Abstract:
A film deposition method of depositing a thin film by alternately supplying at least a first source gas and a second source gas to a substrate is disclosed. The film deposition method includes steps of evacuating a process chamber where the substrate is accommodated, without supplying any gas to the process chamber; supplying an inert gas to the process chamber until a pressure within the process chamber becomes a predetermined pressure; supplying the first source gas to the process chamber filled with the inert gas at the predetermined pressure without evacuating the process chamber; stopping supplying the first source gas to the process chamber and evacuating the process chamber; supplying the second source gas to the process chamber; and stopping supplying the second source gas to the process chamber and evacuating the process chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on Japanese Patent Application No. 2011-038509 filed with the Japanese Patent Office on Feb. 24, 2011, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a film deposition method and apparatus where at least a first source gas and a second source gas are alternately supplied to a substrate, thereby depositing a film formed through chemical reaction between the first and the second source gases on the substrate. 
     2. Description of the Related Art 
     In a fabrication process of semiconductor integrated circuits (ICs), in order to deposit, for example, an insulating film, an atomic layer deposition (ALD) method may be used where a first source gas and a second source gas for forming the insulating film are alternately supplied to a substrate, thereby depositing the insulating film on the substrate. Because the ALD method can utilize self-limiting adsorption of the source gases on the substrate, film thickness uniformity of the deposited film is expected to be excellent. In addition, because a film thickness can be determined by the number of cycles of alternately supplying the first and the second source gases, thickness controllability is expected to be excellent (see Patent Document 1, for example).
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2004-6801.   

     SUMMARY OF THE INVENTION 
     When a thin film is deposited on a substrate, it is not always preferable that the deposited film has excellent film thickness uniformity. For example, in the case of a silicon nitride film deposited by a conventional low pressure chemical vapor deposition method, it may be preferable that the deposited film has a thickness profile where a film is thick in the center area and becomes thinner toward the circumferential area on the substrate. This is because the film having such a thickness distribution is deposited and then an etching profile in a subsequent etching process is determined taking into consideration such a thickness distribution. This can be useful for suppressing a micro loading effect, which is caused from pattern miniaturization. 
     Therefore, when the film is deposited on the substrate by the ALD method, even if the ALD method can achieve excellent thickness uniformity, it is desired that the film deposited by the ALD method have such a film thickness distribution. 
     The present invention has been made in view of the above, and is directed toward providing a film deposition method and apparatus that are capable of depositing a thin film having a desired film thickness distribution. 
     According to an aspect of the present invention, there is provided a film deposition method of depositing a thin film by alternately supplying at least a first source gas and a second source gas to a substrate thereby allowing the first and the second source gases to react with each other on the substrate. The film deposition method includes steps of: evacuating a process chamber where the substrate is accommodated, without supplying any gas to the process chamber; supplying an inert gas to the process chamber until a pressure within the process chamber becomes a predetermined pressure; supplying the first source gas to the process chamber filled with the inert gas at the predetermined pressure without evacuating the process chamber; stopping supplying the first source gas to the process chamber and evacuating the process chamber; supplying the second source gas to the process chamber; and stopping supplying the second source gas to the process chamber and evacuating the process chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a film deposition apparatus according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the film deposition apparatus according to the embodiment of the present invention; 
         FIG. 3  is a flowchart illustrating a film deposition method according to an embodiment of the present invention; 
         FIG. 4  is a pressure chart illustrating a pressure within a process chamber of the film deposition apparatus according to the embodiment of the present invention; 
         FIG. 5  is a graph that illustrates a film thickness distribution of a silicon nitride film deposited by the film deposition method according to the embodiment of the present invention; 
         FIG. 6  is graph that illustrates a film thickness uniformity of the silicon nitride film deposited by the film deposition method according to the embodiment of the present invention; and 
         FIG. 7  is a view for explaining how a film thickness distribution can be controlled by the film deposition method according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Non-limiting, exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings, the same or corresponding reference symbols are given to the same or corresponding members or components. It is to be noted that the drawings are illustrative of the invention, and there is no intention to indicate scale or relative proportions among the members or components, or between thicknesses of various layers. Therefore, the specific thickness or size should be determined by a person having ordinary skill in the art in view of the following non-limiting embodiments. 
       FIG. 1  is a cross-sectional elevation view of schematically illustrating an atomic layer deposition (ALD) apparatus according to an embodiment of the present invention, and  FIG. 2  is a cross-sectional plan view of schematically illustrating the ALD apparatus. 
     Referring to  FIG. 1 , an ALD apparatus  80  includes a process tube  1  that has a shape of a cylinder with a closed top and a bottom opening and is made of, for example, quartz glass. The process tube  1  is provided in its upper inside part with a top plate  2  made of, for example, quartz glass. In addition, a manifold  3  that has a cylindrical shape and is made of, for example, stainless steel is connected to the bottom opening of the process tube  1  via a sealing member  4  such as an O-ring. 
     The manifold  3  allows predetermined gases to be introduced into the process tube  1 , while serving as a supporting member that supports a bottom end of the process tube  1 . Namely, plural through holes (not shown) are formed on a side wall of the manifold  3  and plural gas pipes (described later) are connected to the corresponding through holes. 
     The manifold  3  has a bottom opening, and a lid member  9  is coupled to the bottom end of the manifold  3  via a sealing member  12  such as an O-ring, in order to open or close the bottom opening of the manifold  3 . The lid member  9  has a center opening through which a rotational shaft passes in an airtight manner. A table  8  is placed on an upper end of a rotational shaft  10 ; a heat retention cylinder  7 , which is made of, for example, quartz glass is placed on the table  8 ; and a wafer boat  5  is placed on the heat retention cylinder  7 . As shown in  FIG. 2 , the wafer boat  5  has three pillars  6 . The three pillars  6  have plural grooves, so that plural wafers W are supported by the grooves. The rotational shaft  10  may be rotated by a rotation mechanism (not shown), so that the rotational shaft  10  and thus the wafer boat  5  are rotated around a vertical axis. 
     A bottom end of the rotational shaft  10  is attached to an arm  13  that is elevatably supported by an elevation mechanism (not shown). By moving the arm  13  upward and downward, the wafer boat  5  is transferred into and out from the process tube  1  by the arm  13 . Incidentally, a magnetic fluid seal  11  is provided between the rotational shaft  10  and the lid member  9 , so that the process tube  1  can be sealed in an airtight manner. 
     In addition, the ALD apparatus  80  is provided with a nitrogen-containing gas supplying mechanism  14  that supplies a nitrogen-containing gas to the process tube  1 , a silicon-containing gas supplying mechanism  15  that supplies a silicon-containing gas to the process tube  1 , and an inert gas supplying mechanism  16  that supplies an inert gas to the process tube  1 . 
     The nitrogen-containing gas supplying mechanism  14  includes a nitrogen-containing gas supplying source  17 , a nitrogen-containing gas supplying pipe  17 L that guides the nitrogen-containing gas from the nitrogen-containing gas supplying source  17 , and a nitrogen-containing gas distribution nozzle  19 . The nitrogen-containing gas distribution nozzle  19  is connected to the nitrogen-containing gas supplying pipe  17 L, passes through the manifold  3 , and is bent upward within the process tube  1 . The nitrogen-containing gas distribution nozzle  19  is made of, for example, quartz glass. Plural gas ejection holes  19   a  are formed at predetermined intervals in a vertically extending part of the nitrogen-containing gas distribution nozzle  19 , so that the nitrogen-containing gas is uniformly ejected in a horizontal direction from each of the plural gas ejection holes  19   a.    
     In addition, the nitrogen-containing gas supplying pipe  17 L is provided with an open/close valve  17   a  and a flow rate controller  17   b  that controls a flow rate of the nitrogen-containing gas. With these, the start/stop of supplying the nitrogen-containing gas and the flow rate of the nitrogen-containing gas are controlled. 
     The silicon-containing gas supplying mechanism  15  includes a silicon-containing gas source  20 , a silicon-containing gas supplying pipe  20 L that guides the silicon-containing gas from the silicon-containing gas supplying source  20 , and a silicon-containing gas distribution nozzle  22 . The silicon-containing gas distribution nozzle  22  is connected to the silicon-containing gas supplying pipe  20 L, passes through the manifold  3 , and is bent upward within the process tube  1  to extend in a vertical direction. The silicon-containing gas distribution nozzle  22  is made of, for example, quartz glass. Referring to  FIG. 2 , two silicon-containing gas distribution nozzles  22  are provided in this embodiment. Plural gas ejection holes  22   a  are formed at predetermined intervals in a vertically extending part of each of the silicon-containing gas distribution nozzles  22 , so that the silicon-containing gas is uniformly ejected in a horizontal direction from each of the plural gas ejection holes  22   a . Incidentally, the number of the silicon-containing gas distribution nozzles  22  is not limited to two, but may be only one, or three or more. 
     In addition, the silicon-containing gas supplying pipe  20 L is provided with an open/close valve  20   a , a flow rate controller  20   b , a buffer tank  180 , and an open/close valve  20   c . For example, when the open/close valve  20   a  is opened while the open/close valve  20   c  is closed and the silicon-containing gas is supplied from the silicon-containing gas supplying source  20 , the silicon-containing gas is temporarily retained in the buffer tank  180 . Then, when the open/close valve  20   a  is closed and the open/close valve  20   c  is opened, a predetermined amount of the silicon-containing gas retained in the buffer tank can be supplied to the process tube  1 . 
     The inert gas supplying mechanism  16  includes an inert gas source  41 , an inert gas supplying pipe  41 L that guides the inert gas from the inert gas supplying source  41  and is merged into the silicon-containing gas supplying pipe  20 L. Because the inert gas supplying pipe  41 L is merged into the silicon-containing gas pipe  41 L, the inert gas is ejected from the silicon-containing gas distribution nozzle  22  into the process tube  1 . In addition, the inert gas supplying pipe  41 L is provided with an open/close valve  41   a  and a flow rate controller  41   b  that controls a flow rate of the inert gas. With these, the start/stop of supplying the inert gas and the flow rate of the inert gas are controlled. 
     A plasma generation mechanism  30  is formed in a part of the circumferential wall of the process tube  1 . The plasma generation mechanism  30  includes an opening  31  that is made in the circumferential wall of the process tube  1  and has the shape of a vertically oblong rectangle, and a plasma partitioning wall  32  that is welded to cover the opening  31  from the outside. Specifically, the plasma partitioning wall  32  has a box shape that has a vertical length sufficient to cover the opening  31 , and is made of, for example, quartz glass. Because of the plasma partitioning wall  32 , it appears that a part of the circumferential wall of the process tube  1  is indented outward. An inner space of the plasma partitioning wall  32  communicates with an inner space of the process tube  1 . In addition, the opening  31  is long enough in a vertical direction to span from the lowest wafer W to the highest wafer W loaded in the wafer boat  5 . 
     In addition, the plasma generation mechanism  30  includes a pair of plasma electrodes  33 ,  33  and a high frequency power source  35  that supplies high frequency power to the plasma electrodes  33 ,  33  via a feed line  34 . One of the plasma electrodes  33 ,  33  extends in a vertical direction near one of outer side surfaces of the plasma partitioning wall  32 , and the other one of the plasma electrodes  33 ,  33  extends in a vertical direction near the other one of the outer side surfaces of the plasma partitioning wall  32 , so that the plasma electrodes  33 ,  33  oppose each other across the plasma portioning wall  32 . When electric power at a frequency of 13.56 MHz is applied from the high frequency power source  35  to the plasma electrodes  33 ,  33 , plasma is generated within the plasma partitioning wall  32 . Incidentally, the frequency of the electric power is not limited to 13.56 MHz, but may be 400 kHz, for example. 
     Incidentally, as shown in  FIG. 1 , the nitrogen-containing gas distribution nozzle  19  is bent in an outward direction and then bent again upward near the inner surface of the plasma partitioning wall  32 , thereby to extend upward along the inner surface of the plasma partitioning wall  32 . Therefore, the nitrogen-containing gas ejected from the nitrogen-containing gas distribution nozzle  19  flows through the inner space of the plasma partitioning wall  32 , and is electromagnetically excited by the electric power supplied to the plasma electrodes  33 ,  33 , thereby generating the plasma. In other words, the nitrogen-containing gas is excited sufficiently to be transformed into plasma and flows toward the center of the process tube  1 . 
     An insulating protection cover  36  is attached on the outer surface of the plasma partitioning wall  32 , so that the plasma partitioning wall  32  and the plasma electrodes  33 ,  33  are covered by the insulating protection cover  36 . In addition, a cooling fluid conduit (not shown) is formed inside of the insulating protection cover  36 . When cooled nitrogen gas is supplied to the cooling fluid conduit, the plasma electrodes  33 ,  33  can be cooled. 
     The two silicon-containing gas distribution nozzles  22  stand one on one side of the opening  31  and the other on the other side of the opening  31  of the process tube  1 . The two silicon-containing gas distribution nozzles  22  eject the silicon-containing gas toward a center part of the process tube  1  from the plural ejection holes  22   a  of the corresponding silicon-containing gas distribution nozzles  22 . 
     Incidentally, as the silicon-containing gas, dichlorosilane (DCS), hexachlorodisilane (HCD), monosilane (SiH 4 ), disilane (Si 2 H 6 ), hexamethyldisilazane (HMDS), tetrachlorosilane (TCS), disilylamine (DSA), trisilylamine (TSA), bis(tertiary-butylamino)silane (BTBAS, SiH 2 (NH(C 4 H 9 )) 2 ), or the like may be used. In addition, as the nitrogen-containing gas, ammonia (NH 3 ) gas, hydrazine (N 2 H 2 ), or the like may be used. 
     An evacuation opening  37  for evacuating the process tube  1  is provided on the other side of the opening  31  in the process tube  1 . The evacuation opening  37  has a vertically oblong rectangular shape in this embodiment, and is formed by removing a part of the circumferential wall of the process tube  1 . As shown in  FIG. 2 , an evacuation opening cover member  38 , which has a substantially U-shaped cross-section, is welded onto the outer circumferential surface of the process tube  1  in order to cover the evacuation opening  37 . The evacuation opening cover member  38  extends upward along the outer circumferential wall of the process tube  1 , and defines a gas outlet port  39  in an upper part of the process tube  1 . The gas outlet port  39  is connected to a vacuum pump VP via a main valve MV and a pressure controller PC, so that the process tube  1  is evacuated at a controlled pressure by the vacuum pump VP. The vacuum pump VP may include a mechanical booster pump and a turbo molecular pump. 
     In addition, a heating unit  40  having a cylindrical shape is provided in order to surround the process tube  1 , so that the wafers W in the process tube  1  are heated, as shown in  FIG. 1 . Incidentally, the heating unit  40  is omitted in  FIG. 2 . 
     The ALD apparatus  80  is provided with a controller  50  including a microprocessor (or computer) that controls operations of the ALD apparatus  80 . For example, the controller  50  controls on/off operations of the open/close valves  17   a ,  20   a  to  20   c , and  41   a , thereby controlling starting/stopping the gases, and controls the flow rate controllers  17   b ,  20   b ,  41   b , thereby adjusting flow rates of the gases. In addition, the controller  50  controls the heating unit  40 , thereby heating the wafers W at a predetermined temperature. The controller  50  is connected to a user interface  51  composed of a keyboard (not shown) through which an operator can input process parameters or commands and a display (not shown) that may illustrate process situations. 
     In addition, the controller  50  is connected to a memory part  52  that stores programs or recipes for the controller  50  to cause the ALD apparatus to carry out various treatments with respect to the wafers W. The programs include a film deposition program by which a film deposition method (described later) is carried out by the ALD apparatus  80  under control of the controller  50 . In addition, the programs are stored in a computer readable storage medium  52   a  and downloaded to the memory part  52 . The computer readable storage medium  52   a  may be a hard disk, a semiconductor memory, a compact disk-read only memory (CD-ROM), a digital versatile disk (DVD), a flash memory or the like. In addition, the programs may be downloaded to the memory part  52  from another apparatus through, for example, a dedicated network. 
     When needed, an arbitrary program is read out from the memory part  52  in response to instructions from the user interface  51 , and is executed by the controller  50 , so that a corresponding treatment is carried out under control of the controller  50 . When the film deposition program is carried out, the controller  50  serves as a controlling unit that controls the components and parts of the ALD apparatus  80 , thereby carrying out the film deposition method. 
     Next, referring to  FIGS. 3 and 4  in addition to  FIGS. 1 and 2 , a film deposition method according to an embodiment of the present invention is explained, taking an example where the film deposition method is carried out in the ALD apparatus  80 . In addition, the NH 3  gas is used as the nitrogen-containing gas and the DCS gas is used as the silicon-containing gas. 
     First, the wafers W are loaded into the wafer boat  5 , and the wafer boat  5  is transferred into the process tube  1  by the arm  13 . The wafer boat  5  is rotated around a vertical axis. Then, the main valve MV is opened while no gas is supplied to the process tube  1  (or while the open/close valves  17   a ,  20   c , and  41   a  are closed), and a pressure controlling valve of the pressure controller PC is fully opened, so that the process tube  1  is evacuated to the lowest reachable pressure by the vacuum pump VP (Step S 31  of  FIG. 3 ). 
     After the process tube  1  is evacuated for a predetermined time period, the main valve MV is closed, and a nitrogen gas as the inert gas is supplied with its flow rate controlled by the flow rate controller  41   b  to the process tube  1  through the inert gas supplying pipe  41 L, the silicon-containing gas supplying pipe  20 L, and the silicon-containing gas distribution nozzle  22 , at Step S 32  ( FIG. 3 ). With this, a pressure within the process tube  1  is increased to, for example, 3, 4, or 5 Torr (400, 533, 667 Pa, respectively) depending on a flow rate of the nitrogen gas (or an amount of the nitrogen gas) supplied to the process tube  1 , as shown in  FIG. 4 . The pressure within the process tube  1  may be, for example, 0.05 Torr (6.67 Pa) or more. 
     When the nitrogen gas is supplied to the process tube  1  with the main valve MV is closed, the open/close valve  20   a  is opened while the open/close valve  20   c  is closed in the silicon-containing gas supplying pipe  20 L. In addition, the DCS gas is supplied with its flow rate controlled by the flow rate controller  20   b  from the silicon-containing gas source  20  to the buffer tank  180 , and thus the buffer tank  180  is filled with the DCS gas. In this case, an amount of the DCS gas filling the buffer tank  180  (or the number of DCS gas molecules) may be determined so that upper surfaces of the wafers W supported by the wafer boat  5  are covered with the DCS gas molecules, and specifically, may be determined by carrying out a preliminary experiment. 
     Next, while keeping the main valve MV closed, the open/close valve  41   a  of the inert gas supplying pipe  41 L is closed thereby stopping supplying the N 2  gas, and then, the DCS gas filling the buffer tank  180  is supplied to the process tube  1  by opening the open/close valve  20   c  at Step S 33  ( FIG. 3 ). With this, an inner space of the process tube  1  is under environment of a mixed gas of the N 2  gas and the DCS gas, and the pressure within the process tube  1  is increased depending on the amount of the DCS gas in the buffer tank  180  (see  FIG. 4 ). The DCS gas is adsorbed on the upper surfaces of the wafers W. 
     After the DOS gas is supplied to the process tube  1 , the open/close valve  20   c  is closed and the main valve MV is opened, thereby evacuating the process tube  1  to the lowest reachable pressure at Step S 34  ( FIG. 3 ). With this, the DCS gas within the process tube  1  is evacuated and the pressure within the process tube  1  is decreased as shown in  FIG. 4 . 
     Next, the open/close valve  17   a  is opened thereby supplying the NH 3  gas from the nitrogen-containing gas source  17  to the process tube  1 , and the high frequency electric power of 13.56 MHz is supplied from the high frequency power source  35  to the plasma electrodes  33 ,  33  at Step S 35 . With this, the pressure within the process tube  1  is maintained at a certain pressure depending on a flow rate of the NH 3  gas supplied to the process tube  1 , as shown in  FIG. 4 . In addition, plasma is generated from the NH 3  gas between the plasma electrodes  33 ,  33 , and thus the NH 3  gas is excited thereby generating an active species such as ions and radicals. The active species flow toward the wafers W supported by the wafer boat  3 , and react with the DCS gas adsorbed on the upper surfaces of the wafers W, thereby producing silicon nitride on the upper surfaces of the wafers W. 
     After a time period that allows the active species originating from the NH 3  gas to fully react with the DCS gas has passed, supplying the NH 3  gas is terminated, and the main valve MV of the process tube  1  is opened, thereby evacuating the process tube  1  to the lowest reachable pressure at Step S 36  ( FIG. 3 ). 
     Subsequently, the Steps S 31  through S 36  described above are repeated when the expected number of repetitions is not reached (Step S 37 : NO). On the other hand, the deposition of the silicon nitride film is terminated when the expected number of repetitions is reached (Step S 37 : YES). Specifically, after the main valve MV has been opened once thereby evacuating the process tube  1  to the lowest reachable pressure, the main valve MV is closed and the N 2  gas is supplied into the process tube  1  until the pressure within the process tube  1  is increased to the atmospheric pressure. Next, the wafer boat  5  is transferred out from the process tube  1  by the arm  13 , and the wafers W are taken out from the wafer boat  5  by a loader/unloader (not shown), and thus the film deposition process is completed. 
     Next, an experiment was carried out to deposit a silicon nitride film on a silicon wafer in accordance with the film deposition method and the results are explained with reference to  FIGS. 5 and 6 . In this experiment, the silicon nitride films were deposited on the wafers while the pressure within the process tube  1  at Step  32  ( FIG. 3 ) where the N 2  gas was supplied to the process tube  1  (or before the DCS gas was supplied to the process tube  1 ) was set to be 0.08, 2.67, 3.24, and 3.91 Torr (10.7, 356, 432, and 521 Pa, respectively). The thicknesses and thickness distributions of the silicon nitride films across the wafers were measured. 
       FIG. 5  is a graph illustrating the results of the experiment, where a horizontal axis represents a position along a diameter of the wafer in the units of mm, and a vertical axis represents a film thickness in the units of nm. As shown, when the pressure within the process tube  1  is 0.08 Torr (10.7 Pa) at Step S 32 , the silicon nitride film has a concave thickness distribution. Namely, the silicon nitride film is thinner in a center part thereof and thicker in a circumferential area. On the other hand, when the pressure within the process tube  1  at Step S 32  is 2.67, 3.24, and 3.91 Torr, the silicon nitride film has a convex thickness distribution. Namely, the silicon nitride film is thicker in the center part and thinner in the circumferential area. Namely, when the pressure within the process tube  1  is increased from 0.08 Torr to 2.67 Torr, the film thickness distribution is changed from a concave pattern to a convex pattern. Therefore, it has been confirmed that the film thickness distribution can be controlled by adjusting the pressure within the process tube  1  before supplying the DCS gas into the process tube  1 . 
     In addition,  FIG. 6  is a graph illustrating a film thickness uniformity of the silicon nitride film obtained in the experiment. As shown in  FIG. 6 , the film thickness uniformity becomes degraded as the pressure within the process tube  1  is increased from 2.67 Torr to 3.24 Torr and then to 3.91 Torr. It may be thought that this result indicates that the film thickness becomes more convexly distributed as the pressure within the process tube  1  is increased from 2.67 Torr. In addition, it may be thought from  FIG. 6  that the concave distribution is changed to the convex distribution at a pressure of about 0.5 Torr (66.7 Pa) within the process tube  1  at Step S 32 . In other words, when the pressure within the process tube  1  is in a range from 0.08 Torr to 0.5 Torr the silicon nitride film thickness is concavely distributed, and when the pressure within the process tube  1  exceeds 0.5 Torr the silicon nitride film thickness is convexly distributed. 
     An arrangement by which film thickness distribution can be controlled by the pressure within the process tube  1  before supplying the DCS gas into the process tube  1  may be understood in the following manner. 
     First, when the pressure within the process tube  1  is relatively low, the DCS gas supplied into the process tube  1  can reach a point that is relatively far away from the gas ejection holes of the silicon-containing gas distribution nozzle  22 , as shown by arrows A in an upper section of Section (a) of  FIG. 7 . This is because a mean free path of gas molecules becomes longer when the pressure within the process tube  1  is lower. In this case, if the DCS gas and the NH 3  gas are alternately supplied to the process tube  1  in the aforementioned manner without rotating the wafer boat  5 , the silicon nitride film becomes gradually thinner in a direction from a front edge near the gas ejection holes  22   a  to a distal edge of the wafer W (or along a gas flowing direction), as shown in a middle section of Section (a) of  FIG. 7 . In this situation, when the wafer boat  5  is rotated, the film thickness in a front edge area and the film thickness in a distal edge area can be offset, so that the film thicknesses in the front and the distal edge areas become substantially (the film thickness in the front edge area+the film thickness in the distal edge area)/2, which is still greater than the film thickness of the silicon nitride film in a center area of the wafer W. Therefore, the silicon nitride film thickness becomes concavely distributed, as shown in a lower section of Section (a) of  FIG. 7 . 
     On the other hand, when the pressure within the process tube  1  before the DCS gas is supplied to the process tube  1  is relatively high with the N 2  gas, the DCS gas is impeded by the nitrogen gas molecules and thus can only reach substantially halfway along the diameter of the wafer W, as shown by arrows B in an upper section of Section (b) of  FIG. 7 . In this case, if the DCS gas and the NH 3  gas are alternately supplied to the process tube  1  in the aforementioned manner without rotating the wafer boat  5 , the silicon nitride film becomes gradually thinner in the direction from the front edge to the center area of the wafer W (or along the gas flowing direction) and suddenly thinner in an area slightly beyond the center area of the wafer W, as shown in a middle section of Section (b) of  FIG. 7 . In this situation, when the wafer boat  5  is rotated, the film thickness in the front edge area and the film thickness in the distal edge area can be offset, so that the film thicknesses in the front and the distal edge areas become substantially (the film thickness in the front edge area+the film thickness in the distal edge area)/2. Here, the film thickness in the distal edge area is substantially zero; the average thickness becomes less than the film thickness of the silicon nitride film in the center area of the wafer W. Therefore, the silicon nitride film thickness becomes convexly distributed, as shown in a lower section of Section (b) of  FIG. 7 . 
     As explained above, according to the embodiment of the present invention, a thin film having a desired film thickness distribution can be obtained by the ALD method. 
     While the present invention has been described in reference to the foregoing embodiments, the present invention is not limited to the disclosed embodiments, but may be modified or altered within the scope of the accompanying claims. 
     For example, when the NH 3  gas is supplied to the process tube  1  at Step S 35 , high frequency electric power is supplied to the plasma electrodes  33 ,  33 , thereby activating the NH 3  gas to be plasma, in the above embodiment. However, the NH 3  gas may be supplied to the wafers W in the process tube  1  without utilizing the plasma in other embodiments. In this case, the NH 3  gas may be thermally decomposed by the heat of the wafers W thereby nitriding the DCS gas adsorbed on the upper surfaces of the wafers W. Even in this case, the film thickness distribution of the silicon nitride film can be controlled by the pressure within the process tube  1  before supplying the DCS gas into the process tube  1 . 
     After the main valve MV is closed, the nitrogen gas is supplied to the process tube  1  at Step S 32  in the above embodiment. In other embodiments, the nitrogen gas may be supplied to the process tube  1  while the main valve MV is kept open. In this case, when the pressure within the process tube  1  becomes a predetermined value with the nitrogen gas, supplying the nitrogen gas may be terminated and the main valve MV may be closed, and then the DCS gas is supplied to the process tube  1 . Namely, the main valve MV may be closed when the DCS gas is supplied to the process tube  1 . In addition, when the nitrogen gas is supplied to the process tube  1  while the main valve MV is kept open, the pressure within the process tube  1  may be controlled by the pressure controller PC. 
     In addition, when the nitrogen gas is supplied to the process tube  1  at Step S 32 , the buffer tank  180  may be used in the same manner as the buffer tank  180  is used for the silicon-containing gas. Namely, the nitrogen gas is supplied to the buffer tank  180  in advance and the nitrogen gas may be supplied in a single burst to the process tube  1  from the buffer tank  180  at Step S 32 . With this, the pressure within the process tube  1  rapidly becomes a predetermined value, thereby reducing a process time. 
     In addition, the DCS gas filling the buffer tank  180  is supplied to the process tube  1  at Step S 33  in the above embodiment. However, in other embodiments, the DCS gas may be supplied at a flow rate controlled by the flow rate controller  17   b  from the nitrogen-containing gas source  17  to the process tube  1  without using the buffer tank  180 . 
     Moreover, the nitrogen gas is supplied to the process tube  1  at Step S 32  in the above embodiment; a noble gas such as helium (He) gas, argon (Ar) gas or the like may be used instead of the nitrogen gas. 
     Furthermore, the present invention is applicable to a silicon oxide film deposition carried out by employing the silicon-containing gas and an oxygen-containing gas. As the oxygen-containing gas, ozone (O3) gas may be used. In addition, oxygen gas plasma may be used. 
     The pressure within the process tube  1  before supplying the silicon-containing gas is adjusted by supplying an inert gas to the process tube  1  in an embodiment of the present invention. The pressure may be determined taking into consideration a size of the process tube  1 , a kind of inert gas, source gases to be used, or the like. In addition, the pressure may be determined taking into consideration a film thickness distribution suitable for the subsequent process. A preliminary experiment or a computer simulation is preferably carried out in order to determine the pressure. 
     In addition, the ALD apparatus  80  may be provided with and a purge gas supplying nozzle that goes through the manifold  3 , and a purge gas supplying source that is connected to the purge gas supplying nozzle, in order to supply a purge gas to the process tube  1 . With such a configuration, the purge gas may be supplied to the process tube  1  after the wafer boat  5  is transferred into the process tube  1 , so that remaining air can be easily purged out from the process tube  1  with the purge gas. In addition, the DCS gas (or the NH 3  gas) supplied to the process tube  1  may be purged with the purge gas, before the NH 3  gas (or the DCS gas) is supplied to the process tube  1 . With this, the DCS gas and the NH 3  gas are efficiently impeded from being intermixed with each other within the process tube  1 , thereby assuredly realizing the ALD of the silicon nitride film. 
     Incidentally, it is preferable that the inert gas is supplied to the process tube  1  at Step S 32  ( FIG. 3 ), namely before supplying the DCS gas, through the silicon-containing gas supplying pipe  20 L and the silicon-containing gas distribution nozzle  22 . This is because a flow pattern of the inert gas in the process tube  1  is substantially the same as a flow pattern of the silicon-containing gas that is supplied to the process tube  1  after the inert gas, and thus the silicon-containing gas can reach the upper surfaces of the wafers W in the wafer boat  5  without being disturbed. If the inert gas is supplied to the process tube  1  through the purge gas supplying nozzle described above, the inert gas may excessively perturb the flow pattern of the silicon-containing gas, so that the silicon-containing gas cannot be uniformly adsorbed on the upper surfaces of the wafers W. However, the silicon-containing gas may be supplied to the process tube  1  without being disturbed after the inert gas supplied through the purge gas supplying nozzle calms down and is distributed uniformly in the process tube  1 .