Patent Publication Number: US-8975697-B2

Title: Integrated circuit having MOSFET with embedded stressor and method to fabricate same

Description:
CROSS-REFERENCE TO A RELATED PATENT APPLICATION 
     This patent application is a continuation patent application of copending U.S. patent application Ser. No. 13/900,142, filed May 22, 2013, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The exemplary embodiments of this invention relate generally to semiconductor devices such as metal oxide semiconductor field effect transistors (MOSFETS) and fabrication techniques for same and, more specifically, relate to the fabrication of semiconductor transistor devices where a stressor layer or structure is employed in order to enhance charge carrier (electron or hole) mobility. Such semiconductor transistor devices can be used in, for example, random access memory (RAM) and logic circuitry. 
     BACKGROUND 
     In silicon on insulator (SOI) technology a thin semiconductor layer is formed over an insulating layer, such as silicon oxide, which in turn is formed over a bulk substrate. This insulating layer is often referred to as a buried oxide (BOX) layer or simply as a BOX. For a single BOX SOI wafer the thin semiconductor layer can be divided into active regions by shallow trench isolation (STI) which intersects the BOX and provides total isolation for active device regions formed in the semiconductor layer. Sources and drains of field effect transistors (FETs) are formed, for example, by ion implantation of N-type and P-type dopant material into the thin semiconductor layer and/or by the formation of raised source/drain (RSD) structures. A channel region between a source/drain (S/D) pair can be created so as to underlie a gate structure using, for example, a fin that is defined in the semiconductor layer when a FinFET device is being fabricated. 
     A strained semiconductor layer can be used to enhance the performance of integrated circuits. Charge carrier mobility enhancement results from a combination of reduced effective carrier mass and reduced phonon scattering. In an n-channel MOS field effect FET with a silicon channel improved performance can be achieved with induced biaxial tensile stress in a silicon layer along both width and length axes of an active area or with uniaxial tensile stress along the length axes. In a p-channel MOSFET improved performance can be achieved with induced uniaxial tensile stress in the silicon layer along the width axis only (transverse tensile stress). The p-channel MOSFET can also show enhanced performance with induced uniaxial compressive stress in the top silicon layer along the length axis only (longitudinal compressive stress). Compressive stress can be provided selectively in a silicon surface layer, for example, by using selective epitaxial SiGe stressors in the source and drain regions of a p-channel MOSFET to induce a desired compressive stress along the length axis (longitudinal). Similarly, tensile strain can be provided, for example, by using selective epitaxial Si:C stressors in the source and drain regions of an n-channel MOSFET. 
     SUMMARY 
     In a first non-limiting aspect thereof the embodiments of this invention provide a method that comprises forming a recess into a crystalline semiconductor substrate, the recess being disposed beneath and surrounding a channel region of a transistor; depositing a layer of crystalline dielectric material onto a surface of the crystalline semiconductor substrate that is exposed within the recess; and depositing stressor material into the recess such that the layer of crystalline dielectric material is disposed between the stressor material and the surface of the crystalline semiconductor substrate. 
     In another non-limiting aspect thereof the embodiments of this invention provide a structure that comprises a transistor gate stack or gate stack precursor disposed on a semiconductor-on-insulator (SOI) layer that in turn is disposed upon a buried oxide layer disposed upon a surface of a crystalline semiconductor substrate, where a transistor channel is disposed within the SOI layer. The structure further comprises a channel stressor layer disposed at least partially within a recess formed in the crystalline semiconductor substrate and disposed about the channel, and a layer of crystalline dielectric material disposed between the channel stressor layer and a surface of the crystalline semiconductor substrate. 
     In still another non-limiting aspect thereof the embodiments of this invention provide a method that comprises forming a recess into a crystalline semiconductor substrate, the recess being disposed beneath and surrounding a channel region of a transistor. The method further comprises depositing a layer of crystalline dielectric material onto a surface of the crystalline semiconductor substrate that is exposed within the recess. The layer of crystalline dielectric material is comprised of at least one of a rare earth oxide or a combination of rare earth oxides, a Perovskite or an aluminum oxide or an aluminum oxide compound; and. The method further comprises depositing channel region stressor material into the recess such that the layer of crystalline dielectric material is disposed between the stressor material and the surface of the crystalline semiconductor substrate. The stressor material is deposited so as to at least surround the channel region and is comprised of at least one of silicon, germanium, silicon germanium, carbon doped silicon, a silicon-germanium alloy and a compound semiconductor material. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIGS. 1A-1J , collectively referred to as  FIG. 1 , illustrate a first embodiment of this invention to provide an epitaxial dielectric material to isolate a stressor from a substrate. 
         FIGS. 2A-2G , collectively referred to as  FIG. 2 , illustrate a second embodiment of this invention to provide an epitaxial dielectric material to isolate a stressor from a substrate. 
         FIGS. 3A-3G , collectively referred to as  FIG. 3 , illustrate a still further embodiment of this invention to provide an epitaxial dielectric material to isolate a stressor from a substrate. 
     
    
    
     DETAILED DESCRIPTION 
     Thin channel (e.g., extremely thin SOI or ETSOI) fully depleted MOSFETs are considered as one of the most promising candidates for scaling in 14 nm geometry technologies and beyond. However, the use of embedded stressors is not straightforward with such structures. 
     When a thin BOX is used it may be possible to recess the BOX and the substrate in the S/D area and form embedded stressors. In this approach an undoped or oppositely doped epitaxy layer can be used at a bottom portion of the embedded stressor to minimize a penalty related to a short-channel effect and junction capacitance. In this approach the source and drain can be isolated from the substrate with a p-n junction. 
     One advantage of using thin BOX SOI devices is that there is provided an opportunity to use a back gate and back bias to adjust the voltage threshold (Vt) of the FET. However, the presence of the p-n junction isolation limits a range of voltages that can be applied to the back gate. 
     Clearly a FET device structure having an embedded stressor that is isolated from the substrate without requiring a voltage-sensitive p-n junction would be desirable. 
     Non-limiting aspects of the embodiments of this invention use an epitaxial oxide layer to isolate an embedded stressor from the underlying substrate. 
     It should be noted that while the embodiments of  FIGS. 1 ,  2  and  3  will be described in the context of a gate-first type of processing the embodiments could also be realized using a replacement gate (replacement metal gate or RMG) process, where a sacrificial dummy gate structure or plug is formed and subsequently removed and replaced with a desired gate dielectric and gate conductor/metal. 
     It should be further realized that while the embodiments of  FIGS. 1 ,  2  and  3  will be described in the context of ETSOI structures at least certain aspects of this invention can be employed as well when using FinFET structures, bulk silicon or silicon-containing substrates. 
       FIGS. 1A-1J  illustrate in enlarged cross-section (not to scale) a first embodiment of this invention to provide an epitaxial dielectric material to isolate a stressor from a substrate. 
       FIG. 1A  shows a portion of a semiconductor wafer  10  having a substrate  12 , a BOX layer  14  and an overlying semiconductor-on-insulator (SOI) layer  16 . What will become a transistor (a MOSFET) active area is delineated by trench isolation structures  18 . The substrate  12  can have any desired thickness and can contain silicon or any crystalline semiconductor. The BOX  14  can have a thickness in a range of about 10 nm to about 30 nm, with 20 nm being a suitable value. The BOX  14  may thus be characterized for convenience as being a ‘thin’ BOX layer. The SOI layer  16  can have a thickness in a range of about 2 nm to about 10 nm and may thus be characterized as being an ETSOI layer. The trench isolation structures  18  can be any suitable dielectric material such as an oxide or a nitride and can extend from the top surface of the SOI  16  into the substrate  12  for about 30 nm or deeper, with 200 nm to about 300 nm being one suitable range of dimensions. In general the depth of the trench isolation  18  is made to be deeper into the substrate  12  than the overall depth of the later fabricated structures (e.g., deeper than ETSOI layer  16  plus thin BOX layer  14  plus a subsequently formed substrate recess  12 B shown in  FIG. 1E  into which an epitaxial dielectric layer  30  and stressor material  32  are grown.) A backgate may be subsequently formed in the substrate  12 , such as by implanting a desired dopant species, between the trench isolation structures  18 . 
     It is pointed out that all thicknesses and dimensions and ranges of dimensions stated herein are non-limiting examples of suitable thicknesses and dimensions and ranges of dimensions. 
       FIG. 1B  shows the result of the formation on the SOI  16  of a gate structure also referred to as a stack  20  containing a gate dielectric  22 , an overlying gate electrode  24  and dielectric spacers  26 . Source/drain (SD) extension regions (not shown) could be formed by implanting and/or diffusing suitable dopant species into the SOI  16  at least partially beneath the gate stack  20 . As was noted above the embodiments of this invention could also be used with a RMG process wherein the gate structure would actually at this point be a gate stack precursor structure (e.g., a dummy gate plug). 
     As a non-limiting example the gate dielectric  22  can be formed as a layer of high dielectric constant (high-k) material comprising a dielectric metal oxide and having a dielectric constant that is greater than the dielectric constant of silicon nitride of 7.5. The high-k dielectric layer  36  be formed by methods well known in the art including, for example, chemical vapor deposition (CVD), atomic layer deposition (ALD), molecular beam deposition (MBD), pulsed laser deposition (PLD), liquid source misted chemical deposition (LSMCD), etc. The dielectric metal oxide comprises a metal and oxygen, and optionally nitrogen and/or silicon. Exemplary high-k dielectric materials include 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  36  may be from 1 nm to 10 nm, and more preferably from about 1.5 nm to about 3 nm. The high-k dielectric layer  22  can have an effective oxide thickness (EOT) on the order of, or less than, about 1 nm. The gate metal  24  can be deposited directly on the top surface of the high-k dielectric layer  22  by, for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD). As non-limiting examples the gate metal  24  can include a metal system selected from one or more of TiN, TiC, TaN, TaC, TaSiN, HfN, W, Al and Ru, and can be selected at least in part based on the desired work function (WF) of the device (NFET or PFET), as is known. 
       FIG. 1C  shows, in accordance with the first embodiment of this invention, a result of recessing the SOI  16  and the BOX  14  in what will be a S/D region between the trench isolation  18 . The recessing of the SOI  16  and the BOX  14  can be achieved by using, for example, one or more reactive ion etch (RIE) processes having a chemistry or chemistries selected for removing the SOI  16  and the BOX  14 . This process basically exposes the top surface  12 A of the substrate  12  over an area that surrounds the gate stack  20  and the underlying portion of the SOI  16  and BOX  14 . 
       FIG. 1D  shows the formation of a spacer  28 , e.g., a silicon nitride or a silicon oxide spacer, on sidewalls of the spacer  26 , gate dielectric  22 , SOI  16  and BOX  14 . The spacer  28  is formed to protect during subsequent processing the sidewalls of a channel that will exist in the SOI  16  and can have any desired thickness (e.g., about 3 nm to about 10 or more nm). A portion of the spacer  28  will be subsequently removed as shown in  FIG. 1I . 
       FIG. 1E  shows a result of recessing the substrate  12  by RIE or some other suitable process to form substrate recess  12 B. The substrate  12  is recessed to a depth that will accommodate a subsequently formed dielectric layer  30  ( FIG. 1F ) and subsequently deposited embedded stressor material  32  ( FIG. 1G ). 
       FIG. 1F  shows the formation of the dielectric layer  30  by blanket deposition. The dielectric layer  30  can be characterized as being a crystalline (epitaxial) layer  30 A where it contacts and overlies the crystalline semiconductor substrate  12 , and it can be characterized as being an amorphous layer  30 B where it contacts and overlies the amorphous dielectric material of the trench isolation  18  and the spacer  28 . The dielectric layer  30  may have a thickness in a range of, for example, about 2 nm to about 10 nm, with a thickness of approximately 5 nm being suitable for many embodiments. In general the characteristic of the dielectric layer  30  (amorphous or crystalline) assumes the characteristic of the material upon which it is deposited. The use of the crystalline dielectric layer  30 A is preferred due to the subsequent growth (e.g., see  FIG. 1G ) of crystalline stressor material  32  upon the crystalline dielectric layer  30 A. 
     In an embodiment, the crystalline dielectric layer  30 A may be formed by epitaxial growth on top of an underlying crystalline layer. The crystalline dielectric layer  30 A may be formed of an epitaxial oxide grown on the semiconductor substrate  12  and may include a rare earth oxide (e.g., cerium oxide (CeO 2 ), lanthanum oxide (La 2 O 3 ), yttrium oxide (Y 2 O 3 ), gadolinium oxide (Gd 2 O 3 ), europium oxide (Eu 2 O 3 ), terbium oxide (Tb 2 O 3 )). In one embodiment, crystalline dielectric layer  30 A may include combinations of rare earth oxides (e.g., a material such as ABO 3 , where ‘A’ and ‘B’ may be any rare earth metal (e.g., lanthanum scandium oxide (LaScO 3 )). In one embodiment, crystalline dielectric layer  30 A may include Perovskites (e.g., strontium titanate (SrTiO 3 ) or barium titanate (BaTiO 3 )). In yet another embodiment, crystalline dielectric layer  30 A may include aluminum oxide Al 2 O 3  or aluminum oxide compounds (e.g., lanthanum aluminum LaAlO 3 ) which may be deposited by pulsed laser deposition (PLD). It is understood that the description of crystalline dielectric layers herein are for illustrative purposes, and that any number, orientation, configuration, or combination of crystalline dielectric layers may be used in accordance with embodiments of the invention. As was noted above, when the dielectric layer  30  is deposited on an amorphous material such as dielectric material it will exhibit an amorphous, non-crystalline characteristic. 
       FIG. 1G  shows a result of the growth of the embedded stressor  32  within the substrate recess  12 B. The embedded stressor  32  functions to apply a desired tensile or compressive stress to the channel of the completed FET device that will exist within the SOI layer  16  beneath the gate stack  20 . In an embodiment the stressor  32 , when deposited in what will be a pFET region, can be comprised of Si:Ge material at a Ge percentage ratio of 20-80%, with 30-60% being typical. In an embodiment the stressor  32 , when deposited in what will be an nFET region can be comprised of Si:C material (carbon-doped silicon). 
     In general the crystalline semiconductor layer of the stressor  32  may be doped (e.g., in situ doped) or un-doped and may include: silicon, germanium, a silicon-germanium alloy and/or carbon doped silicon (Si:C). In one embodiment, the crystalline semiconductor layer of the stressor  32  may include carbon doped silicon with an atomic carbon concentration of between about 0.2% to about 4.0% substitutional carbon. In one embodiment the crystalline semiconductor layer of the stressor  32  may include a carbon doped silicon type material having a concentration of about 0.3% to about 2.5% substitutional Carbon. It is understood that the total amount of carbon in the crystalline semiconductor layer of the stressor  32  may be higher than the substitutional amount. In an exemplary embodiment the crystalline semiconductor layer of the stressor  32  may include silicon, germanium, silicon germanium, carbon doped silicon, a silicon-germanium alloy, and compound (e.g. III-V and II-VI) semiconductor materials, etc. 
     The crystalline dielectric layer  30 A, in accordance with an aspect of this invention, functions to beneficially electrically and physically isolate the stressor  32  from the substrate  12 . 
       FIG. 1H  shows a result of the removal of the exposed portions of the amorphous dielectric layer  30 B. This can be accomplished by RIE or by the use of a wet chemical etch selective to the material of the dielectric layer  30 . 
       FIG. 1I  shows a result of the selective removal of the spacer  28  to form recessed spacer portions  28 A. Any suitable removal process can be applied. Note that it may be preferred that the material of the spacer  26  differs from the material of the spacer  28 , and that the removal process is selective to the material of the spacer  28  in order to leave the spacer  26  substantially intact. 
       FIG. 1J  shows a result of a continued epitaxial growth of the crystalline semiconductor layer of the stressor  32  to form stressor regions  32 A that are contiguous with the underlying stressors  32 . The same material can be used for the stressor regions  32 A that was used for the stressors  32 . The stressor regions  32 A can be in situ doped or they can be undoped. If doped they can have the same dopant concentration as the underlying stressors  32  or a different dopant concentration. The stressor regions  32 A are grown to a height such that a top surface  32 B thereof is about coplanar with the top surface  16 A of the SOI  16  beneath the gate stack  20  (or higher, such as to a height about equal to the top surface of the gate dielectric  22  as shown in  FIG. 1J ). 
     Processing of the FET can then continue in a conventional fashion to form S/Ds, such as a raised source drain (RSD), over the stressor regions  32 A, to deposit an interlayer dielectric (ILD), to form contacts to the gate metal  24  and the S/Ds and to form any contact or contacts if desired to a backgate (if present) in the substrate  12 . 
       FIGS. 2A-2G  illustrate in enlarged cross-section (not to scale) a second embodiment of this invention to provide an epitaxial dielectric material to isolate a stressor from a substrate. 
       FIG. 2A  shows a result of processing of the wafer  10  to a point where the gate stack  20  is present and the recess has been formed in the substrate  10 . 
       FIG. 2B  shows the result of the formation of the second spacer  28  to protect the sidewalls of the channel during subsequent processing. Note in this case, as contrasted with  FIGS. 1D and 1E , the second spacer  28  extends also over the recessed portion of the substrate to the bottom of the recess. 
       FIG. 2C  shows a result of the growth of the dielectric layer  30 . Note in this embodiment, due to the presence of the spacer  28  on the sidewall portion of the substrate recess beneath the gate stack  20 , that the crystalline (epitaxial) layer  30 A is present only on the surface of the substrate  12  at the bottom of the recess, while in all of the other locations the dielectric layer  30  has the form of the amorphous dielectric layer  30 B. 
     The use of this embodiment can be beneficial during the bottom-up growth of the dielectric layer  30  in that there is a reduced risk of a defect to be introduced into the crystalline (epitaxial) layer  30 A, as could possibly occur along the vertical sidewalls of the substrate  12  during the dielectric deposition. 
       FIGS. 2D-2G  are basically similar or identical to the process steps of  FIGS. 1G-1J  explained above, i.e., growing the doped or undoped stressor  32  over the dielectric layer  30  in the substrate recess ( FIG. 2D ); removal of the exposed portions of the amorphous dielectric layer  30 B ( FIG. 2E ); the removal of exposed portions of the second spacer  28  and the recessing of same to form recessed portions  28 A ( FIG. 2F ); and the continued epitaxial growth of the crystalline semiconductor layer of the stressor  32  to form stressor regions  32 A ( FIG. 2G ). 
       FIGS. 3A-3G  illustrate in enlarged cross-section (not to scale) a third exemplary embodiment of this invention to provide an epitaxial dielectric material to isolate a stressor from a substrate. 
       FIG. 3A  shows a result of processing of the wafer  10  to a point as in  FIG. 1D  where the gate stack  20  is present, the SOI layer  16  and the BOX  14  have been removed to expose the surface  12 A of the substrate  12 , and the second spacer  28  has been formed to protect the sidewalls of the channel during subsequent processing. 
       FIG. 3B  shows a result of an anisotropic etch that serves to remove more of the substrate  12  volume than the embodiments of  FIGS. 1 and 2  thereby enabling the resulting deposited stressor  32  to have a larger volume directed towards the channel and to thus apply proportionally more strain to the Si channel. If one assumes for convenience a &lt;100&gt; crystalline orientation of the substrate  100  then the anisotropic etch stops at the &lt;111&gt; plane. The resulting “sigma” substrate recess  12 B can be seen to partially undercut the overlying gate structure  20  and second spacer  28 . This etching profile can be achieved by first etching partially downwards into the substrate  12  to form a substantially rectangular box shaped recess (e.g., as in  FIGS. 1E and 2A ) and then performing, as a non-limiting example, a chemical etch using Tetramethylammonium hydroxide (TMAH). Note that the etched recess will typically not extend to the bottom of the trench isolation  18  as shown. 
     Thus, in some embodiments of this invention the recess can exhibit substantially vertical sidewall surfaces and an exposed surface of the substrate  12  upon which the crystalline dielectric material  30 A is formed is present at least along a bottom surface of the recess, where the bottom surface is substantially perpendicular to the vertical sidewall surfaces. In an embodiment of this invention where the recess is formed using an anisotropic etch process in the crystalline semiconductor substrate  12  the recess comprises at least one sidewall that extends at an angle upwardly towards and at least partially beneath the channel region, and where the exposed surface of the substrate  12  upon which the crystalline dielectric material  30 A is formed is present at least along the at least one upwardly angled sidewall. 
       FIG. 3C  shows a result of the growth of the dielectric layer  30 . Note in this embodiment, due to the larger exposed surface area of the crystalline substrate  12  resulting from the anisotropic etch of  FIG. 3B , that there will be a corresponding larger area covered by the crystalline spacer  30 A beneath the gate stack  20  and extending to the trench isolation  18 . In all other locations the dielectric layer  30  has the form of the amorphous dielectric layer  30 B. 
       FIGS. 3D-3G  are basically similar or identical to the process steps of  FIGS. 1G-1J  explained above, i.e., growing the doped or undoped stressor  32  over the dielectric layer  30  within the (sigma) substrate recess  12 B ( FIG. 3D ); removal of the exposed portions of the amorphous dielectric layer  30 B ( FIG. 3E ); the removal of exposed portions of the second spacer  28  to form recessed portions  28 A ( FIG. 3F ); and the continued epitaxial growth of the crystalline semiconductor layer of the stressor  32  to form stressor regions  32 A ( FIG. 3G ). 
     It is to be understood that the exemplary embodiments discussed above with reference to  FIGS. 1-3  can be used on common variants of the FET device including, e.g., FET devices with multi-fingered FIN and/or gate structures, and FET devices of varying gate width and length. Moreover, transistor devices can be connected to metalized pads or other devices by conventional ultra-large-scale integration (ULSI) metalization and lithographic techniques. 
     Integrated circuit dies can be fabricated with various devices such as a field-effect transistors, bipolar transistors, metal-oxide-semiconductor transistors, diodes, resistors, capacitors, inductors, etc. An integrated circuit in accordance with the present invention can be employed in applications, hardware, and/or electronic systems. Suitable hardware and systems in which such integrated circuits can be incorporated include, but are not limited to, personal computers, communication networks, electronic commerce systems, portable communications devices (e.g., cell phones), solid-state media storage devices, functional circuitry, etc. Systems and hardware incorporating such integrated circuits are considered part of this invention. Given the teachings of the invention provided herein, one of ordinary skill in the art will be able to contemplate other implementations and applications of the techniques of the invention. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
     As such, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. As but some examples, the use of other similar or equivalent semiconductor fabrication processes, including deposition processes and etching processes, may be used by those skilled in the art. Further, the exemplary embodiments are not intended to be limited to only those materials, metals, insulators, dopants, dopant concentrations, layer thicknesses and the like that were specifically disclosed above. Any and all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.