Abstract:
A method for inducing a tensile stress in a channel of a field effect transistor (FET) includes forming a nitride film over the FET; forming a contact hole to the FET through the nitride film; and performing ultraviolet (UV) curing of the nitride film after forming the contact hole to the FET through the nitride film, wherein the UV cured nitride film induces the tensile stress in the channel of the FET.

Description:
BACKGROUND 
       [0001]    This disclosure relates generally to the field of semiconductor fabrication, and more specifically to ultraviolet (UV) curing of nitride film to enhance a tensile stress in the nitride film. 
         [0002]    Manipulating stress is an effective way of improving the carrier mobility in a complementary metal oxide semiconductor (CMOS) field effect transistor (FET). When stress is applied to the channel of a FET, the mobility of carriers, and as a consequence, the transconductance and the on-current of the FET, are altered from their original values for an unstressed channel FET. This is because the applied stress and the resulting strain on the semiconductor structure within the channel affects the band gap structure (i.e., breaks the degeneracy of the band structure) and changes the effective mass of the carriers. The effect of the stress depends on the crystallographic orientation of the plane of the channel, the direction of the channel within the crystallographic orientation, and the direction of the applied stress. 
         [0003]    The effect of uniaxial stress, i.e., a stress applied along one crystallographic orientation, on the performance of semiconductor devices, especially on the performance of a FET device built on a silicon substrate, has been extensively studied in the semiconductor industry. For a p-type FET (PFET) with a silicon channel, the mobility of carriers (which are holes in the case of a PFET) in the channel increases under uniaxial compressive stress along the direction of the channel, i.e., the direction of the movement of holes or the direction connecting the drain to the source. Conversely, for an n-type FET (NFET) device with a silicon channel, the mobility of carriers (which are electrons in the case of an NFET) increases under uniaxial tensile stress along the direction of the channel, i.e., the direction of the movement of electrons or the direction connecting the drain to the source. 
         [0004]    The stress in an FET channel may be induced by a stressed nitride film that is formed over the FET device. A nitride film formed over a PFET device may have a compressive stress, and a nitride film formed over an NFET device may have a tensile stress. UV curing of a tensile stressed nitride film may enhance the tensile stress in the nitride film by reconfiguring silicon-hydrogen/nitrogen-hydrogen bonds present in the nitride film. UV curing of nitride is performed by exposing the nitride to UV radiation, which has a wavelength in a range from about 10 nanometers (nm) to about 400 nm. The enhanced stress in the nitride film induces a corresponding enhanced stress in a channel of the FET over which the UV cured nitride film is located, increasing the carrier mobility in the FET channel. 
       BRIEF SUMMARY 
       [0005]    In one aspect, a method for inducing a tensile stress in a channel of a field effect transistor (FET) includes forming a nitride film over the FET; forming a contact hole to the FET through the nitride film; and performing ultraviolet (UV) curing of the nitride film after forming the contact hole to the FET through the nitride film, wherein the UV cured nitride film induces the tensile stress in the channel of the FET. 
         [0006]    In another aspect, a method forming a field effect transistor (FET), the FET including a gate, source/drain regions, and a channel, includes forming the gate on a substrate; forming the source/drain regions in the substrate on either side of the gate, wherein the channel is located in the substrate underneath the gate and between the source/drain regions; forming a nitride film over the gate and the source/drain regions; forming contact holes to the gate and source/drain regions through the nitride film; and performing ultraviolet (UV) curing of the nitride film after forming the contact holes to the gate and source/drain regions through the nitride film, wherein the UV cured nitride film induces a tensile stress in the channel of the FET. 
         [0007]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0009]      FIG. 1  is a flowchart illustrating an embodiment of a method for tensile stress enhancement of a nitride film for stressed channel FET fabrication by UV curing after contact hole etch. 
           [0010]      FIG. 2  is a schematic block diagram illustrating an embodiment of a nitride film formed over FET devices. 
           [0011]      FIG. 3  illustrates an embodiment of the device of  FIG. 2  after contact hole etching through the nitride film. 
           [0012]      FIG. 4  illustrates an embodiment of the device of  FIG. 3  after UV curing of the nitride film. 
           [0013]      FIG. 5  illustrates an embodiment of the device of  FIG. 4  after filling the etched contact holes with a contact metal. 
           [0014]      FIG. 6  illustrates an embodiment of the device of  FIG. 2  after contact hole etching through the nitride film and filling the etched contact holes with a contact metal. 
           [0015]      FIG. 7  illustrates an embodiment of the device of  FIG. 6  after UV curing of the nitride film. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Embodiments of a method for tensile stress enhancement of nitride film by performing UV curing after contact hole etch, with corresponding enhancement of channel tensile stress in a FET on which the nitride film is deposited, are provided, with exemplary embodiments being discussed below in detail. Formation of a nitride film over an FET and UV curing of the nitride film to enhance the stress in the nitride film are typically performed in succession. Contact holes, which are necessary to form electrical connections to the FET source/drain and gate regions, are then typically formed in the UV cured nitride film. However, forming contact holes in a tensile stressed nitride film after UV curing may cause significant relaxation of the tensile stress in the nitride film, which may largely negate the enhanced tensile stress induced in the nitride film by UV curing. This tensile stress relaxation may be greater in magnitude for smaller FET sizes, and may also be greater in magnitude for larger contact holes, which may be necessary to provide lowered contact resistance. By forming contact holes in the nitride film, and then performing UV curing of the nitride film to enhance the tensile stress of the nitride film, the relaxation of the tensile stress in the nitride film that is caused by contact hole formation may be reduced. The final tensile stress induced by the UV cured nitride film in the FET channel over which the UV cured nitride film is located is also correspondingly increased. The FET may be an NFET, and the tensile stress in the UV cured nitride film and corresponding tensile stress induced the NFET channel result in enhancement of electron mobility in the NFET channel. 
         [0017]    A stressed nitride film may be deposited over a FET with a tensile stress from about 1.1 gigapascals (GPa) to about 1.2 GPa without UV curing. However, a nitride film that is to be UV cured may be deposited over a FET with a lower initial tensile stress, about 0.7 GPa in some embodiments. UV curing of a nitride film having an initial tensile stress of about 0.7 GPa may result in a tensile stress of about 1.6 GPa to 1.7 GPa in the nitride film after UV curing is completed. The tensile stress in the nitride film induces a corresponding tensile stress in the FET channel that is dependent on the geometry of the particular FET. For an example FET having a particular FET geometry, the tensile stress induced by the nitride film after UV curing in the channel of the FET may be about 720 megapascals (MPa). However, contact hole etching in the UV cured nitride film causes relaxation of the tensile stress in the nitride film. For example, the above-mentioned FET having a 720 MPa tensile stress in the channel after UV curing of the nitride film may see the tensile stress in the channel drop to a final tensile stress of about 470 MPa after contact hole etching through the UV cured nitride film. However, performing UV curing of the nitride film after contact hole etching yields a higher nitride film tensile stress and corresponding channel tensile stress. For the same example FET having the particular geometry, the final tensile stress induced in the channel by a nitride film that is UV cured after contact hole etching may be about 520 MPa, showing an increase in channel tensile stress of about 10% over the example FET with the nitride film that is UV cured before contact hole etching. 
         [0018]      FIG. 1  illustrates an embodiment of a method  100  for tensile stress enhancement of a nitride film by UV curing after contact hole etch.  FIG. 1  is discussed with respect to  FIGS. 2-7 . In block  101 , a plurality of FET devices are formed on a substrate and then, in block  102 , a nitride film  208  is formed over the plurality of FET devices that were formed in block  101 . The plurality of FET devices are formed on a substrate  201 , as shown in  FIG. 2 . The substrate  201  may include any appropriate substrate that may be used for CMOS device formation, such as silicon. The FET devices located on substrate  201  include source/drain silicide  202  on either side of a gate that includes gate dielectric  203  (which may be a high-k gate dielectric), gate metal  204 , gate polysilicon  205 , and gate silicide  206 . The FET devices may be NFETs. Spacers  207  are located adjacent to the FET gates. Spacers  207  may include nitride or oxide in various embodiments. Nitride film  208  may be formed over the FET devices by deposition, which may include chemical vapor deposition (CVD) in some embodiments. Nitride film  208  may be formed with an initial tensile stress that may be about 0.7 GPa in some embodiments. 
         [0019]    In some embodiments, the contact holes may be formed in the nitride film  208  before the UV curing is performed, and then the contact holes may be filled with a contact metal after the UV curing is performed, as shown in  FIGS. 3-5 . Referring again to  FIG. 1 , in block  103 , contact holes  301  are formed in nitride film  208 , as shown in  FIG. 3 . First, oxide  302  may be formed over nitride film  208 , and the top surface of oxide  302  may be polished flat. The contact holes  301 , which allow electrical connections to the gate silicide  206  and the source/drain silicide  202 , are then formed through the oxide  302  and the nitride film  208 . Contact holes  301  may be formed by etching, which may include reactive ion etching (RIE) in some embodiments. Then, in block  104 , nitride film  208  is UV cured (i.e., exposed to UV radiation) to form UV cured nitride film  401  as shown in  FIG. 4 . The UV cure of block  104  acts to increase the tensile stress in UV cured nitride film  401  as compared to nitride film  208 . The tensile stress in UV cured nitride film  401  induces a tensile stress in the channels of the FET devices that are located underneath UV cured nitride film  401 ; the FET channels are located underneath the gate dielectric  203  and between source/drain silicide  202  in the substrate  201 . The tensile stress induced in the FET channels increases the electron mobility in the FET channels. The final tensile stress induced in the FET channels by UV cured nitride  401  may be about 520 MPa in some embodiments. The UV cure of block  104  may be performed at a temperature of about 400° C. or greater in some embodiments. After UV curing to form UV cured nitride film  401  in block  104 , contact holes  301  are filled with a contact metal  501  as shown in  FIG. 5 . Contact metal  501  provides the electrical connections to the FET gate silicide  206  and source/drain silicide  202 . Contact metal  501  may be any appropriate electrically conductive metal. 
         [0020]    In other embodiments, the contact holes may be filled with contact metal before UV curing of nitride film  208  is performed, as shown in  FIGS. 6-7 . Referring again to  FIG. 1 , in block  103 , contact holes are formed in nitride film  208 , and the contact holes are filled with a contact metal  601 , as shown in  FIG. 6 . First, oxide  602  may be formed over nitride film  208 , and the top surface of oxide  602  may be polished flat. The contact holes, which allow electrical connections to the gate silicide  206  and the source/drain silicide  202 , are then formed through the oxide  602  and the nitride film  208  and filled with the contact metal  601 . The contact holes may be formed by etching, which may include RIE in some embodiments. Contact metal  601  provides the electrical connections to the FET gate silicide  206  and source/drain silicide  202 . Contact metal  601  may be any appropriate electrically conductive metal. Then, in block  104 , nitride film  208  is UV cured (i.e., exposed to UV radiation) to form UV cured nitride film  701  as shown in  FIG. 7 . The UV cure of block  104  acts to increase the tensile stress in UV cured nitride film  701  as compared to nitride film  208 . The tensile stress in UV cured nitride film  701  induces a tensile stress in the channels of the FET devices that are located underneath UV cured nitride film  701 ; the FET channels are located underneath the gate dielectric  203  and between source/drain silicide  202  in the substrate  201 . The increased stress in the FET channels increases the electron mobility in the FET channels. The final tensile stress induced in the FET channels by UV cured nitride  401  may be about 520 MPa in some embodiments. The UV cure of block  104  may be performed at a temperature of about 400° C. or greater in some embodiments. 
         [0021]    The FET devices shown in  FIG. 2-7  are shown for illustrative purposes only. Method  100  may be performed on any tensile stressed nitride film that is located over any appropriate number and type of FET devices, having, for example, any appropriate source/drain and gate composition, to increase the tensile stress induced in the FET channels by the UV cured nitride film. The FET devices may be located on any appropriate type of substrate, and any appropriate number and type of additional CMOS devices that are not covered by the nitride film may also be located on the substrate in various embodiments. 
         [0022]    The technical effects and benefits of exemplary embodiments include enhanced tensile stress in an FET channel by reducing the relaxation of tensile stress in a UV cured nitride film that is caused by contact hole etching. 
         [0023]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0024]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.