Patent Publication Number: US-11393673-B2

Title: Deposition method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based upon and claims the benefit of priority of Japanese Patent Application No. 2019-094832, filed on May 20, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a deposition method. 
     BACKGROUND 
     There is known a method in which a first reaction gas and a second reaction gas that react with each other are alternately supplied into a recessed portion formed in a substrate, to deposit a reaction product of the first reaction gas and the second reaction gas in the recessed portion (see, for example, Patent Document 1). In this method, prior to supplying the first reaction gas, a step of causing a hydroxyl group to be adsorbed by a desired distribution onto the inner surface of the recessed portion formed in the substrate, is performed. Further, as an example of causing the hydroxyl group to be adsorbed by a desired distribution, Patent Document 1 describes an example in which the substrate is exposed to an oxygen plasma generated from a gas including a hydrogen-including gas to supplement an insufficient hydroxyl group, in the step of adsorbing the hydroxyl group.
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2013-135154   

     SUMMARY 
     In view of the above, an aspect of the present disclosure relates to a technology of depositing a silicon oxide film having a good film quality on a substrate, while preventing the oxidation of the substrate. 
     According to one aspect of the present invention, there is provided a deposition method including a first process performed by repeating causing aminosilane gas to be adsorbed on a substrate; causing a first silicon oxide film to be stacked on the substrate by supplying oxidation gas to the substrate to oxidize the aminosilane gas adsorbed on the substrate; and performing a reforming process on the first silicon oxide film by activating a first reformed gas by plasma and supplying the first reformed gas to the first silicon oxide film, and a second process, performed after the first process, by repeating causing aminosilane gas to be adsorbed on the substrate; causing a second silicon oxide film to be stacked on the substrate by supplying oxidation gas to the substrate to oxidize the aminosilane gas adsorbed on the substrate; and performing a reforming process on the second silicon oxide film by activating a second reformed gas by plasma and supplying the second reformed gas to the second silicon oxide film, wherein the first reformed gas has a smaller effect of oxidizing the substrate than the second reformed gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an example of a configuration of a deposition, apparatus according to one embodiment; 
         FIG. 2  is a perspective view of the configuration in a vacuum vessel of the deposition apparatus of  FIG. 1 ; 
         FIG. 3  is a plan view illustrating a configuration in the vacuum vessel of the deposition apparatus of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of the vacuum vessel along a concentric circle of a rotation table rotatably provided in the vacuum vessel of the deposition apparatus of  FIG. 1 ; 
         FIG. 5  is another cross-sectional view of the deposition apparatus of  FIG. 1 ; 
         FIG. 6  is a cross-sectional view of a plasma source provided in the deposition apparatus of  FIG. 1 ; 
         FIG. 7  is another cross-sectional view of the plasma source provided in the deposition apparatus of  FIG. 1 ; 
         FIG. 8  is a top view of the plasma source provided in the deposition apparatus of  FIG. 1 ; 
         FIG. 9  is a flow chart illustrating a deposition method according to one embodiment; 
         FIGS. 10A to 10H  are diagrams illustrating a first process of the deposition method according to one embodiment; 
         FIGS. 11A to 11G  are diagrams illustrating a second process of the deposition method according to one embodiment; 
         FIG. 12  is a diagram for explaining a method for evaluating the wafer oxidation amount; 
         FIG. 13  is a diagram illustrating a relationship between the type of reformed gas and the wafer oxidation amount; and 
         FIG. 14  is a diagram illustrating the relationship between the type of reformed gas and the wet etching rate. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, non-limiting exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings. In all the accompanying drawings, the same or corresponding reference numerals will be applied to the same or corresponding members or components, and overlapping descriptions will be omitted. 
     (Deposition Apparatus) 
     A deposition apparatus suitable for carrying out a deposition method according to one embodiment will be described. Referring to  FIGS. 1 to 3 , a deposition apparatus includes a flat vacuum vessel  1  having a substantially circular planar shape, and a rotation table  2 , which is disposed in the vacuum vessel  1  and which has a rotational center at the center of the vacuum vessel  1 . The vacuum vessel  1  has a vessel body  12  having a cylindrical shape with a bottom and a top plate  11  which is disposed in a detachable manner with respect to the top surface of the vessel body  12  in an airtight manner through a sealing member  13  ( FIG. 1 ), such as an O-ring, for example. 
     The rotation table  2  is fixed to a cylindrical core portion  21  at the center. The core portion  21  is fixed to the upper end of a rotating shaft  22  extending in a vertical direction. The rotating shaft  22  passes through a bottom portion  14  of the vacuum vessel  1 , and a lower end of the rotating shaft  22  is attached to a driving unit  23  which rotates the rotating shaft  22  ( FIG. 1 ) about a vertical axis. The rotating shaft  22  and the driving unit  23  are accommodated in a cylindrical case body  20  having an open top surface. A flange portion provided on the upper surface of the case body  20  is attached in an airtight manner to the lower surface of the bottom portion  14  of the vacuum vessel  1 , and the internal atmosphere of the case body  20  are maintained in an airtight state with respect to the external atmosphere. 
     The surface of the rotation table  2  is provided with circular recessed portions  24  for mounting semiconductor wafers (hereinafter, referred to as a “wafer W”) that are a plurality of (five sheets in the example illustrated) substrates, along the rotational direction (the circumferential direction) as illustrated in  FIGS. 2 and 3 . Note that as a matter of convenience, in  FIG. 3 , the wafer W is illustrated only in one recessed portion  24 . The recessed portion  24  has an inner diameter that is greater than the diameter of the wafer W by, for example, 4 mm, and has a depth that is approximately equal to the thickness of the wafer W. Therefore, when the wafer W is accommodated in the recessed portion  24 , the surface of the wafer W and the surface of the rotation table  2  (the region in which the wafer W is not mounted) will have the same height. In the bottom surface of the recessed portion  24 , through holes (not illustrated) are formed, through which, for example, three raising/lowering pins are penetrated to raise and lower the wafer W while supporting the back surface of the wafer W. 
       FIGS. 2 and 3  illustrate the structure within the vacuum vessel  1 , and as a matter of convenience for explanation, the top plate  11  is not illustrated. As illustrated in  FIGS. 2 and 3 , above the rotation table  2 , reaction gas nozzles  31 ,  32 , and  33  and separation gas nozzles  41  and  42  made of, for example, quartz, are disposed in a circumferential direction of the vacuum vessel  1  (the rotational direction of the rotation table  2  illustrated by an arrow A of  FIG. 3 ) so as to be spaced apart from each other. In the example illustrated, the reaction gas nozzle  33 , the separation gas nozzle  41 , the reaction gas nozzle  31 , the separation gas nozzle  42 , and the reaction gas nozzle  32  are arranged in the stated order in a clockwise manner (rotational direction of the rotation table  2 ) from a conveying port  15  to be described below. The reaction gas nozzles  31 ,  32 , and  33  and the separation gas nozzles  41  and  42  respectively have base end parts that are gas introduction ports  31   a ,  32   a ,  33   a ,  41   a , and  42   a  ( FIG. 3 ), which are fixed to the outer peripheral wall of the vessel body  12 . The reaction gas nozzles  31 ,  32 , and  33  and the separation gas nozzles  41  and  42  are introduced from the outer peripheral wall of the vacuum vessel  1  into the vacuum vessel  1  and are attached along the radial direction of the vessel body  12  so as to extend horizontally with respect to the rotation table  2 . 
     Note that above the reaction gas nozzle  33  in  FIG. 3 , a plasma source  80  is provided, as indicated in a simplified manner by dashed lines. The plasma source  80  will be described later. 
     The reaction gas nozzle  31  is connected to a supply source (not illustrated) of aminosilane gas via piping, a flow control device, and the like (not illustrated). For example, as the aminosilane gas, DIPAS [diisopropylaminosilane], 3DMAS [tris-dimethylaminosilane] gas, and BTBAS [(bistertial butylamino)silane] may be used. 
     The reaction gas nozzle  32  is connected to a supply source (not illustrated) of oxidation gas via piping, a flow control device, and the like (not illustrated). For example, an ozone (O 3 ) gas may be used as the oxidation gas. 
     The reaction gas nozzle  33  is connected to a supply source (not illustrated) of reformed gas via piping, a flow control device, and the like (not illustrated). For example, as the reformed gas, argon (Ar) gas, helium (He) gas, and oxygen (O 2 ) gas may be used. 
     The separation gas nozzles  41  and  42  are connected to a supply source (not illustrated) of separation gas via piping, flow control valves, and the like (not illustrated). For example, as the separation gas, Ar gas and nitrogen (N 2 ) gas may be used. 
     In the reaction gas nozzles  31  and  32 , a plurality of discharge holes  31   h  and  32   h  ( FIG. 4 ) that open toward the rotation table  2 , are arranged along the length direction of the reaction gas nozzles  31  and  32 , respectively, for example, with intervals of 10 mm. The lower region of the reaction gas nozzle  31  is an aminosilane gas adsorption region P 1  for causing the aminosilane gas to be adsorbed onto the wafer W. The lower region of the reaction gas nozzle  32  is an oxidation gas supply region P 2  which oxidizes the aminosilane gas adsorbed on the wafer W in the aminosilane gas adsorption region P 1 . Note that the configuration of the reaction gas nozzle  33 , which is not illustrated in  FIG. 4 , will be described later. 
     Referring to  FIGS. 2 and 3 , two projecting portions  4  are provided in the vacuum vessel  1 . The projecting portion  4  constitutes a separation region D together with the separation gas nozzles  41  and  42 , and is attached to the back surface of the top plate  11  so as to protrude toward the rotation table  2  as described later. The projecting portion  4  has a fan-like planar shape in which the top portion is cut in an arc-like manner, and in one embodiment, the inner arc is connected to a protrusion  5  (described below) and the outer arc is disposed along the inner peripheral surface of the vessel body  12  of the vacuum vessel  1 . 
       FIG. 4  illustrates a cross-section of the vacuum vessel  1  along the concentric circle of the rotation table  2  from the reaction gas nozzle  31  to the reaction gas nozzle  32 . As illustrated in  FIG. 4 , the projecting portion  4  is attached on the back surface of the top plate  11 . Therefore, within the vacuum vessel  1 , there exists a flat low ceiling surface (first ceiling surface  44 ) that is a lower surface of the projecting portion  4  and a ceiling surface (second ceiling surface  45 ) that is higher than the first ceiling surface  44  and that is located on both sides of the first ceiling surface  44  in the circumferential direction. The first ceiling surface  44  has a fan-like planar shape with the top part cut into an arc-shape. As illustrated, the projecting portion  4  is provided with a radially extending groove portion  43  in the center in the circumferential direction, and the separation gas nozzle  42  is accommodated within the groove portion  43 . Similarly, in the other projecting portion  4 , the groove portion  43  is formed, and the separation gas nozzle  41  is accommodated in the groove portion  43 . Further, the reaction gas nozzles  31  and  32  are provided in the space below the second ceiling surface  45 . These reaction gas nozzles  31  and  32  are provided in the vicinity of the wafer W so as to be spaced apart from the second ceiling surface  45 . As illustrated in  FIG. 4 , the reaction gas nozzle  31  is provided in a space  481  below the second ceiling surface  45  on the right side of the projecting portion  4 , and the reaction gas nozzle  32  is provided in a space  482  below the second ceiling surface  45  on the left side of the projecting portion  4 . 
     In the separation gas nozzle  42  accommodated in the groove portion  43  of the projecting portion  4 , a plurality of discharge holes  42   h  (see  FIG. 4 ) that open toward the rotation table  2  are arranged, along the length direction of the separation gas nozzle  42 , with intervals of, for example, 10 mm. Similarly, in the separation gas nozzle  41  accommodated in the groove portion  43  of the other projecting portion  4 , a plurality of discharge holes  41   h  that open toward the rotation table  2  are arranged, along the length direction of the separation gas nozzle  41 , with intervals of, for example, 10 mm. 
     The first ceiling surface  44  forms a separation space H, which is a narrow space, with respect, to the rotation table  2 . When Ar gas is supplied from the discharge holes  42   h  of the separation gas nozzle  42 , the Ar gas flows through the separation space H toward the spaces  481  and  482 . At this time, the capacity of the separation space H is smaller than the capacity of the spaces  481  and  482 , and, therefore, the pressure of the separation space H can be increased, by the Ar gas, compared to the pressure of the spaces  481  and  482 . That is, the separation space H having high pressure is formed between the spaces  481  and  482 . Further, the Ar gas flowing from the separation space H into the spaces  481  and  482  also serves as a counter-flow with respect to the aminosilane gas from the aminosilane gas adsorption region P 1  and the oxidation gas from the oxidation gas supply region P 2 . Thus, the aminosilane gas from the aminosilane gas adsorption region P 1  and the oxidation gas from the oxidation gas supply region P 2  are separated by the separation space H. Therefore, in the vacuum vessel  1 , the aminosilane gas and the oxidation gas are prevented from mixing with each other and reacting with each other. 
     A height h 1  of the first ceiling surface  44  relative to the upper surface of the rotation table  2  is set to a height suitable for increasing the pressure of the separation space H compared to the pressure of the spaces  481  and  482 , in consideration of the pressure in the vacuum vessel  1 , the rotational speed of the rotation table  2 , the flow rate of the separation gas (Ar gas), etc., at the time of deposition. 
     On the other hand, on the lower surface of the top plate  11 , the protrusion  5  ( FIGS. 2 and 3 ) that surrounds the outer circumference of the core portion  21  that fixes the rotation table  2  is provided. In one embodiment, the protrusion  5  is continuous with a portion of the projecting portion  4  on the side of the rotational center, and the lower surface of the protrusion  5  is formed at the same height as the first ceiling surface  44 . 
       FIG. 1  referred to above is a cross-sectional view along the I-I′ line of  FIG. 3 , and illustrates the region where the second ceiling surface  45  is provided.  FIG. 5  is a cross-sectional view illustrating a region where the first ceiling surface  44  is provided. As illustrated in  FIG. 5 , a bent portion  46  that bends in an L-shape facing the outer end surface of the rotation table  2 , is formed at the periphery of the fan-shaped projecting portion  4  (a portion on the outer edge of the vacuum vessel  1 ). Similar to the projecting portion  4 , the bent portion  46  prevents the reaction gas from entering from both sides of the separation region D, thereby preventing the mixing of the aminosilane gas and the oxidation gas. The fan-shaped projecting portion  4  is provided on the top plate  11  and the top plate  11  can be removed from the vessel body  12 , and, therefore, there is a slight gap between the outer peripheral surface of the bent portion  46  and the vessel body  12 . The gap between the inner peripheral surface of the bent portion  46  and the outer end surface of the rotation table  2  and the gap between the outer peripheral surface of the bent portion  46  and the vessel body  12  are set, for example, to a dimension similar to the height of the first ceiling surface  44  relative to the upper surface of the rotation table  2 . 
     The inner peripheral wall of the vessel body  12  is formed in a vertical plane in proximity with the outer peripheral surface of the bent portion  46  in the separation region D ( FIG. 5 ), but in portions other than the separation region D, for example, the inner peripheral wall is recessed outwardly from the portion facing the outer end surface of the rotation table  2  to the bottom portion  14  ( FIG. 1 ). Hereinafter, as a matter of convenience for explanation, the recessed portion having a substantially rectangular cross-sectional shape is referred to as an exhaust region E. Specifically, the exhaust region communicating with the aminosilane gas adsorption region P 1  is referred to as a first exhaust region E 1 , and the region communicating with the oxidation gas supply region P 2  is referred to as a second exhaust region E 2 . At the bottom of the first exhaust region E 1  and the second exhaust region E 2 , a first exhaust port  61  and a second exhaust port  62  are formed, respectively, as illustrated in  FIGS. 1 to 3 . The first exhaust port  61  and the second exhaust port  62  are each connected, via an exhaust pipe  63 , to a vacuum pump  64  that is a vacuum exhaust part, for example, as illustrated in  FIG. 1 . Note that in  FIG. 1 , a pressure controller  65  is illustrated. 
     The space between the rotation table  2  and the bottom portion  14  of the vacuum vessel  1  is provided with a heater unit  7  which is a heater as illustrated in  FIGS. 1 and 5 , and the wafer W on the rotation table  2  is heated, via the rotation table  2 , to a temperature (e.g., 400° C.) determined by a process recipe. Below the vicinity of the circumferential edge of the rotation table  2 , a ring-shaped cover member  71  is provided ( FIG. 5 ). The cover member  71  partitions the atmosphere from the upper space of the rotation table  2  to the first exhaust region E 1  and the second exhaust region E 2 , and the atmosphere in which the heater unit  7  is disposed, to prevent gas from entering the lower region of the rotation table  2 . The cover member  71  includes an inner member  71   a  disposed to face, from below, the outer edge of the rotation table  2  and the outer peripheral side with respect to the outer edge, and an outer member  71   b  disposed between the inner member  71   a  and the inner peripheral surface of the vacuum vessel  1 . The outer member  71   b  is provided below the bent portion  46  formed on the outer edge of the projecting portion  4  in the separation region D, and is in close proximity to the bent portion  46 . The inner member  71   a  surrounds the heater unit  7  throughout the entire circumference below the outer edge of the rotation table  2  (and below the portion that is slightly on the outer side with respect to the outer edge). 
     The bottom portion  14 , at a portion on the side closer to the rotational center than the space in which the heater unit  7  is disposed, protrudes upwardly into proximity to the core portion  21  near the center of the lower surface of the rotation table  2 , thereby forming a protrusion  12   a . The space between the protrusion  12   a  and the core portion  21  is narrow, and the gap between the inner peripheral surface of the through hole of the rotating shaft  22  passing through the bottom portion  14  and the rotating shaft  22  is narrow, and these narrow spaces communicate with the case body  20 . The case body  20  is provided with a purge gas supply pipe  72  for supplying Ar gas that is purge gas into the narrow space to purge the narrow space. Further, at the bottom portion  14  of the vacuum vessel  1 , a plurality of purge gas supply pipes  73  are provided to purge the space in which the heater unit  7  is disposed, at predetermined angular intervals in the circumferential direction below the heater unit  7  (in  FIG. 5 , one of the purge gas supply pipes  73  is illustrated). Further, a lid member  7   a  is provided between the heater unit  7  and the rotation table  2 , so as to cover the portion between the inner peripheral wall of the outer member  71   b  (the upper surface of the inner member  71   a ) and the upper end of the protrusion  12   a  across the circumferential direction, in order to prevent gas from entering the region where the heater unit  7  is provided. The lid member  7   a  is formed, for example, of quartz. 
     Further, to the center of the top plate  11  of the vacuum vessel  1 , a separation gas supply pipe  51  is connected, which is configured to supply Ar gas, which is the separation gas, to a space  52  between the top plate  11  and the core portion  21 . The separation gas supplied to the space  52  is discharged toward the peripheral edge along the surface of the wafer mounting region side of the rotation table  2  via a narrow gap  50  between the protrusion  5  and the rotation table  2 . The gap  50  can be maintained at a pressure higher than the pressure in the spaces  481  and  482  by the separation gas. Accordingly, the gap  50  prevents the aminosilane gas supplied to the aminosilane gas adsorption region P 1  and the oxidation gas supplied to the oxidation gas supply region P 2 , from passing through a central region C and mixing with each other. That is, the gap  50  (or the central region C) functions in the same manner as the separation space H (or the separation region D). 
     Further, as illustrated in  FIGS. 2 and 3 , on the side wall of the vacuum vessel  1 , there is formed the conveying port  15  for transferring the wafer W between an external conveying arm  10  and the rotation table  2 . The conveying port  15  is opened and closed by a gate valve (not illustrated). Below the rotation table  2 , at a portion corresponding to the transferring position of the wafer W, a raising/lowering pin and a raising and lowering mechanism thereof (not illustrated) for lifting the wafer W from the back surface through the recessed portion  24  and transferring the wafer W, are provided. 
     Next, the plasma source  80  will be described with reference to  FIGS. 6 to 8 .  FIG. 6  is a cross-sectional view of the plasma source  80  provided in the deposition apparatus of  FIG. 1 , illustrating a cross-section of the plasma source  80  along the radial direction of the rotation table  2 .  FIG. 7  is another cross-sectional view of the plasma source  80  provided in the deposition apparatus of  FIG. 1 , illustrating a cross-section of the plasma source  80  along a direction perpendicular to the radial direction of the rotation table  2 .  FIG. 8  is a top view of the plasma source  80  provided in the deposition apparatus of  FIG. 1 . As a matter of convenience for illustration, some members are simplified in these figures. 
     Referring to  FIG. 6 , the plasma source  80  includes a frame member  81 , a Faraday shield plate  82 , an insulating plate  83 , and an antenna  85 . The frame member  81  is made of a radio frequency transmissive material, and has a recessed portion recessed from the top surface, and is fitted into an opening portion  11   a  formed in the top plate  11 . The Faraday shield plate  82  is accommodated within the recessed portion of the frame member  81  and has a substantially box-like shape with the top portion opened. The insulating plate  83  is disposed on the bottom surface of the Faraday shield plate  82 . The antenna  83  is supported above the insulation plate  83  and is formed into a coil having a substantially octagonal planar shape. 
     The opening portion  11   a  of the top plate  11  has a plurality of steps, and one of the steps forms a groove portion around the entire circumference, and a seal member  81   a , such as an O-ring, is fitted into the groove portion. On the other hand, the frame member  81  has a plurality of steps corresponding to steps of the opening portion  11   a . Accordingly, when the frame member  81  is fitted into the opening portion  11   a , the back surface of one of the steps of the frame member  81  comes into contact with the seal member  81   a  fitted into the groove portion of the opening portion  11   a , so that the airtightness between the top plate  11  and the frame member  81  is maintained. Further, as illustrated in  FIG. 6 , a pressing member  81   c  is provided along an outer periphery of the frame member  81  fitted into the opening portion  11   a  of the top plate  11  so that the frame member  81  is pressed downward against the top plate  11 . Thus, the airtightness between the top plate  11  and the frame member  81  is more reliably maintained. 
     The lower surface of the frame member  81  faces the rotation table  2  in the vacuum vessel  1 , and the outer periphery of the lower surface is provided with a projection  81   b  projecting downward (toward the rotation table  2 ) along the entire circumference. The lower surface of the projection  81   b  is close to the surface of the rotation table  2 , and a plasma processing region P 3  is defined above the rotation table  2  by the projection  81   b , the surface of the rotation table  2 , and the lower surface of the frame member  81 . Note that the interval between the lower surface of the projection  81   b  and the surface of the rotation table  2  may be approximately the same as the height h 1  of the first ceiling surface  44  with respect to the upper surface of the rotation table  2  in the separation space H ( FIG. 4 ). 
     Further, the reaction gas nozzle  33 , which passes through the protrusion  81   b , extends in the plasma processing region P 3 . In one embodiment, the reaction gas nozzle  33  is connected with an argon gas source  90  to be filled with Ar gas, a helium gas source  91  to be filled with He gas, and an oxygen gas source  92  to be filled with O 2  gas, as illustrated in  FIG. 6 . Ar gas, He gas, and O 2  gas, for which the flow rates are controlled by corresponding flow rate controllers  93 ,  94 , and  95 , respectively, are supplied from the argon gas source  90 , the helium gas source  91 , and the oxygen gas source  92 , to the plasma processing region P 3  at a predetermined flow rate ratio (mixing ratio). 
     Further, a plurality of discharge holes  33   h  are formed in the reaction gas nozzle  33  along a longitudinal direction at predetermined intervals (for example, 10 mm), and the aforementioned Ar gas, He gas, and O 2  gas are discharged from the discharge holes  33   h . As illustrated, in  FIG. 7 , the discharge hole  33   h  is inclined from a direction perpendicular to the rotation table  2  toward the upstream side of the rotational direction of the rotation table  2 . For this reason, the mixture gas supplied from the reaction gas nozzle  33  is discharged in a direction opposite to the rotational direction of the rotation table  2 ; specifically toward a gap between the lower surface of the projection  81   b  and the surface of the rotation table  2 . This prevents oxidation gas or separation gas from flowing into the plasma processing region P 3  from the space below the second ceiling surface  45  located upstream of the plasma source  80  along the rotational direction of the rotation table  2 . Further, as described above, the projection  81   b  formed along the outer periphery of the lower surface of the frame member  81  is close to the surface of the rotation table  2 , and, therefore, the pressure in the plasma processing region P 3  can be easily maintained at a high level by the gas from the reaction gas nozzle  33 . This also prevents oxidation gas or separation gas from flowing into the plasma processing region P 3 . 
     The Faraday shield plate  82  is made of a conductive material such as metal, and is grounded, although not illustrated. As clearly illustrated in  FIG. 8 , a plurality of slits  82   s  are formed at the bottom of the Faraday shield plate  82 . Each of the slits  82   s  extends substantially perpendicular to the corresponding side of the antenna  85  having a substantially octagonal planar shape. 
     Further, the Faraday shield plate  82  also includes outwardly bending supports  82   a  at two top end locations, as illustrated in  FIGS. 7 and 8 . The support  82   a  is supported on the upper surface of the frame member  81 , and, therefore, the Faraday shield plate  82  is supported at a predetermined location within the frame member  81 . 
     The insulating plate  83  is made of, for example, quartz glass, and is slightly smaller than the bottom surface of the Faraday shield plate  82 , and is mounted on the bottom surface of the Faraday shield plate  82 . The insulating plate  83  insulates the Faraday shield plate  82  and the antenna  85  from each other, while transmitting downward the high frequencies radiated from the antenna  85 . 
     The antenna  85  is formed by winding a hollow tube (pipe) made of copper, for example, in triples, such that the planar shape is substantially octagonal. Cooling water can be circulated in the pipe to prevent the antenna  85  from being heated to a high temperature by high frequencies supplied to the antenna  85 . Further, the antenna  85  is provided with a standing portion  85   a , and a supporting portion  85   b  is attached to the standing portion  85   a . The supporting portion  85   b  maintains the antenna  85  at a predetermined position within the Faraday shield plate  82 . Further, a radio frequency power source  87  is connected to the supporting portion  85   b  via a matching box  86 . The radio frequency power source  87  generates a radio frequency having, for example, a frequency of 13.56 MHz. 
     According to the above plasma source  80 , when radio frequency power is supplied to the antenna  85  from the radio frequency power source  87  via the matching box  86 , an electromagnetic field is generated by the antenna  85 . The electric field components of the electromagnetic field are shielded by the Faraday shield plate  82  and thus cannot propagate downwardly. On the other hand, the magnetic field components propagate into the plasma processing region P 3  through the plurality of slits  82   s  in the Faraday shield plate  82 . According to the magnetic field components, plasma is generated from the reformed gas supplied from the reaction gas nozzle  33  to the plasma processing region P 3  at a predetermined flow rate ratio (mixing ratio). The plasma generated in this manner can reduce the radiation damage caused on the thin film stacked on the wafer W and the damage caused on each member in the vacuum vessel  1 , etc. 
     Further, as illustrated in  FIG. 1 , the deposition apparatus includes a controller  100  that is formed of a computer for controlling the operations of the entire apparatus. In the memory of the controller  100 , a program is stored for causing the deposition apparatus to perform a deposition method to be described later under the control of the controller  100 . In the program, a group of steps for executing the deposition method described below, is incorporated. The program is stored in a medium  102 , such as a hard disk, a compact disk, a magneto-optical disk, a memory card, a flexible disk, and the like. The program is read into a storage  101  by a predetermined reading device and is installed in the controller  100 . 
     Next, the reformed gas activated by the plasma generated by the plasma source  80  will be described. Generally, when a silicon oxide film (SiO 2  film) is deposited by using aminosilane gas as the raw material gas, a hydroxyl group (OH group) becomes an adsorption site, and the aminosilane gas is adsorbed on the OH group. However, when a recessed portion such as a trench or a via is formed on the surface of the wafer W, and embedding deposition is performed in the recessed portion, a bottom-up deposition method is preferable, in which the deposition is gradually advanced upward from the bottom surface of the recessed portion, in order to block the opening of the upper portion of the recessed portion and prevent a void from being formed in the recessed portion. In order to perform such a bottom-up deposition method, it is necessary to not form an OH group on the surface of the wafer W. For example, when a mixed gas of hydrogen (H 2 ) gas and O 2  gas (hereinafter, referred to as “H 2 /O 2  gas”) as a reformed gas is activated by plasma, a silicon oxide film having a good film quality can be deposited. However, when H 2 /O 2  gas is used as the reformed gas, an OH group is formed from H 2 /O 2 , and an OH group, which is an adsorption site of the aminosilane gas, is formed on the upper surface of the wafer W, and, therefore, it is difficult to perform the bottom-up deposition method. 
     Further, for example, when a mixed gas of He gas and O 2  gas (hereinafter, referred to as “H 2 /O 2  gas”) is activated by plasma, a silicon oxide film having a good film quality can be deposited. However, when He/O 2  gas is used as the reformed gas, the surface of the wafer W is more likely to be oxidized than when Ar/O 2  gas is used as the reformed gas. Therefore, He/O 2  gas is not preferable in a process when it is desired to prevent the oxidation of the surface of the wafer W. The reason why a difference arises in the wafer oxidation amount, is that the ionization energy of He is greater than the ionization energy of Ar, i.e., He is less likely to become ions than Ar, and hence He/O 2  gas has a higher proportion of O 2  gas that becomes activated than Ar/O 2  gas. 
     Accordingly, in one embodiment, first, the first layer of the silicon oxide film is deposited on the wafer W by a first process including a reforming process in which a mixed gas of Ar gas and O 2  gas (hereinafter, referred to as “Ar/O 2  gas”) is activated by plasma and supplied. Subsequently, by a second process including a reforming process of activating He/O 2  gas with plasma and supplying the activated He/O 2  gas, a second layer of the silicon oxide film is formed on top of the first layer of the silicon oxide film. In this case, the plasma-activated Ar/O 2  gas has a smaller effect of oxidizing the surface of the wafer W, than the plasma-activated He/O 2  gas. Therefore, it is possible to deposit a silicon oxide film while preventing the oxidation of the wafer W. Also, with the plasma-activated He/O 2  gas, it is possible to deposit a silicon oxide film having better film quality than the plasma-activated Ar/O 2  gas. Note that the plasma-activated He/O 2  gas has a greater effect of oxidizing the surface of the wafer W than the plasma-activated Ar/O 2  gas, but when supplying the plasma-activated He/O 2  gas, the surface of the wafer W is covered with the first layer of the silicon oxide film. Thus, the first layer of silicon oxide film functions as a protective layer that prevents the oxidation of the surface of the wafer W by plasma-activated He/O 2  gas. As a result, the oxidation of the surface of the wafer W is prevented. In this manner, in one embodiment, a silicon oxide film having a good film quality can be deposited while preventing the oxidation of the wafer W. 
     (Deposition Method) 
     The deposition method according to one embodiment will be described with reference to  FIG. 9 .  FIG. 9  is a flowchart illustrating a deposition method according to one embodiment. 
     As illustrated in  FIG. 9 , the deposition method according to one embodiment is a method of depositing a silicon oxide film on the wafer W by executing a first process (step S 91 ) and a second process (step S 92 ) in the stated order. The first process (step S 91 ) is a process of depositing a first layer of the silicon oxide film on the wafer W by an ALD process including a reforming process with Ar/He gas. The second process (step S 92 ) is a process of depositing a second layer of the silicon oxide film on the first layer of the silicon oxide film by an ALD process including a reforming process with He/O 2  gas. The first process (step S 91 ) and the second process (step S 92 ) are performed continuously, for example, without exposing the wafer W to air. Further, it is preferable that the number of times of repeating the ALD process in the first process (step S 91 ) is smaller than the number of times of repeating the ALD process in the second process (step S 92 ), particularly in view of the ability to deposit a silicon oxide film having a good film quality. However, for example, the number of times of repeating the ALD process in the first process (step S 91 ) may be greater than the number of times of repeating the ALD process in the second process (step S 92 ). 
     Hereinafter, a deposition method according to one embodiment will be described with reference to an example in which a silicon oxide film is deposited by using the above-described deposition apparatus.  FIGS. 10A to 10H  are diagrams for describing the first process (step S 91 ) of the deposition method according to one embodiment.  FIGS. 11A to 11G  are diagrams for describing the second process (step S 92 ) of the deposition method according to one embodiment. In one embodiment, a description is given of an example in which a silicon oxide film is deposited by being embedded in a trench T of the wafer W in which the trench T is formed on the surface of the wafer W, as illustrated in  FIG. 10A . The trench T is an example of a recessed portion formed on a surface U of the wafer W; other than the trench T, a via hole and the like may be formed. 
     First, the gate valve is opened and the conveying arm  10  transfers the wafer W from the outside into the recessed portion  24  of the rotation table  2  via the conveying port  15 . The wafer W is transferred by raising and lowering the raising/lowering pin from the bottom side of the vacuum vessel  1  through a through hole in the bottom surface of the recessed portion  24  when the recessed portion  24  stops at a position facing the conveying port  15 . The transferring of the wafer W is performed by intermittently rotating the rotation table  2 , so that each of the wafers W is mounted on one of the five recessed portions  24  of the rotation table  2 . 
     Next, the gate valve is closed and the inside of the vacuum vessel  1  is exhausted to a vacuum level attainable by the vacuum pump  64 . Subsequently, Ar gas is discharged at a predetermined flow rate as the separation gas from the separation gas nozzles  41  and  42 , and Ar gas is discharged at a predetermined flow rate from the separation gas supply pipe  51  and the purge gas supply pipe  72 . Further, the inside of the vacuum vessel  1  is controlled to a preset processing pressure by the pressure controller  65 . Then, the wafer W is heated to, for example, 400° C. by the heater unit  7  while rotating the rotation table  2  clockwise at, for example, a rotation speed of 5 rpm. 
     Subsequently, aminosilane gas is supplied from the reaction gas nozzle  31  and O 3  gas is supplied from the reaction gas nozzle  32 . Further, Ar/O 2  gas is supplied from the reaction gas nozzle  33 , and a radio frequency electric power having a frequency of 13.56 MHz is supplied at a magnitude of, for example, 4000 W, to the antenna  85  of the plasma source  80 . Accordingly, an oxygen plasma is generated in the plasma processing region P 3  between the plasma source  80  and the rotation table  2 . In the oxygen plasma, active species, such as oxygen ions and oxygen radicals, and high-energy particles are generated. 
     According to the rotation of the rotation table  2 , the wafer W repeatedly passes through the aminosilane gas adsorption region P 1 , the separation region D, the oxidation gas supply region P 2 , the plasma processing region P 3 , and the separation region, in the stated order. As illustrated in  FIG. 10B , in the aminosilane gas adsorption region P 1 , molecules Ms of aminosilane gas are adsorbed on the surface U of the wafer W and the inner surface of the trench T to form a molecular layer  110  of the aminosilane. After passing through the separation region D, as illustrated in  FIG. 10C , in the oxidation gas supply region P 2 , the aminosilane gas adsorbed on the surface U of the wafer W and the inner surface of the trench T are oxidized by O 3  gas molecules Mo. Accordingly, as illustrated in  FIG. 10D , a layer of a silicon oxide film  111  is deposited along the inner surface of the trench T. Further, when the aminosilane gas is oxidized, an OH group Hy is generated as a by-product, and the generated OH group Hy is adsorbed onto the surface of the silicon oxide film  111 . 
     Next, when the wafer W reaches the plasma processing region P 3  of the plasma source  80 , as illustrated in  FIG. 10E , the wafer W is exposed to oxygen plasma Po. At this time, a portion of the OH group Hy adsorbed on the silicon oxide film  111  is desorbed from the layer of the silicon oxide film  111  by, for example, collision of high energy particles in the oxygen plasma Po. The oxygen plasma Po reaches the surface U of the wafer W and near the opening of the trench T, but does not appreciably reach near the bottom of the trench T. Thus, on the surface U of the wafer W and on the side surfaces near the opening of the trench T, a relatively large amount of the OH group Hy is desorbed. As a result, as illustrated in  FIG. 10E , the OH group Hy is distributed so that the density of the OH group Hy is high at the bottom and at the side surfaces near the bottom of the trench T, and the density is low toward the opening of the trench T and the surface U of the wafer W. In this case, the Ar/O 2  gas, which has a small effect of oxidizing the surface of the wafer W, as the reformed gas, is activated by plasma and supplied, and, therefore, oxidation of the wafer W is prevented. 
     Next, when the wafer W again reaches the aminosilane gas adsorption region P 1  by rotation of the rotation table  2 , the molecules Ms of the aminosilane gas supplied from the reaction gas nozzle  31  are adsorbed to the surface U of the wafer W and the inner surface of the trench T. At this time, because the molecules Ms of the aminosilane gas are easily adsorbed by the OH group Hy, as illustrated in  FIG. 10F , the molecules Ms are adsorbed to the surface U of the wafer W and the inner surface of the trench T in a distribution according to the distribution of the OH group Hy. That is, the molecules Ms of the aminosilane gas are adsorbed to the inner surface of the trench T such that the density is high at the bottom and the side surfaces near the bottom of the trench T and the density is low toward the opening of the trench T. 
     Subsequently, as the wafer W passes through the oxidation gas supply region P 2 , the aminosilane gas adsorbed on the surface U of the wafer W and the inner surface oil the trench T is oxidized by O 3  gas, and as illustrated in  FIG. 10G , the silicon oxide film  111  is further deposited. At this time, the density of the aminosilane gas adsorbed on the inner surface of the trench T is applied to the thickness distribution of the silicon oxide film  111 . That is, the silicon oxide film  111  thickens at the bottom and at the side surfaces near the bottom of the trench T and thins toward the opening of the trench T. Then, the OH group Hy generated by the oxidation of the aminosilane gas is adsorbed to the surface of the silicon oxide film  111 . 
     Next, as the wafer W again reaches the plasma processing region P 3  of the plasma source  80 , as described above, the OH group Hy is distributed such that the density of the OH group Hy is high at the bottom and side surfaces near the bottom of the trench T and the density is low toward the opening of the trench T. 
     Subsequently, as the above-described process is repeated, the silicon oxide film  111  thickens from the bottom of the trench T, as illustrated in  FIG. 10H . 
     Next, in a state where the aminosilane gas is supplied from the reaction gas nozzle  31  and O 3  gas is supplied from the reaction gas nozzle  32 , the reformed gas supplied from the reaction gas nozzle  33  is switched from Ar/O 2  gas to He/O 2  gas. Further, a radio frequency electric power having a frequency of 13.56 MHz is supplied at a magnitude of, for example, 4000 W, to the antenna  85  of the plasma source  80 . Accordingly, an oxygen plasma is generated in the plasma processing region P 3  between the plasma source  80  and the rotation table  2 . In the oxygen plasma, active species, such as oxygen ions and oxygen radicals, and high-energy particles, are generated. 
     According to the rotation of the rotation table  2 , the wafer W repeatedly passes through the aminosilane gas adsorption region P 1 , the separation region D, the oxidation gas supply region P 2 , the plasma processing region P 3 , and the separation region D in the stated order. As illustrated in  FIG. 11A , in the aminosilane gas adsorption region P 1 , the molecules Ms of the aminosilane gas are adsorbed on the surface of the silicon oxide film  111  deposited on the surface U of the wafer W and the inner surface of the trench T, to form a molecular layer  120  of the aminosilane. After passing through the separation region D, in the oxidation gas supply region P 2 , as illustrated in  FIG. 11B , the aminosilane gas adsorbed to the surface of the silicon oxide film  111  is oxidized by the O 3  gas molecules Mo. Accordingly, as illustrated in  FIG. 11C , a layer of a silicon oxide film  121  is deposited along the surface of the silicon oxide film  111 . Further, when the aminosilane gas is oxidized, an OH group Hy is generated as a by-product, and the generated OH group Hy is adsorbed onto the surface of the silicon oxide film  121 . 
     Next, when the wafer W reaches the plasma processing region P 3  of the plasma source  80 , as illustrated in  FIG. 11D , the wafer W is exposed to the oxygen plasma Po. At this time, a portion of the OH group Hy adsorbed on the silicon oxide film  121  is desorbed from the layer of the silicon oxide film  121 , for example, by collision of high energy particles in the oxygen plasma Po. The oxygen plasma Po reaches the surface U of the wafer W and near the opening of the trench T, but does not appreciably reach near the bottom of the trench T. Thus, a relatively large amount of the OH group Hy is desorbed from the surface U of the wafer W and from the side surfaces near the opening of the trench T. As a result, as illustrated in  FIG. 11D , the OH group Hy is distributed such that the density of the OH group Hy is high at the bottom and the side surfaces near the bottom of the trench T, and the density is low toward the opening of the trench T and the surface U of the wafer W. In this case, He/O 2  gas, by which a silicon oxide film with good film quality can be deposited, is activated by plasma and supplied, and, therefore, a silicon oxide film with good film quality can be deposited. Note that the plasma-activated He/O 2  gas has a greater effect of oxidizing the surface of the wafer W than the plasma-activated Ar/O 2  gas, but when supplying the plasma-activated He/O 2  gas, the surface of the wafer W is covered with the silicon oxide film  111 . Thus, the silicon oxide film  111  functions as a protective layer to prevent the surface of the wafer W from being oxidized by the plasma-activated He/O 2  gas. As a result, the oxidation of the surface of the wafer W is prevented. 
     Next, when the wafer W again reaches the aminosilane gas adsorption region P 1  by the rotation of the rotation table  2 , the molecules Ms of the aminosilane gas supplied from the reaction gas nozzle  31  are adsorbed to the surface U of the wafer W and the inner surface of the trench T. At this time, because the molecules Ms of the aminosilane gas are easily adsorbed by the OH group Hy, as illustrated in  FIG. 11E , the molecules Ms are adsorbed to the surface U of the wafer W and the inner surface of the trench T in a distribution according to the distribution of the OH group Hy. That is, the molecules Ms of the aminosilane gas are adsorbed to the inner surface of the trench T such that the density is high at the bottom and the side surfaces near the bottom of the trench T and the density is low toward the opening of the trench T. 
     Then, as the wafer W passes through the oxidation gas supply region P 2 , the aminosilane gas that has adsorbed on the surface U of the wafer W and the inner surface of the trench T is oxidized by O 3  gas, and the silicon oxide film  121  is further deposited. At this time, the density of the aminosilane gas adsorbed on the inner surface of the trench T is applied to the thickness distribution of the silicon oxide film  121 . That is, the silicon oxide film  121  thickens at the bottom and the side surfaces near the bottom of the trench T and thins toward the opening of the trench T. Then, the OH group Hy generated by the oxidation of the aminosilane gas is adsorbed to the surface of the silicon oxide film  121 . 
     Next, as the wafer W again reaches the plasma processing region P 3  of the plasma source  60 , the OH group Hy is distributed such that the density of the OH group Hy is high at the bottom and the side surfaces near the bottom of the trench T and the density is low toward the opening of the trench T as described above. 
     Subsequently, as the above-described process is repeated, as illustrated in  FIG. 11F , the silicon oxide film  121  thickens from the bottom of the trench T, and as illustrated in  FIG. 11G , the embedding of the trench T is completed. 
     As described above, according to the deposition method according to one embodiment, in the reforming process of the silicon oxide film  111  performed in the plasma processing region P 3 , first, the reforming process is performed using Ar/O 2  gas having a small effect of oxidizing the wafer W. Subsequently, a reforming process is performed using He/O 2  gas capable of depositing a silicon oxide film having good film quality on the silicon oxide film  111 . Thus, it is possible to deposit a silicon oxide film with good film quality while preventing the oxidation of the wafer W. 
     Note that in the deposition method according to one embodiment, plasma-activated H 2 /O 2  gas is not supplied, and, therefore, an OH group is not generated by the reformed gas. Therefore, the thickness of the silicon oxide film on the surface U of the wafer W tends to be thin, and bottom-up deposition is performed. 
     Further, before the above-described deposition process is performed, a silicon nitride film is formed in advance as an under layer on the surface of the wafer W, and the deposition process described in  FIGS. 10A to 11G  may be performed on the underlayer. 
     (Practical Example) 
     With respect to the deposition method according to one embodiment, a description is given of a practical example performed in order to identify gas that can be suitably used as the reformed gas used in the first process (step S 91 ) and the reformed gas used in the second process (step S 92 ). Hereinafter, the reformed gas used in the first process (step S 91 ) and the reformed gas used in the second process (step S 92 ) are also referred to as the first reformed gas and the second reformed gas, respectively. In the practical example, four types of gas were used as reformed gases: Ar/O 2  gas (Ar/O 2 =15000/75 sccm), He/O 2  gas (He/O 2 =15000/75 sccm), Ar gas (Ar=15000 sccm), and He gas (He=15000 sccm). 
       FIG. 12  is a diagram for explaining a method for evaluating the wafer oxidation amount. In  FIG. 12 , the horizontal axis represents the time [min] and the vertical axis represents the film thickness [nm]. The circles in  FIG. 12  represent the relationship between the time of depositing the silicon oxide film on a silicon (Si) wafer by an ALD process including a reforming process using Ar/O 2  gas as the reformed gas and the film thickness of the silicon oxide film. On the other hand, the triangles in  FIG. 12  represent the relationship between the time of depositing the silicon oxide film on a thermal oxide film by an ALD process including a reforming process using Ar/O 2  gas as the reformed gas and the film thickness of the silicon oxide film. 
     As illustrated in  FIG. 12 , when a silicon oxide film is deposited on a thermal oxide film by an ALD process including a reforming process using Ar/O 2  gas, the film thickness of the silicon oxide film increases in proportion to the deposition time. On the other hand, when a silicon oxide film is deposited on a Si wafer by an ALD process including a reforming process using Ar/O 2  gas, the film thickness of the silicon oxide film increases in proportion to the deposition time during a period after a predetermined time (15 minutes in the case of  FIG. 12 ) elapses from the start of deposition. However, during the period until the predetermined time elapses, the rate of increase in the thickness of the silicon oxide film is greater than in the period after the predetermined time elapses. This is probably due to the fact that the silicon oxide film is stacked by vapor phase growth on the thermal oxide film, whereas on the Si wafer, the silicon oxide film is formed by surface oxidation of the Si wafer in addition to the stacking of the silicon oxide film by vapor phase growth during the initial deposition period. 
     Accordingly, in the practical example, a difference ΔY between the thickness of the silicon oxide film deposited on the Si wafer after the predetermined time has elapsed and the thickness of the silicon oxide film deposited on the thermal oxide film, was calculated, and the calculated difference ΔY was evaluated as the wafer oxidation amount. 
       FIG. 13  is a diagram illustrating the relationship between the type of reformed gas and the wafer oxidation amount. In  FIG. 13 , the horizontal axis indicates the type of reformed gas and the vertical axis indicates the wafer oxidation amount [Å]. 
     As illustrated in  FIG. 13 , the wafer oxidation amounts when using Ar/O 2  gas, He/O 2  gas, Ar gas, and He gas as the reformed gas, were 40.9 Å, 56.1 Å, 19.2 Å, and 35.1 Å, respectively. From these results, it can be seen that, from the viewpoint of reducing the wafer oxidation amount, the suitable reformed gas is Ar gas, He gas, Ar/O 2  gas, and He/O 2  gas, stated in preferential order. 
       FIG. 14  is a diagram illustrating the relationship between the type of reformed gas and the wet etching rate. In  FIG. 14 , the horizontal axis represents the type of reformed gas and the vertical axis represents the wet etching rate. The wet etching rate illustrated on the vertical axis is the wet etching rate of the silicon oxide film when the wet etching rate of the thermal oxide film is 1. 
     As illustrated in  FIG. 14 , the wet etching rates when using Ar/O 2  gas, He/O 2  gas, Ar gas, and He gas as the reformed gases were 1.50, 1.45, 1.75, and 1.69, respectively. From these results, it can be seen that, from the viewpoint of depositing a fine film having a low wet etching rate, that is, a film having good film quality, the suitable reformed gas is He/O 2  gas, Ar/O 2  gas, He gas, and Ar gas, stated in preferential order. 
     As described above, determining from the results illustrated in  FIGS. 13 and 14 , from the viewpoint of depositing a silicon oxide film with good film quality while preventing oxidation of the wafer, it is preferable to use Ar/O 2  gas as the first reformed gas and to use He/O 2  gas as the second reformed gas. However, as the first reformed gas and the second reformed gas, any kind of relevant gas can be used as long as the first reformed gas has a smaller effect of oxidizing the wafer than the second reformed gas. For example, the first reformed gas may be Ar gas, and the second reformed gas may be He gas. In this case, the film reality is not as good as the case of using Ar/O 2  gas as the first reformed gas and using He/O 2  gas as the second reformed gas; however, the wafer oxidation amount can be particularly reduced. 
     Further, for example, Ar gas may be used as the first reformed gas, Ar/O 2  gas may be used as the second reformed gas, He gas may be used as the first reformed gas, and He/O 2  gas may be used as the second reformed gas. 
     According to one embodiment of the present invention, a silicon oxide film having a good film quality can be deposited on a substrate, while preventing the oxidation of the substrate. 
     The deposition method according to the embodiment disclosed herein are to be considered exemplary in all respects and not limiting. The above embodiment and its variations may include omissions, substitutions, or modifications in various forms without departing from the appended claims and the gist thereof.