Patent Publication Number: US-2020294768-A1

Title: Plasma processing apparatus and etching method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-048799, filed on Mar. 15, 2019; the entire contents of which are incorporated herein by reference. 
     FIELD 
     An embodiment described herein relates generally to a plasma processing apparatus and an etching method. 
     BACKGROUND 
     A plasma processing apparatus is known in which an electric field and a magnetic field are applied to a process gas supplied between two electrodes to generate plasma, and this plasma is used to perform a process to a processing object mounted on one of the electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically illustrating a configuration example of a plasma processing apparatus according to an embodiment; 
         FIGS. 2A and 2B  are diagrams schematically illustrating the mechanism of operation of an electric current control unit; 
         FIG. 3  is a diagram illustrating an example of sequence control information; 
         FIGS. 4A and 4B  are diagrams schematically illustrating the relationship of electric currents flowing through coils with magnetic fields; 
         FIGS. 5A and 5B  are diagrams schematically illustrating an example of an etching process according to the embodiment; 
         FIGS. 6A and 6B  are diagrams illustrating examples of the sectional shape of a hole according to the embodiment; 
         FIG. 7  is a sectional view schematically illustrating another configuration example of a plasma processing apparatus according to the embodiment; and 
         FIG. 8  is a top view schematically illustrating an example of a coil shape. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a plasma processing apparatus includes a chamber, a substrate stage configured to support a substrate inside the chamber, and a plasma generation structure configured to generate plasma processing the substrate, in a space above the substrate inside the chamber. Further, the plasma processing apparatus includes an electromagnet including coils configured to apply a magnetic field to the space, and an electromagnet controller configured to cause pulsed electric currents, in each of which its direction and ON/OFF are pulsed, to flow through the coils. 
     An exemplary embodiment of a plasma processing apparatus and an etching method will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the following embodiment. 
     The density of the plasma is preferably uniform, but is difficult to control to be uniform in practice. For example, in an etching process using plasma, non-uniformity of the plasma density causes holes to be formed in a state tilted relative to the vertical direction. 
       FIG. 1  is a sectional view schematically illustrating a configuration example of a plasma processing apparatus according to an embodiment. Here, an explanation will be given of a case where the plasma processing apparatus is exemplified by a plasma etching apparatus. 
     The plasma processing apparatus  10  includes a chamber  11 , an Electrostatic Chuck (ESC)  12  serving as a substrate stage, an upper electrode  13 , an Alternating Current (AC) power supply  14 , a process gas supply section  15 , a coolant supply section  16 , and a controller  19 . The ESC  12 , the upper electrode  13 , the AC power supply  14 , and the process gas supply section  15  provide an example of a plasma generation structure. The coolant supply section  16  is an example of a gas supply unit. The ESC  12  includes a High Voltage (HV) electrode (lower electrode)  21 , an insulating film  22 , an ESC base  23 , an HV power supply  24 , and an ESC power supply  25 . 
     The chamber  11  has a cylindrical shape, for example, and is configured to accommodate a substrate  100 , such as a wafer, to be treated as a processing object. The ESC  12  serves to hold the substrate  100  inside the chamber  11 . While the upper electrode  13  is disposed outside the ESC  12 , the HV electrode  21  is disposed inside the ESC  12 . The HV electrode  21  is covered with the insulating film  22 , and is arranged above the ESC base  23 . The HV power supply  24  is a variable voltage source for adjusting the electric potential of the HV electrode  21 . The ESC power supply  25  is a variable voltage source for adjusting the electric potential of the ESC base  23 . The substrate  100  is mounted above the HV electrode  21  with the insulating film  22  interposed therebetween. The ESC  12  attracts and holds the substrate  100  by the HV electrode  21  with an electrostatic force. The ESC  12  includes an upper face for mounting the substrate  100  thereon, a lower face opposite to the upper face, and a lateral face. The ESC  12  is configured to move the substrate  100  up and down by a plurality of pins (not illustrated) provided at the upper face of the ESC  12 . 
     The upper electrode  13  is disposed above the HV electrode  21 . For example, the upper electrode  13  is arranged in parallel with the HV electrode  21 . The upper electrode  13  is provided with through-holes (not illustrated) to supply a process gas from the process gas supply section  15  into the space between the upper electrode  13  and the HV electrode  21 . For example, the upper electrode  13  is formed of a plate member including a plurality of through-holes that extend therethrough in the thickness direction. The plasma processing apparatus  10  is configured to generate plasma between the upper electrode  13  and the HV electrode  21 , and to supply the plasma toward the front face S 1  of the substrate  100  to process the substrate  100  by the plasma. Specifically, the front face S 1  of the substrate  100  is etched by dry etching using the plasma. 
     The AC power supply  14  serves to supply an AC current to the upper electrode  13 . Consequently, plasma is generated between the upper electrode  13  and the HV electrode  21 . 
     The process gas supply section  15  serves to supply a process gas for plasma generation into the chamber  11 . The upper electrode  13  and the HV electrode  21  use the AC current from the AC power supply  14  to generate plasma of the process gas. 
     The coolant supply section  16  serves to supply a coolant to the substrate  100  through a plurality of flow passages  12   a  formed in the ESC  12 . The coolant is an inactive gas, such as a rare gas, which is, for example, helium (He) gas. 
     The controller  19  is configured to control the operation of the plasma processing apparatus  10 . For example, the controller  19  controls the operation of the chamber  11 , the operation of the ESC  12 , the ON/OFF and/or electric current of the AC power supply  14 , the ON/OFF and/or process gas supply amount of the process gas supply section  15 , the ON/OFF and/or coolant supply amount of the coolant supply section  16 , and so forth. 
     Further, the plasma processing apparatus  10  includes an electromagnet  30  above the top plate  11   a  of the chamber  11 . The electromagnet  30  includes a core member  31  and a plurality of coils  32 - 1  to  32 - 4 . In the example illustrated here, four ring coils  32 - 1  to  32 - 4  arranged in order from inside are set in a concentric state such that the center of the coils agrees with the center of the top plate  11   a . Hereinafter, when the respective coils  32 - 1  to  32 - 4  do not need distinction, each of the coils will be referred to as “coil  32 ”. 
     Further, the respective coils  32  are connected to an electromagnet controller  34 . The electromagnet controller  34  includes plasma control units  341 , electric current control units  342 , and a sequence control unit  343 . Each of the plasma control units  341  serves to control the intensity of a magnetic field to be generated by the corresponding coil  32 . More specifically, each plasma control unit  341  supplies an electric current of a predetermined magnitude to the corresponding coil  32  during one plasma process, under the control of the sequence control unit  343 . Further, each plasma control unit  341  can change the direction of an electric current caused to flow through the corresponding coil  32 . Every coil  32  may be provided with one plasma control unit  341 , or a plurality of coils  32  may be provided with one common plasma control unit  341 . In the latter case, the plasma control unit  341  is configured to individually control the direction and magnitude of an electric current to be given to each coil  32 . 
     Each of the electric current control units  342  serves to switch the ON/OFF of an electric current to be supplied from the corresponding plasma control unit  341  to the coil  32 , under the control of the sequence control unit  343 .  FIGS. 2A and 2B  are diagrams schematically illustrating the mechanism of operation of each electric current control unit. For example, each electric current control unit  342  includes a power supply  3421  and electrodes  3422  and  3423  connected to the power supply  3421 . 
     As illustrated in  FIG. 2A , in a state where a constant electric current is flowing through the coil  32 , the power supply  3421  applies a positive bias voltage between the electrodes  3422  and  3423  such that the electrode  3422  is higher than the electrode  3423  in electric potential. At this time, electrons flowing through a wiring line  321  connecting the plasma control unit  341  to the coil  32  are attracted by the positive bias voltage applied by the electric current control unit  342 , and an electric current is thereby caused to flow through the coil  32 . As a result, a magnetic field is generated at the coil  32 . 
     On the other hand, as illustrated in  FIG. 2B , in a state where a constant electric current is flowing through the coil  32 , the power supply  3421  applies a negative bias voltage between the electrodes  3422  and  3423  such that the electrode  3422  is lower than the electrode  3423  in electric potential. At this time, electrons flowing through the wiring line  321  receive a repulsive force due to the negative bias voltage applied by the electric current control unit  342 , and the electric current is thereby caused to stop flowing through the coil  32 . As a result, the magnetic field generated at the coil  32  disappears. 
     As described above, the electric current control unit  342  can switch the ON/OFF of an electric current caused to flow through the coil  32 . Further, also when the direction of an electric current is changed by the plasma control unit  341 , the electric current control unit  342  can switch the ON/OFF of the electric current similarly by inverting the sign of a voltage to be applied between the electrodes  3422  and  3423 . As a result, the coil  32  comes to be supplied with a pulsed electric current obtained by pulsing. 
     The sequence control unit  343  is configured to control the plasma control units  341  and the electric current control units  342  to pulse the ON/OFF and direction of electric currents to the respective coils  32  and to synchronize the electric currents with each other among the coils  32 , in accordance with sequence control information.  FIG. 3  is a diagram illustrating an example of the sequence control information. For example, the sequence control information is formed of a timing chart of pulsed electric currents caused to flow through the coils  32 . In  FIG. 3 , the horizontal axis indicates the time, and the vertical axis indicates the electric current magnitude. Further,  FIG. 3  illustrates pulsed electric currents to be given to the respective coils  32 - 1  to  32 - 4 , in order from above. Here, the pulsed electric currents to be given to the respective coils  32 - 1  to  32 - 4  are different from each other in a single period T, and these pulsed electric currents are repeated periodically. Further, the starting time points of each period T at the respective coils  32 - 1  to  32 - 4  are set in synchronism with each other, under the control of the sequence control unit  343 . Here, this T denotes a time period far shorter than the total processing time necessary for performing a plasma process, and has a length of several seconds or less, for example. 
     For the coil  32 - 1 , positive electric current pulses are generated at regular intervals. At the beginning of each period T, a positive electric current pulse rises up. Each positive electric current pulse has a width of T/4, and each off-time also has a width of T/4. 
     For the coil  32 - 2 , negative electric current pulses are generated. At the beginning of each period T, a negative electric current pulse rises up. Each negative electric current pulse has a width of 3T/4, and each off-time has a width of T/4. 
     For the coil  32 - 3 , positive electric current pulses are generated. At the beginning of each period T, a positive electric current pulse rises up. Each positive electric current pulse has a width of T/2, and each off-time has a width of T/2. 
     For the coil  32 - 4 , negative electric current pulses are generated at regular intervals. At the beginning of each period T, an off-time is first present, and then a negative electric current pulse rises up after the lapse of T/4. Each off-time has a width of T/4, and each negative electric current pulse also has a width of T/4. 
     In accordance with the sequence described above, the sequence control unit  343  controls the respective plasma control units  341 , or the electric current control units  342  in addition to the plasma control units  341 . Here, as illustrated in  FIG. 3 , adjacent coils  32  are preferably supplied with respective electric currents such that the directions of the electric currents are opposite to each other. 
       FIGS. 4A and 4B  are diagrams schematically illustrating the relationship of electric currents flowing through the coils with magnetic fields.  FIG. 4A  is a diagram schematically illustrating the directions of the electric currents and the directions of the magnetic fields.  FIG. 4B  is a diagram illustrating an example of the magnitude of the magnetic fields generated by the respective coils above the processing face of the processing object. 
     As illustrated in  FIG. 4A , when electric currents I 1  to I 4  are caused to flow through the coils  32 - 1  to  32 - 4  respectively, magnetic fields Br 1  to Br 4  are generated around the coils  32 - 1  to  32 - 4  in accordance with the right-handed screw rule. As illustrated in  FIG. 4B , the magnetic fields Br 1  to Br 4  are formed above the processing face of the processing object by the electric currents I 1  to I 4  flowing through the respective coils  32 - 1  to  32 - 4 . Here, the magnetic fields Br 1  to Br 4  given by the respective coils  32 - 1  to  32 - 4  are added up to form a magnetic field above the processing face. Here, when the magnitudes and/or directions of the electric currents I 1  to I 4  caused to flow through the coils  32 - 1  to  32 - 4  are changed, the magnitudes of the magnetic fields Br 1  to Br 4  by the respective coils  32 - 1  to  32 - 4  are changed, and thus the magnetic field above the processing face can be changed. As illustrated in  FIG. 3 , when the pulsed electric currents are caused to flow through the respective coils  32 - 1  to  32 - 4 , the magnetic field above the processing face can be formed while being changed dynamically. Here, in general, as the intensity of the magnetic field is higher, the plasma density is increased. 
     For example, sequence control information such as that illustrated in  FIG. 3  is obtained in the following way. First, the relationship between the magnetic field and the plasma density is obtained by a simulation. Then, on the basis of the simulation result, the direction, magnitude, and ON/OFF timing of each of the electric currents are obtained by experiments to obtain a desired plasma density by the plasma processing apparatus  10  in practice. Then, the direction, magnitude, and ON/OFF timing of each of the electric currents are collected for the respective coils  32 , and the synchronization timing of the electric currents is further set, to obtain the sequence control information. 
     Next, an explanation will be given of an etching method to be performed by using this plasma processing apparatus  10 .  FIGS. 5A and 5B  are diagrams schematically illustrating an example of an etching process according to the embodiment.  FIGS. 6A and 6B  are diagrams illustrating examples of the sectional shape of a hole according to the embodiment. 
     First, a substrate  100  to be treated as a processing object is mounted on the ESC  12  serving as a substrate stage. Then, a process gas is supplied from the process gas supply section  15  into the space between the substrate  100  and the upper electrode  13 , and an AC current is supplied from the AC power supply  14  to the upper electrode  13 . Further, the sequence control unit  343  controls at least one of the groups of the plasma control units  341  and the electric current control units  342 , such that pulsed electric currents flow through the respective coils  32 - 1  to  32 - 4  of the electromagnet  30  in synchronism with each other, in accordance with the sequence control information. Consequently, a magnetic field having a predetermined intensity is generated between the ESC  12  and the upper electrode  13 , and thus plasma is generated between the upper electrode  13  and the HV electrode  21 . With the plasma thus generated, the substrate  100  is subjected to an etching process. 
     The plasma P generated at this time is assumed to have the shape illustrated in  FIG. 5A . In general, it is preferable to form uniform plasma above the substrate  100 . However, due to unevenness of the magnetic field in the space, it is difficult to generate uniform plasma above the substrate  100 . For example, as illustrated in  FIG. 5A , the plasma P is in a state where the region corresponding to the peripheral side of the substrate  100  is sagging down as compared with the region corresponding to the central side of the substrate  100 . In the case of an etching process, ions fly out in directions perpendicular to surfaces that define the contour of the plasma P. As a result, in this example, holes  110  are formed to be perpendicular to the processing face of the substrate  100  on the central side of the substrate  100 , but holes  110  are formed to be tilted outward, relative to the direction perpendicular to the processing face, on the peripheral side. 
     As illustrated in  FIG. 5A , holes  110  are formed in a state tilted outward on the peripheral side of the substrate  100 . In consideration of the above, according to this embodiment, after an etching step is performed for a predetermined time under first conditions set as described above, an etching step is performed under second conditions set to generate plasma P for causing holes  110  on the peripheral side of the substrate  100  to be tilted inward, i.e., in the opposite direction. 
     The plasma P generated at this time is assumed to have the shape illustrated in  FIG. 5B . For example, as illustrated in  FIG. 5B , the plasma P is in a state where the region corresponding to the central side of the substrate  100  is sagging down as compared with the region corresponding to the peripheral side of the substrate  100 . As a result, in this example, holes  110  are formed to be perpendicular to the processing face of the substrate  100  on the central side of the substrate  100 , but holes  110  are formed to be tilted inward, relative to the direction perpendicular to the processing face, on the peripheral side. 
     After the etching step is performed for a predetermined time under the second conditions set as described above, an etching step is performed under the first conditions set to generate plasma P for causing holes  110  on the peripheral side of the substrate  100  to be tilted outward, i.e., in the opposite direction. In this way, until the bottoms of holes  110  reach a desired depth, etching steps are repeatedly performed by alternately using the first conditions and the second conditions. However, for example, as shown in the sequence control information illustrated in  FIG. 3 , if the conditions, which include the ON/OFF timing, magnitude, and direction, of the pulsed electric currents to the respective coils  32 - 1  to  32 - 4  are not synchronized with each other, the plasma P illustrated in  FIG. 5A  and the plasma P illustrated in  FIG. 5B  cannot be alternately generated. Accordingly, it is important to synchronize the pulsed electric currents caused to flow through the respective coils  32 - 1  to  32 - 4  with each other. 
     As described above, the etching steps are alternately performed such that the etching directions become opposite to each other on the peripheral side of the substrate  100 . In this case, as illustrated in  FIG. 6A , a region etched in a first etching direction D 1  and a region etched in a second etching direction D 2  opposite to the first etching direction D 1  are alternately formed in the depth direction. In  FIG. 3 , it is assumed that, where ΔT 1  denotes the first half of each period T, and ΔT 2  denotes the second half thereof, the etching in the first etching direction D 1  is performed in ΔT 1 , and the etching in the second etching direction D 2  is performed in ΔT 2 . 
     Consequently, for example, in ΔT 1  of the first cycle, the etching in the first etching direction D 1  is performed down to a depth DE 1 . Here, the reference of the depth direction is the position of the upper face of the substrate  100 . In ΔT 2  of the first cycle, the etching in the second etching direction D 2  is performed from the depth DE 1  to a depth DE 2 . Then, in ΔT 1  of the second cycle, the etching in the first etching direction D 1  is performed from the depth DE 2  to a depth DE 3 . In ΔT 2  of the second cycle, the etching in the second etching direction D 2  is performed from the depth DE 3  to a depth DE 4 . Thereafter, the substantially the same cycles are repeatedly performed. In this way, the etching in the first etching direction D 1  and the etching in the second etching direction D 2  are alternately performed, and a hole is thereby formed. The hole  110  thus formed comes to have a sidewall of a bellows-like shape. 
     Here, when each period T is set to several seconds or less, the etching amount by each period T becomes small. Thus, as illustrated in  FIG. 6B , the hole  110  comes to have a sidewall that extends almost perpendicular to the processing face of the substrate  100 . 
     When the etching reaches a predetermined depth, the etching sequence ends. 
     In the above description, a case is illustrated where the electromagnet  30  is arranged above the top plate  11   a  of the chamber  11 . However, the embodiment is not limited to this example. The electromagnet  30  may be arranged at an arbitrary position of the chamber  11 .  FIG. 7  is a sectional view schematically illustrating another configuration example of a plasma processing apparatus according to the embodiment.  FIG. 7  illustrates a case where an electromagnet  30  is arranged around the lateral face of the chamber  11 . Also with this configuration, the distribution of a magnetic field above the upper face of the substrate  100  can be changed. Here, the constituent elements corresponding to those of  FIG. 1  are denoted by the same reference symbols, and their description will be omitted. 
     Further, in the above description, a case is illustrated where the coils  32  of the electromagnet  30  are arranged in a circular shape. However, the embodiment is not limited to this example.  FIG. 8  is a top view schematically illustrating an example of a coil shape. Where “n” is an integer of 2 or more, a coil  32  is divided into an n-number of arc-shaped coil segments, such that the n-number of coil segments are arranged in a ring shape. In the case of  FIG. 8 , “n” is 2, and the coil  32  is divided into two coil segments  32   a  and  32   b . Further, the two coil segments  32   a  and  32   b  are arranged around the chamber  11  and form a single ring. Here, each of the coil segments  32   a  and  32   b  may be provided with a plasma control unit  341  and an electric current control unit  342 . Alternatively, all the coil segments  32   a  and  32   b  may be provided with a plasma control unit  341  and an electric current control unit  342  in common. In the latter case, the plasma control unit  341  and the electric current control unit  342  are configured to individually control each of the coil segments  32   a  and  32   b.    
     Further, in the above description, the plasma processing apparatus is exemplified by a parallel-plate type. However, the embodiment described above may be applied to a plasma processing apparatus of an Inductive Coupled Plasma (ICP) type. Further, the plasma processing apparatus has been described by taking a plasma etching apparatus as an example. However, the embodiment described above may be applied to a film formation apparatus using plasma, such as a plasma Chemical Vapor Deposition (CVD) apparatus. 
     In the embodiment, the electromagnet controller  34  executes control to cause pulsed electric currents, in each of which its direction and ON/OFF are pulsed, to flow thorough the plurality of coils  32  of the electromagnet  30 , which is arranged at an arbitrary position of the chamber  11  of the plasma processing apparatus  10 . At this time, the electromagnet controller  34  causes the pulsed electric currents to flow so as to generate first state plasma and second state plasma alternately. Consequently, during the time period of performing a plasma process, the plasma density can be made uniform on average. Further, the result of the plasma process performed to the substrate  100  at a position where plasma is not uniform by the first state plasma becomes opposite in direction relative to the result of the plasma process performed to the substrate  100  at the position where plasma is not uniform by the second state plasma, and thus these results are offset each other. As a result, it is possible to give a desired result to the substrate  100  even in a state where the plasma density is not made uniform. 
     For example, during a plasma etching process, pulsed electric currents are caused to flow such that the first state plasma forms holes on the peripheral side of the substrate  100  to be directed outward relative to the processing face, and the second state plasma forms holes on the peripheral side of the substrate  100  to be directed inward relative to the processing face. Further, the first state plasma and the second state plasma are switched in a short time as compared with the total time of the etching process, and thus holes can be formed in a state almost perpendicular to the processing face. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.