Abstract:
A multiple-gate semiconductor structure is disclosed which includes a substrate, a fin formed of a semi-conducting material that has a top surface and two sidewall surfaces. The fin is subjected to a strain of at least 0.01% and is positioned vertically on the substrate; source and drain regions formed in the semi-conducting material of the fin; a gate dielectric layer overlying the fin; and a gate electrode wrapping around the fin on the top surface and the two sidewall surfaces of the fin overlying the gate dielectric layer. A method for forming the multiple-gate semiconductor structure is further disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to a multiple-gate transistor and method for fabricating and more particularly, relates to a strained-channel multiple-gate transistor and method for fabrication.  
         BACKGROUND OF THE INVENTION  
         [0002]    Size reduction of the metal-oxide-semiconductor field-effect transistor (MOSFET) has enabled the continued improvement in speed performance, density, and cost per unit function of integrated circuits over the past few decades. As the gate length of the conventional bulk MOSFET is reduced, the interaction of the source and drain with the channel is increased resulting in gained influence on the channel potential. Consequently, a transistor with a short gate length suffers from problems related to the inability of the gate to substantially control the on/off states of the channel. Phenomena such as reduced gate control associated with transistors having short channel lengths are known as short-channel effects. Increased body doping concentration, reduced gate oxide thickness, and ultra-shallow source/drain junctions are ways to suppress short-channel effects.  
           [0003]    When the gate length is scaled down into the sub-50 nanometer (nm) regime, the requirements for body-doping concentration, gate oxide thickness, and source/drain (S/D) doping profiles to control short-channel effects become increasingly difficult to meet when conventional device structures based on bulk silicon (Si) substrates are employed. For device scaled down well into the sub-50 nm regime, a promising approach for controlling short-channel effects is to use an alternative device structure and the surround-gate or wrap-around gate structure. The multiple-gate transistor structure is expected to extend the scalability of CMOS technology beyond the limitations of the conventional bulk MOSFET. The introduction of additional gates improves the capacitance coupling between the gates and the channel, increases the control of the channel potential by the gate, helps the suppression of short channel effects, and prolongs the scalability of the MOS transistor.  
           [0004]    Strain-induced mobility enhancement is another approach to improve transistor performance in addition to device scaling. Significant mobility enhancement has been reported for both electrons and holes in conventional bulk transistors using silicon channel under biaxial tensile strain. It would be desirable to exploit the effects of strain-induced band structure modification to enhance carrier mobilities in multiple-gate transistors. However, there has been no research publications on the use of strained channel for enhancement of performance in multiple-gate transistors. This is because techniques used to induce the biaxial tensile strain for enhancement of carrier mobilities in conventional bulk transistors are not applicable to multiple-gate transistor structures.  
           [0005]    A conventional technique used to induce a biaxial tensile strain in the channel region of a conventional bulk MOSFET makes use of a relaxed silicon germanium (SiGe) buffer layer, as shown in device  10  of FIG. 1A. The relaxed SiGe layer  12  which is formed on top of the graded SiGe buffer layer  14  on top of Si substrate  16  has a larger lattice constant compared to relaxed Si. A thin layer  20  of epitaxial Si grown on relaxed SiGe  12  will be under biaxial tensile strain, as shown in FIG. 1B. Both hole and electron mobilities are enhanced in the strained Si layer  20 . Since a multiple gate device such as the double-gate fin FET structure resides on a silicon-on-insulator substrate and employs a fin-like channel, the conventional approach that employs a relaxed SiGe buffer is not applicable. In addition, the device structure and channel orientation of a multiple-gate transistor may be significantly different than that of the conventional bulk MOSFET. This means that the nature of strain required for enhancement of carrier mobilities in a multiple-gate device may be significantly different than that used for the conventional bulk MOSFET.  
           [0006]    To overcome the shortcomings of the prior art, it is therefore an object of the invention to provide a multiple-gate transistor structure with a strained channel.  
           [0007]    It is another object of the invention to illustrate the nature and direction of a useful strain required for a non-conventional multiple-gate device structure.  
           [0008]    It is a further object of the invention to provide a method of manufacture for the strained-channel multiple-gate transistor.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with the present invention, a multiple-gate semiconductor structure and a method for fabricating the multiple-gate semiconductor structure are provided.  
           [0010]    In a preferred embodiment, a multiple-gate semiconductor structure is provided which includes a substrate; a fin formed of a semi-conducting material that has a top surface and two sidewall surfaces, the fin is subjected to a strain of at least 0.01% and is positioned vertically on the substrate; source and drain regions formed in the semi-conducting material of the fin; a gate dielectric layer overlying the fin; and a gate electrode wrapping around the fin on the top surface and the sidewall surfaces of the fin overlying the gate dielectric layer.  
           [0011]    In the multiple-gate semiconductor structure, the fin maybe under tensile strain in the source-to-drain direction, or maybe under compressive strain in the source-to-drain direction. A magnitude of the strain is between 0.01% and 2%. The semi-conducting material may be silicon, or a material including silicon and germanium. The fin may be rounded at the top corners. The gate dielectric layer may include silicon oxide or silicon oxynitride. The gate dielectric layer may include a high permittivity material such as, but not limited to, La 2 O 3 , Al 2 0 3 , Hf0 2 , HfON and Zr0 2 . The relative permittivity of the gate dielectric layer is at least 5, while a thickness of the gate dielectric layer may be featuring 3 Å and 100 Å. The gate dielectric layer may have a different thickness on the two sidewall surfaces and the top surface of the fin. The gate dielectric layer may have a first thickness on the top surface of the fin and a second thickness on the sidewall surfaces of the fin, wherein the first thickness is smaller than the second thickness. The gate dielectric layer may have a thickness of less than 20 Å on the top surface of the fin. The gate electrode may include polycrystalline silicon. The gate electrode may further include polycrystalline silicon germanium, or metal. The source and drain regions each may have a lightly doped or extension region. The source and drain regions may be straddled by a conductive material such as metal or silicide. Contacts between the gate electrode and the source and drain regions may be on the sidewall surfaces or on the top surface of the fin.  
           [0012]    The present invention is further directed to a multiple-gate semiconductor structure which includes a substrate that has a depression in a top surface, the depression may be an undercut or a notch; a fin formed of a semi-conducting material that has a top surface and two sidewall surfaces, the fin is subjected to a strain of at least 0.01% and is positioned vertically juxtaposed to the depression in the top surface of the substrate; a gate dielectric layer overlying the fin; and a gate electrode wrapping around the fin on the two sidewall surfaces, the top surface and a bottom of a base of the fin.  
           [0013]    The present invention is still further directed to a method for fabricating a multiple-gate semiconductor structure that can be carried out by the operating steps of providing a substrate that is formed of an insulating material that has a semi-conducting layer on top; patterning and forming a fin in the semi-conducting layer, the fin has two sidewall space surfaces and a top surface; depositing a layer of a dielectric material overlying the fin; depositing a layer of a conducting material overlying the layer of dielectric material; patterning the layer of conductive material forming a gate straddling across the two sidewall surfaces and the top surface of the fin; forming a source and a drain region in the semi-conducting layer; and depositing a layer of a thin film overlying at least partially the source and drain regions.  
           [0014]    The method for fabricating a multiple-gate semiconductor structure may further include the step of forming the semi-conducting layer in a material including silicon, or including silicon and germanium. The method may further include the step of forming the insulating layer in a material including silicon oxide. The fin forming step may further include a fin surface smoothing step, while the fin surface smoothing step may further include sub-steps of sacrificial oxidation in high temperature annealing in a hydrogen ambient. The method may further include the step of forming the gate dielectric layer in a material including silicon oxide, or in a material including silicon oxynitride. The method may further include the step of forming the gate dielectric layer using a high permittivity material such as La 2 O 3 , Al 2 0 3 , Hf0 2 , HfON and Zr0 2 . The high permittivity material may have a relative permittivity of at least 5. The method may further include the step forming the gate dielectric layer to a thickness between 3 Å and 100 Å. The method may further include the step of forming a different thickness of the gate dielectric layer on the sidewall surfaces and on the top surface of the fin.  
           [0015]    The method may further include the step of forming the gate dielectric layer to a smaller thickness on the top surface of the fin compared to the gate dielectric layer on the sidewall surfaces of the fin, or the step of forming the gate dielectric layer on the top surface of the fin to a thickness of less than 20 Å. The method may further include the step of forming the gate in a material including polycrystalline silicon, or in a material including polysilicon germanium. The method may further include the step of forming the source and drain regions with a lightly doped or extension region, or the step of forming a layer of a conductive material overlying the source and drain regions. The method may further include the step of forming the layer of conductive material in a material selected from the group consisting of metal, metallic silicide, and metallic nitride. The method may further include the step of forming the layer of a thin film in a high-stress material, or forming the layer of thin film in silicon nitride. The method may further include the step of forming the layer of thin film to include a tensile strain component in a channel region that is in the source-to-drain direction. The method may further include the step of forming the layer of thin film to induce a compressive strain component in a channel region that is in the source-to-drain direction.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    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:  
         [0017]    [0017]FIGS. 1A and 1B illustrate a conventional bulk MOSFET that utilizes a relaxed silicon germanium buffer layer to induce a biaxial tensile strain in the channel region.  
         [0018]    [0018]FIG. 2 is a perspective view of a present invention multiple-gate transistor.  
         [0019]    [0019]FIGS. 3A and 3B illustrate the six-fold degenerate conduction band valleys of relaxed silicon.  
         [0020]    [0020]FIGS. 4A and 4B are plane view and perspective view of the present invention multiple-gate transistor under a tensile strain.  
         [0021]    [0021]FIGS. 5A and 5B are plane view and perspective view of the present invention multiple-gate transistor under a compressive strain.  
         [0022]    [0022]FIG. 6 is a plane view of the present invention multiple-gate transistor with a gate electrode and a thin film formed on top.  
         [0023]    [0023]FIG. 7 is a plane view of another embodiment of the present invention multiple-gate transistor with a gate electrode and a thin film and two spacers formed on top.  
         [0024]    FIGS.  8 A˜ 8 E are enlarged cross-sectional views illustrating areas process steps for the present invention transistor taken along lines A-A of FIG. 7.  
         [0025]    FIGS.  9 A- 9 E are enlarged, cross-sectional views of the various process steps for the present invention transistor taken along lines B-B of FIG. 7.  
         [0026]    FIGS.  10 A- 10 E are enlarged, cross-sectional views of the various process steps for the present invention transistor taken along lines C-C of FIG. 7.  
         [0027]    [0027]FIG. 11 is a perspective view of still another embodiment of the present invention having an undercut or a notch in the substrate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    The present invention discloses a method of manufacturing for strained channel field effect transistors with multiple-gates capable of scaling down the complementary metal-oxide-semiconductor (CMOS) technology well into the sub-50 nanometer (NM) regime.  
         [0029]    [0029]FIG. 1A illustrates a technique to induce strain in the Si channel of a conventional bulk MOSFET  10  by growing an epitaxial silicon (Si) layer  22  on a relaxed silicon-germanium (SiGe) larger overlying a silicon substrate  16 . Relaxed SiGe layer  12  has a larger lattice constant compared to relaxed Si, and the thin layer  22  of epitaxial Si grown on the relaxed SiGe layer  12  will be under biaxial tensile strain, as shown in FIG. 1B. In the bulk MOSFET  10 , both hole and electron mobilities are enhanced in the Si layer under biaxial tensile strain.  
         [0030]    [0030]FIG. 2 illustrates a perspective view of a present invention multiple-gate transistor  30  formed with a fin-like channel  32 . The fin-like channel  32  extend vertically from an insulating substrate  34  with a height t h  and a width t w . An overlying gate  36  wraps around the gate dielectric layer  38  on the fin  32 . The carriers in the channel flow in the source-to-drain direction, i.e., the y direction from source  40  to drain  42 . It is advantageous to utilize a tall and narrow fin  32  to maximize the on state current while maintaining a good control of the short-channel effects. The height h t  of the fin  32  is typically larger than 200 Å. A thickness of the fin width t w  is between 50 Å and 500 Å. Given this range of thickness for t w , it is difficult and impractical to fabricate a strained fin comprising of multiple-layers of semiconductors with different lattice constants. Therefore, it is obvious that the technique used to induce strain in the bulk Si channel cannot be easily applied in the case of the multiple-gate transistor structure  10 .  
         [0031]    In addition, the device structure and orientation of the plane of the inversion sheet charge in a multiple-gate transistor is significantly different from that of the conventional bulk MOSFET. The conventional transistor has a channel inversion sheet charged oriented in the x-y plane, while the inversion sheet charged induced by the gates on the sidewalls of the multiple-gate transistor  30  of FIG. 2 is oriented in the y-z plane. This means that the nature of strain required for enhancement of carrier mobilities in a multiple-gate device may be significantly different from that used for the conventional bulk MOSFET.  
         [0032]    The useful strain required for mobility enhancement in a multiple-gate transistor is analyzed next. FIG. 3A shows the six-fold degenerate conduction band valleys of relaxed silicon. The electron population distributes equally among the six conduction band valleys. The net electrons motion in the channel is in the y direction. Therefore, as far as carrier transport is concerned, it would be beneficial to lift the energy of conduction valleys with higher effective mass in the y direction. By reducing the population of electrons in the valleys with higher effective mass in the y-direction, as shown in FIG. 3B, carrier transport may be improved. This can be done by the introduction of tensile strain in the y direction of fin  50 , as diagrammatically illustrated in FIGS. 4A and 4B. The magnitude of tensile strain may range from 0.01% to 2%. In the preferred embodiment, the tensile strain magnitude is about 1% to 2%. Tensile strain in the y-direction is beneficial for both electron and hole transport in the y-direction. In addition, compressive strain in the y-direction is also beneficial for hole transport, and this can also be used for the P-channel multiple-gate transistors.  
         [0033]    The compressive strained channel fin  60  is illustrated in FIGS. 5A and 5B. The magnitude of compressive strain may range from 0.01% to 2%. In the preferred embodiment, the compressive strain magnitude is about 1% to 2%. A plane view of the present invention preferred embodiment device  30  (shown in FIG. 2) is shown in FIG. 6 further illustrating the thin film layer  48 . A plane view of a present invention alternate embodiment device  70 , which further incorporates a pair of sidewall spacers  72 ,  74 , is shown in FIG. 7. The lightly-doped drain (LDD) or source/drain extension regions underlie the spacers  72 ,  74 .  
         [0034]    A method for the manufacture of the present invention multiple-gate transistors with tensile or compressive strain in the channel region is now described. The strain may have a component in the source to drain direction. An effective way to induce strain in the fin-like structure is to encapsulate the fin with a high stress thin film material after gate electrode formation. The fin surface in contact with a high-stress thin film material will be under stress as well. Due to the large difference in thermal expansion coefficient and Young&#39;s modulus between the fin and the thin film, significant stress of several hundred MPa to more than 1 GPa can be produced in the fin.  
         [0035]    The thin film material may be a dielectric such as silicon nitride or silicon oxynitride deposited by various methods including chemical vapor deposition. In the thin film material that has a lower thermal expansion coefficient compared with the fin material, the part of the fin experiences compressive stress. If the thin film material has a higher thermal expansion coefficient compared with the fin material, the part of the fin experiences tensile stress.  
         [0036]    As an example, plasma-enhanced chemical vapor deposited silicon nitride can be used to introduce tensile or compressive stress in the channel region, The residual film stress impacts the strain components in the channel. The residual film stress can be tailored from a high state of tension, for stoichiometric silicon nitride, to one of compression, for silicon-rich films. The tensile or compressive nature of the strain in the channel region can therefore be adjusted by varying process conditions such a temperature, pressure, and the ratio of the flow rate of as precursor gas, e.g., dichlorosilane, to the total gas flow rate.  
         [0037]    Table I shows the thermal expansion coefficient α and Young&#39;s modulus E of other thin film materials that may be chosen. In addition, the thermal expansion coefficient and Young&#39;s modulus of silicon dioxide may be varied by the choice of the doping materials, such as germanium, nitrogen, or refractory metals, and also by the doping densities of these materials.  
                                         TABLE I                                   α (K −1 )   E (GPa)                                    Zirconium oxide   11.1E−6    200       Steatite (MgOSiO2)   8.0E−6   —       Aluminum oxide   7.7E−6   390       Aluminum nitride   5.1E−6   380       Silicon carbide   4.3E−6   400       Silicon nitride   2.8E−6 to 3.6E−6   306       Silicon   2.0E−6   156       Silicon dioxide   5.0E−7   —                  
 
         [0038]    The process for fabricating the present invention multiple-gate transistor is shown in FIGS.  8 A˜ 10 E. FIGS.  8 A˜ 8 E are taken along line A-A, FIGS.  9 A˜ 9 E are taken along line B-B, while FIGS.  10 A˜ 10 E are taken along line C-C in FIG. 7. Referring now to FIGS. 8A, 9A and  10 A wherein a silicon-on-insulator substrates  34  are used to form silicon fins  32  on the buried oxide. The silicon fins  32  are patterned using an etch mask (not shown). The etch mask may comprise of a material commonly used for masking an etch process, such as photoresist, silicon oxide, silicon nitride, etc. In the preferred embodiment, the etch mask is silicon oxide. The silicon fin surface may be smoothened to improve or reduce the surface roughness of the fin sidewalls  44 ,  46 . This is performed by subjecting the fin to a sacrificial oxidation and silicon sidewall treatment (e.g. high temperature anneal at 1000° C. in H 2  ambient). The surface smoothing of the fin sidewalls  44 ,  46  contributes to the achievement of good carrier mobilities and good gate oxide reliability. This is followed by removal of the silicon oxide etch mask. The removal of the etch mask on the silicon fin  32  allows the device to have a triple-gate structure since the gate electrode  36  (shown in FIGS.  8 D˜ 8 E) is formed on each of the two sidewalls  44 ,  46  as well as the top surface  58  of the fin. The buried oxide under the silicon fin  32  may be undercut or notched using an etch process. This is shown in FIG. 11, in an alternate embodiment, wherein an undercut area  92  is shown. The etch process may be a plasma etch process or a wet etch process using dilute hydrofluoric acid (HF). The undercutting or notching of the substrate insulating layer  34  underlying the silicon fin  32  allows the formation of an omega-shaped gate electrode  32  as shown in FIGS.  8 D˜ 8 E. This feature allows additional capacitive coupling between the gate and the channel underneath the fin, and the structure thus emulates a wrap-around gate but avoids the process difficulties associated with the implementation of the conventional wrap-around structure. The preceding description completes the fin formation process.  
         [0039]    The process is then followed by the gate dielectric layer  38  shown in FIGS. 8B, 9B and  10 B. The gate dielectric layer  38  may be formed by thermal oxidation, chemical vapor deposition, sputtering, etc. In general, the thickness of the gate dielectric may be different on the sidewall  44 ,  46  of the fin and the top  58  of the fin  32 . Depending on the technique of the gate dielectric formation, the gate dielectric thickness on the fin top may be thinner than the thickness on the fin sidewall. In one embodiment, the gate dielectric thickness on the top surface of the fin is less than 20 Å. The gate dielectric may comprise of a conventional material such as silicon dioxide or silicon oxynitride with a thickness ranging from 3 Å to 100 Å, preferably 10 Å or less. The gate dielectric may also comprise of high permittivity (high-k) materials such as aluminum oxide Al 2 0 3 , hafnium oxide HfO 2 , zirconium oxide ZrO 2 , with an equivalent oxide thickness between 3 angstroms and 100 angstroms.  
         [0040]    Next, the gate material is deposited and formed into a gate electrode  36 . This is shown in FIG. 8C. The gate material may be polycrystalline-silicon (poly-Si), poly-crystalline silicon-germanium (poly-SiGe), a refractory metal such as molybdenum and tungsten, compounds such as titanium nitride, or other conducting materials. Implants known as work function implants may be introduced in the gate to alter the work function of the electrode. A gate mask is defined and the underlying gate material is etched to form the gate electrode. The gate etch stops on the gate dielectric, and the gate is electrically isolated from the transistor structure by the gate dielectric. In the preferred embodiment, the gate material is poly-Si and the gate oxide is silicon oxynitride. A plasma etch using chlorine and bromine chemistry may be used to achieve a high etch selectivity in excess of 2000. A high etch selectivity is critical for device structures with a tall fin and aggressively scaled gate oxide thickness. After the definition of the gate, the masking material is removed.  
         [0041]    The lightly-doped drain (LDD) or source/drain extension  76  is formed next. This is shown in FIGS.  9 D˜ 9 E. This may be achieved by ion implantation, plasma immersion ion implantation (PIII), or other techniques known and used in the art. Next, a spacer is formed on the sidewalls of the gate and the channel by deposition and selective etching of the spacer material. The spacer material may comprise of a dielectric material such as silicon nitride or silicon dioxide. In the preferred embodiment, the spacer comprises a silicon nitride/silicon oxide composite. After the spacer formation, source and drain regions are doped by ion implantation, PIII, gas or solid source diffusion, or any other techniques known and used in the art. Any implant damage or amorphization can be annealed through subsequent exposure to elevated temperatures. The resistant of the source and drain can also be reduced by strapping the source/drain with a conductive material. This can be achieved by self-aligned silicide (salicide) process, by other metal, poly-Si, or by epitaxial silicon deposition. The conductive material may be formed on both the top of the fin as well as the sidewalls of the fin. In the preferred embodiment, the conductive material utilized is nickel silicide.  
         [0042]    A high-stress thin film  68  is then deposited. This is shown in FIGS. 8E, 9E and  10 E. The thin film  68  may have a thickness of between 50 Å and 1000 Å. A heavily doped source region  78  is also shown in FIG. 10E. In the preferred embodiment, the thin film material is silicon nitride with a thickness of about 600 Å. The thin film material introduces significant strain in the channel region of the multiple-gate transistor. In one embodiment, a thin film material is selected such that the strain in the channel in the multiple-gate transistor of FIGS. 4A and 4B has a tensile component in the source-to-drain direction for enhancement of the electron as well as hole mobilities. In another embodiment, a thin film material is selected such that the strain is in the source-to-drain direction for enhancement of the electron as well as hole mobilities. In still another embodiment, a thin film material is selected such that the strain in the p-channel transistor is compressive (as shown in FIGS.  5 A˜ 5 B) for enhancement of hole mobility. The high-stress thin film material may be patterned, and another high-stress material may be deposited so that channels with different strain may be incorporated on the same wafer. Passivation, contact etch, metallization are then performed by conventional methods to complete the devices.  
         [0043]    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 than of limitation.  
         [0044]    Furthermore, while the present invention has been described in terms of a preferred and alternate embodiments, it is to be appreciated that those skilled in the art will readily apply these teachings to other possible variations of the inventions.  
         [0045]    The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows.