Abstract:
A method of forming semiconductor fins includes forming a plurality of sacrificial template fins from a first semiconductor material; epitaxially growing fins of a second semiconductor material on exposed sidewall surfaces of the sacrificial template fins; and removing the plurality of sacrificial template fins.

Description:
BACKGROUND 
     The present invention relates generally to semiconductor device manufacturing and, more particularly, to forming epitaxial silicon germanium (SiGe) and fins for finFET devices using sacrificial silicon fin templates. 
     Field effect transistors (FETs) are widely used in the electronics industry for switching, amplification, filtering, and other tasks related to both analog and digital electrical signals. Most common among these are metal-oxide-semiconductor field-effect transistors (MOSFET or MOS), in which a gate structure is energized to create an electric field in an underlying channel region of a semiconductor body, by which electrons are allowed to travel through the channel between a source region and a drain region of the semiconductor body. Complementary MOS (CMOS) devices have become widely used in the semiconductor industry, wherein both n-type and p-type transistors (NFET and PFET) are used to fabricate logic and other circuitry. 
     The source and drain regions of an FET are typically formed by adding dopants to targeted regions of a semiconductor body on either side of the channel. A gate structure is formed above the channel, which includes a gate dielectric located over the channel and a gate conductor above the gate dielectric. The gate dielectric is an insulator material, which prevents large leakage currents from flowing into the channel when a voltage is applied to the gate conductor, while allowing the applied gate voltage to set up a transverse electric field in the channel region in a controllable manner. Conventional MOS transistors typically include a gate dielectric formed by depositing or by growing silicon dioxide (SiO 2 ) or silicon oxynitride (SiON) over a silicon wafer surface, with doped polysilicon formed over the SiO 2  to act as the gate conductor. 
     The escalating demands for high density and performance associated with ultra large scale integrated (ULSI) circuit devices have required certain design features, such as shrinking gate lengths, high reliability and increased manufacturing throughput. The continued reduction of design features has challenged the limitations of conventional fabrication techniques. 
     For example, when the gate length of conventional planar metal oxide semiconductor field effect transistors (MOSFETs) is scaled below 100 nm, problems associated with short channel effects (e.g., excessive leakage between the source and drain regions) become increasingly difficult to overcome. In addition, mobility degradation and a number of process issues also make it difficult to scale conventional MOSFETs to include increasingly smaller device features. New device structures are therefore being explored to improve FET performance and allow further device scaling. 
     Double-gate MOSFETs represent one type of structure that has been considered as a candidate for succeeding existing planar MOSFETs. In double-gate MOSFETs, two gates may be used to control short channel effects. A finFET is a double-gate structure that exhibits good short channel behavior, and includes a channel formed in a vertical fin. The finFET structure may be fabricated using layout and process techniques similar to those used for conventional planar MOSFETs. 
     SUMMARY 
     In one aspect, a method of forming semiconductor fins includes forming a plurality of sacrificial template fins from a first semiconductor material; epitaxially growing fins of a second semiconductor material on exposed sidewall surfaces of the sacrificial template fins; and removing the plurality of sacrificial template fins. 
     In another aspect, a method of forming semiconductor fins includes forming a plurality of sacrificial silicon template fins from a silicon layer; epitaxially growing silicon germanium (SiGe) fins on exposed sidewall surfaces of the sacrificial silicon template fins; and removing the plurality of sacrificial silicon template fins. 
     In another aspect, an intermediate semiconductor structure includes a plurality of sacrificial silicon template fins formed from a silicon layer; a hardmask layer on top surfaces of the sacrificial silicon template fins; epitaxial silicon germanium (SiGe) fins grown on exposed sidewall surfaces of a lower portion of the sacrificial silicon template fins; and sidewall spacers disposed over the epitaxial SiGe fins, and adjacent sidewall surfaces of an upper portion of the sacrificial silicon template fins and the hardmask layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIGS. 1 through 11  are a series of cross sectional views of an exemplary embodiment of a method of forming epitaxially grown fins for finFET transistor devices, in accordance with an exemplary embodiment, in which: 
         FIG. 1  illustrates the formation of template fins patterned from a silicon-on-insulator substrate; 
         FIG. 2  illustrates the formation and recessing of a protective, flowable oxide layer the covers lower portions of the silicon template fins; 
         FIG. 3  illustrates the formation of sidewall spacers adjacent upper portions of the silicon template fins; 
         FIG. 4  illustrates the removal of the flowable oxide layer to expose lower portions of the silicon template fins; 
         FIG. 5  illustrates the formation of SiGe fins epitaxially grown on exposed sidewall surfaces of the Si template fins; 
         FIG. 6  illustrates the removal of a hardmask layer from atop the Si template fins; 
         FIG. 7  illustrates a dry etching process to remove the exposed Si template fins; 
         FIG. 8  illustrates the removal of the Si template fins; 
         FIG. 9  illustrates the removal of the sidewall spacers atop the epitaxially grown SiGe fins; 
         FIG. 10  illustrates an exemplary gate formation process on the SiGe fins; 
         FIG. 11  illustrates a perspective view of one of the fins shown in  FIG. 10 ; and 
         FIG. 12  illustrates the formation of template fins patterned from a bulk semiconductor substrate, in accordance with an alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Silicon germanium (SiGe) is a promising channel material for FET devices because of its high carrier mobility. In particular, a compressively strained SiGe material provides superior hole mobility as the majority carrier in PFET devices, whether the devices are of a planar geometry or a fin geometry. The epitaxial growth of a SiGe layer on a silicon (Si) substrate, followed by a patterning operation on the SiGe layer (e.g., either by lithography or a spacer image transfer process) is a conventional process for forming the SiGe fins. However, the direct epitaxial growth of a SiGe layer on Si a substrate has a critical thickness limit, which limit also decreases as the concentration of germanium in the SiGe layer increases. Above this critical thickness, the crystal structure of the SiGe becomes defective, and therefore not good for a device channel material. As a result, there is a practical limit for the height of a SiGe fin that may be formed in this matter. 
     Accordingly, disclosed herein is a method of forming SiGe fins for semiconductor devices in which (in lieu of patterning an epitaxially grown SiGe layer) the SiGe fins are grown from sidewall surfaces of sacrificial silicon template fins. The template fins may be formed at a height that exceeds a desired height of the SiGe fins, such that the once the template fins are removed the remaining SiGe fins have the desired height, and with the desired carrier mobility properties and Ge concentration. 
     Referring generally now to  FIGS. 1 through 11 , there is shown a series of cross sectional views of a method of forming epitaxially grown fins for finFET transistor devices, in accordance with an exemplary embodiment. As shown in  FIG. 1 , a starting semiconductor structure  100  includes a bulk semiconductor layer  102 , a buried insulator layer, or more specifically a buried oxide (BOX) layer  104  formed on the bulk semiconductor layer  102 , and a plurality of sacrificial template fins  106  patterned from a semiconductor-on-insulator layer, or more specifically a silicon-on-insulator (SOI) layer formed on BOX layer  104 . As is known in the art, the bulk semiconductor layer  102  may include a material such as, for example, silicon, germanium, silicon-germanium alloy, silicon carbon alloy, silicon-germanium-carbon alloy, gallium arsenide, indium arsenide, indium phosphide, III-V compound semiconductor materials, II-VI compound semiconductor materials, organic semiconductor materials, and other compound semiconductor materials. 
     The patterning of the sacrificial template fins  106  may be implemented in any suitable manner known in the art, such as by lithographic patterning of a hardmask layer  108 , and etching the pattern through the SOI layer down to the BOX layer  104 , thereby defining the fins  106 . Although the hardmask layer  108  atop the template fins  106  may be any suitable hardmask material, it is preferable that the hardmask layer have an etch selectivity to oxide. Thus, an amorphous carbon material is one such suitable example for the hardmask layer  108 . 
     The initial thickness of the SOI layer should exceed a final desired height of the epitaxially grown SiGe fins, for reasons that will become apparent hereinafter. In one exemplary embodiment, the SOI layer thickness (and hence the height of the sacrificial template fins  106 ) exceeds 30 nanometers (nm). In addition, the initial pitch (i.e., spacing between adjacent template fins  106 ) is on the order of about 120 nm, or less. It will also be appreciated, however, that the figures are illustrative only, and that the features shown therein are not necessarily depicted to scale. 
     Referring now to  FIG. 2 , a protective layer  110  is deposited over the structure and thereafter recessed to expose upper portions of the sacrificial silicon template fins  106 , temporarily covering lower portions of the template fins  106 . In one exemplary embodiment, the protective layer  110  is a flowable oxide (FOX) material that is deposited and recessed. Other materials are also contemplated, however, so long as the protective layer  110  has an etch selectivity with respect to the hardmask layer  108 . 
     As then shown in  FIG. 3 , sidewall spacers  112  are formed on sidewalls of the hardmask layer  108  and the exposed upper portions of the sacrificial silicon template fins  106 . This may be carried out by, for example, conformally depositing a nitride layer over the top of the protective layer  110 , upper portions of the sacrificial silicon template fins  106  and the hardmask layer  108 , followed by directional (anisotropic) etching to result in the sidewall spacers  112 . In  FIG. 4 , once the sidewall spacers  112  are formed to cover the upper portions of the sacrificial silicon template fins  106 , the protective layer  110  is then removed. Where the protective layer  110  is a flowable oxide layer, a suitable etch is performed to remove the protective layer  110  such that the sidewall spacers  112  and hardmask layer  108  remain substantially intact. 
     Referring now to  FIG. 5 , SiGe fins  114  are epitaxially grown on exposed sidewall surfaces of the lower portion of the sacrificial silicon template fins  106 . The height of the SiGe fins corresponds to the height of the lower portion of the template fins  106  uncovered by the sidewall spacers  112 . This height may be on the order of about 20 nm to about 60 nm, and in one exemplary embodiment, about 30 nm. Again, by using the silicon fins  106  as a growth template instead of growing an entire layer of SiGe on a planar silicon substrate and subsequent patterning/etching, higher SiGe fins may be formed with the desired compressive strain and carrier mobility performance. The Ge content, x, in Si 1-x Ge x  may range from about 0.1 to about 0.9, and more particularly, from about 0.2 to about 0.6. 
     As will be noted, due to the use of the silicon template fins  106  to grow the SiGe fins  114 , the resulting pitch of the SiGe fins  114  is double that of the template fins  106 . That is, where the silicon template fins  106  are formed at an exemplary pitch where the spacing between adjacent template fins is about 120 nm, then the spacing between adjacent SiGe fins may be on the order of 60 nm or less. 
       FIG. 6  illustrates the removal of the hardmask layer  108  from atop the Si template fins  106 . This exposes the sacrificial Si template fins  106  for removal, while the spacers  112  are still temporarily left in place. A dry etch process, such as a reactive ion etch (RIE) indicated by the arrows in  FIG. 7 , is used to remove the Si template fins  106 . As such an RIE process may otherwise attack the SiGe fins  114 , the nitride spacers  112  protect the integrity of the SiGe fins  114  until the sacrificial Si template fins  106  are completely removed, as shown in  FIG. 8 . The nitride spacers  112  are thereafter removed, leaving the epitaxially grown SiGe fins  114 , as shown in  FIG. 9 . 
     At this point, additional processing is performed in accordance with FET device techniques including, for example: dummy gate stack formation in the case of replacement gate FET devices (e.g., gate oxide deposition, amorphous or polysilicon deposition, hardmask deposition, lithography and gate patterning), spacer formation (e.g., silicon nitride, oxide), epitaxial source/drain fin merging, source/drain formation (implantation/anneal), ILD formation, and dummy gate removal. As such processing operations are known to those skilled in the art, the details thereof are omitted herein. However, by way of one specific (but non-limiting) example,  FIG. 10  illustrates an exemplary gate formation process on the SiGe fins  114 , with  FIG. 11  further illustrating a perspective view of one of the fins  114  shown in  FIG. 10 . 
     In the example depicted, a high-k metal gate stack is formed over the SiGe fins  114 , including one or more high-k dielectric layers  116  and one or more metal workfunction and metal gate layers  118 . Specific examples of high-k dielectric materials include, but are not limited to: HfO 2 , ZrO 2 , La 2 O 3 , Al 2 O 3 , TiO 2 , SrTiO 3 , LaAlO 3 , Y 2 O 3 , HfO x N y , ZrO x N y , La 2 O x N y , Al 2 O x N y , TiO x N y , SrTiO x N y , LaAlO x N y , Y 2 O x N y , a silicate thereof, and an alloy thereof. Each value of x is independently from 0.5 to 3 and each value of y is independently from 0 to 2. The thickness of the high-k dielectric layer  118  may be from about 1 nm to about 10 nm, and more specifically from about 1.5 nm to about 3 nm. The one or more metal workfunction and metal gate layers  118  may include, for example, a wetting titanium nitride deposition layer, and one or more of aluminum, titanium-doped aluminum, tungsten or copper. 
     From this point, conventional processing as known in the art may continue including, for example, chemical mechanical polishing (CMP) of the gate metal layers, silicide contact formation for gate, source and drain terminals, upper level wiring formation, etc. It should be appreciated that although the above described embodiments are presented in terms of an SOI substrate, the techniques are also equally applicable to bulk semiconductor substrates. For example,  FIG. 12  illustrates a starting semiconductor structure  100 ′ in accordance with an alternative embodiment, and includes a bulk semiconductor substrate  102 ′, a plurality of shallow trench isolation regions  104 ′ formed in the bulk semiconductor substrate  102 ′, and a plurality of sacrificial template fins  106  with hardmask layer  108  patterned from the bulk semiconductor material. Again, the bulk semiconductor substrate  102 ′ may include a material such as, for example, silicon, germanium, silicon-germanium alloy, silicon carbon alloy, silicon-germanium-carbon alloy, gallium arsenide, indium arsenide, indium phosphide, III-V compound semiconductor materials, II-VI compound semiconductor materials, organic semiconductor materials, and other compound semiconductor materials. Once the sacrificial fins are formed, the same processing operations may be performed as described in conjunction with  FIGS. 2-11 . 
     It will also be appreciated that the above described process of epitaxially growing SiGe fins on sidewall surfaces of sacrificial template Si fins provides the capability of forming taller SiGe fins above a critical thickness limit that results from growing a SiGe layer on a planar Si substrate, and then patterning the SiGe layer to form the fins. 
     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.