Abstract:
A recording medium suitable for use in an impedance matching apparatus adapted to a waveguide transmitting a microwave to a load. The apparatus includes detecting diodes and a computer. The recording medium includes a program, which includes predetermined approximate expressions. The predetermined approximate expressions recorded in the program correspond to the detecting diodes. The program causes the computer to compute an approximation value of input power to the waveguide using the respective predetermined approximate expressions and output voltages of the detecting diodes. The program also causes the computer to compute coefficient of reflection and phase of the input power using the approximation value; determine whether impedances of the waveguide and the load match with each other based on the computed coefficient of reflection and phase of the input power; and execute automatic impedance matching of the impedance matching apparatus when the impedances of the waveguide and the load do not match with each other.

Description:
This application is a divisional application filed under 37 CFR § 1.53(b) of parent application Ser. No. 08/946,004, filed Oct. 7, 1997, now U.S. Pat. No. 5,939,953. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an automatic matching device provided in a waveguide, and more particularly, to an automatic matching device used for transmitting a microwave and reducing power losses of the microwave, which is to be transmitted to a load, by attenuating a standing wave developed in the waveguide to match the impedance of the waveguide to that of the load. 
     2. Description of the Related Art 
     It is known to provide an automatic matching device in a waveguide, which transmits a microwave generated by a magnetron to a load, in order to efficiently transmit the power of the microwave to the load by matching the impedance of the waveguide to the load. This automatic matching device efficiently transmits the power of the microwave to the load by detecting a standing wave developed in the waveguide and by automatically operating to attenuate this standing wave. In order to increase the efficiency of transmission of the microwave power to a much greater extent, the speed of automatic impedance matching operations must be increased, and the automatic matching device must be compact. 
     The automatic matching device includes a detector section for detecting the standing wave developed in the waveguide; a matching section for attenuating the standing wave by matching the impedance of the waveguide to that of the load; and a control section for operating the matching device in accordance with the signal received from the detector section to attenuate the standing wave. 
     As known matching devices, there are stub matching devices,  4 -E matching devices, and E-H matching devices. 
     The stub matching device has two or three stubs inserted into a waveguide, and impedance matching of the waveguide to the load is performed by adjusting the lengths of the inserted portions of the stubs. 
     In an automatic matching device that employs such a stub matching device, the control section adjusts the lengths of the inserted portions of the stubs in accordance with the signal received from the detector section so as to match the impedance of the waveguide to that of the load. 
     As shown in FIGS. 1A and 1B, the  4 -E matching device comprises a waveguide  1  having a rectangular cross section and four E-plane branch waveguides  3   a  to  3   d  connected to a wider side, or an E-plane  2 , of the waveguide  1 . Provided that one wavelength of the microwave that travels along the waveguide  1  is λg, the distance between the waveguides  3   a ,  3   b  and that between  3   c ,  3   d  is set to λg/4, and the distance between the waveguides  3   b ,  3   c  is set to 3λg/8. 
     A short-circuiting plunger  4  (hereinafter “short plunger”) is provided in each of the waveguides  3   a  to  3   d , and the impedance of the waveguide  1  is matched to that of the load by adjusting the position of the short plunger  4  within each of the waveguides  3   a  to  3   d.    
     In an automatic matching device that employs such an  4 -E matching device, the control section adjusts the positions of the short plungers  4  in accordance with the signal received from the detector section so as to match the impedance of the waveguide  1  to that of the load. 
     As shown in FIG. 2, the conventional E-H matching device comprises the waveguide  1 , an E-plane branch waveguide  5  connected to the E-plane  2  of the waveguide  1 , and an H-plane branch waveguide  6  connected to an H-plane, or a narrower side of the waveguide  1 . As shown in FIG. 3, a short plunger  7  is provided in each of the waveguides  5  and  6 , and impedance matching of the waveguide  1  to the load is performed by adjusting the positions of the short plungers  7  in the waveguides  5  and  6  within the range of λg/2. 
     In an automatic matching device that employs such an E-H matching device, the control section adjusts the positions of the short plungers  7  in accordance with the signal received from the detector section so as to match the impedance of the waveguide  1  to that of the load. 
     The detector section detects a standing wave developed in the waveguide and outputs the result of such detection to the control section. The detector section comprises three or more detecting diodes provided along the axis of the waveguide such that the tip ends of the detecting diodes are exposed to the interior of the waveguide. Output voltages from the diodes are fed to the control section as a power-distribution signal. In accordance with this power-distribution signal, the control section detects the presence/absence of a standing wave and controls the matching device so as to attenuate the standing wave or to match the impedance of the waveguide to that of the load. 
     Each of the detecting diodes has a varying input-power-to-output-voltage characteristic. The input-power-to-output-voltage characteristic of the detecting diode comprises a linear region, a square-curve region, and a saturation region, in which the output voltage changes very little with respect to a variation in the input power. 
     If a dynamic range of the power distribution within the waveguide exceeds the liner region and reaches the square-curve region of the detecting diode, for example, the output characteristic of the diode in the square-curve region must be corrected such that it becomes the same as the output characteristic in the linear region. To this end, the output voltage of each detecting diode is corrected by an analog circuit for characteristic correction purposes, and the thus-corrected output voltage is provided to the control section. 
     Further, if the dynamic range of the power distribution within the waveguide increases and reaches the saturation region of the detecting diode, the analog circuit corrects the output characteristic of the diode in the saturation range such that it becomes the same as the output characteristic in the linear region. The thus-corrected voltage is output. With such a circuit configuration, even if the input power of the detecting diode increases with the result that characteristic range of the detecting diode shifts to a different range, an error in the output voltage is corrected by the analog circuit, and the thus-corrected voltage is output to the control section. 
     In the stub matching device of the foregoing automatic matching devices, if the power of the microwave transmitted to the load by the waveguide is increased, an electric discharge is likely to occur between the tip end of the stub and the interior surface of the waveguide. Further, if the amount of insertion of the stub is increased in order to sufficiently match the impedance of the waveguide to that of the load, an electric discharge is likely to occur between the tip end of the stub and the interior surface of the waveguide. 
     As a result, it is difficult to ascertain the amount of insertion of the stub in order to sufficiently guarantee the range of impedance matching and to transmit a microwave having sufficient power to the load. Accordingly, it is difficult for the stub matching device to perform impedance matching with regard to a microwave having high power. More specifically, in the case of impedance matching with regard to a microwave of 2.45 GHz, impedance matching with regard to about 2 kW is the limit of the stub matching device. 
     The  4 -E matching device is superior to the stub matching device in terms of resistance to power. However, as shown in FIG. 1A, the four E-plane branch waveguides  3   a  to  3   d  must be provided at predetermined intervals on the E-plane of the waveguide  1 . Accordingly, if the length L of the matching device is increased, the size of a three-dimensional circuit constituting the matching device is increased accordingly. 
     Like the  4 -E matching device, the E-H matching device is superior to the stub matching device in terms of resistance to power. However, as shown in FIG. 2, the E-plane branch waveguide protrudes from the plane E in the vertical direction, whereas the H-plane branch waveguide protrudes from the plane H in the horizontal direction, which causes the three-dimensional circuit constituting the matching device to be bulky. 
     Further, in the E-H matching device, an unwanted high-order mode with respect to the frequency λg of the microwave to be transmitted is likely to be generated due to the presence of the H-plane branch waveguide, so that the power distribution of the standing wave is susceptible to disturbance. As shown in FIG. 16, at the time of a matching  25  operation, when an attempt is made to move a normalized resistance R to a matching point P along a circle R=1 by moving the short plunger provided in the E-plane branch waveguide or to move a normalized conductance G to the matching point P along a circle G=1 by moving the short plunger provided in the H-plane branch waveguide, neither the normalized resistance R nor the normalized conductance G moves along the corresponding circle. As a result, it becomes considerably difficult for the automatic matching device, which employs such an E-H matching device, to perform automatic matching operations. 
     Another problem suffered by the E-H matching device is that when the detector section detects the standing wave by detecting the distribution of power in the waveguide, owing to the disturbance of the power distribution, it becomes impossible for the detector section to accurately detect the standing wave. In order to reduce the influence of the E-H matching device exerted on the detector section, the distance between the E-H matching device and the detector section must be increased. However, such a circuit configuration results in an increase in the size of the three-dimensional circuit constituting the matching device. 
     The detector section must correct the characteristic variation of each detecting diode by using an analog circuit. If the dynamic range of the power distribution in the waveguide extends from the linear region to the square curve region of the detecting diode, the output characteristic variation of the diode must be corrected by the analog circuit, and the power distribution must be detected in accordance with the corrected output voltage of the detecting diode. Further, if the dynamic range of the power distribution in the waveguide reaches the saturation region of the detecting diode, a corresponding output voltage variation must also be corrected by the analog circuit. 
     As a result, the analog circuit becomes complicated, and the adjustment of the output characteristic of the circuit becomes considerably complicated. Moreover, it is impossible to completely correct all the variations in the characteristics. Accordingly, the distribution of power in the waveguide, or the standing wave, cannot be accurately detected. 
     If the detecting diode must be exchanged with a new one, the analog circuit must be readjusted in accordance with the output characteristic of the new detecting diode. In contrast, if the analog circuit must be replaced with a new one, the output characteristic of the new analog circuit must be adjusted in accordance with the output characteristics of the detecting diodes to be connected to the analog circuit. As described above, replacement of the detecting diode or the analog circuit requires very complicated exchange operations. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an E-H matching device that reduces the size of a three-dimensional circuit and ensures sufficient resistance to power. 
     Another object of the invention is to provide an automatic matching device that reduces the size of a three-dimensional circuit and accurately detects a standing wave within a waveguide. 
     Still another object of the invention is to provide an automatic matching device equipped with a detector section for accurately detecting the distribution of power in a waveguide by readily and accurately correcting variations in and the output characteristics of detecting diodes. 
     Yet another object of the invention is to provide an automatic matching device equipped with a detector section that readily adjusts the output characteristic of a detecting diode after replacement of the diode. 
     A further object of the invention is to provide an automatic matching device and an automatic matching method, both of which permit high-speed impedance matching. 
     One aspect of the apparatus invention is directed to an E-H matching apparatus for a waveguide body. The E-H matching apparatus includes: an E-plane branch waveguide connected to the waveguide body; an H-plane branch waveguide connected to the waveguide body; and plungers provided in the E-plane branch waveguide and the H-plane branch waveguide, respectively. The plungers are moved to establish impedance matching between the waveguide body and a load. The H-plane branch waveguide has a bend portion formed in close proximity to the waveguide body. 
     Another aspect of the apparatus invention is directed to an automatic microwave impedance-matching apparatus adapted to a waveguide body transmitting a microwave to a load. The automatic microwave impedance-matching apparatus includes: an E-H matching apparatus including an H-plane branch waveguide and H-plane branch waveguide connected to the waveguide body, the E-H matching apparatus further including plungers provided in the E-plane branch waveguide and the H-plane branch waveguide; a detector unit for detecting the distribution of power within the waveguide body; and a control unit for receiving a detection result from the detector unit and detecting a standing wave developed as a result of an impedance mismatch between the load and the waveguide body, the control unit further controlling the plungers so as to attenuate the standing wave. 
     One aspect of the method invention is directed to a method for automatically matching microwave impedance between a load and a waveguide. The method includes the steps of: detecting the power of the inside of the waveguide four or more points defined at intervals corresponding to one-eighth of the wavelength of the microwave to be transmitted along the waveguide and generating a plurality of detection signals; eliminating, from the detection signals, a detection signal associated with one of the points that exceeds the dynamic range of the detection signal; detecting the standing wave developed in the waveguide in accordance with the remaining detection signals; and attenuating the detected standing wave. 
     Another aspect of the method invention is directed to a method for automatically matching microwave impedance between a load and a waveguide. The method includes the steps of: providing an impedance matching apparatus, which is connected to the waveguide, for detecting and attenuating a standing wave developed in the waveguide, the impedance matching apparatus including a movable plunger used for attenuating the standing wave; moving the plunger at high speed when the standing wave is greater than a predetermined level; and moving the plungers at low speed when the standing wave is smaller than the predetermined level. 
     One aspect of the invention is directed to a recording medium suitable for use in an impedance matching apparatus adapted to a waveguide transmitting a microwave to a load. The apparatus has detecting diodes and a computer. The recording medium has a program, which includes a predetermined approximate expression, recorded therein. The program causes the computer to: compute an approximation value of input power to the waveguide using the predetermined approximate expression and output voltages of detecting diodes; compute coefficient of reflection and phase of the input power; determine whether impedances of the waveguide and the load match with each other; and execute automatic impedance matching of the impedance matching apparatus when the impedances of the waveguide and the load do not match with each other. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which; 
     FIGS. 1A and 1B are explanatory views showing a conventional  4 -E matching device; 
     FIG. 2 is a perspective view showing a conventional E-H matching device; 
     FIG. 3 is a cross-sectional view showing the conventional E-H matching device; 
     FIG. 4 is a perspective view showing the outline of an E-H matching device of the present invention; 
     FIG. 5 is a block diagram of a plasma generator of one embodiment of the present invention; 
     FIG. 6 is a perspective view showing an E-H matching device used in one embodiment of the invention; 
     FIG. 7 is a partially sectioned front view showing the E-H matching device used in the embodiment of FIG. 6; 
     FIG. 8 is a partially sectioned side view showing the E-H matching device used in the embodiment of FIG. 6; 
     FIG. 9 is a plan view showing the E-H matching device, in which an E-bend is brought in proximity to a waveguide; 
     FIG. 10 is an explanatory view showing the operation of a short plunger used in the embodiment of FIG. 6; 
     FIG. 11 is an explanatory view showing the state of a reflected waveform within the E-H matching device; 
     FIG. 12 is an explanatory view showing the operation of the short plunger of the E-H matching device; 
     FIG. 13 is a block diagram showing an automatic matching device of the present invention; 
     FIG. 14 is an explanatory diagram showing the operation of a detector section; 
     FIG. 15 is an explanatory diagram showing the matching operation of the E-H matching device of the present invention; 
     FIG. 16 is an explanatory diagram showing the matching operation of the conventional E-H matching device; 
     FIGS. 17-20 are explanatory views showing the matching operation of the E-H matching device of the present invention; 
     FIGS. 21-26 are flowcharts illustrating the automatic matching operation of the device of the present invention; 
     FIG. 27 is a perspective view showing a modification of the E-H matching device in accordance with the present invention; 
     FIG. 28 is a perspective view showing another modification of the E-H matching device in accordance with the present invention; and 
     FIG. 29 is a perspective view showing still another modification of the E-H matching device in accordance with the present invention. 
     FIG. 30 is an explanatory diagram illustrating a detection section at the time an approximate expression is generated; 
     FIG. 31 is a flowchart showing a first approximation system; 
     FIG. 32 is a flowchart showing a second approximation system; 
     FIG. 33 is a flowchart illustrating an automatic matching operation of an automatic impedance matching device; 
     FIG. 34 is a schematic diagram of semiconductor equipment having an automatic impedance matching device according to the invention; and 
     FIG. 35 is a schematic diagram of semiconductor equipment having a conventional automatic impedance matching device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 is a perspective view showing the outline of an E-H matching device according to the present invention. The E-H matching device includes a waveguide  14 , an E-plane branch waveguide  20  connected to an E plane of the waveguide  14 , and an H-plane branch waveguide  25  connected to an H plane of the waveguide  14 . Impedance matching of the waveguide  14  to a load is effected by moving short plungers  21 ,  26  provided in the respective waveguides  20  and  25 . The H-plane branch waveguide  25  is provided with an E-bend  30  which is formed in proximity to the waveguide  14 . 
     First Embodiment 
     FIG. 5 is a block diagram of a plasma generator equipped with a microwave automatic matching device. A magnetron  11  generates a microwave having a predetermined frequency when power is supplied thereto from a power source  12 . The microwave is transmitted to the waveguide  14  via an isolator  13  and is further transmitted to a chamber  15  from the waveguide  14 . 
     Plasma is generated from the thus-supplied microwave in the chamber  15  and is then used in, e.g., a process of manufacturing semiconductor devices. 
     An automatic impedance matching device  16  is provided for the waveguide  14  to efficiently transmit the power of the microwave to the chamber  15  by matching the impedance of the waveguide  14  to that of a load, or to that of the chamber  15 . 
     The automatic matching device  16  comprises a detector section  17  for detecting a developed standing wave by detecting the distribution of power within the waveguide  14 ; a control section  18 , which receives a signal from the detector section  17  and calculates and outputs a control signal for attenuating the standing wave; and an E-H matching device  19 , which receives the control signal from the control section  18  and attenuates the developed standing wave in the waveguide  14  in accordance with the control signal. 
     The specific configuration of the E-H matching device  19  and the detector section  17  is described with reference to FIGS. 6 to  8 . The E-plane branch waveguide  20  is formed on the E-plane of the waveguide  14 , and, as shown in FIG. 8, an E-plane short plunger  21  is provided in the waveguide  20 . 
     The E-plane short plunger  21  is in screw-engagement with a feed screw  22 , and a pulley  23  is attached to the upper end of the feed screw  22 . The pulley  23  is rotatively driven by an E-plane motor  24  via a belt. As a result of the pulley  23  being rotated by the E-plane motor  24 , which is controlled in accordance with the control signal received from the control section  18 , the feed screw  22  is rotated. In association with the rotation of the feed screw  22 , the E-plane short plunger  21  moves up and down within the E-plane branch waveguide  20 . 
     The H-plane branch waveguide  25  is provided on the H plane of the waveguide  14 . The H-plane branch waveguide  25  is bent at the E-bend  30  so that the H-plane branch waveguide  25  extends in the vertical direction, i.e., in a direction parallel to the E-plane branch waveguide  20 . 
     As shown in FIG. 8, an H-plane short plunger  26  is provided in the H-plane branch waveguide  25 . The H-plane short plunger  26  is in screw-engagement with a feed screw  27 , and a pulley  28  is attached to the upper end of the feed screw  27 . The pulley  28  is rotatively driven by an H-plane motor  29  via a belt. As a result of the pulley  28  being rotated by the H-plane motor  29  which is controlled in accordance with the control signal received from the control section  18 , the feed screw  27  is rotated. In association with the rotation of the feed screw  27 , the H-plane short plunger  26  moves up and down within the H-plane branch waveguide  25 . 
     As shown in FIG. 9, if a microwave having a frequency of 2.45 GHz is to be transmitted along a WRJ- 2  waveguide, the E-H matching device has a dimension L 1  of the E plane measuring 108.2 mm, a dimension L 2  of the H plane measuring 54.6 mm, and a distance L 3  between the waveguide  14  and the H-plane branch waveguide  25  measuring 162.8 mm. 
     In the H-plane branch waveguide  25 , provided that one wavelength of the microwave of 2.45 GHz is λg, the distance between the E-bend  30  and the H plane of the waveguide  14  is set to λg/4 or less. Specifically, a distance L 4  (FIG. 8) between the interior surface of the H plane of the waveguide  14  and the point of the bend on the center axis of the H-plane branch waveguide  25  within the E-bend  30  is set to λg/4 or less. 
     As shown in FIG. 10, the waveguide  25  for transmitting a microwave equipped with an E-bend  30  has the effect of shortening the wavelength λg of the microwave passing through the E-bend  30 . Further, the waveguide has the effect of removing a part of an unwanted frequency band of the microwave passing through the E-bend  30  and cuts off the disturbance of an electromagnetic field occurring along the interface between the waveguide  14  and the H-plane branch waveguide  25 . Accordingly, disturbance in the distribution of power is prevented in the interior of the waveguide  14  in the vicinity of the interface between the waveguide  14  and the H-plane branch waveguide  25 . 
     In the conventional E-H matching device as shown in FIG. 2, the H-plane branch waveguide  6  is ended without any bend, and a high-order mode occurs depending on the position of the H-plane short plunger. In contrast, in accordance with the present embodiment, the influence of the high-order mode is reduced by the E-bend  30 , enabling improvements in the characteristics of the matching device. 
     FIG. 11 shows the state of reflection of a conventional E-H matching device in which the degrees of reflection when the respective short plungers are positioned at different positions are shown. In the case of the construction of a full-range matching device in which the short plungers are operated or moved while the interior surfaces  1   a  and  1   b  of the waveguide  1  are regarded as the points of origin, the plungers must pass through the positions of complete reflection. More specifically, the short plungers are usually moved while the positions of no reflection, which are spaced λg/2 away from the interior surfaces  1   a  and  1   b  of the waveguide  1 , are used as the centers of the operation ranges. In this case, full-range matching becomes feasible by movement of the short plungers  7  within the respective ranges of operation D 1 , D 2  shown in FIG.  12 . 
     In the present embodiment, the H-plane short plunger  26  must be moved within the operation range D 3  shown in FIG. 10 because the E-bend  30  is directly connected to the interface between the waveguide  14  and the H-plane branch waveguide  25 . That is, the position of no reflection must be spaced λg apart from the interior surface  14   a  of the waveguide  14 . The λg a parting position is used as the center of the operation range D 3 . 
     The wavelength λg of the microwave becomes shorter by virtue of the wavelength-shortening effect of the E-bend  30 . Therefore, the distance between the center of the operation range D 3  of the H-plane short plunger  26  and the interior surface  14   a  of the waveguide  14  is shortened. As a result, the length of the H-plane branch waveguide  25  is reduced. 
     As shown in FIGS. 6 and 7, four detecting diodes W 1  to W 4  are arranged on the E plane of the waveguide  14  in the vicinity of the E-plane branch waveguide  20  such that they are arranged in a line at intervals of λg/8 in the axial direction of the waveguide  14 . The tip ends of the detecting diodes W 1  to W 4  are exposed inside the waveguide  14 . Each of the detecting diodes W 1  to W 4  detects input power in accordance with the distribution of power within the waveguide  14  and outputs to the control section  18  a voltage corresponding to the input power. Here, the detecting diodes W 1  to W 4  need not be spaced with the interval of λg/8. Also, the diodes W 1  to W 4  need not be arranged in a line as shown. 
     As shown in FIG. 14, the wavelength of the standing wave SW developed in the waveguide  14  becomes λg/2. Since the four detecting diodes W 1  to W 4  are provided at intervals of λg/8, at least three of the detecting diodes do not correspond to the valley of the standing wave SW. Accordingly, the distribution of power within the waveguide  14  is accurately detected in accordance with the voltages output from the three detecting diodes that do not correspond to the valley of the standing wave SW, and the standing wave SW can be accurately detected. 
     If a detecting diode W 1  to W 4  corresponds to the valley of the standing wave SW, the power level of the standing wave may exceed the dynamic range DM of the detecting diode. However, in this embodiment at least three of the four detecting diodes W 1  to W 4  do not correspond to the valley of the standing wave SW. 
     In a case where none of the detecting diodes W 1  to W 4  correspond to the valley of the standing wave SW, in consideration of the disturbance of an electric field occurring in the interface between the waveguide  14  and the H-plane branch waveguide  25 , the distribution of power is detected in accordance with the voltages output from the three detecting diodes W 1  to W 3  provided away from the interface and the standing wave SW can be more accurately detected. 
     The specific configuration of the control section  18  is described with reference to FIG.  13 . The voltages output from the detecting diodes W 1  to W 4  are converted from analog to digital by an A/D converter  31 , and the thus-converted signals are sent to a CPU  32 . 
     A memory section  33  is connected to the CPU  32  and stores a program used for causing the E-H matching device to automatically match the impedance of the waveguide  14  to that of the chamber  15 . 
     This memory section  33  further stores an approximate expression used for calculating the power input to each of the detecting diodes W 1  to W 4  in accordance with the voltage output from each of the detecting diodes W 1  to W 4 . More specifically, the memory section  33  stores an approximate expression used for calculating input power while compensating for the output characteristic variations of the detecting diodes W 1  to W 4  and variations in the output voltage ranging across the linear, square curve, and saturation regions. Based on the relationship between the input power and the output voltage previously measured with regard to each of the detecting diodes W 1  to W 4 , the approximate expression is determined such that the relationship between the input power and the output voltage is accurately obtained even if the relationship between the input power and the output voltage is nonlinear. Accordingly, the number of detecting diodes is not necessarily limited to four. Since the input power is calculated from the output voltage of each detecting diode by the approximate expression, any number of detecting diodes can be provided, as long as at least three detecting diodes are provided. 
     The CPU  32  is connected to a motor control section  35  and input/output section  34 . The motor control section  35  inputs control signals output from the CPU  32  and outputs motor control signals to an H-plane motor driver  36   a  and an E-plane motor driver  36   b  in accordance with the control signals. The drivers  36   a ,  36   b  respectively drive an H-plane motor  29  and an E-plane motor  24  in accordance with the motor control signals. 
     Connected to a CPU  32  are an input/output section  34  and a motor control section  35 , which receives a control signal output from the CPU  32 . Based on the control signal, the motor control section  35  sends a motor control signal to an H-side motor driver  36   a  and an E-side motor driver  36   b , which respectively drive an H-side motor  29  and an E-side motor  24  based on the motor control signals. 
     Stored in a memory section  33  is a program for generating an approximate expression in accordance with the operation of the CPU  32 . Prior to the automatic impedance matching, an automatic impedance matching device  16  performs an approximate expression generating operation in accordance with the program. This operation will now be discussed. 
     As shown in FIG. 30, a detection section  17  comprises detecting diodes W 1  to W 4 , pickups PU 1  to PU 4  and amplifiers AM 1  to AM 4 . The pickups PU 1 -PU 4  are normally connected to the detecting diodes W 1 -W 4 , respectively, and their antenna sections are exposed inside a waveguide  14  to supply the microwave power in the waveguide  14  to the corresponding detecting diodes W 1 -W 4  at given degrees of coupling. 
     The detecting diodes W 1 -W 4  detect the output powers of the respective pickups PU 1 -PU 4  and send output voltages corresponding to the output powers to the corresponding amplifiers AM 1 -AM 4 . The amplifiers AM 1 -AM 4  amplify the output voltages of the detecting diodes W 1 -W 4  and send the amplified voltages to a control section  18 . 
     The detecting diodes W 1 -W 4  are detachably connected to the associated pickups PU 1 -PU 4 ; in the approximate expression generating operation prior to the automatic impedance matching operation, the detecting diodes W 1 -W 4  are removed from the pickups PU 1 -PU 4 . 
     A plurality of reference powers (dBm) of different levels are sequentially input to the detecting diodes W 1 -W 4  from a microwave signal generator  37 , and the control section  18  receives the output voltages output from the amplifiers AM 1 -AM 4 . The control section  18  also inputs the reference powers (dBm) via the input/output section  34 . 
     In the control section  18 , the output voltages of the amplifiers AM 1 -AM 4  and the reference powers are subjected to A/D conversion by an A/D converter  31  before being input to the CPU  32 . The degrees of coupling of the pickups PU 1 -PU 4 , which have been measured in advance by a measuring device (e.g., a vector network analyzer), are input to the control section  18  from the input/output section  34 . 
     First Approximation System 
     The first approximation system will be discussed with reference to FIG.  31 . The CPU  32  approximates the relationship between the input reference powers and the output voltages of the detecting diodes W 1 -W 4 , diode by diode, using a polynomial approximate expression and stores the resultant approximate expressions in the memory section  33  (step  51 ). The CPU  32  receives the degrees of coupling of the pickups PU 1 -PU 4  from the input/output section  34  and stores them in the memory section  33  (step  52 ). 
     At this time, the input reference powers should have a wider range than the value of the input power in the waveguide  14  obtained during actual usage, and the CPU  32  generates the approximate expressions based on the output voltages of the detecting diodes W 1 -W 4  corresponding to the reference powers. This approximate expression generating process permits more accurate approximate expressions to be generated. 
     In actual usage, when a microwave is input to the waveguide  14 , the CPU  32  computes reference power levels corresponding to the output voltages of the detecting diodes W 1 -W 4  based on the output voltages of the detecting diodes W 1 -W 4  and the previously generated approximate expressions. The CPU  32  then computes power in the waveguide  14  based on the computed reference power levels and the degrees of coupling. 
     If a polynomial approximate expression is to be generated by using least square, for example, this approximation system requires polynomial approximate expression of about an order of ten for accurate approximation. 
     Second Approximation System 
     The second approximation system will be discussed with reference to FIG.  32 . The CPU  32  operates based on the program previously stored in the memory section  33 , and stores the relationship between the input reference powers and the output voltages of the detecting diodes W 1 -W 4  in the memory section  33  for the respective detecting diodes W 1 -W 4  (step  61 ). The CPU  32  receives the degrees of coupling of the pickups PU 1 -PU 4  from the input/output section  34  and stores them in the memory section  33  (step  62 ). At this time, the input reference powers should have a wider range than the value of the input power in the waveguide  14  obtained in the actual usage, as in the case of the first approximation system. 
     Then, the CPU  32  computes the power actually input in the waveguide  14  based on the reference powers and the degrees of coupling, computes the relationship between that input power and the output voltages of the detecting diodes W 1 -W 4 , and stores the relationship in the memory section  33  (step  63 ). 
     Next, the CPU  32  generates a polynomial approximate expression based on the relationship between that input power and the output voltages of the detecting diodes W 1 -W 4 , and stores the approximate expression in the memory section  33  (step  64 ). 
     In actual usage, when a microwave is input to the waveguide  14 , the CPU  32  computes input power in the waveguide  14  corresponding to each of the output voltages of the detecting diodes W 1 -W 4  based on the output voltages of the detecting diodes W 1 -W 4  and the previously generated approximate expression. 
     In this approximation system, the input power in the waveguide  14  is directly calculated on the basis of the output voltages of the detecting diodes W 1 -W 4  and the approximate expression stored in the memory section  33 , so that approximation with a precision as high as or higher than the precision of the first approximation system can be accomplished, even with about a fifth order polynomial approximate expression. 
     It is therefore possible to directly calculate the input power based on the output voltages of the detecting diodes W 1 -W 4  and to compute the input power from a low order polynomial approximate expression, thus reducing the burden on the CPU  32 . This improves the response speed of the automatic impedance matching device at the time of automatic impedance matching. 
     The operation of the thus constituted automatic impedance matching device will be briefly explained with reference to FIG.  33 . 
     Before going into the automatic impedance matching operation, approximate expressions for the respective detecting diodes W 1 -W 4  are set based on the above-described approximation systems (step  71 ). 
     When a microwave is input to the waveguide  14  under this situation, the CPU  32  receives the output voltages of the individual detecting diodes W 1 -W 4  (step  72 ) and computes the input power of the microwave input to the waveguide  14  based on the set approximate expressions (step  73 ). 
     Then, the CPU  32  calculates the coefficient of reflection and the phase of the input power in the waveguide  14  (step  74 ) and detects from the calculation results if the impedance of the waveguide  14  matches with the impedance of the load (step  75 ). 
     When there is an impedance match, the matching operation is terminated. In the case of no impedance matching, the automatic impedance matching operation is executed (step  76 ), and, when an impedance match occurs through the repeated processing of steps  72  to  75 , the matching operation is terminated. 
     With reference to FIGS. 21 to  26 , the automatic matching operation of the device will now be described. 
     When a microwave is transmitted to the chamber  15  from the magnetron  11  via the waveguide  14 , if there is an impedance mismatch between the waveguide  14  and the chamber  15 , a standing wave develops in the waveguide  14 . 
     When the microwave is output from the magnetron  11 , the CPU  32  receives, as output voltage data, a voltage output from each of the detecting diodes W 1  to W 4 , which has been converted from the form of analog signal to the form of digital signal by the A/D converter  31  (step  1 ). 
     Subsequently, based on the output voltage data of each of the detecting diodes W 1  to W 4 , the CPU  32  determines whether or not the input power exceeds the measuring range, or the dynamic range, of the detecting diodes (step  2 ). If one of the four detecting diodes W 1  to W 4  corresponds to the valley of the standing wave SW and the input power exceeds the dynamic range, the detecting diode that corresponds to the valley is removed (step  3 ). Input power of each of the remaining three detecting diodes is calculated from the output voltage data regarding the remaining diodes and the approximate expression stored in the memory section  33  (step  4 ). 
     Next, the CPU  32  uses a known expression to calculate the coefficient of reflection and phase of the standing wave from the calculated input power of the three detecting diodes (step  5 ). 
     In contrast, if, in step  2 , none of the powers input to the detecting diodes W 1  to W 4  are determined to exceed the dynamic range, the CPU  32  uses the approximate expression to calculate the input power of each of the detecting diodes W 3  to W 1  from the output voltage data concerning the detecting diodes W 1  to W 3  (step  6 ). Further, the CPU  32  calculates the coefficient of reflection and phase of the standing wave from the calculated input power of the three detecting diodes (step  7 ). 
     The CPU  32  then determines whether or not the thus-calculated phase and coefficient of reflection of the standing wave are numerical values representing the state of impedance match (step  8 ). If there is an impedance match, the matching operation of the matching device is terminated. 
     If the phase and coefficient of reflection do not represent an impedance match in step  8 , the CPU  32  determines the phase of the standing wave from among the region ranging from 0 to 90°, that ranging from 90° to 180°, that ranging from 180° to 270°, and that ranging from 270° to 360° (steps  9  to  11 ). Impedance matching for the appropriate region is commenced. 
     FIGS. 17 and 18 are Smith charts, which represent the impedance calculated from the phase and coefficient of reflection of the standing wave. In a case where the phase is in the region ranging from 0 to 90°, the impedance matching operation is commenced in an area A 1  shown in FIG.  17 . 
     The CPU  32  calculates the value of the normalized resistance R on the basis of the impedance calculated from the phase and coefficient of reflection of the standing wave (step  12 ) and then determines whether or not the normalized resistance R is located on the circle R=1 shown in FIG. 17 (step  13 ). 
     If the normalized resistance R is not located on the circle R=1, the CPU  32  determines whether or not the normalized resistance R is larger than 1; namely, whether the normalized resistance R is inside or outside the circle R=1 (step  14 ). If the normalized resistance R is larger than 1 or if the resistance R is outside the circle R=1, the H-plane plunger  26  is moved toward the positive side, or in an upward direction H ├  in FIG. 10 (step  15 ). As a result, the normalized resistance R moves toward the circle R=1. In contrast, if the normalized resistance R is smaller than 1; namely, if the resistance R is inside the circle R=1, the H-plane plunger  26  is moved toward the negative side, or in a downward direction H −  shown in FIG. 10 (step  16 ). As a result, the normalized resistance R moves toward the circle R=1. 
     When the normalized resistance R reaches the circle R=1 as a result of such operations, the CPU  32  moves the E-plane plunger  21  in one direction, namely, in a downward direction E −  in FIG. 10 (step  17 ). As a result, the normalized resistance R travels along the circle R=1 and approaches a point P. If the normalized resistance R has reached the point P, an impedance match is realized, and the CPU  32  terminates impedance matching operations (step  18 ). 
     In a case where the phase is in the region ranging from 270° to 360°, the matching operation is performed in an area A 4  shown in FIG.  17 . 
     The CPU  32  calculates the value of the normalized resistance R on the basis of the impedance calculated from the phase and coefficient of reflection of the standing wave (step  21 ) and then determines whether or not the normalized resistance R is located on the circle R=1 shown in FIG. 17 (step  22 ). 
     If the normalized resistance R is not located on the circle R=1, the CPU  32  determines whether or not the normalized resistance R is larger than 1; namely, whether the normalized resistance R is inside or outside the circle R=1 (step  23 ). If the normalized resistance R is larger than 1 or if the resistance R is outside the circle R=1, the H-plane plunger  26  is moved toward the negative side H −  (step  24 ). 
     In contrast, if the normalized resistance R is smaller than 1; namely, if the resistance R is inside the circle R=1, the H-plane plunger  26  is moved toward the positive side H ′  (step  25 ). 
     When the normalized resistance R reaches the circle R=1 as a result of such operations, the CPU  32  moves the E-plane plunger  21  toward the positive direction E +  (step  26 ). As a result, if the normalized resistance R has reached the point P, the state of an impedance match is realized, and the CPU  32  terminates impedance matching operations (step  27 ). 
     In a case where the phase is in the region ranging from 90° to 180°, the impedance matching operation is performed in an area A 2  shown in FIG.  18 . 
     The CPU  32  calculates the value of the normalized conductance G on the basis of the impedance calculated from the phase and coefficient of reflection of the standing wave (step  31 ) and then determines whether or not the normalized conductance G is located on the circle G=1 shown in FIG. 17 (step  32 ). 
     If the normalized conductance G is not located on the circle G=1, the CPU  32  determines whether or not the normalized conductance G is larger than 1; namely, whether the normalized conductance G is inside or outside the circle G=1 (step  33 ). If the normalized conductance G is larger than 1, or if the conductance G is outside the circle G=1, the E-plane plunger  21  is moved toward the negative side E −  (step  34 ). 
     In contrast, if the normalized conductance G is smaller than 1; namely, if the conductance G is inside the circle G=1, the E-plane plunger  21  is moved toward the positive side E ′  (step  35 ). 
     When the normalized conductance G reaches the circle G=1 as a result of such operations, the CPU  32  moves the H-plane plunger  26  toward the negative direction H −  (step  36 ). As a result, if the normalized conductance G has reached the point P, the state of an impedance match is realized, and the CPU  32  terminates impedance matching operations (step  37 ). 
     In a case where the phase is in the region ranging from 180° to 270°, the impedance matching operation is performed in an area A 3  shown in FIG.  18 . 
     The CPU  32  calculates the value of the normalized conductance G on the basis of the impedance calculated from the phase and coefficient of reflection of the standing wave (step  41 ) and then determines whether or not the normalized conductance G is located on the circle G=1 shown in FIG. 18 (step  42 ). 
     If the normalized conductance G is not located on the circle G=1, the CPU  32  determines whether or not the normalized conductance G is larger than 1; namely, whether the normalized conductance G is inside or outside the circle G=1 (step  43 ). If the normalized conductance G is larger than 1, or if the conductance G is outside the circle G=1, the E-plane plunger  21  is moved toward the positive side E ├  (step  44 ). 
     In contrast, if the normalized conductance G is smaller than 1; namely, if the conductance G is inside the circle G=1, the E-plane plunger  21  is moved toward the negative side E −  (step  45 ). 
     When the normalized conductance G reaches the circle G=1 as a result of such operations, the CPU  32  moves the H-plane plunger  26  toward the positive direction H +  (step  46 ). As a result, if the normalized conductance G has reached the point P, the state of an impedance match is realized, and the CPU  32  terminates impedance matching operations (step  47 ). 
     In the foregoing impedance matching operations, in order to improve the efficiency of transmission of power to the load by reducing the elapsed time between commencement and termination of the impedance matching operation, the short plungers  21 ,  26  must be moved at high speed. However, it takes a predetermined period of time for the CPU  32  to calculate the state of reflection of the microwave from the data received from the detector section, and also it takes time for the short plungers  21 ,  26  to reach stable positions when they are moved. For these reasons, the impedance matching operations can be performed in the following manner. 
     As shown in FIG. 19, an area AR 1  including the circle R=1 is set. When high speed movement of the H-plane short plunger  26  causes the normalized resistance R to move, e.g., from Q 1  to Q 2 , so that the normalized resistance R reaches the area AR 1 , the E-plane short plunger  21  is moved at high speed in a direction in which the reflection is reduced. If the normalized resistance R moves from Q 2  to Q 3  and is thus outside the area AR 1 , the H-plane short plunger  26  is moved in such a way that the normalized resistance R returns to a location within the area AR 1 . As a result, the normalized resistance R moves from Q 3  to Q 4 . Subsequently, the E-plane short plunger  21  is moved at high speed in a direction in which the reflection is reduced. These operations are performed repeatedly. 
     If the normalized resistance R approaches the matching point P and is within an area AR 2 , the speed of actuation of the H-plane short plunger  26  and the E-plane short plunger  21  is reduced, and the foregoing operations are then repeated. If the normalized resistance R reaches an area AR 3  in the vicinity of the matching point, the matching operations are terminated. Similar operations are carried out with regard to the circle G=1. 
     Through the above-described operation, the short plungers  21 ,  26  are moved at high speed such that the normalized resistance R moves within the AR 1  so as to gradually approach the area AR 2  until the normalized resistance R reaches the inside of the area AR 2 . Accordingly, in the state where the reflection is large, the normalized resistance R can be immediately moved to the inside of the area AR 2  by high-speed actuation of the short plungers. 
     When the normalized resistance R has reached the inside of the area AR 2 , the speed of actuation of the plungers  21 ,  26  is reduced so that the normalized resistance R can accurately move to the inside of the area AR 3 . 
     In these matching operations, the range over which the speed of actuation of the short plungers  21 ,  26  is changed may be divided into a larger number of ranges. Further, the speed of actuation of the H-plane short plunger  26  and that of the E-plane short plunger  21  may be set individually. 
     The user may set the area AR 1  in accordance with the phase of the microwave or as an arbitrary area, instead of setting it to include the circles R=1 and G=1, as in the previous embodiment. 
     Although the oscillation frequency of the magnetron is set to 2.45 GHz in the previous embodiment, the frequency may slightly deviate from the frequency of 2.45 GHz. In such a case, the wavelength of the standing wave developed in the waveguide  14  also deviates from the distance between the detecting diodes. As a result, a difference arises between the data detected by the detecting diodes and an actual standing wave. 
     As shown in FIG. 20, for example, a difference arises between an impedance Q 11  calculated from the signal received from the detector section and an actual impedance Q 12 . If the impedance Q 11  and the impedance Q 12  are located on opposite sides with respect to the interface between the areas A 1  and A 2 , for example, the impedance Q 12  is matched to the matching point P within the area A 1  thereby lengthening the adjustment time or rendering impedance matching impossible. 
     In this case, an interface area AR 4  having a predetermined phase angle α is set along the interface between the areas A 1 , A 2 . If the impedance Q 11  and the impedance Q 12  are positioned in the interface area AR 4  such that they are located on the opposite sides with respect to the interface, the matching device is set to match the impedance Q 12  to the matching point P in the area A 2  in accordance with the pre-input oscillation frequency data concerning the magnetron. As a result, stable automatic impedance matching operations become feasible. 
     Modification of the Matching Device 
     In the E-H matching device of the foregoing description, the distance between the interior wall of the waveguide and the center of the H-plane branch waveguide equipped with the E-bend is λg/4. However, the E-H matching device may be constructed in the following manner: 
     (1) As shown in FIG. 27, the distance between the waveguide and the H-plane branch waveguide is reduced further until the distance between the interior wall of the waveguide and the center of the H-plane branch waveguide equipped with the E-bend becomes smaller than λg/4. With such a configuration, the E-bend can cut off unwanted frequencies to a much greater extent, and the E-H matching device can be made more compact. 
     (2) As shown in FIG. 28, the waveguide, the E-plane branch waveguide, and H-plane branch waveguide may be formed into flattened waveguides. With this configuration, the occurrence of a high-order mode in the interface between the waveguide and the H-plane branch waveguide can be reduced further. 
     (3) As shown in FIG. 29, in an E-H matching device that comprises the waveguide, the E-plane branch waveguide, and the H-plane branch waveguide, all being formed into flattened waveguides, the distance between the interior wall of the waveguide and the center of the H-plane branch waveguide is reduced to λg/4 or less by reducing the distance between the waveguide and the H-plane branch waveguide. With this configuration, in comparison with the E-H matching device shown in FIG. 28, the E-H matching device is more compact and is improved with regard to the wavelength-shortening effect and the unwanted-frequency cut-off effect of the E-bend. 
     The automatic impedance matching device having the foregoing configurations can operate and yield advantageous results in the following manner: 
     (A) Since the E-bend  30  is provided for the H-plane branch waveguide  25  of the E-H matching device  19  in proximity to the waveguide  14 , the three-dimensional circuit is made compact while the power resistance of the impedance matching device is ensured. 
     (B) As a result of the E-bend  30  being provided for the H-plane branch waveguide  25  of the E-H matching device  19  in proximity to the waveguide  14 , a microwave having a high-order mode occurring in the vicinity of the interface between the waveguide  14  and the H-plane branch waveguide  25  of the E-H matching device is reduced. As shown in FIG. 16, in the conventional E-H matching device, it is difficult to accurately move the normalized resistance R and the normalized conductance G along the respective circles R=1 and G=1, even by adjustment of the E-plane plunger and the H-plane plunger. In contrast, as shown in FIG. 15, the device of the present invention enables the normalized resistance R and the normalized conductance G to accurately move along the circles R=1 and G=1. As a result, automatic impedance matching is controlled accurately and readily. 
     (C) The distance between the center of the operation range of the H-plane short plunger  26  and the interior of the waveguide  14  is reduced by virtue of the wavelength-shortening effect of the E-bend  30 . Accordingly, since the length of the H-plane branch waveguide  25  is shortened, the E-H matching device  19  is more compact. 
     (D) The detector section  17  is positioned in proximity to the E-H matching device  19  by virtue of the high-order mode propagation reduction effect of the E-bend  30 . Accordingly, the three-dimensional circuit can be made more compact. 
     (E) An approximate expression for each detecting diode is stored in the memory section  33  in advance. This expression is used for calculating the power input to each of the detecting diodes W 1  to W 4  in accordance with the voltage output from each of the detecting diodes W 1  to W 4 . The CPU  32  calculates the input power on the basis of the voltage output from each of the detecting diodes W 1  to W 4 . 
     Even if there are variations in the output characteristic of each of the detecting diodes W 1  to W 4 , or, if the input power extends over the linear, square curve, and saturation regions of each of the detecting diodes W 1  to W 4 , the input power can be accurately calculated by means of the approximate expression. 
     Even when the detecting diode is replaced, the replacement of the detecting diode can be performed readily by changing a corresponding approximate expression in conjunction with the replacement of the detecting diode. 
     (F) Since the four detecting diodes are provided at intervals of λg/8, if one of the detecting diodes corresponds to the valley of the standing wave and the input power exceeds the dynamic range of this detecting diode, the remaining three detecting diodes do not correspond to the valley of the standing wave. Hence, the power input to these detecting diodes does not exceed the dynamic range of each of the detecting diodes. Accordingly, the input power of each of the detecting diodes is calculated on the basis of the voltage output from each of the remaining three detecting diodes. As a result, the distribution of power within the waveguide  14 , namely, the standing wave, can be reliably detected, which enables correct automatic matching operations. 
     (G) Where none of the detecting diodes W 1  to W 4  correspond to the valley of the standing wave, the standing wave is detected in accordance with the voltages output from the three detecting diodes W 1  to W 3  provided away from the H-plane branch waveguide  25 . As a result, the influence of a high-order mode occurring in the interface between the waveguide  14  and the H-plane branch waveguide  25  is reduced further, which enables accurate detection of the standing wave. 
     (H) When automatic impedance matching operations are performed, the region (A 1  to A 4 ) is detected on the basis of the calculated phase of the standing wave, and the load is matched to the impedance matching point P from each region. As a result, the distance over which the E-plane and H-plane short plungers  21 ,  26  are moved is reduced, which improves the speed of impedance matching operations. 
     (I) Instead of bringing the normalized resistance R and the normalized conductance G calculated from the phase and coefficient of reflection of the standing wave in alignment with the circle R=1 or G=1, the H-plane and E-plane short plungers  26  and  21  are moved at high speed along the predetermined area AR 1  so as to cause the normalized resistance R and the normalized conductance G to approach the area AR 2  in the vicinity of the matching point P. The waveguide is immediately changed to the state of a small standing wave from the state of a large standing wave. Accordingly, great reflection losses caused by the standing wave are eliminated, which improves power efficiency. Further, if the oscillation frequency of the magnetron  11  becomes deviated, the impedance matching operations can be performed in such a way that the impedance of the waveguide approaches the matching point P without the influence of the deviation of the oscillation frequency. 
     (J) If the normalized resistance R and the normalized conductance G are within the predetermined area AR 2  in the vicinity of the matching point P as a result of improvements in the state of an impedance match, impedance matching operations can be performed while the speed of actuation of the H-plane and E-plane short plungers  26 ,  21  is reduced. Impedance matching operations can be accurately performed while the normalized resistance R and the normalized conductance G follow the circles R=1 and G=1. Accordingly, the impedance of the waveguide can be accurately matched to the matching point P. 
     (K) In the first approximation system, the relationship between the reference powers and the output voltages of the detecting diodes W 1 -W 4  is approximated by disconnecting the detecting diodes W 1 -W 4  from the associated pickups PU 1 -PU 4  and inputting the reference powers to the respective detecting diodes W 1 -W 4  from the microwave signal generator  37 . In the actual use of the automatic impedance matching device  16 , the reference powers corresponding to the output voltages of the detecting diodes W 1 -W 4  are computed based on the output voltages of the detecting diodes W 1 -W 4  and the approximate expressions, and the input power of the microwave provided to the waveguide  14  is calculated based on the reference powers and the degrees of coupling of the pickups PU 1 -PU 4 . 
     (l) In the second approximation system, the relationship between the output voltages of the detecting diodes W 1 -W 4  and the input power to the waveguide  14  is approximated by disconnecting the detecting diodes W 1 -W 4  from the associated pickups PU 1 -PU 4  and inputting the reference powers to the respective detecting diodes W 1 -W 4  from the microwave signal generator  37 . In actual use of the automatic impedance matching device  16 , the input power of the microwave input to the waveguide  14  is computed based on the output voltages of the detecting diodes W 1 -W 4  and the approximate expressions. The second approximation system therefore reduces the load on the CPU  32  more than the first approximation system. 
     (m) To generate approximate expressions, the reference powers have a wider range than the powers that are input to the detecting diodes W 1 -W 4  during actual use. It is therefore possible to generate approximate expressions that accurately approximate the powers to be input in the actual use. 
     (n) Since the automatic impedance matching device  16  can be made compact by compactly designing the EH matching device  19  compact, the site space W 1  of semiconductor equipment having a plurality of chambers  15  can be reduced as shown in FIG.  34 . As shown in FIG. 35, in semiconductor equipment that has the automatic impedance matching devices  8  using the conventional EH matching devices, the H-branch waveguide of the EH matching device significantly protrudes sideways, which requires a larger site space W 2 . The semiconductor equipment also includes the magnetrons  11 , the isolators  13  and the waveguides  14  provided independently with respect to a plurality of chambers  15 . 
     (o) Semiconductor equipment that uses the above-described EH matching device  19  may be embodied as a plasma CVD system, a plasma etching system, a plasma ashing system, a downflow plasma etching system, a downflow plasma ashing system, and ECR plasma etching system, and the like, and can contribute to reducing the site space for those systems. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.