Abstract:
Embodiments of the present invention provide transistors with controlled junctions and methods of fabrication. A dummy spacer is used during the majority of front end of line (FEOL) processing. Towards the end of the FEOL processing, the dummy spacers are removed and replaced with a final spacer material. Embodiments of the present invention allow the use of a very low-k material, which is highly thermally-sensitive, by depositing it late in the flow. Additionally, the position of the gate with respect to the doped regions is highly controllable, while dopant diffusion is minimized through reduced thermal budgets. This allows the creation of extremely abrupt junctions whose surface position is defined using a sacrificial spacer. This spacer is then removed prior to final gate deposition, allowing a fixed gate overlap that is defined by the spacer thickness and any diffusion of the dopant species.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to semiconductor fabrication, and more particularly, to controlled junction transistors and methods of fabrication. 
       BACKGROUND 
       [0002]    As transistors disposed on integrated circuits (ICs) become smaller, transistors with source/drain extensions have become more difficult to manufacture. As critical dimensions shrink, forming source and drain extensions becomes very difficult using conventional fabrication techniques. Conventional ion implantation techniques have difficulty maintaining shallow source and drain extensions because of dopant diffusion. The diffusion often extends the source and drain extension vertically into the semiconducting channel and underlying layers, while the use of alternative channel materials such as silicon-germanium or III-V materials may enhance dopant diffusivity, degrading resultant junction profiles. Highly scaled, advanced transistors benefit from precisely defined junction profiles and well-controlled gate overlap geometry to achieve well-behaved short-channel characteristics. Positioning the junction correctly with respect to the gate is challenging. Therefore, it is desirable to have improved and controllable methods of fabrication to address the aforementioned challenges. 
       SUMMARY 
       [0003]    Embodiments of the present invention provide transistors with controlled junctions and methods of fabrication. A dummy spacer is used during the majority of front end of line (FEOL) processing. Towards the end of the FEOL processing, the dummy spacers are removed and replaced with a final spacer material. As the final spacer material is deposited after the high temperature FEOL processes, the final spacer material may be a thermally-sensitive material which can withstand only lower processing temperatures, allowing for greater flexibility in selection of the final spacer material. That is, embodiments of the present invention allow the use of a very low-k material, which is highly thermally-sensitive, by depositing it late in the flow. Additionally, the position of the gate with respect to the doped regions is highly controllable, while dopant diffusion is minimized through reduced thermal budgets. This allows the creation of extremely abrupt junctions whose surface position is defined using a sacrificial spacer. This spacer is then removed prior to final gate deposition, allowing a fixed gate overlap that is defined by the spacer thickness and any diffusion of the dopant species. In this way, there is a high degree of control over the regions where dopants are placed. These controlled junctions can enable improved device performance. As an added benefit, embodiments of the present invention require no additional patterning, and can easily be integrated into an existing replacement metal gate (RMG) flow. 
         [0004]    In a first aspect, embodiments of the present invention provide a method of forming a semiconductor structure, comprising: forming a plurality of doped regions in a semiconductor channel disposed on a semiconductor substrate, wherein the plurality of doped regions are formed adjacent to a dummy gate disposed on a dummy gate oxide, the dummy gate oxide disposed on the semiconductor substrate; forming a plurality of dummy spacers, wherein the plurality of dummy spacers are disposed adjacent to the dummy gate; forming a plurality of source/drain regions adjacent to the dummy gate; depositing a dielectric layer over the source/drain regions; removing the dummy gate to form a gate cavity; removing the plurality of dummy spacers; depositing a final spacer layer; performing an etch of the final spacer layer to form final spacers; removing the dummy gate oxide; and forming a metal gate in the gate cavity. 
         [0005]    In a second aspect, embodiments of the present invention provide a method of forming a semiconductor structure, comprising: forming a metal gate on the semiconductor structure; forming a plurality of dummy spacers adjacent to the metal gate; depositing a contact metal adjacent to the plurality of dummy spacers; removing the plurality of dummy spacers to form a plurality of spacer cavities; forming doped regions in a semiconductor channel disposed at a bottom portion of each spacer cavity of the plurality of spacer cavities; and depositing a final spacer material in each spacer cavity of the plurality of spacer cavities. 
         [0006]    In a third aspect, embodiments of the present invention provide a semiconductor structure comprising: a semiconductor substrate; two doped regions formed on a semiconductor channel disposed on the semiconductor substrate, having a gap between them; a metal gate disposed on the semiconductor substrate over the gap and extending over each of the two doped regions; a plurality of spacers formed adjacent to the metal gate and in contact with one of the two doped regions; and a cap region disposed on the metal gate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the present teachings and together with the description, serve to explain the principles of the present teachings. Certain elements in some of the figures may be omitted, or illustrated not-to-scale, for illustrative clarity. The cross-sectional views may be in the form of “slices”, or “near-sighted” cross-sectional views, omitting certain background lines which would otherwise be visible in a “true” cross-sectional view, for illustrative clarity. Furthermore, for clarity, some reference numbers may be omitted in certain drawings. 
           [0008]      FIG. 1  shows a semiconductor structure at a starting point for embodiments of the present invention. 
           [0009]      FIG. 2  shows a semiconductor structure after a subsequent process step of forming doped regions. 
           [0010]      FIG. 3  shows a semiconductor structure after a subsequent process step of dummy spacer formation. 
           [0011]      FIG. 4  shows a semiconductor structure after a subsequent process step of forming source/drain epitaxial regions. 
           [0012]      FIG. 5  shows a semiconductor structure after a subsequent process step of forming a dielectric region. 
           [0013]      FIG. 6  shows a semiconductor structure after a subsequent process step of removing the dummy gate. 
           [0014]      FIG. 7  shows a semiconductor structure after a subsequent process step of removing the dummy spacers. 
           [0015]      FIG. 8  shows a semiconductor structure after a subsequent process step of depositing the final spacers. 
           [0016]      FIG. 9  shows a semiconductor structure after a subsequent process step of etching the final spacers. 
           [0017]      FIG. 10  shows a semiconductor structure after a subsequent process step of removing the dummy gate oxide. 
           [0018]      FIG. 11  shows a semiconductor structure after a subsequent process step of depositing a metal gate. 
           [0019]      FIG. 12  is a flowchart indicating process steps for embodiments of the present invention. 
           [0020]      FIG. 13  shows a semiconductor structure at a starting point for alternative embodiments of the present invention. 
           [0021]      FIG. 14  shows a semiconductor structure after a subsequent replacement metal gate process. 
           [0022]      FIG. 15  shows a semiconductor structure after a subsequent process of depositing a contact metal. 
           [0023]      FIG. 16  shows a semiconductor structure after a subsequent process of exposing the gate cap region. 
           [0024]      FIG. 17  shows a semiconductor structure after a subsequent process of forming spacer cavities. 
           [0025]      FIG. 18  shows a semiconductor structure after a subsequent process of forming doped regions at the bottom of the spacer cavities. 
           [0026]      FIG. 19  shows a semiconductor structure after a subsequent process of depositing a final spacer material. 
           [0027]      FIG. 20  is a flowchart indicating process steps for alternative embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Illustrative embodiments will now be described more fully herein with reference to the accompanying drawings, in which embodiments are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this disclosure to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments. 
         [0029]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms “a”, “an”, etc., do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term “set” is intended to mean a quantity of at least one. It will be further understood that the terms “comprises” and/or “comprising”, or “includes” and/or “including”, when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
         [0000]    Reference throughout this specification to “one embodiment,” “an embodiment,” “embodiments,” “exemplary embodiments,” “some embodiments”, or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” “in embodiments”, “in some embodiments”, and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. One or more features of an embodiment may be “mixed and matched” with features of another embodiment. 
         [0030]    The terms “overlying” or “atop”, “positioned on, “positioned atop”, or “disposed on”, “underlying”, “beneath” or “below” mean that a first element, such as a first structure (e.g., a first layer) is present on a second element, such as a second structure (e.g. a second layer) wherein intervening elements, such as an interface structure (e.g. interface layer) may be present between the first element and the second element. 
         [0031]      FIG. 1  shows a semiconductor structure at a starting point for embodiments of the present invention. Semiconductor structure  100  comprises a semiconductor channel  101  formed above a semiconductor substrate  102 . In embodiments, the semiconductor substrate  102  may be silicon (Si) or silicon on insulator (SOI). In some embodiments, the semiconductor channel  101  may comprise silicon, silicon-germanium, germanium, a III-V compound semiconductor material or materials, or 2D materials such as graphene or metal dichalcogenides. Additionally, the channel  101  may be realized in non-planar architectures such as fin or nanowire configurations by incorporating earlier etch steps. A dummy gate interfacial material  104  is disposed over the semiconductor substrate  102 . In embodiments, the dummy gate interfacial material  104  may be an oxide such as silicon oxide and/or aluminum oxide. A dummy gate  106  is disposed on the dummy gate interfacial material  104 . In embodiments, the dummy gate  106  may be polysilicon or other suitable material. A gate capping layer  108  may be disposed over the dummy gate  106 . In embodiments, the gate capping layer  108  may be a nitride such as silicon nitride (SiN), or it may be comprised of multiple layers. 
         [0032]      FIG. 2  shows a semiconductor structure after a subsequent process step of forming doped regions. Exposed portions of dummy gate oxide  104  are removed from over substrate  102 , leaving the dummy gate oxide  104  only below dummy gate  106 . In embodiments, the removal is achieved with a selective wet or dry etch process. A plurality of doped regions  110   a - 110   n  are formed on the semiconductor channel  101  that is disposed on the semiconductor substrate  102  adjacent the dummy gate  106 . The two regions  110   a  and  110   n  are formed on the semiconductor substrate having a gap between them, with the dummy gate  106  disposed over the gap. In embodiments, the plurality of doped regions  110   a  and  110   n  are formed by ion implantation, plasma doping, or monolayer doping, shown generally at  112 . In embodiments featuring a Group IV channel  101 , dopants may include one or more of, for example, arsenic, phosphorous, antimony, and/or boron. In embodiments featuring a Group III-IV channel  101 , dopants may include, for example, silicon (Si), carbon (C), magnesium (Mg), sulfur, (S), zinc (Zn), or tellurium (Te). 
         [0033]      FIG. 3  shows a semiconductor structure after a subsequent process step of dummy spacer formation. A plurality of dummy spacers  114   a  and  114   n  are formed adjacent dummy gate  106 . The width of each spacer is shown as Tsac. In embodiments, Tsac may range from 2-20 nanometers. 
         [0034]      FIG. 4  shows a semiconductor structure  100  after a subsequent process step of forming source/drain regions  116   a  and  116   n  adjacent to the dummy gate  106 . In embodiments, the plurality of source/drain regions  116   a  and  116   n  may be epitaxial source/drain regions. In embodiments, the plurality of source/drain regions  116   a  and  116   n  may include, for example, silicon, silicon germanium, and/or a III-V material. In other embodiments, not shown, these regions may be embedded into the channel by first partially or fully recessing the channel, then epitaxially growing the source/drain material. 
         [0035]      FIG. 5  shows a semiconductor structure  100  after a subsequent process step of forming a dielectric region. A dielectric layer  118  is deposited over the plurality of source/drain regions  116   a  and  116   n . In embodiments, the dielectric layer may be comprised of silicon oxide, or multiple layers of various oxides and/or other dielectric layers. In embodiments, the dielectric layer  118  may be deposited by chemical vapor deposition (CVD) or another suitable method. The layer may be planarized using chemical mechanical planarization (CMP), which may also be used to control the geometry of the sacrificial spacers. 
         [0036]      FIG. 6  shows a semiconductor structure  100  after a subsequent process step of removing the dummy gate. Removing the dummy gate  106  creates gate cavity  120 . Dummy gate oxide  104  remains intact at the base of cavity  120 . In some embodiments, the removal of the dummy gate  106  is performed by, for example, a reactive ion etch (RIE) process, or by ammonia-containing wet etch. 
         [0037]      FIG. 7  shows a semiconductor structure  100  after a subsequent process step of removing the plurality of dummy spacers  114   a  and  114   b . In embodiments, the removal is achieved by, for example, a hot phosphoric acid etch process, a reactive ion etch (RIE) process, or other etch chemistries highly selective to oxide. The removal creates an expanded gate cavity  120  and leaves dummy gate oxide layer  104  intact. 
         [0038]      FIG. 8  shows a semiconductor structure  100  after a subsequent process step of depositing the final spacers. A final spacer layer  122  is deposited, in some embodiments, by atomic layer deposition, or another suitable conformal method. The final spacer layer may be deposited over the walls of the cavity  120 , and the top of the dielectric layer  118 . In some embodiments, final spacer material may be, for example, silicon nitride, or low-dielectric constant materials such as silicon boron carbon nitride (SiBCN) or silicon oxycarbonnitride (SiOCN). 
         [0039]      FIG. 9  shows a semiconductor structure  100  after a subsequent process step of etch-back of the final spacers. An etch of the final spacer layer  122  is performed to form final spacers  124   a  and  124   n  using anisotropic RIE or atomic layer etch (ALE) techniques. The final spacer material therefore becomes constrained to the inner walls of gate cavity  120 . In embodiments, the thickness of the final etched spacers Tsp is 0.5-10 nanometers. The length of the channel overlap region, Tol, is then defined by the difference in thicknesses of the sacrificial spacer and the final spacer, Tsac-Tsp. In embodiments, the Tol is 0.1-10 nanometers. In some embodiments, the dummy gate oxide  104  may be left intact after spacer etchback, or may be removed during processing of the dummy and final spacers. 
         [0040]      FIG. 10  shows a semiconductor structure  100  after a subsequent process step of removing the dummy gate oxide. Dummy gate oxide  104  is removed from the gate cavity  120 . In embodiments, the removal may be achieved using, for example, a chemical oxide removal (COR) process, Siconi or Frontier process, or a wet etch. 
         [0041]      FIG. 11  shows a semiconductor structure  100  after a subsequent process step of depositing a metal gate. A replacement metal gate  126  is deposited into gate cavity  120 . In embodiments, replacement metal gate  126  may include tungsten, aluminum, multiple workfunction metal layers, such as titanium, and gate dielectric layers (not shown), which may include hafnium oxide, silicon oxide, aluminum oxide, zirconium oxide, or other dielectrics with desirable interfacial and electrical properties. A self-aligned capping layer  128  is deposited in gate cavity  120  over the replacement metal gate  126 . It should be recognized that the final replacement metal gate  126  extends over the plurality of doped regions  110   a - 110   n , an example of which is shown generally at reference number  130 . The gate  126  extends by distance Tol ( FIG. 9 ), which in embodiments, ranges from about 0.1 nanometers to about 10 nanometers. From this point forward, industry standard techniques may be used to complete fabrication of the integrated circuit. This may include, without limitation, gate contact formation and/or filling the remaining cavity with a dielectric layer. 
         [0042]      FIG. 12  is a flowchart indicating process steps for embodiments of the present invention. At  202 , a plurality of doped regions are formed on a semiconductor substrate, wherein the plurality of doped regions are formed adjacent to a dummy gate disposed on a dummy gate oxide, the dummy gate oxide disposed on the semiconductor substrate. At  204 , a plurality of dummy spacers are formed, wherein the plurality of dummy spacers are disposed adjacent to the dummy gate. At  206 , a plurality of source/drain regions are formed adjacent to the dummy gate. At  208 , a dielectric layer is deposited over the source/drain regions. At  210 , the remaining dummy gate is removed to form a gate cavity. At  212 , the plurality of dummy spacers is removed. At  214 , a final spacer layer is deposited. At  216 , an etch of the final spacer layer is performed to form final spacers. At  218 , the dummy gate oxide is removed. At  220 , a metal gate is formed in the gate cavity. 
         [0043]      FIG. 13  shows a semiconductor structure  300  at a starting point for alternative embodiments of the present invention. Semiconductor structure  300  comprises a semiconductor substrate  302 . In embodiments, the semiconductor substrate  302  may be silicon (Si) or silicon-on-insulator (SOI). A dummy gate oxide  304  is disposed over the semiconductor substrate  302 . In embodiments, the dummy gate oxide  304  may be an oxide such as silicon oxide. A dummy gate  306  is disposed on the dummy gate oxide  304 . In embodiments, the dummy gate may be polysilicon or other suitable material. A gate capping layer  308  may be disposed over the dummy gate  306 . In embodiments, the gate capping layer  308  may be a nitride such as silicon nitride (SiN), or a low-k material such as SiBCN or SiOCN. A plurality of dummy spacers  314   a  and  314   n  are formed adjacent dummy gate  306 . In some embodiments, the plurality of dummy spacers  314   a  and  314   n  are silicon nitride, SiBCN, and/or SiOCN. A plurality of source/drain regions  316   a  and  316   n  are formed adjacent the dummy spacers  314   a  and  314   n . In embodiments, the plurality of source/drain regions  316   a  and  316   n  may be epitaxial source/drain regions. In embodiments, the plurality of source/drain regions  316   a  and  316   n  may include silicon, silicon germanium, or another suitable material. A dielectric layer  318  is disposed over the plurality of source/drain regions  316   a  and  316   n . In embodiments, the dielectric layer  318  may be silicon oxide, or multiple layers of various oxides and/or other dielectric layers previously mentioned, such as hafnium oxide, silicon oxide, and/or zirconium oxide. 
         [0044]      FIG. 14  shows a semiconductor structure  300  after a subsequent replacement metal gate process. The dielectric layer  318  is opened up, and the gate capping layer  108  is removed from over the gate  306 . In some embodiments, the removal is achieved by a reactive ion etch process or another suitable method. The dummy gate  306  is then removed, in some embodiments, by, for example, a selective etch process. A gate cavity  320  is therefore formed between the plurality of spacers. A gate oxide  330  is deposited into the gate cavity. In some embodiments, the gate oxide  330  may be hafnium oxide. A replacement metal gate  332  is deposited into the cavity over the gate oxide  330 . In some embodiments, the metal gate  332  is tungsten, aluminum, and/or other work function metals (not shown). A self-aligned gate capping layer  334  is deposited over the replacement metal gate  332 . In embodiments, the gate capping layer  334  may be a nitride such as silicon nitride. Additional dielectric material  318  then covers the capping layer  334 . In embodiments, the dielectric material  318  may be comprised of an oxide, such as silicon oxide or tetraethyl orthosilicate (TEOS). 
         [0045]      FIG. 15  shows a semiconductor structure  300  after a subsequent process of depositing a contact metal. The dielectric layer  318  is removed except for region  336 . In some embodiments, the removal is achieved by, for example, a selective wet or dry etch. A liner  338  is deposited, in some embodiments, by atomic layer deposition or another suitable method. In embodiments, liner  338  may include titanium nitride or another suitable material. A source/drain contact metal  340  is deposited over liner  338 . In embodiments, the source/drain contact metal  340  may be tungsten. The structure  300  may be planarized using, for example, a chemical mechanical planarization process to leave the contact metal  340  flush with the dielectric layer  336 . 
         [0046]      FIG. 16  shows a semiconductor structure  300  after a subsequent process of exposing the gate cap region. The dielectric layer  336  may be selectively removed to expose the capping layer  334  by forming opening  342 . 
         [0047]      FIG. 17  shows a semiconductor structure  300  after a subsequent process of forming spacer cavities. Each spacer cavity  344   a  and  344   n  is formed by selectively removing the dummy spacers. This can be achieved by a wet etch, for example a hot phosphoric acid etch process selective to silicon nitride, a reactive ion etch process (RIE), or other selective dry removal processes, such as Frontier, or a combination of these processes. 
         [0048]      FIG. 18  shows a semiconductor structure  300  after a subsequent process of forming doped regions at the bottom of the spacer cavities. Doped regions  346   a  and  346   n  are formed at a bottom of each spacer cavity  344   a  and  344   n . In embodiments, the doping can be achieved by a plasma doping process, or a monolayer doping process. In embodiments, doped regions  346   a  and  346   n  may be arsenic, boron, antimony, phosphorus, and/or other elements suitable to the channel material. 
         [0049]      FIG. 19  shows a semiconductor structure  300  after a subsequent process of depositing a final spacer material. A final spacer material  348  is deposited. In some embodiments, the deposition may be achieved by, for example, atomic layer deposition, or chemical vapor deposition. In some embodiments, final spacer material  348  may be, for example, silicon nitride and/or SiBCN or SiOCN. From this point forward, industry standard techniques may be used to complete fabrication of the integrated circuit. This may include, without limitation, gate contact formation and/or filling the remaining cavity with a dielectric layer. 
         [0050]      FIG. 20  is a flowchart indicating process steps for alternative embodiments of the present invention. At  402 , a metal gate is formed on semiconductor structure. At  404 , a plurality of dummy spacers are formed adjacent to the metal gate. At  406 , a contact metal is deposited adjacent to the plurality of dummy spacers. At  408 , the plurality of dummy spacers is removed to form a plurality of spacer cavities. At  410 , doped regions are formed at a bottom portion of each spacer cavity of the plurality of spacer cavities. At  412 , a final spacer material is deposited in each spacer cavity of the plurality of spacer cavities. 
         [0051]    While the invention has been particularly shown and described in conjunction with exemplary embodiments, it will be appreciated that variations and modifications will occur to those skilled in the art. For example, although the illustrative embodiments are described herein as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events unless specifically stated. Some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein, in accordance with the invention. In addition, not all illustrated steps may be required to implement a methodology in accordance with the present invention. Furthermore, the methods according to the present invention may be implemented in association with the formation and/or processing of structures illustrated and described herein as well as in association with other structures not illustrated. Moreover, in particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.) the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several embodiments, such feature may be combined with one or more features of the other embodiments as may be desired and advantageous for any given or particular application. Therefore, it is to be understood that the appended claims are intended to cover all such modifications and changes that fall within the true spirit of the invention.