Abstract:
An impedance matching network includes a first input port that receives radio frequency (RF) power and includes an input impedance, an output port that provides the RF power and includes an output impedance, and a variable capacitance module that varies the output impedance. The variable capacitance module includes a first variable capacitor, a second variable capacitor, a first motor, and a second motor that adjusts a capacitance of the second variable capacitor. A relationship between a desired value of the capacitance and an actual value of the capacitance is dependent on a capacitance of the first variable capacitor.

Description:
FIELD 
       [0001]    The present disclosure relates to controlling an impedance match between an output and an input of respective radio frequency (RF) devices. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    Referring now to  FIG. 1 , a functional block diagram is shown of a plasma processing system  10 . Plasma processing system  10  includes RF generator  12  that provides RF power for a plasma chamber  14 . A matching network  16  communicates the RF power to plasma chamber  14 . Matching network  16  includes a variable capacitance module  26  that determines a tuning range and resolution of impedances for matching network  16 . 
         [0004]    Referring briefly to  FIG. 2 , a schematic diagram is shown of a portion of variable capacitance module  26 . Variable capacitance module  26  includes a first adjustable capacitor  18 - 1  and a second adjustable capacitor  18 - 2 , collectively referred to as capacitors  18 . Capacitors  18  are adjusted by respective stepper motors  20 - 1  and  20 - 2  such that an output impedance of matching network  16  matches an input impedance of plasma chamber  14 . 
         [0005]    Returning to  FIG. 1 , matching network  16  includes a phase and magnitude detection module  22 . Phase and magnitude detection module  22  generates signals based on respective ones of the phase and magnitude of the RF power from RF generator  12 . Phase and magnitude detection module  22  communicates the signals to a control module  24  and communicates the RF power to variable capacitance module  26 . Variable capacitance module  26  communicates the RF power to fixed inductance and capacitance (LC) module  28 . Fixed LC module  28  cooperates with variable capacitance module  26  to vary the impedance of matching network  16 . Fixed LC module  28  also communicates the RF power to an output sensing module  30 . Output sensing module  30  generates signals based on the respective magnitude and/or phase of the RF power that is communicated to plasma chamber  14 . The signals are communicated to a performance monitoring module  32  that performs diagnostic monitoring functions. 
         [0006]    Control module  24  also generates stepper motor control signals that are communicated to respective ones of motors  20 . The stepper motor control signals are based on the signals from phase and magnitude detection module  22 , performance monitoring module  32 , and/or user input settings. 
         [0007]    Referring now to  FIG. 3A , a graph  40  shows a relationship between a user input setting for capacitor  18 - 1  and an associated number of steps or position for motor  20 - 1 . The user input setting for 100% capacitance is shown at  42 . The user input setting for 0% capacitance is shown at  44 . A vertical axis  46  indicates the number of steps for motor  20 - 1 . A horizontal axis  48  indicates the number of steps for motor  20 - 2 . Graph  40  shows that the user input settings (e.g.  42  and  44 ) for capacitor  18 - 1  are associated with respective numbers of steps for motor  20 - 1  independent of the position of motor  20 - 2  (and hence, the capacitance of capacitor  18 - 2 ). 
         [0008]    Referring now to  FIG. 3B , a graph  50  shows a relationship between a user input setting for capacitor  18 - 2  and the associated number of steps or position of motor  20 - 2 . The user input setting for 100% capacitance is shown at  52 . The user input setting for 0% capacitance is shown at  54 . A vertical axis  56  indicates the number of steps for motor  20 - 2 . A horizontal axis  58  indicates the number of steps for motor  20 - 1 . Graph  50  shows that the user input settings (e.g.  52  and  54 ) for capacitor  18 - 2  are associated with respective numbers of steps for motor  20 - 2  independent of the position of motor  20 - 1  (and hence, the capacitance of capacitor  18 - 1 ). 
         [0009]    Referring now to  FIG. 4 , a Smith chart  60  illustrates an effect of the motor position and capacitance relationships that are shown in  FIGS. 3A and 3B . Generally the minimum and maximum capacitances of capacitors  18  are chosen such that matching network  16  has enough range to allow tuning to several impedances on the Smith chart  60 . An example tuning range is shown at  62 . The range  62  may be much larger than what is needed to tune to a single impedance  64  during operation. Since range  62  may be larger than what is needed the adjustment resolutions of capacitors  18  may also be more coarse than necessary. 
       SUMMARY 
       [0010]    An impedance matching network includes a first input port that receives radio frequency (RF) power and includes an input impedance, an output port that provides the RF power and includes an output impedance, and a variable capacitance module that varies the output impedance. The variable capacitance module includes a first variable capacitor, a second variable capacitor, a first motor, and a second motor that adjusts a capacitance of the second variable capacitor. A relationship between a desired value of the capacitance and an actual value of the capacitance is dependent on a capacitance of the first variable capacitor. 
         [0011]    In other features the first and second motors are stepper motors. The impedance matching network further includes a control module that determines the desired value based on a portion of the RF power that is reflected from a load that is driven by the output port. The control module drives the second motor. The control module includes a lookup table that is stored in a memory and represents the relationship between the desired and actual values of the capacitance. 
         [0012]    In other features the relationship between the desired and actual values of the capacitance is described by a polynomial relationship. The relationship between the desired and actual values of the capacitance can also be described by a piecewise linear relationship. 
         [0013]    An impedance matching network includes a first input port that receives radio frequency (RF) power and includes an input impedance, an output port that provides the RF power and includes an output impedance, and a variable capacitance circuit that varies the output impedance. The variable capacitance circuit includes a first variable capacitor, a second variable capacitor, and a first motor that includes an output shaft that rotates to a first position. The first position corresponds with a first capacitance of the first variable capacitor. A second motor includes an output shaft that rotates to a second position. The second position corresponds with a second capacitance of the second variable capacitor. A relationship between a desired value of the first capacitance and an actual value of the first capacitance is dependent on the second position. 
         [0014]    In other features a relationship between a desired value of the second capacitance and an actual value of the second capacitance is dependent on the first position. The first and second motors are stepper motors. The impedance matching network also includes a control module that determines the desired value based on a portion of the RF power that is reflected from a load that is driven by the output port. The control module determines the first and second positions. The control module includes a lookup table that is stored in memory and represents the relationship between the desired and actual values of the capacitance. 
         [0015]    In other features the relationship between the desired and actual values of the first capacitance is described by a polynomial equation. The relationship between the desired and actual values of the first capacitance may also be described by a piecewise linear equation. 
         [0016]    A method of operating an impedance matching network includes receiving radio frequency (RF) power via an input that includes an input impedance, providing the RF power at an output that includes an output impedance, and adjusting first and second capacitances to vary the output impedance. A relationship between a desired value of the second capacitance and an actual value of the second capacitance is dependent on the value of the first capacitance. 
         [0017]    In other features adjusting first and second capacitances includes driving respective first and second motors to positions that determine the respective first and second capacitances. The method includes determining the desired value based on a portion of the RF power that is reflected from a load that is driven by the output port. The method includes storing a lookup table in a computer memory. The lookup table represents the relationship between the desired and actual values of the second capacitance. 
         [0018]    In other features the relationship between the desired and actual values of the second capacitance is described by a polynomial relationship. The relationship between the desired and actual values of the capacitance may also be described by a piecewise linear relationship. 
         [0019]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0020]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0021]      FIG. 1  is a functional block diagram of a radio frequency plasma processing system; 
           [0022]      FIG. 2  is a schematic drawing of adjustable tuning capacitors and associated stepper motors; 
           [0023]      FIGS. 3A and 3B  are graphs that show relationships between desired positions and actual positions of the adjustable tuning capacitors of  FIG. 2 ; 
           [0024]      FIG. 4  is a Smith chart that shows a tuning range of a match network that employs the adjustable tuning capacitors and the relationships of  FIGS. 3A and 3B ; 
           [0025]      FIGS. 5A and 5B  are graphs that show sloped relationships between desired positions and actual positions of the adjustable tuning capacitors of  FIG. 2 ; 
           [0026]      FIG. 6  is a chart of adjustment points for the relationships of  FIGS. 5A and 5B ; 
           [0027]      FIG. 7  is a flowchart for determining the positions of the adjustable tuning capacitors of  FIG. 2 ; 
           [0028]      FIG. 8  is a Smith chart that shows a tuning range of a match network that employs the adjustable tuning capacitors and the relationships of  FIGS. 5A and 5B ; 
           [0029]      FIG. 9  is a graph that shows a polynomial relationship between desired positions and actual positions of the adjustable tuning capacitors of  FIG. 2 ; and 
           [0030]      FIG. 10  is a chart that shows a piecewise-linear relationship between desired positions and actual positions of the adjustable tuning capacitors of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
         [0032]    Referring now to  FIGS. 5A and 5B , graphs  100  and  120  show one of various embodiments of relationships between user input settings for capacitors  18  and associated number of steps for respective motors  20 . The relationships determine the number of motor steps or positions of each motor  20  based on the user input settings and the number of motor steps of the other one of motors  20 . The relationships allow matching network  16  to tune to a wide range of impedances while also providing improved tuning resolution around a particular impedance. The user input settings can be generated by control module  24  of  FIG. 1 . As shown the user input settings are scaled as a percentage of maximum capacitance of respective capacitors  18 , however it should be appreciated that other scales may also be used. 
         [0033]    Referring now to  FIG. 5A , graph  100  shows relationships that can be used to determine the position of motor  20 - 1  and, consequently, the capacitance of capacitor  18 - 1 . The user input setting for 100% capacitance of capacitor  18 - 1  is shown at  102 . The user input setting for 0% capacitance of capacitor  18 - 1  is shown at  104 . A vertical axis  106  indicates the number of steps for motor  20 - 1 . A horizontal axis  108  indicates the number of steps for motor  20 - 2 . Graph  100  shows that for a given user input setting (e.g.  102  or  104 ) the position of motor  20 - 1  is dependent on the user input setting and the position of motor  20 - 2 . If the user input setting is between 0% and 100% then the number of motor steps can be interpolated from relationships  102  and  104 . 
         [0034]    Referring now to  FIG. 5B , graph  120  shows relationships that can be used to determine the position of motor  20 - 2  and, consequently, the capacitance of capacitor  18 - 2 . The user input setting for 100% capacitance of capacitor  18 - 2  is shown at  122 . The user input setting for 0% capacitance of capacitor  18 - 2  is shown at  124 . A vertical axis  126  indicates the number of steps for motor  20 - 2 . A horizontal axis  128  indicates the number of steps for motor  20 - 1 . Graph  120  shows that for a given user input setting for capacitor  18 - 2  (e.g.  122  or  124 ) the position of motor  20 - 2  is dependent on the user input setting and the position of motor  20 - 1 . 
         [0035]    Graphs  100  and  120  can be implemented as look-up tables that are stored in a computer-readable memory. The memory can be included in control module  24 . In other embodiments control module  24  can include a computer program that is stored in a computer-readable memory. The computer program can be executed by a processor. The processor can be included with control module  24 . The computer program can implement graphs  100  and  120  with respective equations. One set of equations can estimate the position of motor  20 - 1  and another set of equations can estimate the position of motor  20 - 2 . Each set of equations can take the form 
         [0000]        C   1UserPosition   =C   1MotorStepValue   *C   1Gain   +C   1Offset ,  (1) 
         [0000]    where C 1UserPosition  is the user input setting for the capacitor  18  of interest, C 1MotorStepValue  is the number of steps of the motor  20  that is associated with the capacitor  18  of interest, and C 1Gain  and C 1Offset  are point-slope equation variables that can be determined based on the equations 
         [0000]        C   1Gain =((100− C   2Position )/100)* C   1Gain0 +( C   2Position /100)* C   1Gain1  and  (2) 
         [0000]        C   1Offset =((100− C   2Position )/100)* C   1Offset0 +( C   2Position /100)* C   1Offset1 .  (3) 
         [0000]    C 2Position  is the position of the stepper motor  20  that is associated with the other one of the capacitors  18 . C 1Gain0  and C 1Gain1  are respective slopes. 
         [0036]    Referring now to  FIG. 6 , a chart  130  lists endpoints of position relationships (e.g.  102  and  104 , or  122  and  124 ) and corresponding variables that determine the locations of the endpoints. Chart  130  includes a first column  132  and a second column  134 . First column  132  lists the endpoints of the relationship of interest (e.g.  102  and  104 , or  122  and  124 ). Second column  134  lists the corresponding variables that determine the positions of the relationship of interest. The variables can be changed to provide a desired combination of tuning range and/or resolution for tuning capacitors  18 . 
         [0037]    Referring now to  FIG. 7 , a flowchart illustrates a method  140  for estimating the positions of motors  20 . Method  140  can be implemented as computer-readable instructions that are stored in computer memory associated with control module  24 . The instructions can be executed by a processor that can also be included with control module  24 . 
         [0038]    Control enters through block  142  and proceeds to block  144 . In block  144  control determines a maximum capacitance that is needed from variable capacitance module  26  (see  FIG. 1 ). The maximum capacitance can be determined based on the inductance and/or capacitance of fixed LC module  28  and an anticipated scope of impedance mismatches between the output of RF generator  12  and the input of plasma chamber  14 . Control then proceeds to block  146  and estimates the capacitances that are needed from adjustable capacitors C 18 . The capacitances can be represented as a number of motor steps (e.g. motor position) of motor  20 . Control may estimate the capacitances and associated motor steps by solving equation (1) above for each capacitor C 18  while simultaneously satisfying the conditions that 1) neither capacitor  18  is allowed to be at 0% capacitance and 2) a sum of the positions of motors  20  is less than or equal to the maximum number of steps that was determined in block  144 . Control then returns to other processes via block  148 . 
         [0039]    Referring now to  FIG. 8 , a Smith chart  160  illustrates a tuning range  162 . Tuning range  162  is a result of the motor position and capacitance relationships that are shown in  FIGS. 3A and 3B  together with method  140 . Comparing tuning range  162  to tuning range  62  ( FIG. 4 ) of the prior art, it can be seen that circuits and methods that are disclosed herein limit the tuning range in the capacitive region (bottom half) of Smith chart  160 . The tune range has therefore been made considerably smaller while allowing the impedance  64  to be reached. By limiting the tune range a unit-to-unit repeatability between a plurality of matching networks  16  should be increased. Also the conformity within a single matching network  16  in terms of efficiency will be increased. That is, prior art impedance matching networks with large tuning ranges often have widely varying impedance values. By decreasing the range as disclosed herein the impedance will vary less from capacitance limits of capacitors  18 . 
         [0040]    Referring now to  FIG. 9 , a chart  170  shows that the user position/capacitor position relationships, such as  102 ,  104 ,  122 , and/or  124  can also have a curve shape. The curve can be implemented in a look-up table or be approximated by a polynomial equation. 
         [0041]    Referring now to  FIG. 10 , a chart  180  shows that the user position/capacitor position relationships, such as  102 ,  104 ,  122 , and/or  124  can also be represented by a piecewise linear approximation of a curve. The piecewise linear approximation can be implemented in the look-up table. Each segment of the piecewise linear approximation may also be approximated by respective instances of equations (1)-(3). Each slope and endpoint would then be associated with the slope and endpoints of a respective one of the segments of the piecewise linear approximation.