Patent Publication Number: US-2009233430-A1

Title: Semiconductor device manufacturing method, semiconductor device manufacturing apparatus, and semiconductor device manufacturing system

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 of Japanese Patent Application Nos. 2008-037821, filed on Feb. 19, 2008, 2008-284646, filed on Nov. 5, 2008, and 2009-033493, filed on Feb. 17, 2009, in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device manufacturing method, a substrate processing apparatus, and a substrate processing system for manufacturing a semiconductor device and forming a high-dielectric gate insulating film (hereinafter, referred to as a high-k gate insulating film) on a substrate, and more particularly, to improved technology for nitrogenizing a high-k gate insulating film. 
     2. Description of the Prior Art 
     With the development of highly-integrated, high speed semiconductor devices in theses days, there has been advance in miniaturization of metal oxide semiconductor field effect transistors (MOSFETs). However, in the case where the thickness of a gate insulating film becomes thinner as a result of the miniaturization of MOSFETs, a large gate leakage current flows through the gate insulating film hitherto made of an silicon oxide (SiO 2 ) due to the tunneling effect or other reasons. Particularly, unlike a capacitor insulating film, the gate insulating film required to be thin does not function as an insulating film if a large leakage current flows through the gate insulating film, and thus it is necessary to suppress the leakage current of the gate insulating film. For these reasons, as a method of suppressing gate leakage current, the physical thickness of the gate insulating film is increased while maintaining the equivalent oxide thickness of the gate insulating film at a low level by forming the gate insulating film using a high-k material such as hafnium oxide (HfO 2 ) or hafnium silicate (HfSiO x ). 
     However, a high-k film such as a HfO 2  or HfSiO x  film is crystallized by a heat treatment at about 700° C. If such a high-k film made of HfO 2  or HfSiO x  is directly used as a gate insulating film after a film forming process, since the gate insulating film is heated to 1000° C. or more in a later annealing process, the crystal structure of HfO 2  or HfSiO x  may be changed from amorphous to polycrystalline. Since crystal grain boundaries always exist in the polycrystalline structure, leakage current increases along defects of the crystal grain boundaries when a voltage is applied to a gate electrode. 
     Therefore, in a method proposed to prevent crystallization, a high-k gate insulating film is nitrogenized. If a high-k gate insulating film is nitrogenized, the degree of bond between the high-k gate insulating film and nitrogen is increased so that crystallization of the high-k gate insulating film can be suppressed. In the past, a thermal nitrogenizing method using nitrogen-containing gas such as ammonia gas (NH 3 ) was commonly used to introduce nitrogen into a gate insulating film; in recent days, however, a plasma nitrogenizing method using nitrogen gas has been favored (For example, refer to Patent Document 1). 
     In the plasma nitrogenizing method, elemental nitrogen gas (N 2 ) is activated through a plasma process to obtain an activated nitrogen species, and a bias voltage is applied to a silicon substrate to introduce the activated nitrogen species into a high-k gate insulating film formed on the silicon substrate. Unlike the plasma nitrogenizing method, the thermal nitrogenizing method is performed at a high temperature close to 1000° C. If a high-k gate insulating film is processed at a high temperature. crystallization occurs in the high-k gate insulating film. As described above, crystallization increases current leakage. 
     However, the plasma nitrogenizing method is performed at a lower temperature than the thermal nitrogenizing method. Therefore, a high-k gate insulating film is less crystallized when it is nitrogenized by plasma than when it is crystallized by heat. 
     Therefore, current leakage can be suppressed more effectively when a high-k gate insulating film is nitrogenized by plasma than when the high-k gate insulating film is nitrogenized by heat. 
     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-57163 
     However, the plasma nitrogenizing method has the following disadvantages. 
     The amount of nitrogen escaping from a gate insulating film should be small as compared with the amount of nitrogen escaping from a capacitor. 
     It is not necessary to strictly control the amount of nitrogen escaping from a device such as a capacity; however, the amount of nitrogen escaping from a gate insulating film should be strictly controlled for suppressing current leakage or increasing electric permittivity. 
     However, in a plasma nitrogenizing process using only nitrogen gas, a large portion of nitrogen, once introduced into a high-k gate insulating film, escapes from the high-k gate insulating film. Therefore, it is difficult to introduce a sufficient amount of nitrogen into the high-k gate insulating film for suppressing crystallization of the high-k gate insulating film. 
     In addition, since a silicon substrate is heated in a post nitridation anneal (post nitrogenization anneal, PNA) process after the plasma nitrogenizing process, a large amount of nitrogen escapes from the high-k gate insulating film during the PNA process. Therefore, in the case where a high-k gate insulating film is plasma-nitrogenized by only using nitrogen gas, it is more difficult to keep a sufficient amount of nitrogen in the high-k gate insulating film for suppressing crystallization of the high-k gate insulating film. As a result, it is difficult to suppress a large gate leakage current flowing to a gate electrode through the high-k gate insulating film. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a semiconductor device manufacturing method, a substrate processing apparatus, and a substrate processing system for forming a high-k gate insulating film adapted to suppress nitrogen leakage and gate leakage current. 
     According to an aspect of the present invention, there is provided a method of manufacturing a semiconductor device, the method comprising: forming a high-k gate insulating film on a silicon substrate in a first process unit; carrying the silicon substrate to a second process unit; nitrogenizing the high-k gate insulating film using gas comprising nitrogen gas and rare gas; and annealing the silicon substrate in the second process unit. 
     According to another aspect of the present invention, there is provided a system for manufacturing a semiconductor device, the system comprising: a first processing unit configured to form a high-k gate insulating film on a silicon substrate; a second processing unit configured to nitrogenize the high-k gate insulating film using gas comprising nitrogen gas and rare gas and anneal the silicon substrate; a third processing unit configured to form an electrode on the high-k gate insulating film; a fourth processing unit configured to form an insulating layer enclosing the electrode; and a substrate carrying unit configured to carry the substrate among the processing units. 
     According to another aspect of the present invention, there is provided a semiconductor device manufacturing apparatus for nitrogenizing a high-k gate insulating film formed on a silicon substrate, the semiconductor device manufacturing apparatus comprising: a reaction gas introduction unit configured to introduce reaction gas comprising nitrogen gas and rare gas into a substrate process chamber; a plasma generating unit configured to ionize the reaction gas into plasma; a substrate stage installed inside the process chamber for placing a substrate on the substrate stage; a substrate heating unit configured to heat a substrate; and a control unit configured to control an operation of introducing reaction gas comprising nitrogen gas and rare gas after a substrate comprising a high-k gate insulating film is carried into the process chamber, an operation of ionizing the introduced reaction gas into plasma for nitrogenizing the high-k gate insulating film of the substrate, and an operation of stopping introduction of the rare gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a modified magnetron typed (MMT) apparatus as a substrate processing apparatus relevant to a first embodiment of the present invention. 
         FIG. 2  is a view for explaining a method of manufacturing a semiconductor device relevant to the first embodiment of the present invention. 
         FIG. 3  is a nitrogen concentration table, in which nitrogen concentrations in a HfSiO x  film after a plasma nitrogenizing process at 400° C. and a PNA process are compared for the cases of using different reaction gases. 
         FIG. 4  is a nitrogen concentration table, in which nitrogen concentrations in a HfSiO x  film after a plasma nitrogenizing process at 200° C. and a PNA process are compared. 
         FIG. 5  is a schematic view illustrating an inductively coupled plasma (ICP) processing apparatus as a substrate processing apparatus relevant to a second embodiment of the present invention. 
         FIG. 6  is a schematic view illustrating an electron cyclotron resonance (ECR) plasma processing apparatus as a substrate processing apparatus relevant to a third embodiment of the present invention. 
         FIG. 7  is a schematic view illustrating a cluster apparatus according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Explanation will now be given on an exemplary method of manufacturing a semiconductor device using a high-k gate insulating film. 
     According to the present invention, a high-k gate insulating film is plasma-nitrogenized using a mixture of nitrogen gas and rare gas. Therefore, as compared with a plasma nitrogenizing process using only nitrogen gas, the degree of bond between nitrogen and the high-k gate insulating film can be increased, and thus escaping of nitrogen from the high-k gate insulating film can be reduced. As a result, crystallization of the high-k gate insulating film can be suppressed although the high-k gate insulating film is treated at a high temperature, and thus gate current leakage can be suppressed. 
     An embodiment of the present invention will be described hereinafter with reference to  FIG. 2 . 
     As shown in  FIG. 2 , a semiconductor device to be treated is a metal oxide semiconductor field effect transistor (MOSFET). The MOSFET includes a source  101  and a drain  102  formed in a silicon substrate (silicon wafer)  200 , a high-k gate insulating film  30  formed between the source  101  and the drain  102 , and a polysilicon electrode  32  formed on the high-k gate insulating film  30 . 
     In the current embodiment, formation of the high-k gate insulating film  30 , nitrogenization of the high-k gate insulating film  30 , annealing of the nitrogenized high-k gate insulating film  30 , and formation of the polysilicon electrode  32  after the annealing are performed using a semiconductor device manufacturing system such as a cluster apparatus shown in  FIG. 7 . The cluster apparatus of  FIG. 7  includes carrier stations  11   a ,  11   b , and  11   c  (load ports), an atmospheric transport unit  12 , a substrate position correction unit  13 , a loadlock chamber  14 , a carrying chamber  15  at which a vacuum transfer unit  21  (substrate carrying unit) is installed, a first process unit  16 , a second process unit  17 , a third process unit  18 , and a fourth process unit  19 . The process units  16  to  19  are controlled by a controller  22 . The process units  16  to  19  will be described later. 
     Hereinafter, a detailed explanation will be given on the flow of processes performed using the cluster apparatus. In the following explanation, operations of parts of the cluster apparatus are controlled by the controller  22  (control unit). 
     To form a high-k gate insulating film  30  on the surface of a wafer  200 , the wafer  200  is loaded into the first process unit  16  used as a high-k film forming apparatus by the substrate carrying unit  21 . In the high-k film forming apparatus  16 , according to a predetermined film forming method such as poly-atomic layer chemical vapor deposition (PLCVD) method, remote plasma oxygen or ozone (O 3 ), and an organic metallic material containing hafnium (Hf) or silicon (Si) are introduced to the surface of the wafer  200  to form a metal oxide film such as a HfO 2  or HfSiO x  film as the high-k gate insulating film  30  (step  301 ). 
     After the high-k gate insulating film  30  is formed, the wafer  200  is loaded into the second process unit  17  (MMT apparatus) by the substrate carrying unit  21 . The MMT apparatus  17  will be described later in detail. In the MMT apparatus  17 , the high-k gate insulating film  30  is nitrogenized (step  302 ). 
     After the nitrogenization, the wafer  200  is loaded into the third process unit  18  (rapid thermal processing (RTP) apparatus) by the substrate carrying unit  21 . In the RTP apparatus  18 , the nitrogenized high-k gate insulating film  30  of the wafer  200  is annealed. Through the annealing, the bond strength between the high-k gate insulating film  30  and nitrogen can be increased, and thus current leakage can be prevented more efficiently (step  303 ). 
     After the annealing, the wafer  200  is carried to the fourth process unit  19  (electrode forming apparatus) by the substrate carrying unit  21 . A gate electrode  32  (polysilicon electrode) is formed on the nitrogenized and annealed high-k gate insulating film  30 . 
     After the polysilicon electrode  32  is formed, the wafer  200  is carried from the cluster apparatus to an insulating layer forming apparatus. In the insulating layer forming apparatus, an insulating layer  34  is formed using a material such as silicon nitride (Si 3 N 4 ) to cover the polysilicon electrode  32 . The insulating layer  34  may be formed by a chemical vapor deposition (CVD) method using SiH 2 Cl 2  gas and NH 3  gas. 
     After the insulating layer  34 , the wafer  200  is loaded into an ion implanting apparatus, and a source  101  and a drain  102  (impurity regions) are formed using a method such as an ion implanting method by adding an n-type impurity to a main surface of the wafer  200 . Between the source  101  and the drain  102 , a channel  103  is formed. 
     In this way, the gate part of a semiconductor device is formed. 
     In the system or between the apparatuses, a substrate is carried using one or more carrying devices. 
     Next, a modified magnetron typed (MMT) apparatus will be described as a substrate processing apparatus (nitrogenizing apparatus) for nitrogenizing a high-k gate insulating film according to the present invention. 
     In an embodiment of the present invention, a plasma processing furnace is a substrate processing furnace configured to process a substrate such as a wafer using an MMT plasma source, which is provided for generating high-density plasma using electric and magnetic fields (hereinafter, the plasma processing furnace will be referred as an MMT apparatus). In the MMT apparatus, a substrate is loaded in an air-tightly sealed process chamber; reaction gas is introduced into the process chamber through a shower head; and while maintaining the process chamber at a predetermined pressure, high-frequency power is supplied to a discharge electrode to generate an electric field and a magnetic field at the same time for magnetron discharging. Electrons emitted from the discharge electrode revolve while continuing cycloidal drifting so that the electrodes exist for a long time. Therefore, the rate of ionization can be increased, and thus high-density plasma can be generated. In this way, reaction gas can be activated or decomposed to perform various plasma processes on a substrate, such as a diffusion process (e.g., diffusion oxidization and diffusion nitrogenizing) a thin film forming process, and an etching process. 
       FIG. 1  is a schematic view illustrating such an MMT apparatus as a substrate processing apparatus relevant to a first embodiment of the present invention. 
     The MMT apparatus includes a process vessel  203 , and the process vessel  203  includes a dome-shaped upper vessel  210  as a first vessel and a bowl-shaped lower vessel  211  as a second vessel. The upper vessel  210  covers the upper side of the lower vessel  211 . The upper vessel  210  is made of an aluminum oxide or a nonmetallic material such as quartz, and the lower vessel  211  is made of quartz. In addition, a susceptor  217  (described later) used as a substrate holder (substrate holding unit) integrated with a heater  217   b  is made of an aluminum nitride, or a nonmetallic material such as a ceramic material or quartz, so as to prevent contamination of a film during a process. 
     A shower head  236  is installed at an upper side of a process chamber  201 . The shower head  236  includes a cap-shaped cover  233 , a gas inlet  234 , a buffer chamber  237 , an opening  238 , a shield plate  240 , and a gas blow outlet  239 . The buffer chamber  237  provides a space for dispersing gas introduced through the gas inlet  234 . 
     A gas introduction pipe  232  is connected to the gas inlet  234 , and a gas cylinder (not shown) in which a reaction gas  230  is stored is connected to the gas introduction pipe  232  through an on-off valve  243   a  and a mass flow controller  241  used as a flow rate controller. 
     A reaction gas  230  is introduced to the process chamber  201  through the shower head  236  (reaction gas introduction unit),. and a gas exhaust outlet  235  is installed at a sidewall of the lower vessel  211  to allow the reaction gas  230  to flow from the vicinity of the susceptor  217  to the bottom side of the process chamber  201  after the reaction gas  230  is used for processing a substrate. A gas exhaust pipe  231  is connected to the gas exhaust outlet  235  for exhausting gas, and a vacuum pump  246  (exhaust device) is connected to the gas exhaust pipe  231  through an automatic pressure control (APC) valve  242  (pressure regulator) and an on-off valve  243   b.    
     As a discharging structure (discharging unit or plasma generating unit) for exciting a reaction gas  230  into plasma, a tubular electrode  215  (first electrode) having a tubular shape such as a cylindrical shape is installed. The tubular electrode  215  is installed around the circumference of the process vessel  203  (the upper vessel  210 ) to surround a plasma generating region  224  defined inside the process chamber  201 . A high-frequency power supply  273  is connected to the tubular electrode  215  through a matching device  272  used for impedance matching. 
     In addition, as magnetic field forming structures (magnetic field forming units), magnets  216  having a tubular shape such as a cylindrical shape and configured by permanent magnets are provided. The tubular magnets  216  are disposed close to upper and lower ends of the outer surface of the tubular electrode  215 . The upper and lower tubular magnets  216  have magnetic poles at both ends (inner and outer circumferential ends) along a radial direction of the process chamber  201  with opposite poles of the upper and lower tubular magnets  216  being aligned with each other. That is, inner-circumferential magnetic poles having opposite polarities are aligned in a manner such that magnetic field lines exist along the inner circumference of the tubular electrode  215  in the center-axis direction of the tubular electrode  215 . 
     At the lower center side of the process chamber  201 , the susceptor  217  is installed as a substrate holder (substrate stage) for holding a substrate such as a wafer  200 . The susceptor  217  is made of an aluminum nitride or a nonmetallic material such as a ceramic material or quartz, and the heater  217   b  is integrally buried in the susceptor  217  as a heating structure (substrate heating unit) for heating a wafer  200 . The heater  217   b  is configured to heat a wafer  200  up to about 500° C. when power is supplied to the heater  217   b . In the current embodiment, the heating unit has a structure buried in the susceptor  217 . However, the present invention is not limited thereto. For example, a heater may be installed outside the process chamber  201  for heating a wafer. In another example, a heater may be installed inside the process chamber  201  away from the susceptor  217 . 
     In addition, if the substrate processing apparatus is used as an RTP apparatus, the heater  217   b  is set to heat a wafer to about 1050° C. Alternatively, the heater  217   b  may be set to heat a wafer to about 500° C., and an additional RTP heater may be used; in this case, the heater  217   b  installed in the susceptor  217  and the RTP heater may be used together to heat the wafer. For example, in a wafer nitrogenizing process the heater  217   b  may heat a wafer to about 500° C., and then the heater  217   b  and the RTP heater may be used together to heat the wafer to about 1050° C. for annealing the wafer. 
     Inside the susceptor  217 , a second electrode (not shown) is equipped to adjust impedance again. The second electrode is grounded via an impedance varying device  274 . The impedance varying device  274  is configured by a coil or a variable condenser so that the potential of a wafer  200  can be controlled via the second electrode and the susceptor  217  by varying the number of patterns of the coil or the capacitance of the variable condenser. 
     A process furnace  202  configured to process a wafer  200  by magnetron-discharging of a magnetron type plasma source is constituted by at least the process chamber  201 , the process vessel  203 , the susceptor  217 , the tubular electrode  215 , the tubular magnets  216 , the shower head  236 , and the gas exhaust outlet  235 . In the process chamber  201 , a wafer  200  can be treated using plasma. 
     Around the tubular electrode  215  and the tubular magnets  216 , a shield plate  223  is installed to prevent electric and magnetic fields formed by the tubular electrode  215  and the tubular magnets  216  from affecting outside environments or other apparatuses such as other furnaces. 
     The susceptor  217  is insulated from the lower vessel  211 , and a susceptor elevating mechanism (elevating unit)  268  is installed to move the susceptor  217  upward and downward. Penetration holes  217   a  are formed through the susceptor  217 , and wafer lifting pins  266  are installed at at least three positions of the bottom surface of the lower vessel  211  for lifting a wafer  200 . The penetration holes  217   a  and the wafer lifting pins  266  are arranged in a manner such that the wafer lifting pins  266  can pass through the penetration holes  217   a  without making contact with the susceptor  217  when the susceptor  217  is moved downward by the susceptor elevating mechanism  268 . 
     At the sidewall of the lower vessel  211 , a gate valve  244  is installed. The gate valve  244  can be opened to load/unload a wafer  200  into/from the process chamber  201  by using the substrate carrying unit  21 , and the gate valve  244  can be closed to make the process chamber  201  airtight. 
     A controller  121  is used as a control part (control unit) for controlling the APC valve  242 , the on-off valve  243   a , the vacuum pump  246  via a signal line A; the susceptor elevating mechanism  268  via a signal line B; the gate valve  244  via a signal line C; the matching device  272  and the high-frequency power supply  273  via a signal line D; the mass flow controller  241  and the on-off valve  243   a  via a signal line E; and the heater  217   b  buried in the susceptor  217  and the impedance varying device  274  via a signal line (not shown). 
     Next, an explanation will be given on a method of nitrogenizing a high-k gate insulating film using the MMT apparatus  17 . In the following explanation, parts of the MMT apparatus  17  are controlled by the controller  121 . 
     After a high-k gate insulating film  30  is formed on a wafer  200 , the wafer  200  is carried from the high-k film forming apparatus  16  to the MMT apparatus illustrated  FIG. 1  by the substrate carrying unit  21 . 
     By the substrate carrying unit  21 , the wafer  200  is carried from the outside of the process chamber  201  constituting the process furnace  202  into the process chamber  201  and is loaded on the susceptor  217 . The wafer  200  loaded on the susceptor  217  is a silicon wafer on which a high-k gate insulating film such as a HfO 2  or HfSiO x  film is formed. In some cases, a HfSiO x  film may be layered between the surface of a silicon wafer and an HfO 2  film. 
     In detail, the wafer  200  is carried as follows. When the susceptor  217  is moved down to a wafer receiving position, the wafer lifting pins  266  passes through the penetration holes  217   a  of the susceptor  217 . At this time, the wafer lifting pins  266  protrude from the surface of the susceptor  217  by a predetermined length. Next, the gate valve  244  installed at the lower vessel  211  is opened, and the carrying unit  21  places the wafer  200  on ends of the wafer lifting pins  266 . Then, after the carrying unit  21  is moved to the outside of the process chamber  201 , the gate valve  244  is closed. Thereafter, the susceptor  217  is moved upward by the susceptor elevating mechanism  268  so that the wafer  200  can be placed on the surface of the susceptor  217  and moved upward to a processing position. 
     The heater  217   b  buried in the susceptor  217  is previously operated, and the wafer  200  is heated by the heater  217   b  to a predetermined wafer processing temperature in the range from 200° C. to 500° C. 
     At this time the heater  217   b  is controlled such that the wafer  200  can be kept at a temperature equal to or higher than 200° C. but lower than 500° C. 
     By performing a nitrogenizing process using plasma at a temperature equal to or higher than 200° C. but lower than 500° C,, the bond strength between the high-k gate insulating film  30  and nitrogen can be increased. Therefore, less nitrogen escapes from the high-k gate insulating film  30  in a post nitridation anneal (post nitrogenization anneal, PNA) process, and thus crystallization and current leakage can be suppressed more surely. 
     The temperature of the nitrogenizing process is kept lower than 500° C. If the temperature is equal to or higher than 500° C., the mobility of an MOSFET is lowered due to thermal diffusion of nitrogen to interfacial silicon (Si). In addition, gate leak current increases due to crystallization at a high temperature. Therefore, it is preferable that the temperature of the nitrogenizing process be kept equal to or higher than 200° C. but lower than 500° C. 
     In addition, it is more preferable that the temperature of a wafer be kept equal to or higher than 300° C. but lower than 500° C., particularly, at about 400° C. If the temperature of the wafer is kept equal to or higher than 300° C., the amount of escaping nitrogen can be significantly reduced as compared with the case where the temperature of the wafer is kept equal to or higher than 200° C. but lower than 300° C. 
     After the wafer  200  reaches the predetermined processing temperature and is stably maintained at the predetermined processing temperature, a previously prepared mixture of nitrogen gas and rare gas is introduced from the gas inlet  234  to the top surface (processing surface) of the wafer  200  disposed in the process chamber  201  through the gas blow outlet  239  of the shield plate  240 . At this time, the gas mixture is introduced at a predetermined flow rate. The rare gas may be argon (Ar) or helium (He). When the rare gas is added to nitrogen gas, the amount of nitrogen gas may be increased according to the amount of the added rare gas. 
     The process chamber  201  is maintained at a predetermined pressure in the range from 1 Pa to  10  Pa by using the vacuum pump  246  and the APC valve  242 . 
     While maintaining the process chamber  201  at the predetermined pressure, high-frequency power is applied to the tubular electrode  215  from the high-frequency power supply  273  through the matching device  272 . The high-frequency power applied to the tubular electrode  215  has a predetermined level in the range from 300 W to 700 W. At this time, a desired impedance valve can be obtained by previously adjusting the impedance varying device  274 . By this bias adjustment, the energy of plasma incident on the wafer  200  can be controlled. 
     Owing to a magnetic field formed by the tubular magnets  216 ,  216 , magnetron discharging occurs, and electric charge is trapped above the wafer  200 , so that the mixture of nitrogen gas and rare gas can be ionized into high-density plasma. The high-density plasma produces an activated nitrogen species. The high-k gate insulating film  30  formed on the wafer  200  is plasma-nitrogenized using the activated nitrogen species to introduce nitrogen into the high-k gate insulating film  30  (refer to step  302  of  FIG. 2 ). 
     The gas mixture is introduced at a nitrogen gas: rare gas flow rate ratio of 1:9 to 1:19. In this condition, high-concentration nitrogenization is possible. The activated nitrogen species produced from the high-density plasma can be controlled using the impedance varying device  274  capable of adjusting the energy of ions entering into the wafer  200  placed on the susceptor  217 , so as to control the depth profile of nitrogen introduced into the high-k gate insulating film  30 . 
     After the plasma nitrogenizing process, introduction of rare gas is interrupted, and nearly at the same time, magnetron discharging stops. Then, the inside atmosphere of the process chamber  201  is only nitrogen (N 2 ) gas. 
     The wafer  200  treated by plasma nitrogenizing is carried out from the process chamber  201  using the carrying unit  21  in the reverse order to the carrying-in order. 
     The wafer  200  carried out from the process chamber  201  after the plasma nitrogenizing process is carried to an annealing apparatus. For example, the annealing apparatus may be an RTP apparatus. The RTP apparatus is a rapid heating device using, for example, a lamp. While the temperature of the wafer  200  is feedback controlled using temperature information measured at a side of the wafer  200  by a temperature probe, the wafer  200  is heated using a lamp disposed at the other side of the wafer  200 . Power to the lamp is controlled so that each surface zone of the wafer  200  can be uniformly heated for uniform temperature distribution, and moreover the wafer  200  is rotated by a rotary mechanism while the wafer  200  is heated for further improving the temperature distribution uniformity. 
     In the RTP apparatus, a PNA process is performed on the wafer  200  to enhance the degree of bond between the high-k gate insulating film  30  and nitrogen. 
     Instead of carrying the wafer  200  from the MMT apparatus to another process chamber after the nitrogenizing process, the wafer  200  may be processed by PNA in the MMT apparatus. 
     In this case, introduction of rare gas used for the nitrogenizing process is first stopped. At approximately the same time, magnetron discharging is stopped. Introduction of nitrogen is continued from the nitrogenizing process. Therefore, the inside of the process chamber  201  can be filled with only N 2 . In this nitrogen atmosphere, the wafer  200  placed in the process chamber  201  is continuously heated by the heater  217   b.    
     Nitrogen introduced into the high-k gate insulating film  30  may be released from the high-k gate insulating film  30  with the lapse of time. However, as described above, in the case where the PNA process is performed on the wafer  200  in the same process chamber without moving the wafer  200  to another process chamber after the nitrogenizing process, more nitrogen couples to the high-k gate insulating film  30  and remains in the high-k gate insulating film  30 . 
     Particularly, in the case where the annealing process is performed in the same process chamber after the nitrogenizing process, time necessary for initializing the annealing process can be reduced, and thus the amount of nitrogen escaping from the high-k gate insulating film  30  can be efficiently reduced. 
     Furthermore, in this case, since the wafer  200  is not moved in the process chamber  201 , the processed wafer  200  can be less contaminated. 
     (Refer to step  303  of  FIG. 2 ) 
     After the annealing process, the wafer  200  is carried out from the process chamber by the carrying unit  21  in the reverse order to the carrying-in order. 
     Next, a specific example of the nitrogenizing process will be described. 
     A plasma nitrogenizing process was performed using the MMT apparatus illustrated in  FIG. 1 . 
     As shown in  FIG. 3  and  FIG. 4 , a plurality of gas mixtures was used. 
     Experiments were conducted while varying reaction gas introduced into the process chamber of the MMT apparatus from nitrogen gas (N 2 ), mixture gas of N 2  and Ar, to mixture gas of N 2  and He. HfSiO x  gate insulting films formed on silicon wafers were plasma-nitrogenized using the reaction gases, respectively. 
     The experiment with (N 2 +He) mixture gas was performed at a wafer temperature of 200° C. and at a wafer temperature of 400° C., and both the experiment with N 2  gas and the experiment with (N 2 +Ar) mixture gas were performed at a wafer temperature of 400° C. 
     Other experimental conditions of the process chamber are as follows: pressure is 1 pa to 10 pa, radio frequency (RF) power is 300 W to 700 W, N 2 :Ar flow rate ratio is 1:9 to 1:19, and N 2 :H2 flow rate ratio is 1:9 to 1:19. 
     After the plasma nitrogenizing processes, the wafers were not moved to another process chamber, and PNA processes were performed on the wafers. The PNA processes were performed in the process chamber under a nitrogen atmosphere at a wafer temperature of 1050° C. during 10 seconds. 
     Referring to  FIG. 3 , nitrogen concentrations in high-k gate insulating films were measured by X-ray photoelectron spectroscopy (XPS) after the high-k gate insulating films were treated by plasma nitrogenization at a wafer temperature of 400° C. using different reaction gases and after the PNA processes were performed on the high-k gate insulating films, so as to compare relative nitrogen atom amounts contained in the high-k gate insulating films. As shown in  FIG. 3 , when the nitrogen atom amounts are compared for different reaction gases, the nitrogen atom amounts increase in terms of reaction gases as follows: N 2 &lt;(N 2 +Ar)&lt;(N 2 +He). Particularly, when helium (He) is used as rare gas, the nitrogen gas amount is large. In the case where (N 2 +He) mixture gas is used for plasma nitrogenizing, the nitrogen concentration (XPS) in the high-k gate insulating film approaches 15% after a PNA process. That is, the nitrogen concentration approaches to a higher level than the cases where only N 2  gas or (N 2 +Ar) mixture gas are used for plasma nitrogenizing. 
     Referring to  FIG. 4 , nitrogen concentrations in the high-k gate insulating film were measured by XPS after the high-k gate insulating film was treated by plasma nitrogenizing at a wafer temperature of 200° C. using (N 2 +He) mixture as a reaction gas and after the PNA process was performed on the high-k gate insulating film, so as to compare relative nitrogen atom amounts contained in the high-k gate insulating film. As shown in  FIG. 4  and  FIG. 3 , if the nitrogenizing temperature is low in the case of using (N 2 +He) mixture gas as a reaction gas, nitrogen leakage is large during a PNA process. 
     The above-described embodiments provide a semiconductor device manufacturing method including a process of forming a high-k gate insulating film on a substrate and a process of performing a plasma nitrogenizing process on the high-k gate insulating film by using a mixture of nitrogen gas and rare gas. According to the embodiments, one or more of the following effects can be attained. 
     (1) Since a plasma nitrogenizing process is performed using a mixture of nitrogen gas and rare gas, the amount of nitrogen escaping from a high-k gate insulating film can be reduced as compared with the case of performing a simple nitrogenizing process. Therefore, increase of gate leakage current can be surely prevented. 
     (2) An activated nitrogen species produced from plasma is introduced into a high-k gate insulating film in a way that hydrogen (H) of Si-OH bonds of the high-k gate insulating film is replaced with the activated nitrogen species, so that the nitrogen concentration in the high-k gate insulating film can be increased. 
     (3) An activated nitrogen species produced from plasma of a nitrogen and rare gas mixture is used to introduce nitrogen into a high-k gate insulating film formed on a silicon wafer using HfO 2  or HfSO x . In this case, the energy of ionized rare gas having a higher ionization potential is transferred to nitrogen molecules (this is known as Penning effect), so that an activated nitrogen species can be produced more efficiently, and more nitrogen can be introduced into the high-k gate insulating film. In addition, since the high-k gate insulating film is treated using plasma, the bond between the high-k gate insulating film and nitrogen can be enhanced so that the amount of nitrogen leaking (escaping) from the high-k gate insulating film can be reduced in a later PNA process. Therefore, a sufficient amount of nitrogen can be ensured. 
     Accordingly, crystallization of the high-k gate insulating film can be sufficiently suppressed, and thus an increase of gate leakage current caused by crystal grain boundaries can be surely prevented. 
     (4) Nitrogen leakage can be largely reduced by using helium (He) as compared with the case of using other rare gas such as argon (Ar). 
     (5) Performing a plasma nitrogenizing process at a temperature of 300° C. to 500° C. is advantageous in that: the bond between a high-k gate insulating film and nitrogen can be enhanced; nitrogen leakage can be reduced in a later PNA process; crystallization of the high-k gate insulating film can be surely suppressed; and current leakage through the gate insulating film can be surely suppressed. If the plasma nitrogenizing process is performed at a temperature lower than 300° C., specific permittivity cannot be improved due to nitrogen leakage. Furthermore, gate leakage current through the high-k gate insulating film cannot be reduced, or heat resistance of the high-k gate insulating film cannot be improved in a later process. In addition, nitrogen leakage in a PNA process increases. Therefore, it is preferable that the plasma nitrogenizing process be performed at a temperature of 300° C. or higher. However, the plasma nitrogenizing process may not be performed at 500° C. or higher. If the plasma nitrogenizing process is performed at 500° C. or higher, thermal diffusion of nitrogen can reach an interfacial silicon (Si) to reduce MOSFET mobility. Furthermore, the high-k gate insulating film can be crystallized at such a high temperature, and thus gate leakage current can be increased. Therefore, it is preferable that the plasma nitrogenizing process be performed at 300° C. to 500° C. 
     (6) In addition, the degree of bond between a high-k gate insulating film and nitrogen can be increased after a PNA process is performed on the high-k gate insulating film. Therefore, gate leakage current can be suppressed more reliably. 
     (7) Furthermore, in the MMT apparatus, the energy of plasma incident on a wafer can be adjusted by bias controlling. Therefore, when a high-k gate insulating film made of a material such as HfO 2  and HfSiO x  is nitrogenized using plasma, the profile and amount of nitrogen introduced into the high-k gate insulating film can be easily controlled. This allows high-density nitrogenizing and nitrogen profile controlling, and thus crystallization of the high-k gate insulating film can be effectively suppressed by the plasma nitrogenizing process. Therefore, gate leakage current can be suppressed more surely. 
     In the above-described embodiments. an MMT apparatus is used for performing a plasma nitrogenizing process. However, the present invention is not limited to the MMT apparatus. For example, other apparatuses such as an inductively coupled plasma (ICP) apparatus and an electron cyclotron resonance (ECR) apparatus may be used. 
       FIG. 5  is a schematic view illustrating ICP processing apparatus as a substrate processing apparatus relevant to a second embodiment of the present invention. In the description of the current embodiment, elements having the same functions as the elements of the first embodiment will be denoted by the same reference numerals, and descriptions of the elements having the same functions will be omitted. 
     The ICP processing apparatus  10 A of the current embodiment includes an induction coil  15 A as a plasma generating unit for generating plasma using electricity, and the induction coil  15 A is installed at the outside of a ceiling wall of a process vessel  202 . In the current embodiment, a mixture of nitrogen gas and rare gas is introduced to the process vessel  202  from a gas introduction pipe  232  through a gas blow outlet  234 , like in the previous embodiment. Before or after the introduction of the gas mixture, high-frequency power is supplied to the induction coil  15 A used as a plasma generating unit to generate an electric field by electromagnetic induction. The introduced gas mixture is ionized into plasma by the energy of the electric field, and a high-k gate insulating film formed on a wafer  200  is nitrogenized by an activated nitrogen species produced from the plasma. 
       FIG. 6  is a schematic view illustrating an ECR plasma processing apparatus as a substrate processing apparatus relevant to a third embodiment of the present invention. In the description of the current embodiment, elements having the same functions as the elements of the previous embodiments will be denoted by the same reference numerals, and descriptions of the elements having the same functions will be omitted. 
     The ECR plasma processing apparatus  10 B of the current embodiment includes a microwave guide pipe  17 B as a plasma generating unit for generating plasma using microwaves. In the current embodiment, a mixture of nitrogen gas and rare gas is introduced to a process vessel  202  from a gas introduction pipe  232  through a gas blow outlet  234 , like in the previous embodiments. Before or after the introduction of the gas mixture, microwaves  18 B are introduced through the microwave guide pipe  17 B into a process chamber  201 . The introduced gas mixture is ionized into plasma by the microwaves  18 B, and a high-k gate insulating film formed on a wafer  200  is nitrogenized by an activated nitrogen species produced from the plasma. 
     In the above-described embodiments, explanations are given on a method of heating a wafer using an RTP apparatus after the wafer is nitrogenized, and a method of heating a wafer in a second process chamber (MMT apparatus) after nitrogenization and post nitrogenization processes are performed on the wafer in the second process unit. However, the present invention is not limited to thereto. 
     For example, after a wafer is heated in a second process chamber, the wafer may be carried to an RTP apparatus and be heated in the RTP apparatus. 
     In this case, since the wafer is heated in the second process chamber (MMT apparatus), the bond between nitrogen and a high-k gate insulating film can be increased. In addition, since the wafer is heated to a high temperature in the RTP apparatus, more nitrogen can be coupled to the high-k gate insulating film, and thus current leakage can be suppressed more surely. 
     According to the present invention, a high-k gate insulating film can be formed, which makes nitrogen difficult to escape from the high-k gate insulating film and suppresses a gate leakage current flowing across the high-k gate insulating film. 
     (Supplementary Note) 
     The present invention also includes the following embodiments. 
     (Supplementary Note 1) 
     According to an embodiment of the present invention, there is provided a semiconductor device manufacturing method comprising performing a plasma nitrogenizing process on a high-k gate insulating film formed on a substrate by using a mixture of nitrogen gas and rare gas. 
     According to the method. one of the following effects or other effects can be attained. Since the plasma nitrogenizing process is performed using a mixture of nitrogen gas and rare gas, escaping of nitrogen from the high-k gate insulating film can be prevented more surely than the case of simple nitrogenization. Owing to this, crystallization can be suppressed, and gate leak current can be reduced. In addition, owing to the improved bond strength between nitrogen and the high-k gate insulating film, heat-resisting characteristics can be improved. Moreover, the relative permittivity of a gate insulating film can be high since the high-k gate insulating film is used as the gate insulating film. 
     (Supplementary Note 2) 
     In the semiconductor device manufacturing method of Supplementary Note 1, the rare gas may be helium (He). 
     In this case, escaping of nitrogen can be suppressed more surely than the case where other rare gas such as argon (Ar) is used. 
     (Supplementary Note 3) 
     In the semiconductor device manufacturing method of Supplementary Note 1, the plasma nitrogenizing process may be performed at a temperature equal to or higher than 300° C. but lower than 500° C. 
     It is preferable that the temperature of the plasma nitrogenizing process be equal to or higher than 300° C. but lower than 500° C. In this case, during a PNA process, less nitrogen escapes from the high-k gate insulating film, and crystallization of the high-k gate insulating film can be suppressed. 
     (Supplementary Note 4) 
     The semiconductor device manufacturing method of Supplementary Note 1 may further comprise performing an annealing process after the plasma nitrogenizing process. 
     In this case, the bond strength between nitrogen and the high-k gate insulating film can be increased, and thus gate leak current can be suppressed more surely. 
     (Supplementary Note 5) 
     According to another embodiment of the present invention, there is provided a substrate processing method comprising forming a metal oxide film containing nitrogen by performing a plasma nitrogenizing process on a high-k metal oxide film formed on a surface of a semiconductor substrate by using a mixture of nitrogen gas and helium gas. 
     (Supplementary Note 6) 
     In the semiconductor device manufacturing method of Supplementary Note 1, wherein the plasma nitrogenizing process may be performed at a pressure of 1 Pa to 10 Pa, RF power of 300 W to 700 W, a nitrogen gas:helium gas flow rate ratio of 1:9 to 1:19. 
     (Supplementary Note 7) 
     In the semiconductor device manufacturing method of Supplementary Note 1, wherein the plasma nitrogenizing process may be performed at a pressure of 1 Pa to 10 Pa, RF power of 300 W to 700 W. a nitrogen gas:argon gas flow rate ratio of 1:9 to 1:19. 
     (Supplementary Note 8) 
     According to an embodiment of the present invention, there is provided a method of manufacturing a semiconductor device, the method comprising: forming a high-k gate insulating film on a silicon substrate; nitrogenizing the high-k gate insulating film using gas comprising nitrogen gas and rare gas; forming an electrode on the high-k gate insulating film; and forming an insulating layer enclosing the electrode. 
     (Supplementary Note 9) 
     In the method of Supplementary Note 8, the rare gas may be helium (He). 
     (Supplementary Note 10) 
     The method of Supplementary Note 8 or 9 may further comprise performing a heat treatment on the substrate after the nitrogenizing of the high-k gate insulating film but prior to the forming of the electrode. 
     (Supplementary Note 11) 
     In the method of any one of Supplementary Notes 8 to 10, the nitrogenizing of the high-k gate insulating film may be performed while maintaining the substrate at a temperature of 200° C. to 500° C. 
     (Supplementary Note 12) 
     In the method of any one of Supplementary Notes 8 to 10, the nitrogenizing of the high-k gate insulating film may be performed while maintaining the substrate at a temperature equal to or higher than 300° C. but lower than 500° C. 
     (Supplementary Note 13) 
     According to another embodiment of the present invention, there is provided a system for manufacturing a semiconductor device, the system comprising: a first processing unit configured to form a high-k gate insulating film on a silicon substrate; a second processing unit configured to nitrogenize the high-k gate insulating film using gas comprising nitrogen gas and rare gas; a third processing unit configured to form an electrode on the high-k gate insulating film; a fourth processing unit configured to form an insulating layer enclosing the electrode; and a substrate carrying unit configured to carry the substrate among the processing units. 
     (Supplementary Note 14) 
     The system of Supplementary Note 13 may further comprise a substrate heating unit configured to perform a heat treatment after the high-k gate insulating film is nitrogenized. 
     (Supplementary Note 15) 
     According to another embodiment of the present invention, there is provided a semiconductor device manufacturing apparatus for nitrogenizing a high-k gate insulating film formed on a silicon substrate, the semiconductor device manufacturing apparatus comprising: a reaction gas introduction unit configured to introduce reaction gas comprising nitrogen gas and rare gas to a substrate process chamber; a plasma generating unit configured to ionize the reaction gas into plasma; a substrate stage installed inside the process chamber for placing a substrate on the substrate stage; a substrate heating unit configured to heat a substrate; and a control unit configured to control an operation of introducing reaction gas comprising nitrogen gas and rare gas after a substrate comprising a high-k gate insulating film is carried into the process chamber and an operation of ionizing the introduced reaction gas into plasma for nitrogenizing the high-k gate insulating film of the substrate. 
     (Supplementary Note 16) 
     In the semiconductor device manufacturing apparatus of Supplementary Note 15, the rare gas may be helium (He). 
     (Supplementary Note 17) 
     In the semiconductor device manufacturing apparatus of Supplementary Note 15 or 16, the control unit may stop the plasma generating unit after the high-k gate insulating film is nitrogenized, and then, the control unit may control the substrate heating unit to heat the substrate. 
     (Supplementary Note 18) 
     In the semiconductor device manufacturing apparatus of any one of Supplementary Notes 15 to 17, the control unit may control the substrate heating unit to maintain the substrate at a temperature equal to or higher than 200° C. but lower than 500° C. when the high-k gate insulating film of the substrate is nitrogenized. 
     (Supplementary Note 19) 
     In the semiconductor device manufacturing apparatus of any one of Supplementary Notes 15 to 17. the control unit may control the substrate heating unit to maintain the substrate at a temperature equal to or higher than 300° C. but lower than 500° C. when the high-k gate insulating film of the substrate is nitrogenized.