Abstract:
The present invention provides a method of integrated semiconductor devices such that different types of devices are formed upon a specific crystallographic orientation of a hybrid substrate. In accordance with the present invention, junction capacitance of one of the devices is improved in the present invention by forming the source/drain diffusion regions of the device in an epitiaxial semiconductor material such that they are situated on a buried insulating layer that extends partially underneath the body of the second semiconductor device. The second semiconductor device, together with the first semiconductor device, is both located atop the buried insulating layer. Unlike the first semiconductor device in which the body thereof is floating, the second semiconductor device is not floating. Rather, it is in contact with an underlying first semiconducting layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to high-performance metal oxide semiconductor field effect transistors (MOSFETs) for digital or analog applications, and more particularly to MOSFETs utilizing carrier mobility enhancement from surface orientation. 
     BACKGROUND OF THE INVENTION 
     In present semiconductor technology, complementary metal oxide semiconductor (CMOS) devices, such as nFETs or pFETs, are typically fabricated upon semiconductor wafers, such as Si, that have a single crystal orientation. In particular, most of today&#39;s semiconductor devices are built upon Si having a (100) crystal orientation. 
     Electrons are known to have a high mobility for a (100) Si surface orientation, but holes are known to have high mobility for a (110) surface orientation. That is, hole mobility values on (100) Si are roughly 2×-4× lower than the corresponding electron mobility for this crystallographic orientation. To compensate for this discrepancy, pFETs are typically designed with larger widths in order to balance pull-up currents against the nFET pull-down currents and achieve uniform circuit switching. pFETs having larger widths are undesirable since they take up a significant amount of chip area. 
     On the other hand, hole mobilities on (110) Si are 2× higher than on a (100) Si; therefore, pFETs formed on a (110) surface will exhibit significantly higher drive currents than pFETs formed on a (100) surface. Unfortunately, electron mobilities on (110) Si surfaces are significantly degraded compared to (100) Si surfaces. 
     As can be deduced from the above discussion, the (110) Si surface is optimal for pFET devices because of excellent hole mobility, yet such a crystal orientation is completely inappropriate for nFET devices. Instead, the (100) Si surface is optimal for nFET devices since that crystal orientation favors electron mobility. 
     Co-pending and co-assigned U.S. patent application Ser. No. 10/250,241, filed Jun. 17, 2003, provides an approach to fabricate CMOS devices on hybrid orientations wherein the pFETs are formed on a (110) surface orientation and nFETs are formed on a (100) surface orientation. Because hole mobility is greater than 150% higher on a (110) orientation than on a (100) orientation, the drive current of pFETs is greatly enhanced from conventional CMOS technology. However, this prior art approach puts one type of device on a silicon-on-insulator (SOI) and the other type of device on bulk (epitaxial layer) depending on the surface orientation of the SOI and the handle wafer. For example, with a (100) SOI and a (110) handle wafer, nFETs will be SOI devices and pFETs will be bulk-like devices. With a (110) SOI and a (100) handle wafer, nFETs will be bulk-like devices and pFETs will be SOI devices. 
     In general, bulk-like devices have larger junction capacitance than SOI devices and will affect circuit performance under AC conditions. Techniques are known which are capable of fabricating both types of devices on SOI, however the processing steps involved in these prior art techniques are very complicated. 
     In view of the above, there is a need to reduce the junction capacitance for the bulk-like device in the hybrid orientation structure using a simple process. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of integrating semiconductor devices such that different types of devices are formed upon a specific crystal orientation of a hybrid substrate that enhances the performance of each type of device. Specifically, the present invention provides a method of integrating semiconductor devices such that the pFETs are located on a (110) crystallographic plane, while the nFETs are located on a (100) crystallographic plane of a hybrid substrate. The method of the present invention also improves the performance of the device on an epitaxial semiconductor layer by reducing its junction capacitance using a source/drain diffusion layer on a buried insulator. 
     The method of the present invention can also be used in integrating different CMOS devices onto a substrate having different crystallographic surface planes wherein isolation regions are formed between CMOS devices. 
     The above are achieved in the present invention by utilizing a method in which a hybrid substrate comprising first and second semiconducting layers having different crystal orientations is first provided. After providing the hybrid substrate having different crystal orientations, the substrate is subjected to a first patterning and etching process which forms a first opening in the hybrid substrate that exposes a buried insulating layer that is located between the first and second semiconducting layers. A second patterning and etching process is then performed that forms a second opening within the hybrid substrate which exposes the underlying first semiconducting layer. The second opening is located in the area defined by the first opening. A semiconductor material having the first crystallographic orientation is regrown atop the exposed portion of the first semiconducting layer by epitaxy and a planarization process follows which provides a substrate that has substantially coplanar surfaces having different crystallographic planes. In particular, the substrate has a first crystallographic plane comprising the regrown semiconductor material that has the first crystallographic orientation and a second crystallographic plane comprising the second semiconducting layer which has the second crystallographic orientation. In both planes, subsequently formed semiconductor devices such as FETs will both be located on SOI-like areas. 
     In broad terms, the present invention provides a method of forming CMOS devices on a hybrid substrate with reduced junction capacitance for the bulk-like devices that comprises the steps of: 
     providing a hybrid substrate comprising a first semiconducting layer of a first crystallographic orientation, a buried insulating layer located on a surface of the first semiconducting layer and a second semiconducting layer of a second crystallographic orientation which is different from the first crystallographic orientation located on said buried insulating layer; 
     forming a first opening in said hybrid substrate, said first opening exposes a surface of the buried insulating layer, wherein said first opening defines a second area, said second area is located next to a first area; 
     forming a second opening within said first opening that exposes a surface of the first semiconducting layer; 
     growing a semiconductor material on said first semiconducting layer in said second opening by epitaxy, said semiconductor material having the first crystallographic orientation; 
     planarizing the hybrid substrate containing the semiconductor material so that an upper surface of the second semiconducting layer is substantially coplanar with an upper surface of the semiconductor material; and 
     forming at least one first semiconductor device in said first area, while forming at least one second semiconductor device on said semiconductor material in said second area. 
     In accordance with the present invention, the first semiconductor device may be a pFET with the second crystallographic orientation is (110) or (111), while the second semiconductor device may be an nFET with the first crystallographic orientation is (100) or (111). It is also possible in the present invention, that the first semiconductor device is an nFET with the second crystallographic orientation is (100) or (111) and that the second semiconductor device is a pFET with the first crystallographic orientation is (110) or (111). In one embodiment of the present invention, the method future includes forming isolation regions between the first area and the second area. In yet another embodiment of the present invention, the order of forming the first and second openings is reversed such that the second opening is formed prior to forming the first opening. 
     It should be noted that the crystallographic orientations mentioned above are exemplary and that any crystallographic orientation can be used. As stated above, the second semiconducting layer and the regrown semiconductor material include upper surfaces that are substantially coplanar. In accordance with the present invention, both devices are formed on a SOI-like region and the second semiconductor device formed atop the epitaxially grown semiconductor material has reduced junction capacitance since the source/drain diffusion regions of the second semiconductor device are located within the regrown semiconductor material and they are located on a portion of the buried insulating layer. Note that the second semiconductor device is not floating, i.e., the body of the second semiconductor device located on the epitaxial semiconductor material is in contact with the underlying first semiconducting layer through the second opening. 
     In broad terms, the inventive structure provided by the above processing steps comprises: 
     a substrate comprising a second semiconducting layer of a second crystallographic orientation that has an upper surface that is coplanar with an upper surface of an epitaxial semiconductor material of a first crystallographic orientation, said first crystallographic orientation is different from said second crystallographic orientation, wherein a buried insulating layer is located beneath said second semiconducting layer and a portion of the epitaxial semiconductor material; and 
     a first semiconductor device located on said second semiconducting layer and a second semiconductor device located on said epitaxial semiconductor material, wherein said second semiconductor device includes source and drain diffusion regions that are located on said buried insulating layer that is beneath said epitaxial semiconductor material, wherein a body portion of the second semiconductor device is not floating. 
     By not “floating”, it is meant that the body of the second semiconductor device located within the epitaxial semiconductor material is in contact with a first semiconducting layer that is located beneath the buried insulating layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial representation (through a cross sectional view) showing a hybrid substrate that has different surface orientations which can be used in the present invention. 
         FIG. 2  is a pictorial representation (through a cross sectional view) showing the hybrid substrate of  FIG. 1  after forming a surface dielectric layer  18  on the substrate. 
         FIG. 3  is a pictorial representation (through a cross sectional view) showing a first opening formed in the substrate of  FIG. 2  so as to expose a portion of the buried insulating layer. 
         FIG. 4  is a pictorial representation (through a cross sectional view) showing the structure of  FIG. 3  after removing the patterned mask and forming spacers in said first opening. 
         FIG. 5  is a pictorial representation (through a cross sectional view) showing a second opening formed in the structure of  FIG. 4  so as to expose a portion of the bottom semiconductor layer. 
         FIG. 6  is a pictorial representation (through a cross sectional view) illustrating the structure of  FIG. 5  after a semiconductor material is grown from the exposed surface of the bottom semiconductor layer. 
         FIG. 7  is a pictorial representation (through a cross sectional view) illustrating the integrated semiconductor structure of the present invention containing an nFET and a pFET formed on a coplanar surface of different crystallographic orientations. The source/drain of the device on the epitaxial semiconductor material are on the buried insulating layer and thus the junction capacitance is reduced. 
         FIG. 8  is a pictorial representation (through a cross sectional view) illustrating another hybrid substrate that can be used in the present invention in which the device isolation regions are formed prior to regrowing the semiconductor material. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention, which provides a method of fabricating CMOS devices with reduced junction capacitance on a hybrid substrate having first and second semiconducting layers of different crystal orientations, will now be described in greater detail by referring to the drawings that accompany the present application. It is noted that the drawings provided in the present invention are for illustrative purposes and thus they are not drawn to scale. 
     In the drawings, the final hybrid substrate has upper coplanar regions of different crystallographic orientation. 
     Referring to  FIG. 1 , there is shown an initial hybrid substrate  10  that is employed in the present invention. The hybrid substrate  10  comprises a first (or bottom) semiconducting layer  12 , a buried insulating layer  14  located on a surface of the first semiconducting layer  12 , and a second (or top) semiconducting layer  16  located on a surface of the buried insulating layer  14 . In accordance with the present invention, the first semiconducting layer  12  of the hybrid substrate  10  comprises a first semiconducting material that has a first crystallographic orientation and the second semiconducting layer  16  of the hybrid substrate  10  comprises a second semiconducting material that has a second crystallographic orientation that differs from the first crystallographic orientation. 
     The first semiconducting layer  12  of the hybrid substrate  10  is comprised of any semiconducting material including, for example, Si, SiC, SiGe, SiGeC, Ge, GaAs, InAs, InP as well as other III/V or IINI compound semiconductors. Combinations of the aforementioned semiconductor materials are also contemplated herein. The first semiconducting layer  12  may be unstrained, strained or a combination of strained and unstrained layers. The first semiconducting layer  12  is also characterized as having a first crystallographic orientation which may be (110), (111), (100) or any other crystallographic orientation. 
     The first semiconducting layer  12  serves as a handle wafer, and its thickness is close to the thickness of a standard wafer. 
     The second semiconducting layer  16  is comprised of any semiconducting material which may be the same or different from that of the first semiconducting layer  12 . Thus, the second semiconducting layer  16  may include, for example, Si, SiC, SiGe, SiGeC, Ge, GaAs, InAs, InP as well as other III/V or II/VI compound semiconductors. The second semiconducting layer  16  may be unstrained, strained or a combination of strained and unstrained layers. The second semiconducting layer  16  may also include combinations of the aforementioned semiconducting materials. 
     The second semiconducting layer  16  is also characterized as having a second crystallographic orientation that is different from the first crystallographic orientation. Thus, the crystallographic orientation of the second semiconducting layer  16  is (100), (111), (110) or any other crystallographic orientation with the proviso that the crystallographic orientation of the second semiconducting layer  16  is not the same as the crystallographic orientation of the first semiconducting layer  12 . 
     The second semiconducting layer  16  may have a thickness from about 5 nm to about 2 μm, with a thickness from about 10 nm to about 100 nm being more typical. For smaller junction/parasitic capacitance, a thinner second semiconductor layer  16  is desired. 
     The buried insulating layer  14  that is located between the first and second semiconducting layers ( 12  and  16 ) of the hybrid substrate  10  can be an oxide, nitride, oxynitride or any combination of these insulating materials. In some embodiments, the buried insulating layer  14  is an oxide. The buried insulating layer  14  typically has a thickness from about 20 nm to 500 nm, with a thickness of from about 40 nm to 200 nm being more typical. 
     The hybrid substrate  10  shown in  FIG. 1  is formed utilizing a layer transfer process in which bonding is employed. Specifically, the layer transfer process disclosed in co-pending and co-assigned U.S. application Ser. No. 10/250,241, filed Jun. 17, 2003 can be used. The entire content of the &#39;241 application is incorporated herein by reference. 
     The semiconducting layers ( 12  and  16 ) used in fabricating the hybrid substrate  10  may include two SOI wafers wherein one of the wafers includes the first semiconducting layer  12  and the other wafer includes the second semiconducting layer  16 ; an SOI wafer and a bulk semiconductor wafer; two bulk semiconductor wafers in which at least one of the wafers contains an insulating layer  14  thereon. The bulk wafers may include an ion implant region, such as a H 2  implant region, which can be used to split a portion of at least one of the wafers during bonding. A surface dielectric may be present on the wafer that includes the second semiconductoing layer  16 . 
     Bonding is achieved by first bringing the two semiconducting wafers into intimate contact with other; optionally applying an external force to the contacted wafers; and then heating the two contacted wafers under conditions that are capable of bonding the two wafers together. The heating step may be performed in the presence or absence of an external force. The heating step is typically performed in an inert ambient at a temperature from about 600° C. to about 1300° C. for a time period from about 2 hours to about 20 hours. More typically, the bonding used in forming the hybrid substrate  10  is performed at a temperature from about 900° C. to about 1200° C. The term “inert ambient” is used in the present invention to denote an atmosphere in which an inert gas, such as He, Ar, N 2 , Xe, Kr or a mixture thereof, is employed. A preferred ambient used during the bonding process is N 2 . 
     In the embodiment where two SOI wafers are employed, some material layers of at least one of the SOI wafers may be removed after bonding utilizing a planarization process such as chemical mechanical polishing (CMP) or grinding and etching. 
     In the embodiment in which one of the wafers includes an ion implant region, the ion implant region forms a porous region during bonding which causes a portion of the wafer above the ion implant region to break off leaving the hybrid substrate  10  such as is shown, for example, in  FIG. 1 . The implant region is typically comprised of H 2  ions that are implanted into the surface of the wafer utilizing ion implantation conditions that are well known to those skilled in the art. The layer splitting occurs at the implant region during a 350° C.-500° C. anneal after an initial heating step that is performed at a temperature from about 200° C. to about 500° C. directly after bonding. 
     In embodiments where the wafers to be bonded do not include a surface dielectric layer therein, the surface dielectric layer  18  may be formed atop the hybrid substrate  10  by a thermal process, such as oxidation, or by a conventional deposition process such as chemical vapor deposition (CVD), plasma-enhanced CVD, atomic layer deposition, chemical solution deposition as well as other like deposition processes. The dielectric layer  18  serves as a mask for subsequent selective epitaxy. The structure including the surface dielectric layer  18  is shown, for example, in  FIG. 2 . The surface dielectric layer  18  typically has a thickness from about 5 nm to about 500 nm, with a thickness from about 20 nm to about 100 nm being more typical. It is again noted that the surface dielectric layer  18  can be present on the surface of the wafer including the second semiconducting layer  16  prior to bonding as well. 
     A first mask  20  is then formed on a predetermined portion of the hybrid substrate  10  shown in  FIG. 2  so as to protect a portion of the hybrid substrate  10 , while leaving another portion of the hybrid substrate  10  unprotected. The protected portion defines a first area (or region)  23  of the structure, whereas the unprotected portion defines a second area (or region)  22 . First mask  20  can be a photoresist patterned by lithography, which includes the steps of applying, exposing and developing. The photoresist usually is removed after pattern transfer. The first mask  20  may also be hard material such as a nitride, oxynitride or an oxide layer that is formed and patterned utilizing deposition, lithography and etching. 
     After providing the first mask  20 , the structure is subjected to one or more etching steps so as to expose the surface of the buried insulating layer  14 . The etching may include a dry etching process such as reactive-ion etching, ion beam etching, plasma etching or laser etching, a wet etching process wherein a chemical etchant is employed or any combination thereof. In one preferred embodiment of the present invention, reactive-ion etching (RIE) is used in this step of the present invention. The structure formed after the etching step has been performed is shown, for example, in  FIG. 3 . The structure includes a first opening  25  defined by the first patterning and etching process defined above which exposes a surface of the buried insulating layer  14 . It is noted that the first opening  25  is located within the second area  22  of structure. As shown in  FIG. 3 , the sidewalls  24  within the first opening  25  are bare at this point of the inventive process. 
     Next, an optional spacer  26  can be formed in the first opening  25  on the exposed sidewall  24  of at least the patterned and etched second semiconducting layer  16  provided by the above processing steps. The optional spacer  26  is formed by deposition and etching. The optional spacer  26  can be comprised of an insulating material such as, for example, an oxide, nitride, oxynitride or any combination thereof. The optional spacer  26  may be a single spacer, as shown, or it may comprise multiple spacers.  FIG. 4  shows the presence of the optional spacers  26  in the structure after removing the first mask  20 . The optional spacer  26  is used in the present invention to prevent epitaxy from the sidewall  24  of the semiconducting layer  16  to ensure high quality single grain crystals in the second region  22 . It is noted that although the drawings in the subsequent process sequence show the presence of the optional spacer  26  the process flow works equally well when the optional spacers  26  are not present. 
     Next, a second mask  42  is formed on the structure using a similar approach as the first mask  20  and following a subsequent etch that exposes a portion of the first semiconducting layer  12  the structure shown in  FIG. 5  is provided. The structure shown in  FIG. 5  includes a second opening  44 . The size of the second opening  44  in the second area  22  is determined by the channel. The second opening  44  can be smaller or slightly larger than the channel (or gate conductor). In the present invention, the second opening  44  is preferred to be centered with the channel. There can be multiple channel regions within the second area  22  although only one is shown in  FIG. 5 . Using the second mask  42 , the first semiconducting layer  12  is exposed by a subsequent etching step. After pattern transfer the second mask  42  is removed. 
     An optional spacer (not shown) can be formed after the second opening  44  is formed. 
     The sequence of forming the second opening  44  and first opening  25  used in defining second area  22  can be reversed, i.e., the second mask  42  can be applied first to define second opening  44 , and the first mask  20  is then applied to define second region  22 . Similar pattern transfer technique can be used as described above. 
     Semiconductor material  50  is then epitaxially grown from the exposed surface of the first semiconducting layer  12 . In accordance with the present invention, the semiconductor material  50  has a crystallographic orientation that is the same as the crystallographic orientation of the first semiconducting layer  12 . Although this epitaxial semiconductor material  50  will have the same surface orientation as the first semiconducting layer  12 , it can be of a different semiconductor material than the first semiconducting layer  12 . 
     The epitaxial semiconductor material  50  may comprise any semiconductor material, such as Si, strained Si, SiGe, SiC, SiGeC or combinations thereof, which is capable of being formed utilizing a selective epitaxial growth method. The epitaxial material  50  can be strained, unstrained or a combination thereof. In one embodiment, the epitaxial semiconductor material  50  is an unstrained semiconductor material. 
     To achieve a high quality epitaxial semiconductor material  50 , selective epitaxy is recommended where there is no polysilicon or amorphous silicon formed on top of the surface dielectric layer  18 . The regrown semiconductor material  50  should be grown thick enough so that the lateral growth portion can fill up the whole of area  22  including the second opening  44 . To eliminate a facet formation during the epitaxy, the regrown semiconductor material  50  can be grown, in some embodiments, higher than the surface dielectric layer  18 , then polished down to the surface dielectric layer  18 , and finally recessed to be coplanar with the top surface of the second semiconductor layer  16 . The structure after semiconductor material regrowth is shown in  FIG. 6   
     Following the removal of the surface dielectric layer  18  and the formation of a substantially coplanar surface, isolation regions  68 , such as shallow trench isolation regions, are typically formed so as to isolate the first region  23  from the second region  22 , and isolate devices within the same region. The isolation regions  68  are formed utilizing processing steps that are well known to those skilled in the art including, for example, trench definition and etching; optionally lining the trench with a diffusion barrier; and filling the trench with a trench dielectric such as an oxide. After the trench fill, the structure may be planarized and an optional densification process step may be performed to densify the trench dielectric. 
     The resultant substantially coplanar structure containing isolation region  68  is shown, for example in  FIG. 7 . As shown, the structure of  FIG. 7  includes an exposed second semiconducting layer  16  of a second crystal orientation and an epitaxial semiconductor material  50  that has the same crystal orientation as that of the first semiconducting layer  12 . 
       FIG. 7  also shows the integrated structure that is formed after a first semiconductor device  61  is formed on a portion of the first area  23  and a second semiconductor device  63  is formed on the epitaxial semiconductor material  50  in the second area  22 . Despite showing the presence of only one device in each region, the present invention contemplates forming a plurality of each type of device in the specific device region. In accordance with the present invention, the first semiconductor device  61  may be a pFET or an nFET, whereas the second semiconductor device  63  may be an NFET or a pFET, with the proviso that the first semiconductor device  61  is different from the second semiconductor device  63  and that the specific device is fabricating on a crystal orientation that provides high performance. 
     The pFETs and nFETs are formed utilizing standard CMOS processing steps that are well known to those skilled in the art. Each FET includes a gate dielectric, a gate conductor (labeled as  66  and  56 ), spacers located on the sidewalls of at least the gate conductor, and the source/drain diffusion regions (labeled as  52 ,  62 ). Note that pFET is formed over the semiconductor material having a (110) or (111) orientation, whereas the nFET is formed over a semiconductor having a (100) or (111) orientation. Regions  64  and  54  are source/drain extension regions. 
     To reduce the junction capacitance for the second semiconductor device  63  on the epitaxial material, the diffusion regions  52  are located on top of the buried insulating layer  14 . To achieve this, the gate conductor  56  should be centered to the now filled opening  44 . The width of the now filled opening  44  can be smaller than the gate conductor  56 . Note that the size of the filled opening  44  is determined by the channel region. In the present invention, the now filled opening  44  is preferred at least as large as the gate conductor width in order to reduce the amount of epitaxial growth. However, the now filled opening  44  should be small enough so that at least most of the diffusion region  52  is on the buried insulating layer  14 . 
     The doping of the first semiconducting layer  12  and the epitaxial semiconductor material  50  can be the same type depending on the type of the device. For example, when the device  63  is pFET on (110) orientation, n-type doping is desired. In this case, the body of device  63  is connected to the bottom of the wafer through opening  44 . Unlike a regular SOI device, the body of device  63  is not floating, although its source/drain region  52  are on a buried insulating layer. 
     In another embodiment when floating body is desired, the epitaxial semiconductor material  50  can be doped to the opposite type from the bottom semiconductor  12 . For example, when the device  63  is pFET on (110) orientation, the epitaxial semiconductor material  50  can be doped to n-type as n-well and the bottom semiconducting layer  12  can be p-type. In this case the n-well of the pFET  63  is floating. Device operation range is limited, however, so that no forward bias of the floating well can occur to prevent leakage current. 
     Doping of the epitaxial semiconductor material  50  can be done in-situ with the epitaxy process or can be done during the well implantation before the gate stack formation in the standard CMOS process. 
     In the above embodiment, the device isolation is performed after epitaxy and surface planarization. In another embodiment (See  FIG. 8 ), the device isolation  68  can be done first on the hybrid substrate  10  prior to applying mask  20  or  42 . In this case, the surface dielectric layer  18  also serves as chemical mechanical polishing stop layer for the isolation region  68 . The processing described above then can be performed. Note that is the structure of  FIG. 8  is employed, there is no need to form the optional spacers  26 . 
     While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention is not limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.