Abstract:
A substrate processing method includes a first step of exposing a silicon substrate surface to mixed gas plasma of an inert gas and hydrogen, and a second step of conducting any of oxidation processing, nitridation processing and oxynitridation processing to said silicon substrate surface by plasma processing after said first step, wherein an organic substance remaining on said substrate surface is removed in said first step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention is a continuation-in-part application of PCT/JP2004/002013 field on Feb. 20, 2004 based on Japanese priority application 2003-054242 filed on Feb. 28, 2003, the entire contents of each are incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention generally relates to substrate processing technology, and more particularly to a substrate processing method for forming an insulation film on a silicon substrate.  
         [0003]     In semiconductor production technology, formation of insulation film on a silicon substrate is most fundamental and yet important technology. Especially, a very high quality insulation film is required for the gate insulation film, or the like, of MOS transistors. Meanwhile, the film thickness of the gate insulation film has been decreased to about 1 nm with device miniaturization with recent ultra-miniaturized high-speed semiconductor devices, and there is a need for the technology capable of forming such a thin insulation film with high quality.  
         [0004]     Conventionally, high quality silicon oxide films used for the gate insulation film of a MOS transistor have been formed by thermal oxidation processing of a silicon substrate surface. A thermal oxide film of silicon thus formed has the feature of small number of dangling bonds, and there is caused little trapping of carriers even in the case the film is used for an insulation film covering the channel region and thus used in the environment in which the film is subjected to high electric field. Thereby, stable threshold characteristics are realized.  
         [0005]     On the other hand, with the progress in the miniaturization technology, it is now becoming possible these days to produce ultra-miniaturized semiconductor devices having a gate length of 0.1 μm or less.  
         [0006]     In order to improve the operational speed of semiconductor device with such ultra miniaturized semiconductor devices by way of reducing the gate length, there is a need of reducing the thickness of the gate insulation film in accordance with scaling law. In the case of a MOS transistor having the gate length of 0.1 μm, for example, there is a need of reducing the thickness of the gate insulation film to 2 nm or less. On the other hand, when the film thickness is decreased like this with a conventional thermal oxide film, there occurs an increase of gate leakage current in the form of tunneling current. From this reason, it has been thought that film thickness of 2 nm would be the limit of gate insulation film formed by a thermal oxide film. With a thermal oxide film having the film thickness of 2 nm, a gate leakage current of 1×10 −2 A/cm 2  has been realized.  
         [0007]     Contrary to this, there is proposed a technology capable of forming a higher quality silicon oxide film by conducting oxidation processing to a silicon substrate, by using microwave plasma.  
         [0008]     With the silicon oxide film thus formed by microwave plasma oxidation of silicon substrate, it has been confirmed that leakage current of 1×10 −2 A/cm 2  is possible with the application voltage of 1V, even in the case the film has a film thickness of 1.5 nm. Thus, it is expected that the silicon oxide film formed by microwave plasma enables breaking through of the foregoing limit of device miniaturization encountered in the conventional semiconductor devices that use a conventional thermal oxide film. Further, with the substrate processing that uses the microwave plasma, it becomes possible to form an oxynitride film or nitride having a large specific dielectric constant on a silicon substrate with the film quality exceeding the film quality of a thermal oxide film. In the case of using an oxynitride film for gate insulation film, a leakage current of 1×10 −2 A/cm 2  or less is realized at the application voltage of 1V for an oxynitride film having a film thickness equivalent to the film thickness of 1 nm of silicon oxide film.  
         [0009]     Substrate processing by microwave plasma can be performed at a low temperature typically below 500° C., and because of this, it becomes possible to reduce the time needed for raising and lowering the substrate temperature. Thereby, it becomes possible to produce the semiconductor device with large throughput. Further, with such low temperature processing, there occurs no change of impurity concentration profile of diffusion regions even when the diffusion regions are already formed in the substrate, and it becomes possible to realize desired device characteristics with reliability.  
         [0010]     Meanwhile, a gate insulation film is required to provide the feature of small leakage current and high reliability.  
         [0011]      FIG. 1  shows the relationship between the accumulated defect rate F and integral electric charge amount (Qbd) leading to breakdown (TDDB: time dependent dielectric breakdown characteristic) for a silicon oxide film formed on a silicon substrate surface with the thickness of 10 nm by a microwave plasma oxidation processing conducted by the inventor of the present invention (shown in the drawing as “plasma oxide film”), in comparison with a thermal oxide film of the same thickness, wherein the vertical axis represents the accumulated defect rate F while the horizontal axis represents the integral electric charge amount Qbd that leads to insulation breakdown. It should be noted that the plasma oxide film has been formed by using a microwave plasma substrate processing apparatus to be explained later with  FIG. 2 , by oxidizing the silicon substrate surface already applied with removal process of native oxide film in the mixed gas plasma of argon and oxygen at the substrate temperature of 400° C.  
         [0012]     Referring to  FIG. 1 , the line representing the accumulated defect rate F forms a steep gradient with regard to the integral electric charge amount Qbd in the case of the thermal oxide film, and thus, insulation breakdown occurs when the integral electric charge amount Qbd has reached a predetermined value. Such an insulation film has excellent reliability characterized predictable lifetime.  
         [0013]     In the case of the plasma oxide film, on the other hand, the slope of the line representing the accumulated defect rate F is small, indicating that breakdown of the insulation film occurs with various values of the integrated electric charge amount. With such an insulation film, it is not possible to predict the device lifetime with certainty and no reliability is attained for the semiconductor device.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, it is a general object of the present invention to provide a novel and useful substrate processing method wherein the foregoing problems are eliminated.  
         [0015]     Another and more specific object of the present invention is to provide a substrate processing method capable of forming an oxide film, a nitride film, or an oxynitride film on a silicon substrate surface by oxidation processing, nitridation processing or oxynitridation processing conducted in plasma with improved reliability and thus capable of assuring long device lifetime with the semiconductor device that uses such an insulation film.  
         [0016]     Another object of the present invention is to provide a substrate processing method, comprising:  
         [0017]     a first step of exposing a silicon substrate surface to mixed gas plasma of an inert gas and hydrogen; and  
         [0018]     a second step of conducting any of oxidation processing, nitridation processing and oxynitridation processing to said silicon substrate surface by plasma processing after said first step.  
         [0019]     According to the present invention, organic substance remaining on the substrate surface, is removed effectively by exposing the silicon substrate surface to the mixture gas plasma of the inert gas and the hydrogen gas before the substrate processing by plasma, and it becomes possible to form a very high-quality insulation film on a fresh silicon surface.  
         [0020]     Other objects and further features of the present invention will become apparent from the detailed explanation of invention hereinafter when read in conjunction with the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0021]      FIG. 1  is a diagram showing the Qbd characteristic of a conventional thermal oxide film and a plasma oxide film;  
         [0022]      FIGS. 2A and 2B  are diagrams showing the construction of a plasma processing apparatus used with the present invention;  
         [0023]      FIGS. 3A and 3B  are diagrams showing the substrate processing according to a first embodiment of the present invention;  
         [0024]      FIG. 4  is a diagram showing the Qbd characteristic of a plasma oxide film obtained with the first embodiment of the present invention;  
         [0025]      FIG. 5  is a diagram showing the leakage current characteristic of the plasma oxide film obtained according to the first embodiment of the present invention;  
         [0026]      FIG. 6A and 6B  are diagrams showing the substrate processing according to a second embodiment of the present invention; and  
         [0027]      FIGS. 7A and 7B  are diagrams respectively showing the overall construction of the substrate processing system according to a third embodiment of the present invention including the substrate processing apparatus of  FIGS. 2A and 2B . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     First Embodiment  
       [0028]     The inventor of the present invention has acquired the knowledge, in an experimental investigation on the formation process of oxide films, nitride films and oxynitride films on a silicon substrate by microwave plasma processing, suggesting that organic substance remaining on the silicon substrate surface exerts a significant effect on the reliability of insulation film formed on the substrate.  
         [0029]      FIGS. 2A and 2B  schematically show the construction a microwave plasma substrate processing apparatus  10  used by the inventor of the present invention.  
         [0030]     Referring to  FIG. 2A , the plasma substrate processing apparatus  10  includes a processing vessel  11  in which a processing space  11 A is formed such that a stage  12  holding a substrate W to be processed thereon is formed in the processing space  11 A, wherein the processing vessel  11  is evacuated by an evacuation system  11 E at an evacuation port  11 C via a space  11 B surrounding the stage  12  and an adaptive pressure controller  11 D.  
         [0031]     The stage  12  is provided with a heater  12 A, wherein the heater  12 A is driven by a power source  12 C via a line  12 B.  
         [0032]     Further, the processing vessel  11  is provided with a substrate in/out opening  11   g  and a gate valve  11 G cooperating therewith for loading and unloading of the substrate W to be processed to and from the processing vessel  11 .  
         [0033]     On the processing vessel  11 , there is formed an opening in correspondence to the substrate W to be processed on the stage  12 , and the opening is closed by a top plate  13  of quartz or a low-loss dielectric such as alumina or AlN. Further, underneath the top plate  13 , there are formed a gas ring  14  formed with a gas inlet path and a large number of nozzle openings communicating therewith such that the gas ring  14  faces the substrate W to be processed.  
         [0034]     It should be noted that the cover plate  13  forms a microwave window, and a flat microwave antenna  15  of a radial line slot antenna is provided on the top part of the top plate  13 . In place of the radical line slot antenna, it is also possible to use a horn antenna.  
         [0035]     In the illustrated example, a radial line slot antenna is used for the flat microwave antenna  15 , wherein it should be noted that the antenna  15  includes a flat conductor part  15 A and a plane antenna plate  15 C, wherein the plane antenna plate  15 C is provided at the opening part of the flat conductor part  15 A via a retardation plate  15 B of quartz or alumina.  
         [0036]     The plane antenna plate  15 C is provided with a large number of slots  15   a  and  15   b  as will be explained with reference to  FIG. 1B , wherein the antenna  15  is connected to a coaxial waveguide  16  having an outer conductor  16 A connected to the conductor part  15 A of the antenna  15  and a central conductor  16 B connected to the plane antenna plate  15 C through the retardation plate  15 B. The coaxial waveguide  16  is connected to a rectangular waveguide  110 B via a mode conversion part  110 A, wherein the rectangular waveguide  110 B is connected to a microwave source  112  via an impedance matcher  111 . Thereby, the microwave source  112  supplies a microwave to the antenna  15  via the rectangular waveguide  110 B and the coaxial waveguide  16 .  
         [0037]     Further, a cooling unit  15 D is provided on the conductor part  15 A.  
         [0038]      FIG. 2B  shows the construction of the radial line slot antenna.  
         [0039]     Referring to  FIG. 2B  showing the radiation plate  15 C in a plan view, it can be seen that the slots  15   a  and  15   b  are formed in a concentric relationship in such a manner that a slot  15   a  and an adjacent slot  15   b  form an angle of 90 degrees.  
         [0040]     Thereby, the microwave supplied from the coaxial waveguide  16  spreads in the radial direction in the radial line slot antenna  15  with wavelength compression caused by the retardation plate  15 B. Thereby, the microwave is emitted from the slits  15   a  and  15   b  generally in the direction perpendicular to the plane of the radiation plate  15 C in the form of a circular polarized microwave.  
         [0041]     Further, as shown in  FIG. 2A , a rare gas source  101 A such as an Ar gas source and a hydrogen gas source  101 H are connected to the gas ring  14  via respective mass flow controllers  103 A and  103 H and via respective corresponding valves  104 A,  104 H,  105 A,  105 H and a common valve  106 . As noted before, the gas ring  14  is provided with a large number of gas inlet ports around the stage  12  uniformly, and the rare gas and the hydrogen gas supplied to the gas ring  14  are introduced into the processing space  14 A inside the processing vessel  11  uniformly. In addition, an oxygen gas source  1010  is connected to the gas ring  14  via a mass flow controller  1030  and valves  1040  and  1050  in the illustrated example for supplying oxygen to the processing vessel  11 .  
         [0042]     Further, although not illustrated, there may be provided other gas sources such as a nitrogen gas source, an ammonia gas source, a NO gas source, a N 2 O gas source, a H 2 O gas source, or the like.  
         [0043]     In operation, the processing space inside the processing vessel  11  is set to a predetermined pressure by evacuating through the evacuation port  11 C, and an oxidizing gas or a hydrogen gas is introduced from the gas ring  14  together with an inert gas such as Ar, Kr, Xe, Ne, Ne (rare gas) and the like.  
         [0044]     Further, a microwave having the frequency of several GHz such as 2.45 GHz is introduced from the microwave source  112  via the antenna  15 , and there is excited high-density microwave plasma in the processing vessel  11  at the surface of the substrate W to be processed with a plasma density of 10 11 -10 13 /cm 3 . By exciting the plasma by the microwave introduced via the antenna, the plasma has low electron temperature of 0.7-2 eV or less, preferable 1.5 eV or less, with the substrate processing apparatus of  FIG. 1A , and damaging of the substrate W or the inner wall of the processing vessel is avoided. Further, the radicals thus formed are caused to flow in the radial direction along the surface of the substrate W to be processed and are evacuated promptly. Thereby, recombination of the radicals is suppressed, and an extremely uniform and efficient substrate processing is realized at the low temperature of 550° C. or less.  
         [0045]      FIGS. 3A-3C  are diagrams showing the substrate processing conducted by the inventor of the present invention in the investigation constituting the foundation of the present invention and corresponding to a first embodiment of the present invention, while using substrate processing apparatus  10  of  FIG. 1 .  
         [0046]     Referring to  FIG. 3A , a silicon substrate  21 , from which the native oxide film is removed by a diluted HF solution (1% HF concentration, for example), is introduced to the processing vessel  11  of the substrate processing apparatus  10  as the substrate W to be processed, and a mixed gas of argon and hydrogen is introduced from the shower plate  14 . Further, plasma is formed by exciting the mixed gas by a microwave. Thereby, it is possible to form the plasma stably and with uniformity as a result of use of the Ar gas for the plasma gas.  
         [0047]     In an example, the process pressure inside the processing vessel  11  is set to 7 Pa, and an argon gas and a hydrogen gas are supplied with respective flow rates 1000 SCCM and 40 SCCM. Further, a microwave of 2.4 GHz in frequency is supplied to the microwave antenna  15  with the power of 1500 W at the substrate temperature of 400° C., and high density plasma is formed uniformly and stably in the vicinity of the surface of the substrate W to be processed.  
         [0048]     With the step of  FIG. 3A , an organic substance remaining on the substrate surface is removed effectively in the form of hydrocarbons as a result of exposing the surface of the silicon substrate  21  to the plasma thus formed, even at a low substrate temperature of 400° C., and a fresh silicon surface is exposed at the substrate surface.  
         [0049]     Next, in the step of  FIG. 3B , a silicon oxide film  22  is formed on the silicon substrate  21  thus applied with the processing of  FIG. 3A  with the thickness of 1-10 nm, by setting the processing pressure inside the processing vessel  11  to typically 7 Pa and supplying an argon gas and an oxygen gas with respective flow rates of 1000 SCCM and 40 SCCM, while setting the substrate temperature to 400° C. and by supplying the microwave of 2.4 GH frequency to the microwave antenna  15  with the power of 1500 W.  
         [0050]      FIG. 4  shows the relationship between the accumulated defect rate F and the breakdown electric charge amount Qbd for the silicon oxide film thus obtained in comparison with the result of  FIG. 1 . Further,  FIG. 4  also shows the result for the case in which the silicon substrate  21  is exposed to the argon plasma in the step of  FIG. 3A . In  FIG. 4 , the silicon oxide film is formed to the thickness of 10 nm.  
         [0051]     Referring to  FIG. 4 , it should be noted that the process in which the pre-processing of  FIG. 3A  is omitted, and thus, the silicon oxide film  22  is formed on the silicon substrate  21  directly with the thickness of 10 nm, corresponds to the plasma oxide film explained previously with reference to  FIG. 1 , wherein it will be noted that there appears a large variation of breakdown electric charge amount Qbd as explained already with reference to  FIG. 1 .  
         [0052]     Contrary to this, in the case the argon plasma processing is conducted in the pre-processing step of  FIG. 3A , it can be seen that variation of the breakdown electric charge amount Qbd is decreased. Particularly, in the case the pre pre-processing is conducted in the mixed gas plasma of argon and hydrogen as shown in  FIG. 3A , the variation of the breakdown electric charge mount Qbd is decreased further, and a result comparable to the case of a thermal oxide film is attained. Thus, by carrying out the pre-processing process of  FIG. 3A  in the mixture gas plasma of argon and hydrogen, it can be seen that a plasma oxide film having the reliability comparable to that of a thermal oxide film is obtained.  
         [0053]     Moreover, as can be seen from  FIG. 4 , the absolute value of the breakdown electric charge amount Qbd of the plasma oxide film of the present embodiment is increased further as compared with the case of thermal oxide film, indicating that the lifetime of the obtained plasma oxide film is increased.  
         [0054]     The fact shown in  FIGS. 1 and 4  that the variation of the breakdown electric charge amount Qbd is small in the thermal oxide film formed in the oxidizing ambient at high temperatures and that the breakdown electric charge amount Qbd is large in the plasma oxide film formed at the low temperature of about 400° C., suggests that this phenomenon is related to organic substances remaining on the surface of the silicon substrate  21 . In the present embodiment, it is thought that, as a result of the processing of the surface of the silicon substrate  21  in the mixed gas plasma of argon and hydrogen in the step of  FIG. 3A , the organic substance remaining on the silicon substrate surface is removed therefrom in the form of hydrocarbons, and a fresh silicon surface is exposed at the silicon substrate at the commencement of the step of  FIG. 3B .  
         [0055]      FIG. 5  shows the leakage current characteristics of the silicon oxide film  22  thus formed with the film thickness of 10 nm, wherein the measurement of  FIG. 5  is conducted under the condition of applying a voltage of 12V, and thus, the values are different from the case explained previously in which the measurement was made by applying a voltage of 1V.  
         [0056]     Referring to  FIG. 5 , with the plasma oxide film shown in  FIG. 1  in which the pre-processing step of  FIG. 3A  is omitted, a leakage current density comparable with that of a conventional thermal oxide film is obtained, while in the case in which the plasma pre-processing by argon gas is conducted in the step of  FIG. 3A , there is caused a decrease of leakage current, particularly in the case the pre-processing process of  FIG. 3A  is conducted in the mixed gas plasma of argon and hydrogen.  
         [0057]     Further, while formation of silicon oxide film has been made in the present embodiment on the surface of the silicon substrate  21  in the step of  FIG. 3B  by the mixed gas plasma of argon and hydrogen, it is also possible to form a silicon nitride film  23  by using argon and nitrogen, or argon and ammonia, or argon and a mixed gas of nitrogen and hydrogen. Further, it is also possible to form a silicon oxynitride film  24  by using argon and nitrogen and oxygen, or argon and ammonia and oxygen, or argon and a mixed gas of nitrogen and hydrogen and oxygen.  
         [0058]     Further, it is also possible to use an inert gas of other rare gas such as helium, krypton and xenon, in place of argon with the present embodiment.  
         [0059]     Further, it is possible with the present embodiment to use other oxidizing gas or nitriding gas such as NO, N 2 O, H 2 O, or the like in the present invention, in place of the oxygen gas, nitrogen gas and ammonia gas.  
       Second Embodiment  
       [0060]      FIGS. 6A and 6B  show the substrate processing method according to a second embodiment of the present invention.  
         [0061]     Referring to  FIG. 6A , there is formed a silicon oxide film  22  on a silicon substrate  21  by the process of  FIGS. 3A and 3B  explained before or by other process, wherein the surface of the silicon oxide film  22  is processed by the mixture gas plasma of argon and hydrogen under the condition similar to the process of  FIG. 3A , and the organic substance remaining on the surface of the silicon oxide film  22  is removed.  
         [0062]     Next, in the step of  FIG. 6B , there is caused a growth of the oxide film on the silicon oxide film  22  thus processed by applying the mixed gas plasma of argon and oxygen under the similar condition as  FIG. 3B , and with this, there is formed an oxide film  25 .  
         [0063]     It should be noted that the oxide film  25  thus formed has excellent reliability and leakage current density similarly to the plasma oxide film that explained with the previous embodiment.  
         [0064]     Further, in the process of  FIG. 6B , it is possible to form a silicon oxynitride film  26  by nitriding the silicon oxide film  22  by using argon and nitrogen, or argon and ammonia, or the mixed gas plasma of argon and nitrogen and hydrogen.  
         [0065]     Further, while explanation has been made for the present embodiment for the case of using the microwave plasma substrate processing apparatus of  FIGS. 2A and 2B  that uses the radial line slot antenna  15 , it is also possible to omit the shower plate  14  in the construction of  FIG. 2A  and introduce the gases from the gas inlet part  14 A directly into the processing vessel  11 . Further, the present invention is not limited to such a particular substrate processing apparatus, but is effective also in a parallel plate plasma processing apparatus, an ICP plasma processing apparatuses, an ECR plasma processing apparatus, and the like.  
       Third Embodiment  
       [0066]      FIGS. 7A  shows the construction of an overall substrate processing system  100  that includes the substrate processing apparatus  10  of  FIGS. 2A and 2B  and used for the processing of the present invention of  FIGS. 3A and 3B  or  FIGS. 6A and 6B , while  FIG. 7B  shows a computer used for controlling the substrate processing apparatus  10  of  FIGS. 2A and 2B  in the system of  FIG. 8A .  
         [0067]     Referring to  FIG. 7A , the system  100  includes the Ar gas source  101 A, the hydrogen gas source  101 H and the oxygen gas source  1010 , wherein the Ar gas source  101 A supplies an Ar gas to the gas ring  14  of the substrate processing apparatus  10  via the mass flow controller  103 A and via the valves  104 A and  105 A and further via the valve  106 , while the hydrogen gas source  101 H supplies a hydrogen gas to the gas ring  14  via the mass flow controller  103 H and via the valves.  104 H and  105 H and further via the valve  106  coupled to the gas ring  14  commonly to the gas supply path of the Ar gas and the gas supply path of the hydrogen gas. Further, the oxygen gas source  1010  supplies an oxygen gas to the gas ring of the substrate processing apparatus  10  via the mass flow controller  1030  and the valves  1040 ,  1050  and the valve  106 .  
         [0068]     Further, the system  100  includes the microwave power source  112  that supplies the microwave power to the radial line slot antenna  15  via an impedance matcher  111 .  
         [0069]     Further, the heating mechanism  12 A is provided in the stage  12  for temperature control of the substrate W to be processed.  
         [0070]     Further, the system  100  includes the evacuation system  11 E coupled to the evacuation port  11 C via the adaptive pressure controller  11 D.  
         [0071]     Further, the system  100  includes the gate valve  11 G cooperating with the substrate in/out opening  11   g  provided on the processing vessel  11  for loading and unloading the substrate W to be processed to and from the processing vessel  11 .  
         [0072]     Further, it should be noted that there is provided a system controller  100 C that controls the mass flow controllers  103 A,  103 B, and  1030 , valves  104 A,  104 H,  1040 ,  105 A,  105 H,  1050  and  106 , the heating mechanism  12 H, an evacuation pump not illustrated, and further the gate valve  11 G according to the program held therein, and the substrate processing apparatus  10  performs the foregoing hydrogen radical processing or oxidation processing under control of the controller  100 C.  
         [0073]      FIG. 7B  shows the construction of the controller  100 C.  
         [0074]     Referring to  FIG. 7B , the controller  100 C is a general purpose computer and includes a CPU  1001 , a memory  1002  holding a program and data, an interface unit  1003  connected to the system  100 , and an I/O interface  1005  connected with each other by a system bus  1004 , wherein the computer  100 C is provided with the control program of the substrate processing system  100  from a recording medium  1006  such as an optical disk or a floppy disk or from a network  1007  and controls the substrate processing system  100  of  FIG. 7A  including the substrate processing apparatus  10  via the interface unit  1003 .  
         [0075]     Thus, the present invention also includes such a computer configured by the program code means recorded on a processor-readable medium and also the processor readable medium that carries such a program code.  
         [0076]     Further, while the present invention has been explained heretofore with regard to preferred embodiments, the present invention is not limited to such a particular embodiment but various variations and modifications may be made within the subject matter recited in claims.