Abstract:
Vertical GAA FET structures are disclosed in which a current-carrying nanowire is oriented substantially perpendicular to the surface of a silicon substrate. The vertical GAA FET is intended to meet design and performance criteria for the 7 nm technology generation. In some embodiments, electrical contacts to the drain and gate terminals of the vertically oriented GAA FET can be made via the backside of the substrate. Examples are disclosed in which various n-type and p-type transistor designs have different contact configurations. In one example, a backside gate contact extends through the isolation region between adjacent devices. Other embodiments feature dual gate contacts for circuit design flexibility. The different contact configurations can be used to adjust metal pattern density.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure generally relates to various geometries for gate all-around transistor devices built on a silicon substrate and, in particular, to vertically oriented gate all-around transistors in which current flows in a direction transverse to the surface of the silicon substrate. 
         [0003]    2. Description of the Related Art 
         [0004]    Conventional integrated circuits incorporate planar field effect transistors (FETs) in which current flows through a semiconducting channel between a source and a drain, in response to a voltage applied to a control gate. The semiconductor industry strives to obey Moore&#39;s law, which holds that each successive generation of integrated circuit devices shrinks to half its size and operates twice as fast. As device dimensions have shrunk below 100 nm, however, conventional silicon device geometries and materials have experienced difficulty maintaining switching speeds without incurring failures such as, for example, leaking current from the device into the semiconductor substrate. Several new technologies emerged that allowed chip designers to continue shrinking gate lengths to 45 nm, 22 nm, and then as low as 14 nm. One particularly radical technology change entailed re-designing the structure of the FET from a planar device to a three-dimensional device in which the semiconducting channel was replaced by a fin that extends out from the plane of the substrate. In such a device, commonly referred to as a FinFET, the control gate wraps around three sides of the fin so as to influence current flow from three surfaces instead of one. The improved control achieved with a 3-D design results in faster switching performance and reduced current leakage. Building taller devices has also permitted increasing the device density within the same footprint that had previously been occupied by a planar FET. Examples of FinFET devices are described in further detail in U.S. Pat. No. 8,759,874 and U.S. Patent Application Publication US2014/0175554, assigned to the same assignee as the present patent application. 
         [0005]    The FinFET concept was further extended by developing a gate all-around FET, or GAA FET, in which the gate fully wraps around the channel for maximum control of the current flow therein. In the GAA FET, the channel can take the form of a cylindrical nanowire that is isolated from the substrate, in contrast to the peninsular fin. In the GAA FET the cylindrical nanowire is surrounded by the gate oxide, and then by the gate. Existing GAA FETs are oriented horizontally, such that the nanowire extends in a direction that is substantially parallel to the surface of the semiconductor substrate. GAA FETs are described in, for example, U.S. Patent Application Publication No. 2013/0341596 to Chang et al., of IBM and in U.S. patent application Ser. No. 14/312,418, assigned to the same assignee as the present patent application. 
       BRIEF SUMMARY 
       [0006]    Vertical GAA FET structures are disclosed in which a current-carrying nanowire is oriented substantially perpendicular to the surface of a silicon substrate. The vertical GAA FET is intended to meet design and performance criteria for the 7 nm technology generation. In some embodiments, electrical contacts to the drain and gate terminals of the vertically oriented GAA FET can be made via the backside of the substrate. Examples are disclosed in which various n-type and p-type transistor designs have different contact configurations. In one example, a backside gate contact extends through the isolation region between adjacent devices. Other embodiments feature dual gate contacts for circuit design flexibility. The different contact configurations can be used to adjust metal pattern density. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
           [0008]      FIG. 1  is a cross-sectional view of n-type and p-type vertical gate-all-around (GAA) transistors, according to one embodiment described herein. 
           [0009]      FIGS. 2-4  are cross-sectional views of alternative embodiments of the vertical GAA transistors shown in  FIG. 1 , wherein each embodiment has a different gate contact configuration, as described herein 
           [0010]      FIG. 5  is a flow diagram summarizing a sequence of processing steps that can be used to fabricate the vertical GAA transistors shown in  FIGS. 1-4 , according to a first exemplary embodiment described herein. 
           [0011]      FIGS. 6-9  are cross-sectional views of the vertical GAA transistor configuration shown in  FIG. 1 , at various steps during the processing sequence shown in  FIG. 5 . 
           [0012]      FIG. 10  is a cross-sectional view of completed n-type and p-type vertical gate-all-around (GAA) transistors having the gate contact configuration shown in  FIG. 1 , according to one embodiment described herein. 
           [0013]      FIG. 11  is a flow diagram summarizing a sequence of processing steps that can be used to fabricate the vertical GAA transistors shown in  FIGS. 1-4 , according to a second exemplary embodiment described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of semiconductor processing comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
         [0015]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
         [0016]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects of the present disclosure. 
         [0017]    Reference throughout the specification to integrated circuits is generally intended to include integrated circuit components built on semiconducting substrates, whether or not the components are coupled together into a circuit or able to be interconnected. Throughout the specification, the term “layer” is used in its broadest sense to include a thin film, a cap, or the like and one layer may be composed of multiple sub-layers. 
         [0018]    Reference throughout the specification to conventional thin film deposition techniques for depositing silicon nitride, silicon dioxide, metals, or similar materials include such processes as chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), metal organic chemical vapor deposition (MOCVD), plasma-enhanced chemical vapor deposition (PECVD), plasma vapor deposition (PVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), electroplating, electro-less plating, and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. For example, in some circumstances, a description that references CVD may alternatively be done using PVD, or a description that specifies electroplating may alternatively be accomplished using electro-less plating. Furthermore, reference to conventional techniques of thin film formation may include growing a film in-situ. For example, in some embodiments, controlled growth of an oxide to a desired thickness can be achieved by exposing a silicon surface to oxygen gas or to moisture in a heated chamber. 
         [0019]    Reference throughout the specification to conventional photolithography techniques, known in the art of semiconductor fabrication for patterning various thin films, includes a spin-expose-develop process sequence typically followed by an etch process. Alternatively or additionally, photoresist can also be used to pattern a hard mask (e.g., a silicon nitride hard mask), which, in turn, can be used to pattern an underlying film. 
         [0020]    Reference throughout the specification to conventional etching techniques known in the art of semiconductor fabrication for selective removal of polysilicon, silicon nitride, silicon dioxide, metals, photoresist, polyimide, or similar materials includes such processes as wet chemical etching, reactive ion (plasma) etching (RIE), washing, wet cleaning, pre-cleaning, spray cleaning, chemical-mechanical planarization (CMP) and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. In some instances, two such techniques may be interchangeable. For example, stripping photoresist may entail immersing a sample in a wet chemical bath or, alternatively, spraying wet chemicals directly onto the sample. 
         [0021]    Specific embodiments are described herein with reference to vertical gate-all-around devices that have been produced; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and should not be limited to those shown. 
         [0022]    Turning now to the figures,  FIGS. 1-4  show various different embodiments of vertical GAA transistors.  FIG. 1  shows CMOS n-type and p-type vertical gate all-around (GAA) transistors, an n-FET device  100  and a p-FET device  101 , respectively, built on a silicon substrate  102 , according to one embodiment described herein. Each one of the vertical GAA transistors is essentially a linear, or 1-D device in the form of a nanowire  104  oriented in a direction transverse to planar front and back surfaces of the silicon substrate  102 . Two such nanowires are shown in  FIGS. 1, 104   n  and  104   p . The nanowire  104   n  is the channel region of the n-FET device  100  and the nanowire  104   p  is the channel region of the p-FET device  101 . The nanowire  104   n  includes a silicon channel  106   n  that couples an N+ drain  105   n  located below the channel  106   n  to an N+ source  107   n  located above the channel  106   n  along a channel axis  108 ; the nanowire  104   p  includes a SiGe channel  106   p  that couples a P+ drain  105   p  below the channel  106   p  to a P+ source  107   p  above the channel  106   p.  Alternatively, one or both channels can be made of a III-V semiconducting material such as InAs, as suggested by Ionescu and Riel in “Tunnel Field-Effect Transistors as Energy-Efficient Electronic Switches,” [Nature, Vol. 479, November 17, 201, p. 379]. The vertical nanowire  104  desirably has a diameter in the range of 6-10 nm. 
         [0023]    The n-type material in the source and drain of the n-FET device  100  can be, for example, epitaxially grown indium-doped silicon. The p-type material in the source and drain of the p-FET device  101  can be, for example, epitaxially grown SiGe. In one embodiment the channel length can be as long as 100 nm. A long channel length having an aspect ratio in the range of about 4:1-10:1 provides a high gate contact area to maintain low resistance contacts. Backside nanowire contacts  110   n  and  110   p  are also shown in  FIG. 1 , along with front side nanowire contacts  112   n  and  112   p.    
         [0024]    Metal gates  114  wrap around each of the nanowires  104 . In one embodiment, the metal gates  114  include a stack of work function materials. For example, the metal gate for the n-FET device  100  is a three-layer stack that includes a 4-nm thick layer of titanium carbide (TiC) sandwiched between two 3-nm layers of titanium nitride (TiN). The metal stack for the p-FET device  101  is a three-layer stack of TiN that yields a total thickness of about 10 nm. The metal gates  114  are spaced apart from the channel by a wrap-around gate dielectric  115  made of a high-k material, e.g., HfO 2 . The n-FET has a front side gate contact  116   n  and the p-FET has a backside gate contact  116   p.  Each contact contains a bulk metal and a liner, as is customary in the art. The gate contacts  116  are isolated from the source regions by a thick hard mask  117  made of silicon nitride (SiN) or silicon carbide (SiC). 
         [0025]    The transistors  100  and  101  are separated by an isolation region  118  that is filled with an insulator, e.g., an oxide material with a silicon nitride liner. The backside gate contact  116   p  passes through the isolation region  118 . 
         [0026]    Finally, the nanowires  104  are covered by a low-k encapsulant  120 , which is, in turn, covered, by an insulating material  122 . In one embodiment, the low-k encapsulant  120  is made of SiOCN or SiBCN, having a thickness in the range of about 8-30 nm. 
         [0027]    Comparing  FIGS. 1-4 , it becomes apparent that the different embodiments shown present alternative contact arrangements to the two nanowire devices, and in particular, alternative gate contact arrangements. The nanowires  104  and the geometries of the source/drain contacts  110  and  112  are substantially the same throughout  FIGS. 1-4 . However, the structure and placement of the various gate contacts  116  differs. For example, in  FIG. 1 , each device has a single gate contact, wherein the n-FET gate is accessible from the front side of the silicon, while the p-FET gate contact is accessible from the back side. Whereas, in the arrangement  102  shown in  FIG. 2 , each nanowire has a dual gate contact. For example, the n-FET gate  114   n  is accessible via two front side gate contacts  116   n,  while the p-FET gate  114   p  is accessible via two back side gate contacts  116   p,  each of which is disposed in an isolation region  118 . A symmetric contact design in which an equal number of connections is made on the top side and the back side of the silicon maintains a balanced metal line pattern density. Maintaining consistent pattern density facilitates processes that are particularly sensitive to pattern uniformity such as photolithography and planarization processes. Another advantage of using back side contacts is that they can be large compared with front side contacts, for example 10-100 nm or larger, and can thus serve as heat sinks. 
         [0028]    In  FIG. 3 , each transistor gate  114  is accessible by a single front side gate contact  116 . 
         [0029]    In  FIG. 4 , each transistor gate  114  is accessible by both a front side gate contact  116  and a backside gate contact  116 . In addition, the n-FET has a dual front-side contact  116   n.  Providing more than one gate contact is helpful in the design of Boolean logic circuit applications, for example. 
         [0030]      FIG. 5  shows steps in a method  200  of fabricating the vertical GAA transistors shown in  FIG. 1 , as an example. The method shown in  FIG. 5  is further illustrated by  FIGS. 6-10 , and described below. The exemplary method  200  uses a technique that entails coating and removal of a sacrificial polymer material, for example, benzocyclobutene (BCB). An alternative method of fabrication may use a method known as inlay banding. 
         [0031]    At  201 , isolation regions are formed in the substrate  202  as shown in  FIG. 6  by known methods. 
         [0032]    At  202 , drain regions are formed in the substrate  102 , as shown in  FIG. 6  by, for example, implant doping, in the usual way. The N+ concentration, typically boron, is desirably in the range of about 1E16-3E20 cm −3 , with a target concentration of 2E19 cm −3 . The P+ concentration, typically arsenic or phosphorous, is desirably in the range of about 1E16-5E20 cm −3,  with a target concentration of 5E19 cm −3 . 
         [0033]    At  203 , following an anneal step to drive the dopants to a desired depth, the insulating layer  117  is formed. 
         [0034]    At  204 , a first thick coating of the polymer BCB  150  is applied. 
         [0035]    At  206 , the BCB  150  is patterned using a reactive ion etch (RIE) process to form trenches for the nanowires  104 . The trenches diameter is in the range of about 2-200 nm and the trench height is in the range of about 8-800 nm. 
         [0036]    At  208 , the channels  106  and then the source regions are formed by epitaxial growth within the high aspect ratio trenches, as shown in  FIG. 6 . The n-FET channel can be formed of silicon or indium arsenide (InAs), and the p-FET channel is formed of SiGe. The source regions can be formed by in-situ doping during epitaxy. 
         [0037]    At  210 , the metal gates  114   n  and  114   p  are formed, as shown in  FIG. 7 . First, a gate trench is formed using an RIE process that stops on the silicon nitride (SiN) layer  117 . Then, the gate trenches are filled with the high-k wrap-around gate dielectric  115 , a metal liner, one or more work function metals as discussed above, and then the metal gates  114  are polished to stop on the BCB  150 . 
         [0038]    At  212  The BCB  150  and the metal gates  114   n  and  114   p  are recessed, by selective etching, to reveal the source regions  107   n  and  107   p.    
         [0039]    At  214 , the BCB  150  is removed, as shown in  FIG. 8 . In addition, the high-k dielectric material  115  on the outsides of the metal gates  114  is removed. Some high-k material may remain on the exposed source regions  107 . 
         [0040]    At  216 , the devices are encapsulated with the low-k encapsulant  120  for capacitance reduction. 
         [0041]    At  218 , inter-device regions are filled with the inter-layer dielectric (ILD)  122 , and the ILD  122  is then planarized to stop on the low-k encapsulant  120 , as shown in  FIG. 9 . 
         [0042]    Opening contacts to the source, drain, and gate terminals of the n-FET and p-FET devices then produces the structure shown in  FIG. 10 , which is a reproduction of  FIG. 1 , or alternatively, the structures shown in  FIGS. 2-4 , which have different gate contact arrangements. The vertical geometry of the GAA transistors this formed allows flexibility in circuit design because it is possible to access the devices from different front side and back side locations by simply changing the contact configuration. For example, the NFET gate contact can extend from the front side, while the PFET gate contact can extend from the back side, or vice versa. One or more gate contacts can pass through isolation regions  118  for an even more compact design, as shown in  FIGS. 1, 2, and 4 . 
         [0043]    An alternative method  300  of fabricating the vertical GAA devices shown in  FIGS. 1-4  is shown in  FIG. 11 , in accordance with methods described in a manuscript by Bjork et al. in “Si—InAs Heterojunction Esaki Tunnel Diodes with High Current Densities”. Steps  302 - 306  of the method  300  are the same as steps  201 - 203  of  FIG. 5 . 
         [0044]    Then, at  308 - 310 , instead of forming trenches in a layer of BCB  150  and filling the trenches to form the nanowires  104 , the drain regions are exposed at  308 , and then at  310  vertical nanowires  104  are selectively grown from the drain regions  105 . In one example, after opening the SiN layer  117  to expose the drain regions, selective nanowire growth is performed in an MOCVD system at 400-600 C and a reactor pressure of 60 Torr, using a trimethyl-indium (TMIn) and a tertiarybutyl-arsine molar flow of 0.7 μMol/min and 12.6 μMol/min, respectively to create InAs nanowires. Doping of the InAs is achieved by injecting disilane (Si 2 H 6 ) during growth at Si 2 H 6 /TMIn ratios of 1E-6 to 1E-2. 
         [0045]    At  312 , once the nanowires  104  are in place, the metal gates  114  are formed by depositing the gate stack, including the high-k dielectric, the metal liner, the work function material, and the bulk metal gate layer, conformally over the nanowires  104 , and etching away portions outside a desired radius from the nanowires  104 . A BCB layer can then be used to mask the gate structure around the channel portions of the nanowires  104  while selectively etching gate stack material from the source region portions of the nanowires  104 . 
         [0046]    Steps  314 - 316  of the method  300  are the same as steps  216 - 218  of the method  200  described above. 
         [0047]    It will be appreciated that, although specific embodiments of the present disclosure are described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, the present disclosure is not limited except as by the appended claims. 
         [0048]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 
         [0049]    The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.