Patent Publication Number: US-8536650-B2

Title: Strained ultra-thin SOI transistor formed by replacement gate

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of currently co-pending U.S. patent application Ser. No. 12/057,443, filed on Mar. 28, 2008, the subject matter of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates generally to semiconductor devices and their fabrication. In particular, the present disclosure relates to a strained ultra-thin silicon-on-insulator transistor formed by replacement gate. 
     2. Description of Related Art 
     Ongoing scaling efforts of semiconductor devices not only contribute to higher integrated circuit packing density, but also improve integrated circuit performance. As the scaling process proceeds towards the physical limits of currently available semiconductor technologies and techniques, newer technologies and techniques are developed to further decrease device size and increase device performance. As device size decreases, tremendous challenges arise in the areas of device modeling accuracy and process integration. The latest technologies for fabricating integrated circuits (or ICs) using “silicon-on-insulator” (or SOI) substrates have propelled semiconductor technology ahead for another generation or two of scaling. These SOI-based technologies accomplish this by balancing more expensive SOI wafer substrates with more advanced lithographic patterning tools and techniques. Integrated semiconductor devices based on thinner SOI substrates provide fully depleted transistor bodies, effectively eliminating undesirable floating body effects. Accordingly, there is a trend in the semiconductor industry towards ultra-thin semiconductor devices based upon ever-thinner SOI substrates. Another advantage of using ultra-thin SOI substrates is that they permit the body regions of semiconductor devices to experience a “strain” condition such that carrier mobility (both electrons and holes) is enhanced. The thinner the silicon layer of the SOI substrate, the greater the strain applied to it by the gate dielectric and buried oxide layer (BOX). In addition, ultra-thin SOI transistors have the advantages of improved short-channel effect, improved sub-threshold swing, and enhanced carrier mobility. It is one of the upfront approaches for continued complementary metal oxide semiconductor (CMOS) scaling. Another approach for CMOS scaling is strain engineering. One of widely adopted strain techniques is forming embedded SiGe (eSiGe) in the source/drain (S/D) of a PFET and embedded Si:C (eSi:C) in the source/drain of an NFET to produce a strain in the channel to enhance carrier mobility. Unfortunately, it is extremely difficult, if not impossible, to form eSiGe and/or eSi:C in ultra-thin SOI devices. eSiGe and eSi:C are formed by recessing a portion of the SOI in the source/drain region and then filling the recessed portion with SiGe for PFET and Si:C for NFET. Given the fact that the silicon layer is already very thin in ultra-thin SOI, it is very difficult to recess a portion of such thin SOI layer with a precise control. Furthermore, the strain is strongly dependent on the depth of the recessed S/D. Shallow recess in ultra-thin SOI results in very limited strain effect. 
     Therefore, there is a need for an improvement in forming embedded S/D in UTSOI. 
     SUMMARY OF THE INVENTION 
     The present disclosure is directed to structure and method of forming a strained ultra-thin silicon-on-insulator transistor having embedded source/drain (e.g. embedded SiGe). In one embodiment, a semiconductor structure is described. The structure includes a transistor formed in a semiconductor substrate, the semiconductor substrate having a semiconductor-on-insulator (SOI) layer; a channel associated with the transistor and formed on a first portion of the SOI layer; and a source/drain region associated with the transistor and formed in a second portion of the SOI layer and in a recess at each end of the channel, wherein the second portion of the SOI layer is substantially thicker than the first portion of the SOI layer; and wherein the source/drain region includes a stressor material. The structure further includes a high-k metal gate disposed above the channel, and a source/drain extension formed between the channel and a corresponding the source/drain region, each the source/drain extension and the corresponding source/drain region being aligned to the high-k metal gate and the channel. In one particular embodiment, the SOI layer is formed over a stair-shaped buried insulating (BOX) layer. In another embodiment, the semiconductor substrate includes further includes a BOX layer and formed over a base substrate layer, wherein the SOI layer is formed over the BOX layer. The stressor material is selected from a group consisting of eSiGe, eSi:C and a combination thereof. In addition, the stressor material in the source/drain region is substantially thicker than the first portion of the SOI layer. In one particular embodiment, the first portion of the SOI layer includes a thickness ranging from about 5.0 nm to about 70.0 nm, and wherein the second portion of the SOI layer includes a thickness ranging from about 20.0 nm to about 70.0 nm. The first portion of the SOI layer includes a thickness ranging from about 5.0 nm to about 70.0 nm. The transistor is a strained filed effect transistor (FET). 
     In another embodiment, a semiconductor device is described. The device includes a field effect transistor including: a thin channel formed in a first portion of a semiconductor-on-insulator (SOI) layer; a high-k metal gate disposed above the thin channel; and a source/drain region formed in a second portion of the SOI layer and in a recess at each end of the thin channel, wherein the second portion of the SOI layer is substantially thicker than the first portion of the SOI layer; and a stair-shaped buried insulating (BOX) layer insulating the SOI layer from a base semiconductor substrate; wherein the source/drain region includes a stressor material selected from a group consisting of eSiGe, eSi:C and a combination thereof; wherein the stressor material is substantially thicker that the first portion of the SOI layer. The device further includes a source/drain extension formed between the thin channel and the stressor material, wherein each of the source/drain extension and the corresponding stressor material is aligned to the metal gate and the thin channel. The device further includes a source/drain extension formed between the channel and a corresponding the source/drain region, each the source/drain extension and the corresponding source/drain region being aligned to the high-k metal gate and the channel. In one particular embodiment, the stressor material includes a thickness ranging from about 20.0 nm to about 70.0 nm; and the first portion of the SOI layer includes a thickness ranging from about 5.0 nm to about 70.0 nm. In addition the first portion of the stressor material includes a thickness ranging from about 20.0 nm to about 70.0 nm, and wherein the second portion of the SOI layer includes a thickness ranging from about 20.0 nm to about 70.0 nm. Moreover, the first portion of the SOI layer includes a thickness ranging from about 5.0 nm to about 70.0 nm, and wherein the second portion of the SOI layer includes a thickness ranging from about 20.0 nm to about 70.0 nm. 
     A method of forming a semiconductor structure is also described. The method includes forming a dummy gate in a semiconductor substrate; performing a SIMOX process to form a semiconductor-on-insulator (SOI) layer such that a first portion of the SOI layer under the dummy gate is substantially thinner than a second portion of the SOI layer; forming a source/drain extension in the SOI layer; and recessing the source/drain extension for forming a source/drain region; epitaxially growing the second portion of the SOI layer; forming an insulating layer over the epitaxial growth; removing the dummy gate for forming a gate opening; and filling the gate opening with a gate dielectric material and a gate conductor material. The SOI layer is formed over a stair-shaped buried insulating (BOX) layer. In addition, the source/drain region includes a stressor material, wherein the stressor material is selected from a group consisting of eSiGe, eSi:C and a combination thereof. In one particular embodiment, the first portion of the SOI layer includes a thickness ranging from about 5.0 nm to about 70.0 nm, and wherein the second portion of the SOI layer includes a thickness ranging from about 20.0 nm to about 70.0 nm. 
     In a second embodiment of a method of forming a semiconductor structure, the method includes forming a dummy gate in thinned portion of a semiconductor-on-insulator (SOI); forming source/drain extensions in the SOI layer abutting the thinned portion of the SOI layer; forming an interlayer dielectric; removing the dummy gate for forming a gate opening; and forming a gate dielectric and a gate conductor in the gate opening. The SOI layer is formed over a stair-shaped buried insulating (BOX) layer. 
     Other features of the presently disclosed structure and method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate will become apparent from the following detailed description taken in conjunction with the accompanying drawing, which illustrate, by way of example, the presently disclosed structure and method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the presently disclosed structure and method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate will be described hereinbelow with references to the figures, wherein: 
         FIGS. 1-8  illustrate simplified cross-sectional views of progressive stages of a method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate, in accordance with one embodiment of the present disclosure; and 
         FIG. 9  is an exemplary flow diagram illustrating a method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate, in accordance with one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing figures, wherein like references numerals identify identical or corresponding elements, an embodiment of the presently disclosed structure and method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate, will be disclosed in detail. In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide a thorough understanding of the present invention. However, it will be appreciated by one skilled in the art that the invention may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail to avoid obscuring the disclosure. Thus, the materials described herein are employed to illustrate the disclosure in one application and should not be construed as limiting. 
     The present disclosure provides a structure and method for forming an ultra-thin silicon-on-insulator transistor having embedded source/drain, such as, for example, SiGe. In one particular aspect of the disclosure, a high-k metal gate is provided by replacement gate method. A dummy gate is used for forming a stair buried oxide for facilitating the formation of the embedded SiGe, in a manner described in detail hereinbelow. 
       FIGS. 1-8  illustrate a structure and method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate. In particular, the structure includes a field effect transistor formed on a semiconductor-on-insulator (SOI) layer having a first portion and a second portion, where the second portion of the SOI layer is substantially thicker than the first portion of the SOI layer. In particular, the field effect transistor includes a thin channel formed in the first portion of the semiconductor-on-insulator (SOI) layer; a metal gate disposed above the thin channel; and a source/drain region formed in the second portion of the SOI layer and in a recess at each end of the thin channel. The source/drain region includes a stressor material selected from a group consisting of eSiGe, eSi:C and a combination thereof. In addition, the stressor material is substantially thicker than the first portion of the SOI layer. In one particular embodiment, the SOI layer is formed on a stair-shaped buried insulating (BOX) layer insulating the SOI layer from a base semiconductor substrate. 
       FIGS. 1-8  further illustrate a method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate. The method includes forming a dummy gate in a semiconductor substrate; performing a SIMOX process to form a semiconductor-on-insulator (SOI) layer such that a first portion of the SOI layer under the dummy gate is substantially thinner than a second portion of the SOI layer; forming a source/drain extension in the SOI layer; and recessing a portion of the second SOI layer for forming a source/drain region; epitaxially growing the second portion of the SOI layer; forming an insulating layer; removing the dummy gate for forming a gate opening; and filling the gate opening with a gate dielectric material and gate conductor material. 
     With initial reference to  FIG. 1 , an embodiment of a bulk silicon wafer, in accordance with the present disclosure, is illustrated and is designated generally as silicon wafer  100 . Silicon wafer  100  includes a handle substrate or base semiconductor substrate  102  and a dielectric (e.g. oxide and/or nitride) layer  104  formed in an upper surface of base semiconductor substrate  102  using conventional techniques such as deposition or oxidation. Dielectric layer  104  includes a thickness ranging from about 2 nm to about 10 nm. A dummy gate  106  is then formed by a conventional pattering method (e.g. lithography and reactive ion etch (RIE)) atop dielectric layer  104 . Dummy gate  106  includes a polysilicon layer  108  and a cap (e.g. nitride) layer  110  formed on top of polysilicon layer  108 . Polysilicon layer  108  includes a thickness ranging from about 10 nm to about 100 nm and it may be formed by deposition, such as, for example chemical vapor deposition (CVD). Cap layer  110  includes a thickness ranging from about 1 nm to about 10 nm may be formed by nitridation or deposition. 
     Base semiconductor substrate  102  may include any of several semiconductor materials well known in the art, such as, for example, a bulk silicon substrate, silicon-on-insulator (SOI) and silicon-on-sapphire (SOS). Other non-limiting examples include silicon, germanium, silicon-germanium alloy, silicon carbide, silicon-germanium carbide alloy and compound (i.e. III-V and II-VI) semiconductor materials. Non-limiting examples of compound semiconductor materials include gallium arsenide, indium arsenide and indium phosphide semiconductor material. Typically, base semiconductor substrate  102  may be about, but is not limited to, several hundred microns thick. For example, base semiconductor substrate  102  may include a thickness ranging from about 0.5 mm to about 1.5 mm. 
     With reference to  FIG. 2 , a buried insulating (e.g. buried oxide (BOX)) layer  112  is formed on base semiconductor substrate  102 . In addition, silicon-on-insulator (SOI) structure  114  is formed on BOX layer  112 , where BOX layer  112  isolates SOI structure  114  from base semiconductor substrate  102 . The BOX layer includes a first surface under a first portion of the SOI layer and a second surface under a second portion of the SOI layer. The first surface and the second surface form a right angle step. In one particular embodiment, SOI structure  114  is formed using a technique referred to as separation by implanted oxygen (SIMOX) wherein ions, typically oxygen, are implanted into a bulk Si-containing substrate (i.e. base semiconductor substrate  102 ). Base semiconductor substrate  102  having the implanted ions is then annealed under conditions that are capable of forming BOX layer  112 . Other SIMOX processes and conditions are also envisioned. For example, the various SIMOX processes and conditions mentioned in U.S. Pat. No. 6,074,928 and co-assigned U.S. Patent Application Publication Nos. 20020190318 and 20020173114, and U.S. Pat. Nos. 5,930,634, 6,486,037, 6,541,356 and 6,602,757, the entire contents of which are incorporated herein by reference. Other alternative methods of forming SOI structure  114  and BOX layer  112 , such as, for example, a layer transfer process such as, a bonding process, as also envisioned. 
     With continued reference to  FIG. 2 , it is noted that dummy gate  106  causes the implanted oxygen (from the SIMOX process) to be substantially shallower in the areas directly under dummy gate  106  than other areas not covered by dummy gate  106 , as illustrated by the figure. In particular, SOI structure  114  includes a first portion  114   a  having a thickness t 1  and a second portion  114   b  having a thickness t 2 , where t 2  is substantially thicker than t 1  (i.e. t 1 &lt;t 2 ). In one particular embodiment, t 1  ranges from about 2 nm to about 100 nm and more preferably, from 5 nm to about 20 nm and t 2  ranges from about 20 nm to about 200 nm, and more preferably from about 50 nm to about 100 nm, greater than t 1 . Moreover, BOX layer  112  takes on a stair-shape due to the presence of dummy gate  106 . BOX layer  112  has a thickness ranging from about 20 nm to about 500 nm, and more preferably of about 100 nm 
     With reference to  FIG. 3 , source/drain (S/D) extension  116   a ,  116   b  and spacers  118   a ,  118   b  are formed using conventional methods. Optionally, a halo  120   a ,  120   b  is also formed adjacent S/D extensions  116   a ,  116   b  respectively. Halo  120   a ,  120   b  and S/D extensions  116   a ,  116   b  may be formed by ion implantation, plasma doping, and/or any other suitable techniques known in the art. In one embodiment, halo  120   a ,  120   b  are butted to BOX layer  112  and are formed in first portion  114   a  of SOI structure  114  (i.e. under dummy gate  106 ). S/D extensions  116   a ,  116   b  is also butted to BOX layer  112  and is formed in second portion  114   b  of SOI structure  114 . Spacers  118   a ,  118   b  are formed on the sidewalls of dummy gate  106  by deposition (e.g. CVD) and directional etch such as reactive ion etch (RIE). Spacers  118   a ,  118   b  may include any of several materials such as oxide, nitride, low-k material, high-k material, or the combination of those materials. For simplicity, halo  120   a ,  120   b  is omitted in subsequent figures. 
     With reference to  FIG. 4 , using conventional methods well known in the art, S/D extensions  116   a ,  116   b  are recessed (i.e. etched), for example, by RIE, to a predetermined depth, for forming a S/D region  115   a ,  115   b  adjacent dummy gate  106 . The depth of S/D region  115   a ,  115   b  is about 20 nm to 100 nm, depending on the thickness of SOI layer  114 . 
     With reference to  FIG. 5 , a stress material (e.g. SiGe for PFET stack and Si:C for NFET stack) is epitaxially grown in S/D regions  115   a ,  115   b . For example, a highly compressive selective epitaxial SiGe layer  122   a ,  122   b  is grown in S/D regions  115   a ,  115   b  of a pFET stack, fully filling S/D etched regions  115   a ,  115   b  of a pFET stack. SiGe layer  122   a ,  122   b  may be grown to a thickness of about 10 nm to 100 nm thick, although other thicknesses are also contemplated by the disclosure. Alternatively, a highly tensile selective epitaxial Si:C layer is grown to a thickness of about 10 nm to 100 nm thick in S/D regions  115   a ,  115   b  of an nFET stack. In one embodiment, the SiGe and/or eSi:C layer can be in-situ doped (i.e. doping during epitaxial growth). Alternatively, SiGe and/or Si:C is doped after the epitaxial growth, for example, by ion implantation, plasma doping, and/or any other suitable doping techniques. 
     With reference to  FIGS. 6-8 , conventional replacement gate processes is followed, as described, for example, in U.S. Pat. No. 6,885,084. In particular, with reference to FIG.  6 , a silicide layer  124  is formed. In one particular embodiment, silicide layer  124  includes a nickel silicide, which is formed by deposition of nickel (Ni) which reacts with epitaxial layer  122   a ,  122   b  to form silicide by thermal annealing. An interlayer dielectric (ILD) layer  126  is then deposited and planarized for exposing dummy gate  106 . The ILD layer  126  may comprise oxide, nitride, low-k dielectric, high-k dielectric, or any combination of those materials. The exposed dummy gate  106  (i.e. gate oxide  104 , polysilicon layer  108  and cap layer  110 ) is removed using conventional methods well known in the art, for forming gate opening  128 , as illustrated in  FIG. 7 . Dummy gate  106  may be removed, for example, by dry etch, wet etch, or a combination of both. 
     With reference to  FIG. 8  gate dielectric  130  and gate conductor  132  are formed in gate opening  128  by using conventional steps. In particular, gate dielectric  130  is formed in gate opening  128 . Gate conductor  132  is then formed by deposition and planarization. Examples of gate dielectric  130  include but are not limited to silicon oxide, silicon nitride, metal oxides such as hafnium oxide, hafnium silicon oxide, hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, lead zinc niobate, and any combination of these materials. Examples of gate conductor includes but are not limited to Zr, W, Ta, Hf, Ti, Al, Co, Ni, Ru, Pa, Pt, metal oxide, metal carbide, transition metal aluminides such as Ti3Al, ZrAl, TaC, TaMgC, TiAlN, WCN, metal oxide, metal nitride such as Mo2N, MoAlN, TiN, TaN, or any combination of those materials. 
     Gate dielectric  130  and gate conductor  132  can be formed by conventional methods, including but not limited to, atomic layer deposition (ALD), chemical vapor deposition (CVD), high temperature oxide deposition (HTO), low temperature oxide deposition (LTO), chemical oxidation, thermal oxidation, thermal nitridation, ultrahigh vacuum chemical vapor deposition (UHVCVD), metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), physical vapor deposition, sputtering, plating, evaporation, spin-on-coating, ion beam deposition, electron beam deposition, laser assisted deposition, chemical solution deposition, or any combination of those methods. 
     With reference to  FIG. 9 , in conjunction with  FIGS. 1-8 , a flow diagram of an exemplary method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate, in accordance with the present disclosure, is illustrated. Initially, at step  200 , a dummy gate  106  if formed over a device structure, such as, for example a silicon substrate  102 , as discussed hereinabove. In accordance with the present disclosure, at step  202 , a SIMOX process is performed to form a SOI structure  114  having a first portion  114   a  substantially thinner than a second portion  114   b . At step  204 , a S/D extension is formed in the second portion  114  of the SOI structure  114 . At step  206 , the S/D extension is recessed by conventional methods. At step  208 , the second portion  114   b  of SOI structure  114  is epitaxially grown for filling the recessed S/D extension with a stress material, such as, for example, SiGe for PFET and Si:C for NFET. At step  210 , an insulating layer  124  is formed over the epitaxial growth. At step  212 , dummy gate  106  is removed for forming a gate opening  128 . Finally, at step  214 , gate opening  128  is filled with gate dielectric material and gate conductor material. 
     It will be understood that numerous modifications and changes in form and detail may be made to the embodiments of the presently disclosed structure and method of forming a strained ultra-thin silicon-on-insulator transistor formed by replacement gate method. It is contemplated that numerous other configuration of the interconnect structure may be formed, and the material of the structure and method may be selected from numerous materials other than those specifically disclosed. Therefore, the above description should not be construed as limiting the disclosed structure and method, but merely as exemplification of the various embodiments thereof. Those skilled in the art will envisioned numerous modifications within the scope of the present disclosure as defined by the claims appended hereto. Having thus complied with the details and particularity required by the patent laws, what is claimed and desired protected is set forth in the appended claims.