# Patent

## Title
Strained semiconductor by wafer bonding with misorientation

## Abstract

One aspect of the present invention relates to a method for forming a strained semiconductor structure. In various embodiments, at least two strong bonding regions are defined for a desired bond between a crystalline semiconductor membrane and a crystalline semiconductor substrate. The two strong bonding regions are separated by a weak bonding region. The membrane is bonded to the substrate at a predetermined misorientation. The membrane is pinned to the substrate in the strong bonding regions. The predetermined misorientation provides the membrane in the weak bonding region with a desired strain. In various embodiments, the membrane is bonded to the substrate at a predetermined twist angle to biaxially strain the membrane in the weak bonding region. In various embodiments, the membrane is bonded to the substrate at a predetermined tilt angle to uniaxially strain the membrane in the weak bonding region. Other aspects are provided herein.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to the following commonly assigned U.S. patent applications which are herein incorporated by reference in their entirety: “Output Prediction Logic Circuits With Ultra-Thin Vertical Transistors and Methods of Formation,” U.S. application Ser. No. 10/164,611, filed on Jun. 10, 2002; “Micro-Mechanically Strained Semiconductor Film,” U.S. application Ser. No. 10/379,749, filed on Mar. 5, 2003 and “Localized Strained Semiconductor on Insulator,” U.S. application Ser. No. 10/425,797, filed on Apr. 29, 2003. This application is also related to the following commonly assigned U.S. patent applications: “Strained Si/SiGe Structures By Ion Implantation,” U.S. application Ser. No. 10/431,134, filed on May 7, 2003 and “Micromechanical Strained Silicon By Wafer Bonding,” U.S. application Ser. No. 10/431,137, filed on May 7, 2003.


TECHNICAL FIELD
This disclosure relates generally to semiconductor devices, and more particularly, to devices formed with strained semiconductor films.
BACKGROUND
The semiconductor industry continues to strive for improvements in the speed and performance of semiconductor devices. Strained silicon technology enhances carrier mobility in both n-channel and p-channel devices, and thus improves device speed and performance.
One technique for producing strained silicon involves growing silicon on relaxed silicon germanium (Si/SiGe) structures. There is a large mismatch in the cell structure between the Si and SiGe layers. This mismatch causes a pseudomorphic layer of Si on relaxed SiGe to be under a biaxial tensile strain that modifies the band structure and enhances carrier transport in the Si layer. In an electron inversion layer, the subband splitting is larger in strained Si because of the strain-induced band splitting in addition to that provided by quantum confinement. For example, the ground level splitting (E0(d4)−E0(d2)) in a MOS inversion layer at 1 MV/cm transverse field is  ̃120 meV for unstrained Si and  ̃250 meV for strained Si. The increase in energy splitting reduces inter-valley scattering and enhances NMOSFET mobility, as demonstrated at low (<0.6 MV/cm) and higher ( ̃1 MV/cm) vertical fields. The scaled transconductance (gm) is also improved due to the reduced density of states and enhanced non-equilibrium transport.
One method for forming the Si/SiGe layer involves epitaxially growing the Si and SiGe layers using an ultra-high vacuum chemical vapor deposition (UHVCVD) process. The UHVCVD process is a costly and complex process. The Ge content is graded in steps to form a fully relaxed SiGe buffer layer before a thin ( ̃20 nm) strained Si channel layer is grown. X-ray diffraction analysis can be used to quantify the Ge content and strain relaxation in the SiGe layer. The strain state of the Si channel layer can be confirmed by Raman spectroscopy. One proposed back end approach for straining silicon applies uniaxial strain to wafers/dies after the integrated circuit process is complete. The dies are thinned to membrane dimensions and then affixed to curved substrates to apply an in-plane, tensile strain after device manufacture.
Research indicates that uniaxial strained silicon has advantages over biaxial strained silicon. Less strain is required to obtain an improvement factor if the silicon is strained uniaxially rather than biaxially. Uniaxial strained silicon reduces contact potentials, alters the work function, and reduces band gap and in-plane effective mass to improve conduction.
There is a need in the art to provide improved strained semiconductor films and devices that incorporate the strained films, and to provide improved methods for forming strained semiconductor films.
SUMMARY
The above mentioned problems are addressed and will be understood by reading and studying this specification. Strained semiconductor films are disclosed herein. Also disclosed herein are methods of forming the strained semiconductor films along with methods of forming structures and devices that include strained semiconductor films. A relatively simple and low cost wafer bonding technique to produce strained semiconductor layers is described herein. A thin semiconductor layer is bonded to a semiconductor substrate with a desired tilt/twist misorientation designed to produce a desired strain in the thin semiconductor layer. The thickness of the thin semiconductor layer is sufficiently small such that the film is capable of being sufficiently strained to enhance carrier mobility before plastic deformation.
Disclosed herein is a method for forming a strained semiconductor structure. In various embodiments, at least two strong bonding regions are defined for a desired bond between a crystalline semiconductor membrane and a crystalline semiconductor substrate. The two strong bonding regions are separated by a weak bonding region. The membrane is bonded to the substrate at a predetermined misorientation. The membrane is pinned to the substrate in the strong bonding regions. The predetermined misorientation provides the membrane in the weak bonding region with a desired strain. In various embodiments, the membrane is bonded to the substrate at a predetermined twist angle to biaxially strain the membrane in the weak bonding region. In various embodiments, the membrane is bonded to the substrate at a predetermined tilt angle to uniaxially strain the membrane in the weak bonding region.
In various embodiments, a crystalline semiconductor membrane is defined in a surface layer of a sacrificial crystalline semiconductor wafer. The surface layer of the sacrificial wafer is bonded to a crystalline semiconductor substrate at a predetermined misorientation. The crystalline membrane is pinned to the crystalline substrate in two or more strong bonding regions that are separated by a weak bonding region. The sacrificial wafer and the substrate are heat-treated. The sacrificial layer is removed from the membrane such that the membrane remains strongly bonded to the substrate at the predetermined misorientation. The crystalline membrane in the weak bonding region has a desired strain attributed to the predetermined misorientation.
Disclosed herein is a strained semiconductor structure. The structure includes a crystalline silicon substrate, and a crystalline silicon membrane strongly bonded to the substrate in at least two predetermined strong bonding regions and weakly bonded to the substrate in a weak bonding region located between the strong bonding regions. The membrane is strained in between the strong bonding regions. The membrane includes a regular array of screw dislocations in the strong bonding regions. In various embodiments, the membrane is bonded to the substrate at a predetermined twist misorientation such that the membrane in the weak bonding region has a biaxial strain. In various embodiments, the membrane is bonded to the substrate at a predetermined tilt misorientation such that the membrane in the weak bonding region has a uniaxial strain.
These and other aspects, embodiments, advantages, and features will become apparent from the following description and the referenced drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a relationship between elastic strain and semiconductor layer thicknesses.
FIG. 2 illustrates the lattice constant of a silicon germanium (Si1−xGex) substrate for different percentages (X) of germanium.
FIG. 3 illustrates the mobility enhancement for strained silicon for different percentages (X) of germanium in a silicon germanium (Si1−xGex) substrate.
FIG. 4 illustrates bonding of a thin semiconductor film with respect to a substrate at a predetermined misorientation, according to various embodiments of the present invention.
FIGS. 5A-5D illustrate a method to form a strained semiconductor membrane using a bond cut process, according to various embodiments of the present invention.
FIGS. 6A-6B illustrate a method to biaxially strain a semiconductor membrane by bonding the membrane to the substrate at a predetermined twist misorientation, according to various embodiments of the present invention.
FIGS. 7A-7B illustrate a method to uniaxially strain a semiconductor membrane by bonding the membrane to the substrate at a predetermined tilt misorientation, according to various embodiments of the present invention.
FIG. 8 illustrates a misorientation angle (α) for crystalline semiconductor cells having a length (a) for use to determine a strain associated with the misorientation.
FIG. 9 illustrates a transistor fabricated with a strained semiconductor membrane, according to various embodiments of the present invention.
FIG. 10 illustrates a method for forming a strained semiconductor structure, according to various embodiments of the present invention.
FIG. 11 illustrates a method for forming a strained semiconductor structure using a bond cut process, according to various embodiments of the present invention.
FIG. 12 is a simplified block diagram of a high-level organization of various embodiments of a memory device according to various embodiments of the present invention.
FIG. 13 is a simplified block diagram of a high-level organization of various embodiments of an electronic system according to the present invention.



DETAILED DESCRIPTION
The following detailed description refers to the accompanying drawings which show, by way of illustration, specific aspects and embodiments in which the present invention may be practiced. The various embodiments of the present invention are not necessarily mutually exclusive as aspects of two or more embodiments can be combined to form other embodiments. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. In the following description, the terms wafer and substrate are interchangeably used to refer generally to any structure on which integrated circuits are formed, and also to such structures during various stages of integrated circuit fabrication. Both terms include doped and undoped semiconductors, epitaxial layers of a semiconductor on a supporting semiconductor or insulating material, combinations of such layers, as well as other such structures that are known in the art. The terms “horizontal” and “vertical”, as well as prepositions such as “on”, “over” and “under” are used in relation to the conventional plane or surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.
Various embodiments of the present invention provides methods to strain silicon, and structures formed using strained silicon. A crystalline semiconductor membrane is bonded to a crystalline semiconductor substrate at a predetermined misorientation to provide the membrane with a desired strain. Various embodiments mechanically strain a thin silicon layer by bonding the thin silicon layer to a silicon substrate with a desired tilt misorientation to provide the silicon layer with a desired uniaxial strain. Various embodiments mechanically strain a thin silicon layer by bonding the thin silicon layer to a silicon substrate with a desired twist misorientation to provide the silicon layer with a desired biaxial strain. Various embodiments mechanically strain a thin silicon layer by bonding the thin silicon layer to a silicon substrate with a desired tilt and twist misorientation. The silicon layer is sufficiently thin such that it does not plastically deform due to dislocations, yields and/or fractures. One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that other semiconductor materials can be strained in accordance with this disclosure.
Silicon is intentionally strained with a mechanical strain to enhance carrier mobility. Desirable mechanical strain can be determined using research findings for Si/SiGe structures. Si has a lattice constant of 5.43095 Å, and Ge has a lattice constant of 5.64613 Å. The lattice constant of SiGe is between the lattice constant of Si and the lattice constant of Ge, and depends on the percentage of Ge in the SiGe layer. FIG. 1 illustrates the lattice constant of a Si1−xGex substrate for different percentages (X) of Ge. As indicated by FIG. 1, a Si1−xGex substrate containing about 30% Ge (X≈0.3) has a lattice constant of about 5.50 Å. The biaxial strain of the Si on the SiGe can be calculated as follows: 



  
    
      
        Biaxial_Strain
        =
        
          
            
              SiGe
              LC
            
            -
            
              Si
              LC
            
          
          
            Si
            LC
          
        
      
    
    
      
        (
        1
        )
      
    
  




where the subscript LC represents the lattice constant of the SiGe or Si. Thus, as shown in Equation 2, the Si on the SiGe substrate has a biaxial strain of about 1.28%. 



  
    
      
        
          Biaxial_Strain
          ≈
          
            
              5.50
              -
              5.43
            
            5.43
          
        
        =
        
          1.28
          ⁢
          
            %
            .
          
        
      
    
    
      
        (
        2
        )
      
    
  




FIG. 2 illustrates the mobility enhancement for strained Si for different percentages (X) of Ge in a Si1−xGex substrate. The mobility enhancement increases as the percentage of Ge in the Si1−xGex increases, and levels off to around 1.6 when the percentage of Ge is around 22% or larger. Referring to FIG. 1, 22% Ge provides the Si1−xGex substrate with a lattice constant (SiGeLC) of around 5,485. Using Equation 1, it is determined that the corresponding strain for 22% Ge (the approximate point where the mobility enhancement levels off) is about 1%.

When the percentage of Ge in the Si1−xGex is about 20% (near the knee of the curve), it can be calculated that the resulting strain is about 0.75%. When the percentage of Ge in the Si1−xGex is about 40%, it can be calculated that the resulting strain is about 1.5%. Referring again to FIG. 1, it can be seen that a Si1−xGex substrate having just under 10% Ge still provides considerable mobility enhancement (1.3). A Si1−xGex substrate having just under 10% Ge provides the Si1−xGex substrate with a lattice constant (SiGeLC) of around 5.457. Using Equation 1, it is determined that the corresponding strain is around 0.5%. Thus, it is desirable to achieve a biaxial strain around or greater than 0.5%, and preferably around 1% or greater to obtain the desired enhanced mobility associated with strained Si.
A strain is mechanically induced in thin semiconductor layers. FIG. 3 illustrates a relationship between elastic strain and semiconductor layer thicknesses. The semiconductor yield is plotted with respect to plastic deformation and defects in bulk samples. The illustrated values represent the relationship of thin SiGe layers on silicon. FIG. 3 illustrates that thin layers of silicon or other semiconductor materials are more tolerant of strain than thick bulk samples. Previously, thin layers of SiGe have been fabricated on silicon because of the tolerance of the thin layers to strain. FIG. 3 indicates that 1000 Å thin semiconductor layers can be strained up to about 1%, that 100 Å thin semiconductor layers can be strained up to about 2% and thinner semiconductor layers can be strained up to about 2.5%. However, as illustrated earlier with respect to FIG. 2, the mobility enhancement levels off when the strain reaches about 1%.
It is thus desirable to strain a thin semiconductor layer, such as a silicon layer, with a strain greater than 0.5% to achieve significant mobility enhancement. It is desirable to strain a thin semiconductor layer, such as an ultra-thin silicon layer approximately 100 Å or less, with a strain within a range of approximately 0.75% to approximately 1.5% where the mobility enhancement levels off. It is also desirable to reduce unnecessary strain and provide a margin for error without unduly affecting the mobility enhancement. Thus, it is desirable to strain a thin semiconductor layer, such as a thin silicon layer, with a strain in the range of approximately 1% to approximately 1.2%.
A thin silicon film is strained by bonding the thin silicon film on a silicon substrate with a predetermined misorientation. In various embodiments, the predetermined misorientation is a predetermined tilt and/or twist misorientation. Twist-bonding has been explored in wafer bonding research, where it is known that a lattice-mismatch between an epitaxial layer and the substrate material causes structural defects when a film is grown to a certain thickness. The film grows pseudomorphically until dislocations form to accommodate the strain energy. Thus, misorientation during wafer bonding can cause a stress that results in plastic deformation. The stress associated with the misorientation causes dislocations to be created, and/or causes the bonded material to yield. It has been proposed in literature to twist-bond a thin crystalline membrane onto a crystalline substrate to form a structure that has been identified in literature as a “compliant substrate,” a “universal substrate,” and a “compliant universal substrate.” These substrates involve twist-bonding a crystallized membrane (such as GaAs) onto a crystallized substrate (such as GaAs). The membrane is rotated with respect to and wafer-fused to the substrate such that the crystals of the membrane and the substrate are not orientated. A dense network of screw dislocations are formed in the membrane to bond the thin membrane to the substrate. This dense network is formed as a regular array of small islands that twist into alignment with and firmly bond to the substrate. Flexible distorted regions are formed between the regular array of islands (screw dislocations). These flexible distorted regions between the screw dislocations are not securely bonded to the substrate, and form a spring-like flexible layer capable of absorbing lattice mismatches. Thus, when a lattice mismatched epitaxial layer is grown on the membrane, the membrane deforms elastically to accommodate the strained energy to prevent misfit dislocations in the epitaxial layer. It is known from experience with Si/SiGe structures that thin films are able to be significantly strained before plastic deformation or yield. Thus, thin films of GaAs have been previously described for producing compliant universal substrate.
A thin silicon film is bonded on a silicon substrate with a predetermined misorientation (tilt and/or twist) to produce a sufficient strain for mobility enhancement without causing dislocations, plastic deformation, yield and/or fracture. A bond between a crystalline membrane and a crystalline substrate is designed with two or more strong bonding regions that are separated by a weak bonding region. The strong bonding regions include a regular array of screw dislocations. The membrane in the weak bonding region has a desired strain. Various embodiments of the present invention provide biaxial strained silicon and various embodiments provide uniaxial-strained silicon.
FIG. 4 illustrates bonding of a thin semiconductor film with respect to a substrate at a predetermined misorientation, according to various embodiments of the present invention. The figure illustrates a crystalline semiconductor substrate 402, such as a crystalline semiconductor substrate, and further illustrates a crystalline thin semiconductor film 404, layer or membrane, such as a crystalline silicon membrane. The membrane 404 is oriented to the substrate 402 at an angle α, and is bonded to the substrate. Due to the crystalline nature of the membrane 404 and the substrate 402, the crystal orientation of the membrane is considered to be misorientated with the crystal orientation of the substrate. This misorientation is intentional, and as is described below, is chosen to provide a desired strain in the membrane.
The membrane and the substrate are defined to have strong bonding regions 406 and a weak bonding region 408 between the strong bonding regions. The nomenclature for these regions 406 and 408 characterize a bonding interface between the membrane 404 and the substrate 402. In various embodiments, the membrane and the substrate are appropriately prepared to provide an oxide-free bonding interface between the membrane and the substrate in the strong bonding regions 406 to provide a stronger bond. In various embodiments, the membrane and the substrate are appropriately prepared to provide an oxide in the bonding interface between the membrane and the substrate in the weak bonding region 408 to provide a weaker bond. One of ordinary skill in the art will appreciate, upon reading and comprehending this disclosure, that these preparations may only require preparing one of the membrane 404 and the substrate 402. The strong bonding regions 406 have a regular array of screw dislocations that bond the membrane to the substrate. The membrane 404 is strained in the weak bonding region 408. A desired strain is achieved by controlling the misorientation angle α.
Thinner semiconductor layers are capable of being under greater strain before yielding. The semiconductor membrane has a thickness such that it does not yield under a desired strain achieved by bonding the membrane to the substrate at a desired misorientation angle. In various embodiments, the semiconductor membrane is ultra thin. In various embodiments, the semiconductor membrane is approximately 1000 Å or less. In various embodiments, the semiconductor membrane is approximately 100 Å or less.
One process for forming the membrane 404 involves a bond cut process to form a surface silicon layer from a sacrificial wafer, and bond the surface silicon layer to a silicon substrate in a manner to provide the desired twist and/or tilt orientation between the crystalline structure in the surface layer and the crystalline structure in the substrate. The bond cut process has been referred to in various literature as a “smart-cut process.”
FIGS. 5A-5D illustrate a method to form a strained semiconductor membrane using a bond cut process, according to various embodiments of the present invention. The bond cut process involves bonding together two substrates, or wafers, and breaking off a section of at least one of the two substrate after the substrates have been bonded together.
FIG. 5A illustrates a sacrificial semiconductor wafer 510, and FIG. 5B illustrates a semiconductor substrate 502. The substrate 502 includes a semiconductor material. In various embodiments, the semiconductor material includes one of the following materials: silicon; germanium; silicon-germanium; gallium arsenide; indium phosphide; and other semiconductor materials. This list of potential semiconductor materials is not intended to be an all-inclusive list. The silicon substrate is cut into wafer size patterns, and integrated circuits are formed thereon. In various embodiments, the sacrificial wafer includes various semiconductor material including but not limited to silicon, germanium, silicon-germanium, gallium arsenide, indium phosphide, and other semiconductor materials.
The sacrificial wafer 510 is a single crystal wafer, and is conditioned by implanting ions 512 into a surface. The ions are implanted along a plane, represented in FIG. 6 as a line 514, to define a surface layer 516 with a predetermined thickness. The plane is approximately parallel to the surface in which the ions are implanted. In various embodiments, hydrogen ions are used as implantation ions. The hydrogen ions can include H+, H2+, D+, and/or D2+ ions. The implanted ions act 512 to form cavities along the plane 514. The cavities are joined through thermal processing, allowing the surface layer 516 to be removed from the remaining portion of the sacrificial wafer 518 at the cleavage plane 514. In various embodiments, this thermal processing occurs while the surface layer 516 is being bonded to the substrate 502, as shown in FIG. 5C. Once these cavities join and the surface layer is bonded to the substrate, the surface layer breaks off of the sacrificial wafer at the cleavage plane and remains bonded to the substrate. The remaining portion of the sacrificial wafer 518 can be used to form membranes for other substrates, thus reducing waste the overall cost for the manufacturing process of a wide variety of electronic devices.
At least one of the membrane and the substrate are prepared to define strong and weak bond regions 506 and 508, respectively. FIG. 5B illustrates an oxide 520 on the substrate 502 where it is desired to weakly bond the membrane to the substrate. The strong bonding regions are oxide-free to provide strongly bond the interface of the membrane to the substrate. One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that there are a number of ways to define strong and weak bonding regions on the substrate 502 and/or on the surface layer 516.
FIG. 5C illustrates the surface layer 516 of the sacrificial wafer 510 bonded to the substrate 502. Before the surface layer is bonded to the substrate, the sacrificial wafer and the substrate can be cleaned using conventional cleaning procedures. In various embodiments, the bonding force includes the strong Van der Waal's force that naturally bonds surfaces together as the bonding force. In various embodiments, the Van der Waal's force provides an initial bonding force that is strengthened during subsequent thermal processing.
The bonded wafers are heated to further bond the surface layer to the substrate and to cut the surface layer 516 from the sacrificial wafer 510. Heating the sacrificial wafer joins the cavities in the cleavage plane, allowing the remaining portion of the sacrificial wafer to be removed from the surface layer, which remains bonded to the substrate. The remaining portion of the sacrificial wafer can be prepared and conditioned for another bond cut process.
The thickness of the silicon layer 516 bonded to the substrate 502 is defined by the depth of ion implantation 512 during the bond cut process. In various embodiments, the thickness of the silicon layer is such that it does not yield or otherwise plastically deform under the desired mechanical strained induced by the twist/tilt bond. In various embodiments, the silicon layer has a thickness of about 0.1 microns (100 nm or 1000 Å). In various embodiments, the silicon layer has a thickness less than 0.1 microns. In various embodiments, the silicon layer has a thickness of around 100 Å or less.
In various embodiments, the silicon film is prepared for transistor fabrication. In various embodiments, the preparation of the film includes chemical and/or mechanical polishing. Thus, the membrane bonded to the substrate illustrated in FIG. 5D can be thinner than the surface layer defined in the sacrificial layer in FIG. 5A. Device processing can be accomplished using conventional processes and procedures.
FIGS. 6A-6B illustrate a method to biaxially strain a semiconductor membrane by bonding the membrane to the substrate at a predetermined twist misorientation, according to various embodiments of the present invention. The figure illustrates a top view of a silicon membrane 604 bonded to a silicon substrate 602 with a predetermined twist misorientation. The misorientation angle α is along the plane where the membrane 604 interfaces with the substrate 602. The present invention is applicable to semiconductor material other than silicon.
Strong bonding regions 606 are provided, and a weak boding region 608 is provided between the strong bonding regions 606. The membrane 604, or thin film, is pinned to the substrate in the strong bonding regions. These strong bonding regions are made free of an oxide. The weak bonding region between the pinned regions where the film will be strained is left with a thin oxide or native oxide, or otherwise is provided with an oxide, to provide a low bonding strength.
FIG. 6B is a top view of the membrane that is pinned to the substrate in the strong bonding regions. The membrane in the weak bonding region has a biaxial strain, as indicated by the arrows 622.
FIGS. 7A-7B illustrate a method to uniaxially strain a semiconductor membrane by bonding the membrane to the substrate at a predetermined tilt misorientation, according to various embodiments of the present invention. FIG. 7A illustrates a side view of a silicon membrane 704 bonded to a silicon substrate 702 with a predetermined tilt orientation. The misorientation angle α is vertical with respect to a flat wafer surface. Again, the present invention is applicable to semiconductor material other than silicon.
FIG. 7B illustrates a top view of the silicon layer. The silicon is uniaxially strained if the misorientation is attributed to a tilt misorientation. Strong bonding regions 706 are provided, and a weak bonding region 708 is provided between the strong bonding regions. The membrane 704, or thin film, is pinned to the substrate 702 in the strong bonding regions 706. These strong bonding regions are made free of an oxide. The weak bonding region 708 between the pinned regions where the film will be strained is left with a thin oxide or native oxide, or otherwise is provided with an oxide, to provide a low bonding strength. The membrane 704 in the weak bonding region 708 has a uniaxial strain, as indicated by the arrows 724.
Less strain is required to obtain an improvement factor if the silicon is strained uniaxially rather than biaxially. Uniaxial strained silicon reduces band gap and in-plane effective mass to improve conduction, reduces contact potentials, and alters the work function.
FIG. 8 illustrates a misorientation angle (α) for crystalline semiconductor cells having a length (a) for use to determine a strain associated with the misorientation. The figure illustrates a calculation of the amount of strain as a function of a twist misorientation. As is calculated below, 1.8° twist misorientation produces a 1% strain. This strain stretches the silicon bonds. A 1% strain is sufficient to produce a significant enhancement in the electron mobility. As provided earlier, a silicon strain greater than 0.5% achieves significant mobility enhancement. Various embodiments provide a strain up to about 2.5%. Various embodiments mechanically strain the silicon with a strain within a range of approximately 0.75% to approximately 1.5% where the mobility enhancement levels off. Various embodiments mechanically strain the silicon with a strain within a range of approximately 1% to approximately 1.2%.
The amount of strain the layers allow before plastic deformation and/or yielding is determined by the thickness of the film. In various embodiments, a thin silicon film is bonded to a silicon substrate. In various embodiments, the thin silicon film approximately 1000 Å (100 nm) or less. Silicon films having a thickness of approximately 1000 Å (100 nm) or less can be referred to as ultra-thin films. In various embodiments, the thin silicon film is approximately 100 Å or less.
In FIG. 8, the length of the crystalline atoms is identified as “a” and the length for a number (n) of crystalline atoms is “n·a.” The length of a number of crystalline atoms increases from “n·a” to d when a crystalline membrane is bonded to a crystalline substrate with a misorientation angle α because the silicon bonds are stretched. Referring to the figure, it is determined that:

d=√{square root over ((n·a)2+y2;)}and  (3)




  
    
      
        
          tan
          ⁢
          
             
          
          ⁢
          α
        
        =
        
          
            y
            
              n
              ·
              a
            
          
          .
        
      
    
    
      
        (
        4
        )
      
    
  



 y2=(n·a)2·tan2 αand  (5)

d=√{square root over ((n·a)2+(n·a)2·(tan2 α))}{square root over ((n·a)2+(n·a)2·(tan2 α))}{square root over ((n·a)2+(n·a)2·(tan2 α))}  (6)

Equation 6 is simplified into equation 7 as follows.

d=(n·a)·√{square root over (1+(tan2 α))}.  (7)

If “x” is small, the following approximation (equation 8) can be made. 



  
    
      
        
          
            1
            +
            
              x
              2
            
          
        
        ≈
        
          1
          +
          
            
              x
              2
            
            .
          
        
      
    
    
      
        (
        8
        )
      
    
  




Substituting into equation 7, 



  
    
      
        
          d
          =
          
            n
            ·
            
              a
              ⁡
              
                (
                
                  1
                  +
                  
                    
                      tan
                      ⁢
                      
                         
                      
                      ⁢
                      α
                    
                    2
                  
                
                )
              
            
          
        
        ,
        or
      
    
    
      
        (
        9
        )
      
    
  
  
    
      
        
          
            d
            
              n
              ·
              a
            
          
          =
          
            1
            +
            
              
                
                  tan
                  ⁢
                  
                     
                  
                  ⁢
                  α
                
                2
              
              .
            
          
        
        ⁢
        
           
        
      
    
    
      
        (
        10
        )
      
    
  




If α small, the following approximation (equation 11) can be made.

tan α≈α, where α is in radians.   (11)

Substituting into equation 10, 



  
    
      
        
          d
          
            n
            ·
            a
          
        
        =
        
          1
          +
          
            
              α
              2
            
            .
          
        
      
    
    
      
        (
        12
        )
      
    
  




An estimated strain when α is 1.15°, which equals π(1.15/180) or 0.02 radians, is provided in equation 13. 



  
    
      
        strain
        =
        
          
            
              (
              
                1
                +
                
                  0.02
                  2
                
              
              )
            
            -
            1
          
          =
          
            1
            ⁢
            
              %
              .
            
          
        
      
    
    
      
        (
        13
        )
      
    
  




Thus, the induced strain is approximately equal to one half the misorientation angle α in radians, where α is small. Thus, a 2.5% strain corresponds to a misorientation angle of 0.050 radians or 2.9°, and a 0.75% strain corresponds to a misorientation angle of 0.015 radians or 0.86°.

FIG. 9 illustrates a transistor fabricated with a strained semiconductor membrane, according to various embodiments of the present invention. The illustrated transistor 930 includes a crystalline semiconductor substrate 902, and a crystalline semiconductor membrane 904 bonded to the substrate 902 with a desired misorientation to provide the membrane 904 with a desired strain. A gate dielectric 932 is formed on the strained membrane, and a gate 934 is formed on the gate dielectric 932. First and second diffusion regions 936 and 938 are formed in the structure. A channel region 940 is formed in the strained semiconductor membrane 904 between the first and second diffusion regions 936 and 938.
FIG. 10 illustrates a method for forming a strained semiconductor structure, according to various embodiments of the present invention. In various embodiments, at least two strong bonding regions are defined at 1050 for a desired bond between a crystalline semiconductor membrane and a crystalline semiconductor substrate. The strong bonding regions are separated by a weak bonding region. The membrane is bonded to the substrate at a predetermined misorientation at 1052. The membrane is pinned to the substrate at the strong bonding regions by a regular array of screw dislocations. The membrane is strained in the weak bonding region. The strain corresponds to the misorientation angle of the membrane bonded to the substrate.
FIG. 11 illustrates a method for forming a strained semiconductor structure using a bond cut process, according to various embodiments of the present invention. At 1154, a membrane is defined in a surface layer of a sacrificial wafer. The membrane is defined by implanting ions into the surface layer of the sacrificial wafer to form cavities along a cleavage plane in the sacrificial wafer. At 1156, the surface layer of the sacrificial wafer is bonded to a substrate at two or more strong bonding regions in a predetermined misorientation. The naturally occurring Van der Waal's force provides an initial bonding force. At 1158, the sacrificial wafer and the substrate is heat treated. This heat-treating process further bonds the surface layer of the sacrificial wafer to the substrate, and joins the cavities along the cleavage plane. At 1160, the sacrificial wafer (or the remaining portions thereof) is removed from the membrane with the membrane left bonded to the substrate at the predetermined misorientation. The membrane has a desired strain in a weak bonding region positioned between strong bonding regions.
FIG. 12 is a simplified block diagram of a high-level organization of various embodiments of a memory device according to various embodiments of the present invention. The illustrated memory device 1268 includes a memory array 1270 and read/write control circuitry 1272 to perform operations on the memory array via communication line(s) 1274. The illustrated memory device 1268 may be a memory card or a memory module such as a single inline memory module (SIMM) and dual inline memory module (DIMM). One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that semiconductor components in the memory array 1270 and/or the control circuitry 1272 are able to be fabricated using the strained semiconductor films, as described above. For example, in various embodiments, the memory array 1270 and/or the control circuitry 1272 include transistors with strained body layers formed using a strained silicon on silicon germanium (Si/SiGe) structure. The structure and fabrication methods for these strained body layers have been described above.
The memory array 1270 includes a number of memory cells 1278. The memory cells in the array are arranged in rows and columns. In various embodiments, word lines 1280 connect the memory cells in the rows, and bit lines 1282 connect the memory cells in the columns. The read/write control circuitry 1272 includes word line select circuitry 1274, which functions to select a desired row. The read/write control circuitry 1272 further includes bit line select circuitry 1276, which functions to select a desired column.
FIG. 13 is a simplified block diagram of a high-level organization of various embodiments of an electronic system according to the present invention. In various embodiments, the system 1300 is a computer system, a process control system or other system that employs a processor and associated memory. The electronic system 1300 has functional elements, including a processor or arithmetic/logic unit (ALU) 1302, a control unit 1304, a memory device unit 1306 (such as illustrated in FIG. 12) and an input/output (I/O) device 1308. Generally such an electronic system 1300 will have a native set of instructions that specify operations to be performed on data by the processor 1302 and other interactions between the processor 1302, the memory device unit 1306 and the I/O devices 1308. The control unit 1304 coordinates all operations of the processor 1302, the memory device 1306 and the I/O devices 1308 by continuously cycling through a set of operations that cause instructions to be fetched from the memory device 1306 and executed. According to various embodiments, the memory device 1306 includes, but is not limited to, random access memory (RAM) devices, read-only memory (ROM) devices, and peripheral devices such as a floppy disk drive and a compact disk CD-ROM drive. As one of ordinary skill in the art will understand, upon reading and comprehending this disclosure, any of the illustrated electrical components are capable of being fabricated to include strained silicon on silicon germanium (Si/SiGe) in accordance with the present invention.
The illustration of the system 1300 is intended to provide a general understanding of one application for the structure and circuitry of the present invention, and is not intended to serve as a complete description of all the elements and features of an electronic system using strained semiconductor films according to the present invention. As one of ordinary skill in the art will understand, such an electronic system can be fabricated in single-package processing units, or even on a single semiconductor chip, in order to reduce the communication time between the processor and the memory device.
Applications containing strained semiconductor films, such as transistors with a strained semiconductor body layer, as described in this disclosure include electronic systems for use in memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. Such circuitry can further be a subcomponent of a variety of electronic systems.
CONCLUSION
The present invention provides strained semiconductor layers by bonding, or wafer bonding, a crystalline semiconductor membrane to a crystalline semiconductor substrate at a predetermined misorientation. The membrane is strongly bonded to the substrate in at least two strong bonding regions and is weakly bonded to the substrate in a weak bonding region between the strong bonding regions. The membrane in the weak bonding region has a strain attributable to the misorientation. A regular array of screw dislocations bonds the membrane to the substrate in the strong bonding regions. In various embodiments, an interface between the membrane and the substrate is free of an oxide in the strong bonding regions, and includes an oxide, such as a thin oxide or native oxide, in the weak bonding regions.
This disclosure includes several processes, circuit diagrams, and structures. The present invention is not limited to a particular process order or logical arrangement. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments, and other embodiments, will be apparent to those of skill in the art upon reviewing the above description. The scope of the present invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

## Claims

1. A method for forming a strained semiconductor structure, comprising:
with respect to a desired bond between a crystalline semiconductor membrane and a crystalline semiconductor substrate, defining at least two strong bonding regions separated by a weak bonding region; and 
bonding the membrane to the substrate at a predetermined misorientation, including pinning the membrane to the substrate in the strong bonding regions, wherein the predetermined misorientation provides the membrane in the weak bonding region with a desired strain.

2. The method of claim 1, wherein defining at least two strong bonding regions separated by a weak bonding region includes:
forming an interface between the membrane and the substrate to be free of an oxide in the strong bonding regions to strengthen bonding between the membrane and the substrate; and forming the interface between the membrane and the substrate to include an oxide in the weak bonding region to weaken bonding between the membrane and the substrate.

3. The method of claim 1, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined twist angle to biaxially strain the membrane in the weak bonding region.

4. The method of claim 1, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined tilt angle to uniaxially strain the membrane in the weak bonding region.

5. The method of claim 1, further comprising forming the membrane to have a thickness sufficiently thin to not plastically deform under the desired strain.

6. The method of claim 5, wherein the thickness of the membrane is approximately 1000 Å or less.

7. The method of claim 1, wherein bonding the membrane to the substrate at a predetermined misonentation includes bonding the membrane to the substrate at a misorientation to provide the membrane with a strain greater than 0.5%.

8. The method of claim 1, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a misorientation to provide the membrane with a strain within a range between approximately 0.75% and approximately 1.5%.

9. The method of claim 1, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a misorientation to provide the membrane with a strain within a range between approximately 1.0% and approximately 1.2%.

10. The method of claim 1, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane at an angle with respect to the substrate to provide a strain within a range between approximately 0.75% and approximately 2.5%.

11. A method for forming a strained silicon structure, comprising:
with respect to a desired bond between a crystalline silicon layer and a crystalline silicon substrate, defining at least two strong bonding regions separated by a weak bonding region; and 
bonding the silicon layer to the substrate at a predetermined misorientation, including pinning the silicon layer to the substrate in the strong bonding regions, wherein the predetermined misorientation provides the membrane in the weak bonding region with a desired strain.

12. The method of claim 11, wherein pinning the silicon layer to the substrate in the strong bonding regions includes forming a regular array of screw dislocations in the silicon layer.

13. The method of claim 11, wherein bonding the silicon layer to the substrate uses a naturally-occurring Van der Waal's force.

14. The method of claim 13, further comprising heat-treating the silicon layer and the substrate to further bond the silicon layer to the substrate.

15. The method of claim 11, further comprising forming the silicon layer from a sacrificial silicon wafer using a bond-cut process.

16. A method for forming a biaxially strained silicon structure, comprising:
defining at least two strong bonding regions separated by a weak bonding region for a bond between a crystalline silicon layer and a crystalline silicon substrate, including forming both the silicon layer and the substrate in the strong bonding regions to be oxide free, and forming at least one of the silicon layer and the substrate in the weak bonding region with an oxide; and 
bonding the crystalline silicon layer to the crystalline substrate at a predetermined twist misorientation, including pinning the silicon layer to the substrate in the strong bonding regions, wherein the crystalline membrane in the weak bonding region has a desired biaxial strain.

17. The method of claim 16, wherein pinning the silicon layer to the substrate in the strong bonding regions includes forming a regular array of screw dislocations in the silicon layer.

18. The method of claim 16, further comprising forming the silicon layer to have a thickness sufficiently thin to not plastically deform under the desired strain.

19. The method of claim 16, wherein bonding the silicon structure to the substrate includes forming the silicon layer in a surface layer of a sacrificial silicon wafer, and performing a bond-cut process to bond the silicon layer to the substrate and remove a remaining portion of a sacrificial wafer from the silicon layer.

20. The method of claim 16, wherein bonding the crystalline silicon layer to the crystalline substrate further includes bonding the crystalline silicon layer to the crystalline substrate at a predetermined tilt misorientation such that the crystalline membrane in the weak bonding region also has a uniaxial strain component.

21. A method for forming a uniaxially strained silicon structure, comprising:
defining at least two strong bonding regions separated by a weak bonding region for a bond between a crystalline silicon layer and a crystalline silicon substrate, including forming both the silicon layer and the substrate in the strong bonding regions to be oxide free, and forming at least one of the silicon layer and the substrate in the weak bonding region with an oxide; and 
bonding the crystalline silicon layer to the crystalline substrate at a predetermined tilt misorientation, including pinning the silicon layer to the substrate in the strong bonding regions, wherein the crystalline membrane in the weak bonding region has a desired uniaxial strain.

22. The method of claim 21, wherein pinning the silicon layer to the substrate in the strong bonding regions includes forming a regular array of screw dislocations in the silicon layer.

23. The method of claim 21, further comprising forming the silicon layer to have a thickness sufficiently thin to not plastically deform under the desired strain.

24. The method of claim 21, wherein bonding the silicon structure to the substrate includes forming the silicon layer in a surface layer of a sacrificial silicon wafer, and performing a bond-cut process to bond the silicon layer to the substrate and remove a remaining portion of a sacrificial wafer from the silicon layer.

25. The method of claim 21, wherein bonding the crystalline silicon layer to the crystalline substrate further includes bonding the crystalline silicon layer to the crystalline substrate at a predetermined twist misorientation such that the crystalline membrane in the weak bonding region also has a biaxial strain component.

26. A method for forming a strained semiconductor structure, comprising:
defining a crystalline semiconductor membrane in a surface layer of a sacrificial crystalline semiconductor wafer; 
bonding the surface layer of the sacrificial wafer to a crystalline semiconductor substrate at a predetermined misorientation, including pinning the crystalline membrane to the crystalline substrate in two or more strong bonding regions that are separated by a weak bonding region; 
heat-treating the sacrificial wafer and the substrate; and 
separating the sacrificial layer from the membrane such that the membrane remains strongly bonded to the substrate at the predetermined misorientation, 
wherein the crystalline membrane in the weak bonding region has a desired strain attributed to the predetermined misorientation.

27. The method of claim 26, wherein:
defining a crystalline semiconductor membrane in a surface layer of a sacrificial crystalline semiconductor wafer includes implanting helium ions into the saerificial wafer to form cavities along a cleavage plane; and 
heat-treating the sacrificial wafer and the substrate combines cavities along the cleavage plane such that the sacrificial layer is separated from the membrane along the cleavage plane.

28. The method of claim 26, wherein bonding the surface layer of the sacrificial wafer to a crystalline semiconductor substrate at a predetermined misorientation includes bonding the surface layer to the substrate at a predetermined twist misorientation such that the crystalline membrane in the weak bonding region has a desired biaxial strain.

29. The method of claim 26, wherein bonding the surface layer of the sacrificial wafer to a crystalline semiconductor substrate at a predetermined misorientation includes bonding the surface layer to the substrate at a predetermined tilt misorientation such that the crystalline membrane in the weak bonding region has a desired uniaxial strain.

30. A method for forming a strained silicon structure, comprising:
forming a crystalline silicon membrane and a crystalline silicon substrate with an oxide-free area to form a strong bonding region; 
forming at least one of the membrane and the substrate with an oxide area to form a weak bonding region; and 
bonding the crystalline silicon membrane to the crystalline silicon substrate at a predetermined misorientation using a bond cut process such that the membrane is pinned to the substrate in the strong bonding region and has a predetermined strain in the weak bonding region corresponding to the predetermined misorientation.

31. The method of claim 30, wherein bonding the crystalline silicon membrane to the crystalline silicon substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined twist misorientation such that the crystalline membrane in the weak bonding region has a desired biaxial strain.

32. The method of claim 30, wherein bonding the crystalline silicon membrane to the crystalline silicon substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined tilt misorientation such that the crystalline membrane in the weak bonding region has a desired uniaxial strain.

33. The method of claim 30, wherein the bond cut process includes:
implanting helium ions into a surface layer of a sacrificial wafer to form cavities positioned to define the crystalline membrane and a remaining portion in the sacrificial wafer; 
placing the surface layer in contact with the substrate; 
heat treating the sacrificial wafer and the substrate to bond the surface layer of the sacrificial wafer to the substrate and to join cavities; and 
removing the remaining portion of the sacrificial wafer such that the crystalline membrane is left bonded to the substrate.

34. A method for forming a transistor, comprising:
forming a semiconductor structure with a strained crystalline semiconductor membrane bonded to a crystalline semiconductor substrate, comprising: 
defining at least two strong bonding regions separated by a weak bonding region on at least one of the membrane and the substrate; and 
bonding the membrane to the substrate at a predetermined misorientation, including pinning the crystalline membrane to the crystalline substrate in the strong bonding regions, wherein the crystalline membrane in the weak bonding region has a desired strain; 

forming a first diffusion region and a second diffusion region separated by a channel region in the strained membrane; 
forming a gate dielectric over the channel region; and 
forming a gate over the gate dielectric.

35. The method of claim 34, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined twist misorientation to provide the membrane with a predetermined biaxial strain.

36. The method of claim 34, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined tilt misorientation to provide the membrane with a predetermined uniaxial strain.

37. The method of claim 34, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined misorientation to provide the membrane with a strain within a range of approximately 0.75% to 1.5%.

38. A method for forming a memory device, comprising:
forming a memory array in a semiconductor substrate, including forming a plurality of memory cells in rows and columns and forming at least one transistor for each of the plurality of memory cells; 
forming a plurality of word lines, including connecting each word line to a row of memory cells; 
forming a plurality of bit lines, including connecting each bit line to a column of memory cells; 
forming control circuitry in the semiconductor substrate, including forming word line select circuitry and bit line select circuitry for use to select a number of memory cells for writing and reading operations, 
wherein at least one of forming the memory array and forming the control circuitry includes forming at least one transistor, including: 
forming a semiconductor structure with a strained crystalline semiconductor membrane bonded to a crystalline semiconductor substrate, comprising: 
defining at least two strong bonding regions separated by a weak bonding region; and 
bonding the silicon membrane to the substrate at a predetermined misorientation, including pinning the membrane to the substrate in the strong bonding regions, wherein the crystalline membrane in the weak bonding region has a desired strain; 

forming a first diffusion region and a second diffusion region separated by a channel region in the strained membrane; 
forming a gate dielectric over the channel region; and 
forming a gate over the gate dielectric.

39. The method of claim 38, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined twist misorientation to provide the membrane with a predetermined biaxial strain.

40. The method of claim 38, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined tilt misorientation to provide the membrane with a predetermined uniaxial strain.

41. The method of claim 38, wherein bonding the membrane to the substrate at a predetermined misorientation includes bonding the membrane to the substrate at a predetermined misorientation to provide the membrane with a strain within a range of approximately 1.0% to 1.2%.