Abstract:
A disclosed film deposition method comprises alternately repeating an adsorption step and a reaction step with an interval period therebetween. The adsorption step includes opening a first on-off valve of a source gas supplying system for a predetermined time period thereby to supply a source gas to a process chamber, closing the first valve after the predetermined time period elapses, and confining the source gas within the process tube, thereby allowing the source gas to be adsorbed on an object to be processed, while a third on-off valve of a vacuum evacuation system is closed. The reaction step includes opening a second on-off valve of a reaction gas supplying system thereby to supply a reaction gas to the process chamber, thereby allowing the source gas and the reaction gas to react with each other thereby to produce a thin film on the object to be processed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of Japanese Patent Applications No. 2010-170758 and 2011-105146, filed on Jul. 29, 2010, and May 10, 2011, respectively with 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 film deposition method and a film deposition apparatus where a silicon oxide film is deposited on an upper surface of an object to be processed such as a semiconductor wafer. 
     2. Description of the Related Art 
     In order to fabricate semiconductor integrated circuits, various processes such as film deposition, etching, oxidation, diffusion, and alteration processes are carried out with respect to a semiconductor wafer (referred to as a wafer hereinafter) made of silicon or the like. For example, the film deposition process is carried out in a single-wafer film deposition apparatus disclosed, for example, in Patent Document 1 below and in a batch-type film deposition apparatus disclosed, for example, in Patent Document 2. As shown in  FIG. 1 , the batch-type film deposition apparatus has a wafer boat  4  that is accommodated in a process tube  2  and holds plural wafers serving as objects to be processed in a multistage manner, distribution nozzles  8 ,  10  that extend along a vertical direction within the process tube  2  and supply source gases toward the wafer boat  4 , an evacuation opening  12  provided in a lower portion of the process tube  2 , a vacuum evacuation system  14  including a vacuum pump  16 , and a heating part  6  that surrounds the process tube  2 . 
     In addition, when a silicon oxide film is deposited on the wafers in the film deposition shown in  FIG. 1 , the wafer boat  4  that holds plural wafers is accommodated in the process tube  2 , and the wafers are heated to about 600° C. by the heating part  6 . A silicon-containing gas as a source gas is supplied to the distribution nozzle  8  from a gas supplying part  7 , and then supplied toward the wafers W from plural gas ejection holes  8 A provided in and along the longitudinal direction of the distribution nozzle  8 . In addition, ozone gas as a reaction gas is supplied to the distribution nozzle  10  from the gas supplying part  7 , and then supplied toward the wafers W from gas ejection holes  10 A provided in and along the longitudinal direction of the distribution nozzle  10 . On the other hand, the interior of the process tube  2  is evacuated by the vacuum evacuation system  14 , and maintained at a predetermined pressure. The silicon-containing gas and the ozone gas are reacted, and thus the silicon oxide film is deposited on the wafers W. 
     In addition, another film deposition method may be carried out where the silicon-containing gas and the ozone gas, which is an oxidizing gas, are alternately repeatedly supplied by alternately opening and closing an on-off valve  88  and an on-off valve  10 B of the gas supplying part  7 , thereby allowing the source gas adsorbed on the wafers W to react with the ozone gas and depositing the silicon oxide film on the wafers W. This film deposition method is advantageous in that properties of the film obtained are relatively excellent and the film deposition can be carried out at relatively lower temperatures.
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. H09-077593.   Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2009-246318.   Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2006-054432.   

     SUMMARY OF THE INVENTION 
     Incidentally, the silicon oxide film serving as an insulation film may be deposited on a metal film serving as an electrical line in order to form electrical lines in a semiconductor fabrication process. In this case, the metal film, for example, a tungsten film is deposited on a wafer such as a silicon substrate in a previous process, and the wafer is brought to the film deposition apparatus for the silicon oxide deposition while the wafer is exposed to a clean atmospheric environment in a clean room. Alternatively, the wafer is brought within an air-tight wafer carrier inside of which is kept in a clean inert gas environment. 
     In this case, the upper surface of the tungsten film may be reacted with oxygen or moisture within the clean atmospheric environment in the clean room or only a small amount of oxygen or moisture, which may exist even in the clean inert gas environment within the wafer carrier. As a result, a natural metal oxide film having only a small thickness is formed on or in the upper surface of tungsten film. 
     Because the naturally-grown metal oxide film may degrade electric properties of a semiconductor device, it is preferable that a thickness of the metal oxide film is prevented from being increased as much as possible or the metal oxide film is removed before the silicon oxide deposition. However, removal of the metal oxide film is not advisable because it leads to an increased number of processes. Therefore, the silicon oxide film is not purposefully deposited on the metal film having the metal oxide film on the top without removing the metal oxide film in the conventional process. 
     On the other hand, when the silicon oxide film is deposited by a conventional chemical vapor deposition method on the metal film, the metal oxide film on and in the metal film may be thickened by an oxidation gas that is one of the source gases in addition to the silicon-containing gas. As a result, the electric properties of the semiconductor device are relatively greatly degraded (see Patent Document 3). Especially, when a thickness of the metal oxide film is increased, not only the electric properties are degraded but also a needle-like crystal is created, which causes defects. 
     The present invention has been made in view of the above, and provides a film deposition method and a film deposition apparatus that are capable of controlling a thickness of the metal oxide film that may exist at a boundary of an underlying metal film and a silicon oxide film deposited on the metal film, thereby reducing the thickness of the metal oxide film. 
     Having vigorously investigated deposition of the silicon oxide film, the inventors of the present application found that diffusion of ozone as the reaction gas may contribute to an increase in the metal oxide film thickness. Based on this finding, the inventors further found that the ozone contribution to the thickness increase can be controlled by increasing an amount of the source gas adsorbed on an upper surface of the object to be processed through confinement of the source gas within the process chamber. With these findings, the present invention has been arrived at. 
     According to a first aspect of the present invention, there is provided a film deposition method that may be carried out to deposit a silicon oxide film on a metal containing film formed on an object to be processed, using a film deposition apparatus including a process chamber that accommodates the object to be processed; a source gas supplying system that includes a first on-off valve and supplies a source gas to the process chamber; a reaction gas supplying system that includes a second on-off valve and supplies a reaction gas to the process chamber; and a vacuum evacuation system that includes a third on-off valve and evacuates the process chamber to vacuum. The film deposition method comprises alternately repeating an adsorption step and a reaction step with an interval period therebetween. The adsorption step includes opening the first on-off valve of the source gas supplying system for a predetermined time period thereby to supply the source gas to the process chamber, closing the first valve after the predetermined time period elapses, and confining the source gas within the process tube, thereby allowing the source gas to be adsorbed on the object to be processed, while the third on-off valve of the vacuum evacuation system is closed. The reaction step includes opening the second on-off valve of the reaction gas supplying system thereby to supply the reaction gas to the process chamber, thereby allowing the source gas and the reaction gas to react with each other thereby to produce a thin film on the object to be processed. 
     According to a second embodiment, there is provided a film deposition apparatus comprising a process chamber that accommodates an object to be processed; a holding part that holds the object to be processed; a heating part that heats the object to be processed; a source gas supplying system that includes a first on-off valve and supplies a source gas to the process chamber; a reaction gas supplying system that includes a second on-off valve and supplies a reaction gas to the process chamber; a vacuum evacuation system that includes a third on-off valve and evacuates the process chamber to vacuum; and an apparatus controlling part that controls the film deposition apparatus so that the film deposition method according to the first aspect is carried out therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a schematic view of a related art batch-type film deposition apparatus; 
         FIG. 2  is a schematic view of a film deposition apparatus according to the embodiment of the present invention; 
         FIG. 3  is a timing chart illustrating operations of on-off valves of the film deposition apparatus of  FIG. 2  when a film deposition method according to an embodiment of the present invention is carried out; 
         FIG. 4  is an enlarged cross-sectional view that schematically illustrates a cross section of a multilayer film formed by the film deposition method; 
         FIG. 5  is a graph that illustrates a relationship between a hold period of the source gas and a film deposition rate per cycle; 
         FIG. 6  is a graph that illustrates a relationship between the film deposition rate per cycle and a film thickness of a metal oxide film; 
         FIG. 7  is an explanatory view for explaining a reason why a metal film is oxidized at a boundary between a metal containing film and a silicon oxide film deposited on the metal film; and 
         FIG. 8  is a graph that illustrates a relationship between a film deposition rate per cycle and a thickness of a metal oxide film of a TiN film. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to an embodiment of the present invention, there is provided a film deposition method and a film deposition apparatus that are capable of controlling a thickness of the metal oxide film that may exist at a boundary of an underlying metal film and a silicon oxide film deposited on the metal film, thereby reducing the thickness of the metal oxide film. As a result, not only the thickness of the metal oxide film is reduced thereby preventing degradation of electric properties of a semiconductor device but also structural defects are reduced. 
     A film deposition method and a film deposition apparatus according to embodiments of the present invention are described with reference to the accompanying drawings.  FIG. 2  is a schematic view of an example of the film deposition apparatus according to the embodiment of the present invention, and  FIG. 3  is a time chart illustrating operations of on-off valves of the film deposition apparatus when the film deposition method according to the embodiment of the present invention is carried out. 
     In the following an example is described where a silicon oxide film is deposited using a tris(dimethylamino)silane (SiH(N(CH3)2)3): 3DMAS) gas, which is a silicon-containing organic material, a source gas, and an ozone gas, which is an oxidation gas, as a reaction gas. 
     As shown in  FIG. 2 , a film deposition apparatus  20  includes a process tube  22  that may accommodate plural semiconductor wafers W as objects to be processed. The process tube  22  has a double tube structure composed of a vertically long inner tube  24  having a cylindrical shape with a lid at the top and a vertically long outer tube  26  having a cylindrical shape with a lid at the top. The outer tube  26  surrounds the inner tube  24  with a predetermined gap between the outer circumferential surface of the inner tube  24  and the inner circumferential surface of the outer tube  26 . 
     The inner tube  24  and the outer tube  26  are made of, for example, quartz, and have bottom openings. A diameter of the process tube  22  (the outer tube  26 ) is about 400 to 500 mm when the wafers W have diameters of 300 mm. An inner volume of the process tube  22  depends on the number of the wafers W accommodated in the process tube  22 , and is about 200 liters when up to 150 wafers are accommodated. 
     A bottom end of the outer tube  26  is connected to a manifold  28  that is made of, for example, stainless steel and has a cylindrical shape, by way of a sealing member  30  such as an O-ring. In other words, the outer tube  26  is supported at the bottom end thereof by the manifold  28 . Incidentally, the manifold  28  is supported by a base plate (not shown). In addition, a supporting pedestal  32  having a ring shape is provided in the inner wall of the manifold  28 . The supporting pedestal  32  supports a bottom end of the inner tube  24 . 
     A wafer boat  34  serving as a wafer holding part is accommodated within the inner tube  24  of the process tube  22 . The wafer boat  34  holds plural wafers W as objects to be processed at a predetermined pitch. In this embodiment, for example, the 50 to 100 wafers W having a diameter of 300 mm are held at an equal pitch in a multistage manner by the wafer boat  34 . The wafer boat  34 , which is elevatable as described later, is loaded/unloaded into/from the inner tube  24  through the bottom opening of the manifold  28 . The wafer boat  34  is made of, for example, quartz. 
     In addition, when the wafer boat  34  is accommodated in the inner tube  24 , the bottom opening of the manifold  28 , which corresponds to a bottom end of the process tube  22 , is closed by a lid part  36 , which is made, for example, quartz or stainless steel. A sealing member  38  such as an O-ring is provided between the bottom end of the process tube  22  and the lid part  36 . The wafer boat  34  is placed on a table  42  by way of a warmth keeping cylinder  40 , which is made of, for example, quartz. The table  42  is supported on an upper part of a rotation shaft  44  that goes through the lid part  36  that opens and closes the bottom opening of the manifold  28 . 
     A magnetic fluid seal  46  is provided between the rotation shaft  44  and a hole that the rotation shaft  44  goes through, and thus the rotation shaft  44  is rotatably supported in an air-tight manner. The rotation shaft  44  is attached on a distal end of an arm  50  supported by an elevation mechanism  48  such as a boat elevator, and thus is capable of moving the wafer boat  34 , the lid part  36 , or the like in unison. Incidentally, the table  42  may be fixed on the lid part  36 . In this case, the film is deposited on the wafers W without rotating the wafer boat  34 . 
     A heating part  52 , which is made of, for example, a carbon wire heater, is provided so that the process tube  22  is surrounded by the heating part  52 . With this, the process tube  22  and the wafers W within the process tube  22 , which are positioned inside the heating part  52 , are heated. A source gas supplying system  54  that supplies a source gas, a reaction gas supplying system  56  that supplies a reaction gas, and a purge gas supplying system  58  that supplies an inert gas as a purge gas are provided in the manifold  28 . 
     Specifically, the source gas supplying system  54  includes a gas nozzle  60  that is made of, for example, quartz, has an L-shape, and goes through the manifold  28  in an air-tight manner. The gas nozzle  60  extends inside and substantially entirely along the inner tube  24 . The gas nozzle  60  has plural gas ejection holes  60 A that are formed at predetermined pitches. The source gas is supplied transversely toward the wafers W held by the wafer boat  34  through the gas ejection holes  60 A. The gas nozzle  60  is connected to a gas passage  62 . 
     The gas passage  62  is provided with a flow rate controller  62 A such as a mass flow controller and an on-off valve  62 B that starts and terminates (or shuts off) a flow of gas, so that the source gas can be supplied to the gas nozzle  60  at a controlled flow rate as may be necessary and terminated. In this embodiment, the 3DMAS, which contains silicon is used as the source gas. The 3DMAS, which is liquid at a normal temperature, is vaporized by an evaporator  63  and transported by a carrier gas. Specifically, a vapor pressure of the 3DMAS is controlled by adjusting a temperature of the 3DMAS using the evaporator  63 , thereby controlling an amount of the evaporated 3DMAS. As the carrier gas, an inert gas including a nitrogen gas and a noble gas such as argon (Ar) and helium (He) is used. However, the vaporized 3DMAS gas may be transported without using the carrier. 
     In this embodiment, a baking type evaporator is used as the evaporator  63 . The evaporator of this type includes a source tank  63 T that stores the 3DMAS and a heater  63 A that heats the source tank  63 T. The 3DMAS is heated and evaporated by heating the 3DMAS within the source tank  63 T and the evaporated 3DMAS is directly introduced into the process tube  22 . In this case, the carrier gas is not used. In addition, the evaporator  63  is provided with a pressure gauge  63 B, and a temperature of the 3DMAS is controlled so that a vapor pressure of the 3DMAS is greater than a pressure in the process tube  22 . Such control is carried out by the temperature controller (not shown). 
     In addition, an injection type evaporator may be used as the evaporator  63 . In this type of the evaporator, after the evaporator is started, the evaporated 3DMAS needs to be evacuated through a by-pass line, without flowing through the process tube  22 , for 5 to 10 seconds in order to stabilize a flow rate of the evaporated 3DMAS. This means that the organic material, which is relatively expensive, needs to be wasted. Therefore, the baking type evaporator is preferably used from a viewpoint of effective utilization of the source material. 
     In addition, the reaction gas supplying system  56  includes a gas nozzle  64  that is made of, for example, quartz, has an L-shape, and goes through the manifold  28  in an air-tight manner. The gas nozzle  64  extends inside and substantially entirely along the inner tube  24 . The gas nozzle  64  has plural gas ejection holes  64 A that are formed at predetermined pitches. The reaction gas is supplied transversely toward the wafers W held by the wafer boat  34  through the gas ejection holes  64 A. The gas nozzle  64  is connected to a gas passage  66 . The gas passage  66  is provided with a flow rate controller  66 A such as a mass flow controller and an on-off valve  66 B that starts and terminates (or shuts off) a flow of gas, so that the reaction gas can be supplied to the gas nozzle  64  at a controlled flow rate as may be necessary and terminated. In this embodiment, ozone (O 3 ) gas is used as the reaction gas, described above. 
     In addition, the purge gas supplying system  58  includes a gas nozzle  68  that is made of, for example, quartz, has an L-shape, and goes through the manifold  28  in an air-tight manner. The gas nozzle  68  extends inside and substantially entirely along the inner tube  24 . The gas nozzle  68  has plural gas ejection holes  68 A that are formed at predetermined pitches. The purge gas is supplied transversely toward the wafers W held by the wafer boat  34  through the gas ejection holes  68 A. The gas nozzle  68  is connected to a gas passage  70 . The gas passage  70  is provided with a flow rate controller  70 A such as a mass flow controller and an on-off valve  70 B that starts and terminates (or shuts off) a flow of gas, so that the purge gas can be supplied to the gas nozzle  68  at a controlled flow rate as may be necessary and terminated. In this embodiment, a nitrogen gas is used as the purge gas, described above. In other embodiments, an inert gas such as argon (Ar) and helium (He) may be used as the purge gas instead of the nitrogen gas. 
     In addition, the gas nozzles  60 ,  64 ,  68  are positioned in one side of the inside of the inner tube  24 , although the gas nozzle  68  is illustrated on the other side of the gas nozzles  60  and  64 , for the sake of illustrating convenience. Plural openings  72  having relatively large diameters are provided in the circumferential wall of the inner tube  24  along the vertical direction so that the openings  72  are positioned across the wafer boat  34  with respect to the gas nozzles  60 ,  64 ,  68  when the wafer boat  34  is accommodated in the inner tube  24 . With this, the gases supplied to the corresponding gas nozzles  60 ,  64 ,  68  horizontally flow between the wafers W and are guided to a gap  74  between the inner tube  24  and the outer tube  26  through the openings  72 . 
     An evacuation opening  76  is formed to allow the inner tube  24  to be in gaseous communication with the outer tube  26  on an upper part of the manifold  28 . The evacuation opening  76  is connected to a vacuum evacuation system  78  that evacuates the process tube  22  to vacuum. Specifically, the vacuum evacuation system  78  has an evacuation passage  80  connected to the evacuation opening  76 . In addition, the evacuation passage  80  has an on-off valve  80 B that changes an opening degree of a closure element inside thereof in order to control an inner pressure of the process tube  22 , and a vacuum pump  82  in this order. With this, the process tube  22  can be evacuated at a controlled pressure. The closure element of the on-off valve  80 B provided in the evacuation passage  80  can be arbitrarily adjusted within a range from “fully open position” through “fully closed position”. In addition, when the closure element is fully closed, the gas flow from the process tube  22  is completely shut off by the on-off valve  80 B. 
     In addition, the film deposition apparatus  20  is provided with an apparatus control part  84  that is composed of, for example, a computer and controls entire operations of the film deposition apparatus  20 . The apparatus control part  84  controls starting and terminating the flows of gas, on-off operations of the valves including adjustment of the opening degree of the closure element of the pressure control valve  80 B, a process pressure, a process temperature, and the like, thereby carrying out the film deposition process in the film deposition apparatus  20 . Such control is carried out by executing a computer program that controls the entire operations of the film deposition apparatus  20 . The computer program is stored in a computer readable storage medium  86 , and downloaded into the apparatus control part  84 . As the computer readable storage medium  86 , a flexible disk, a compact disk (CD), a hard disk, a flash memory, a digital versatile disk (DVD), and the like may be used. 
     Next, a film deposition method according to an embodiment of the present invention, which is preferably carried out in the film deposition apparatus  20  described above, is explained with reference to  FIG. 4 through 7 .  FIG. 4  is an enlarged cross-sectional view that schematically illustrates a cross section of a multilayer film formed by the film deposition method;  FIG. 5  is a graph that illustrates a relationship between a hold period of the source gas and a film deposition rate per cycle;  FIG. 6  is a graph that illustrates a relationship between the film deposition rate per cycle and a film thickness of a metal oxide film; and  FIG. 7  is an explanatory view for explaining a reason why a metal film is oxidized at a boundary between the metal film and a silicon oxide film deposited on the metal film. Incidentally, the operations of the film deposition apparatus  20  are carried out based on the program stored in the computer readable storage medium  86 . 
     In addition, the wafers W are silicon wafers on which a tungsten film as a metal-containing film is deposited in a previous process, in this embodiment. The wafers W are housed in a wafer carrier and transported from a metal film deposition used in the previous process to the film deposition apparatus  20 . An atmospheric environment that is the same as the environment of a clean room, or a clean inert gas environment, is kept in the wafer carrier. The upper surface of the tungsten film is exposed to oxygen or moisture in the atmospheric environment, or oxygen or moisture that may exist, even if only a tiny amount, in the clean inert gas environment. As a result, a natural metal oxide film is formed on or in the upper surface of the tungsten film. 
     The wafer boat  34  is removed downward through the bottom opening of the manifold  28  and held below the process tube  22 , so that the wafer boat  34  is cooled to a normal temperature. Then, plural (e.g., fifty) of the wafers W are loaded in the wafer boat  34 , and the wafer boat  34  is brought upward into the process tube  22 . The bottom opening of the process tube  22  (the bottom opening of the manifold  22 ) is closed by the lid part  36 , and thus the process tube  22  is closed in an air-tight manner. 
     Incidentally, a temperature inside the process tube  22  is kept at a temperature that is higher than a normal temperature and lower than a film deposition temperature. 
     Next, the vacuum pump  82  of the vacuum evacuation system  78  is started, so that the process tube  22  is evacuated to vacuum; and the inner pressure of the process tube  22  is kept at a predetermined process pressure by the on-off valve  80 B. During the film deposition process, the vacuum pump  82  and the on-off valve  80 B are continuously operated. 
     In addition, by increasing electric power supplied to the heating part  52 , the wafers W are heated to the film deposition temperature. After the film deposition temperature is stabilized, the source gas, the reaction gas, and the like are supplied at controlled flow rates. 
     Specifically, the 3DMAS gas as the silicon-containing gas is supplied from the gas nozzle  60  of the source gas supplying system  54 ; the ozone gas is supplied from the gas nozzle  64  of the reaction gas supplying system  56 ; and N 2  gas as a purge gas is supplied from the gas nozzle  68  of the purge gas supplying system  58 . 
     The gases supplied from the corresponding gas nozzles  60 ,  64 ,  68  flow through spaces between the wafers W supported in a multistage manner by the wafer boat  34 , and further through the opening  72  to reach the gap  74  between the inner tube  24  and the outer tube  26 . Then, the gases are evacuated through the evacuation opening  76  in the manifold  28  by the vacuum evacuation system  78 . 
     Next, a method of supplying each gas is specifically explained with reference to  FIG. 3 . As described above, the source gas and the reaction gas are alternately repeatedly supplied.  FIG. 3(A)  illustrates open/close operations of the on-off valve  62 B for the source gas;  FIG. 3(B)  illustrates open/close operations of the on-off valve  66 B for the reaction gas; and  FIG. 3(C)  illustrates open/close operations (or an opening degree of the closure element) of the on-off valve  80 B of the vacuum evacuation system. 
     A thin film is deposited on the wafers W by repeating an adsorption process where the on-off valve  62 B for the source gas is opened for a predetermined period of time, thereby allowing the silicon-containing source gas to be adsorbed on the upper surface of the wafers W as shown in  FIG. 3(A) , an evacuation process where supplying the source gas is terminated and the process tube  22  is evacuated, a reaction process where the on-off valve  66 B for the reaction gas is opened to supply the ozone gas, thereby allowing the source gas adsorbed on the upper surface of the wafers W to be reacted with the reaction gas to form a silicon oxide film having a small thickness, and an evacuation process where the flow of the reaction gas is terminated and the process tube  22  is evacuated, in this order. 
     Here, processes from one adsorption process to the next adsorption process are referred to as one cycle, and therefore the thin film having an extremely small thickness corresponding to one monolayer or several monolayers is formed in one cycle. A time period T 1  of the adsorption process and a time period T 2  of the reaction process within the one cycle are about 60 seconds; and a time period T 3  of the evacuation process in the first half of the one cycle and a time period T 4  of the evacuation process in the second half of the one cycle are about 10 seconds. Incidentally, N 2  purge gas may be supplied in the evacuation processes. After one such cycle, the thin film is formed as shown in  FIG. 4 . Namely, before the film deposition, a metal containing film  100  made of, for example, a tungsten film is deposited on the upper surface of the wafer W, and the natural metal oxide film  102  is formed on and in the upper surface of the metal containing film  100  during transporting the wafers W, as described above, as shown in  FIG. 4(A) . The metal oxide film  102  is expressed as, for example, WOx (x: an integer greater than 1) because the metal containing film  100  is made of tungsten, in this embodiment. After the film deposition, a thin film  104  made of silicon oxide is formed on the metal oxide film  102 , as shown in  FIG. 4(B) . In this case, the metal oxide film  102  is reduced and thus a thickness of the metal oxide film  102  can be controlled because the source gas is supplied and held within the process tube  22 , as described later. 
     In the film deposition method according to this embodiment, the on-off valve  62 B is opened thereby supplying the source gas for a first predetermined time period t 1  in the adsorption process; and then the on-off valve  62 B is closed for a predetermined time period h and the source gas is held inside the process tube  22 , as shown in  FIG. 3(A) . Namely the source gas is not supplied in the entire adsorption process. In addition, the on-off valve  80 B of the vacuum evacuation system is kept closed in the entire adsorption process, as shown in  FIG. 3(C) . In other words, the source gas supplied to the process tube  22  for the first predetermined time period t 1  is confined (or stays) in the process tube  22  in the next time period h because the on-off valve  80 B of the vacuum evacuation system is closed. As a result, a relatively large amount of the source gas is adsorbed on the upper surface of the metal oxide film  102 . In this case, a thickness of the metal oxide film  102  can be controlled by adjusting length of the time period (referred to a hold time hereinafter) h. 
     For example, a flow rate of the source gas is about 10 to about 500 sccm in the first predetermined time period t 1 . In addition, an inner pressure of the process tube  22  in the adsorption process T 1  is first rapidly increased and becomes constant (e.g., at 667 Pa) at the same time when the on-off valve  62 B is closed. 
     Next, when the adsorption process is completed, the procedure moves onto the first evacuation process T 3 . Namely, the on-off valve  80 B of the vacuum evacuation system is fully opened while no gases are supplied to the process tube  22 , thereby rapidly evacuating the remaining gas in the process tube  22 . Incidentally, the N 2  gas as a purge gas may be supplied into the process tube  22  in order to facilitate evacuating the remaining gas from the process tube  22 . With the first evacuation process, a concentration of the source gas (3DMAS) is rapidly decreased. 
     Next, the procedure moves on to the reaction process T 2 . Here, the on-off valve  66 B for the reaction gas is opened as shown in  FIG. 3(B) , thereby supplying the ozone gas as the reaction gas to the process tube  22  during the entire time period of the reaction process T 2 . In this case, the on-off valve  80 B of the vacuum evacuation system may be fully opened. Alternatively, the opening degree of the closure element of the on-off valve  80 B may be less than or equal to 100% if an evacuation flow rate is sufficient. In the illustrated example, the opening degree of the closure element is constant at 50%. 
     Incidentally, an oxygen gas is supplied at a flow rate of 6.5 standard liters per minute (slm) to an ozone generator, so that the ozone gas of 200 g/Nm3 is produced, and thus the oxygen gas including this amount of the ozone gas is supplied to the process tube  22 . By supplying the ozone gas, the source gas adsorbed on the upper surface of the wafer W is reacted with the ozone gas, thereby forming a thin film  104  made of silicon oxide. In a conventional film deposition method, the ozone gas diffuses through the source gas adsorbed on the upper surface of the wafer W and the thin film  104  to reach the underlying metal-containing film  100 , so that the upper surface of the metal-containing film  100  is oxidized, leading to an increased thickness of the metal oxide film  102 . 
     However, because the source gas can be appropriately adsorbed on the upper surface of the wafer W during the hold time h, the ozone diffusion is suppressed, and thus the thickness of the metal oxide film  102  is suppressed. Moreover, the thickness of the metal oxide film  102  may be reduced depending on the length of the hold time h. 
     In such a manner, when the reaction process T 3  is completed, the procedure moves onto the second evacuation process T 4 . Namely, the on-off valve  80 B of the vacuum evacuation system is fully opened while no gases are supplied to the process tube  22 , thereby rapidly evacuating the remaining gas in the process tube  22 . Incidentally, the N 2  gas as a purge gas may be supplied into the process tube  22  in order to facilitate evacuating the remaining gas from the process tube  22 . With these procedures, the one cycle is completed, and the same cycle is repeated plural times afterwards. As a result, the thin film  104 , which is made of silicon oxide, having a desired thickness is obtained. Incidentally, the inner pressure of the process tube  22  (film deposition pressure) is maintained at a pressure with a range from a base pressure (e.g., about 13.3 Pa) to 133.3 Pa. 
     As described above, according to this embodiment, there is provided the hold time h in the adsorption process T 1  for supplying the source gas to the wafer W, where the source gas is confined within the process tube  22  by closing the on-off valve  62 B for the source gas and the on-off valve  80 B of the vacuum evacuation system, so that the source gas does not flow out from the process tube  22 . Therefore, the thickness of the metal oxide film  102 , which has been naturally formed, is prevented from being increased. In addition, the metal oxide film  102  may be reduced by adjusting the length of the hold time h. Namely, the thickness of the metal oxide film  102  can be controlled. As a result, not only the electric properties of the semiconductor device are prevented from being degraded by preventing the increased thickness of the metal oxide film  102 , but also the structural defects are prevented. 
     &lt;Evaluation Tests and Their Results&gt; 
     Next, results of evaluation experiments that were carried out in order to confirm advantages of the film deposition method according to embodiments of the present invention are explained.  FIG. 5  is a graph illustrating a relationship between the hold time and the film deposition rate per cycle; and  FIG. 6  is a graph illustrating a relationship between the film deposition rate, the upper surface of the metal-containing film, and the thickness of the metal oxide film. Here, the reaction period T 2  is fixed between 1 through 30 seconds and the predetermined time period t 1  where the on-off valve  62 B for the source gas is opened is fixed between 1 through 30 seconds. In addition, the hold time h (=T 1 −t 1 ) where the on-off valve  62 B for the source gas is closed in the adsorption process is changed by changing the time period of the adsorption process T 1 . 
     In  FIG. 5 , the hold time h is taken as a horizontal axis, and the film deposition rate per cycle is taken as a vertical axis. In addition, the 3DMAS, which contains silicon, was used as the source gas, and the ozone gas (or the oxygen containing ozone of 10 vol. %) was used as the reaction gas. A silicon wafer having the tungsten film deposited thereon, as the metal-containing film  100 , and the tungsten oxide film  102  naturally formed on and in the tungsten film was used. Here, a total flow amount of the 3DMAS was set to be one-fourth of that in a comparative example (a conventional condition). A wafer temperature at the time of film deposition was 550° C., and the process pressure is 1.2 kPa at maximum. 
     In addition, the source gas and the reaction gas were alternately supplied without the hold time, thereby depositing the silicon oxide film in the comparative example. Other film deposition conditions were the same as those in the evaluation experiments according to the embodiment of the present invention. Namely, the adsorption process T 1  is 30 seconds, and the total flow amount of 3DMAS is four times of that in the embodiment of the present invention 
     As understood from  FIG. 5 , as the hold time h is increased from 10 seconds to 115 seconds, the film deposition rate per cycle is linearly increased from 0.1 nm/cycle to 0.21 nm/cycle. This is because an amount of the source gas adsorbed on the upper surface of the wafer W is increased as the hold time h is increased, which leads to an increased film deposition rate. 
     In addition, while the film deposition rate is about 0.13 nm/cycle when the 3DMAS of X g is supplied in the comparative example, the film deposition rate per cycle can be increased with an increase in the hold time h according to the embodiment of the present invention, although the flow rate of the 3DMAS is reduced to X/4. Specifically, when the hold time is about 40 seconds, the film deposition rate is the same as that in the comparative example. Therefore, it has been found that the film deposition rate that is the same as that in the comparative example can be obtained when the hold time h is set as 40 seconds or more, even when the total flow amount of the source gas is reduced to one-fourth of that in the comparative example. In other words, the total flow amount of the source gas can be greatly reduced by operating the on-off valves according to the embodiment of the present invention. 
     Next, the thicknesses of the tungsten oxide (WOx) film were measured depending on the film deposition rates. The results are illustrated in  FIG. 6 . The thickness of the metal oxide film  102 , which exists at a boundary between the metal containing film  100  made of tungsten and the silicon oxide film  104 , was measured using X-ray electron spectrometry (XPS). 
     An initial thickness of the tungsten oxide film  102  naturally formed before the film deposition was about 1.1 nm. Incidentally, typical values of the hold time h are written in  FIG. 6 . A curve A indicates the thickness dependence on the film deposition rate in the case where the film deposition was carried out at a temperature of 550° C. using the 3DMAS as the source gas. As shown in  FIG. 6 , when the film deposition rate is small, the thickness of the metal oxide film  102  is rather greater than the initial thickness of the naturally formed tungsten oxide film  102 . As the film deposition rate becomes greater, the thickness of the metal oxide film  102  is rapidly decreased. Especially, when the film deposition rate is about 0.115 nm/cycle, the thickness of the metal oxide film  102  is substantially the same as the initial thickness. When the film deposition rate becomes further greater, the thickness of the metal oxide film  102  is rather slowly decreased. 
     From the above results, it has been found that the thickness of the metal oxide (e.g., WOx) film  102  that exists at the boundary between the metal containing film  100  and the thin film (silicon oxide)  104  can be controlled by changing the film deposition rate, which can be adjusted by the hold time h where the source gas is confined in the process tube  22 . Specifically, the thickness of the metal oxide film  102  can be smaller than the initial thickness by setting the film deposition rate to be about 0.115 nm/cycle or smaller. In other words, when the hold time h is set to be about 23 seconds or greater (film deposition rate: 0.115 nm/cycle or greater), the thickness of the metal oxide film  102  can be smaller than the initial thickness. 
     Here, a reason why the thickness of the metal oxide film  102  becomes greater than the initial thickness when the film deposition rate per cycle is smaller than 0.115 nm/cycle may be explained as follows. As shown in  FIG. 7(A) , when the number of source gas molecules  110  that are adsorbed on the upper surface of the metal containing film  100  is small, there are many gaps between the molecules  110 . Therefore, ozone molecules  112  introduced after the source gas molecules  110  are adsorbed on the upper surface of the metal containing film  100  can go through the gaps, as shown by dotted arrows. As a result, the ozone molecules  110  can diffuse into the metal oxide film  102  and the metal containing film  100 , and thus the metal containing film  100  is oxidized, thereby increasing the thickness of the metal oxide film  102 . 
     As the film deposition rate per cycle is increased, the number of the silicon source gas molecules  110  adsorbed on the upper surface of the metal oxide film  102  is increased as shown in  FIG. 7(B) , which decreases the gaps between the source gas molecules  110 . As a result, the ozone molecules  112  are impeded from diffusing into the metal oxide film  102  and the metal containing film  100 , and thus oxidation of the metal containing film  100  is also impeded. 
     In addition, a reason why the thickness of the metal oxide film  102  becomes smaller than the initial thickness when the film deposition rate per cycle is 0.115 nm/cycle or greater may be explained as follows. Because the source gas containing silicon per se provides reduction ability at the film deposition temperature of 550° C., the naturally formed metal oxide film  102  can be reduced. As a result, the thickness of the metal oxide film  102  after the film deposition of silicon oxide (see  FIG. 4(B) ) is smaller than the initial thickness (see  FIG. 4(A) ). 
     From the above results, the hold time h is preferably set to be 23 seconds or longer in order to obtain the thickness of the metal oxide film  102  that is smaller than the initial thickness, at the film deposition temperature of 550° C. or more in each process when the 3DMAS is used as the source gas. Incidentally, the film deposition temperature is preferably about 600° C. at a maximum. When the film deposition temperature is greater than 600° C., the source gas adsorbed on the upper surface of the wafer W may be thermally decomposed, rather than oxidized by the ozone gas, which is not advisable from the viewpoint of the ALD (or MLD). 
     In addition, an additional experiment was carried out using the 3DMAS at a film deposition temperature of 450° C. at the hold time h of only 23 seconds. The other conditions are the same as the above. As shown by a point B in  FIG. 6 , the result was that the film deposition rate was 0.088 nm/cycle and the thickness of the tungsten oxide film was 1.77 nm, which is greater than the initial thickness of 1.1 nm. This result indicates that the film deposition temperature is preferably 550° C. rather than 450° C. 
     Moreover, another experiment was carried out using di-iso-propylaminosilane (DIPAS), which belongs to the same aminosilane organic source material family as the 3DMAS, instead of the 3DMAS under the same conditions as the above, except for the hold time h and the film deposition temperature. Point C in  FIG. 6  indicates the result of the experiment carried out at the hold time h of 23 seconds at the film deposition temperature of 450° C.; and point D in  FIG. 6  indicates the result of the experiment carried out at the hold time h of 23 seconds at the film deposition temperature of 300° C. 
     As shown in  FIG. 6 , the film deposition rate is 0.15 nm/cycle and the thickness of the tungsten oxide film is 1.1 nm, which is the same as the initial thickness at the point C. In addition, the film deposition rate is 0.185 nm/cycle and the thickness of the tungsten oxide film is 1.1 nm, which is the same as the initial thickness at the point D. 
     As shown above, when the DIPAS is used instead of the 3DMAS as the source gas, the film deposition rate is rather greater even at a relatively low film deposition temperature of 450° C., for example, in a range from 300° C. through 450° C. In addition, because the number of the source molecules adsorbed on the upper surface of the wafer W is greater, the oxidation of the tungsten containing film (or growth of the tungsten oxide film) is suppressed. 
     Incidentally, while the tungsten film was used as the metal containing film  100  in the above experiments, a titanium nitride (TiN) film was used as the metal containing film  100  in yet another experiment.  FIG. 8  illustrates a relationship between a film deposition rate per cycle and a thickness of a metal oxide film of the TiN film. An initial thickness of the metal oxide film was 2.7 nm. In the same manner as that in the experiments whose results are shown in  FIG. 6 , the hold time h was changed from 5 seconds to 113 seconds. 
     As shown in  FIG. 8 , as the hold time h becomes greater, the film deposition rate is increased and the thickness of the titanium oxide film is linearly decreased. When the film deposition rate is relatively small, the thickness of the metal oxide film is greater than the initial thickness, and the thickness of the metal oxide film is substantially the same as the initial thickness at the film deposition rate of 0.11 nm/cycle. As the film deposition rate becomes further greater, the thickness of the metal oxide film becomes decreased. Specifically, the thickness of the metal oxide film is decreased to 1.7 nm from the initial thickness of 2.7 nm. 
     Incidentally, while the tungsten film and the titanium nitride film were used as the metal containing film in the above experiments, the metal containing film includes other metal films and metal nitride films. Specifically, one selected from a group of a tungsten film, a tungsten nitride film, a titanium film, a titanium nitride film, a tantalum film, and a tantalum nitride film is used as the metal containing film. 
     In addition, while the 3DMAS was used as the source gas in the above experiments, other aminosilane organic source material such as bis(tertiary-butylamino)silane (BTBAS), tetrakisdimethylaminosilane (4DMAS), di-iso-propylaminosilane (DIPAS), or the like may be used as the source gas. 
     Moreover, while the ozone gas, which is an oxidizing gas, was used as the reaction gas in the above experiments, one selected from a group of O 3 , O 2 , O 2  plasma, N 2 O, and NO may be used as the reaction gas. Furthermore, oxygen active species and hydroxyl group active species that can be produced under a relatively lower pressure of 133 Pa, which are disclosed in Japanese Patent Application Laid-Open Publication, may be used as the reaction gas. In addition, the process tube  22  is merely exemplified. The process tube  22  is not limited to one having the double-tube structure, but may be composed of a single tube. 
     While the semiconductor substrate is taken as an example of the object to be processed, the semiconductor substrate includes a silicon wafer and a compound semiconductor wafer made of gallium arsenide, silicon carbide, gallium nitride, or the like. In addition, a glass substrate, which is used to fabricate a flat panel display or a liquid crystal display, a ceramic material substrate, and the like are also used in an embodiment of the present invention. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the sprit or scope of the general inventive concept as defined by the appended claims and their equivalents.