Abstract:
A method for fabricating a surrounding-gate silicon nanowire transistor with air sidewalls is provided. The method is compatible with the CMOS process; the introduced air sidewalls can reduce the parasitic capacitance effectively and increase the transient response characteristic of the device, thus being applicable to a high-performance logic circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a manufacturing technology of an ultra-large-sale integrated circuits, more particularly, to a method for fabricating a surrounding-gate silicon nanowire transistor with air sidewalls. 
       BACKGROUND OF THE INVENTION 
       [0002]    As a device size has been reduced to a deep sub-micrometer, gate control capability of a conventional planar transistor is getting down, and a short channel effect is becoming more and more obvious, thus leading to a series of problems, such as drift of threshold voltage, increase of sub-threshold slope and leakage current in sub-threshold region, drain induced barrier lowering effect and the like. In order to suppress the short channel effect, a multi-gate structure can be used to enhance the control capability for the gate to the channel. An ultimate of the multi-gate structure is a surrounding-gate silicon nanowire structure. Due to an excellent gate-control capability and a one-dimensional quasi-ballistic transport of the surrounding-gate structure, the surrounding-gate silicon nanowire transistor can have an excellent sub-threshold characteristic, an increased current on-off ratio, an enhanced current drivability. Further, it is also well compatible with a CMOS process, and therefore, the surrounding-gate silicon nanowire transistor has been considered to be a promising device that will replace the planar transistor in the future. 
         [0003]    However, as shown in  FIG. 1 , in the surrounding-gate silicon nanowire transistor, a fringing capacitance between the gate and the source/drain may be an unnegligible issue because of the structural characteristics of the surrounding gate and the nanometer-sized channel, thus adversely affecting the transient response characteristics of the device. 
         [0004]    In order to reduce the parasitic capacitance, a material with a lower dielectric constant can be used as sidewalls to reduce a capacitive coupling effect between the gate and the source/drain, so that the fringing capacitance can be reduced. To this end, air is one of the materials having a very low dielectric constant.  FIG. 2  illustrates a cross-sectional view along a channel direction of the surrounding-gate silicon nanowire transistor with air sidewalls. 
         [0005]    Due to a peculiar three dimensional structure of the nanowire, it is required certain special processes to form the air sidewalls, which has not been reported so far in this research area. 
       SUMMARY OF THE INVENTION 
       [0006]    A purpose of the present invention is to provide a method for fabricating a surrounding-gate silicon nanowire transistor with air sidewalls. The transistor is fabricated over a SOI (Silicon-On-Insulator) substrate. 
         [0007]    A technical solution of the invention is described as follows. 
         [0008]    A method for fabricating a surrounding-gate silicon nanowire transistor with air sidewalls is characterized in that the transistor is fabricated over a SOI substrate, including the following steps: 
         [0009]    1) performing an isolation process; 
         [0010]    2) depositing a SiO 2  layer and depositing a SiN layer; 
         [0011]    3) performing a photolithography process to define a channel region and a large source/drain region; 
         [0012]    4) transferring a pattern on a photoresist onto SiN and SiO 2  hard masks by etching; 
         [0013]    5) depositing a layer of material A (such as SiO 2 , SiN and etc.) having a high etching selection ratio with respect to Si; 
         [0014]    6) performing a photolithography process to define a Fin (fin-shaped thin lines) bar; 
         [0015]    7) transferring the pattern of the photoresist to the layer of the material A by etching, so as to form a hard mask for the Fin bar and the large source/drain region; 
         [0016]    8) etching the Si layer by using the layer of material A and the SiN layer the hard mask, so as to form the Fin bar of Si and the large source/drain; 
         [0017]    9) depositing a SiN layer; 
         [0018]    10) etching the SiN layer to form SiN sidewalls; 
         [0019]    11) performing an oxidization process to form nanowires; 
         [0020]    12) removing the oxide layer by using a wet process, so as to form suspended nanowires; 
         [0021]    13) forming a gate oxide layer; 
         [0022]    14) depositing a polysilicon layer; 
         [0023]    15) performing a photolithography process to define a gate line; 
         [0024]    16) performing an etching process to transfer the pattern of the photoresist to the polysilicon layer; 
         [0025]    17) performing an implantation process on the polysilicon layer and the source/drain region; 
         [0026]    18) performing a wet etching process on the SiN layer; 
         [0027]    19) depositing a SiO 2  layer to form air sidewalls; 
         [0028]    20) performing an annealing process to activate impurities; 
         [0029]    21) performing subsequent processes by using conventional processes, so that the fabrication of the device is completed. 
         [0030]    In the step 1), a silicon island isolation or a local oxidation of silicon (LOCOS) isolation is used. 
         [0031]    In the steps 2), 5), 9), 14), 19), a chemical vapor deposition is used. 
         [0032]    In the steps 4), 7), 8), 16), an anisotropic dry etching is used. 
         [0033]    In the step 10), an anisotropic dry etching is used to ensure SiN remain above the source/drain without being fully removed. 
         [0034]    In the step 11), a dry-oxygen oxidation or a hydrogen and oxygen combination oxidation is used. 
         [0035]    In the step 12), the oxide layer is removed by using a hydrofluoric acid. 
         [0036]    In the step 13), a SiO 2  dielectric layer formed by a dry-oxygen oxidation or other dielectric layer with high dielectric constant is used. 
         [0037]    In the step 18), SiN is removed by using a concentrated phosphoric acid at 170° C. 
         [0038]    The beneficial effects of the invention lie in that, the method for fabricating the surrounding-gate silicon nanowire transistor with air sidewalls according to the invention is compatible with the CMOS process; the introduced air sidewalls can reduce the parasitic capacitance effectively and increase the transient response characteristic of the device, thus being applicable to high-performance logic circuits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a schematic view showing fringing capacitances of a surrounding-gate silicon nanowire device; 
           [0040]      FIG. 2  is cross-sectional views along channel direction of surrounding-gate silicon nanowire devices with SiO 2  and air sidewalls respectively; 
           [0041]      FIGS. 3 to 13  are flow diagrams showing processes implemented in the embodiments of the present invention, in which material of layers are described as follows: 
           [0000]    
         
           
                 
                 
                 
               
             
                 
                     
                 
               
               
                 
                     
                   1-Si 
                   2-a buried oxide layer 
                 
                 
                     
                   3-SiN 
                   4-polysilicon 
                 
                 
                     
                   5-SiO 2   
                   6-air 
                 
                 
                     
                 
               
            
           
         
       
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0042]    Hereinafter, the present invention will be further described with reference to accompany drawings and specific embodiments. 
       Embodiment 1 
       [0043]    a process flow for fabricating a surrounding-gate silicon nanowire device with air sidewalls are as follows. 
         [0044]    Starting from a SOI substrate (a thickness of a Si layer over a buried oxide layer is 2500 Å): 
         [0045]    1. A silicon island isolation is performed. 
         [0046]    2. A SiO 2  layer is deposited with a thickness of 300 Å by using a low pressure chemical vapor deposition (LPCVD). 
         [0047]    3. A SiN layer is deposited with a thickness of 1500 Å by using a low pressure chemical vapor deposition (LPCVD), as shown in  FIG. 3 . 
         [0048]    4. A photolithography process is performed to define a channel region and a large source/drain region. 
         [0049]    5. As shown in  FIG. 4 , the SiN layer is etched by 1500 Å and the SiO 2  layer is etched by 300 Å by using a reactive ion etching (RIE) technology, so as to form a hard mask for the large source/drain region; and then the photoresist is removed. 
         [0050]    6. A SiN layer is deposited with a thickness of 300 Å, as shown in  FIG. 5 . 
         [0051]    7. A photolithography process is performed to define a hard mark for a Fin bar. 
         [0052]    8. As shown in  FIG. 6 , the SiN layer is etched by 300 Å through a reactive ion etching (RIE) technology, so as to transfer patterns of the photoresist onto the SiN layer to form a Fin hard mask of SiN; and then the photoresist is removed. 
         [0053]    9. As shown in  FIG. 7 , using the SiN layer as a hard mask, the Si layer is etched by 2500 Å using an inductively coupled plasma (ICP) etching, so as to form a Si Fin bar and the large source/drain region. 
         [0054]    10. The SiN layer is deposited with a thickness of 1500 Å by using a low pressure chemical vapor (LPCVD) deposition. 
         [0055]    11. As shown in  FIG. 8 , the SiN layer is etched by 2500 Å using a reactive ion etching (RIE), so as to form the SiN sidewalls. 
         [0056]    12. A dry-oxygen oxidation is performed to form Si nanowires. 
         [0057]    13. As shown in  FIG. 9 , the SiO 2  layer obtained through the dry-oxygen oxidation is etched by using a buffer hydrofluoric acid (BHF), so as to form suspended nanowires. 
         [0058]    14. A gate oxidation is performed to form a gate oxide layer with a thickness of 5 nm. 
         [0059]    15. Polysilicon is deposited with a thickness of 2000 Å by using a low pressure chemical vapor deposition (LPCVD), as shown in  FIG. 10 . 
         [0060]    16. A photolithography process is performed to define a gate line. 
         [0061]    17. As shown in  FIG. 11 , the polysilicon layer is etched by 2000 Å using an inductively coupled plasma (ICP) etching, so as to form a polysilicon gate; and the photoresist is removed. 
         [0062]    18. As shown in  FIG. 12 , As is implanted with an implantation energy of 50K ev and a implantation dose of 4×10 15  cm −2 . 
         [0063]    19. The SiN layer is selectively etched by using concentrated phosphoric acid at 170° C., so as to completely remove SiN layer over the source/drain as well as the SiN sidewalls. 
         [0064]    20. As shown in  FIG. 13 , a SiO 2  layer is deposited with a thickness of 3000 Å by using a low pressure chemical vapor deposition (LPCVD), so as to form air sidewalls. 
         [0065]    21. A rapid thermal annealing (RTP) is performed for 5 seconds at 1050° C. in nitrogen atmosphere to activate impurities. 
         [0066]    22. A photolithography process is performed to define metal contact holes. 
         [0067]    23. The SiO 2  layer is etched by 3000 Å using a reactive ion etching (RIE), SiO 2  remaining in the metal contact holes is etched by using a buffer hydrofluoric acid (BHF), and the photoresist is removed. 
         [0068]    24. Ti and Al layers are sputtered with thicknesses of 700 Å and 1 μm respectively. 
         [0069]    25. A photolithography process is performed to define metal leading wires. 
         [0070]    26. The Al and Ti layers are etched by 1 μm and 700 Å respectively by using a RIE etching, and then the photoresist is removed. 
         [0071]    27. An annealing is performed for 30 minutes at 430° C. in N 2 +H 2  atmosphere for alloying and the process is finished. 
       Embodiment 2 
       [0072]    to the difference between Embodiment 1 and Embodiment 2 are as follows. 
         [0073]    1. A LOCOS isolation is performed. 
         [0074]    6. The SiO 2  layer is deposited with a thickness of 300 Å. 
         [0075]    8. The SiO 2  layer is etched by 300 Å using a reactive ion etching (RIE) technology, so as to transfer the pattern of the photoresist onto the SiO 2  layer to form a Fin hard mask of SiO 2 ; and then the photoresist is removed. 
         [0076]    9. By using the SiO 2  and SiN layers as hard masks, the Si layer is etched by 2500 Å using inductively coupled plasma (ICP) etching, so as to form a Si Fin bar and the large source/drain region. 
         [0077]    12. A hydrogen and oxygen combination oxidation is performed to form Si nanowires. 
         [0078]    13. The SiO 2  layer obtained through the hydrogen and oxygen combination oxidation is etched by using a buffer hydrofluoric acid (BHF), so as to form suspended nanowires.