Abstract:
A dual dielectric tri-gate field effect transistor, a method of fabricating a dual dielectric tri-gate field effect transistor, and a method of operating a dual dielectric tri-gate effect transistor are disclosed. In one embodiment, the dual dielectric tri-gate transistor comprises a substrate, an insulating layer on the substrate, and at least one semiconductor fin. A first dielectric having a first dielectric constant extends over sidewalls of the fin, and a metal layer extends over the first dielectric, and a second dielectric having a second dielectric constant is on a top surface of the fin. A gate electrode extends over the fin and the first and second dielectrics. The gate electrode and the first dielectric layer form first and second gates having a threshold voltage Vt 1 , and the gate electrode and the second dielectric layer form a third gate having a threshold voltage Vt 2  different than Vt 1.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention generally relates to semiconductor devices, and more specifically, to tri-gate field effect transistors. 
         [0003]    2. Background Art 
         [0004]    Due to the increasing difficulty in shrinking complementary metal-oxide-semiconductor (CMOS) transistor gate lengths while simultaneously controlling leakage current, the traditional single-gate metal-oxide-semiconductor field-effect transistor (MOSFET) structure may be supplanted by dual- or triple-gate MOSFET structures. These structures, by increasing the gate&#39;s control of the channel potential, allow greater ability to turn off MOSFETs with ultra-short channel lengths. Of the various multi-gate MOSFETs structures explored in recent years, the most promising in terms of manufacturability and performance are typically variations of the so-called “FinFET” structure. In these devices, strip or “fin” of silicon is formed, and subsequently the gate material is deposited and etched, so that the resulting gate surrounds the fin on the three exposed sides. The channel region of the device is located in the fin. Because the gate electrode and the gate dielectric surround the semiconductor body on three sides, the transistor essentially has three separate channels and gates. 
         [0005]    Tri-gate device structures, in particular, are receiving substantial attention as a candidate for 22 nm technologies and beyond. Because there are three separate channels formed in the semiconductor body, the semiconductor body can be fully depleted when the transistor is turned on, thereby enabling the formation of a fully depleted transistor with gate lengths of less than 30 nanometers without requiring the use of ultra-thin semiconductor bodies or requiring photolithographic patterning of the semiconductor bodies to dimensions less than the gate length of the device. 
         [0006]    Tri-gate device structures offer better electrostatic control, permitting gate length scaling. In addition, the current available per planar layout is potentially increased, as the sidewalls are gated regions. 
       BRIEF SUMMARY 
       [0007]    Embodiments of the invention provide a dual dielectric tri-gate field effect transistor, a method of fabricating a dual dielectric tri-gate field effect transistor, and a method of operating a dual dielectric tri-gate field effect transistor. In one embodiment, the dual dielectric tri-gate field effect transistor comprises a semiconductor substrate, an insulating layer on said substrate, and at least one semiconductor fin on and extending upward from said insulating layer. A first dielectric layer having a first dielectric constant extends over first and second sidewalls of the fin. A metal layer extends over this first dielectric layer, and this metal layer and the first dielectric form a metal-dielectric layer. A second dielectric layer having a second dielectric constant, different than the first dielectric constant, is on a top surface of the fin. A gate electrode extends over the fin, the metal-dielectric layer, and the second dielectric layer. The gate electrode and the metal-dielectric layer form first and second gates having a threshold voltage Vt 1 , and the gate electrode and the second dielectric layer form a third gate having a threshold voltage Vt 2  different than Vt 1 . 
         [0008]    In one embodiment, the first dielectric layer is a high-k dielectric, and the metal layer and the first dielectric layer form a metal-high-k dielectric. For example, the high-k dielectric may be HfO 2 , ZrO 2  or Hf/Zr, and the metal layer may be comprised of TiN or TaN. 
         [0009]    An embodiment of the invention provides a method of fabricating a dual dielectric tri-gate field effect transistor. This method comprises providing a base structure comprising a semiconductor substrate, an insulating layer, and at least one semiconductor fin extending upward from the insulating layer, said fin having first and second lateral sides and a top. This method further comprises forming a first dielectric material layer extending over the first and second lateral sides of the fin, forming a metal layer over the first dielectric material layer, and forming a second dielectric material layer, different that the first dielectric material layer, extending over the top of the fin. A gate electrode is formed extending over the fin and the first and second dielectric layers; and the gate electrode and the first dielectric layer form first and second gates having a threshold voltage Vt 1 , and the gate electrode and the second dielectric layer form a third gate having a threshold voltage Vt 2  different than Vt 1 . 
         [0010]    In an embodiment, the first dielectric material is a high-k dielectric and the metal layer and the first dielectric material form a metal-high-k dielectric. In an embodiment, the first dielectric layer extends over substantially all of the first and second sides of the fin, the second dielectric layer extends over substantially all of the top surface of the fin, and the gate electrode is comprised of an electrode material extending over both the first and second dielectric layers. 
         [0011]    An embodiment of the invention provides a method of operating a dual dielectric tri-gate field effect transistor (FET) comprising first, second and third gates, where the first and second gates have a threshold voltage of Vt 1 , and the third gate has a threshold voltage of Vt 2  that is greater than Vt 1 . This method comprises applying a supply voltage Vdd to the first, second and third gates of the FET, and operating the FET in a low power mode when Vdd is less than Vt 2  and greater than Vt 1 . 
         [0012]    In an embodiment of the invention, the top surface of the gated region is engineered to have a threshold voltage Vt 1  with a polysilicon gated SiON based dielectric and with metal high-k gated side surfaces to both have Vt 2 . A device with these properties will operate excellently in low Vdd (Vt 2 &gt;Vdd&gt;Vt 1 ), low power mode, and when Vdd is increased above Vt 2 , the device will operate in a high performance mode. In the low power mode, the device will also consume lower active power, as the gate capacitance of polysilicon gated SiON FETs will be much lower than MHK gated devices. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]      FIG. 1  shows a dual dielectric tri-gate structure in accordance with an embodiment of the invention. 
           [0014]      FIG. 2  illustrates a base structure from which the tri-gate structure of  FIG. 1  is fabricated. 
           [0015]      FIG. 3  depicts the formation of a high-k dielectric on the structure of  FIG. 2   
           [0016]      FIG. 4  illustrates a metal deposition on the high-k dielectric. 
           [0017]      FIG. 5  shows a SiO 2  deposited on the structure of  FIG. 4 . 
           [0018]      FIG. 6  illustrates a SiON grown on the Si fins shown in  FIG. 5 . 
           [0019]      FIG. 7  shows a fabrication flow chart diagram according to an embodiment of the invention. 
           [0020]      FIG. 8  depicts a bulk semiconductor substrate that may also be used, in an embodiment of the invention, in the fabrication of a transistor. 
           [0021]      FIG. 9  illustrates an oxide layer on the bulk semiconductor substrate of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide a thorough understanding of the present invention. However, it will be appreciated by one of ordinary skill in the art that the invention may be practiced with a wide range of specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the invention. 
         [0023]      FIG. 1  shows a dual dielectric tri-gate structure according to one embodiment of the present invention. Structure  10  comprises a base semiconductor substrate  12 , an insulator layer  14 , a plurality of semiconductor fins  16 , Hi-K dielectric  20 , metal layer  22 , top gate dielectric  24 , and gate electrode  26 . 
         [0024]    The base semiconductor substrate layer  12  may comprise any semiconductor material including, but not limited to: Si, SiC, SiGe, SiGeC, Ge alloys, GaAs, InAs, InP, other III-V or II-VI compound semiconductors, or organic semiconductor structures. In some embodiments of the present invention, the base semiconductor substrate layer  12  may be comprised of a Si-containing semiconductor material, i.e., a semiconductor material that includes silicon. Further, the base semiconductor substrate layer  12  may be doped or contain both doped and undoped regions. Although the base semiconductor substrate layer  12  may be a bulk semiconductor structure, it may also include a layered structure with one or more buried insulator layers (not shown). 
         [0025]    The insulator layer  14  may comprise any suitable insulator material(s), and it typically comprises a buried oxide (BOX), a nitride, or an oxynitride in either a crystalline phase or a non-crystalline phase. The buried insulator layer  14  may be a homogeneous, continuous layer, or it may contain relatively large cavities or micro- or nano-sized pores (not shown). The physical thickness of the buried insulator layer  14  may vary widely depending on the specific applications, but it typically ranges from about 10 nm to about 500 nm, with from about 20 nm to about 200 nm being more typical. The present invention, in an embodiment, may utilize a bulk substrate, referred to as bulk FinFET or Trigat/FinFET on bulk substrate, discussed in more detail below. 
         [0026]    The semiconductor fins  16  may comprise any semiconductor material including, but not limited to: Si, SiC, SiGe, SiGeC, Ge alloys, GaAs, InAs, InP, other III-V or II-VI compound semiconductors, or organic semiconductor structures. In some embodiments of the present invention, it may be preferred that the semiconductor fins  16  be comprised of a Si-containing semiconductor material, i.e., a semiconductor material that includes silicon. Further, the semiconductor fins  16  may be doped or contain both doped and undoped regions therein. The physical thickness of the fins  16  may vary widely depending on the specific applications. As will be understood by those of ordinary skill in the art, fins  16  may be formed in other ways. For example, Side wall Image Transfer (SIT) may be used to define the fins. 
         [0027]    Gate dielectric layer  20  extends over the sidewalls of semiconductor fins  16  and on or adjacent the insulating layer  14 . Gate dielectric layer  20  can be any suitable dielectric material. For instance, the gate dielectric layer may be a silicon dioxide (SiO 2 ), silicon oxynitride (SiO x N y ) or a silicon nitride (Si 3 N 4 ) dielectric layer. In an embodiment of the present invention, the gate dielectric layer  20  may be a silicon oxynitride film formed to a thickness of between 5-20 Å. In an embodiment of the present invention, gate dielectric layer  20  may be a high K gate dielectric layer, such as a metal oxide dielectric, such as but not limited to tantalum pentaoxide (Ta 2 O 5 ), and titanium oxide (TiO 2 ). Gate dielectric layer  20  can be other types of high K dielectric, such as but not limited to PZT. 
         [0028]    Metal layer  22  extends over gate dielectric layer  20 , and this layer  22  may be formed of a variety of suitable materials such as, but not limited to, tungsten, tantalum, titanium, and their nitrides. As another example, layer  22  may comprise polycrystalline silicon doped to a concentration density between 10 19 -10 20  atoms/cm 3 . Also, layer  22  need not necessarily be a single material and can be a composite stack of thin films, such as but not limited to a polycrystalline silicon/metal electrode or a metal/polycrystalline silicon electrode. 
         [0029]    Top gate dielectrics  24  are positioned on or adjacent top surfaces of fins  16 . Dielectrics  24 , similar to dielectric layer  20 , can be any suitable dielectric material; and, for example, dielectrics  24  may be a silicon oxynitride SiON or a silicon nitride dielectric layer. In an embodiment of the invention, the gate dielectric  24  may be a silicon oxynitride film formed to a thickness of between 5-20 Å. 
         [0030]    The gate electrode layer  26  may comprise polycrystalline silicon (poly-silicon), metal such as Al, Cu, W, Ti, Ta, TiN, TaN, NiSi, CoSi, and/or other proper conductive materials. The gate electrode layer  26  may be formed by CVD, PVD, plating, ALD, and other suitable processes. The gate electrode layer  26  may have a multilayer structure and may be formed in a multiple-step process. 
         [0031]      FIGS. 2-6  illustrate processing steps in the fabrication of the structure  10  shown in  FIG. 1 , and  FIG. 7  shows a fabrication flow chart diagram according to an embodiment of the invention. 
         [0032]    Generally, in embodiments of the invention, conventional fabrication steps can be used to form semiconductor substrate  12 , insulator layer  14  and fins  16 , as shown in  FIG. 2 . For example, in the manufacture of the device  10 , a silicon substrate  12  forming a silicon semiconductor body may be provided with an insulating layer  14  and on top thereof a monocrystalline silicon layer. Such a semiconductor body can, for example, be obtained by implanting oxygen ions into a monocrystalline silicone substrate. However, other techniques to obtain such a start-point semiconductor body are feasible, such as using thermal oxidation of a semiconductor substrate. Subsequently, an implant may be performed to tune the electrical properties of the semiconductor/silicon layer  12 . 
         [0033]    After this, a hard mask layer, e.g., of silicon nitride or a silicon oxide, may be deposited and patterned on the semiconductor layer at the location at which fins are to be formed and where source and drains regions are envisaged for forming a FinFET device. This may be followed by an etching step to form the fins  16 . Optionally, this may be followed by a surface treatment like an H 2  annealing step. Then, a poly silicon layer or hard mask layer is deposited and patterned, after which source and drain implants are done for forming source and drain regions that border the fin. During each of these two implants, the other regions of the structure are protected by, for example, a photo resist spot. After the source and drain implants are completed, the hard mask layer N is removed, also by (selective) etching. 
         [0034]    With reference to  FIGS. 3 and 7 , after the formation of structure  30  of  FIG. 2 , high-k dielectric layer  20  is formed at step  102 . This may be done, for example, by a chemical vapor deposition (CVD) or an atomic layer deposition (ALD) hi-k HfO 2 , ZiO 2  or Hf/Zr silicate deposition. The high-k dielectric layer  20  may contain any of the materials known in the art, including, but not limited to oxides of Zr, Hf, Al, HfSi, HfSiN, and combinations thereof. The thickness of high-k dielectric layer  20  may be between about 1.0 nm and about 2.5 nm. 
         [0035]    An optional step  104  is to deposit a band-edge metal on the hi-k dielectric layer  20 . For an NFET, this may be done, for instance, by depositing any II/II column element such as La, MG, or Ba. For a PFET, AlO 2  or Rh may be deposited to form a base-edge metal. 
         [0036]    Step  106 , illustrated in  FIG. 4 , is a metal deposition step to form metal layer  22 . This layer extends over gate dielectric layer  20 , and the metal layer  22  may be formed of a variety of suitable materials such as, but not limited to, tungsten, tantalum, titanium, and their nitrides. As another example, layer  22  may comprise polycrystalline silicon doped to a concentration density between 10 19 -10 20  atoms/cm 3 . Also, layer  22  need not necessarily be a single material and can be a composite stack of thin films, such as but not limited to a polycrystalline silicon/metal electrode or a metal/polycrystalline silicon electrode. 
         [0037]    At step  110 , silicon dioxide (SiO 2 ), shown in  FIG. 5  at  32 , is deposited to fill the trenches between the fins  16  and to cover layer  22 . In one embodiment, a silicon oxide layer is thickly deposited to cover the entire structure. Subsequently, Chemical-Mechanical Polishing (CMP) is performed to planarize the silicon oxide layer and to expose the tops of the fin-shaped structures  16 . 
         [0038]    At step  112 , the oxide is removed from the trenches, and top gate dielectrics  24  are then grown, at step  114 , on the tops of fins  16 , as shown in  FIG. 6 . These top gate dielectrics can be any suitable dielectric material, and for example, dielectrics  24  may be a silicon oxynitride SiON or a silicon nitride dielectric layer. In an embodiment of the invention, the gate dielectric  24  may be a silicon oxynitride film formed to a thickness of between 5-20 Å. Dielectrics  24  may be formed, for example, by rapid thermal processing (RTP) oxide, decoupled plasma nitridation (DPN) or by rapid thermal oxidation using NO gas (RTNO). 
         [0039]    At step  116 , polysilicon gate  26  (shown in  FIG. 1 ) is deposited over the metal-hi-k (MHK) sidewalls and the SiON top gate dielectrics  24 . This gate electrode layer  26  may comprise polycrystalline silicon (poly-silicon), metal such as Al, Cu, W, Ti, Ta, TiN, TaN, NiSi, CoSi, and/or other proper conductive materials. The gate electrode layer  26  may be formed by CVD, PVD, plating, ALD, and other suitable processes. Also, the gate electrode layer  26  may have a multilayer structure and may be formed in a multiple-step process. 
         [0040]    With the above-described design, FET device  10  essentially has three separate channels and gates. Each fin  16  forms a top channel and two side channels. The top dielectric  24  and polysilicon material  26  form a first, top gate with a threshold voltage Vt 1 , and metal-hi-k dielectric layer  20 ,  22  and the polysilicon material  26  form two additional side gates with a threshold voltage Vt 2 . 
         [0041]    As mentioned above, in an embodiment, the invention may be fabricated using a bulk substrate, referred to as a bulk Fin FET or Trigat/FinFET on bulk Si substrate.  FIG. 8  shows such a bulk substrate, with fins  42 . Any suitable bulk substrate may be used, and fins  42  may be formed thereon in any suitable way. As shown in  FIG. 9 , an oxide insulator layer  44  is deposited on substrate  44 , between the fins  42 . Any suitable oxide material may be used, and the oxide layer  44  may be formed or deposited on substrate  40  in any suitable manner. After layer  44  is formed, the resulting structure may be processed, for example, as described above in connection with  FIG. 2-7  to fabricate a dual dielectric trigate field effect transistor. 
         [0042]    Embodiments of the invention have significant utility. For instance, in an embodiment of the invention, the top surface of the gated region may be engineered to have a threshold voltage Vt 1  that is less than the threshold voltage Vt 2  of the metal high-k gated side surfaces. A device with these properties will operate excellently in low Vdd (Vt 2 &gt;Vdd&gt;Vt 1 ), low power mode, and when Vdd is increased above Vt 2 , the device will operate in a high performance mode. In the low power mode, the device will also consume lower active power, as the gate capacitance of polysilicon gated SiON FETs will be much lower than MHK gated devices. 
         [0043]    While it is apparent that the invention herein disclosed is well calculated to fulfill the objects discussed above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true scope of the present invention.