Abstract:
A silicon-on-insulator semiconductor device which includes a substrate; and insulator layer overlying the substrate; a plurality of strained silicon islands overlying the insulator layer, the strained silicon islands are isolated from each other by mesa isolation; and a plurality of transistors formed on the strained silicon islands. A method for fabricating the silicon-on-insulator semiconductor device is further disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to semiconductor devices and a method for making the same. More particularly, the invention is related to strained silicon-on-insulator transistors formed with mesa isolation.  
         BACKGROUND OF THE INVENTION  
         [0002]    Complementary metal-oxide-semiconductor (CMOS) technology is the dominant semiconductor technology used for the manufacture of ultra-large scale integrated (ULSI) circuits today. Size reduction of the metal-oxide-semiconductor field-effect transistor (MOSFET) has provided significant improvement in the speed performance, circuit density, and cost per unit function of semiconductor chips over the past few decades. Significant challenges are faced when CMOS devices are scaled into the sub-100 nm regime. An attractive approach for additional improvement of CMOS transistor performance exploits strain-induced band-structure modification and mobility enhancement to increase the transistor drive current. Enhanced electron and hole mobility in silicon (Si) under biaxial tensile strain can be achieved. Enhanced electron and hole mobilities improve the drive currents of N-channel and P-channel MOSFETs, respectively.  
           [0003]    Various designs of strained silicon layers for transistor fabrication utilize thick buffer layers or complex multi-layer structures on bulk silicon substrates. As shown in FIG. 1A, a device  10  utilizing conventional strained Si substrate technology employs a thick silicon-germanium (SiGe) graded buffer layer  12  with thickness in the order of micrometers formed on a Si substrate  14 . Such a thick graded buffer layer  12  requires an extended length of time, i.e., of several tens of minutes to several hours, to grow and is an expensive process. A relaxed SiGe layer  16  is then deposited overlies the graded buffer layer  12 . The relaxed SiGe layer  16  has a larger natural lattice constant than that of silicon. As a result, a thin layer  18  of silicon that is epitaxially grown on the relaxed SiGe layer  16  will be under biaxial tensile strain. This is shown in FIG. 1B. Transistors fabricated on this strained silicon layer  18  will have enhanced electrical performance. However, such a substrate might not be easily or economically introduced into a conventional CMOS process. The integration of strained layers into a conventional CMOS process introduce significant difficulties. This is because conventional CMOS process utilize high processing temperatures, especially during the formation of isolation structures.  
           [0004]    Isolation structures such as shallow trench isolation (STI), local oxide of silicon (LOCOS), and their variants are in widespread use on bulk substrates today. High temperatures favor the relaxation of strained layers and the formation of dislocations. The dislocation density in strained layers has been observed to increase with prolonged annealing at high temperatures. Since starting substrates with strained layers are subjected to the high thermal budget of the isolation formation process, the integration of strained layers on bulk substrates is very challenging. In U.S. Pat. No. 6,429,061, the strained silicon layer is grown selectively after isolation structures are formed to avoid the high temperatures of the isolation formation process. In spite of this, U.S. Pat. No. 6,429,061 employs an expensive thick SiGe buffer layer and a selective strained silicon epitaxial process.  
           [0005]    Another type of substrate, silicon-on-insulator (SOI) substrates, though offer several advantages over bulk substrates, are not as widely used as bulk substrates. In SOI technology, MOSFETs are formed on a thin layer of silicon overlying a layer of insulating material such as silicon oxide. Devices formed on SOI substrates offer many advantages over their bulk counterparts, including reduced junction capacitance, absence of reverse body effect, soft-error immunity, full dielectric isolation, and absence of latch-up. SOI technology therefore enables higher speed performance, higher packaging density, and reduced power consumption. Since it is not straightforward to incorporate a strained silicon layer on a SOI substrate, strained silicon-on-insulator (SSOI) substrates are in an initial stage of research.  
           [0006]    Others have attempted to fabricate transistors on a strained silicon layer overlying two SiGe layers with different Ge contents which is provided on an insulator layer. In U.S. Pat. No. 6,410,371, a method of forming a semiconductor-on-insulator (SOI) wafer with a Si/SiGe/Si heterostructure is disclosed, where the topmost silicon layer is strained. These works teach the formation of SOI substrates comprising of Si and SiGe layers where the Si layer is strained. The presence of SiGe in the semiconductor layer introduces problems in the formation of liner oxide in the STI process. STI process also potentially relaxes the strained silicon layer.  
           [0007]    MOSFETs with strained silicon channels have enhanced carrier mobilities. Most of the research work on strained silicon transistors employed bulk substrates, where a pseudomorphic strained silicon layer is epitaxially grown on a relaxed silicon-germanium (Si—Ge) layer. The relaxed SiGe layer overlies a thick SiGe graded buffer layer on a silicon substrate (as shown in FIGS. 1A and 1B). Such bulk substrates employ isolation structures such as shallow trench isolation (STI) to isolate devices from one another. The STI formation process typically comprises of a trench formation step, a liner oxide formation step, a trench filling step, and an annealing step. The liner oxide formation step, in the case of the strained silicon substrate, involves the oxidation of SiGe. This results in a liner oxide of poor quality and leads to leakage problems and poor device isolation.  
           [0008]    Research work on a SiGe-free SOI substrate where strained silicon is incorporated has also been reported. The elimination of SiGe in the semiconductor layer of the SOI substrate alleviates problems with liner oxide formation in the STI process. However, the STI process still involves high temperatures that might relax the strained silicon layer.  
           [0009]    It is therefore an object of the present invention to provide a silicon-on-insulator semiconductor device utilizing strained silicon technology.  
           [0010]    It is another object of the present invention to provide a silicon-on-insulator semiconductor device that can be fabricated without the drawbacks or shortcomings of the conventional manufacturing methods.  
           [0011]    It is a further object of the present invention to provide a highly manufacturable strained silicon-on-insulator transistor with mesa isolation.  
           [0012]    It is another further object of the present invention to provide a fabrication process for strained silicon-on-insulator technology with reduced thermal budget.  
         SUMMARY OF THE INVENTION  
         [0013]    In accordance with the present invention, a silicon-on-insulator semiconductor device and a method for fabricating the device are provided.  
           [0014]    In a preferred embodiment, a silicon-on-insulator semiconductor device is provided which includes a substrate; an insulator layer overlying the substrate; a plurality of strained silicon islands overlying the insulator layer, the strained silicon islands are isolated from each other by mesa isolation; and a plurality of transistors formed on the strained silicon islands.  
           [0015]    In the silicon-on-insulator semiconductor device, the substrate may be formed of a semi-conducting material, or may be a silicon wafer. The insulator layer may be a dielectric material, or may be silicon oxide. The strained silicon islands have thicknesses in the range of between 10 angstroms and 500 angstroms. A strain in the strained silicon islands may be tensile in nature in a plane of the wafer, wherein the strain may have a magnitude between 0.1% and 2%. A strain in the strained silicon islands may be compressive in nature in the plane of the wafer, wherein the strain may have a magnitude between 0.1% and 2%. The strained silicon islands may have rounded corners at isolation edges.  
           [0016]    The present invention is further directed to a method for forming strained silicon-on-insulator transistors which can be carried out by the operating steps of providing a strained silicon-on-insulator substrate including a strained silicon layer overlying an insulator layer; forming strained silicon islands in the strained silicon layer that are electrically isolated from each other by mesa isolation; and forming transistors on the strained silicon islands.  
           [0017]    The -method for forming strained silicon-on-insulator transistors may further include the step of forming the insulator layers in a material including a dielectric, or in a material including silicon oxide. The method may further include the step of forming the strained silicon layer to a thickness in the range of 10 angstroms to 500 angstroms. A strain in the strained silicon layer may be tensile in nature in a plane of the substrate, wherein a magnitude of the strain may be between about 0.1% and about 2%. A strain in the strained silicon layer may be compressive in nature in a plane of the substrate, wherein a magnitude of the strain may be between about 0.1% and about 2%.  
           [0018]    The present invention is still further directed to a method for forming strained silicon-on-insulator transistors which can be carried out by the operating steps of providing a strained silicon-on-insulator substrate including a strained silicon layer overlying an insulator layer; forming strained silicon islands in the strained silicon layer that are electrically isolated from each other by mesa isolation; forming rounded corners on the strained silicon islands; and forming transistors on the strained silicon islands.  
           [0019]    The method for forming strained silicon-on-insulator transistors may further include the step of forming the insulator layer in a material including a dielectric, or the step of forming the insulator layer in a material including silicon oxide. The method may further include the step of forming the strained silicon layer to a thickness in the range of 10 angstroms to 500 angstroms, wherein a strain in the strained silicon layer may be tensile in nature in a plane of the substrate. A magnitude of the tensile strain may be between about 0.1% and about 2%. A strain in the strained silicon layer may be compressive in nature in a plane of the substrate, wherein a magnitude of the strain may be between about 0.1% and about 2%. The method may further include the step of forming the rounded corners by process steps including etching, oxidation, and annealing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    These and other objects, features and advantages of the present invention will become apparent from the following detailed description and the appended drawings in which:  
         [0021]    [0021]FIGS. 1A and 1B are enlarged, cross-sectional views illustrating conventional strained silicon technology implemented using a bulk substrate incorporating a thick graded silicon-germanium buffer layer.  
         [0022]    [0022]FIG. 2A is an enlarged, cross-sectional view of a present invention silicon-on-insulator semiconductor device with strained silicon islands formed on an insulator layer.  
         [0023]    [0023]FIG. 2B is an enlarged, cross-sectional view of the present invention silicon-on-insulator semiconductor device with strained silicon islands that have rounded corners formed on an insulator layer.  
         [0024]    [0024]FIG. 2C is a plane view of the present invention silicon-on-insulator semiconductor device illustrating the strained silicon islands.  
         [0025]    FIGS.  3 A- 3 D are enlarged, cross-sectional views illustrating the process steps for forming strained silicon islands for the present invention silicon-on-insulator semiconductor device.  
         [0026]    FIGS.  4 A- 4 D are enlarged, cross-sectional view of strained silicon islands that have rounded corners for the present invention silicon-on-insulator semiconductor device.  
         [0027]    [0027]FIG. 5 is an enlarged, cross-sectional view of the present invention strained silicon transistors with mesa isolation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    In the present invention, a strained silicon on insulator structure with mesa isolation is disclosed.  
         [0029]    The present invention avoids the shortcomings of conventional devices and processes by disclosing a structure and a process where a strained silicon layer does not experience device isolation formation processes that require high temperatures, and where problems of liner oxide formation is eliminated. One isolation scheme without the use of high temperatures is mesa isolation. Mesa isolation relies on the formation of islands of active regions where device or transistors reside. The islands of active regions overlie an insulating substrate. Transistors residing on separate islands are therefore isolated from each other. The formation of islands of active regions does not involve high temperature steps. This is important for the retention of strain in the strained silicon layer. However, mesa isolation cannot be used in the conventional strained silicon substrate shown in FIG. 1A. This is because it is not an easily implemented process to form islands of strained silicon overlying an insulating layer using the substrate of FIG. 1A.  
         [0030]    By using a strained silicon-on insulator substrate, it is possible to form islands of strained silicon overlying an insulator layer. A cross-sectional view of device  30  having strained silicon islands  32 ,  34 ,  36  on insulator  38  is shown in FIG. 2A. The strained silicon islands  32 ,  34 ,  36  do not need to have the same film thickness. For example, one strained silicon island may have a film thickness t si1 , and another island may have a thickness t si2 . In addition, the cross-sectional view of the islands  42 ,  44 ,  46  for device  40  may feature rounded corners, as shown in FIG. 2B. In FIG. 2C, the top view of the strained silicon islands  32 ,  34 ,  36  are shown. The islands  32 ,  34 ,  36  are electrically isolated from each other.  
         [0031]    According to the present invention, a strained silicon-on-insulator transistor  50  may be fabricated by a process shown in FIGS.  3 A- 3 D. A strained silicon-on-insulator substrate  52  is used as the starting material, as shown in FIG. 3A. In the preferred embodiment, the insulator layer  54  (or buried oxide layer) is comprised of silicon oxide. The thickness of the silicon oxide is preferably between 100 angstroms and 2000 angstroms. However, it may be comprised of any other insulating material or dielectric such as silicon nitride and aluminum oxide. The substrate is preferably a silicon substrate, but also may include other semiconductor substrates such as silicon-germanium substrates or gallium arsenide substrates. The strained silicon layer  56  may have a strain magnitude of between 0.1% and 2%. The strain magnitude is defined to be the percentage change in the lattice constant of the strained material from its natural lattice constant. The nature of the strain may be tensile or compressive. In the preferred embodiment, the strain is tensile in nature in the in-plane direction, that is, the lattice constant of the strained silicon layer is larger than its natural lattice constant in the plane of the wafer. The thickness of the strained silicon layer ranges from 10 angstroms to 500 angstroms which may be formed by a layer transfer technique. An example of a layer transfer technique is a wafer bonding step followed by a wafer separation step. In the wafer bonding step, a donor wafer comprising a strained silicon layer overlying a relaxed silicon-germanium layer is bonded to a target wafer comprising a silicon oxide layer overlying a silicon substrate, such that the strained silicon layer is in atomic contact with the silicon oxide layer. In the subsequent wafer separation step, the strained silicon layer is separated from the donor wafer so that a new final wafer is formed comprising a strained silicon layer overlying a silicon oxide layer which is turn overlies a silicon substrate.  
         [0032]    Next, a masking layer (not shown) is deposited over the strained silicon layer  56  and patterned using standard photolithographic techniques to form a mask  62 . This is shown in FIG. 3B. The mask is used to define active regions  72 ,  74  (shown in FIG. 3C) in the strained silicon layer  56 . The mask  62  may be comprised of any masking material known and used in the art, such as silicon oxide, silicon nitride, or silicon oxynitride. The mask may also be formed of a stack including a silicon nitride layer overlying a silicon oxide layer.  
         [0033]    Following active region definition using the mask, the strained silicon layer  56  is etched using techniques known and used in the art. An example is a dry plasma etch using flourine chemistry. The mask  62  is then removed to yield the strained silicon islands  82 ,  84 , the cross-sections of which are shown in FIG. 3D.  
         [0034]    Silicon islands  92 ,  94  with rounded corners may also be formed in a slightly modified process in a second embodiment of the present invention. In the second embodiment, the process of active region definition is performed and the strained silicon layer  56  is etched, as shown in FIG. 4A. The size of the mask  62  is then slightly shrunk by etching. The result is that the edges  62  of the mask is pulled-back to reveal a portion of the strained silicon island  82 ,  84 , as shown in FIG. 4B. A corner rounding process is then performed as shown in FIG. 4C. The corner rounding process may be an oxidation step followed by oxide removal. An example of the oxidation step is a thermal oxidation conducted at a temperature between 550 degrees Celsius to 950 degrees Celsius in an oxygen ambient. An example of the oxide removal step is a wet etch process carried out in a diluted hydrofluoric acid. Corner rounding of the active regions  72 ,  74  may also be achieved by various combinations of processes such as etching, oxidation, and/or annealing, performed after the active region  72 ,  74  definition. The mask  62  is then removed to result in strained silicon islands  92 ,  94  with rounded corners. This is shown in FIG. 4D. Si corner rounding at the isolation edge is a feature that helps alleviate problems associated with a double-hump in the drain current versus gate voltage (IDS-VGS) characteristic.  
         [0035]    Transistors may then be fabricated on the strained silicon islands using techniques known and used in the art. FIG. 5 shows a completed device  100  featuring N-channel and P-channel MOSFETs  102 ,  104  with strained silicon channels and electrically isolated using mesa isolation.  
         [0036]    While the present invention has been described in an illustrative manner, it should be understood that the terminology used is intended to be in a nature of words of description rather of limitation.  
         [0037]    Furthermore, while the present invention has been described in terms of a preferred and an alternate embodiment, it is to be appreciated that those skilled in the art will readily apply these teachings to other possible variations of the inventions.  
         [0038]    The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows.