Abstract:
A method is disclosed for forming a semiconductor wafer having a strained Si or SiGe layer on an insulator layer. The method produces a structure having a SiGe buffer layer between the insulator layer and the strained Si or SiGe layer, but eliminates the need for Si epitaxy after bonding. The method also eliminates interfacial contamination between strained Si and SiGe buffer layer, and allows the formation of Si/SiGe layers having a total thickness exceeding the critical thickness of the strained Si layer.

Description:
BACKGROUND OF INVENTION 
   This invention relates to integrated circuit (IC) structures and processes that include a strained silicon or silicon germanium (Si/SiGe) layer. More particularly, this invention relates to formation of a structure having a strained Si/SiGe layer on an insulator layer which is useful for fabricating high speed devices such as complementary metal-oxide-semiconductor (CMOS) transistors and other metal-oxide-semiconductor field effect transistor (MOSFET) applications. 
   Electron and hole mobility in strained silicon or silicon germanium layers has been shown to be significantly higher than that in bulk silicon. For example, measured values of electron mobility in strained Si at room temperature are about 3000 cm 2 /Vs as opposed to 400 cm 2 /Vs in bulk silicon. Similarly, hole mobility in strained SiGe with high Ge concentration (60%˜80%) reaches up to 800 cm 2 /Vs, which is about 5 times the hole mobility in bulk silicon of 150 cm 2 /Vs. MOSFETs with strained-Si channels have been experimentally demonstrated to have enhanced device performance compared to devices fabricated in conventional (unstrained) silicon substrates. Potential performance improvements include increased device drive current and transconductance, as well as the added ability to scale the operation voltage without sacrificing circuit speed in order to reduce the power consumption. 
   Strained-Si layers are the result of biaxial tensile stress induced in silicon grown on a substrate formed of a material whose lattice constant is greater than that of silicon. The lattice constant of germanium is about 4.2 percent greater than that of silicon, and the lattice constant of a silicon-germanium alloy is nearly linear with respect to its germanium concentration. As a result, the lattice constant of a SiGe alloy containing fifty atomic percent germanium is about 1.02 times greater than the lattice constant of silicon. Epitaxial growth of silicon on such a SiGe substrate will yield a silicon layer under tensile strain, with the underlying SiGe substrate being essentially unstrained, or “relaxed.” A structure and process that realize the advantages of a strained-Si channel structure for MOSFET applications are taught in commonly-assigned U.S. Pat. No. 6,059,895, which discloses a technique for forming a CMOS device having a strained-Si channel on a SiGe layer, all on an insulating substrate. 
   The underlying conducting substrate for MOSFETs and bipolar transistors or the interaction of the underlying substrate with the active device regions in CMOS are undesirable features which limit the full performance of high speed devices. To resolve the problem, in Si technology, an insulating layer is usually used to isolate the active device region from the substrate, by creating Silicon-On-insulator (SOI) wafers to replace bulk Si material for device fabrication. Available technology to achieve SOI wafers includes Separation by Implanted Oxygen (SIMOX), bonding and etchback Silicon-On-Insulator (BESOI), separation by implanted hydrogen also known as the Smart-Cut® process which is described in U.S. Pat. No. 5,374,564, or the combination of the last two processes for making ultra-thin SOI, described in U.S. Pat. No. 5,882,987. 
   When Si in an SOI wafer is substituted by strained Si or SiGe (Si/SiGe) layers for high speed applications, two methods are generally used to produce strained Si/SiGe-on-insulator structures. In one method, thermal mixing is used to produce a relaxed SiGe-on-insulator structure (SGOI), followed by epitaxial growth of strained Si on SGOI. This thermal mixing method is illustrated in FIGS.  1 ( a )-( c ). A SiGe layer  13  is deposited on an SOI substrate comprising silicon substrate  10 , insulator or oxide layer  11  and silicon layer  12 , as shown in FIG.  1 ( a ). Thermal mixing is then performed, to produce the structure shown in FIG.  1 ( b ) which comprises substrate  10 , insulator layer  11 , and SiGe layer  14 . During thermal mixing, germanium is rejected from the oxide during high temperature oxidation, and the final SiGe concentration and relaxation in layer  14  is a function of the initial SiGe concentration in layer  13 , its thickness, and the final thickness of SiGe layer  14 . Following thermal mixing, oxide is stripped from the top surface of the structure. Finally, strained Si layer  15  is grown on SiGe layer  14 , as shown in FIG.  1 ( c ). 
   While thermal mixing is a promising method to make strained Si/SiGe-on-insulator, it has draw backs. In the thermal mixing method, a SiGe-on-insulator structure is first formed, then strained Si is grown on the SiGe. Strained Si deposition on SiGe may leave a non-ideal interface with O and C residue, which may affect device performance or yield. In addition, SiGe after thermal mixing is usually not fully relaxed. In order to achieve high strain in the strained Si, high concentration SiGe is needed as the template for strained Si growth. The high concentration SiGe will lead to integration complexity and potentially yield degradation. 
   The other method generally used to produce strained Si/SiGe on insulator structures involves wafer bonding. Specifically, a first wafer bonding method involves bonding relaxed SiGe onto an insulator followed by strained Si/SiGe growth. This first wafer bonding method is described in U.S. Pat. No. 6,524,935, and is illustrated in FIGS.  2 ( a )- 2 ( d ). The method begins with growing an epitaxial relaxed SiGe layer  21  on a first silicon substrate  20 , as shown in FIG.  2 ( a ). Next, hydrogen is implanted into the SiGe layer  21  to form a hydrogen-rich defective layer (not shown). The surface of the SiGe layer  21  is smoothed by chemical-mechanical polishing (CMP). Then, the surface of the first substrate is bonded to the surface of a second substrate comprising bulk silicon  22  and an insulator layer  23 , as shown in FIG.  2 ( b ). Specifically, the smoothed surface of the SiGe layer  21  is bonded to the insulator layer  23 , which is typically SiO 2 . Bonding the two substrates together is accomplished by placing the surface of the first substrate against the surface of the second substrate resulting in a weak chemical bond which holds the two substrates together. A thermal treatment is usually performed to the bonded wafer pair to strengthen the chemical bonds at the joined interface. Following bonding, the two substrates are separated at the hydrogen-rich defective layer, resulting in the structure shown in FIG.  2 ( c ) which comprises second substrate  22 , insulator layer  23  and a portion of SiGe layer  21 . The top surface of SiGe layer  21  in this separated structure may be smoothed by CMP. Finally, in FIG.  2 ( d ), strained Si layer  24  is epitaxially grown on SiGe layer  21 . 
   This wafer bonding method suffers from process complications. The as-bonded SiGe on insulator is usually too thick, and therefore thinning of SiGe is required before strained Si deposition, which is a non-trivial process. In addition, strained Si deposition on SiGe may leave a non-ideal interface with O and C residue, which may affect device performance or yield. 
   A second wafer bonding method involves directly bonding strained Si/SiGe onto an insulator. This second wafer bonding method is described in U.S. Pat. No. 6,603,156, and is illustrated in FIGS.  3 ( a )- 3 ( e ). The method begins with growing a relaxed SiGe layer  31  on a first silicon substrate  30 , as shown in FIG.  3 ( a ). A strained-Si layer  32  is next formed on strain-inducing SiGe layer  31 , as shown in FIG.  3 ( b ). Then, the first substrate is bonded to a second substrate comprising bulk silicon  33  and an insulator layer  34 , as shown in FIG.  3 ( c ). Specifically, the two structures are bonded such that the insulating layer  34  is between strained-Si layer  32  and second substrate  33 , and the strained-Si layer  32  directly contacts the insulating layer  34 , as shown in FIG.  3 ( d ). The initial strain-inducing layer  31  is then removed to expose the surface of the strained-Si layer  32  and yield a strained-Si-on-insulator (SSOI) structure. Strain-inducing layer  31  may be removed by CMP, wafer cleaving (smart cut), or chemical etching. A chemical etching process such as HHA (hydrogen peroxide, hydrofluoric acid, and acetic acid) selective to Si is preferred so that the SiGe layer  31  is fully removed stopping on the strained-Si layer  32 . 
   This second wafer bonding method eliminates the steps of thinning of SiGe and the interface left by strained-Si growth on SiGe, as needed by the first wafer bonding method. U.S. Pat. No. 6,603,156 also teaches that a structure without SiGe between the strained-Si and the insulator is advantageous, as SiGe usually complicates CMOS processes. However, with strained-Si directly on insulator, the thickness of Si is limited due to the critical thickness of the strained layer. For example, strained-Si with 1% of strain is limited to a thickness of about 100 Å, beyond which defects may form in the strained-Si during high temperature process steps. The critical thickness of Si with higher strain is even less. Given that current CMOS technologies require various Si thicknesses for SOI structures, there is a need in the art for a method of forming strained SOI or SGOI structures having the required total Si/SiGe thickness without exceeding the critical thickness of the strained layer. 
   SUMMARY OF INVENTION 
   The aforementioned deficiencies of the prior art methods for forming a strained Si/SiGe-on-insulator structure are alleviated through use of the method of the present invention, in which a SiGe buffer is added in between the strained layer and the insulator to achieve the required total Si/SiGe thickness without exceeding the critical thickness of the strained layer. 
   Specifically, the invention is directed to a method of forming a strained Si 1-y Ge y  layer above an insulator layer. The method comprises the steps of: forming a relaxed Si 1-x Ge x  layer on a first crystalline semiconductor substrate; forming a strained Si 1-y Ge y  layer on said relaxed Si 1-x Ge x  layer; forming a Si 1-z Ge z  layer on said strained silicon layer; forming a hydrogen-rich defective layer in said relaxed Si 1-x Ge x  layer; providing a second crystalline semiconductor substrate having an insulator layer thereover; bonding a top surface of said Si 1-z Ge z  layer on said first substrate to said insulator layer on said second substrate; separating said relaxed Si 1-x Ge x  layer at said hydrogen-rich defective layer to form a structure comprising said second substrate with said insulator layer, said Si 1-z Ge z  layer on said insulator layer, said strained Si 1-y Ge y  layer on said Si 1-z Ge z  layer, and a portion of said relaxed Si 1-x Ge x  layer on said strained Si 1-y Ge y  layer; and removing said portion of said relaxed Si 1-x Ge x  layer. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The drawings are for illustration purposes only and are not drawn to scale. Furthermore, like numbers represent like features in the drawings. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
     FIGS.  1 ( a )- 1 ( c ) illustrate a prior art method for forming a strained Si/SiGe-on-insulator structure using thermal mixing; 
     FIGS.  2 ( a )- 2 ( d ) illustrate a prior art method for forming a strained Si/SiGe-on-insulator structure using a first wafer bonding method which involves bonding relaxed SiGe onto an insulator and then growing strained Si/SiGe; 
     FIGS.  3 ( a )- 3 ( e ) illustrate a prior art method for forming a strained Si-on-insulator structure using a second wafer bonding method which involves directly bonding strained Si onto an insulator; and 
     FIGS.  4 ( a )- 4 ( f ) illustrate a preferred embodiment of the method of the present invention for forming a strained Si/SiGe-on-insulator structure. 
   

   DETAILED DESCRIPTION 
   The invention will now be described by reference to the accompanying figures. In the figures, various aspects of the structures have been shown and schematically represented in a simplified manner to more clearly describe and illustrate the invention. For example, the figures are not intended to be drawn to scale. In addition, the vertical cross-sections of the various aspects of the structures are illustrated as being rectangular in shape. Those skilled in the art will appreciate, however, that with practical structures these aspects will most likely incorporate more tapered features. Moreover, the invention is not limited to constructions of any particular shape. 
   A preferred embodiment of the method of the present invention is illustrated in FIGS.  4 ( a )- 4 ( f ). The method begins with formation of a relaxed Si x Ge x  layer  41  on a first crystalline semiconductor substrate  40 , as shown in FIG.  4 ( a ). First substrate  40  may be any single crystal material suitable for forming epitaxial layers thereon. Examples of such suitable single crystal materials include Si, SiGe, SiGeC and SiC, with Si being preferred. 
   The upper surface of layer  41  should be substantially relaxed or completely relaxed. The relaxation may be due to a modified Frank-Read mechanism as described in U.S. Pat. No. 5,659,187, the disclosure of which is incorporated herein by reference. Layer  41  may be formed by growing a relatively thick graded SiGe layer followed by a constant concentration SiGe layer having a total thickness of greater than 1 μm, where the SiGe is fully or partially relaxed, followed by CMP smoothing. Alternatively, layer  41  may be formed by growing a medium thickness SiGe layer having a thickness of about 500 to 3000 Å, followed by He implant and anneal, and CMP smoothing if necessary. 
   The concentration x of Ge in layer  41  may range from about 0.05 up to about 1.0, and is preferably in the range of about 0.15 to about 0.40. 
   Next, a strained Si 1-y Ge y  layer  42  is grown epitaxially on the top surface of layer  41 , and then a Si 1-z Ge z  layer  43  is grown on top of strained layer  42 , as shown in FIG.  4 ( b ). The concentration y of Ge in layer  42  may range from zero up to 0.05. The concentration y in layer  42  should be less than the concentration x in layer  41 , such that layer  41  has a greater lattice constant than layer  42 , thereby forming a strained layer  42  which is under biaxial tension. In a preferred embodiment, concentration y in layer  42  is zero, such that layer  42  is a strained-Si layer. Layer  42  preferably has a thickness of about 50 Å to about 300 Å. The thickness of layer  42  is related to the strain in the film. For higher strain, the thickness of layer  42  should be smaller to avoid film relaxation and additional defect formation in the film. 
   Si 1-z Ge z  layer  43  may be strained or unstrained, depending on the concentration z of Ge and the process needs. Specifically, the concentration z may range from about 0.05 to about 1.0, more preferably about 0.10 to about 0.30, and may be less than or greater than the concentration x of Ge in layer  41 . The thickness of Si 1-z Ge z  layer  43  may be selected so that the total thickness of layers  42  and  43  is as required by the specific CMOS technology needs. In a preferred embodiment, layer  43  may have a thickness of about 50 Å to about 600 Å, more preferably about 100 Å to about 300 Å. 
   Si 1-z Ge z  layer  43  may be epitaxially grown following growth of the strained Si 1-y Ge y  layer  42 , preferably without taking the wafer out from the epitaxy chamber, so that the interface between Si 1-z Ge z  layer  43  and strained Si 1-y Ge y  layer  42  is clean. 
   Next, a hydrogen implantation step is performed to form a hydrogen-rich defective layer  44 , as shown in FIG.  4 ( c ). Specifically, layer  41  is subjected to ion bombardment or the implantation of hydrogen ions, which may be implanted at an energy of about 10 KeV to about 200 KeV at a dose of about 5×10 16  to about 1×10 17  ions/cm 2 . The hydrogen implantation results in the formation of a hydrogen-rich layer  44  comprising hydrogen-containing SiGe point defects and planar micro cracks residing in principle crystallographic planes of SiGe. The energy of the hydrogen ions is selected to place the peak dose in layer  41  below the top surface of layer  41 , preferably at a depth of about 100 nm to 1000 nm. The hydrogen-rich defective layer  44  will form at the peak dose location of hydrogen. 
   After forming hydrogen-rich defective layer  44 , the first structure comprising layers  40 ,  41 ,  42  and  43  is bonded to a second structure comprising layers  45  and  46 , as shown in FIG.  4 ( d ). Specifically, second structure comprises substrate  46  and insulating layer  45 . Suitable materials for substrate  46  include single-crystal silicon, polysilicon, SiGe, GaAs and other III-V semiconductors, with single-crystal silicon being particularly preferred. The insulating layer  45  may be formed of any suitable material, including silicon oxide (SiO 2 ), silicon nitride (SiN) and aluminum oxide (Al 2 O 3 ), although other electrically insulating materials could be used, including silicon oxynitride, hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ) and doped aluminum oxide. SiO 2  is particularly preferred for insulating layer  45 . While the individual thicknesses of insulating layer  45  and substrate  46  are not generally critical to the invention, thicknesses of up to about 1 μm are suitable for the insulating layer  45 . 
   The first structure may be bonded to the second structure using any suitable wafer bonding technique. Prior to wafer bonding, the top surface of layer  43  may be polished by a touch up Chemical Mechanical Polishing (CMP) process to provide a smooth top surface if necessary, with minimum removal of film in layer  43 . This polishing may be performed before or after formation of hydrogen-rich defective layer  44 . This top surface of layer  43  shown in FIG.  4 ( c ) then may be turned upside down and brought into contact with the top surface of layer  45 . The bonding between the surfaces of layers  43  and  45  may be strengthened by annealing at a temperature of about 50° C. to about 500° C., for a time period of about 2 hours to about 50 hours. 
   Layer  41  is then separated at the hydrogen-rich defective layer  44  by any suitable technique, without disturbing the mechanical bond between layers  43  and  45 . For example, layer  41  may be separated into two portions by annealing, preferably at a temperature of about 200° C. to about 600° C. After separation, the remaining structure comprises substrate  46 , insulating layer  45 , Si 1-z Ge z  layer  43 , strained Si 1-y Ge y  layer  42 , and a portion of relaxed Si 1-x Ge x  layer  41 , a shown in FIG.  4 ( e ). 
   It is possible at this point to perform an optional bond strengthening anneal at a temperature between 500° C. to 900° C. for a period of time ranging from a few seconds (using rapid thermal annealing) to 3 hours. The purpose of this anneal is to both strengthen the bonds at the joined interface as well as remove any residual hydrogen which may interfere with the subsequent selective removal of the remaining portion of layer  41 . 
   Finally, the remaining portion of layer  41  is removed by any suitable method, preferably by selective etch such as using HHA stopping on strained layer  42 . The resulting structure, shown in FIG.  4 ( f ), comprises substrate  46 , insulating layer  45 , Si 1-z Ge z  buffer layer  43 , and strained Si 1-y Ge y  layer  42 . The interface between strained Si 1-y Ge y  layer  42  and Si 1-z Ge z  buffer layer  43  is clean, as the two films were grown in the same epitaxy step. 
   The process steps of the present invention are similar to the method described in U.S. Pat. No. 6,603,156, with the addition of Si 1-z Ge z  layer  43  on top of strained Si 1-y Ge y  layer  42  before wafer bonding. As a result, a strained Si/SiGe on SiGe on insulator structure similar to that disclosed in U.S. Pat. No. 6,524,935 is obtained, but without the need for non-trivial SiGe thinning and no contaminated interface between strained layer  42  and underlying  43 . 
   An alternative embodiment would allow the possibility of a SiGe buffer when epitaxially growing Si 1-y Ge y  on Si 1-x Ge x . This may be used for better growth of strained Si on SiGe. The SiGe buffer may have a Ge concentration of x, or less than x, and preferably the SiGe buffer is lattice matched to Si 1-x Ge x . For example, when x is 0.3 and is 90% relaxed, then the SiGe buffer has a Ge concentration of 0.27. 
   Another alternative embodiment would allow the possibility of having an insulator layer on top of Si 1-z Ge z  before wafer bonding, instead of or in addition to having it on the second substrate, similar to U.S. Pat. No. 6,603,156. 
   While the present invention has been particularly described in conjunction with a specific preferred embodiment and other alternative embodiments, it is evident that numerous alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore intended that the appended claims embrace all such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.