Abstract:
Transistors and methods for fabricating the same include forming one or more semiconductor fins on a substrate; covering source and drain regions of the one or more semiconductor fins with a protective layer; annealing uncovered channel portions of the one or more semiconductor fins in a gaseous environment to reduce fin width and round corners of the one or more semiconductor fins; and forming a dielectric layer and gate over the thinned fins.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to transistor fabrication and, more particularly, to the fabrication of transistors with thinned channel fins. 
         [0003]    2. Description of the Related Art 
         [0004]    Limits on length of gates in fin field effect transistors (FinFETs) are imposed by fin width. As the gate length decreases, short channel current leakage increases. Because new technologies are capable of forming progressively smaller transistors, these short channel effects become significant challenges in transistor design. Forming transistors with thinner fins helps enhance gate control over the transistor channel, thereby reducing current leakage. 
         [0005]    However, uniformly decreasing the size of fins presents its own challenges. When the width of a fin is reduced, current crowding at the fin&#39;s exterior increases. This increases the effective resistance of the thinner fin, which impacts the drive current performance of the FinFET. This compromises the benefits to the subthreshold leakage control provided by having a thinner fin. 
         [0006]    Existing processes for thinning fins include oxidation. However, such thinning processes may undercut spacers around the channel region, resulting in physical faults or defects in the resulting transistors. As such, existing oxidation processes are inadequate for producing small-scale devices. 
       SUMMARY 
       [0007]    A method for fabricating a transistor includes forming one or more semiconductor fins on a substrate; covering source and drain regions of the one or more semiconductor fins with a protective layer; annealing uncovered channel portions of the one or more semiconductor fins in a gaseous environment to reduce fin width and round corners of the one or more semiconductor fins; and forming a dielectric layer and gate over the thinned fins. 
         [0008]    A method for fabricating a transistor includes forming a plurality of semiconductor fins on a substrate; merging respective source and drain regions of the plurality of semiconductor fins by epitaxially growth; covering source and drain regions of the plurality of semiconductor fins with a protective layer; annealing uncovered channel portions of the plurality of semiconductor fins in a high-temperature, low-pressure hydrogen environment to reduce fin width; forming a high-k dielectric layer on the thinned fins; and forming a metal gate structure on the high-k dielectric layer. 
         [0009]    A transistor includes one or more semiconductor fins formed on a substrate, wherein the one or more semiconductor fins are thinner in a channel region than in source and drain regions and have rounded corners formed by an anneal in a gaseous environment; a gate dielectric layer on the channel region of the one or more semiconductor fins, conforming to the contours of the one or more semiconductor fins; and a gate structure on the gate dielectric layer. 
         [0010]    These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
           [0012]      FIG. 1  is a cross-sectional view of a step in forming a locally thinned channel FinFET in accordance with the present principles; 
           [0013]      FIG. 2  is a top-down view of a step in forming a locally thinned channel FinFET in accordance with the present principles; 
           [0014]      FIG. 3  is a cross-sectional view of a step in forming a locally thinned channel FinFET in accordance with the present principles; 
           [0015]      FIG. 4  is a top-down view of a step in forming a locally thinned channel FinFET in accordance with the present principles; 
           [0016]      FIG. 5  is a cross-sectional view of a step in forming a locally thinned channel FinFET in accordance with the present principles; 
           [0017]      FIG. 6  is a block/flow diagram of a method of forming a locally thinned channel FinFET in accordance; 
           [0018]      FIG. 7  is a cross-sectional view of fins before and after annealing in accordance with the present principles; 
           [0019]      FIG. 8  is a top-down view of fins before and after annealing in accordance with the present principles; 
           [0020]      FIG. 9  is a cross-sectional view of a step in forming a locally thinned channel FinFET in accordance with the present principles; 
           [0021]      FIG. 10  is a cross-sectional view of a step in forming a locally thinned channel FinFET in accordance with the present principles; and 
           [0022]      FIG. 11  is a cross-sectional view of a step in forming a locally thinned channel FinFET in accordance with the present principles. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    The present principles provide fin field effect transistors (FinFETs) that have fins with reduced width localized to the channel region. Short channel effects in transistors can be controlled if W fin &lt;0.7 L gate , where W fin  is the width of the fin and L gate  is the length of the gate. Current crowding is a further concern, because reducing the fin width in the area of the source and drain region can cause a nonhomogeneous distribution of current density, which can lead to an increased effective resistance and shortened device lifetime. 
         [0024]    To keep fin widths low and allow for further reduction in gate length, the present principles apply a hydrogen anneal that removes material only from the channel region of the fins, preserving the width of the fins in the source/drain region. In this way, leakage current in the transistor&#39;s off state is reduced without increasing the exterior resistance of the fins in the source and drain regions. The fins are epitaxially grown in the source/drain regions to bring them into contact. The thinning provided by the present principles is compatible with standard replacement gate FinFET process flows, with no additional maskset being needed. 
         [0025]    It is to be understood that the present invention will be described in terms of a given illustrative architecture having a wafer; however, other architectures, structures, substrate materials and process features and steps may be varied within the scope of the present invention. 
         [0026]    It will also be understood that when an element such as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
         [0027]    A design for an integrated circuit chip of photovoltaic device may be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer may transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
         [0028]    Methods as described herein may be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
         [0029]    Reference in the specification to “one embodiment” or “an embodiment” of the present principles, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment of the present principles. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
         [0030]    It is to be appreciated that the use of any of the following “/”, “and/or”, and “at least one of”, for example, in the cases of “A/B”, “A and/or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and/or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This may be extended, as readily apparent by one of ordinary skill in this and related arts, for as many items listed. 
         [0031]    Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG. 1 , a step in forming a locally thinned channel FinFET is shown. This view shows a cut-away perspective along fins  104 . Fins  104  are formed on a substrate  102 . The substrate  102  may be formed from a bulk semiconductor, such as silicon or germanium. The substrate  102  may also be formed from alternative semiconductor materials such as a silicon-germanium composite, silicon carbide, or a III-V composite semiconductor such as gallium arsenide or indium arsenide. The substrate  102  may be a semiconductor on insulator substrate, where a semiconductor layer is formed on top of a dielectric material such as, e.g., silicon dioxide. In the present embodiments, a semiconductor-on-insulator structure is shown, with a buried oxide layer  102  supporting semiconductor fins  104 . A top dielectric layer  106  is formed over the fins  104  and may include, e.g., silicon dioxide or any other appropriate insulating material. The top dielectric layer  106  is, e.g., a flowable oxide layer that serves to protect the fins  104  during a gate replacement process. 
         [0032]    The fins  104  may be formed from any appropriate fabrication process. In particular, it is contemplated that the fins  104  may be etched from a layer of semiconductor using a lithographic or reverse ion etching process. Those having ordinary skill in the art will be able to select an appropriate mask and etch according to the particular materials used. 
         [0033]    Spacers  110  separate a region of fins  104  that will form the channel. The spacers  110  may be formed from an appropriate hardmask material such as, e.g., silicon nitride. A dummy gate structure  108  is formed over the fins  104  in the channel region to protect them to this stage and may be formed from any appropriate material including, e.g., polysilicon. 
         [0034]    Referring now to  FIG. 2 , a step in forming a locally thinned channel FinFET is shown. This view shows a top-down perspective on the fins  104  with the top dielectric layer  106  removed for the sake of illustration. The fins  104  are epitaxially grown to form bridging connections  202 , thereby merging the fins  104  in the source/drain region. The epitaxial growth causes each fin  104  to expand from its sides until the fins  104  come into contact. 
         [0035]    Referring now to  FIG. 3 , a step in forming a locally thinned channel FinFET is shown. The dummy gate  108  is removed using, e.g., a wet etch or polysilicon chemical mechanical polish, exposing the channel region  302  of the fins  104 . If the fins  104  had caps, those caps may be removed using any appropriate process including, e.g., a chemical oxide removal process. The top dielectric layer  106  remains over the fins  104  in the source/drain regions, protecting these portions of the fins  104 . At this point, an anneal may be performed that thins the fins  104  only in the channel region  302 . The anneal may be performed using, e.g., hydrogen (H 2 ) in a high-temperature, low-pressure environment. The anneal causes fin material in the channel region  302  to be removed. In an embodiment where the fins  104  are formed from silicon and a hydrogen anneal is used, the chemical reaction is either Si+H 2 →SiH 2  or Si+2H 2 →SiH 4 . The hydrogen anneal produces superior results to fin thinning produced by, e.g., oxidation processes, which would undercut the spacers  110  and separate them from the substrate  102 . It should be recognized that this anneal will also work on non-silicon semiconductor materials such as germanium or III-V composite semiconductors. 
         [0036]    In an exemplary anneal, hydrogen gas is applied at a temperature of about 800 degrees Celsius at a pressure of 10 Torr for 300 seconds. As will be described in greater detail below, the quality of a given set of anneal parameters may be quantified as the line edge roughness (LER) of the fins  104  in the channel region  302 . Fins  104  in a realistic embodiment will not be truly straight—the anneal helps to smooth the fins  104  and decrease LER. Anneals performed at temperatures that are too high (e.g., 850 degrees Celsius) cause too much reflow in the material of the fins  104  and may cause the LER to increase again. 
         [0037]    It should be noted that an anneal under these conditions may cause longer fins to migrate. When fins  104  migrate, the fins  104  may physically break away from their place on substrate  102  and merge with a neighboring fin  104 . This concern is less prominent in the present embodiments due to the short length of the fins of the channel region  302 . 
         [0038]    Referring now to  FIG. 4 , a step in forming a locally thinned channel FinFET is shown. Again, the top dielectric layer  106  is not shown in this top-down view to illustrate the thinning of the fins  104 . In particular, the anneal removes material only from the fins in the channel region, producing thinned channel fins  402 , without the epitaxially grown semiconductor material  202  to connect them. The fins  402  may be thinned to about 10 nm to about 8 nm. Thinning to smaller widths than this can impede functionality, as the threshold voltage of a transistor will increase due to quantum effects and carrier mobility penalties when the fins  402  are about 5 nm thick. 
         [0039]    Referring now to  FIG. 5 , a step in forming a locally thinned channel FinFET is shown. A dielectric layer  504  is deposited in the channel region  302  conforming to the contours of thinned fins  402 . The dielectric layer  504  may be formed from any appropriate dielectric material including, e.g., a high-k dielectric such as hafnium dioxide, hafnium silicate, zirconium dioxide, or zirconium silicate. The dielectric layer  504  may be formed using any appropriate deposition process such as, e.g., chemical vapor deposition (CVD), atomic layer deposition (ALD), or physical vapor deposition (PVD). A gate  502  is formed over the dielectric layer  504 . It is specifically contemplated that the gate  502  may be a metal gate formed from, e.g., tantalum, tantalum nitride, niobium, etc. 
         [0040]    Referring now to  FIG. 6 , a method for forming a locally thinned channel FinFET is shown. As shown above, a standard FinFET process may be used to produce fins in block  602 , where the source/drain regions of the fins  104  are protected by a top dielectric layer  106  and the channel region  302  of the fins  104  is covered by a dummy gate  108 . Block  604  removes the dummy gate  108  to uncover the channel region  302  of the fins  104 . 
         [0041]    As described above, block  606  performs an anneal using, e.g., hydrogen gas, to remove material from the fins  104  in the channel region  302 . It is specifically contemplated that the anneal may be performed at 800 degrees Celsius and a pressure of 10 Torr. Those having ordinary skill in the art will be able to select an appropriate duration for the anneal according to the size and thickness of the fins  104 . After the anneal, thinned fins  402  are covered by a gate stack in block  608  by applying a dielectric layer  504  and a gate material  502 . 
         [0042]    The present principles enhance the shape of the fins  104  in multiple ways. The hydrogen anneal described above improves uniformity in the profile of the fins  104  and also improves the morphology along the direction of the fins  104 , described above as the LER. 
         [0043]    Referring now to  FIG. 7 , a cross-section of two fins is shown, pre- and post-anneal. A first fin,  702 , shows an exemplary fin before anneal, where the chemical etch that produced the fin  702  left irregularities in the fin profile. The anneal reduces the irregularities, producing an improved fin profile  704 . The anneal also reduces the overall fin width in proportion to the anneal&#39;s duration. Suboptimal fin profiles lead to increased irregularities along the sides of the fins. In particular, it should be noted that the anneal has removed the concavities of the fin profile in  702 , such that the resulting fin profile is entirely convex. 
         [0044]    In one experimental trial, initial fin width was measured at the top, middle, and bottom with widths of 9.231 nm, 8.951 nm, and 10.91 nm respectively, at a height of 27.97 nm and having relatively sharp corners. As shown in fin  702 , the top and bottom measurements were greater than the middle width, producing a concave cross-section, which is not optimal. After annealing, the fin had measurements of 6.373 nm, 6.918 nm, and 7.397 nm respectively, with a height of 26.81 nm and rounded corners. In addition to an overall reduction in width, this produces the superior profile shown as fin  704  with better electrical properties than the original fin  702 . Notably, this profile is most effectively formed in a low-pressure environment. 
         [0045]    Experiments with high-pressure environments have resulted in a fin profile that bulges in the middle, with a middle width greater than the width of the top and bottom. In one exemplary test, a pressure of 600 Torr was used, resulting in widths of 7.84 nm, 8.64 nm, and 8.25 nm respectively. Similarly, anneals at temperatures higher than about those discussed above (e.g., at 850 degrees Celsius) result in similar bulges in the middle width measurement post-anneal. 
         [0046]    Referring now to  FIG. 8 , a top-down view of two fins is shown, pre- and post-anneal. A first fin  802  shows an exemplary fin before anneal, where the formation of the fin  802  produced surface irregularities. The irregularities are shown here as “wavy” sides and may be quantified by a measure of the LER, where LER may be defined as the root-mean-squared of the fin&#39;s edge. After the anneal, the thinned fins  804  exhibit decreased LER because convex portions of the fins  802  are preferentially etched in the anneal. Rough surfaces may impede carrier transport and thereby increase the resistance of the channel fins  402 . 
         [0047]    Referring now to  FIG. 9 , an alternative embodiment of a step in forming a locally thinned channel FinFET is shown. In this embodiment, an oxide capping layer  902  is left on the fins  104 . The presence of a capping layer  902  may help prevent damage to the fins  104  during the anneal. The hydrogen anneal may be performed with the oxide cap  902  remaining on the fins  104 , such that the oxide cap  902  is etched and thinned along with the fins in the channel region  302 . 
         [0048]    Referring now to  FIG. 10 , an optional step in forming a locally thinned channel FinFET is shown. After the removal of dummy gate  108 , the substrate layer  102  may be etched to expose the underside of the channel region  302  of the fins  104 . This allows the formation of nanowires from the fins  104  by rounding the rough edges after the hydrogen anneal. The etch of the fins  104  into nanowires may be performed by any appropriate process including, e.g., a wet etch that preferentially etches and softens the corners of the fins  104 . 
         [0049]    Referring now to  FIG. 11 , an optional step in forming a locally thinned channel FinFET is shown. After removal of the dummy gate  108 , additional liner material is deposited and etched from the channel region  302  to increase the width of spacers  1102 . The additional liner material should match the material of spacers  110  and thus may be, for example, formed from silicon nitride. 
         [0050]    Increasing the width of spacers  1102  helps provide additional protection for fins  104  in the source and drain regions by preventing hydrogen gas from seeping inward during the anneal process. Allowing the hydrogen to etch into the source and drain regions could result in thinning of the fins  104  in these regions and cause dopant loss. The loss of dopant in these regions would mean a higher resistance and decreased functionality for the finished device. 
         [0051]    Having described preferred embodiments of localized fin width scaling using a hydrogen anneal (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.