Abstract:
A method for fabricating an ultra-high tensile-stressed nitride film is disclosed. A PECVD process is first performed to deposit a transitional silicon nitride film over a substrate. The transitional silicon nitride film has a first concentration of hydrogen atoms. The transitional silicon nitride film is subjected to UV curing process for reducing the first concentration of hydrogen atoms to a second concentration of hydrogen atoms.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application Ser. No. 60/593,781, filed Feb. 13, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to the fabrication of semiconductor devices, and more particularly, to a method for forming an ultra-high tensile-stressed nitride film and strained-silicon transistor devices thereof.  
         [0004]     2. Description of the Prior Art  
         [0005]     For decades, chip manufacturers have made metal-oxide-semiconductor (MOS) transistors faster by making them smaller. As the semiconductor processes advance to very deep sub micron era such as 65-nm node or beyond, how to increase the driving current for MOS transistors has become a critical issue.  
         [0006]     In order to improve device performance, crystal strain technology has been developed. Crystal strain technology is becoming more and more attractive as a means for getting better performance in the field of CMOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strain makes CMOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel.  
         [0007]     Generally, strain in silicon can be induced in different ways: through stresses created by films in a form of poly stressor or contact etch stop layer (CESL) and structures that surround the transistor, called process-induced strain, or by employing a strained silicon wafer, where the top layer of silicon has typically been grown on top of a crystalline lattice that is larger than that of silicon. Most leading-edge chip manufacturers employ process-induced stress in some form in production today, typically tensile nitrides to improve NMOS device performance. As known in the art, tensile stress improves electron mobility and compressive stress improves hole mobility.  
         [0008]     It is desirable to employ a nitride film having a tensile stress as high as possible in the fabrication of transistor devices. Specifically, according to the roadmap, a nitride CESL film with a tensile stress that is greater than 1.8 Gpa is required in the next-generation 45 nm process. However, so far the nitride film formed by conventional plasma-enhanced chemical vapor deposition (PECVD) methods can only reach a tensile stress of 1.2 GPa at the best.  
         [0009]     In light of the above, there is a need in this industry to provide a method of forming nitride films having an ultra-high tensile stress for the next-generation process.  
       SUMMARY OF THE INVENTION  
       [0010]     It is one object of the present invention to provide a method of forming an ultra-high tensile-stressed nitride film for the next-generation process.  
         [0011]     It is another object of the present invention to provide a strained-silicon transistor device employing an ultra-high tensile-stressed nitride film as a contact etch stop layer (CESL) or a poly stressor.  
         [0012]     According to the claimed invention, a method for fabricating an ultra-high (≧1.8 GPa) tensile-stressed nitride film is disclosed. The method includes the steps of:  
         [0013]     (1) providing a substrate;  
         [0014]     (2) performing a plasma-enhanced chemical vapor deposition (PECVD) process to deposit a transitional silicon nitride film having a first concentration of hydrogen atoms over the substrate; and  
         [0015]     (3) performing a means for reducing the first concentration of hydrogen atoms of the silicon nitride film to a second concentration of hydrogen atoms.  
         [0016]     From one aspect of this invention, a method for fabricating a strained-silicon transistor is provided. The method for fabricating a strained-silicon transistor includes the steps of:  
         [0017]     (1) providing a semiconductor substrate;  
         [0018]     (2) forming a gate structure on the semiconductor substrate;  
         [0019]     (3) forming source/drain regions on the semiconductor substrate adjacent to the gate structure;  
         [0020]     (4) performing a plasma-enhanced chemical vapor deposition (PECVD) process to deposit a transitional silicon nitride film having a first concentration of hydrogen atoms over the substrate, wherein the transitional silicon nitride film covers the gate structure and the source/drain regions; and  
         [0021]     (5) performing a means for reducing the first concentration of hydrogen atoms of the silicon nitride film to a second concentration of hydrogen atoms, thereby transforming the transitional silicon nitride film into the ultra-high tensile-stressed nitride film.  
         [0022]     From another aspect of this invention, a metal-oxide-semiconductor (MOS) transistor device is disclosed. The MOS transistor device includes a semiconductor substrate; a gate structure on the semiconductor substrate; source/drain regions on the semiconductor substrate adjacent to the gate structure; an ultra-high tensile-stressed nitride film having a tensile stress being equal to or greater than 1.5 GPa covering the gate structure and the source/drain regions; and an inter-layer dielectric (ILD) film over the ultra-high tensile-stressed nitride film.  
         [0023]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0025]      FIGS. 1-8  are schematic, cross-sectional diagrams illustrating a method of fabricating a semiconductor NMOS transistor device in accordance with one preferred embodiment of the present invention;  
         [0026]      FIG. 9  is a plot showing the relation between the concentration of hydrogen atom in the post-treated nitride film and the stress value in accordance with this invention;  
         [0027]      FIG. 10  is a plot showing the relation between ΔH (removed amount of H atoms) and the stress value of the post-treated nitride film;  
         [0028]      FIG. 11  is a schematic diagram of FTIR comparing the Si—H bond and N—H bond of the PECVD nitride film before UV curing with the Si—H bond and N—H bond of the PECVD nitride film after UV curing; and  
         [0029]      FIGS. 12-15  are schematic, cross-sectional diagrams showing the steps of poly stressor process according to one preferred embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]     The present invention is directed to a method of forming an ultra-high (≧1.8 GPa) tensile-stressed PECVD nitride film and strained-silicon transistor devices employing such high tensile-stressed nitride film.  
         [0031]      FIGS. 1-8  are schematic, cross-sectional diagrams showing the steps for fabricating a strained-silicon NMOS transistor according to one preferred embodiment of this invention. As shown in  FIG. 1 , a semiconductor substrate  10  is provided and a gate structure  12  is formed on the semiconductor substrate  10 . Shallow trench isolation (STI)  20  is also provided for isolation purpose. The gate structure  12  comprises a gate oxide layer  14 , a gate electrode layer  16  stacked on the gate oxide layer  14 , and an offset spacer  18 . Preferably, the gate oxide layer  14  is composed of silicon dioxide, and the gate electrode layer  16  is composed of doped polysilicon, but not limited thereto.  
         [0032]     The semiconductor substrate  10  may be a silicon substrate, silicon-on-insulator (SOI) substrate or any suitable semiconductor substrate with epitaxial layers. Such epitaxial layers include, but not limited to, silicon epitaxial layer, silicon germanium epitaxial (SiGe) layer or the like.  
         [0033]     As shown in  FIG. 2 , an ion implantation process  30  is performed to form a lightly doped source/drain region  26  in the semiconductor substrate  10  adjacent to the offset spacer  18 .  
         [0034]     As shown in  FIG. 3 , silicon nitride spacers  38  are formed on sidewalls of the gate structure  12 . The formation of the silicon nitride spacers  38  includes the steps of depositing a silicon nitride film over the semiconductor substrate  10 , followed by anisotropically etching back the silicon nitride film.  
         [0035]     As shown in  FIG. 4 , an ion implantation process  40  is then carried out to implant dopants species such as arsenic or phosphorus into the semiconductor substrate  10  thereby forming heavily doped source/drain regions  46  adjacent to the spacers  38 . A rapid thermal annealing (RTA) process is performed to activate the dopants within the source/drain regions  26  and  46 . The RTP process also repairs the damage of the lattice structure of the semiconductor substrate  10  resulting from the ion implantation process.  
         [0036]     As shown in  FIG. 5 , a conventional self-aligned silicide or salicide process is carried out to form salicide layers  52  on the source/drain regions  46  and on the gate electrode layer  16 . For example, the formation of the salicide layers  52  includes the steps of sputter depositing a metal layer such as cobalt or nickel over the semiconductor substrate  10 , reacting the metal with the underlying silicon surfaces, and then removing the un-reacted metal layer.  
         [0037]     As shown in  FIG. 6 , a PECVD process is carried out to deposit a tensile-stressed nitride film  60  having a thickness of about 500-1000 angstroms over the semiconductor substrate  10 . According to the preferred embodiment, the tensile-stressed nitride film  60  is deposited by utilizing silane (SiH 4 ) and ammonia (NH 3 ) as main reaction gases. Preferably, the aforesaid PECVD is carried out at a low-frequency RF power ranging between 50 Watts and 2700 Watts, a high-frequency RF power ranging between 100 Watts and 200 Watts, preferably less than 150 Watts, and a silane/ammonia ratio (flowrate ratio) ranging between 2 and 20.  
         [0038]     According to the preferred embodiment, the tensile-stressed nitride film  60  has a controlled Si—H/N—H ratio ranging between 0.1 and 10, preferably about 1.0. The concentration of Si—H bond ranges between 1E22 atoms/cm 3  and 5E23 atoms/cm 3 . The concentration of N—H bond ranges between 1E22 atoms/cm 3  and 5E23 atoms/cm 3 . The concentration of hydrogen atoms in the tensile-stressed nitride film  60  ranges between 1E22 atoms/cm 3  and 1E24 atoms/cm 3 , preferably greater than 3E22 atoms/cm 3 . The nitride film  60  has a tensile-stress that is less than 1.8 GPa, for example, 0-1.2 GPa. According to this invention, the concentration of hydrogen atoms in the tensile-stressed nitride film  60  is preferably kept as high as possible within the aforesaid range.  
         [0039]     Subsequently, as shown in  FIG. 7 , a post treatment  70  is performed to cure the nitride film  60 . The post treatment  70  is to reduce the concentration of hydrogen atom in the nitride film  60  as much as possible. According to the preferred embodiment, the post treatment  70  is UV curing process executed in vacuum environment, at a temperature of 300-1200° C. for a time period of about 1-30 seconds. In another case, the post treatment  70  may comprise a rapid thermal process (RTP). After the UV curing process, an ultra-high tensile stressed nitride film  62  is produced.  
         [0040]     According to the preferred embodiment, the post-treated nitride film  62  has a Si—H/N—H ratio ranging between 0.1 and 10, preferably about 0.3. The concentration of Si—H bond ranges between 1 E18 atoms/cm 3  and 5E22 atoms/cm 3 . The concentration of N—H bond ranges between 1 E18 atoms/cm 3  and 5E22 atoms/cm 3 . The concentration of hydrogen atoms in the nitride film  62  ranges between 1 E18 atoms/cm 3  and 5E22 atoms/cm 3 , preferably less than 1E22 atoms/cm 3 , more preferably less than 5E21 atoms/cm 3 .  
         [0041]     As shown in  FIG. 8 , an inter-layer dielectric (ILD) film  80  such as silicon oxide is deposited over the post-treated nitride film  62 . Conventional lithographic and etching processes are performed to form contact holes  82  in the ILD film  80  and the nitride film  62 . The contact holes  82  expose a portion of the underlying salicide layers  52  formed on the source/drain regions  46 .  
         [0042]     Please refer briefly to  FIGS. 9-11 . As shown in  FIG. 9 , it has been experimentally confirmed that the post-treated nitride film  62  with a final concentration of hydrogen atoms of less than 1E22 atoms/cm 3  has a tensile-stress that is above 1.6 GPa.  FIG. 10  is a plot showing the relation between removed amount of H atoms (ΔH) and the stress value of the post-treated nitride film. It has been experimentally confirmed that the more the amount of H atoms is removed the more the stress of the nitride film can be increased. Preferably, in order to get a higher tensile stress, ΔH is greater than 1E22 atoms/cm 3 , more preferably greater than 1.4E22 atoms/cm 3 . It is believed that the tensile stress of the post-treated nitride film  62  is increased due to that the significant concentration drop of the hydrogen atoms in the film  62  causes rearrangement of other atoms.  
         [0043]      FIG. 11  is a diagram of FTIR showing that both of the concentration of Si—H bond and the concentration of N—H bond of the nitride film decrease after UV curing. The UV curing process is able to increase the stress of the nitride film  60  by breaking Si—H bond and N—H bond of the nitride film and removing released hydrogen atoms.  
         [0044]     As previously mentioned, the ultra-high tensile-stressed nitride film may be applied to poly stressor process.  FIGS. 12-15  are schematic, cross-sectional diagrams showing the steps of poly stressor process according to one preferred embodiment of this invention, wherein like numeral numbers designate like elements, regions or layers. As shown in  FIG. 12 , a semiconductor substrate  10  is provided and a gate structure  12  is formed on the semiconductor substrate  10 . Shallow trench isolation (STI)  20  is also provided for isolation purpose. Likewise, the gate structure  12  comprises a gate oxide layer  14 , a gate electrode layer  16  stacked on the gate oxide layer  14 , and sidewall spacer  18 . Preferably, the gate oxide layer  14  is composed of silicon dioxide, and the gate electrode layer  16  is composed of doped polysilicon, but not limited thereto.  
         [0045]     The semiconductor substrate  10  may be a silicon substrate, silicon-on-insulator (SOI) substrate or any suitable semiconductor substrate with epitaxial layers. Such epitaxial layers include, but not limited to, silicon epitaxial layer, silicon germanium epitaxial (SiGe) layer or the like.  
         [0046]     As shown in  FIG. 13 , an ion implantation process  30  is performed to form a source/drain region  26  in the semiconductor substrate  10  adjacent to the sidewall spacer  18 .  
         [0047]     As shown in  FIG. 14 , a PECVD process is carried out to deposit a tensile-stressed nitride film  60  over the semiconductor substrate  10 . The tensile-stressed nitride film  60  covers the gate structure  12  and the source/drain region  26 . The tensile-stressed nitride film  60  is deposited by utilizing silane and ammonia as main reaction gases. Preferably, the PECVD is carried out at a low-frequency RF power ranging between 50 Watts and 2700 Watts, a high-frequency RF power ranging between 100 Watts and 200 Watts, preferably less than 150 Watts, and a silane/ammonia ratio (flowrate ratio) ranging between 2 and 20.  
         [0048]     According to this invention, the nitride film  60  has a controlled Si—H/N—H ratio ranging between 0.1 and 10, preferably about 1.0. The concentration of Si—H bond ranges between 1E22 atoms/cm 3  and 5E23 atoms/cm 3 . The concentration of N—H bond ranges between 1E22 atoms/cm 3  and 5E23 atoms/cm 3 . The concentration of hydrogen atoms in the tensile-stressed nitride film  60  ranges between 1E22 atoms/cm 3  and 1E24 atoms/cm 3 , preferably greater than 3E22 atoms/cm 3 . The nitride film  60  has a tensile-stress that is less than 1.8 GPa, for example, 0-1.2 GPa. According to this invention, the concentration of hydrogen atoms in the tensile-stressed nitride film  60  is preferably kept as high as possible within the aforesaid range.  
         [0049]     As shown in  FIG. 15 , a post treatment  70  is performed to cure the nitride film  60 . The post treatment  70  is to reduce the concentration of hydrogen atom in the nitride film  60  as much as possible. According to the preferred embodiment, the post treatment  70  is UV curing process. After the UV curing process, an ultra-high tensile stressed nitride stressor film  62  is produced.  
         [0050]     According to the preferred embodiment, the post-treated nitride film  62  has a Si—H/N—H ratio ranging between 0.1 and 10, preferably about 0.3. The concentration of Si—H bond ranges between 1E18 atoms/cm 3  and 5E22 atoms/cm 3 . The concentration of N—H bond ranges between 1E18 atoms/cm 3  and 5E22 atoms/cm 3 . The concentration of hydrogen atoms in the nitride film  62  ranges between 1E18 atoms/cm 3  and 5E22 atoms/cm 3 , preferably less than 1 E22 atoms/cm 3 , more preferably less than 5E21 atoms/cm 3 . The nitride stressor film  62 , which strains the gate channel, is then removed.  
         [0051]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims