Abstract:
A method for forming a polysilicon film in a plasma-assisted chemical vapor deposition (CVD) system including a chamber in which a first electrode and a second electrode spaced apart from the first electrode are provided comprises providing a substrate on the second electrode, the substrate including a surface exposed to the first electrode, applying a first power to the first electrode for generating a plasma in the chamber, applying a second power to the second electrode during a nucleation stage of the polysilicon film for ion bombarding the surface of the substrate, and flowing an erosive gas into the chamber.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to a method of semiconductor fabrication, and more particularly, to a method of forming a polycrystalline silicon film by a plasma-assisted chemical vapor deposition (“CVD”) process.  
         [0002]     In the fabrication of flat panel display (“FPD”) devices, thin film transistors (“TFTs”) and liquid crystal cells, metal interconnects and other features are formed by depositing and/or removing multiple layers of conducting, semi-conducting and dielectric materials from a glass substrate. One of the conventional methods for fabricating a polycrystalline silicon (hereinafter polysilicon) TFT is solid phase crystallization (“SPC”). In LCD devices, since a normal glass substrate can only work at a temperature below 600° C., fabricating a polysilicon film directly under a high temperature will make the glass substrate twisted. The SPC method overcomes the high temperature issue by using an expensive quartz substrate. However, only a small size LCD panel can be made. In a method called biased-enhanced nucleation (“BEN”), nucleation of diamond on Pt or Pt alloy substrates at high density is possible by applying a bias voltage to the substrate in a plasma atmosphere that includes carbon at the beginning of diamond CVD. In the BEN method, for diamond growth, known CVD methods, such as microwave plasma CVD, radio frequency plasma CVD, hot filament CVD, DC plasma CVD methods, plasma jet, combustion, thermal CVD and the like can be used. An example of the BEN method can be found in “Generation of Diamond Nuclei by Electric Field in Plasma Chemical Vapor Deposition” by Yugo et al., Appl. Phys. Lett., 58 (1991), 1036. The BEN method is effective because the substrate surface is quickly supersaturated with carbon-containing species as ions containing carbon atoms are attracted to the substrate by an electric field due to the bias, and thus diamond nuclei are more readily formed and grown.  
         [0003]     A low temperature polycrystalline silicon (“LTPS”) process has been developed for manufacturing the TFT array of liquid crystal display (“LCD”) devices. The low temperature polysilicon has the characteristic of faster electron mobility than the amorphous silicon (a-Si) over approximately 100 times. Consequently, each pixel of low temperature polysilicon has a faster response time and smaller outlined dimension compared with amorphous silicon.  
         [0004]     Conventional methods for fabricating polysilicon TFTs at low temperature may include excimer laser annealing (“ELA”) and CVD processes. The ELA method requires a lower crystallization temperature than the SPC method. However, the transient temperature provided by a laser may damage a polymeric substrate. Since polymeric substrates are used more often than glass substrates, the ELA method may still have a temperature issue. Furthermore, the ELA method may not be cost efficient in mass production because ELA equipment is expensive.  
         [0005]     In the CVD method, a low temperature polysilicon film is formed by crystallizing an amorphous silicon film. Processing techniques used to create the TFT devices may include plasma-enhanced chemical vapor deposition (“PECVD”) and the like. Plasma processing may be well suited for the production of TFT devices because of the relatively lower processing temperatures required to deposit films and the good film quality which results from plasma processes. Examples of the CVD method can be found in “Structural Properties of Polycrystalline Silicon Films Prepared at Low Temperature by Plasma Vapor Deposition” by Kakinuma et al., J. Appl. Phys., 70 (1991), 7374, and “Study of Polycrystalline Silicon Films Deposited by Inductive Couple Plasma Chemical Vapor Deposition” by Won et al., Journal of the Korean Physical Society, Vol. 39, 123˜126 (2001). Although the CVD method may appear more advantageous than the SPC and ELA methods, published research papers have indicated that in the CVD method a polycrystalline silicon state of a film cannot be achieved until an amorphous incubation layer of approximately 100 to 1000 angstroms (Å) of the film is grown, which disadvantageously requires a longer fabrication time. The CVD method may be generally used for fabricating top-gate TFT devices. However, it is not suggested to fabricate bottom-gate TFT devices with the CVD method. A bottom-gate TFT device thus fabricated may suffer from threshold voltage (V TH ) shift due to stress, resulting in an early degradation. The CVD method therefore is not suitable for use in the fabrication of organic light emitting diodes (OLEDs). Furthermore, the CVD method may have a relatively low deposition rate of, for example, approximately 0.1 to 1 Å/sec, resulting in an undesirable throughput.  
         [0006]     It is therefore desirable to have a method for fabricating polysilicon films that overcomes the disadvantages of the above-mentioned conventional methods in order to obtain a larger production yield, better uniformity over larger surfaces and thinner deposition layers.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to a method that obviates one or more problems resulting from the limitations and disadvantages of the prior art.  
         [0008]     In accordance with an embodiment of the present invention, there is provided a method for forming a polysilicon film in a plasma-assisted chemical vapor deposition (CVD) system including a chamber in which a first electrode and a second electrode spaced apart from the first electrode are provided that comprises providing a substrate on the second electrode, the substrate including a surface exposed to the first electrode, applying a first power to the first electrode for generating a plasma in the chamber, applying a second power to the second electrode during a nucleation stage of the polysilicon film for ion bombarding the surface of the substrate, and flowing an erosive gas into the chamber.  
         [0009]     Also in accordance with the present invention, there is provided a method for forming a polysilicon film in a plasma-assisted chemical vapor deposition (CVD) system including a chamber in which a first electrode and a second electrode spaced apart from the first electrode are provided that comprises providing a substrate on the second electrode, the substrate including a surface exposed to the first electrode, applying a first power to the first electrode for generating a plasma in the chamber, and applying a second power to one of the second electrode or the substrate during a nucleation stage of the polysilicon film for ion bombarding the surface of the substrate.  
         [0010]     Further in accordance with the present invention, there is provided a method for forming a polysilicon film in a plasma-assisted chemical vapor deposition (CVD) system including a chamber in which a first electrode and a second electrode spaced apart from the first electrode are provided that comprises providing a substrate on the second electrode, the substrate including a surface exposed to the first electrode, generating a plasma in the chamber during a nucleation stage and a growth stage after the nucleation stage of the polysilicon film, ion bombarding the surface of the substrate during the nucleation stage of the polysilicon film, and chemically eroding the surface of the substrate.  
         [0011]     Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.  
         [0012]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0013]     The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
         [0014]     In the drawings:  
         [0015]      FIG. 1  is a schematic diagram of a system for forming a polysilicon film in accordance with one embodiment of the present invention;  
         [0016]      FIG. 2  is a schematic diagram illustrating a method for forming a polysilicon film in accordance with one embodiment of the present invention;  
         [0017]      FIGS. 3A and 3B  are TEM (transmission electron microscope) photo diagrams respectively showing a plane view and a cross-sectional view of a polysilicon film formed by a method in accordance with one embodiment of the present invention; and  
         [0018]      FIG. 4  is a plot of a Raman spectrum analysis on the polysilicon film shown in  FIGS. 3A and 3B . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 1  is a schematic diagram of a system  10  for forming a polysilicon film in accordance with one embodiment of the present invention. Referring to  FIG. 1 , system  10 , a plasma assisted chemical reaction system, includes a chamber  12 , a first power generator  14  and a second power generator  16 . System  10 , except second power generator  16 , may include one of a model AKT-1600 PECVD system manufactured by Applied Komatsu Technology, a high density plasma CVD (HDPCVD) system manufactured by Applied Materials, Inc., or an inductively coupled plasma CVD (ICP-CVD) system. The present invention, however, is not limited to the above-mentioned systems and may be used with other commercially available deposition systems.  
         [0020]     A substrate  30 , either glass or a polymeric material, is placed in chamber  12  that is equipped with a pair of parallel plate electrodes including a first electrode  12 - 1  and a second electrode  12 - 2 . First electrode  12 - 1  functions as a gas inlet manifold or shower head through which a reactant gas provided by a gas controller  18  flows into chamber  12 . Second electrode  12 - 2 , separated from first electrode  12 - 1  by several inches, functions to support or hold substrate  30 . During deposition, a radio frequency (RF) voltage provided by first power generator  14  through a matching network  14 - 1  is applied to first electrode  12 - 1  to produce a plasma within the reactant gas in chamber  12 . The plasma causes the reactant gas to decompose and deposit a layer of material onto an exposed surface  30 - 1  of substrate  30 . Second power supply  16  provides an RF voltage, a direct current (DC) voltage, an alternating current (AC) voltage or a pulse voltage to second electrode  12 - 2  to create an electrical field between first electrode  12 - 1  and second electrode  12 - 2 . The deposition process and operation of second power supply  16  will be later discussed in detail by reference to  FIG. 2 .  
         [0021]     System  10  further includes a heat controller  20 , a lift mechanism  22  and a pump  24 . Heat controller  20  powers a heater (not shown) for heating substrate  30  during deposition to achieve or maintain second electrode  12 - 2  at an appropriate temperature level. Lift mechanism  22  is provided to support second electrode  12 - 2  at an appropriate elevation level. Pump  24  is used to evacuate chamber  12  to a state of vacuum.  
         [0022]      FIG. 2  is a schematic diagram illustrating a method for forming a polysilicon film in accordance with one embodiment of the present invention. The deposition process is a result of chemical reactions between reactive molecular precursors and substrate  30 . Initial atoms and molecules that will constitute the film are delivered as precursors, which are fed from gas controller  18  shown in  FIG. 1 . The desired reactions are to deposit a pure film on surface  30 - 1  of substrate  30  and eliminate extra atoms or molecules that comprise the precursors.  
         [0023]     Referring to  FIG. 2 , the deposition process at least includes a nucleation stage and a growth stage. The nucleation stage is assumed when a film of stable material is deposited on nucleation sites on surface  30 - 1  of substrate  30 . Substrate  30  has many bonding locations on surface  30 - 1 , where chemical binding occurs during deposition, causing gaseous atoms and molecules to chemically attach to surface  30 - 1 . However, the reaction does not occur at all of the potential bonding locations. Generally, defect sites, which have irregular topology or impurities, are likely to trap the molecular precursors. To provide more of such defect sites, also referring to  FIG. 1 , second power supply  16  provides a bias voltage to second electrode  12 - 2  during the nucleation stage to generate an electrical field between first electrode  12 - 1  and second electrode  12 - 2 , resulting in an ion bombarding effect on surface  30 - 1 . In another embodiment, second power supply  16  provides a bias voltage to substrate  30  during the nucleation stage. The ion bombarding facilitates formation of defect sites for initial reaction products, that is, nucleation seeds. The nucleation seeds are immobile and diffusing molecular precursors have a high probability to collide with them and react, resulting in the growing of metastable clusters. As the metastable clusters grow larger, most of the collisions occur at the boundaries of the metastable clusters. As the metastable clusters further grow three-dimensionally, most of the binding and reaction processes occur on the upper surfaces of the metastable clusters, resulting in the formation of critical clusters. During the growth stage, eventually, the vertical growth of the critical clusters results in the formation of grains, which finally coalesce into a continuous film.  
         [0024]     In one embodiment according to the present invention, the RF power provided by first power supply is approximately 600 watts at a frequency of approximately 13.56 MHz. The density of plasma generated is approximately 10 11  to 10 13  cm −3 , which facilitates the nucleation with a shorter incubation time and thinner incubation layer as compared to a lower density of one or two orders less. Second power supply  16 , in one aspect, provides an RF power ranging from approximately 100 to 1000 watts at 13.56 MHz. In another aspect, second power supply  16  provides a DC bias ranging from approximately 0 to 600 volts. In still another aspect, second power supply  16  provides an AC bias ranging from approximately 0 to 500 volts at a frequency of approximately 0 to 400 Hz. In yet another aspect, second power supply  16  provides a pulse voltage, for example, in the form of a square wave transmitting in a single direction, i.e., either positive or negative. The pulse voltage ranges from approximately 0 to 500 volts at a frequency of approximately 0 to 400 Hz with a pulse width of approximately 1 to 10 μm/sec. Chamber  12  is evacuated to a pressure of approximately 10 −3  Torr. The reactant gases include silane (SiH 4 ), hydrogen (H 2 ) and Argon (Ar). In one embodiment, Ar is approximately 0 to 50 sccm, SiH 4  is approximately 50 sccm, and the ratio of SiH 4  to H 2  is approximately 1:10 to 1:100. Substrate  30  is maintained at a temperature of approximately 25° C. to 500° C. The incubation layer ranges approximately from 300 Å to 500 Å, under which thickness the amorphous silicon is crystallized into polycrystalline silicon.  
         [0025]     During the growth stage, a chemical erosion process is conducted to remove weakly bonded amorphous or silicon molecules on the upper surface of the incubation layer. In another embodiment, however, the chemical erosion process is conducted during the nucleation stage. Since separated nucleation sites can result in the formation of grain boundaries and voids on surface  30 - 1  of substrate  30 , where potential bonding sites failed to bond with the molecular precursors, the removal of the weakly bonded materials helps to reduce the incubation time and the incubation layer thickness. An erosive gas including SiF 4  and H 2  or SF 6  and H 2  is used in the chemical erosion process. In one embodiment according to the present invention, the ratio of SiF 4  to H 2  ranges from approximately 1:10 to 1:100. In one aspect, SiF 4  is 1 sccm and H 2  is 10 sccm. In another embodiment, during the growth stage, second power supply  16  provides a DC bias of approximately 50 volts to second electrode  12 - 2  or substrate  30  in order to achieve a condensed polysilicon film. In still another embodiment, second power supply  16  provides an AC bias ranging from approximately 0 to 50 volts at a frequency of approximately 0 to 400 Hz. In yet another embodiment, second power supply  16  provides a pulse voltage of approximately 0 to 50 volts at a frequency of approximately 0 to 400 Hz with a pulse width of approximately 1 to 10 μm/sec. Furthermore, during the growth stage, the reactant gas Ar is cut off, SiH 4  is maintained at approximately 50 sccm, and the ratio of SiH 4  to H 2  is approximately 1:10 to 1:100.  
         [0026]      FIGS. 3A and 3B  are TEM (transmission electron microscope) photo diagrams respectively showing a plan view and a cross-sectional view of a polysilicon film formed by a method in accordance with one embodiment of the present invention. Referring to  FIG. 3B , an LTPS state of the film is achieved when the film is only grown to approximately 500 Å.  
         [0027]      FIG. 4  is a plot of a Raman spectrum analysis on the polysilicon film shown in  FIGS. 3A and 3B . Referring to  FIG. 4 , a signal occurs at the wave number of 520 cm −1 , which indicates that a polycrystalline silicon has been formed.  
         [0028]     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.  
         [0029]     Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of the steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.