Patent Publication Number: US-11651938-B2

Title: Impedance matching device provided in high-frequency power system

Description:
TECHNICAL FIELD 
     The present invention relates to an impedance matching device for use in a high-frequency power system configured to supply a load with an output from a high-frequency power source via a matching circuit whose constant is mechanically changed. 
     BACKGROUND ART 
     Semiconductor manufacturing processes, such as etching and thin-film formation, use a plasma treatment device and a high-frequency power system for supplying the plasma treatment device with necessary power. The high-frequency power system normally has the function of achieving an impedance match with the plasma treatment device in order to stably and efficiently supply the plasma treatment device with power. 
     Patent Document 1 discloses a high-frequency power system  100  with an impedance matching function, including a high-frequency power source  101 , an impedance matching device  102 , and a matching circuit  103  (see  FIG.  11   ). The impedance matching device  102  is configured to change an oscillation frequency of the high-frequency power source  101  in accordance with an impedance matching condition between the high-frequency power source  101  and a load  20  (a plasma treatment device). Moreover, the matching circuit  103  consists of passive elements whose constants are mechanically changed. 
     The high-frequency power system  100  is claimed to allow a broader matching range than do conventional systems by combining impedance matching which offers a fast matching speed but a narrow matching range, as achieved by changing the oscillation frequency, and impedance matching which offers a wide matching range but a slow matching speed, as achieved by the matching circuit  103 . 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: International Publication No. WO 2013/132591 
     DISCLOSURE OF THE INVENTION 
     Problems To Be Solved by the Invention 
     However, the conventional high-frequency power system  100  might not be able to improve an impedance mismatch caused by the impedance of the load  20  continuously changing after plasma ignition, resulting in unstable plasma or misfire. 
     The present invention has been achieved in view of the above issue, with a problem to be solved being to provide an impedance matching device capable of promptly improving an impedance mismatch between a high-frequency power source and a load even when the impedance of the load continuously changes. 
     Solution to the Problems 
     To solve the above problem, the present invention provides an impedance matching device for use in a high-frequency power system configured to supply a load with an output from a high-frequency power source via a matching circuit whose constant is mechanically changed, the device including a matching condition value acquisition portion for acquiring a matching condition value indicating a matching condition between the high-frequency power source and the load, and a control portion for controlling an oscillation frequency of the high-frequency power source based on the matching condition value, wherein, when the matching condition value indicates deterioration of the matching condition, the control portion changes the oscillation frequency with a first slope toward improving the matching condition, and thereafter shifts the oscillation frequency back to an original value with a second slope more gradual than the first slope. 
     The control portion of the impedance matching device may change the oscillation frequency with the first slope for a predetermined mismatch reduction period. In this case, when the mismatch reduction period ends, the control portion may start shifting the oscillation frequency back to the original value with the second slope, regardless of the matching condition indicated by the matching condition value. 
     Alternatively, the control portion of the impedance matching device may change the oscillation frequency with the first slope until the matching condition value indicates that the matching condition has been improved to a predetermined degree. 
     Furthermore, when an amount of change in the matching condition value exceeds a predetermined threshold, the control portion of the impedance matching device may start changing the oscillation frequency with the first slope. 
     Furthermore, the matching condition value used in the impedance matching device can be a value related to a reflected wave amplitude. 
     An example of the load on the impedance matching device is a plasma treatment device. 
     Effect of the Invention 
     The present invention renders it possible to provide an impedance matching device capable of promptly improving an impedance mismatch between a high-frequency power source and a load even when the impedance of the load continuously changes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram illustrating a schematic configuration of a high-frequency power system including an impedance matching device according to the present invention. 
         FIGS.  2 (A),  2 (B),  2 (C) and  2 (D)  provide circuit diagrams illustrating specific matching circuit configurations. 
         FIG.  3    is a flowchart showing an operation of a control portion included in an impedance matching device according to a first embodiment of the present invention. 
         FIGS.  4 (A) and  4 (B)  provide graphs describing an operation of the high-frequency power system according to the first embodiment. 
         FIGS.  5 (A) and  5 (B)  provide graphs describing another operation of the high-frequency power system according to the first embodiment. 
         FIGS.  6 (A) and  6 (B)  provide graphs describing still another operation of the high-frequency power system according to the first embodiment. 
         FIGS.  7 (A) and  7 (B)  provide graphs describing an operation of a high-frequency power system according to a comparative example. 
         FIGS.  8 (A) and  8 (B)  provide graphs describing another operation of the high-frequency power system according to the comparative example. 
         FIG.  9    is a flowchart showing an operation of a control portion included in an impedance matching device according to a second embodiment of the present invention. 
         FIGS.  10 (A) and  10 (B)  provide graphs describing an operation of the high-frequency power system according to the second embodiment. 
         FIG.  11    is a block diagram illustrating a schematic configuration of a conventional high-frequency power system. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of an impedance matching device according to the present invention will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG.  1    illustrates a high-frequency power system  10  including an impedance matching device  12  according to a first embodiment of the present invention. The high-frequency power system  10  is intended for supplying a load  20  with necessary power, and further includes a high-frequency power source  11  and a matching circuit  16 , as shown in the figure. 
     The high-frequency power source  11  outputs high-frequency power to the load  20  via the matching circuit  16 . The high-frequency power source  11  is configured such that an oscillation frequency (i.e., a frequency of the high-frequency power) can be adjusted within a certain range (in the present embodiment, the range of ±1.00 MHz from 13.56 MHz at the center) in accordance with a command from the impedance matching device  12 . 
     The matching circuit  16  includes two capacitors, each of which has a variable constant (capacitance), and one inductor, as shown in (A) of  FIG.  2   . The constants of the capacitors are changed by a matching condition sensing portion and a control portion included in the matching circuit  16 , both of which are unillustrated, so as to improve an impedance mismatch between the high-frequency power source  11  and the load  20 . However, such a change is mechanically performed using an electric actuator or suchlike, and therefore is very gradual. 
     It should be noted that the circuit configuration of the matching circuit  16  is not limited to that shown in (A) of  FIG.  2   , and may be, for example, as shown in (B) to (D) of the figure. In the present invention, the circuit configuration of the matching circuit  16  can be suitably selected in accordance with the configuration of the load  20  and/or other factors. 
     As is apparent from  FIG.  1   , the matching circuit  16  is independent from the impedance matching device  12 . Accordingly, it can be said that, when viewed from the impedance matching device  12 , the constants of the matching circuit  16  are changed autonomously. 
     The load  20  is a plasma treatment device (more specifically, a coil wound on a plasma chamber included in the plasma treatment device) for use in semiconductor manufacturing processes such as etching and thin-film formation. The impedance of the load  20  changes with each moment in accordance with, for example, the type and the amount of gas being introduced into the plasma chamber. 
     The impedance matching device  12  includes a matching condition value acquisition portion  13 , a control portion  14 , and a settings memory portion  15 . The impedance matching device  12  may be accommodated as a whole or in part (e.g., the control portion  14  and the settings memory portion  15 ) in a housing of the high-frequency power source  11  or the matching circuit  16 . Alternatively, all of the high-frequency power source  11 , the impedance matching device  12 , and the matching circuit  16  may be accommodated in one housing. 
     The matching condition value acquisition portion  13  is a directional coupler disposed on a power line connecting the high-frequency power source  11  and the matching circuit  16 . The matching condition value acquisition portion  13  outputs a value related to a reflected wave amplitude (reflected power value) as a matching condition value indicating a matching condition between the high-frequency power source  11  and the load  20 . The settings memory portion  15  is volatile or non-volatile memory. The settings memory portion  15  has memorized therein settings information inputted by a user in relation to the operation of the control portion  14 . The control portion  14  is a computer unit such as an MPU (micro-processing unit). The control portion  14  outputs a control signal for controlling the oscillation frequency of the high-frequency power source  11 , on the basis of the matching condition value outputted by the matching condition value acquisition portion  13  and the settings information memorized in the settings memory portion  15 . 
     Next, an operation example of the control portion  14  in the first embodiment will be described with reference to  FIGS.  3  to  5   . Note that for the sake of simplification, the following will be described with respect to the case where the impedance of the load  20  changes under the conditions that plasma ignition has already been completed, the oscillation frequency of the high-frequency power source  11  is a center frequency F 0  (=13.56 MHz), and there is no reflected wave. 
     Initially, the control portion  14  reads the settings information memorized in the settings memory portion  15  (step S 1 - 1 ). The settings information is related to a first slope, a second slope, a mismatch reduction period, and a threshold. 
     Next, the control portion  14  determines whether the matching condition has deteriorated sharply, on the basis of the matching condition value (step S 1 - 2 ). More specifically, the control portion  14  refers to the matching condition value every 100 μs, and when the amount of change between the previous matching condition value and the latest matching condition value exceeds a threshold ΔRth, the control portion  14  determines that the matching condition has deteriorated sharply. The control portion  14  repeats step S 1 - 2  every 100 μs until the matching condition deteriorates sharply. 
     Once the matching condition deteriorates sharply, the control portion  14  changes the oscillation frequency with the first slope toward improving the matching condition for the mismatch reduction period T 1  (step S 1 - 3 ). More specifically, the control portion  14  changes (in the operation example shown in  FIGS.  4  and  5   , decreases) the oscillation frequency every 100 μs by an amount corresponding to the first slope. 
     In the present embodiment, the mismatch reduction period T 1  is set at 500 μs. Accordingly, the control portion  14  changes the oscillation frequency five times during the mismatch reduction period T 1 . Moreover, in the present embodiment, the first slope is set at 5 Hz/μs. Accordingly, the control portion  14  decreases (or increases) the oscillation frequency every 100 μs by 500 Hz. 
     After a lapse of the mismatch reduction period T 1 , the control portion  14  shifts the oscillation frequency back to the center frequency F 0  with the second slope (step S 1 - 4 ). More specifically, the control portion  14  changes (in the operation example shown in  FIGS.  4  and  5   , increases) the oscillation frequency toward the center frequency F 0  every 100 μs by an amount corresponding to the second slope. 
     In the present embodiment, the second slope is set at 2.5 Hz/ms. That is, in the present embodiment, the second slope is set at 1/2000 of the first slope. Accordingly, the control portion  14  decreases (or increases) the oscillation frequency every 100 μs by 0.25 Hz. 
     It should be noted that not only in the case where the mismatch is significantly improved during the mismatch reduction period T 1  (see  FIG.  4   ), but also in the case where the mismatch is not significantly improved during the mismatch reduction period T 1  (see  FIG.  5   ), the control portion  14  executes step S 1 - 4  after a lapse of the mismatch reduction period T 1 , without further changing the oscillation frequency with the first slope. 
     The mismatch that was not improved at step S 1 - 3  is slowly improved by the constants of the matching circuit  16  being changed autonomously. Specifically, during a period T 2  in which the oscillation frequency of the high-frequency power source  11  returns to the center frequency F 0  with the second slope, the constants and the oscillation frequency are changed in parallel, but since the second slope is very gradual, as described earlier, both changes do not compete with each other, with the result that no hunting oscillation occurs. 
     In this manner, the impedance matching device  12  according to the present embodiment changes the oscillation frequency of the high-frequency power source  11  and thereby improves the mismatch to some degree, and thereafter shifts the oscillation frequency back to the original value. Accordingly, even when the matching condition continuously deteriorates, the impedance matching device  12  according to the present embodiment changes the oscillation frequency within the range between an upper frequency F 1  (in the present embodiment, 14.56 MHz) and a lower frequency F 2  (in the present embodiment, 12.56 MHz), thereby improving the matching condition within a short period of time (see  FIG.  6   ). 
     On the other hand, in a comparative example where step S 1 - 4  is not performed after step S 1 - 3  (see  FIGS.  7  and  8   ), when the matching condition continuously deteriorates, the oscillation frequency reaches either the upper frequency F 1  or the lower frequency F 2  (in  FIG.  8   , the lower frequency F 2 ), and thereafter, the matching condition cannot be improved by changing the oscillation frequency. That is, in the comparative example, when the matching condition continuously deteriorates, the matching condition cannot be improved within a short period of time. Note that the high-frequency power system  100  described in Patent Document 1 has a commonality with the comparative example in that no operation corresponding to step S 1 - 4  is performed. 
     Second Embodiment 
     An impedance matching device  12  according to a second embodiment of the present invention includes a matching condition value acquisition portion  13 , a control portion  14 , and a settings memory portion  15 , as in the first embodiment. However, in the present embodiment, the control portion  14  is operated differently from the first embodiment. An operation example of the control portion  14  in the second embodiment will be described below with reference to  FIGS.  9  and  10   . 
     Initially, the control portion  14  reads settings information memorized in the settings memory portion  15  (step S 2 - 1 ). The settings information is related to a first slope, a second slope, a mismatch reduction period, and a threshold. 
     Next, the control portion  14  determines whether the matching condition has deteriorated sharply (step S 2 - 2 ), as in step S 1 - 2  of the first embodiment. The control portion  14  repeats step S 2 - 2  every 100 μs until the matching condition deteriorates sharply. 
     Once the matching condition deteriorates sharply, the control portion  14  starts changing the oscillation frequency with the first slope toward improving the matching condition (step S 2 - 3 ). More specifically, the control portion  14  starts changing (in the operation example shown in  FIG.  10   , decreasing) the oscillation frequency every 100 μs by an amount corresponding to the first slope. The first slope is set at 5 Hz/μs, as in the first embodiment. 
     Next, the control portion  14  determines whether a total period since the start of the oscillation frequency change has reached a mismatch reduction period T 1  in order words, whether the mismatch reduction period T 1  has elapsed (step S 2 - 4 ). 
     In the case where the mismatch reduction period T 1  is determined to have elapsed, the control portion  14  shifts the oscillation frequency back to the center frequency F 0  with the second slope (step S 2 - 7 ), as in step S 1 - 4  of the first embodiment. The second slope is set at 2.5 Hz/ms, as in the first embodiment. That is, in the present embodiment also, the second slope is 1/2000 of the first slope. 
     On the other hand, when the mismatch reduction period T 1  is not determined to have elapsed, the control portion  14  determines whether the matching condition has deteriorated sharply again (step S 2 - 5 ), as in step S 2 - 2 . 
     When the matching condition is determined to have deteriorated sharply again, the control portion  14  resets the total period since the start of the oscillation frequency change (step S 2 - 6 ), and starts changing the oscillation frequency with the first slope toward improving the matching condition (step S 2 - 3 ). 
     The control portion  14  repeats steps S 2 - 4  and S 2 - 5  until the mismatch reduction period T 1  elapses or the matching condition deteriorates sharply during the mismatch reduction period T 1 . 
     In this manner, once the matching condition deteriorates sharply again during the mismatch reduction period T 1 , the impedance matching device  12  according to the present embodiment keeps on changing the oscillation frequency with the first slope from the start of the deterioration onward during the mismatch reduction period T 1 . Accordingly, even when the matching condition continually deteriorates at very short intervals, the impedance matching device  12  according to the present embodiment can improve the matching condition within a short period of time. 
     Variant 
     It should be noted that the impedance matching device according to the present invention has variants as exemplified below. 
     The control portion  14  may use VSWR (voltage standing wave ratio), which is obtained based on reflected wave power and travelling wave power, as a matching condition value to be referred to when determining whether the matching condition has deteriorated sharply. 
     The settings memory portion  15  may memorize combinations of an interval for changing the oscillation frequency and an amount of single change as the “first slope” and the “second slope”. In such a case, as the “mismatch reduction period”, the settings memory portion  15  may memorize the number of times the oscillation frequency is to be changed with the first slope. 
     The settings memory portion  15  may memorize as the “first slope” a combination of an interval for changing the oscillation frequency and a rule for deciding an amount of single change based on the matching condition value (the reflected wave amplitude or the VSWR). In such a case, the control portion  14  changes the oscillation frequency by an amount corresponding to the latest matching condition value. Moreover, in this case, the “first slope” might be changed during the mismatch reduction period. With this configuration, even when there is a large mismatch, the mismatch can be promptly improved by sharply changing the oscillation frequency. 
     The settings memory portion  15  may memorize a “target matching condition” instead of the “mismatch reduction period”. In such a case, the control portion  14  changes the oscillation frequency with the first slope until the matching condition value indicates that the matching condition has been improved to the “target matching condition”, and thereafter, the control portion  14  shifts the oscillation frequency with the second slope. 
     The load  20  does not have to be a plasma treatment device. 
     Specific numerical values in the first and second embodiments are merely illustrative examples and may be suitably changed. Note that from the viewpoint of preventing hunting oscillation, it is preferable to set the second slope as low as possible. However, when the second slope is excessively low, another matching condition deterioration occurs during the period T 2  (i.e., before the oscillation frequency completely returns to the center frequency F 0 ), causing difficulty in improving the matching condition by changing the oscillation frequency within the range between the upper frequency F 1  and the lower frequency F 2 . Accordingly, when setting the second slope, it is necessary to consider these contradictory circumstances. 
     DESCRIPTION OF THE REFERENCE CHARACTERS 
     
         
         
           
               10  high-frequency power system 
               11  high-frequency power source 
               12  impedance matching device 
               13  matching condition value acquisition portion 
               14  control portion 
               15  settings memory portion 
               16  matching circuit 
               20  load