Abstract:
A self-aligned SiGe FinFET device features a relaxed channel region having a high germanium concentration. Instead of first introducing germanium into the channel and then attempting to relax the resulting strained film, a relaxed channel is formed initially to accept the germanium. In this way, a presence of germanium can be established without straining or damaging the lattice. Gate structures are patterned relative to intrinsic silicon fins, to ensure that the gates are properly aligned, prior to introducing germanium into the fin lattice structure. After aligning the gate structures, the silicon fins are segmented to elastically relax the silicon lattice. Then, germanium is introduced into the relaxed silicon lattice, to produce a SiGe channel that is substantially stress-free and also defect-free. Using the method described, concentration of germanium achieved in a structurally stable film can be increased to a level greater than 85%.

Description:
BACKGROUND 
       [0001]    Technical Field 
         [0002]    The present disclosure generally relates to techniques for fabricating an array of high performance FinFET devices. 
         [0003]    Description of the Related Art 
         [0004]    Advanced integrated circuits often feature strained channel transistors, silicon-on-insulator (SOI) substrates, FinFET structures, or combinations thereof, in order to continue scaling transistor gate lengths below 20 nm. Such technologies allow the channel length of the transistor to shrink while minimizing detrimental consequences such as current leakage and other short channel effects. 
         [0005]    A FinFET is an electronic switching device in which a conventional planar semiconducting channel is replaced by a semiconducting fin that extends outward from the substrate surface. In such a device, the gate, which controls current flow in the fin, 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 FinFET design results in faster switching performance and reduced current leakage than is possible with a planar transistor. FinFETs 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. 
         [0006]    Strained silicon transistors have been developed to increase mobility of charge carriers, i.e., electrons or holes, passing through a semiconductor lattice. Incorporating strain into the channel of a semiconductor device stretches the crystal lattice, thereby increasing charge carrier mobility in the channel so that the device becomes a more responsive switch. Introducing a compressive strain into a PFET transistor tends to increase hole mobility in the channel, resulting in a faster switching response to changes in voltage applied to the transistor gate. Likewise, introducing a tensile strain into an NFET transistor tends to increase electron mobility in the channel, also resulting in a faster switching response. 
         [0007]    There are many ways to introduce tensile or compressive strain into transistors, for both planar devices and FinFETs. Such techniques typically entail incorporating into the device epitaxial layers of one or more materials having crystal lattice dimensions or geometries that differ slightly from those of the silicon substrate. The epitaxial layers can be made of doped silicon or silicon germanium (SiGe), for example. Such epitaxial layers can be incorporated into source and drain regions, into the transistor gate that is used to modulate current flow in the channel, or into the channel itself, which is a portion of the fin. For example, one way to introduce strain is to replace bulk silicon from the source and drain regions, or from the channel, with silicon compounds such as silicon germanium. Because Si—Ge bonds are longer than Si—Si bonds, there is more open space in a SiGe lattice. Electrons thus move more freely through a lattice that contains elongated Si—Ge and Ge—Ge bonds than through a lattice that contains shorter Si—Si bonds. The presence of germanium atoms having longer bonds tends to stretch the lattice, causing internal strain. Replacing silicon atoms with SiGe atoms can be accomplished, for example, during a controlled process of epitaxial crystal growth, in which a new SiGe crystal layer is grown from the surface of a bulk silicon crystal, while maintaining the same crystal structure of the underlying bulk silicon crystal. Alternatively, strain can be induced in the fin from below the device by using various types of SOI substrates. An SOI substrate features a buried insulator, typically a buried oxide layer (BOX) underneath the active area. SOI FinFET devices have been disclosed in patent applications assigned to the present assignee, for example, U.S. Patent Application Publication No. 2015/0279970, entitled “SOI FinFET Transistor with Strained Channel,” which is hereby incorporated by reference in its entirety. 
       BRIEF SUMMARY 
       [0008]    Strain and mobility effects in the channel of a FinFET can be tuned by controlling the size and the elemental composition of the fins. It is advantageous for SiGe films to contain a high concentration of germanium, e.g., in the range of at least 25%-40%, to provide enhanced electron mobility compared with lower concentration SiGe films. Carrier mobility in the channel region determines overall transistor performance. Consequently, it is desirable to increase to a level as high as possible the percent concentration of germanium atoms in the fins of a SiGe FinFET. 
         [0009]    While a strained silicon lattice is beneficial, creating strain by incorporating germanium atoms using existing methods tends to damage the crystal lattice. As a result, the lattice structures of germanium-rich films tend to be mechanically unstable, especially if they contain a high number of structural defects such as faults, or dislocations. Furthermore, such a mechanically unstable fin may be structurally limited with regard to its aspect ratio, or height:width ratio. Such a limitation is undesirable because one advantage of a FinFET is that the fin, being a vertical structure, has a small footprint. 
         [0010]    Dislocation defects that cause such instability can be avoided by creating a germanium-rich film that is relaxed, as an alternative to a strained film. A self-aligned SiGe FinFET device described herein features a relaxed channel region having a high germanium concentration. Instead of first introducing germanium into the channel and then attempting to relax the resulting strained film, a relaxed channel is formed initially to accept the germanium. Thus, a presence of germanium can be established without straining or damaging the lattice. In the CMOS FinFET fabrication method described herein, sacrificial gate structures are patterned relative to intrinsic silicon fins, to ensure that the gates are properly aligned, prior to introducing germanium into the fin lattice structure. After aligning the gate structures, the silicon fins are segmented to elastically relax the silicon lattice. Then, germanium is introduced into the relaxed silicon lattice, to produce a SiGe channel that is substantially stress-free and also defect-free. Because stress cannot build up in a segmented fin that has a small volume, lattice defects simply do not emerge when the germanium is introduced. In this way, the concentration of germanium achieved in a structurally stable film can be increased to a level greater than 85%, producing a film that is nearly pure germanium. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    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. 
           [0012]      FIG. 1  is a flow diagram summarizing a sequence of processing steps in a method that can be used to fabricate self-aligned FinFETs having SiGe channels that are substantially free of crystalline structure defects, according to one embodiment as described herein. 
           [0013]      FIGS. 2A-10C  are different views of a FinFET array after completing each processing step of the method illustrated in  FIG. 1 .  FIGS. 2B, 3B, 4B, 5B, 6B, 8B, 9B, and 10B  are top plan views of the FinFET array.  FIGS. 2A, 3A, 4A, 5A, 6A, 8A, 9A, and 10A  are corresponding cross-sectional views of the FinFET array, cut across the fins.  FIGS. 2C, 3C, 4C, 5C, 6C, 8C, 9C, and 10C  are corresponding cross-sectional views of the FinFET array, cut along the fins. 
           [0014]      FIGS. 2A-2C  show the FinFET array following formation of three fins according to one embodiment as described herein. 
           [0015]      FIGS. 3A-3C  show the FinFET array following formation of four gate structures that wrap around three sides of each fin according to one embodiment as described herein. 
           [0016]      FIGS. 4A-4C  show the FinFET array after segmenting the fins to create channel regions having a relaxed lattice, according to one embodiment as described herein. 
           [0017]      FIGS. 5A-5C  show the FinFET array after filling spaces between the gate structures with an oxide, according to one embodiment as described herein. 
           [0018]      FIGS. 6A-6C  show the FinFET array after removal of sacrificial polysilicon gates from the gate structures, according to one embodiment as described herein. 
           [0019]      FIG. 7A  is a cross-sectional view of the FinFET array, cut across the fins, after formation of an overlying film that contains germanium, according to one embodiment as described herein. 
           [0020]      FIG. 7B  is a cross-sectional view of the FinFET array, cut across the fins, after germanium is incorporated into the fin segments, according to one embodiment as described herein. 
           [0021]      FIGS. 8A-8C  show the FinFET array after replacing the polysilicon gates with metal gates. 
           [0022]      FIGS. 9A-9C  show the FinFET array after formation of source and drain regions that include epitaxial extensions. 
           [0023]      FIGS. 10A-10C  show the FinFET array after formation of source and drain contacts. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    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. 
         [0025]    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.” 
         [0026]    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. 
         [0027]    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. 
         [0028]    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. 
         [0029]    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. 
         [0030]    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. 
         [0031]    Specific embodiments are described herein with reference to self-aligned SiGe FinFET 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. 
         [0032]    Turning now to the figures,  FIG. 1  shows an exemplary sequence of steps in a method  200  of fabricating a self-aligned SiGe FinFET, according to one embodiment. Steps  202 - 218  in the method  200  are illustrated in  FIGS. 2A-10C  and described below. In each set of Figures, B is a top plan view, A is a cross-sectional view along a fin, and C is a cross-sectional view across the fins, as indicated by cut lines shown in the top plan view. 
         [0033]    At  202 , fins  304  are formed on a silicon-on-insulator substrate, according to one embodiment as shown in  FIGS. 2A-2C . The SOI substrate includes an active layer over a buried oxide (BOX) layer  302 , on a silicon substrate  300 . The thickness of the active layer can be anywhere within a wide range of about 5-500 nm. SOI wafers are standard starting materials commonly used in the semiconductor industry. Alternatively, the BOX layer  302  can be formed on a bulk silicon wafer by growing, in a diffusion furnace, a thick oxide, typically 80-120 nm, as is well known in the art. Or, the substrate starting material can be a pre-strained SOI (sSOI) substrate or a SiGe-on-insulator (SGOI) substrate, in which the active layer is SiGe instead of silicon. 
         [0034]    The fins  304  can be patterned from the active layer in a conventional fashion using direct photolithography and etching with SiN hard mask. Alternatively, the fins  304  can be formed using, for example, a sidewall image transfer (SIT) process as described in greater detail in U.S. Patent Application Publication No. 2014/0175554, assigned to the same assignee as the present patent application. 
         [0035]    The sidewall image transfer process is capable of defining very high aspect ratio fins  304  using silicon nitride (SiN) sidewall spacers as a hard mask, instead of patterning the fins  304  using a photolithography mask. According to the sidewall image transfer technique, a mandrel, or temporary structure, is formed first, and then silicon nitride is deposited conformally over the mandrel and planarized to form sidewall spacers on the sides of the mandrel. Then the mandrel is removed, leaving behind a pair of narrow sidewall spacers that serve as a mask to create a pair of silicon fins  304 . By either method, the fins  304  extend vertically outward from a top surface of the substrate as shown in  FIG. 2C . The height of the fins  304  is desirably in the range of about 20-200 nm. In one embodiment, the fins formed on an SOI wafer are 50 nm tall. In another embodiment, fins formed on a bulk silicon wafer are 100 nm tall. The width of the fins  304  is desirably in the range of about 5-20 nm. Accordingly, corresponding aspect ratios of the fins are in the range of about 4-10. The resulting high aspect ratio fins  304  are shown in  FIGS. 2A, 2B, and 2C . 
         [0036]    At  204 , dummy gate structures  316  are formed in a transverse direction relative to the fins  304 , according to one embodiment, as shown in  FIGS. 3A-3C . Formation of the dummy gate structures  316  is part of a replacement metal gate (RMG) process well known in the art of semiconductor processing. In the RMG process, the dummy gate structures  316 , made of polysilicon, are later replaced with a permanent metal gate structure. An RMG process is described in greater detail in U.S. Patent Application Publication No. 2014/0175554. 
         [0037]    The dummy gate structures  316  include a thin gate oxide  308 , a sacrificial gate  310 , a hard mask cap  312 , and a pair of sidewall spacers  314 . First, the gate oxide  308  is conformally deposited to cover the fins  304 . The gate oxide  308  is desirably made of a 3-5 nm thick high-k gate material such as, for example, SiO 2  or HfO 2 , as is well known in the art. Next, a layer of amorphous silicon or polysilicon is deposited and patterned using a silicon nitride hard mask to form the sacrificial gates  310 . If amorphous silicon is used in the sacrificial gates  310 , the amorphous silicon material can be transformed into polysilicon by annealing at a later step. The dummy gate structures  316  are then aligned to the fins  304  such that the dummy gate structures  316  are in contact with three sides of the fins, as shown in  FIG. 3C . The sacrificial gate  310  has a height in the range of about 50-100 nm and a width in the range of about 15-25 nm, desirably about 20 nm. The hard mask used to pattern the sacrificial gate  310  is in the range of 20-100 nm thick. The dummy gate structures  316  are shown in  FIGS. 3A, 3B , and  3 C, covered by the hard mask cap  312  and sidewall spacers  314 . 
         [0038]    The sidewall spacers  314  are formed on the sacrificial gate  310  by depositing and patterning a layer of dielectric material, e.g., silicon dioxide (SiO 2 ), silicon nitride (SiN), SiBCN, silicon oxynitride (SiON), SiOCN, silicon carbonate (SiOC), or the like. In one embodiment, SiN sidewall spacers  314  are formed on the sides of the sacrificial gate  310  by atomic layer deposition (ALD). The ALD process deposits SiN conformally over the sacrificial gate  310 , and on top of the fins  304 . Following deposition, the SiN can be etched anisotropically in the usual way using an RIE process to remove SiN on the horizontal surfaces between the gate structures  316  while leaving the SiN cap  312  on top of the sacrificial gates  310  and SiN on the sidewalls of the sacrificial gates  310 . The SiN sidewall spacer thickness is desirably in the range of about 5-20 nm. 
         [0039]    At  206 , the fins  304  are segmented, according to one embodiment, as shown in  FIGS. 4A-4C . Segmentation of the fins  304  can be accomplished by conventional etching with an etching chemistry that is selective to the dielectric sidewall spacers  314  or, alternatively, by a SIT process as described above. The fin segments  313  are cut so as to be spaced at regular intervals as shown in  FIGS. 4A, 4B, and 4C , such that the dummy gate structures  316  are centered on the fin segments  313 . The fin segment lengths are in the range of about 20-100 nm, and the fin segments  313  are spaced apart by gaps  315  of about 10 nm. Accordingly, the footprint area of a fin segment  313  is in the range of about 100-2000 nm 2 . The resulting fin segments  313  have an elastically relaxed silicon lattice in which dislocation defects will not tend to accumulate. Even if the fin is made of strained silicon or SiGe, the fin may be fully relaxed following segmentation. Unstrained silicon fins will tend to remain fully relaxed. 
         [0040]    At  208 , the gaps  315  separating the fin segments  313  are filled with oxide  318  according to one embodiment, as shown in  FIGS. 5A-5C . The oxide  318  is planarized using a conventional CMP process that stops on the sacrificial gates  310 , thereby removing the hard mask caps  312 . 
         [0041]    At  210 , the sacrificial gates  310  are removed, according to one embodiment, as shown in  FIGS. 6A-6C . Removal of the sacrificial gates  310  can be accomplished using a conventional wet chemical etchant such as, for example, hot ammonia, followed by an HF treatment to remove the gate oxide  308  as well. 
         [0042]    At  212 , the silicon in segments  313  are transformed into SiGe-rich fin segments  322  using an oxidizing condensation process, according to one embodiment, as shown in  FIGS. 7A and 7B . First, a cladding  320  is deposited over the silicon fin segments  313  as shown in  FIG. 7A . Alternatively, the cladding  320  can be epitaxially grown outward from the top and the sides of the fin segments  313 . The cladding  320  can be, for example, a crystalline or amorphous form of SiGe that will serve as a source of germanium for creating the SiGe-rich fin segments  322 . Next, the cladding  320  is exposed to an oxygen-rich environment that forms GeO 2  at the surface of the silicon fin segments  313 . However, the chemical bonds of GeO 2  tend to be unstable, allowing the underlying silicon in the fin segments  313  to react with the GeO 2  to form SiGe and SiO 2 . This oxidation reaction effectively causes the germanium to condense into the silicon fin segments  313 , producing SiGe fin segments  322  that are rich in SiGe and surrounded by oxide  324 , as shown in  FIG. 7B . The resulting SiGe-rich fin segments  322  have a substantially uniform structure in which germanium atoms are incorporated throughout the silicon crystal lattice with a concentration that exceeds 85%. 
         [0043]    Alternatively, the cladding  320  can be made of pure germanium, or a III-V material such as, for example, indium phosphide (InP), indium gallium arsenide (InGaAs), or the like. 
         [0044]    More generally, fin segments  322  that incorporate a second semiconductor material can be produced by other processes such as epitaxial growth, a combination of epitaxy and the condensation process described above, or a combination of epitaxy and diffusion. Regardless of the technique used to introduce new materials into channels, the resulting fin segments  322  remain fully relaxed because stress cannot accumulate within the small volume of the small footprint, high aspect ratio fins. 
         [0045]    At  214 , a thin low-k dielectric material such as, for example, HfO 2 , and metal gates  326  are formed on the new fin segments  322  according to one embodiment as shown in  FIGS. 8A-8C . The metal gates  326  are deposited between the pairs of sidewall spacers  314  in a self-aligned process. The gate stack metals may include those typically used in metal gate processes, e.g., tungsten (W), titanium (Ti), titanium nitride (TiN), work function materials, and the like. A hard mask cap  328  is formed over the metal gates  326 . During subsequent contact formation, the oxide  318  separating the gates is recessed down to the base of the fin segments  322 . It is noted that each metal gate  326  is substantially centered over a fin segment  322 . Portions of the fin segments  322  that are located directly underneath the metal gates  326  will serve as transistor channels, while portions of the fin segments  322  outside the influence of the metal gates  326  will serve as source and drain regions of the devices. 
         [0046]    At  216 , source and drain extensions  330  are grown epitaxially on the sidewalls of the fin segments  322 . First, the SiN sidewall spacers  314  are trimmed using a wet chemical process such as, for example, phosphoric acid (H 3 PO 4 ), which will remove SiN selectively to oxide and silicon. Alternatively, an HF-EG wet etch process can be used in which hydrofluoric acid (HF) and ethylene glycol (EG) are combined to form a chemical mixture that removes both SiN and SiO 2  at substantially equal rates. The HF-EG formulation is advantageous in that it has a slow etch rate compared with HF alone that provides superior process control when etching either SiN or SiO 2 . If HF-EG is used, a small amount of the BOX, approximately 5 nm, may be eroded without substantial impact on the device integrity. 
         [0047]    Then, the source and drain extensions  330  are formed by selective epitaxy of SiGe, silicon carbide (SiC), or group V materials from sidewalls of the fin segments  322 . The source and drain extensions  330  extend outward from the ends of each fin segment  322 , expanding the fin segment at acute angles relative to a vertical axis of the fin segment  322  to produce diamond-shaped structures, as shown in  FIG. 10A . The source and drain extensions  330  may be doped in-situ with boron (B), phosphorous (P), or arsenic (As), to produce p-type or n-type devices, respectively, in the usual way. 
         [0048]    At  218 , metal contacts  332  to the source and drain regions and to the source and drain extensions  330  are formed according to one embodiment, as shown in  FIGS. 10A-10C . Formation of the metal contacts  332  uses conventional methods in which a metal liner is first deposited and reacts with the silicon to form a metal silicide. Then, a bulk contact metal is deposited in the usual way, and planarized to stop on the hard mask cap  328 . 
         [0049]    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. 
         [0050]    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. 
         [0051]    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.