Patent Publication Number: US-2020294773-A1

Title: Plasma processing method and plasma processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2019-043693 filed on Mar. 11, 2019, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The various aspects and embodiments described herein pertain generally to a plasma processing method and a plasma processing apparatus. 
     BACKGROUND 
     When etching a multilayered film by using a plasma processing apparatus, a plurality of holes having different depths may be formed in an oxide layer included in the multilayered film. A plasma processing method described in Patent Document 1 is directed to a method of forming multiple holes having different heights in a multilayered film. The multilayered film has an oxide layer, a plurality of etching stop layers and a mask layer. The etching stop layers are made of tungsten. In this method, by supplying a processing gas into a processing vessel and forming plasma from the processing gas, the multilayered film ranging from a top surface of the oxide layer to the plurality of etching stop layers is etched. The multiple holes having the different depths are formed in the oxide layer at the same time through this etching. The processing gas includes a fluorocarbon-based gas, a rare gas, oxygen and nitrogen. 
     Patent Document 1: Japanese Patent Laid-open Publication No. 2014-090022 
     SUMMARY 
     In an exemplary embodiment, there is provided a plasma processing method of processing a processing target object. The processing target object comprises a first layer and a second layer. The second layer is provided with multiple openings and is provided on a top surface of the first layer. The top surface is exposed through the multiple openings. The first layer is provided with multiple etching stop layers. Within the first layer, lengths from the multiple etching stop layers to the top surface are different. The first layer is made of silicon oxide. The second layer is made of a material containing carbon. The method comprises a processing sequence which is performed repeatedly within a chamber of a plasma processing apparatus in which the processing target object is accommodated. The processing sequence comprises: etching the processing target object through the multiple openings with the second layer as a mask by forming plasma from a first gas; and etching, after the etching by forming the plasma from the first gas, the processing target object by forming plasma from a second gas. The first gas includes a gas containing a carbon atom and a fluorine atom. The second gas includes a gas containing a carbon atom, a fluorine atom and a hydrogen atom. High-order fluorocarbon is generated by the plasma from the first gas in the etching performed by forming the plasma from the first gas. Low-order fluorocarbon or low-order hydrofluorocarbon is generated by the plasma from the second gas in the etching performed by forming the plasma from the second gas. 
     The foregoing summary is illustrative only and is not intended to be any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description that follows, embodiments are described as illustrations only since various changes and modifications will become apparent to those skilled in the art from the following detailed description. The use of the same reference numbers in different figures indicates similar or identical items. 
         FIG. 1  is a flowchart illustrating an example of a plasma processing method according to an exemplary embodiment; 
         FIG. 2  is a diagram illustrating a configuration example of a plasma processing apparatus in which the plasma processing method shown in  FIG. 1  is performed; 
         FIG. 3  is a diagram illustrating an example structure of a processing target object on which the plasma processing method of  FIG. 1  is performed; 
         FIG. 4  is a diagram illustrating an example structure obtained while etching the processing target object of  FIG. 3  by the plasma processing method shown in  FIG. 1 ; 
         FIG. 5  is a diagram illustrating an example structure obtained while further etching the processing target object of  FIG. 4  by the plasma processing method shown in  FIG. 1 ; and 
         FIG. 6  is a diagram illustrating an example structure obtained by performing the plasma processing method of  FIG. 1  on the processing target object shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part of the description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Furthermore, unless otherwise noted, the description of each successive drawing may reference features from one or more of the previous drawings to provide clearer context and a more substantive explanation of the current exemplary embodiment. Still, the exemplary embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings, may be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     Hereinafter, various exemplary embodiments will be described. The exemplary embodiments provide a plasma processing method of processing a processing target object. The processing target object has a first layer and a second layer. The second layer is provided with a plurality of openings and is provided on a top surface of the first layer. Through the openings of the second layer, the top surface of the first layer is exposed. The first layer has a plurality of etching stop layers. Within the first layer, lengths from the respective etching stop layers to the top surface of the first layer are all different. The first layer is made of silicon oxide. The second layer contains carbon. In this plasma processing method, a processing sequence is repeatedly performed within a chamber of a plasma processing apparatus in which the processing target object is accommodated. The processing sequence includes etching the processing target object through the openings with the second layer as a mask by forming plasma from a first gas (sometimes referred to as process A). The processing sequence further includes etching, after etching the processing target object by the plasma from the first gas, the processing target object by forming plasma from a second gas (sometimes referred to as process B). The first gas includes a gas containing a carbon atom and a fluorine atom. The second gas includes a gas containing a carbon atom, a fluorine atom and a hydrogen atom. In the etching performed by forming the plasma from the first gas, high-order fluorocarbon is generated by the plasma from the first gas. In the etching performed by forming the plasma from the second gas, low-order fluorocarbon or low-order hydrofluorocarbon is generated by the plasma from the second gas. 
     In the process A, by the etching with the plasma from the first gas, the etching upon the first layer trough the openings of the second layer can be performed. 
     In the process A, however, the high-order fluorocarbon may be generated by the plasma from the first gas. The high-order fluorocarbon is polymer having a high attachment coefficient (hereinafter, sometimes referred to as first polymer). In the process A, this first polymer attaches on the second layer and a side surface of a hole formed by performing the process A. However, it is difficult for this first polymer to reach a bottom of the hole. If the process A is carried on, the first polymer keeps on attaching on a top surface of the second layer and side surfaces of the openings, clogging the openings. Accordingly, it may be difficult to carry on the etching upon the first layer. 
     Further, since it is difficult for the first polymer to reach the bottom of the hole, selectivity with respect to the etching stop layer is relatively low in the etching of the process A. Therefore, in case that the etching stop layer is exposed through the hole, this etching stop layer may not be protected by the first polymer, and, as a result, this etching stop layer may be etched. 
     Particularly, in the above-described method, a plurality of holes having different lengths from the top surface of the first layer to the etching stop layers are formed in parallel, not one by one. Therefore, the etching stop layer in a hole having a comparatively short length may be excessively etched by the etching of the process A. 
     In the process B following the process A, low-order fluorocarbon or low-order hydrofluorocarbon is generated by the plasma from the second gas. The low-order fluorocarbon or the low-order hydrofluorocarbon is polymer having a low attachment coefficient (hereinafter, sometimes referred to as second polymer). In the process B, though it is difficult for this second polymer to attach on the second layer and the side surface of the hole formed by the process A, the second polymer easily reaches the bottom of the hole. Meanwhile, in case that the etching stop layer is exposed through the hole, this second polymer may attach on the etching stop layer through the hole. 
     As stated above, the second polymer easily reaches the bottom of the hole. Accordingly, in case that the etching stop layer is exposed through the hole, the second polymer may be deposited on the etching stop layer (bottom of the hole), and, thus, the etching stop layer can be protected by the deposited second polymer. 
     As described above, in the etching performed in the process A, the selectivity with respect to the etching stop layer is relatively low. Further, in the etching performed in the process A, the opening of the second layer (mask) may be clogged, and it may be difficult to carry on the etching. As a resolution, by performing the process B after performing the process A appropriately, the opening of the second layer which is clogged in the process A can be enlarged. Further, in case that the etching stop layer is exposed through the hole formed by the process A, a protective film (second polymer) can be formed on the etching stop layer by performing the process B. Therefore, at the beginning of the process A performed after the process B, the opening of the second layer is already enlarged. At this time, in case that the etching stop layer is exposed through the hole in the etching of the process A, the protective film (second polymer) is already formed on the etching stop layer. Therefore, in the process A performed after the process B, the excessive etching upon the etching stop layer can be suppressed by the protective film (second polymer) while the opening of the second layer is suppressed from being clogged. 
     Furthermore, in this method, the above-stated processing sequence can be performed repeatedly. Accordingly, by performing the present method, the holes having the different lengths can be formed in parallel, not one by one. In this case, during a period until a hole having the longest length from the top surface of the first layer is formed, it is possible to avoid the clogging of the opening while suppressing the etching stop layer in the hole having the comparatively short length from being excessively etched. 
     In the plasma processing method according to the exemplary embodiment, the first gas may include at least one of a C 4 F 6  gas or a C 4 F 8  gas. 
     In the plasma processing method according to the exemplary embodiment, the second gas may include at least one of a CHF 3  gas, a CH 2 F 2  gas or a CH 3 F gas. 
     In the plasma processing method according to the exemplary embodiment, the second gas may further include at least one of a CO gas, a CO 2  gas, an O 2  gas, a N 2  gas, or a H 2  gas. 
     In the plasma processing method according to the exemplary embodiment, the etching stop layer may be made of tungsten. 
     In the plasma processing method according to the exemplary embodiment, in the etching performed by forming the plasma from the first gas, the high-order fluorocarbon mainly attaches on the second layer. In the etching performed by forming the plasma from the second gas, the low-order fluorocarbon or the low-order hydrofluorocarbon attaches on the etching stop layer through the hole formed by performing the processing sequence. 
     In the exemplary embodiment, a plasma processing apparatus is provided. The plasma processing apparatus includes a chamber, a placing table, a gas supply system, a high frequency power supply and a controller. The placing table is provided within the chamber. The gas supply system is configured to supply a first gas and a second gas into the chamber. The high frequency power supply is configured to supply a high frequency power to excite the first gas and the second gas. The controller is configured to control the gas supply system and the high frequency power supply. The controller controls the gas supply system and the high frequency power supply to perform a processing sequence repeatedly to etch a processing target object, which is placed on the placing table and provided with a first layer and a second layer, by forming plasma from the first gas and plasma from the second gas. The second layer is provided with multiple openings and is provided on a top surface of the first layer. The top surface is exposed through the multiple openings. The first layer is provided with multiple etching stop layers. Within the first layer, lengths from the multiple etching stop layers to the top surface are different. The first layer is made of silicon oxide. The second layer is made of a material containing carbon. The processing sequence comprises: etching the processing target object through the multiple openings with the second layer as a mask by forming plasma from the first gas; and etching, after the etching by forming the plasma from the first gas, the processing target object by forming plasma from the second gas. The first gas includes a gas containing a carbon atom and a fluorine atom. The second gas includes a gas containing a carbon atom, a fluorine atom and a hydrogen atom. High-order fluorocarbon is generated by the plasma from the first gas in the etching performed by forming the plasma from the first gas. Low-order fluorocarbon or low-order hydrofluorocarbon is generated by the plasma from the second gas in the etching performed by forming the plasma from the second gas. 
     In the plasma processing apparatus according to the exemplary embodiment, the first gas may include at least one of a C 4 F 6  gas or a C 4 F 8  gas. 
     In the plasma processing apparatus according to the exemplary embodiment, the second gas may include at least one of a CHF 3  gas, a CH 2 F 2  gas or a CH 3 F gas. 
     In the plasma processing apparatus according to the exemplary embodiment, the second gas may further include at least one of a CO gas, a CO 2  gas, an O 2  gas, a N 2  gas, or a H 2  gas. 
     In the plasma processing apparatus according to the exemplary embodiment, the high-order fluorocarbon attaches mainly on the second layer in the etching performed by forming the plasma from the first gas. In the etching performed by forming the plasma from the second gas, the low-order fluorocarbon or the low-order hydrofluorocarbon attaches on the etching stop layer through the hole formed by performing the processing sequence. 
     Now, various exemplary embodiments will be described in detail with reference to the accompanying drawings. In the various drawings, same or corresponding parts will be assigned same reference numerals. 
       FIG. 1  is a flowchart illustrating a plasma processing method (hereinafter, referred to as “method MT”) according to an exemplary embodiment. The method MT shown in  FIG. 1  can be performed by using, for example, a plasma processing apparatus  10  shown in  FIG. 2 . First, referring to  FIG. 2 , a configuration of the plasma processing apparatus  10  will be explained. 
       FIG. 2  is a diagram illustrating the plasma processing apparatus  10  according to the exemplary embodiment. The plasma processing apparatus  10  shown in  FIG. 2  is configured as a capacitively coupled parallel plate type plasma processing apparatus, and is equipped with a substantially cylindrical chamber  12 . The chamber  12  has, for example, an anodically oxidized aluminum surface. The chamber  12  is frame-grounded. 
     The plasma processing apparatus  10  is equipped with the chamber  12 , a grounding conductor  12   a,  an exhaust port  12   e,  a carry-in/out opening  12   g,  a supporting member  14 , a placing table  16 , an electrostatic chuck  18 , an electrode  20  and a DC power supply  22 . The plasma processing apparatus  10  is also equipped with a coolant path  24 , a pipeline  26   a,  a pipeline  26   b,  a gas supply line  28 , an upper electrode  30 , an insulating shield member  32 , an electrode plate  34 , multiple gas discharge holes  34   a  and an electrode supporting body  36 . 
     The plasma processing apparatus  10  is further equipped with a gas diffusion space  36   a,  multiple gas through holes  36   b,  a gas inlet opening  36   c,  a gas supply line  38 , a gas supply system  40 , a splitter  43 , a deposition shield  46 , an exhaust plate  48 , an exhaust device  50 , an exhaust line  52  and a gate valve  54 . 
     The plasma processing apparatus  10  is also equipped with a conductive member  56 , a power feed rod  58 , a rod-shaped conductive member  58   a,  a cylindrical conductive member  58   b,  an insulating member  58   c,  a DC power supply  60 , a first high frequency power supply  62 , a second high frequency power supply  64 , a matching device  70 , and a matching device  71 . The plasma processing apparatus  10  is further equipped with a controller Cnt, a focus ring FR and a processing space S. 
     The supporting member  14  is placed on a bottom of the chamber  12 . The supporting member  14  may have a cylindrical shape. The supporting member  14  may be made of an insulating material. The supporting member  14  supports the placing table  16 . 
     The placing table  16  is provided within the chamber  12 . The placing table  16  may be made of a metal such as aluminum. In the present exemplary embodiment, the placing table  16  constitutes a lower electrode. 
     The electrostatic chuck  18  is provided on a top surface of the placing table  16 . The electrostatic chuck  18  and the placing table  16  constitute a placing table of the exemplary embodiment. The electrostatic chuck  18  has a structure in which the electrode  20  is embedded in a pair of insulating layers or a pair of insulating sheets. 
     The electrode  20  may be a conductive film. The electrode  20  is electrically connected with the DC power supply  22 . The electrostatic chuck  18  attracts and holds a processing target object (for example, a processing target object W shown in  FIG. 3 ) by an electrostatic force generated by a DC voltage applied from the DC power supply  22 . 
     The focus ring FR is disposed on the top surface of the placing table  16  to surround the electrostatic chuck  18 . The focus ring FR is configured to improve etching uniformity. The focus ring FR may be made of, by way of non-limiting example, silicon or quartz. 
     The coolant path  24  is provided within the placing table  16 . A coolant of a preset temperature, for example, cooling water from a chiller unit provided outside is supplied into and circulated in the coolant path  24  via the pipelines  26   a  and  26   b.  By controlling the temperature of the coolant circulated in the coolant path  24 , a temperature of the processing target object placed on the electrostatic chuck  18  can be controlled. 
     Through the gas supply line  28 , a heat transfer gas, for example, a He gas from a heat transfer gas supply mechanism (not shown) is supplied into a gap between a top surface of the electrostatic chuck  18  and a rear surface of the processing target object. 
     The upper electrode  30  is provided within the chamber  12 . The upper electrode  30  is disposed above the placing table  16  serving as the lower electrode, facing the placing table  16 . The placing table  16  and the upper electrode  30  are arranged to be substantially parallel to each other. Formed between the upper electrode  30  and the lower electrode is the processing space S in which plasma etching is performed on the processing target object. 
     The upper electrode  30  is supported at an upper portion of the chamber  12  with the insulating shield member  32  therebetween. The upper electrode  30  may include the electrode plate  34  and the electrode supporting body  36 . The electrode plate  34  is in direct contact with the processing space S, and is provided with the multiple gas discharge holes  34   a.  The electrode plate  34  may be made of a conductor or semiconductor having low resistance and low Joule heat. 
     The electrode supporting body  36  is configured to support the electrode plate  34  in a detachable manner, and may be made of a conductive material such as, but not limited to, aluminum. The electrode supporting body  36  may have a water-cooling structure. 
     The gas diffusion space  36   a  is formed within the electrode supporting body  36 . The gas diffusion space  36   a  communicates with the processing space S through the multiple gas through holes  36   b  and the multiple gas discharge holes  34   a.    
     The multiple gas through holes  36   b  communicate with the multiple gas discharge holes  34   a,  respectively. The gas through holes  36   b  are formed at the electrode supporting body  36 , and the gas discharge holes  34   a  are formed at the electrode plate  34 . 
     The gas inlet opening  36   c  is connected with the gas supply line  38 . The gas inlet opening  36   c  is formed at the electrode supporting body  36 . Various kinds of gases output from the gas supply system  40  can be introduced into the gas diffusion space  36   a  through the gas inlet opening  36   c.    
     The gas supply system  40  is configured to supply a first gas and a second gas for performing the method MT shown in  FIG. 1  into the chamber  12 . The gas supply system  40  is connected to the gas supply line  38  via the splitter  43 . 
     The first gas includes a gas composed of a carbon atom and a fluorine atom. The first gas may include at least one of, for example, a C 4 F 6  gas or a C 4 F 8  gas. 
     The second gas includes a gas composed of a carbon atom, a fluorine atom and a hydrogen atom. The second gas may include at least one of, for example, a CHF 3  gas, a CH 2 F 2  gas or a CH 3 F gas. 
     The second gas may further include at least one of, for example, a CO gas, a CO 2  gas, an O 2  gas, a N 2  gas, or a H 2  gas. 
     The grounding conductor  12   a  is of a substantially cylindrical shape. The grounding conductor  12   a  extends upward from a sidewall of the chamber  12  to be higher than a height position of the upper electrode  30 . 
     The deposition shield  46  is provided along an inner wall of the chamber  12  in a detachable manner. The deposition shield  46  is also provided on an outer side surface of the supporting member  14 . The deposition shield  46  is configured to suppress an etching byproduct (deposit) from adhering to the chamber  12 . The deposition shield  46  may be formed by coating, for example, an aluminum member with ceramics such as Y 2 O 3 . 
     At a bottom side of the chamber  12 , the exhaust plate  48  is disposed between the supporting member  14  and the inner wall of the chamber  12 . The exhaust plate  48  may be made of, for example, an aluminum member coated with ceramics such as Y 2 O 3 . 
     Within the chamber  12 , the exhaust opening  12   e  is provided under the exhaust plate  48 . The exhaust opening  12   e  is connected with the exhaust device  50  via the exhaust line  52 . 
     The exhaust device  50  includes a vacuum pump such as a turbo molecular pump, and is capable of decompressing the inside of the chamber  12  to a required vacuum level. 
     The carry-in/out opening  12   g  is provided for the processing target object. The carry-in/out opening  12   g  is provided at the sidewall of the processing vessel  12 . The carry-in/out opening  12   g  is opened or closed by the gate valve  54 . 
     The conductive member  56  is provided at the inner wall of the chamber  12 . The conductive member  56  is fixed to the inner wall  12  to be located on a substantially level with the processing target object in a height direction. The conductive member  56  is DC-connected to the ground and has an effect of suppressing an abnormal discharge. 
     The location of the conductive member  56  is not limited to the example shown in  FIG. 2  as long as the conductive member  56  is provided in a plasma formation space. By way of example, the conductive member  56  may be provided near the placing table  16 , for example, around the placing table  16 . Alternatively, the conductive member  56  may be provided near the upper electrode  30 . For example, the conductive member  56  may be provided at an outside of the upper electrode  30  in a ring shape. 
     The power feed rod  58  supplies a high frequency power to the placing table  16  serving as the lower electrode. The power feed rod  58  has a coaxial double pipe structure. The power feed rod  58  includes the rod-shaped conductive member  58   a  and the cylindrical conductive member  58   b.    
     The rod-shaped conductive member  58   a  extends from an outside of the chamber  12  to an inside of the chamber  12  through the bottom of the chamber  12  in a substantially vertical direction. An upper end of the rod-shaped conductive member  58   a  is connected to the placing table  16 . 
     The cylindrical conductive member  58   b  is disposed to be coaxial with the rod-shaped conductive member  58   a,  surrounding the rod-shaped conductive member  58   a . The cylindrical conductive member  58   b  is supported at the bottom of the chamber  12 . Two sheets of substantially annular insulating members  58   c  are disposed between the rod-shaped conductive member  58   a  and the cylindrical conductive member  58   b.  Accordingly, the rod-shaped conductive member  58   a  and the cylindrical conductive member  58   b  are electrically insulated. 
     Lower ends of the rod-shaped conductive member  58   a  and the cylindrical conductive member  58   b  are connected to the matching devices  70  and  71 . The matching device  70  is connected to the first high frequency power supply  62 . The matching device  71  is connected to the second high frequency power supply  64 . 
     The first high frequency power supply  62  is configured to supply a high frequency power to excite the first gas and the second gas. The first high frequency power supply  62  generates a first high frequency power for plasma formation. A frequency of the first high frequency power is in a range from 27 MHz to 100 MHz, for example, 100 MHz. 
     The second high frequency power supply  64  is configured to generate a second high frequency power for ion attraction into the processing target object by applying a high frequency bias power to the placing table  16 . A frequency of the second high frequency power is in a range from 400 kHz to 13.56 MHz, and may be for example, 3 MHz. 
     The DC power supply  60  is connected to the upper electrode  30 . The DC power supply  60  is configured to apply a negative DC voltage to the upper electrode  30 . With the above-described configuration, the two different high frequency powers are applied to the placing table  16  serving as the lower electrode, and the DC voltage is applied to the upper electrode  30 . 
     The controller Cnt is a computer including a processor, a storage, an input device, a display device, and so forth. The controller Cnt controls the individual components of the plasma processing apparatus  10 , for example, the power supply system, the gas supply system, and the driving system. Particularly, the controller Cnt is capable of controlling the gas supply system  40 , the first high frequency power supply  62  and the second high frequency power supply  64 . 
     The storage of the controller Cnt stores therein a control program for implementing various processings performed in the plasma processing apparatus  10  by the processor. The control program that can be executed by the processor includes a computer program for allowing each component of the plasma processing apparatus  10  to perform a processing according to processing conditions, i.e., a processing recipe. 
     The control program stored in the storage of the controller Cnt may particularly include a computer program for implementing a processing described in the flowchart of the method MT of  FIG. 1 . The controller Cnt executes the control program to etch the processing target object placed on the placing table  16  by forming the plasma from each of the first gas and the second gas supplied from the gas supply system  40 . The controller Cnt controls the gas supply system  40  and the first high frequency power supply  62  to repeat a processing sequence SQ of the method MT shown in  FIG. 1 . 
     To perform an etching processing by using the plasma processing apparatus  10 , the processing target object is placed on the electrostatic chuck  18 . By supplying various kinds of gases from the gas supply system  40  into the chamber  12  at preset flow rates while evacuating the chamber  12  by the exhaust device  50 , an internal pressure of the chamber  12  is set to be in a range from, e.g., 0.1 Pa to 50 Pa. 
     The first high frequency power is supplied to the lower electrode from the first high frequency power supply  62 , and the second high frequency power is supplied to the lower electrode from the second high frequency power supply  64 . The first DC voltage is applied to the upper electrode  30  from the DC power supply  60 . Accordingly, a high frequency electric field is formed between the upper electrode  30  and the lower electrode, and the plasma from the various processing gases supplied into the processing space S can be formed. The processing target object can be etched by various ions and radicals in the plasma. 
     The method MT shown in  FIG. 1  may be a method of etching the processing target object W having the structure shown in  FIG. 3 , for example. The processing target object W has a first layer LY 1  and a second layer LY 2 . The first layer LY 1  has a multiple number of etching stop layers (etching stop layers ML 1  to ML 4 , etc.). 
     The etching stop layer ML 3  is provided above the etching stop layer ML 4 . The etching stop layer ML 2  is provided above the etching stop layer ML 3 . The etching stop layer ML 1  is provided above the etching stop layer ML 2 . A top surface SF is provided above the etching stop layer ML 1 . By way of example, a film thickness of the etching stop layers ML 1  to ML 4  ranges from 30 nm to 80 nm. 
     Within the first layer LY 1 , lengths from the respective etching stop layers (the etching stop layers ML 1  to ML 4 ) to the top surface SF are all different. In the present exemplary embodiment, a length L 1  from the etching stop layer ML 1  to the top surface SF is shorter than a length L 2  from the etching stop layer ML 2  to the top surface SF. The length L 2  is shorter than a length L 3  from the etching stop layer ML 3  to the top surface SF. The length L 3  is shorter than a length L 4  from the etching stop layer ML 4  to the top surface SF. By way of example, the length L 1  is in a range from 500 nm to 1000 nm, and the length L 4  is in a range from 7500 nm to 8000 nm. 
     As stated above, in the method MT, a multiple number of holes (openings) (holes HL 1  to HL 4 , etc.) having the different lengths from the top surface SF to the etching stop layers (the etching stop layer ML 1 , etc.) are formed in parallel, not one by one, as in the processing target object W shown in  FIG. 3  to  FIG. 6 . 
     The second layer LY 2  is provided on the top surface SF of the first layer LY 1 . The second layer LY 2  is provided with a multiple number of openings (openings OP 1  to OP 4 , etc.). The top surface SF is exposed through the openings (openings OP 1  to OP 4 , etc.). By way of example, the openings OP 1  to OP 4  have a diameter ranging from 120 nm to 140 nm. 
     In the present exemplary embodiment, the opening OP 1  is overlapped with the etching stop layer ML 1  in a stacking direction DL of the multiple number of etching stop layers (etching stop layers ML 1  to ML 4 , etc.) within the first layer LY 1 . The opening OP 2  is overlapped with the etching stop layer ML 2  in the stacking direction DL. The opening OP 3  is overlapped with the etching stop layer ML 3  in the stacking direction DL. The opening OP 4  is overlapped with the etching stop layer ML 4  in the stacking direction DL. 
     The first layer LY 1  is made of silicon oxide. By way of non-limiting example, the first layer LY 1  may be made of silicon dioxide (SiO 2 ). The second layer LY 2  may be made of a material containing carbon. The second layer LY 2  may be a carbon layer formed by, for example, CVD (Chemical Vapor Deposition). The etching stop layers ML 1  to ML 4  may be made of tungsten. 
     In the present exemplary embodiment, the processing target object W further includes a third layer LY 3 . The first layer LY 1  is provided above this third layer LY 3 . To elaborate, the etching stop layer ML 4  is provided on the third layer LY 3 . 
     Referring back to  FIG. 1 , the method MT will be discussed. The method MT is an example of a plasma processing method of processing the processing target object. To be more specific, the method MT is a method of etching the processing target object W placed on the placing table  16  by forming the plasma from the first gas and the plasma from the second gas. In the method MT, the multiple number of holes (holes HL 1  to HL 4 , etc.) having the different lengths from the top surface SF to the etching stop layers (etching stop layers ML 1  to ML 4 , etc.) are formed in parallel, not one by one, as in the processing target object W shown in  FIG. 3  to  FIG. 6 . 
     The method MT includes the processing sequence SQ. The processing sequence SQ includes a process ST 1  and a process ST 2 . The process ST 2  is performed after the process ST 1 . The multiple number of holes (holes HL 1  to HL 4 , etc.) are formed by performing the processing sequence SQ. 
     The method MT also includes a process ST 3 . The process ST 3  is performed after the processing sequence SQ. 
     In the method MT, the processing sequence SQ is performed repeatedly (to be more specified, a preset number of times) in the chamber  12  of the plasma processing apparatus  10  in which the processing target object W shown in  FIG. 3  is accommodated (placed on the placing table  16 ). The method MT can be performed under the control of the controller Cnt. In performing the etching according to the method MT, the controller Cnt particularly controls the gas supply system  40  and the first high frequency power supply  62 . 
     In the process ST 1  of the processing sequence SQ, the plasma from the first gas is formed, and the processing target object W is etched through the openings (opening OP 1 , etc.) of the second layer LY 2  by using the second layer LY 2  as a mask. Through the etching of the process ST 1  using the plasma from the first gas, the etching upon the first layer LY 1  through the multiple number of openings (openings OP 1  to OP 4 , etc.) of the second layer LY 2  can be performed. 
     As stated above, in the process ST 1 , the etching upon the first layer LY 1  is performed by the plasma from the first gas. In the process ST 1 , however, high-order fluorocarbon may be generated by the plasma from the first gas. The high-order fluorocarbon is first polymer mainly composed of C x F y  (x is equal to or larger than 2) and has a high attachment coefficient. In the process ST 1 , the first polymer mainly attaches on the second layer LY 2  and may also attach to a side surface of the hole such as the hole HL 1  shown in  FIG. 6  which is formed through the process ST 1 . As shown in  FIG. 4 , due to the adhesion of the first polymer, a deposit film DP 1  of the first polymer is formed mainly on the second layer LY 2  and, also, on the side surface of the hole such as the hole HL 1  (particularly, at an upper portion of the corresponding side surface in the opening OP 1  or the like). Further, the first polymer may not reach a bottom of the hole such as the hole HL 1 . 
     Accordingly, if the process ST 1  is carried on over a relatively long period, the first polymer keeps on attaching on the top surface of the second layer LY 2  and on the side surface of the hole such as the hole HL 1  (particularly, at the upper portion of the corresponding side surface in the opening OP 1  or the like), so that a thickness of the deposit film DP 1  may be increased. In such a case, the opening such as the opening OP 1  may be clogged with the deposit film DP 1 , and it may be difficult to carry on the etching upon the first layer LY 1 . The process ST 1  may be continued for an appropriate time period unless the opening of the hole such as the hole HL 1  formed by the etching of the process ST 1  is clogged with the deposit film DP 1 . 
     Further, it is difficult for the first polymer to reach the bottom of the hole such as the hole HL 1 , and selectivity with respect to the etching stop layer such as the etching stop layer ML 1  is relatively low in the etching of the process ST 1 . Thus, in case that the etching stop layer such as the etching stop layer ML 1  is exposed through the hole such as the hole HL 1 , the corresponding etching stop layer is not protected by the first polymer, so that the corresponding etching stop layer may be etched. 
     Particularly, in the method MT, the multiple number of holes (corresponding to the hole HL 1 , etc.) having the different lengths from the top surface SF to the etching stop layers such as the etching stop layer ML 1  are formed in parallel, not one by one. Therefore, in the hole (corresponding to the hole HL 1 , etc.) having a relatively short length, the etching stop layer such as the etching stop layer ML 1  may be excessively etched by the etching of the process ST 1 . 
     The high frequency power for plasma formation from the first high frequency power supply  62  in the process ST 1  may be in a range from, e.g., 300 W to 1000 W. Further, if the high frequency power is larger than 1000 W, the deposit film DP 1  is formed at the upper portion and the sidewall of the second layer LY 2  and the sidewall and the bottom of the hole such as the hole HL 1 . As a result, it may be difficult to carry on the etching. 
     In the process ST 2  following the process ST 1 , the etching is performed on the processing target object W by forming the plasma from the second gas to remove the first polymer formed in the process ST 1  and to suppress the excessive etching upon the etching stop layer in the process ST 1 . 
     In the process ST 2 , low-order fluorocarbon or low-order hydrofluorocarbon is generated by the plasma from the second gas while the deposit film DP 1  formed in the opening such as the opening OP 1  in the process ST 1  is removed. The low-order fluorocarbon or the low-order hydrofluorocarbon is second polymer mainly composed of CF, CF 2 , CF 3 , CHF or CHF 2 , and has a low attachment coefficient. In the process ST 2 , though it is difficult for this second polymer to attach on the second layer LY 2  and the side surface of the hole such as the hole HL 1  formed by the process ST 1 , the second polymer easily reaches the bottom of the hole such as the hole HL 1 . Due to the adhesion of this second polymer, a deposit film DP 2  of the second polymer is formed at the bottom of the hole such as the hole HL 1  and a lower portion of the hole such as the hole HL 1  extending from the corresponding bottom. 
     Meanwhile, in case that the etching stop layer such as the etching stop layer ML 1  is exposed through the hole such as the hole HL 1 , the second polymer may attach on the etching stop layer such as the etching stop layer ML 1  through the corresponding hole. In such a case, the deposit film DP 2  of the second polymer is formed on the etching stop layer such as the etching stop layer ML 1  through the hole such as the hole HL 1 . 
     As stated above, the second polymer easily reaches the bottom of the hole such as the hole HL 1 . Accordingly, if the etching stop layer such as the etching stop layer ML 1  is exposed through the hole such as the hole HL 1 , the second polymer is deposited on the etching stop layer (bottom of the hole), so that the deposit film DP 2  is formed thereat. The etching stop layer can be protected by this deposit film DP 2 . 
     Further, if at least one of a CO gas, a CO 2  gas, an O 2  gas, a N 2  gas or a H 2  gas is added to the second gas, a width of the opening of the hole such as the hole HL 1  may be easily adjusted. 
     Particularly, if at least one of the CO gas or the CO 2  gas is added to the second gas, CO or CO 2  bonds with the fluorine atom of the second polymer, so that COF 2  is generated. In such a case, fluorine atoms in the second polymer and the first polymer are scavenged, so that carbon atoms deposited on the bottom of the hole such as the hole HL 1  may be relatively increased. Accordingly, in the process ST 1  performed after the process ST 2 , the excessive etching upon the etching stop layer such as the etching stop layer ML 1  can be effectively suppressed. 
     In the etching performed in the process ST 1 , selectivity with respect to the etching stop layer such as the etching stop layer ML 1  is comparatively low. Further, in the etching performed in the process ST 1 , the opening (opening OP 1 , etc.) of the second layer LY 2  (mask) may be clogged, and the etching may not be carried on. As a resolution, by performing the process ST 2  after carrying out the process ST 1  appropriately, the opening (opening OP 1 , etc.) of the second layer LY 2  clogged in the process ST 1  may be enlarged. Further, if the etching stop layer such as the etching stop layer ML 1  is exposed through the hole such as the hole HL 1 , the protective film (second polymer) can be formed on this etching stop layer as a result of performing the process ST 2 . 
     Therefore, at the beginning of the process ST 1  performed after the process ST 2 , the opening (opening OP 1 , etc.) of the second layer LY 2  is already enlarged. At this time, if the etching stop layer such as the etching stop layer ML 1  is exposed through the hole such as the hole HL 1  in the etching of the process ST 1 , the protective film (second polymer) is already formed on this etching stop layer. Therefore, in the process ST 1  performed after the process ST 2 , excessive etching upon the etching stop layer such as the etching stop layer ML 1  can be suppressed by the protective film (second polymer) while the opening (opening OP 1 , etc.) of the second layer LY 2  is suppressed from being clogged. 
     A processing time of the process ST 2  may be, for example, 5 seconds to 30 seconds, for example, 10 seconds to 25 seconds. If the processing time of the process ST 2  is relatively short (for example, shorter than 5 seconds), the deposit film DP 2  may not be formed at the bottom of the hole such as the hole HL 1  shown in  FIG. 6 . If the processing time of the process ST 2  is relatively long (for example, longer than 30 seconds), on the other hand, the thickness of the deposit film DP 2  may be excessively increased, and the etching upon the first layer LY 1  may not be carried on in the process ST 1  which may be performed after the process ST 2  through the process ST 3  to be described later. Further, if the processing time of the process ST 2  is relatively long, the opening such as the opening OP 1  may be excessively enlarged. 
     The high frequency power for plasma formation from the first high frequency power supply  62  in the process ST 2  may be equal to or higher than, e.g., 2000 W. If the high frequency power is less than 2000 W, the etching stop layer may be excessively etched. 
     In the method MT, by performing the process ST 3 , the above-described processing sequence SQ can be performed repeatedly a preset number of times. In a first cycle of the processing sequence SQ the processing target object W having the structure shown in  FIG. 4  is obtained from the processing target object W having the structure shown in  FIG. 3  by the process ST 1 , and the processing target object W having the structure shown in  FIG. 5  is obtained by the process ST 2  which is performed after the process ST 1 . Further, by performing the processing sequence SQ the multiple times, the processing target object W having the structure shown in  FIG. 6  can be obtained. Thus, by performing the method MT, the multiple number of holes (corresponding to the hole HL 1 , etc.) having the different lengths can be formed in parallel, not one by one. In such a case, during a period until the hole HL 4  having the largest length from the top surface SF is formed, the clogging of the openings (opening OP 1 , etc.) can be avoided and the excessive etching upon the etching stop layer in the hole having the relatively short length can be suppressed. Further, in the first cycle of the processing sequence SQ, the hole HL 1  of the opening OP 1  need not necessarily reach the etching stop layer ML 1  by the process ST 1  as in the case where the processing target object W having the structure shown in  FIG. 4  is obtained from the processing target object W having the structure shown in  FIG. 3 . Furthermore, the holes HL 2  to HL 4  of the openings OP 2  to OP 4  may etched deeper than a top surface of the etching stop layer ML 1 . 
     As stated above, by performing the method MT in which the processing sequence SQ is repeatedly performed the preset number of times, the holes (holes HL 1  to HL 4 , etc.) having the different lengths can be formed in the first layer LY 1  effectively, as shown in  FIG. 6 . As depicted in  FIG. 6 , by performing the method MT, the hole HL 1  is formed in the first layer LY 1  to reach the etching stop layer ML 1  through the opening OP 1  while the excessive etching upon the etching stop layer ML 1  is suppressed. 
     The hole HL 2  is formed in the first layer LY 1  to reach the etching stop layer ML 2  through the opening OP 2  while the excessive etching upon the etching stop layer ML 2  is suppressed. The hole HL 3  is formed in the first layer LY 1  to reach the etching stop layer ML 3  through the opening OP 3  while the excessive etching upon the etching stop layer ML 3  is suppressed. The hole HL 4  is formed in the first layer LY 1  to reach the etching stop layer ML 4  through the opening OP 4 . 
     According to the exemplary embodiment, it is possible to form a plurality of holes having different lengths effectively in parallel. 
     So far, the various exemplary embodiments have been described. However, the exemplary embodiments are not limiting, and various omissions, substitutions and changes may be made. Further, other exemplary embodiments may be created by combining elements in the various exemplary embodiments. 
     From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting. The scope of the inventive concept is defined by the following claims and their equivalents rather than by the detailed description of the exemplary embodiments. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the inventive concept.