Abstract:
A field effect transistor (FET) includes a substrate; a channel material located on the substrate, the channel material comprising one of graphene or a nanostructure; a gate located on a first portion of the channel material; and a contact aligned to the gate, the contact comprising one of a metal silicide, a metal carbide, and a metal, the contact being located over a source region and a drain region of the FET, the source region and the drain region comprising a second portion of the channel material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/820,341 (Chang et al.), filed on Jun. 22, 2010 the contents of which are incorporated herein by reference in their entirety. 
     
    
     FIELD 
       [0002]    This disclosure relates generally to the field of field effect transistor (FET) fabrication, and in particular to graphene-based FET fabrication. 
       DESCRIPTION OF RELATED ART 
       [0003]    Graphene refers to a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a truly planar graphene structure does not exist in a three-dimensional space, being unstable with respect to formation of curved structures such as soot, fullerenes, nanotubes or buckled two dimensional structures. However, a two-dimensional graphene structure may be stable when supported on a substrate, for example, on the surface of a silicon carbide (SiC) crystal. Free standing graphene films have also been produced, but they may not have the idealized flat geometry. 
         [0004]    Structurally, graphene has hybrid orbitals formed by sp 2  hybridization. In the sp 2  hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp 2  orbitals. The one remaining p-orbital forms a pi (π)-bond between the carbon atoms. Similar to the structure of benzene, the structure of graphene has a conjugated ring of the p-orbitals, i.e., the graphene structure is aromatic. Unlike other allotropes of carbon such as diamond, amorphous carbon, carbon nanofoam, or fullerenes, graphene is only one atomic layer thin. 
         [0005]    Graphene has an unusual band structure in which conical electron and hole pockets meet only at the K-points of the Brillouin zone in momentum space. The energy of the charge carriers, i.e., electrons or holes, has a linear dependence on the momentum of the carriers. As a consequence, the carriers behave as relativistic Dirac-Fermions with a zero effective mass and are governed by Dirac&#39;s equation. Graphene sheets may have a large carrier mobility of greater than 200,000 cm 2 /V-sec at 4K. Even at 300K, the carrier mobility can be as high as 15,000 cm 2 /V-sec. 
         [0006]    Graphene layers may be grown by solid-state graphitization, i.e., by sublimating silicon atoms from a surface of a silicon carbide crystal, such as the (0001) surface. At about 1,150° C., a complex pattern of surface reconstruction begins to appear at an initial stage of graphitization. Typically, a higher temperature is needed to form a graphene layer. Graphene layers on another material are also known in the art. For example, single or several layers of graphene may be formed on a metal surface, such as copper and nickel, by chemical deposition of carbon atoms from a carbon-rich precursor. 
         [0007]    Graphene displays many other advantageous electrical properties such as electronic coherence at near room temperature and quantum interference effects. Ballistic transport properties in small scale structures are also expected in graphene layers. 
         [0008]    While single-layer graphene sheet has a zero band-gap with linear energy-momentum relation for carriers, two-layer graphene, i.e. bi-layer graphene, exhibits drastically different electronic properties, in which a band gap may be created under special conditions. In a bi-layer graphene, two graphene sheets are stacked on each other with a normal stacking distance of roughly 3.35 angstrom, and the second layer is rotated with respect to the first layer by 60 degree. This stacking structure is the so-called A-B Bernel stacking, and is also the graphene structure found in natural graphite. Similar to single-layer graphene, bi-layer graphene has zero-band gap in its natural state. However, by subjecting the bi-layer graphene to an electric field, a charge imbalance can be induced between the two layers, and this will lead to a different band structure with a band gap proportional to the charge imbalance. 
         [0009]    A field effect transistor (FET) may be fabricated using graphene, graphite, which comprises stacked sheets of graphene, or nanostructures formed from graphene such as carbon nanotubes, for the FET channel and source/drain regions. Alternately, the FET may be formed using nanowires made of a semiconducting material for the FET channel and source/drain regions. The source/drain regions of the FET require electrical contacts, which may be formed using a contact material. The contacts to the source/drain regions in a graphene/nanostructure FET need to be formed using a contact material with a relatively low contact resistance, in order to obtain good FET operating characteristics. 
       SUMMARY 
       [0010]    In one aspect, a field effect transistor (FET) includes a substrate; a channel material located on the substrate, the channel material comprising one of graphene or a nanostructure; a gate located on a first portion of the channel material; and a contact aligned to the gate, the contact comprising one of a metal silicide, a metal carbide, and a metal, the contact being located over a source region and a drain region of the FET, the source region and the drain region comprising a second portion of the channel material. 
         [0011]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0013]      FIG. 1  illustrates an embodiment of a method of fabricating a graphene/nanostructure FET with a self-aligned contact and gate. 
           [0014]      FIG. 2A  illustrates an embodiment of a deposited channel material on a substrate. 
           [0015]      FIG. 2B  illustrates a top view of an embodiment of a deposited channel material comprising nanostructures on a substrate. 
           [0016]      FIG. 3  illustrates an embodiment of the device of  FIG. 2A  after gate and spacer formation. 
           [0017]      FIG. 4  illustrates an embodiment of the device of  FIG. 3  after deposition of contact material. 
           [0018]      FIG. 5  illustrates an embodiment of the device of  FIG. 4  after deposition of a dielectric material. 
           [0019]      FIG. 6  illustrates an embodiment of the device of  FIG. 5  after chemical mechanical polishing. 
           [0020]      FIG. 7  illustrates an embodiment of the device of  FIG. 6  after recessing the contact material. 
           [0021]      FIG. 8  illustrates an embodiment of the device of  FIG. 7  after removal of the dielectric material. 
           [0022]      FIG. 9A  illustrates an embodiment of a graphene/nanostructure FET with a self-aligned contact and gate after patterning the contact material. 
           [0023]      FIG. 9B  illustrates a top view of an embodiment of a graphene/nanostructure FET with a self-aligned contact and gate comprising a nanostructure channel material. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Embodiments of a graphene/nanostructure FET with a self-aligned contact and gate, and methods of forming a graphene/nanostructure FET with a self-aligned contact and gate, are provided, with exemplary embodiments being discussed below in detail. The channel and source/drain regions of the graphene/nanostructure FET include one or more sheets of graphene in some embodiments, or nanostructures, such as carbon nanotubes or semiconductor nanowires, in other embodiments. A relatively low-resistance contact that is self-aligned to the FET gate may be formed over the source/drain regions of the graphene/nanostructure FET. The contact may be formed from a material having a relatively low resistance, such as a metal, a silicide, or a carbide. 
         [0025]      FIG. 1  illustrates an embodiment of a method  100  of fabricating a graphene/nanostructure FET having a self-aligned contact and gate.  FIG. 1  is discussed with reference to  FIGS. 2-9 . In block  101 , a channel material  203  is deposited on a substrate. The substrate includes an insulating layer  202  located on a silicon substrate  201 , as shown in  FIG. 2A . Insulating layer  202  may comprise an oxide material such as silicon oxide (SiO 2 ) in some embodiments. Channel material  203  may be one or more sheets of graphene in some embodiments, or may include nanostructures, such as semiconductor nanowires or carbon nanotubes, in other embodiments.  FIG. 2B  illustrates an embodiment of a channel material  203  comprising nanostructures, such as carbon nanotubes or semiconductor nanowires, on the substrate. As shown in  FIG. 2B , the nanostructure channel material  203  is formed on insulating layer  202 . 
         [0026]    Gate  301  is then formed over the channel material  203 , and sidewall spacers  302  are formed adjacent to gate  301  over channel material  203 , as shown in  FIG. 3 . In some embodiments, gate  301  may be patterned from a stack of materials including a thin layer of a gate dielectric, such as hafnium oxide (HfO 2 ), and, on top of the thin layer of gate dielectric, a gate metal such as titanium nitride (TiN) or tungsten (W), and, optionally, on top of the gate metal, a gate hardmask, such as silicon nitride (SiN). Sidewall spacers  302  may comprise a nitride or an oxide in some embodiments, and may be formed by deposition of the spacer material and etching to form the sidewall spacers  302 . 
         [0027]    In block  102 , a contact material  401  is deposited over the device  300  of  FIG. 3 , resulting in the device  400  shown in  FIG. 4 . Contact material  401  may include a metal that has a relatively low resistance, or carbon or silicon in some embodiments. 
         [0028]    In block  103 , a dielectric material  501  is deposited over contact material  401 , as shown in  FIG. 5 . Deposition of dielectric material  501  may be directional in some embodiments, i.e., dielectric material does not form on the horizontal portions of contact material  401  on sidewall spacers  302 , as shown in  FIG. 5 . In other embodiments, if deposition of dielectric material  501  is not directional, any portions of dielectric material  501  that is deposited on the horizontal portion of the contact material  401  on sidewall spacers  302  is be removed using, for example, a hydrofluoric wet etch, as shown in  FIG. 5 . Dielectric material  501  may be a material that chosen such that contact material  401  is removable without removing dielectric material  501 ; dielectric material  501  may include high density plasma (HDP) oxide in some embodiments. 
         [0029]    In block  104 , chemical mechanical polishing (CMP) is performed to remove the top portion of dielectric material  501  and contact material  401 , exposing the top of gate  301 , as is shown in  FIG. 6 . Then, in block  105 , the contact material  401  located adjacent to sidewall spacers  302  is recessed below the level of dielectric material  501 , as shown in  FIG. 7 . In embodiments in which contact material  401  includes carbon and dielectric material  501  includes HDP oxide, contact material  401  may be recessed using oxygen plasma. Use of oxygen plasma allows removal of contact material  401  without removing dielectric material  501  or sidewall spacers  302 . 
         [0030]    In block  106 , the dielectric material  501  is removed as shown in  FIG. 8 . Dielectric material  501  may be removed by any appropriate method. Then, in embodiments in which contact material  401  comprises carbon or silicon, the contact material may be converted to a metal carbide or a metal silicide in order to reduce the contact resistance of contact material  401 . The conversion may be accomplished by depositing a layer of a complementary metal on contact material  401  and annealing the device with the deposited metal layer to a temperature that is higher than the formation temperature of the desired metal silicide or metal carbide that the metal reacts with the silicon or carbon contact material  401 . The annealing causes contact material  401  to be converted into the metal silicide or metal carbide; all of contact material  401  may react with the deposited metal layer in some embodiments. Then, after the anneal, any remaining unreacted metal is removed selectively to the metal silicide or metal carbide. 
         [0031]    In block  107 , the contact material  401  is masked and patterned to remove any contact material  401  that is located on non-FET regions of the substrate, resulting in self-aligned contact  901  as shown in  FIGS. 9A-B . FET  900 A-B of  FIGS. 9A-B  has self-aligned contact  901  and gate  301  formed over channel material  203 . Contact  901  is self-aligned to gate  301  and to the source and drain regions, and may comprise a metal, a carbide, or a silicide, having a relatively low contact resistance to the source and drain regions of the FET. Channel material  203  may include one or more sheets of graphene in some embodiments, or nanostructures such as carbon nanotubes or semiconductor nanowires in other embodiments.  FIG. 9B  illustrates a top view of an embodiment of a FET  900 B showing contact  901  over channel material  203 , in which channel material  203  includes nanostructures such as carbon nanotubes or semiconductor nanowires. The portion of channel material  203  located underneath gate  301  comprises the FET channel, and the portions of channel material  301  located underneath self-aligned contact  901  form the FET source and drain regions. Self-aligned contact  901  acts as an electrical contact for the source and drain regions of the FET during operation. 
         [0032]    The technical effects and benefits of exemplary embodiments include a self-aligned method of forming a FET having a relatively low contact resistance. 
         [0033]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. 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. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0034]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.