Abstract:
A process for forming both tensile and compressive strained silicon layers to accommodate channel regions of MOSFET or CMOS devices has been developed. After formation of shallow trench isolation structures as well as application of high temperature oxidation and activation procedures, the fabrication sequences used to obtain the strained silicon layers is initiated. A semiconductor alloy layer is deposited followed by an oxidation procedure used to segregate a germanium component from the overlying semiconductor alloy layer into an underlying single crystalline silicon body. The level of germanium segregated into the underlying single crystalline silicon body determines the level of strain, which is in tensile state of a subsequently selectively grown silicon layer. A second embodiment of this invention features the thinning of a portion of the semiconductor alloy layer prior to the oxidation procedure allowing a lower level of germanium to be segregated into a first underlying portion of the underlying single crystalline silicon body, while during the same oxidation procedure a second portion of the underlying single crystalline silicon body receives a higher level of germanium segregation. So the subsequently deposited silicon-germanium layer, although the same process and thickness, can be strained in different states (tensile or compressive) and levels, depending different underlying portions&#39; germanium concentration.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     The present invention relates to methods used to fabricate semiconductor devices such as metal oxide semiconductor field effect (MOSFET) devices, and more specifically to devices formed with strained channel regions.  
         [0003]     (2) Description of Prior Art  
         [0004]     Enhanced performance of MOSFET devices has been addressed via device channel regions formed in strained layers. Increased carrier mobility and thus enhanced device performance has been achieved in both P channel (PMOS) as well as N channel (NMOS) structures via formation of channel regions in strained layers. Formation of strained layers such as a strained silicon layer can be accomplished by formation of the silicon layer on an underlying substrate or layer comprised of silicon-germanium or silicon-germanium carbon.  
         [0005]     The magnitude of strain as well as the type of strain, tensile or compressive, is a function of the level of germanium in the underlying materials. To achieve the desired properties of the strained layer the underlying material, such as a silicon-germanium layer or alike substrate, has to remain stable and undisturbed during subsequent device processing, therefore the integration of the silicon-germanium layer or alike substrate into a MOSFET device process sequence has to be carefully implemented.  
         [0006]     The present invention will describe process integration sequences in which the critical silicon-germanium material layer is not adversely effected by subsequent MOSFET processing steps such as formation of shallow trench isolation (STI) elements, or high temperature anneal procedures. Prior art such as Chiu et al in U.S. Pat. No. 2004/0209437 A1, Koester et al in U.S. Pat. No. 2004/0164373 A1, Ngo et al in U.S. Pat. No. 2004/0137742 A1, Wang et al in U.S. Pat. No. 2004/0180509 A1, as well as Comfort et al in U.S. Pat. No. 5,308,785, describe methods of forming isolation structures in strained silicon and in underlying silicon-germanium layers. The above prior art however does not teach the process integration sequence of the present invention in which specific process sequences such as STI formation are integrated into a MOSFET process without adverse consequence of the strained and underlying layers.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of this invention to form a MOSFET device featuring a channel region incorporated in a strained layer.  
         [0008]     It is another object of this invention to form the strained layer and the underlying silicon-germanium bodies only after formation of STI structures.  
         [0009]     It is still another object of this invention to form the underlying silicon-germanium bodies via batch deposition and oxidation procedures applied to an overlying silicon-germanium layer, followed by a selective epitaxial growth of the strained layer.  
         [0010]     It is another object of this invention to allow formation of silicon-germanium (Si—Ge} in selective areas only, so hybrid structures for different circuit applications can be formed and integrated in the same wafer.  
         [0011]     It is yet another object of this invention to form a complimentary metal oxide semiconductor (CMOS) device via formation of a tensile strained layer for a NMOS device, and of a compressive strained layer for a PMOS device.  
         [0012]     It is yet a further object of this invention to optimize NMOS as well as PMOS devices in surface-Channel operating mode thus to achieve better Channel Electrostatic Control to minimize short channel effects.  
         [0013]     In accordance with the present invention methods of forming strained layers for a MOSFET device in which the strained layers and overlying silicon-germanium layers are obtained after formation of specific device features such as STI, are described. A shallow trench isolation area is first defined by a composite layer of silicon nitride-silicon oxide, and in a top portion of either a silicon substrate or in a top portion of a Silicon on Oxide (SOI) substrate. Subsequent to patterning of silicon-nitride, silicon oxide, etching into then exposed silicon region, and formation of a silicon oxide liner layer on exposed silicon surfaces (e.g. sidewall, bottom, and top corners of future Si active areas), the desired STI structure is formed via insulator fill and chemical mechanical polishing (CMP) procedures. Silicon nitride removal and surface cleaning is followed by deposition or sputtering of a polycrystalline silicon-germanium layer. A first embodiment of this invention features an oxidation procedure in which germanium is segregated into the underlying silicon body resulting in the desired silicon-germanium body to be used to accommodate a subsequent overlying strained Silicon layer. Removal of the oxidized material exposes the now formed underlying silicon-germanium crystalline (Si 1-x Gex) body wherein x is the weight percent of germanium in the Si 1-x Ge x  layer. The desired strained silicon layer is next achieved via a selective epitaxial growth (SEG) procedure formed on the underlying exposed Si 1-x Ge x  body.  
         [0014]     A second embodiment of this invention features the formation of both tensile strained silicon-germanium and compressively strained silicon-germanium for the same MOSFET device. After deposition or sputtering of a polycrystalline silicon-germanium layer, photolithographic and dry etching procedures are used to thin a portion of the polycrystalline silicon germanium layer in a region of the MOSFET device to be used for P channel device fabrication. A subsequent oxidation procedure results in germanium segregation into the underlying silicon body with a greater level of germanium incorporated into the portion of silicon body underlying the unetched or thicker polycrystalline silicon-germanium layer, when compared to level of germanium incorporated into the portion of silicon body underlying the thinner polycrystalline silicon-germanium layer. Removal of oxidized material followed by SEG of a thin silicon-germanium layer, with the chosen [Ge] concentration between the two underlying substrates concentration, will result in a silicon-germanium layer featuring tensile strain overlying the silicon-germanium body with the greater germanium incorporation while a compressively strained silicon-germanium layer is grown on the silicon body having the lower germanium concentration. The top layers for all regions will be designated as SiGe z  while SiGe x  and SiGe y  will be the two underlying layers, z chosen to be x&lt;z&lt;y. Subsequent MOSFET processing such as gate insulator growth, conductive gate structure formation, source/drain formation, result in a MOSFET device featuring either PMOS, NMOS or CMOS devices.  
         [0015]     A third iteration of this invention features the growth of a thin silicon layer on underlying layers SiGe x  and SiGe y , with the silicon layer featuring tensile strain overlying the silicon-germanium body (SiGe y ) with greater germanium incorporation while a compressively strained silicon-germanium layer is grown on the silicon-germanium body (SiGe x ) having the lower germanium concentration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The object and other advantages of this invention are best described in the preferred embodiments with reference to the attached drawings that include:  
         [0017]      FIGS. 1-10 , which schematically in cross-sectional style show key stages of fabrication for either a tensile or compressive strained silicon layer on an underlying silicon-germanium body, formed after STI formation.  
         [0018]      FIGS. 11-14 , which schematically in cross-sectional style show key stages for integration of both tensile and compressive strained layers on a underlying silicon-germanium body, again formed after STI formation. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     The method of forming strained silicon layers on an underlying silicon-germanium body for incorporation of MOSFET channel regions, wherein the strained silicon regions and underlying silicon-germanium body were formed after formation of specific device features such as shallow trench isolation (STI) structures, will now be described in detail. Semiconductor substrate  1 , shown schematically in  FIG. 1 , is comprised of single crystalline, P type silicon, featuring a &lt;100&gt; crystallographic orientation. The substrate orientation is not critical for this invention and thus can be any orientation if desired without detracting from the spirit of this invention. If desired a silicon on oxide (SOI) layer  2   a , can be formed on semiconductor substrate  1 , to reduce capacitance and enhance performance of subsequently formed devices. This invention will be described using the SOI option with silicon layer  2   a , at a thickness between about 100 Angstroms to 0.5 um, (100 Angstroms will be called an ultra-thin body and will be a fully depleted body while SOI at a thickness of about 0.5 um will be a partially depleted body). Layer  1 , can also be a buried oxide layer for example with a thickness between about 1000 Angstroms to 1 um. Not shown in the drawings are N well and P well regions formed in SOI layer  2   a , to accommodate the PMOS and NMOS devices. The well regions can be intrinsic or doped, with doped regions formed via ion implantation of specific ions such as boron, arsenic phosphorous, indium, or antimony, followed by high temperature annealing to activate the implanted ions. It is imperative to form the well regions prior to forming the strained silicon and underlying silicon-germanium bodies since the high temperature processes of well formation would result in unwanted out gassing of germanium from a silicon-germanium materials.  
         [0020]     Formation of a shallow trench isolation (STI) structure is next addressed and schematically shown using  FIGS. 2-6 . The process steps, dry etching, liner formation, etc, could again deleteriously influence the integrity of a strained silicon layer as well as an underlying silicon-germanium body, therefore the STI structure is formed prior to formation of these layers. Silicon oxide layer  3 , to be used as a pad oxide, is first obtained at a thickness between about 50 to 150 Angstroms, preferably at 100 Angstroms, via thermal oxidation procedures. This is followed by deposition of silicon nitride layer  4 , at a thickness between about 500 to 1500 Angstroms, preferably at 1000 Angstroms, via low pressure chemical vapor deposition (LPCVD) or via plasma enhanced chemical vapor deposition (PECVD) procedures. The use of an anti-reflective coating film overlying silicon nitride layer  4 , prior to photolithographic processing is optional. Photolithographic and selective anisotropic reactive ion etching (RIE) procedures, are then applied to define shallow trench shape  20 , in silicon nitride layer  4 , in silicon oxide layer  3 , and in the entire or in a top portion of SOI layer  2   a . This is schematically shown in  FIG. 2 . The anisotropic RIE procedure is performed using Cl 2  or CF 4  as an etchant for silicon nitride, CHF 3  as an etchant for silicon oxide, and Cl 2  or SF 6  as an etchant for silicon, and the chemistry or mixed chemistry chosen for etching of each layer is not limited to the above. Shallow trench shape  20 , is formed in the entire thickness of SOI layer  2   a , to a depth between about 100 to 3500 Angstroms. (For a body thickness &lt;3500 Angstroms the depth is all reach through, for &gt;3500 Angstroms in body thickness the thickness of 3500 Angstroms is fixed, just like the formation in the bulk silicon. If desired shallow trench shape  20 , can be formed only in top portion of SOI layer  2   a , terminating in a bottom portion of SOI layer  2   a ).  
         [0021]     With a photoresist shape (not shown in te drawings) used to define shallow trench shape  20 , still in place lateral pullback  5 , of exposed portions of silicon nitride layer  4 , is performed. This is accomplished via an isotropic dry etch procedure using Cl 2  or CF 4  as an etchant, or via a wet etch procedure using hot phosphoric acid as an etchant for silicon nitride. Pullback  5 , shown schematically in  FIG. 3 , is between about 100 to 200 Angstroms. Removal of the shallow trench defining photoresist shape is now accomplished via plasma oxygen ashing procedures followed by wet clean procedures. A buffered hydrofluoric acid component used in the wet clean procedure results in removal of portions of silicon oxide layer  3 , exposed after the silicon nitride pullback procedure resulting in an additional silicon oxide pull back of between about 100 to 200 Angstroms silicon nitride layer  4 . The result of these procedures is schematically shown in  FIG. 3 .  
         [0022]     To repair the etched trench sidewall surfaces and also protect exposed surfaces of SOI layer  2   a , from a subsequent chemically deposited insulator layer used to fill shallow trench shape  20 , liner layer  6 , is formed to a thickness between about 100 to 400 Angstroms, preferably at 200 Angstroms. Liner layer  6 , shown schematically in  FIG. 4 , is comprised of silicon dioxide formed via a thermal oxidation procedure performed at a temperature between about 900 to 1200° C., preferably at 1100° C. The thickness of silicon dioxide liner layer  6 , is optimized to satisfy the corner rounding required by the subsequently formed devices. Silicon-germanium layers if exposed to the above high temperature oxidation procedure would be deleteriously altered as a result of germanium out gassing, and poor silicon-germanium oxide quality.  
         [0023]     Insulator layer  7   a , comprised of silicon oxide is next deposited via LPCVD, PECVD or via (HDP) high density plasma deposition procedures to a thickness between about 5000 to 8000 Angstroms, completely filling shallow trench shape  20 . Removal of unwanted portions of silicon oxide layer  7   a , from the top surface of silicon nitride layer  4 , is accomplished via a CMP procedure for better planarized surface. If desired a RIE procedure can be employed using CHF 3  as a selective etchant for silicon oxide layer  7   a , with the selective RIE procedure terminating at the appearance of the top surface of silicon nitride layer  4 . This is schematically shown in  FIG. 5 . Silicon nitride layer  4 , is next selectively removed preferably via a hot phosphoric acid treatment resulting in the silicon oxide filled shallow trench opening shown schematically in  FIG. 6 . Alternately a selective RIE procedure may be utilized.  
         [0024]     Careful wet clean procedures using BHF as a component are next employed to prepare exposed surfaces of SOI layer  2   a , for deposition of Si 1-x Ge x  layer  8 . Si 1-x Ge x  layer  8 , wherein x is the weight percent of germanium, can be obtained via a low temperature chemical vapor deposition (LPCVD) procedure using the identical apparatus used for polysilicon deposition. The LPCVD procedure is performed at a temperature between about 500 to 700° C., preferably at 600° C., using silane or disilane as a source for the silicon component of Si 1-x Ge x  layer  8 , while germane is employed ass a source for the germanium component. Germanium weight percent x, is adjusted via flow rate levels of silane and germane. Si 1-x Ge x  layer  8 , shown schematically in  FIG. 7 , is deposited to a thickness between about 500 to 1500 Angstroms, preferably at 1000 Angstroms, with a germanium weight percent between about 10% to 70%. If desired Si 1-x Ge x  layer  8 , can be obtained via d.c. or r.f, sputtering procedures, for this case a target comprised with only a specific percent of silicon and of germanium can be employed. Alternately, silicon epitaxial growth (SEG) could be used tp form Si 1-x Ge x  layer  8 .  
         [0025]     A critical oxidation procedure is next performed in an oxygen-steam ambient at a temperature between about 900 to 1100° C. This temperature is selected to be less than the melting point of Si 1-x Ge x , wherein the melting point decreases with increasing x. The oxidation procedure in addition to converting the silicon component of Si 1-x Ge x  layer  8 , to silicon oxide layer  18 , also allows segregation of germanium into the underlying SOI layer to result. The resulting underlying SOI layer is now single crystalline Si 1-x Ge x  body  2   b , comprised with a weight percent of germanium between about 10% up to &gt;90%. This is schematically shown in  FIG. 8 . Selective removal of silicon oxide layer  18 , is next accomplished via a wet BHF procedure, also may resulting in a thinner silicon oxide component  7   b , filling the shallow trench shape. This is schematically shown in  FIG. 9 . Single crystalline Si 1-x Ge x  body  2   b , comprised with a desired germanium weight content is now in the correct form to allow growth of an overlying strained silicon layer to proceed.  
         [0026]     A selective epitaxial growth (SEG) procedure is next employed to grow single crystalline silicon or silicon-germanium layer  9 , on exposed top surfaces of single crystalline Si 1-x Ge x  body  2   b . The SEG procedure is performed at a temperature between about 500 to 700° C., preferably at 600 to 650° C., using silane or disilane for the silicon option, while adding germane for a silicon-germanium option. Single crystalline silicon layer  9 , shown schematically in  FIG. 10 , is selectively grown at a thickness between about 50 to 200 Angstroms, preferably at 100 Angstroms as channel thickness demands, only on exposed top surfaces of single crystalline Si 1-x Ge x  body  2   b . The ability to grow the above layer on underlying single crystalline Si 1-x Ge x  body  2   b , results in a tensile strained silicon layer  9 . The magnitude of tensile strain is a function of the weight percent of germanium in underlying single crystalline Si 1-x Ge x  body  2   b . Strained silicon layer  9 , will now be used to accommodate a MOSFET channel region with enhanced carrier mobility thus resulting in enhanced MOSFET performance. The MOSFET device is completed via: formation of a gate insulator layer on strained silicon layer  9 ; depositing and definition of a conductive gate structure on the underlying gate insulator layer; and formation of a source/drain region in regions of strained silicon layer  9 , not covered by the conductive gate structure.  
         [0027]     A second embodiment of this invention teaches a process which allows a first portion of a silicon-germanium layer to be selectively formed with tensile strain while a second portion of the same selectively grown silicon-germanium layer is formed with compressive strain. Referring to  FIG. 11 , with the insulator filled STI structure in place Si and using the structure of  FIG. 6  as a starting point, Si 1-x Ge x  layer  10   a , is deposited wherein x is the weight percent of germanium. Si 1-x Ge x  layer  10   a , can again be obtained via a low temperature chemical vapor deposition (LPCVD) procedure using the identical apparatus used for polysilicon deposition. The LPCVD procedure is performed at a temperature between about 500 to 700° C., using silane or disilane as a source for the silicon component of Si 1-x Ge x  layer  10   a , while using germane as a source for the germanium component. Germanium weight percent x, is adjusted via flow rate levels of silane and germane. Si 1-x Ge x  layer  10   a , is deposited to a thickness between about 500 to 1500 Angstroms, preferably at 1000 Angstroms, with a germanium weight percent between about 10% to 70%. If desired Si 1-x Ge x  layer  10   a , can be obtained via d.c. or r.f sputtering procedures. Photoresist shape  11 , is next formed and used as an etch mask to allow removal or partial removal of a top portion of Si 1-x Ge x  layer  10   a , to be performed. A RIE procedure employed using Cl 2  or SF 6  as an etchant results in the attainment of thinner Si 1-x Ge x  layer  10   b , now at a thickness between about 0 to 500 Angstroms, overlying a second portion of SOI layer  2   a . Unetched portions of Si 1-x Ge x  layer  10   a , at a thickness between about 500 to 1500 Angstroms remain overlying a first portion of SOI layer  2   a . An option for a hybrid structure is the thinning of layer  10   a , to where the thickness of layer  10   b , is 0 Angstroms (i.e., total removal), resulting in no germanium entering underlying region  2   a . This could be used for the subsequent formation of, for example, leakage sensitive devices or the like.  
         [0028]     After removal of photoresist shape  11 , via plasma oxygen ashing and wet clean procedures a critical oxidation procedure is performed in an oxygen-steam ambient at a temperature between about 900 to 1000° C. The oxidation procedure is employed to completely convert the unetched Si 1-x Ge x  layer  10   a , to silicon oxide layer  20   a , and to completely convert thinner Si 1-x Ge x  layer  10   b , to silicon oxide layer  20   b . The oxidation procedure again results in segregation of germanium into underlying SOI layer  2   a . The extent of Ge segregation into the underlying SOI layer  2   a , is a function of the thickness or amount of germanium in the overlying pre-oxidized Si 1-x Gex layer. Since Si 1-x Ge x  layer  10   b , is thinner than the unetched Si 1-x Ge x  layer  10   a , less germanium segregation into the underlying SOI layer occurs when compared to the level of germanium segregation into the portion of SOI layer  2   a , underlying Si 1-x Ge x  layer  10   a  with full as-deposited thickness. The oxidation procedure of Si 1-x Ge x  layer  10   a , at a pre-oxidized thickness between about 600 to 800 Angstroms, (with [Ge] at 50% as an example), allows germanium segregation to occur in underlying single crystalline body Si 1-x Ge x    2   b , wherein x the weight percent of germanium in single crystalline body Si 1-x Ge x  layer  2   b , is between about 60 to 80 percent, resulting in a final thickness of about 500 Angstroms. The same oxidation procedure applied to thinner Si 1-x Ge x  layer  10   b , results in single crystalline Si 1-x Ge x  body  2   c , wherein the weight percent of germanium is only between about 20% to 40%. This is schematically shown in  FIG. 12 .  
         [0029]     Selective removal of silicon oxide layer  20   a , and of  20   b , is next accomplished via a wet BHF procedure. This procedure also results in thinning of the silicon oxide shape of the STI structure resulting in thinner silicon oxide component  7   b . This is schematically shown in  FIG. 13 . The selective epitaxial growth (SEG) procedure previously described in this first embodiment of this invention is again employed to grow single crystalline silicon-germanium layers on exposed top surfaces of single crystalline Si 1-x Ge x  body  2   b , and Si 1-x Ge x  body  2   c . The SEG procedure is performed at a temperature between about 500 to 700° C., preferably at 600° C. The silicon-germanium layer is achieved via the addition of germane to either a silane or disilane flow. Single crystalline silicon-germanium (Si 1-z Ge z ) layer  12   a , at a thickness between about 50 to 200 Angstroms (thinner than the underlying substrate), preferably at 100 Angstroms, is selectively grown on underlying Si 1-x Ge x  body  2   b , and is comprised with a tensile strain. The tensile strain featured in single crystalline silicon-germanium layer  12   a , is achieved via selective epitaxial growth on an underlying Si 1-x Ge x  body  2   b , comprised with a germanium weight percent between about 60% to 80%. The tensile strain of single crystalline silicon layer  12   a , will allow enhancement of electron mobility to be realized thus enhancing the performance of subsequently fabricated NMOS devices. The same SEG procedure results in the selective growth of single crystalline silicon-germanium(Si 1-z Ge z ) layer  12   b , on underlying Si 1-x Ge x  body  2   c . However single crystalline silicon-germanium layer  12   b , is comprised with compressive strain as a result of growth on underlying Si 1-x Ge x  body  2   c , which is comprised with a lower germanium weight percent between about 20 to 40 weight percent, than counterpart Si 1-x Ge x  body  2   b , because x&lt;z. The compressive nature of single crystalline silicon-germanium layer  12   b , will allow enhancement of hole mobility to be realized thus enhancing the performance of subsequently fabricated PMOS devices. The result of the SEG procedure is schematically shown in  FIG. 14 .  
         [0030]     A third embodiment of this invention features the growth of a single crystalline layer (Si 1-z Ge z )  12   a  or  12   b , on underlying layers Si 1-x Ge x  and Si 1-v Ge v , wherein the germanium content [z] is zero weight percent. Silicon layer  12   a , featuring tensile strain is grown overlying the silicon-germanium body (Si 1-v Ge v ) comprised with the greater germanium incorporation, while compressively strained silicon layer  12   b  is grown on the silicon-germanium body (Si 1-x Ge x ) having the lower germanium concentration.  
         [0031]     Complimentary metal oxide semiconductor (CMOS) devices can now be formed wherein NMOS devices can be formed in with enhanced electron mobility in single crystalline silicon layer  12   a , while counterpart PMOS devices can be formed with enhanced hole mobility in single crystalline silicon layer  12   b . The completion of the CMOS devices feature: gate insulator layer formed on both single crystalline silicon layer  12   a  and  12   b ; definition of a conductive gate structure on the underlying gate insulator layer; and formation of an N type and P type source/drain region in regions of the strained silicon layers not covered by the conductive gate structure. The ability to form the Si 1-x Ge x  bodies and the overlying strained silicon layers after STI formation and after high temperature processing did not allow the integrity of the above layers to be degraded.  
         [0032]     While this invention has been particularly shown and described with reference to, the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.