very close to the gate electrode. This arrangement forms a parasitic capacitor, namely a gate-to-contact capacitor, and the role of this capacitor in determining circuit performance is becoming more important as device and pitch scaling continues. However, further reduction in spacer thickness (width) must be done with extreme caution to avoid an undesirable rapid increase in the contact-to-gate capacitance with degrades the RC delay of the transistors. 
     The present disclosure is directed to various methods of forming gate structures for semiconductor devices, such as transistors, using a replacement gate technique and the resulting semiconductor devices that may avoid, or at least reduce, the effects of 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 gate structures for semiconductor devices, such as transistors, using a replacement gate technique and the resulting semiconductor devices. One method disclosed herein includes, among other things, performing at least one first etching process to remove a portion, but not all, of a sacrificial gate electrode so as to thereby define a recessed sacrificial gate electrode having a recessed upper surface, performing at least one second etching process to define recessed sidewall spacers positioned adjacent the recessed sacrificial gate electrode, wherein the recessed sidewall spacers, recessed sacrificial gate electrode and a layer of insulating material define a gate opening, forming a plurality of sidewall spacers within the gate opening, wherein one of the spacers is comprised of low-k insulating material (an insulating material having a dielectric constant equal to or less than 3.9) that is positioned laterally between two other spacers, after forming the plurality of sidewall spacers, removing at least the recessed sacrificial gate electrode so as to define, at least in part, a replacement gate cavity, forming a replacement gate structure within the replacement gate cavity and forming a gate cap layer within the replacement gate cavity above the replacement gate structure. 
     One illustrative device disclosed herein includes, among other things, a gate structure positioned above an upper surface of a semiconductor substrate, wherein the gate structure comprises a high-k gate insulation layer and a gate electrode comprised of at least one layer of metal, a sidewall spacer positioned adjacent opposite sides of at least a portion of the gate electrode, wherein the sidewall spacers have an upper surface that is positioned above the upper surface of the substrate, a gate cap layer positioned above at least the gate electrode, a layer of insulating material positioned above the surface of the substrate and around the gate structure, a plurality of sidewall spacers positioned laterally between the layer of insulating material and a portion of the gate structure and vertically between the upper surface of the sidewall spacers and a bottom surface of the gate cap layer, wherein the plurality of spacers comprise a low-k spacer comprised of an insulating material having a dielectric constant equal to or less than 3.9. 
    
    
     
       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: 
         FIG. 1A  is a perspective view of one illustrative embodiment of a prior art FinFET device; 
         FIGS. 1B-1G  depict one illustrative prior art method of forming a gate structure of a transistor using a so-called “replacement gate” technique; and 
         FIGS. 2A-2T  depict various illustrative methods disclosed herein for various methods of forming gate structures for semiconductor devices using a replacement gate technique and the resulting semiconductor devices. 
     
    
    
     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 generally relates to various methods of forming gate structures for semiconductor devices using a replacement gate technique. Moreover, as will be readily apparent to those skilled in the art upon a complete reading of the present application, the present method is applicable to a variety of devices, including, but not limited to, logic devices, memory devices, etc., and the methods disclosed herein may be employed to form N-type or P-type semiconductor devices. The methods and devices disclosed herein may be employed in manufacturing products using a variety of technologies, e.g., NMOS, PMOS, CMOS, etc., and they may be employed in manufacturing a variety of different devices, e.g., memory devices, logic devices, ASICs, etc. Of course, the inventions disclosed herein should not be considered to be limited to the illustrative examples depicted and described herein. With reference to the attached figures, various illustrative embodiments of the methods and devices disclosed herein will now be described in more detail. 
     One conceptual technique that may be employed to avoid the problems associated with work-function chamfering, as noted in the background section of this application, would be to increase the width of the upper portion of the gate cavity by reducing the thickness of the spacers. One technique that involves, among other things, reducing the thickness of the spacers is disclosed in a pending U.S. application entitled “Methods of Forming Replacement Gate Structures for Transistors and the Resulting Devices,” Ser. No. 13/959,847, wherein two of the four inventors on that application are the two named inventors in the present application. Another technique that involves, among other things, removing an entire upper portion of the spacers is disclosed in a pending U.S. application entitled “Methods of Forming Semiconductor Device With Self-Aligned Contact Elements and the Resulting Devices,” Ser. No. 13/947,670, wherein one of the four inventors on that application is one of the two named inventors in the present application. However, the conductive source/drain contact structures for a device, the gate electrode and the insulating material positioned therebetween, e.g., the insulating sidewall spacer, define a Gate-to-Contact capacitor. Given the critical nature of the electrical performance of modern devices, all other things being equal, it is desirable that the parasitic Gate-to-Contact capacitance be as low as possible. Unfortunately, by reducing the thickness of the sidewall spacers in an attempt to avoid the problems associated with work-function chamfering, the space between the gate electrode and the conductive source/drain contact structures will be decreased, thereby tending to undesirably increase the parasitic Gate-to-Contact capacitance. The presently disclosed inventions may be useful in resolving or reducing the effects of some of these problems. 
     As will be appreciated by those skilled in the art after a complete reading of the present application, the methods and structures disclosed herein may be used when forming either planar or 3D transistor devices. An illustrative device  100  in the form of a planar device will be depicted for purposes of disclosing the subject matter set forth herein. Additionally, various doped regions, e.g., source/drain regions, halo implant regions, well regions and the like, are not depicted in the attached drawings. Of course, the inventions disclosed herein should not be considered to be limited to the illustrative examples depicted and described herein. Moreover, the transistor devices that are depicted in the attached drawings may be either NMOS or PMOS devices. The illustrative transistor device  100  depicted in the drawings is formed above an illustrative substrate  102  that may have a variety of configurations, such as the depicted bulk silicon configuration. The substrate  102  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. Thus, the terms “substrate” or “semiconductor substrate” should be understood to cover all semiconducting materials and all forms of such materials. 
     In the examples disclosed herein, the device  100  will be formed using a replacement gate technique. Accordingly,  FIG. 2A  depicts the device  100  at a point in fabrication wherein several layers of material and structures have been formed above the substrate  102 . More specifically, at the point of fabrication depicted in  FIG. 2A , the device  100  includes a sacrificial gate insulation layer  104 , a dummy or sacrificial gate electrode  106 , sidewall spacers  110 , a layer of insulating material  114  and an illustrative gate cap layer  108  that has been formed above the sacrificial gate electrode layer  106 . The device depicted in  FIG. 2A  may be comprised of a variety of different materials. For example, the sacrificial gate insulation layer  104  may be comprised of silicon dioxide, the sacrificial gate electrode  106  may be comprised of polysilicon or amorphous, the sidewall spacers  110  and the gate cap layer  108  may be comprised of silicon nitride and the layer of insulating material  114  may be comprised of silicon dioxide. The sidewall spacers  110  may be comprised of silicon nitride and they may formed by depositing a layer of spacer material and thereafter performing an anisotropic etching process to produce the spacers  110 . The base width of the spacers  110  may vary depending upon the particular application, e.g., 6-15 nm. Also depicted in  FIG. 2A  are regions of illustrative epi semiconductor material  112  that were formed in/or above the source/drain regions of the device  100 . Of course, such epi semiconductor material  112  need not be formed to practice the various inventions disclosed herein. The various components and structures of the device  100  disclosed herein may be formed using a variety of different materials and by performing a variety of known techniques, e.g., a chemical vapor deposition (CVD) process, an atomic layer deposition (ALD) process, a thermal growth process, spin-coating techniques, etc. The thicknesses of these various layers of material may also vary depending upon the particular application. 
       FIG. 2B  depicts the device  100  after one or more chemical mechanical polishing (CMP) processes were performed to remove any materials above the sacrificial gate electrode  106  (such as the protective cap layer  108 ) such that the upper surface  106 S of the sacrificial gate electrode  106  is exposed so that the sacrificial gate electrode  106  can be removed. 
       FIG. 2C  depicts the device  100  after a timed, recess etching process was performed to remove a portion, but not all, of the sacrificial gate electrode  106 . This etching process results in the formation of a recessed sacrificial gate electrode  106 R having a thickness  106 T and a recessed surface  106 X. The thickness  106 T of the recessed sacrificial gate electrode  106 R is targeted to be the desired final thickness of the work-function metal(s) to be formed for the device  100 . The thickness  106 T may vary depending upon the particular application. In one illustrative embodiment, the thickness  106 T may fall within the range of about 10-30 nm. 
       FIG. 2D  depicts the device  100  after a timed etching process was performed to remove portions of the sidewall spacers  110  selectively relative to the surrounding materials. This etching process results in the formation of a recessed sidewall spacers  110 R having an upper recessed surface  110 X and a gate opening  115 . In general, the spacers  110  may be formed of a high-k material that, for purposes of this application and the claims as it relates to the spacer materials, is a material having a dielectric constant of 5.5 or greater, e.g., silicon nitride, SiCN, SiBCN, SiBCON, etc. 
     The next sequence of process operations involves formation of a plurality of spacer or liner structures  120 ,  122  and  124  in the gate opening  115  above the upper surface  110 X of the recessed spacers  110 R. The spacers may be formed by depositing a layer of spacer material above the device and in the cavity and thereafter performing an anisotropic etching process to produce the depicted spacers. In general, the spacers  120  and  124  may be formed of a high-k material (k value of 5.5 or greater) while the spacer  122  may be formed of a material that has a dielectric constant of 3.9 or less (a “low-k material” for purposes of this application and the claims). 
       FIG. 2E  depicts the device  100  after a first sidewall spacer  120  has been formed on the exposed portions of the layer of insulating material  114  above the upper surface  110 X of the recessed spacers  110 R. The first sidewall spacer  120  should be formed of a material that may be selectively etched relative to at least the layer of insulating material  114 . In one illustrative example where the layer of insulating material is made of silicon dioxide, the first sidewall spacer  120  may be made of silicon nitride, SiCN, SiBCN or SiBCON, etc. The base thickness of the first sidewall spacer  120  may vary depending upon the particular application. In one illustrative embodiment, the base thickness of the first sidewall spacer  120  may fall within the range of about 1-3 nm. 
       FIG. 2F  depicts the device  100  after a second sidewall spacer  122  has been formed on the first sidewall spacer  120  and above the upper surface  110 X of the recessed spacers  110 R within the gate opening  115 . As noted above, the second sidewall spacer  122  is comprised of low-k material. In one illustrative example, the second sidewall spacer  122  may be made of silicon dioxide. The base thickness of the second sidewall spacer  122  may vary depending upon the particular application. In one illustrative embodiment, the base thickness of the second sidewall spacer  122  may fall within the range of about 3-8 nm. 
       FIG. 2G  depicts the device  100  after a third sidewall spacer  124  has been formed on the second sidewall spacer  122  above the upper surface  110 X of the recessed spacers  110 R. The third sidewall spacer  124  should be formed of a material that may be selectively etched relative to at least the second sidewall spacer  122 . In one illustrative example where the second sidewall spacer  122  is made of silicon dioxide, the third sidewall spacer  124  may be made of a high-k material (k value of 5.5 or greater), such as silicon nitride, SiCN, SiBCN, or SiBCON, etc. The spacers  120  and  124  need not be made of the same material, but that can be the case in some applications. The base thickness of the third sidewall spacer  124  may vary depending upon the particular application. In one illustrative embodiment, the base thickness of the third sidewall spacer  124  may fall within the range of about 1-3 nm. 
     As will be appreciated by those skilled in the art after a complete reading of the present application, the first sidewall spacer  120  and the third sidewall spacer  124  are formed in an effort to protect or encapsulate the low-k second sidewall spacer  122  during subsequent processing operations. In one embodiment, the collective width of the three spacers  120 ,  122  and  124  should be such that, after the formation of the three spacers, a portion of the upper surface  110 X of the recessed spacers  110 R remains exposed. In one embodiment, the remaining exposed portion of the upper surface  110 X has a width  110 W of about 1.5-4 nm, a width which may vary depending upon the particular application. 
       FIG. 2H  depicts the device  100  after one or more wet or dry etching processes were performed to remove the recessed sacrificial gate electrode  106 R and the exposed portions of the sacrificial gate insulation layer  104  to thereby define a replacement gate cavity  130  where a replacement gate structure (the final gate structure) will subsequently be formed for the device  100 . Typically, the sacrificial gate insulation layer  104  is removed as part of the replacement gate technique, as depicted herein. Even in cases where the sacrificial gate insulation layer  104  is intentionally removed, there will typically be a very thin native oxide layer (not shown) that forms on the surface  102 S of the substrate  102  within the replacement gate cavity  130 . 
     The next major process sequence involves formation of the replacement gate structure for the device  100 . The replacement gate structure that will be depicted herein is intended to be representative in nature of any type of gate structure that may be employed in manufacturing integrated circuit products using so-called gate-last (replacement-gate) manufacturing techniques. Accordingly, with reference to  FIG. 2I , a pre-clean process was performed in an attempt to remove all foreign materials from within the gate cavity  130  prior to forming the various layers of material that will become part of the replacement gate structure. Next, a high-k (k value greater than 10) gate insulation layer  132 , such as hafnium oxide (or the other high-k materials noted in the background section of this application), was deposited across the device  100  and within the replacement gate cavity  130  above the substrate  102  by performing a conformal deposition process. If desired, a thin interfacial layer of silicon dioxide (not shown) may be formed prior to the formation the high-k gate insulation layer  132 . Next, at least one work function adjusting metal layer  134  (e.g., a layer of titanium nitride or TiAlC depending upon the type of transistor device being manufactured) was deposited on the high-k gate insulation layer  132  and within the replacement gate cavity  130  by performing a conformal deposition process. Of course, the work function adjusting metal layer  134  may be comprised of any of the metals described in the background section of this application and more than one layer of work function metal may be formed in the replacement gate cavity  130 , depending upon the particular device under construction. Due to the very small sizes of the replacement gate cavities  130  in modern transistor devices, the formation of the work function adjusting metal layer  134  may result in the formation of a very narrow opening or recess  135  within the replacement gate cavity  130 . As noted in the background section of this application, as channel lengths have continued to decrease, the lateral size of such recesses  135  has become so small that it has become difficult to reliably fill openings of this size, even when using specifically designed materials to fill such small openings, such as OPL material. As a result, the recessing of the work function materials becomes more difficult and less reliable. However, in accordance with one aspect disclosed herein, as described more fully below, by using the process flows disclosed herein, the work function metal recessing process may be performed without having to fill the small size openings  135 , to the extent they exist. 
       FIG. 2J  depicts the device  100  after a sacrificial material  136  has been formed in the replacement gate cavity  130  and recessed (by performing an etch-back process) to expose the desired amount of the work function metal layer  134  that is to be removed. In one illustrative embodiment, the sacrificial material  136  may be an OPL material or DUO™ that is formed by performing a spin-coating process, thereafter dried and thereafter etched such that its upper surface  136 S as at the desired height level in the replacement gate cavity  130 . In some cases, the sacrificial material  136  may not fill all or part of the recess  135 , as depicted in  FIG. 2J . However, due to removal of portions of the spacers  110  which effectively widens the opening of the replacement gate cavity  130  above the recessed sacrificial gate structure  106 R, the opening that must be filled by the sacrificial material  136  so that the work function metal recessing process may be performed is much wider than the width of the recess  135 . Thus, even if the sacrificial material  136  does not fill all or part of the recess  135 , the sacrificial material  136  may be reliably formed at the desired level and location within the replacement gate cavity  130 . Of course, if the sacrificial material  136  completely fills the recess  135 , that is acceptable as well. 
     The next process operation involves recessing the exposed upper portions of at least the work function adjusting metal layer  134 . To that end,  FIG. 2K  depicts the device  100  after one or more etching processes were performed to remove at least the work function adjusting metal layer  134  above the level of the surface  136 S of the sacrificial material  136  from within the gate cavity  130 . In the depicted process flow, the high-k gate insulation layer  132  is not removed during this process operation as it serves as an etch-stop layer during this process. Optionally, the exposed portion of the high-k gate insulation layer  132  may be removed during this process operation if desired. 
       FIG. 2L  depicts the device  100  after the sacrificial layer of material  136  has been removed from the replacement gate cavity  130 . The sacrificial layer of material  136  may be removed by using a variety of known techniques depending upon the material selected for the sacrificial layer of material  136 , e.g., a wet stripping process. 
       FIG. 2M  depicts the device  100  after several process operations were performed. First, a bulk conductive material layer  138 , such as tungsten or aluminum, was blanket-deposited above the substrate so as to over-fill the replacement gate cavity  130 . Then, an etching process was performed on the conductive material layer  138  such that the desired amount of the conductive material layer  138  remains positioned within the replacement gate cavity  130 . 
       FIG. 2N  depicts the device  100  after one or more wet or dry etching processes were performed to remove portions of the high-k gate insulation layer  132  and the spacers  120 ,  122  and  124 . These operations result in the formation of the replacement gate structure  139  comprised of the high-k gate insulation layer  132 , the work function adjusting metal layer  134  and the conductive material layer  138 . In one illustrative embodiment, the high-k gate insulation layer  132  and the spacers  124 ,  122  and  120  may be removed by performing a series of sequential etching processes in the same process chamber or in different process tools. That is, the high-k gate insulation layer  132  may be removed using the third sidewall spacer  124  as an etch-stop, the third sidewall spacer  124  may be removed using the second sidewall spacer  122  as an etch-stop, the second sidewall spacer  122  may be removed using the first sidewall spacer  120  as an etch-stop and the first sidewall spacer  120  may be removed using the layer of insulating material  114  as an etch stop. These various etching processes may be timed etching processes. 
       FIG. 2O  depicts the device  100  after an illustrative gate cap layer  140  was formed in the recess above the recessed conductive material layer  138 . The gate cap layer  140  may also be formed of a high-k material (k value of 5.5 or greater), e.g., typically silicon nitride. The gate cap layer  140  may be formed by depositing a layer of gate cap material so as to over-fill the recess formed in the replacement gate cavity  130  above the replacement gate structure  139  and, thereafter, performing a CMP process to remove excess portions of the gate cap material layer positioned above the surface  114 S of the layer of insulating material  114 . The gate cap layer  140  is typically formed so as to protect the underlying gate materials during subsequent processing operations. However, in accordance with one aspect of the inventions disclosed herein, the gate cap layer  140  also serves to protect the low-k spacer  122  during subsequent processing operations. 
       FIG. 2P  depicts the device  100  after several process operations were performed to form an illustrative conductive source/drain contact structure  144  to one of the source/drain regions of the device  100 . A gate contact structure (not shown) that is conductively coupled to the replacement gate structure  139 , i.e., to the conductive gate materials that are part of the replacement gate electrode, may also be formed as part of these process operations. Typically, this processing sequence involves forming a layer of insulating material  142  above the device  100  and thereafter performing one or more etching processes through one or more etch mask layers (not shown) on the exposed portions of the layer of insulating material  142  and the underlying layer of insulating material  114  to define contact openings for the source/drain conductive structures  144 . The source/drain contact structures  144  may be of any desired cross-sectional configuration when viewed from above, e.g., square, rectangular, round, etc. The source/drain contact structure  144  is intended to be schematic and representative in nature, as they may be formed using any of a variety of different conductive materials and by performing traditional manufacturing operations. The source/drain contact structure  144  may also contain one or more barrier layers (not depicted). In one illustrative example, the source/drain contact structures  144  may be formed by depositing a liner, e.g., Ti, TiN, followed by overfilling the contact openings with a conductive material, such as tungsten. Thereafter, a CMP process may be performed to planarize the upper surface of the layer of insulating material  142 , which results in the removal of excess portions of the liner and the tungsten positioned above the layer of insulating material  142  outside of the contact openings and the formation of the source/drain contact structure  144 . 
     The dashed ring  150  in  FIG. 2P  encloses a region of the device  100  that includes the low-k material, i.e., the spacer  122 , which has been enlarged and schematically depicted in  FIGS. 2Q-2T  to discuss various aspects of the presently disclosed invention at the point in the process flow wherein the source/drain conductive structures  144  have been formed. In general, as will be appreciated by those skilled in the art, by forming the second sidewall spacer  122  using a low-k material, and positioning that low-k material between the replacement gate structure  139  and the source/drain contact structure  144 , an undesirable increase in the Gate-to-Contact capacitance may be reduced as compared to prior art structures, even as device pitches continue to decrease. 
     Given the very small size of the spacers disclosed herein and inherent variations in processing techniques, e.g., alignment and etching variation, it is possible that some or all of one or both of the first sidewall spacer  120  (high-k material) and the third sidewall spacer  124  (high-k material) may be consumed during some aspect of performing process operations to manufacture the device  100  at the point in the process flow depicted in  FIG. 2P . In some cases (not depicted) some of the low-k second sidewall spacer  122  may also be consumed. However, due to the protective nature of both the first and third sidewall spacers  120 ,  124  disclosed herein, as well as the gate cap layer  140 , it is believed that the second sidewall spacer  122  will survive normal processing operations. 
       FIG. 2Q  depicts the idealized situation wherein the both the first sidewall spacer  120  and the third sidewall spacer  124  remain substantially intact and, in combination with the gate cap layer  140  and the recessed spacers  110 R serve to substantially encapsulate the remaining portions of the low-k second sidewall spacer  122 . In this example, the first sidewall spacer  120  is positioned on and in contact with the high-k gate insulation layer  132 , while the third sidewall spacer  124  is positioned on and in contact with the source/drain conductive structure  144 . 
       FIG. 2R  depicts the situation wherein, due to various process operations, all (depicted) or a portion of the third sidewall spacer  124  was consumed during processing operations. In this situation, the first sidewall spacer  120  may remain substantially intact. In one example, despite best efforts, some or all of the third sidewall spacer  124  may be undesirably consumed during various etching process that are performed to form the contact opening for the source/drain conductive structure  144 . In a real-world device, should all or a portion of the third sidewall spacer  124  be consumed and thereby expose all or a portion of the low-k second sidewall spacer  122 , portions of the source/drain conductive structure  144  will be positioned laterally closer to (partial consumption of spacer  124 ) or contact (full consumption of spacer  124 —depicted in  FIG. 2R ) the low-k second sidewall spacer  122 . 
       FIG. 2S  depicts the situation wherein, due to various process operations, all (depicted) or a portion of the first sidewall spacer  120  was consumed during processing operations. In this situation, the third sidewall spacer  124  may remain substantially intact. In one example, despite best efforts, some or all of the first sidewall spacer  120  may be consumed during a high-k pre-clean process. In a real-world device, should all or a portion of the first sidewall spacer  120  be consumed and thereby expose all or a portion of the high-k (k value greater than 10) gate insulation layer  132 , then the low-k second sidewall spacer  122  will be positioned laterally closer to (partial consumption of spacer  120 ) or contact (full consumption of spacer  120 —depicted in  FIG. 2S ) the high-k (k value greater than 10) gate insulation layer  132 . 
       FIG. 2T  depicts the situation wherein, due to various process operations, all (depicted) or a portion of the first sidewall spacer  120  and the third sidewall spacer  124  were consumed during processing operations. In this situation, the low-k second sidewall spacer  122  contacts the high-k (k value greater than 10) gate insulation layer  132  and the source/drain conductive structure  144 . Of course, in a real-world device, there are an infinite variations in the amount of possible consumptions of the spacers  120 ,  122  and  124 . 
     Note that, in all of the embodiments disclosed herein, the remaining portion of the low-k second sidewall spacer  122  contacts the bottom surface  140 B of the gate cap layer  140  and the recessed upper surface  110 X of the recessed spacers  110 R. By virtue of the formation of the first sidewall spacer  120 , the third sidewall spacer  124  and the gate cap layer  140  and the recessed spacers  110 R as described herein, the low-k second sidewall spacer  122  may be formed so as to reduce the undesired Gate-to-Contact capacitance, as described above. 
     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. Note that the use of terms, such as “first,” “second,” “third” or “fourth” to describe various processes or structures in this specification and in the attached claims is only used as a shorthand reference to such steps/structures and does not necessarily imply that such steps/structures are performed/formed in that ordered sequence. Of course, depending upon the exact claim language, an ordered sequence of such processes may or may not be required. Accordingly, the protection sought herein is as set forth in the claims below.