Patent Publication Number: US-2022235462-A1

Title: Film forming method and film forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-009876, filed on Jan. 25, 2021, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a film formation method and a film formation apparatus. 
     BACKGROUND 
     Patent Document 1 discloses, as a method of manufacturing a semiconductor device, a technique of forming an adsorption control layer that covers the upper portion of a 3D structure formed on a substrate and forming a material layer on the adsorption control layer and on a lower portion of the 3D structure that is not covered with the adsorption control layer. 
     PRIOR ART DOCUMENT 
     [Patent Document] 
     
         
         Patent Document 1: U.S. Patent Application Publication No. 2020/0111660 
       
    
     SUMMARY 
     According to an embodiment of the present disclosure, there is provided a film forming method includes: placing a substrate on which a pattern, which includes a plurality of convex and concave portions, is formed on a stage disposed inside a chamber; and selectively forming a silicon-containing film on the plurality of convex portions of the pattern by applying a bias power to the stage and introducing microwaves into the chamber while supplying a processing gas containing a silicon-containing gas and a nitrogen-containing gas into the chamber to generate plasma, wherein the selectively forming the silicon-containing film includes a first film formation of forming a silicon-containing film around upper sides of the plurality of convex portions and a second film formation of forming a silicon-containing film on upper portions of the plurality of convex portions. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the present disclosure, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the present disclosure. 
         FIG. 1  is a cross-sectional view schematically illustrating an example of a plasma processing apparatus according to an embodiment. 
         FIG. 2  is a flowchart illustrating an example of a flow of a film forming method according to an embodiment. 
         FIG. 3  is a view illustrating an example of a substrate according to an embodiment. 
         FIG. 4  is a view illustrating film formation results of a first film forming step and a second film forming step according to an embodiment. 
         FIG. 5  is a view illustrating a film formation result of a comparative example. 
         FIG. 6  is a view illustrating an example of film formation results. 
         FIGS. 7A and 7B  are views illustrating an example of subsequent steps performed on a substrate. 
         FIG. 8  is a view illustrating film formation by microwave plasma and capacitively coupled plasma (CCP). 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of a film forming method and a film forming apparatus disclosed herein will be described in detail with reference to the drawings. The film forming method and the film forming apparatus are not limited by the embodiments. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one of ordinary skill in the art that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, systems, and components have not been described in detail so as not to unnecessarily obscure aspects of the various embodiments. 
     In recent years, with the miniaturization of semiconductor devices, the aspect ratios of patterns formed on substrates such as semiconductor wafers has tended to increase. For example, in the manufacture of 3D NANDs, since the opening widths of trenches and vias formed on substrates are narrowed, high aspect ratio structuring is progressing. When a barrier layer or a liner layer is conformally formed in such a trench or via, atomic layer deposition (ALD) is generally used. However, even if ALD film formation, which is excellent in step coverage, is used when trying to conformally form a barrier layer or a liner layer in a trench or via with a high aspect ratio structure, throughput (productivity) becomes a very significant problem because the adsorption and purge time of a raw material gas, which is longer than that of a normal ALD film forming sequence, is required. In a normal ALD film forming sequence, a film thickness is thick at the top of a trench or via and thin at the bottom, causing overhang and poor step coverage, which is problematic. 
     Therefore, as in Patent Document 1, it is conceivable to form an adsorption control layer on the upper portion of a pattern through plasma enhanced chemical vapor deposition (PECVD) and then form a material layer such as a barrier layer or a liner layer through ALD. 
     However, in the PECVD, the shape (profile) of a film cannot be controlled to a shape in which a film having good uniformity is obtained in a concave portion through film formation by the ALD. Therefore, there is a need of a technique for controlling the shape of a silicon-containing film to be formed into a shape in which a film having good uniformity is obtained in a concave portion through film formation by ALD. 
     Embodiments 
     [Configuration of Apparatus] 
     An example of a plasma processing apparatus for executing a film forming method of the present disclosure will be described.  FIG. 1  is a cross-sectional view schematically illustrating an example of a plasma processing apparatus  100  according to an embodiment. 
     The plasma processing apparatus  100  includes a chamber  101 , a stage  102 , a gas supply mechanism  103 , an exhaust mechanism  104 , a microwave plasma source  105 , and a controller  106 . 
     The chamber  101  is made of a metal material, for example, aluminum having an anodized surface, and has a substantially cylindrical shape. The chamber  101  has a plate-shaped ceiling wall  111 , a bottom wall  113 , and a side wall  112  connecting the ceiling wall  111  and the bottom wall  113  to one another. The inner wall of the chamber  101  may be coated with yttria (Y 2 O 3 ) or the like. The stage  102  is disposed inside the chamber  101 . The chamber  101  accommodates a substrate W such as a semiconductor wafer. 
     The ceiling wall  111  includes a plurality of openings into which a microwave radiation mechanism  143  and the gas introduction nozzle  123  (which will be described later) of the microwave plasma source  105  are fitted. The side wall  112  includes a carry-in/out port  114  for performing carry-in/out of the substrate W to/from a transfer chamber (not illustrated) adjacent to the chamber  101 . The carry-in/out port  114  is opened and closed by a gate valve  115 . An exhaust pipe  116  is connected to the bottom wall  113 . 
     The stage  102  is formed in a disk shape, and is made of a metal material, for example, aluminum having an anodized surface, or a ceramic material, for example, aluminum nitride (AlN). The substrate W is placed on the top surface of the stage  102 . The stage  102  is supported by a support member  120 , which is a metal cylinder extending upward from the center of the bottom of the chamber  101  via an insulating member  121 . An electrostatic chuck (not illustrated) that attracts and holds the substrate W by electrostatic force may be provided on the top surface of the stage  102 . 
     Inside the stage  102 , lifting pins (not illustrated) for raising and lowering the substrate W are provided to be capable of protruding and retracting with respect to the top surface of the stage  102 . In addition, a temperature control mechanism (not shown) including a temperature control medium flow path or a heater through which the temperature control medium flows, or both the temperature control medium flow path and the heater are provided inside the stage  102  so that the temperature of the substrate W on the stage  102  can be controlled to a predetermined temperature. 
     From the viewpoint of performing a good plasma process, the stage  102  is provided at a position at which a distance from the bottom surface of the ceiling wall  111 , which is the microwave radiation surface of the microwave radiation mechanism  143 , to the substrate W may be in the range of 40 to 200 mm. The microwave transmission frequency of the microwave radiation mechanism  143  is in the range of 300 MHz to 30 GHz, and the input power may be 500 W or more in order to maintain plasma stably. 
     A high-frequency power supply  122  for ion attraction is electrically connected to the stage  102 . When the stage  102  is made of ceramic, an electrode is provided in the stage  102 , and the high-frequency power supply  122  is electrically connected to the electrode. The high-frequency power supply  122  applies high-frequency (RF) power as the bias power to the stage  102 . The frequency of the high-frequency power applied by the high-frequency power supply  122  may be in the range of 300 KHz to 3 MHz. 
     The gas supply mechanism  103  supplies various processing gases for forming a film into the chamber  101 . The gas supply mechanism  103  includes a plurality of gas introduction nozzles  123 , a gas supply pipe  124 , and a gas supplier  125 . The gas introduction nozzles  123  are fitted into openings formed in the ceiling wall  111  of the chamber  101 . The gas supplier  125  is connected to each gas introduction nozzle  123  via the gas supply pipe  124 . The gas supplier  125  supplies various processing gases. For example, the gas supplier  125  includes sources of various processing gases including a silicon-containing gas and a nitrogen-containing gas, and supplies various processing gases including the silicon-containing gas and the nitrogen-containing gas. The gas supplier  125  may further include a noble gas source and a carbon-containing gas source, and may further supply a noble gas and a carbon-containing gas as the processing gases. In addition, the gas supplier  125  is provided with an opening/closing valve for performing supply and cutoff of a processing gas and a flow rate adjuster for adjusting the flow rate of the processing gas. 
     An exhaust pipe  116  is connected to the bottom wall  113  of the chamber  101 . The exhaust mechanism  104  is connected to the exhaust pipe  116 . The exhaust mechanism  104  includes a vacuum pump and a pressure control valve, and exhausts the interior of the chamber  101  through the exhaust pipe  116  by a vacuum pump. An internal pressure of the chamber  101  is controlled by the pressure control valve based on a value of a pressure gauge. 
     The microwave plasma source  105  is provided above the chamber  101 . The microwave plasma source  105  introduces electromagnetic waves (microwaves) into the chamber  101  to generate plasma. 
     The microwave plasma source  105  includes a microwave output part  130  and an antenna unit  140 . The antenna unit  140  includes a plurality of antenna modules. In  FIG. 1 , the antenna unit  140  includes three antenna modules. Each antenna module includes an amplifier  142  and a microwave radiation mechanism  143 . The microwave output part  130  generates microwaves, distributes microwaves, and outputs the microwaves to each antenna module. The amplifier  142  of the antenna module mainly amplifies the distributed microwaves and outputs the amplified microwaves to the microwave radiation mechanism  143 . The microwave radiation mechanism  143  is provided on the ceiling wall  111 . The microwave radiation mechanism  143  radiates the microwaves output from the amplifier  142  into the chamber  101 . 
     In  FIG. 1 , the case in which three antenna modules are provided in the antenna unit  140  has been described as an example, but the number of antenna modules is not limited. For example, six antenna modules may be provided such that the vertices of a regular hexagon are arranged in the region of the ceiling wall  111  above the stage  102 . By arranging an antenna module at the center position of the hexagon as well, seven antenna modules may be provided. 
     As long as a microwave power density can be appropriately controlled, a microwave plasma source having a single microwave introduction part having a size corresponding to a substrate W may be used. 
     The controller  106  is, for example, a computer including a processor, a storage part, an input device, a display device, and the like. The controller  106  controls each part of the plasma processing apparatus  100 . In the controller  106 , an operator may perform, for example, a command input operation in order to manage the plasma processing apparatus  100  using the input device. In addition, in the controller  106 , the operation situation of the plasma processing apparatus  100  may be visualized and displayed by the display device. Furthermore, the storage part of the controller  106  stores a control program and recipe data for controlling various processes, which are executed in the plasma processing apparatus  100 , by the processor. The processor of the controller  106  executes the control program and controls each part of the plasma processing apparatus  100  according to the recipe data, whereby desired processing is executed in the plasma processing apparatus  100 . For example, the controller  106  controls each part of the plasma processing apparatus  100  to execute the processes of the film forming method according to the embodiment. 
     As described above, the aspect ratios of patterns formed on substrates W tend to increase as semiconductor devices become finer. For example, in the manufacture of 3D NANDs, the opening widths of trenches and vias formed on substrates W are narrowed, so that high aspect ratio structuring is progressing. When a barrier layer or a liner layer is conformally formed in such a trench or via, it is common to use ALD. However, when trying to form a barrier layer or a liner layer through ALD in a trench or via with a high aspect ratio structure, the film thickness is thick at the top of the trench or via and thinner at the bottom, thereby causing overhang or poor step coverage, which is problematic. 
     Therefore, prior to the step of forming a barrier layer or a liner layer in a trench or a via through ALD, an insulating film, such as a silicon-containing film or a carbon-containing film, as an adsorption control layer is formed as a structure using the plasma processing apparatus  100  by the film forming method according to the present disclosure.  FIG. 2  is a flowchart illustrating an example of a flow of a film forming method according to an embodiment. 
     Next, a processing operation when the film forming process of the film forming method according to the embodiment is performed by the plasma processing apparatus  100  configured as described above will be described.  FIG. 2  is a flowchart illustrating an example of the flow of a film forming method according to an embodiment. 
     In the film forming method according to the embodiment, a placing step S 1  is executed. In the placing step S 1 , the substrate W is placed on the stage  102  disposed in the chamber  101 . On the substrate W, a pattern including a plurality of convex and concave portions are formed.  FIG. 3  is a view illustrating an example of the substrate W according to the embodiment.  FIG. 3  illustrates the substrate W on which a pattern including convex and concave portions are formed. Concave portions  12  between convex portions  11  of the substrate W are, for example, trenches or vias. 
     In the film forming method according to the embodiment, a film forming step S 2  is subsequently executed. In the film forming step S 2 , a bias power is applied to the stage  102 , and microwaves are introduced into the chamber  101  while supplying a processing gas containing a silicon-containing gas and a nitrogen-containing gas into the chamber  101  to generate plasma, so that a silicon-containing film is selectively formed on the plurality of convex portions  11  of the pattern. 
     The silicon-containing gas may be, for example, silane-based hydrogen gas or silane-based halogen gas, and may be any one of SiH 4 , Si 2 H 6 , and SiH 2 Cl 4 . The nitrogen-containing gas may be, for example, a hydrogen nitride-based gas of NH 3 , N 2 H 2 , N 3 H 5  or the like, such as ammonia, hydrazine, or triazane, and nitrogen gas N 2  alone. In the film forming step S 2 , another gas such as a noble gas or a carbon-containing gas may be further supplied as a processing gas. For example, by adding a small amount of a carbon-containing gas (e.g., about 0.1 to 5 [sccm] of C 2 H 6  gas), it is possible to form a SiCN film more excellent in chemical resistance (wet etching resistance) without changing a SiN shape characteristic of the film forming step S 2 . The noble gas may be a monatomic element that may be represented in Group 18 of the periodic table, and may be, for example, Ar (argon) gas or He (helium). The carbon-containing gas may be a gas of any of hydrocarbons such as C 2 H 6 , CH 4 , and C 3 H 8 . Here, an organic silane material such as trimethylsilane (TMS) (CH 3 ) 3 SiH may be used as the carbon-containing gas. 
     The internal pressure of the chamber  101  in the film forming step S 2  may be 1.5 Torr or less, specifically 50 to 100 m Torr. Flow rates of the various processing gases are those obtained in advance so that the film quality of a silicon-containing film to be formed is optimized. 
     The film forming step S 2  includes a first film forming step S 21  and a second film forming step S 22 . In the first film forming step S 21 , a silicon-containing film is formed around the upper sides of the plurality of convex portions  11  of the pattern. In the second film forming step S 22 , a silicon-containing film is formed on the upper portions of the plurality of convex portions  11  of the pattern. 
     In the film forming step S 2 , the second film forming step S 22  may be executed after the first film forming step S 21 , or the first film forming step S 21  may be executed after the second film forming step S 22 . In the film forming step S 2 , the first film forming step S 21  and the second film forming step S 22  may be alternately executed multiple times. 
     In the film forming step S 2 , ⅚ to ½ of the total film thickness of the silicon-containing film to be formed is formed through the first film forming step S 21 , and the rest of the total film thickness is formed through the second film forming step  22 . Here, the basis for setting the film thickness in the first film forming step S 21  to ⅚ to ½ of the total film thickness and for forming the film of the rest thickness in the second film forming step S 22  is that the optimum distribution was derived from an experiment based on optimum three-dimensional formation as the “adsorption control layer” in the film forming step S 2 . The most preferable distribution is close to ½, which is the same for both the first film forming step S 21  and the second film forming step S 22 , but even if the thickness of the film formed through the first film forming step S 21  occupies ⅚ of the total film thickness, it is possible to form a three-dimensional object as the desired “adsorption control layer”. 
     Hereinafter, the first film forming step S 21  and the second film forming step S 22  will be described in more detail. 
     In the first film forming step S 21 , a silicon-containing film is formed under at least one condition selected from the group consisting of a higher bias power than that in the second film forming step S 22  is applied to the stage  102  and the refractive index (RI) is higher than that in the second film forming step S 22 . 
     First, the case of changing the bias power will be described. In the first film forming step S 21 , a silicon-containing film is formed by applying a higher bias power than that in the second film forming step S 22  to the stage  102 . For example, in the first film forming step S 21 , a bias power of 200 W to 1,000 W is applied to the stage  102 . In the second film forming step S 22 , a bias power of 0 to 200 W is applied to the stage  102 . 
       FIG. 4  is a view illustrating film formation results of the first film forming step S 21  and the second film forming step S 22  according to the embodiment.  FIG. 4  illustrates an enlarged view of a portion around the upper periphery of a convex portion  11  formed on the substrate W.  FIG. 4  illustrates a case in which a SiN film  20  as the silicon-containing film is formed when the silicon-containing gas is SiH 4  gas, the nitrogen-containing gas is NH 3  gas, and the second film forming step S 22  is performed after the first film forming step S 21 . In the first film forming step S 21  and the second film forming step S 22 , as the processing conditions, only the bias power applied to the stage  102  is changed, and the other condition is the same. In the first film forming step S 21 , the bias power is set to 300 to 600 W. In the second film forming step S 22 , the bias power is set to 0 to 100 W.  FIG. 4  illustrates the region of a SiN film  20   a  formed through the first film forming step S 21  and the region of a SiN film  20   b  formed through the second film forming step S 22  separately. In the first film forming step S 21 , the SiN film  20   a  is selectively formed around the upper side of the convex portion  11 . In the second film forming step S 22 , the SiN film  20   b  is selectively formed on the upper portion of the convex portion  11  on which the SiN film  20   a  is formed. 
     In the second film forming step S 2 , the shape of the SiN film  20  formed by the SiN film  20   a  and the SiN film  20   b  can be controlled by forming the SiN film  20   a  and the SiN film  20   b  in the first film forming step S 21  and the second film forming step S 22 . The SiN film  20  protrudes laterally (toward the adjacent concave portions  12 ) around the upper portion of the convex portion  11  and has an overhanging shape. In  FIG. 4 , the thickness of the SiN film  20  formed on the top surface of the convex portion  11  (top thickness) is indicated as a thickness TH. In addition, the most laterally protruding portion of the SiN film  20  is indicated as an overhang point  21 . Further, the depth from the upper surface of the SiN film  20  to the position of the overhang point  21  (overhang depth) is indicated as OD. For example, by forming the SiN film  20  in the first film forming step S 21  and the second film forming step S 22 , the depth OD of the overhang point  21  can be increased, and the position of the overhang point  21  of the SiN film  20  can be lowered. As a result, for example, the position of the overhang point  21  of the SiN film  20  can be made lower than the position of the top surface of the convex portion  11 . Here, the significance (advantage) of deepening the OD will be explained. Generally, in PECVD performed without the two-step film formation step according to the present proposal, the OD cannot be formed deeply unless a strong RF bias is applied by a high-frequency power source connected to the stage  102 . However, when an RF bias is strongly applied, even if the OD can be formed deeply, erosion (damage) may occur in the SiN film  20  due to a sputtering effect as a side effect, or the SiN film  20  may be formed to a depth of a concave portion of a base pattern. Thus, an optimum shape as a desired “adsorption control layer” cannot be obtained. Therefore, the ability to form the OD deeply is one of the indicators of the required performance of the “adsorption control layer”. 
     Here, as a comparative example, the film forming step S 2  will be described with reference to a case in which a film is formed only using the first film forming step S 21 .  FIG. 5  is a diagram illustrating a film formation result of a comparative example.  FIG. 5  illustrates the result of forming the SiN film  20   a  through the first film forming step S 21  until the thickness TH is equivalent to that of the SiN film  20  of  FIG. 4 . The most laterally protruding portion of the SiN film  20   a  is indicated as an overhang point  22 . In the first film forming step S 21 , since the SiN film  20   a  is formed around the upper side of the convex portion  11 , the depth OD of the overhang point  22  from the top surface of the SiN film  20   a  becomes shallower. As a result, the position of the overhang point  22  of the SiN film  20   a  is higher than that of the overhang point  21  of the SiN film  20  of  FIG. 4 . Here, since the depth OD affects the margin (dimensional margin) of the processed shape after the execution of CMP or the like in the subsequent process, it is ideally desirable to deepen the depth OD to the same position as the TH. 
     The processing conditions such as the types and flow rates of processing gases, and the bias powers in the first film forming step S 21  and the second film forming step S 22  are as follows. 
     &lt;First Film Forming Step S 21 &gt; 
     
         
         
           
             Processing gas
           SiH 4  gas: 3 to 60 sccm (specifically, 7 to 20 sccm)   NH 3  gas: 4 to 100 sccm (specifically, 9 to 40 sccm)   Inert gas for dilution: Ar gas: 50 to 1,000 sccm (specifically, 50 to 300 sccm)
               Microwave power (860 MHz): 1,500 to 10,000 W (specifically, 2,500 to 5,000 W)   Bias power (400 KHz): 200 to 1,000 W (specifically, 300 to 600 W)   
               
         
           
         
       
    
     &lt;Second Film Forming Step S 22 &gt; 
     
         
         
           
             Processing gas
           SiH 4  gas: 3 to 60 sccm (specifically, 7 to 20 sccm)   NH 3  gas: 4 to 100 sccm (specifically, 9 to 40 sccm)   Inert gas for dilution: Ar gas: 50 to 1,000 sccm (specifically, 50 to 300 sccm)
               Microwave power (860 MHz): 1,500 to 10,000 W (specifically, 2,500 to 5,000 W)   Bias power (400 KHz): 0 to 200 W (specifically, 0 to 100 W)   
               
         
           
         
       
    
       FIG. 6  is a view illustrating an example of film formation results. Comparative Example 1 illustrates a case in which a SiN film was formed to have a thickness of 150 Å only through the second film forming step S 22 . Comparative Example 2 illustrates a case in which a SiN film was formed to have a thickness of 150 Å only through the first film forming step S 21 . Example 1 illustrates a case in which a SiN film was formed to have a thickness of 125 Å through the first film forming step S 21 , and then a SiN film was formed to have a thickness of 25 Å through the second film forming step S 22 , thereby forming a SiN film having a total thickness of 150 Å. Example 2 illustrates a case in which a SiN film was formed to have a thickness of 75 Å through the first film forming step S 21 , and then a SiN film was formed to have a thickness of 75 Å through the second film forming step S 22 , thereby forming a SiN film having a total thickness of 150 Å. “Overhang position OD (nm)” indicates the depth from the top surface of each SiN film to the position of the overhang point. As illustrated in  FIG. 6 , the “overhang position OD (nm)” is 10 nm in Comparative Example 1, 13 nm in Comparative Example 2, and 15 nm in Examples 1 and 2. As described above, in Examples 1 and 2, the depth to the overhang point is increased compared with Comparative Examples 1 and 2. 
     In  FIG. 6 , the SiN film was formed by performing the second film forming step S 22  after the first film forming step S 21 . However, even when the first film forming step S 21  is executed after the second film forming step S 22  is executed first, the result that the depth to the position of the overhang point is similarly increased can be obtained. That is, the first film forming step S 21  may be performed after the second film forming step S 22 . In addition, the first film forming step S 21  and the second film forming step S 22  may be alternately performed multiple times. 
     Next, a case in which the refractive index (RI) of the silicon-containing film to be formed is changed will be described. In the first film forming step S 21  and the second film forming step S 22 , the refractive index of the silicon-containing film is controlled by the flow rate ratio of the silicon-containing gas to the nitrogen-containing gas. For example, in the first film forming step S 21 , the flow rate ratio (A/B) of the silicon-containing gas: A to the nitrogen-containing gas: B is set to 0.5 to 0.8. In the second film forming step S 22 , the flow rate ratio (A/B) of the silicon-containing gas: A and the nitrogen-containing gas: B is set to 0.8 to 1.0. 
     The bias power applied to the stage  102  is the same in the first film forming step S 21  and the second film forming step S 22 . For example, in the first film forming step S 21  and the second film forming step S 22 , a bias power of 0 W to 1,000 W is applied to the stage  102 . The bias power applied to the stage  102  may be changed. For example, as described above, in the first film forming step S 21 , a higher bias power may be applied to the stage  102  than that in the second film forming step S 22  to form a silicon-containing film. 
     By forming a silicon-containing film having a low refractive index in the second film forming step S 22 , the insulating property (a leak characteristic) of the silicon-containing film can be improved. However, the silicon-containing film having a low refractive index tends to have a shallower depth to the overhang point than the silicon-containing film having a high refractive index. 
     Therefore, in the film forming step S 2 , the silicon-containing film is formed under the conditions where the flow rate ratio of the silicon-containing gas to the nitrogen-containing gas is changed between the first film forming step S 21  and the second film forming step S 22  and where the refractive index is higher in the first film forming step S 21  than in the second film forming step S 22 . For example, after forming a SiN film  20   a  having a refractive index of 1.95 to 2.05 through the first film forming step S 21  using SiH 4  gas as the silicon-containing gas and NH 3  gas as the nitrogen-containing gas, a SiN film  20   b  having a refractive index of 2.05 or more is formed through the second film forming step S 22 . As a result, the SiN film  20  formed of the SiN film  20   a  and the SiN film  20   b  can be formed as described above with reference to  FIG. 4 . 
     The processing conditions such as the types and flow rates of processing gases, and the bias powers in the first film forming step S 21  and the second film forming step S 22  are as follows. 
     &lt;First Film Forming Step S 21 &gt; 
     
         
         
           
             Processing gas
           SiH 4  gas: 3 to 60 sccm (specifically, 7 to 20 sccm)   NH 3  gas: 4 to 100 sccm (specifically, 9 to 40 sccm)   Inert gas for dilution: Ar gas: 50 to 1,000 sccm (specifically, 50 to 300 sccm)
               Microwave power (860 MHz): 1,500 to 10,000 W (specifically, 2,500 to 5,000 W)   Bias power (400 kHz): 200 to 1,000 W (specifically, 300 to 600 W)   
               
         
           
         
       
    
     &lt;Second Film Forming Step S 22 &gt; 
     
         
         
           
             Processing gas
           SiH 4  gas: 3 to 60 sccm (specifically, 7 to 20 sccm, same as in the first film forming step S 21 )   NH 3  gas: 4 to 100 sccm (specifically, 9 to 40 sccm, same as in the first film forming step S 21 )   Inert gas for dilution: Ar gas: 50 to 1,000 sccm (specifically, 50 to 300 sccm)
               Bias power (400 kHz): 200 to 1,000 W (specifically, 300 to 600 W)   
               
         
           
         
       
    
     In the second film forming step S 22 , by increasing the supply amount of the NH 3  gas compared with the first film forming step S 21 , the flow rate ratio of the SiH 4  gas to the NH 3  gas is changed, and the refractive index of the SiN film to be formed is changed to be high. As a result, the depth of the SiN film formed on the convex portion to the overhang point increases. 
     The substrate W on which the silicon-containing film is formed through the film forming method according to the embodiment in the plasma processing apparatus  100  is carried out of the plasma processing apparatus  100 , and subsequent steps, such as film formation through an ALD, are carried out. 
       FIGS. 7A and 7B  are views illustrating an example of subsequent steps performed on the substrate W.  FIG. 7A  illustrates the substrate W on which a SiN film  20  is formed as a silicon-containing film on the convex portions  11  through the film forming method according to the embodiment. The substrate W on which the SiN film  20  is formed is polished from the top surface side through, for example, chemical mechanical polishing (CMP) or the like. For example, the substrate W is polished to the position of the dotted line below the top surface of the convex portion  11  such that the top surfaces of the convex portions  11  are exposed.  FIG. 7B  illustrates a polished substrate W. Since the SiN film  20  has a deep overhang point, the SiN film may remain thick on the side surfaces of the convex portions  11  and may narrow the openings of the concave portions  12 . In the subsequent step, film formation is executed through ALD on the substrate W in which the openings of the concave portions  12  are narrowed in this way. 
     Conventionally, an ALD process has a problem in large processing capacity (throughput) in a process time for film formation. Thus, in order to reduce the process time for film formation, the ALD process is executed in the state in which the concentration of an ALD precursor is increased. Theoretically, there is no problem if the ALD reaction is ideal and the ALD precursor is only chemically adsorbed to the surface of the material, so that a surplus ALD precursor is not physically adsorbed to the surface to be adsorbed after saturated chemical adsorption. However, in many cases, some of the ALD precursor that has entered deep inside into the underlayer concave portions is physically adsorbed. As a result, when ALD is performed on a substrate W that does not have a SiN film  20 , a high-concentration ALD precursor penetrates from the opening sides of the concave portions  12 . Thus, the film thickness is thicker in the upper portions of the concave portions  12  and thinner in the lower portions of the concave portions  12 , thereby causing overhangs and poor step coverage. Therefore, it is conceivable to reduce the concentration of the ALD precursor, but the process time for film formation increases and the productivity decreases. 
     Meanwhile, when the ALD process is performed on the substrate W of  FIG. 7B , even if a high-concentration ALD precursor is generated, the ALD precursor that penetrates into the concave portions  12  is reduced by narrowing the openings of the upper portions of the concave portions  12  so that the density of the effective ALD precursor that penetrates into the concave portions  12  can be reduced. Thus, a film can be formed with good step coverage. 
     The substrate W subjected to the ALD process is polished from the top surface side through, for example, CMP or the like to remove the SiN film  20 . 
     In the embodiment, a silicon-containing film is formed by using a microwave plasma type plasma processing apparatus  100  that generates plasma by microwaves. However, it is also conceivable to form a silicon-containing film by using a capacitively coupled plasma (CCP) type plasma processing apparatus using parallel flat plates. However, in the CCP type plasma processing apparatus, the ion energy of plasma is high, and a silicon-containing film may be formed deep in the concave portions  12 . 
       FIG. 8  is a view illustrating film formation by microwave plasma and CCP.  FIG. 8  illustrates cases in each of which a pattern of ion energy of plasma in a concave portion  12  when a SiN film  20  is formed on the substrate W by microwave plasma and CCP. In addition,  FIG. 8  schematically illustrates the shapes of the SiN films  20 , each of which is formed on a side wall of a concave portion  12  by microwave plasma and CCP. Since the CCP has a high plasma ion energy, a SiN film  20  is formed deep into a concave portion  12 . Meanwhile, it is possible to reduce the ion energy of microwave plasma compared with that of CCP, and thus it is possible to reduce the range in which the SiN film  20  is formed. As a result, it is possible to form a film from a shallow position in the concave portion  12  in the subsequent step. In addition, for example, when the substrate W is polished through CMP or the like to remove the SiN film  20 , it is possible to reduce the polishing depth. 
     In the above-described embodiments, the case in which a SiN film is formed as the silicon-containing film has been described as an example, but the silicon-containing film to be formed is not limited to the SiN film. For example, a carbon-containing gas, such as a C 2 H 6  gas or a TMS (CH 3 ) 3 SiH gas, may be added to the processing gas to form a SiCN film as the silicon-containing film. In addition, for example, a diborane (B 2 H 6 ) gas may be added to the processing gas to form a SiBN film as the silicon-containing film. For example, by adding a small amount of carbon-containing gas (e.g., about 0.1 to 10 sccm of C 2 H 6  gas), it is possible to form a SiCN film that is more excellent in chemical resistance (HF wet etching resistance) without changing a film shape characteristic of the film forming step S 2  (film number  20 ). Similarly, by adding a small amount of diborane (B 2 H 6 ) gas (e.g., about 0.1 to 15 sccm), the effect of improving the chemical resistance to LAL buffered hydrofluoric acid (mixed solution of NH 4 F and HF) is obtained without changing the film shape characteristic of the film (film number  20 ). 
     As described above, the film forming method according to the embodiment includes the placing step S 1  and the film forming step S 2 . In the placing step S 1 , the substrate W on which a pattern including a plurality of convex and concave portions is formed is placed on the stage  102  disposed in the chamber  101 . In the film forming step S 2 , a bias power is applied to the stage  102 , and microwaves are introduced into the chamber  101  while supplying a processing gas containing a silicon-containing gas and a nitrogen-containing gas into the chamber  101  to generate plasma, so that a silicon-containing film is selectively formed on the plurality of convex portions  11  of the pattern. The film forming step S 2  includes the first film forming step S 21  and the second film forming step S 22 . In the first film forming step S 21 , a silicon-containing film is formed around the upper sides of the plurality of convex portions  11 . In the second film forming step S 22 , a silicon-containing film is formed on the upper portions of the plurality of convex portions  11 . As a result, in the film forming method according to the embodiment, the shape of a silicon-containing film to be formed can be controlled to a shape in which a film having good uniformity is obtained in a concave portion through film formation by ALD. 
     In the first film forming step S 21 , a silicon-containing film is formed under at least one condition selected from the group consisting of a higher bias power than that in the second film forming step S 22  is applied to the stage  102  and the refractive index is higher than that in the second film forming step S 22 . As a result, in the film forming method according to the embodiment, the position of the overhang point  21  of the silicon-containing film to be formed can be deepened. 
     In the first film forming step S 21 , a bias power of 200 W to 600 W is applied to the stage  102 . In the second film forming step S 22 , a bias power of 0 to 200 W is applied to the stage  102 . As a result, in the film forming method according to the embodiment, the position of the overhang point  21  of the silicon-containing film to be formed can be deepened by changing the bias power in this way between the first film forming step S 21  and the second film forming step S 22 . 
     In the first film forming step S 21 , a silicon-containing film having a refractive index of 1.95 to 2.05 is formed. In the second film forming step S 22 , a silicon-containing film having a refractive index of 2.05 or more is formed. As a result, in the film forming method according to the embodiment, the position of the overhang point  21  of the silicon-containing film to be formed can be deepened by changing the refractive indices of the silicon-containing films formed in the first film forming step S 21  and the second film forming step S 22  as described above. 
     In the film forming step S 2 , ⅚ to ½ of the total film thickness of the silicon-containing film to be formed is formed through the first film forming step S 21 , and the rest of the total film thickness is formed through the film forming step S 22 . As a result, in the film forming method according to the embodiment, a silicon-containing film to be formed can be formed in a shape in which a film having good uniformity is obtained in a concave portion through film formation by ALD. 
     Although embodiments have been described above, it should be considered that the embodiments disclosed herein are illustrative and are not restrictive in all respects. Indeed, the above-described embodiments can be implemented in a variety of forms. In addition, the above-described embodiments may be omitted, replaced, or modified in various forms without departing from the scope and spirit of the claims. 
     For example, in the above-described embodiments, although the case in which the substrate W is a semiconductor wafer has been described as an example, the present disclosure is not limited thereto. In addition, as a pattern including a plurality of convex and concave portions, a structure having mainly vertical convex and concave portions, such as vias and trenches has been described as an example, but the present disclosure is not limited thereto. For example, the present disclosure is also useful for a substrate having a pattern including convex and concave portions in the horizontal direction in addition to the vertical direction, such as a 3D structure. 
     According to the present disclosure, a shape of a silicon-containing film to be formed can be controlled to a shape in which a film having good uniformity is obtained in a concave portion through film formation by ALD. 
     It should be understood that the embodiments disclosed herein are examples in all respects and are not restrictive. Indeed, the above-described embodiments can be implemented in various forms. The embodiments described above may be omitted, replaced, or modified in various forms without departing from the scope and spirit of the appended claims.