# Patent

## Title
Micro-mechanically strained semiconductor film

## Abstract

One aspect of the present subject matter relates to a method for forming strained semiconductor film. In various embodiments, a single crystalline semiconductor film is formed on a substrate surface, and a recess is created beneath the film. A portion of the film is influenced into the void and strained. In various embodiments, the naturally-occurring Van der Waal's force is sufficient to influence the film into the void. In various embodiments, a nano-imprint mask is used to assist with influencing the film into the void. In various embodiments, an oxide region is formed in a silicon substrate, and a single crystalline silicon film is formed on the semiconductor substrate and on at least a portion of the oxide region. The oxide region is removed allowing the Van der Waal's force to bond the film to the silicon substrate. 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 its entirety: “Vertical Transistors And Output Prediction Logic Circuits Containing Same,” U.S. application Ser. No. 10/164,611, filed on Jun. 10, 2002, now U.S. Pat. No. 6,900,521. This application is also related to the following: “Localized Strained Semiconductor On Insulator,” U.S. Pat. No. 7,041,575, filed on Apr. 29, 2003; “Strained Si/SiGe Layer Structures By Ion Implantation,” U.S. Pat. No. 6,987,037, filed on May 7, 2003; and “Strained Semiconductor By Wafer Bonding With Misorientation,” U.S. Published application Ser. No. 2004/0217352, filed on Apr. 29, 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 has been shown to enhance carrier mobility in both n and p-channel devices, and thus has been of interest to the semiconductor industry as a means to improve device speed and performance.
One technique for producing strained silicon involves growing silicon (Si) layers on relaxed silicon-germanium (Si—Ge) layers. There is a large mismatch in the cell structure between the Si and Si—Ge layers. This mismatch causes a pseudo-morphic layer of Si on relaxed SiGe to be under biaxial tensile strain. The biaxial strain 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.
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%.

Referring again to FIG. 2, 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.
One method for forming the strained Si layer on the relaxed SiGe layer involves epitaxially growing the Si and SiGe layers using an ultra-high vacuum chemical vapor deposition (UHVCVD) process. The UHVCVD process which 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.
A 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 has found 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 band gap and in-plane effective mass to improve conduction. Additionally, the work function is altered and contact potentials are reduced.
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 by the present subject matter and will be understood by reading and studying the following specification. The present subject matter relates to strained semiconductor films along with the structures and devices that include strained semiconductor films. The present subject matter further relates to methods of forming the strained semiconductor films along with methods of forming structures and devices that include strained semiconductor films.
The present subject provides uniaxially-strained semiconductor films, which has been shown to possess advantages over biaxially strained films. The strained films are produced using semiconductor processes that are simple and low cost. Strained semiconductor layers are provided by forming dimpled semiconductor layers using a micro-mechanical technique. The micro-mechanical technique uses the strong Van der Waal's force that naturally bond surfaces together as the bonding force between a substrate with recesses and the semiconductor layer. The semiconductor layers dimple as they are drawn into the recesses and bonded to the substrate. In various embodiments, an imprint mask is used to provide the dimpling force, in addition to the Van der Waal's force. The strained semiconductor material enhances electron mobility, and is also capable of enhancing hole mobility.
One aspect of the present subject matter relates to a method for forming strained semiconductor film. In various embodiments of the method, a single crystalline semiconductor film is formed on a substrate surface, and a recess is created beneath the film such that a portion of the film is influenced into the recess to strain the film. In various embodiments, the naturally-occurring Van der Waal's force is sufficient to influence the film into the recess. In various embodiments, a nano-imprint mask is used to assist with influencing the film into the recess.
In various embodiments of the method, an oxide region is formed in a silicon substrate using a LOCal Oxidation of Silicon (LOCOS) process. A single crystalline silicon film is formed on the semiconductor substrate and on at least a portion of the oxide region. The oxide region is removed from underneath the film such that a Van der Waal's force acts to draw a portion of the film into the void and bond the film to the silicon substrate.
One aspect of the present subject matter relates to a semiconductor structure. In various embodiments, the structure includes a substrate with a number of recesses and a single crystalline semiconductor film bonded to the substrate within the number of recesses, which micro-mechanically strains the semiconductor film. The structure further includes a gate formed over the semiconductor film over at least a portion of each of the number of recesses. The gate is separated from the semiconductor film by a gate insulator. The semiconductor film formed over at least a portion of each of the number of recesses includes a channel region having a length less than 1000 Å and a depth less than about 300 Å. Thus, an ultra-thin body is capable of being formed in the semiconductor film.
These and other aspects, embodiments, advantages, and features will become apparent from the following description of the present subject matter and the referenced drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the lattice constant of a Si1-xGex substrate for different percentages (X) of Ge.
FIG. 2 illustrates the mobility enhancement for strained Si for different percentages (X) of Ge in a Si1-xGex substrate.
FIG. 3 illustrates a semiconductor device formed on a strained semiconductor film, according to various embodiments of the present subject matter.
FIGS. 4A-4E illustrates a process for forming a strained semiconductor film according to various embodiments of the present subject matter that involves a common LOCal Oxidation of Silicon (LOCOS) process and a solid phase epitaxy process, and that takes advantage of the naturally-occurring Van der Wall's force to strain semiconductor film in a recess after removing the oxide formed by the LOCOS process.
FIGS. 5A-5C illustrate the uniaxial strain in the semiconductor layer achieved by the process illustrated in FIGS. 4A-4E.
FIGS. 6A and 6B provide top and cross section views, respectfully, to illustrate a pattern of silicon nitride on a silicon substrate in preparation for performing a LOCOS process, according to various embodiments of the present subject matter.
FIGS. 7A and 7B provide top and cross section views, respectfully, to illustrate a resulting structure after the oxidized structure is planarized, and the single crystalline film is formed over the oxide-filled recesses using a solid phase epitaxial process, according to various embodiments of the present subject matter.
FIG. 8 provides a cross section view illustrating the single crystalline film being influenced into the recesses after the oxide is removed from beneath the film, according to various embodiments of the present subject matter.
FIG. 9 provides a cross section view of two transistor devices in an array of transistor devices in which the transistor devices are formed using strained semiconductor films, according to various embodiments of the present subject matter.
FIG. 10 illustrates a method for forming a strained semiconductor film according to various embodiments of the present subject matter.
FIG. 11 illustrates a method for forming a strained semiconductor film according to various embodiments of the present subject matter.
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 subject matter.
FIG. 13 is a simplified block diagram of a high-level organization of various embodiments of an electronic system according to the present subject matter.



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 subject matter may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present subject matter. The various embodiments of the present subject matter are not necessarily mutually exclusive as aspects of one embodiment can be combined with aspects of another embodiment. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present subject matter. 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 term “horizontal” as used in this application is defined as a plane parallel to the conventional plane or surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizontal as defined above. Prepositions, such as “on”, “side” (as in sidewall), “higher”, “lower”, “over” and “under” are defined with respect to the conventional plane or surface being on the top surface of the 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.
In various embodiments, the present subject matter micro-mechanically strains ultra-thin transistor body layers using the naturally-occurring Van der Waal's force. The term micro-mechanical refers to the quantitative relations of force and matter at a very small or microscopic level. An ultra-thin body relates to transistor bodies having a length of about 100 nm (1000 Å) or less, and a thickness of about 30 nm (300 Å) or less. Once strained, the ultra-thin body layer is bonded to the substrate by Van der Waal's force, which is the natural force that holds together bonded wafers. The present subject matter uses well-known, low cost semiconductor processing techniques, and takes advantage of the naturally-occurring Van der Waal's force to mechanically flex or bend the semiconductor film to obtain a desired strain in the film. The resulting strain is sufficient to desirably modify the band structure and electron mobility of the semiconductor material.
FIG. 3 illustrates a semiconductor device formed on a strained semiconductor film, according to various embodiments of the present subject matter. The illustrated semiconductor device 300 includes a substrate 301 and a strained semiconductor film 302 bonded to the substrate. The illustrated substrate has a recessed surface 303. The semiconductor film is influenced by the naturally-occurring Van der Waal's force to conform to the shape of the recess. The Van der Waal's force bonds the semiconductor film 302 to the substrate 301.
In various embodiments, the semiconductor film 302 includes a single crystalline semiconductor material such a single crystalline silicon film. In various embodiments, the substrate 301 includes a silicon substrate. In various embodiments, the recess is formed in the substrate so as to cause the semiconductor film to undergo uniaxial strain when it is mechanically flexed into the recess and bonded to the substrate via the Van der Waal's force.
The illustrated semiconductor structure includes a transistor. In various embodiments, the strained semiconductor film includes an ultra thin body region 304 of a transistor that has a channel length less than or equal to about 100 nm and a thickness less or equal to about 30 nm. The transistor includes a first diffusion region 305 and a second diffusion region 306 (also referred to as source/drain regions). In various embodiments, the strained portion of the semiconductor film in the recess includes at least a portion of the first and second diffusion regions. According to various embodiments of the present subject matter, at least a portion of the first and second diffusion regions extend to non-strained portions of the semiconductor film. Furthermore, in various embodiments, at least a portion of the first and second diffusion region extend into the substrate. The illustrated transistor includes a gate insulator 307, such as an oxide and the like, formed above the body region 304 of the transistor and a gate 308 is formed on the gate insulator using relatively inexpensive semiconductor processes that are known to one of ordinary skill in the art.
Upon reading and comprehending this disclosure, one of ordinary skill in the art will understand the benefits associated with having a strained ultra-thin body region to form a channel region in the transistor. The continuous scaling of MOSFET technology to reduce channel lengths to sub-micron dimensions causes significant problems in conventional transistor structures. It is desired that the junction depth is much less than the channel length to minimize short channel effects such as drain induced barrier lowering, threshold voltage roll off, and sub-threshold conduction. In a Dynamic Random Access Memory (DRAM) application, for example, sub-threshold conduction is a significant problem as it reduces the charge storage retention time on capacitor nodes. When a MOSFET is scaled to have sub-micron channel lengths, the junction depths should be around a few hundred Å for channel lengths of about 1000 Å (100 nm). However, such shallow junctions are difficult to form by conventional implantation and diffusion techniques. One known method for suppressing short-channel effects uses extremely high levels of channel doping. However, these extremely high channel doping levels increase leakage and reduce carrier mobility such that the improved performance achieved by making the channel shorter is negated by lower carrier mobility. The strained silicon films of the present subject increase carrier mobility in the channel region, and thus can alleviate problems associated with the high channel doping.
FIGS. 4A-4E illustrates a process for forming a strained semiconductor film according to various embodiments of the present subject matter. The illustrated process involves a common LOCal Oxidation of Silicon (LOCOS) process and a solid phase epitaxy process, and takes advantage of the naturally-occurring Van der Wall's force to strain semiconductor film in a recess after removing oxide formed by the LOCOS process.
FIG. 4A illustrates a semiconductor structure 410 toward the beginning of a LOCOS process. The semiconductor structure 410 includes a silicon substrate 411. A layer of silicon nitride 412 is deposited, such as by Chemical Vapor Deposition (CVD), on the silicon substrate and is etched to expose portions of the silicon substrate for subsequent selective oxidation. One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that the pattern of the silicon nitride affects the pattern and characteristics of the recesses and thus of the strained semiconductor film.
FIG. 4B illustrates the semiconductor structure 410 after the silicon substrate 411 has been oxidized and the silicon nitride mask 412 removed. In various embodiments, the oxide 413 is thermally grown by means of wet oxidation. The oxide grows where there is no masking nitride. At the edges of the nitride, some oxidant diffuses laterally to grow under the nitride edges. This lateral growth has the shape of a slowly tapering oxide wedge 414 and is commonly referred to as a “bird's beak.”
FIG. 4C illustrates the semiconductor structure 410 after the structure has been planarized to leave an oxide-filled recess 414 in the silicon substrate. A single crystalline semiconductor layer 415 is formed over the oxide-filled recess 414. This layer is subsequently micro-mechanically strained into a vacated recess in the process illustrated in FIG. 4D.
The single crystalline semiconductor layer in various embodiments of the present subject matter includes a single crystalline silicon layer. One method for forming the single crystalline silicon layer involves a solid phase epitaxial process. In various embodiments, the solid phase epitaxial process includes depositing a thin amorphous silicon layer and annealing the structure to recrystallize the amorphous silicon. One end of the amorphous layer is seeded to promote a desired crystalline growth. In various embodiments, for example, one end of the amorphous layer is implanted with silicon (illustrated by the vertical arrow and the “Si” reference). Upon annealing, the amorphous silicon laterally recrystallizes (illustrated by the horizontal arrow), thus providing an ultra-thin (less than or equal to about 300 Å thick) single crystalline layer of silicon 415 grown over the buried oxide 414. A grain boundary 416 is illustrated on the non-seeded end of the layer. In various embodiments, the deposited amorphous layer is lightly doped with desired channel impurities for a desired design of a transistor structure.
FIG. 4D illustrates the semiconductor structure 410 after the oxide is removed to form a recess 417 below the single crystalline silicon layer and to form a microscopic bridge across the recess. FIG. 4E illustrates the top view of FIG. 4D. The shape of the oxide-filled recess can vary according to the LOCOS process described earlier. The oxide is removed using a liquid oxide etch. The naturally occurring Van der Waal's force (illustrated using arrows 418 in FIG. 4D) collapses the bridge and bonds the single crystalline silicon layer to the silicon substrate within the recess. If the bridge does not collapse under the Van der Waal's force, a non-scale imprint mask can be used to press on the bridges to cause them to collapse allowing the Van der Waal's force to bond the single crystalline layer to the silicon substrate. Nano-scale imprint masks can be made with very small raised areas which either do or do not adhere to surface upon contact.
One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that the slowly tapering shape of the bird's beak is useful to controllably induce strain in ultra-thin semiconductor films. However, the tapered bird's beak shape is not required to practice the present subject matter. One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that other means for creating a recess or void in the substrate can be used. For example, a grinding process can be used to create a recess or a trench can be otherwise formed in the substrate. The resulting recess can be filled using a material that can be later etched away or otherwise removed after the single crystalline semiconductor layer is formed over the recess.
FIGS. 5A-5C illustrate the uniaxial strain in the semiconductor layer achieved by the process illustrated in FIGS. 4A-4E. Referring to FIG. 5A, the illustrated semiconductor layer 515 is collapsed and bonded to the substrate within the recess 517. In various embodiments, the illustrated semiconductor layer is an ultra-thin single crystalline silicon layer having a thickness (T) of 300 Å (30 nm) suitable for use to form a transistor channel less than or equal to about 1000 Å (100 nm). If the recess is formed with a depth (D) of about 75 Å (7.5 nm) and a length (L) of about 1000 Å (100 nm), the single crystalline silicon layer will undergo a micro-mechanical strain (Δ strain) attributable to the dimpling of the layer into the recess.
Referring to FIG. 5B, the dimpling process can be geometrically represented using a triangle with a first leg representing 1⁄2L (half the length of the recess), and a second leg representing D (the depth of the recess). The hypotenuse of the triangle represents the strained layer (1⁄2L+Δ strain). FIG. 5C illustrates the representation of FIG. 5B with simplified or normalized values. In FIG. 5C, 500 Å is normalized to 1.0, such that 75 Å is normalized to 0.15 (75 Å/500 Å). The normalized hypotenuse is calculated to be 1.011. Thus, in the example shown in FIG. 5A, the induced strain in the single crystalline silicon layer is about 1.1%. As stated earlier in the Background section with respect to Si/SiGe layers, it is desired to provide silicon with a biaxial strain around or greater than 0.5% and preferably a biaxial strain of 1% or greater to obtain the desired enhanced mobility associated with strained silicon. Thus, as shown by the example illustrated in FIGS. 5A-5C, the processes and structures of the present subject matter are capable of providing uniaxial strains in this range. As stated earlier in the Background section, uniaxial strain has advantages over biaxial strain as it reduces band gap and in-plane effective mass to improve conduction.
The present subject matter is capable of being used to create dimpled semiconductor layers such as ultra-thin silicon layers. The dimpled areas of the silicon layers provide a desired strain in the semiconductor. FIGS. 6A-B, 7A-B, 8 and 9 illustrate a process for creating a pattern of dimples in a semiconductor film to create locally strained film, according to various embodiments of the present subject mater. Semiconductor devices are capable of being fabricated to incorporate the strained film. For example, the dimpled or micro-mechanically strained film is capable of being incorporated in the channel region of MOSFETs.
FIGS. 6A and 6B provide top and cross section views, respectfully, to illustrate a pattern of silicon nitride on a silicon substrate in preparation for performing a LOCOS process, according to various embodiments of the present subject matter. These views illustrate bars of silicon nitride 612 deposited on a silicon substrate 611. One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that the pattern of the silicon nitride corresponds to the pattern of the resulting recesses in the substrate, which in turn correspond to the pattern of strained film areas which are incorporated in semiconductor devices. The LOCOS process oxidizes the exposed portions of the substrate. The silicon nitride is removed and the substrate is planarized.
FIGS. 7A and 7B provide top and cross section views, respectfully, to illustrate a resulting structure after the oxidized structure is planarized, according to various embodiments of the present subject matter. The single crystalline film 715 is formed over the substrate 711, including over the oxide-filled recesses 714 .
In various embodiments, a solid phase epitaxial process is performed to form single crystalline silicon films on the substrate. In one solid phase epitaxial process, an amorphous silicon film is deposited on the substrate and patterned to form rows as illustrated in FIG. 7A. The amorphous silicon film is crystallized or recrystallized. In FIG. 7B, for example, silicon is implanted into the amorphous film to seed the recrystallization process. The semiconductor structure is annealed such that the amorphous silicon undergoes a lateral recrystallization process to form single crystalline silicon film. Grain boundaries are illustrated on each end of the single crystalline silicon film. In the illustrated embodiment, after the single crystalline silicon film is formed, the oxide is removed from the recesses using a liquid oxide etch.
FIG. 8 provides a cross section view illustrating the single crystalline film 815 being influenced into the recesses 817 after the oxide is removed from beneath the film, according to various embodiments of the present subject matter. After the oxide is removed, the naturally-occurring Van der Waal's force (illustrated in the figure as arrows 818) acts to influence the single crystalline silicon film into the recesses, and bond the film to the substrate. A nano-imprint mask can be used to assist with collapsing the silicon film bridges if the Van der Waal's force does not collapse the bridge. Thus, a micro-mechanical force dimples the single crystalline film to induce localized strain in the film.
FIG. 9 provides a cross section view of two transistor devices in an array of transistor devices in which the transistor devices are formed using strained semiconductor films, according to various embodiments of the present subject matter. Each transistor 931 and 932 includes an ultra-thin body 904 formed with strained semiconductor film 902. In various embodiments, the channel length of the transistor is less than or equal to about 1000 Å, and the thickness of the film is less than or equal to about 300 Å. The strain enhances mobility in the channel, thus overcoming problems associated with heavy channel doping.
One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that the transistors can be fabricated in the micro-mechanical strained ultra-thin silicon body layer using conventional techniques. Each transistor includes a gate insulator 907, such as a gate oxide and the like, deposited over the channel region, and a gate 908 deposited over the gate insulator. Each transistor includes a first diffusion region 905 and a second diffusion region 906. In the illustrated embodiment, the transistors share second diffusion regions 906. Other layouts are encompassed by the present subject matter. One of ordinary skill in the art will also understand, upon reading and comprehending this disclosure, how to fabricate the appropriate wiring level(s) for these transistors.
The diffusion regions can be formed using a number of processes. In various embodiments, for example, the substrate includes appropriately doped regions beneath the desired diffusion regions in the semiconductor film. These diffusion regions in the substrate may be positioned where the amorphous region contacts the substrate in FIG. 7B. When the amorphous silicon film is annealed during the solid phase epitaxy process, impurities diffuse from the substrate into the desired regions in the single crystalline film.
FIG. 10 illustrates a method for forming a strained semiconductor film according to various embodiments of the present subject matter. In the illustrated method, a single crystalline semiconductor film is formed on a substrate at 1041. At 1042, a void or recess is created beneath the film. Thus, the film forms a bridge that spans across the recess. As described above, various embodiments of the present subject matter use a LOCOS process to create the recess. One benefit of the LOCOS process is that it is a common economical semiconductor fabrication process. Another benefit of the LOCOS process is the tapered bird's beak, which allows for controlled strain in the film. At 1043, the film is influenced into the recess and bonds with the substrate using the naturally-occurring Van der Waal's force. If the characteristics of the film and the recess are such that the Van der Waal's force is not sufficient to collapse the bridge into the recess, a force is applied using a nano-imprint mask at 1044.
FIG. 11 illustrates a method for forming a strained semiconductor film according to various embodiments of the present subject matter. In the illustrated method, an oxide is formed in a silicon substrate using a LOCOS process at 1151. The substrate is planarized at 1152 to form an even surface. The resulting structure includes oxide-filled recesses. At 1153, amorphous silicon film is deposited over the oxide. At 1154, the amorphous silicon film is recrystallized using a solid phase epitaxy process, which involves seeding the amorphous film (such as with silicon implants) and annealing the structure to laterally recrystallize the film into a single crystalline silicon film. At 1155, the oxide is removed from beneath the film to form a void(s) or recess(es) beneath the single crystalline silicon film which forms a bridge that spans across the recess. At 1156, the film is strained as it is influenced into the recess and bonds with the substrate using the naturally-occurring Van der Waal's force. If the characteristics of the film and the recess are such that the Van der Waal's force is not sufficient to collapse the bridge into the recess, a force is applied using a nano-imprint mask at 1157.
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 subject matter. 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 device 1268 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 device 1268 and/or the control circuitry 1272 include transistors with strained, ultra-thin body layers. The structure and fabrication methods for these strained, ultra-thin body layers have been described above.
The memory array 1270 includes a number of memory cells 1200. The memory cells in the array are arranged in rows and columns. In various embodiments, word lines connect the memory cells in the rows, and bit lines 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. The read/write control circuitry 1272 further includes read circuitry 1280, which functions to detect a memory state for a selected memory cell in the memory array 1270.
FIG. 13 is a simplified block diagram of a high-level organization of various embodiments of an electronic system according to the present subject matter. 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 semiconductor films in accordance with the present subject matter.
The illustration of the system 1300 is intended to provide a general understanding of one application for the structure and circuitry of the present subject matter, 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 subject matter. 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 ultra-thin 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, such as a clock, a television, a cell phone, a personal computer, an automobile, an industrial control system, an aircraft, and others.
CONCLUSION
Uniaxially strained semiconductor films are adapted to be fabricated using common and economical semiconductor processing techniques. Semiconductor layers are dimpled using a micro-mechanical technique to provide strained semiconductor material. The micro-mechanical technique uses the strong Van der Waal's force that naturally bond surfaces together to dimple and bond the strained semiconductor layer to recesses in a substrate. In various embodiments, an imprint mask is used to assist the Van der Waal's force to dimple the semiconductor layers. The strained semiconductor material enhances electron mobility, and is also capable of enhancing hole mobility.
This disclosure includes several processes, circuit diagrams, and cell structures. The present subject matter 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 subject matter. 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 subject matter 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 strained semiconductor film, comprising:
forming a single crystalline semiconductor film on a substrate surface; and 
creating a recess beneath the film to allow a portion of the film to be influenced into the recess to strain the film.

2. The method of claim 1, wherein a Van der Waal's force acts to influence the portion of the film into the recess and bond the film to the substrate in the recess.

3. The method of claim 1, further comprising influencing the portion of the film into the recess using an imprint mask.

4. The method of claim 1, wherein forming a single crystalline semiconductor film on a surface includes forming a single crystalline silicon film on the surface.

5. The method of claim 1, wherein creating a recess includes grinding a recess in the substrate surface, filling the recess with a material upon which the single crystalline semiconductor film is formed, and removing the material from beneath the single crystalline semiconductor film.

6. The method of claim 1, wherein creating a recess includes performing a Local Oxidation of Silicon (LOCOS) process to form an oxide region upon which the single crystalline film is formed, and removing the oxide region from beneath the single crystalline film using a liquid oxide etch.

7. A method for forming strained semiconductor film, comprising:
forming a single crystalline semiconductor film on a surface; and 
creating a void beneath the film, wherein a Van der Waal's force acts to draw a portion of the film into the void to strain the film and acts to bond the film to the surface.

8. The method of claim 7, further comprising applying an imprint mask to impress the film into the void.

9. The method of claim 7, wherein forming a single crystalline semiconductor film includes forming an amorphous silicon film and crystallizing the film using a solid phase epitaxial process.

10. A method for straining a silicon film, comprising:
forming an oxide region in a silicon substrate; 
forming a single crystalline silicon film on the semiconductor substrate and on at least a portion of the oxide region; and 
removing the oxide region underneath the film to form a recess such that a Van der Waal's force acts to draw a portion of the film into the recess and bond the film to the silicon substrate.

11. The method of claim 10, wherein forming an oxide region includes performing a Local Oxidation of Silicon (LOCOS) process to form the oxide region, and planarizing the substrate in preparation to form the single crystalline silicon film.

12. The method of claim 10, wherein forming a single crystalline silicon film includes forming an amorphous silicon film and crystallizing the film using a solid phase epitaxial process.

13. The method of claim 10, wherein removing the oxide region includes performing a liquid oxide etch.

14. The method of claim 10, further comprising applying an imprint mask to deform the film into the void.

15. A method for inducing strain in a silicon film on a substrate, comprising:
forming a recess in the substrate beneath at least a portion of the silicon film to form a silicon film bridge such that a Van der Waal's force acts to collapse the bridge and bond the silicon film to the substrate in the recess.

16. The method of claim 15, wherein forming a recess in the substrate includes removing a volume of oxide formed in the substrate beneath the silicon film.

17. The method of claim 16, wherein removing a volume of oxide includes applying an liquid oxide etch.

18. The method of claim 15, further comprising applying an imprint mask to assist with collapsing the silicon film bridge.

19. The method of claim 15, wherein forming a recess includes forming a recess with predetermined dimensions to induce a strain of at least 0.5% in the silicon film when the film is bonded to the substrate within the recess.

20. The method of claim 15, wherein the silicon film forms an ultra-thin silicon body layer, and forming a recess includes forming a recess about 100 nm long and about 7.5 nm deep to induce a strain of about 1% in the silicon film.

21. A method for forming a transistor, comprising:
forming a single crystalline semiconductor film on a substrate; and 
creating a recess in the substrate beneath the film, wherein a Van der Waal's force acts to strain the film and bond the film to the surface in the recess; 
forming a body region for the transistor in the strained film, the body region being operably positioned between two source/drain regions; 
forming a gate insulator over the body region in the strained film; and 
forming a gate over the gate insulator.

22. The method of claim 21, further comprising forming the two source/drain regions in the strained film.

23. The method of claim 21, wherein forming a single crystalline semiconductor film includes performing a solid phase epitaxial process.

24. The method of claim 21, wherein forming a single crystalline semiconductor film includes depositing an amorphous silicon film on the surface, and crystallizing the amorphous silicon film.

25. The method of claim 24, wherein crystallizing the amorphous silicon film includes seeding the amorphous silicon film and annealing.

26. The method of claim 25, wherein seeding the amorphous silicon film includes implanting a portion of the amorphous silicon film with silicon.

27. The method of claim 21, further comprising impressing the semiconductor film into the recesses using an imprint mask to assist the Van der Waal's force.

28. The method of claim 21, wherein the body region includes a body region under at least 0.5% strain.

29. The method of claim 21, wherein the body region includes a body region under about 1% strain.