Abstract:
A plasma generator system for reducing the effects of impedance mismatch. The system has a variable frequency source having an output for emitting an RF signal. A plasma chamber has an input for receiving the RF signal. The variable frequency source modulates at least one of the frequency and phase of the RF signal to improve the system tolerance of impedance mismatches between the output of the variable frequency source and the input of the plasma chamber.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to radio frequency power generators for plasma chambers and, more particularly, to radio frequency power generators that limit the effects of an impedance mismatch between the radio frequency generator and the plasma chamber.  
       BACKGROUND OF THE INVENTION  
       [0002]     Radio frequency (RF) generators are used to drive plasma chambers for various applications such as etching, semiconductor fabrication, and flat panel displays. Industry specified frequencies for operating RF power generators include 2 MHz, 4 MHz, 8 MHz, 3.2 MHz, 13.56 MHz, 60 MHz, and 100 MHz.  
         [0003]     Known RF generators use continuous wave (CW) single-tone narrow-band signals which fix an RF carrier frequency at a desired operating frequency. However, an input impedance of the plasma chamber as seen by the RF generator may change drastically during operation due to perturbations of the plasma. When the input impedance changes, a possibility exists for a mismatch between the input impedance of the plasma chamber and an output impedance of the RF generator. The mismatch may be so significant that reflected waves from the plasma chamber severely impact operation of the RF generator. Alternatively, the RF generator may be self-protected and designed to turn off or reduce its output power upon detecting the significant mismatch. Either of these responses by a self-protected RF generator may cause the plasma to extinguish, which is also undesirable.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     In accordance with the needs identified in the art, a plasma generator system is provided. The system has a variable frequency source having an output for emitting an RF signal. A plasma chamber has an input for receiving the RF signal. The variable frequency source modulates at least one of the frequency and phase of the RF signal to improve the system tolerance of impedance mismatches between the output of the variable frequency source and the input of the plasma chamber.  
         [0005]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0007]      FIG. 1  depicts a block diagram of a generic plasma generator system having an open loop, FM mode, RF signal generation system;  
         [0008]      FIG. 2  depicts a block diagram of a plasma generator system having an open loop, analog FM mode, RF signal generation system;  
         [0009]      FIG. 3  depicts a block diagram of a plasma generator system having an open loop, digital FM mode, RF signal generation system;  
         [0010]      FIG. 4  depicts a block diagram of a generic plasma generator system having a closed loop, FM mode, RF signal generation system;  
         [0011]      FIG. 5  depicts a block diagram of a plasma generator system having a closed loop, analog FM mode, RF signal generation system;  
         [0012]      FIG. 6  depicts a block diagram of a plasma generator system having a closed loop, digital FM mode, RF signal generation system;  
         [0013]      FIG. 7  depicts a block diagram of a generic plasma generator system having an open loop, AM/FM mode, RF signal generation system;  
         [0014]      FIG. 8  depicts a block diagram of a generic plasma generator system having a closed loop, AM/FM mode, RF signal generation system;  
         [0015]      FIG. 9  depicts a block diagram of a generic plasma generator system having an open loop, AM/FM mode, RF signal generation system;  
         [0016]      FIG. 10  depicts a block diagram of a generic plasma generator system having a closed loop, AM/FM mode, RF signal generation system;  
         [0017]      FIG. 11  depicts spectral diagrams of RF signals; and  
         [0018]      FIG. 12  depicts a flowchart of a method for generating an FM RF signal. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0020]      FIG. 1  shows a generic plasma generator system having an open loop, FM mode, RF signal generation system. An RF generator  10  is shown generically as having a variable frequency oscillator (VFO)  12  and an RF amplifier  14 . The RF generator  10  generates a power RF signal  16  for driving a plasma chamber  18  through a matching network  20 . The matching network  20  matches an output impedance of the RF generator  10  to an input impedance of the plasma chamber  18 . A modulation source  22  provides a modulating signal  24  to the RF generator  10  to modulate an output frequency of the power RF signal  16 . The modulating signal  24  is generated according to a function G(t) which is selected as described later. The variable t preferably represents time.  
         [0021]     In some embodiments, a significant impedance mismatch at a particular frequency will have a limited effect on the RF generator  10 . Since the RF generator  10  provides FM RF energy, the amount of time spent at the particular frequency of the impedance mismatch will be minimized. This reduces the possibility of the reflected energy causing an undesirable effect on the RF power generator  10 . Concurrently, the RF energy at the frequencies other than the particular frequency of the impedance mismatch should be sufficient to prevent the plasma from extinguishing in the plasma chamber  18 .  
         [0022]      FIG. 2  shows a plasma generator system having an open loop, analog FM mode, RF signal generation system. A modulating signal generator  26  produces the modulating signal  24 . The modulating signal  24  is applied to the RF generator  10  and modulates the power RF signal  16  to an analog mode FM signal. Examples of analog mode FM signals include direct spread (DS) spectrum, narrow band FM (NFM), and wide band FM (WFM). The signal generator  24  produces the modulating signal  24  in accordance with the function G(t).  
         [0023]     The system of  FIG. 2  may also be used to generate the power RF signal  16  having a digital FM mode. A digital FM mode may be generated by using a function G(t) in the signal generator  26  to create a modulating signal  24  having square rising and falling edges. For example, when the modulating signal  24  is a staircase waveform, the resulting digital FM mode is frequency hopping spread spectrum (FHSS). In another example, when the modulating waveform  24  is a square waveform, the resulting digital FM mode is frequency shift keying (FSK).  
         [0024]     The function G(t) may be determined experimentally and determines the wave shape, magnitude, and frequency of the modulating signal  24  produced by the modulation source  22  and the signal generator  26 . The function G(t) should be chosen such that plasma generation in the plasma chamber  18  is sustained while limiting reflected RF to acceptable levels that do not significantly inhibit operation of the RF generator  10 .  
         [0025]      FIG. 3  shows a plasma generator system having an open loop, digital FM mode, RF signal generation system. The VFO  12  provides an RF signal to a phase modulator  28 . An output signal from the phase modulator  28  is amplified by the RF amplifier  14  and applied to the plasma chamber  18  through the matching network  20 . A counter  30 , which may count in a random or sequential pattern, provides a modulating signal  24  in the form of digital counts m-bits wide and n-bits wide, as shown. The variables n and m are positive integers. The n-bit count controls the VFO  12  to output one of 2 n  distinct frequencies. The m-bit count controls the phase modulator to shift the output signal from the VFO  12  by one of 2 m  phase values. In an alternative embodiment, the n-bit counter may be eliminated (n=0) which would have the effect of fixing the output frequency of the VFO  12 .  
         [0026]     The modulated signal from the phase modulator  28  is an RF signal having a broad frequency spectrum. The modulation modes than can be generated for operating the plasma chamber  18  include a phase shift keying (PSK) mode with n=0 and m=1, a two-bit quadrature phase shift keying (QPSK) mode with n=0 and m=2, a minimum phase shift keying (MSK) mode with n=0 and m=2, a frequency hopping spread spectrum (FHSS) mode with n&gt;0 and m=0, and a frequency shift keying (FSK) mode with n=1 and m=0. QPSK modes of 4-bit and higher are achieved when the counter  30  has n-bits &gt;0 and m-bits &gt;2.  
         [0027]     The values of n and m, as well as the count rate (counts per second) of the counter and the n-bit and m-bit counter step sizes may be determined experimentally. The selected values of n, m, count rates, and step sizes should be chosen such that at least two conditions are satisfied. The first condition to be satisfied is that plasma generation is sustained in the plasma chamber  18 . The second condition to be satisfied is that the device providing RF power to the plasma chamber  18  does not receive a damaging level of reflected RF in the event the plasma chamber  18  creates an impedance mismatch at a particular frequency.  
         [0028]     The bandwidth of the RF signal may also be controlled by using filters. For example, using MSK modulation with a symbol period/filter bandwidth product (Bt) of 0.5, over a 13.5-14 MHz spectrum modulated at 500 KHz, may produce satisfactory results. A frequency spectrum of 12.5-15 MHz modulated at 2.5 MHz may also produce satisfactory results.  
         [0029]      FIG. 4  shows a generic plasma generator system having a closed loop, FM mode, RF signal generation system. The RF generator  10  provides the FM RF signal  16  to the plasma chamber  18  through a matching network  20  and a reflected power meter  32 . The reflected power meter  32  provides a measurement signal  34  indicative of the degree of RF coupling between an output of the matching network  12  and the input of the plasma chamber  18 . The reflected power meter  32  may determine the degree of RF coupling using techniques known in the RF art, such as measuring the standing wave ratio (SWR) between the RF signal applied to the plasma chamber  18  and a reflected RF signal from the plasma chamber  18 . The measurement signal  34  is applied to a feedback block  36  that implements a function H(r), where the variable r is preferably a magnitude of the measurement signal  34 . The feedback block  36  generates a modulation signal  24  in accordance with the function H(r) and applies the modulation signal  24  to the RF generator  10 . The RF generator  10  modulates a frequency and/or phase of the power RF signal  16  in accordance with the modulating signal  24 .  
         [0030]      FIG. 5  depicts a plasma generator system having a closed loop, analog FM mode, RF signal generation system. The RF generator  10 , the matching network  12 , the reflected power meter  32 , and the plasma chamber  18  operate as shown and described in  FIG. 4 . The reflected power meter  32  provides the measurement signal  34  to a controller  38 . The controller  38  may be a PID type controller having any combination of P, I, and D terms such that it may also be one of a P, a PI, and a PD controller. A magnitude of each PID term may be selected experimentally to produce a stable system behavior. The controller  38  applies an output signal  40  to a modulating signal generator  42 . The signal generator  42  uses the signal from the controller  38  to produce the modulating signal  24  which is applied to the RF generator  10 . In alternate embodiments, the controller  38  may be implemented with a fuzzy logic controller or a neural network controller.  
         [0031]     The modulating signal  24  is applied to the RF generator  10  such that it produces an RF signal modulated in an analog FM mode. Examples of such analog FM modes include DS, NFM, and WFM. As in  FIG. 2 , the modulating signal  24  may also have square rising and falling edges, thereby causing the RF generator  10  to produce a power RF signal  16  modulated in a digital FM mode such as FHSS and FSK.  
         [0032]     The modulating signal generator  42  determines the wave shape, magnitude, and frequency of the modulating signal  24  in accordance with the output signal  40  from the controller  38 . Accordingly, the function H(r) implemented in the signal generator  42  may be determined experimentally and determines the wave shape, magnitude, and frequency of the modulating signal  24  produced by the signal generator  42 . The function H(r) should be chosen such that plasma generation in the plasma chamber  18  is sustained while limiting the reflected RF signal to an acceptable level that does not significantly inhibit operation of the RF generator  10 .  
         [0033]      FIG. 6  depicts a plasma generator system having a closed loop, digital FM mode, RF signal generation system. The matching network  12 , the reflected power meter  32 , the controller  38 , and the plasma chamber  18  operate as shown and described in  FIG. 4 . The output signal  40  from the controller  38  is applied to a counter  44 . The counter  44 , which may count in a random or sequential pattern, provides a modulating signal  24  in the form of digital counts m-bits wide and n-bits wide. The variables n and m are positive integers, and counting frequencies of the m-bit and n-bit counters are determined by the output signal from the controller  38 . The n-bit count controls the VFO  12  to generate an RF signal having one of 2ˆn distinct frequencies. The m-bit count controls the phase modulator  28 , which shifts the output signal from the VFO  12  by one of 2ˆm phase values. In an alternative embodiment, the n-bit counter may be eliminated (n=0) which has the effect of fixing the output frequency of the VFS  10 .  
         [0034]     The modulated signal from the phase modulator  28  is an RF signal having a broad frequency spectrum. The modulation modes than can be generated for the plasma chamber  18  include a phase shift keying (PSK) mode with n=0 and m=1, a two-bit quadrature phase shift keying (QPSK) mode with n=0 and m=2, a minimum phase shift keying (MSK) mode with n=0 and m=2, a frequency hopping spread spectrum (FHSS) mode with n&gt;0 and m=0, and a frequency shift keying (FSK) mode with n=1 and m=0. QPSK modes of 4-bit and higher are achieved when the counter  44  has n-bits &gt;0 and m-bits &gt;2.  
         [0035]     In some embodiments of the systems of  FIGS. 5 and 6 , the controller  38  holds the output signal  40  constant while the degree of RF coupling, as indicated by the measurement signal  34 , is greater than a predetermined value. Holding the output signal  40  constant causes the RF generator  10 , and the combination of the VFO  12  and phase modulator  28 , to produce a CW RF signal. When a perturbation in the plasma causes the input impedance of the plasma chamber  18  to change, the degree of RF coupling will decrease. The decrease in the degree of RF coupling will be indicated by the measurement signal  34 . If the RF coupling is less than the predetermined threshold, then the controller  38  resumes varying the output signal  40  in accordance with the measurement signal  34  and a control method executed by the controller  38 .  
         [0036]      FIG. 7  shows a generic plasma generator system having an open loop, AM/FM mode, RF signal generation system. An RF generator  11  is shown generically having the variable frequency oscillator (VFO)  12  and a variable gain RF amplifier  15 . The modulation source  22  provides the modulating signal  24  to the VFO  12  to modulate its output frequency. The modulating signal  24  is generated according to the function G(t). An amplitude modulating signal  25  is generated by an amplitude modulation source  23  according to a function A(t). The variable t preferably represents time. The variable gain RF amplifier  15  has a gain controlled by the amplitude modulating signal  25 . The variable gain RF amplifier  15  receives the constant amplitude FM signal from the VFO  12  and amplifies the FM signal in accordance with the amplitude modulating signal  25 . The variable gain RF amplifier  15  thereby generates a power RF signal  16  that is amplitude and frequency modulated (AM/FM). The functions G(t) and A(t) can also be chosen such that the power RF signal  16  is amplitude and phase modulated, such as quadrature amplitude modulation (QAM). The matching network  20  matches an output impedance of the RF generator  11  to the input impedance of the plasma chamber  18 .  
         [0037]      FIG. 8  shows a generic plasma generator system having a closed loop, AM/FM mode, RF signal generation system. The RF generator  11  provides the AM/FM power RF signal  16  to the plasma chamber  18  through a matching network  20  and a reflected power meter  32 . The reflected power meter  32  provides the measurement signal  34  indicative of the degree of RF coupling between the output of the matching network  20  and the input of the plasma chamber  18 . The measurement signal  34  is applied to feedback blocks  36  and  37 . The feedback block  36  controls the frequency of the VFO  24  as shown and described in  FIG. 4 . The feedback block  37  generates the amplitude modulation signal  25  in accordance with a function A(r) and applies the amplitude modulation signal  25  to the variable gain RF amplifier  15 . The RF generator  11  thereby provides the AM/FM power RF signal  16  in accordance with the modulating signal  24  and the amplitude modulating signal  25 . The functions G(r) and A(r) can also be chosen such that the power RF signal  16  is amplitude and phase modulated, such as QAM.  
         [0038]      FIGS. 9 and 10  show generic plasma generator systems that are similar, except for the architecture of the RF generator  11 , to the systems of  FIGS. 7 and 8 , respectively. In  FIGS. 9 and 10 , the RF generator  11  has a VFO  13  with a variable output amplitude. The gain of the RF amplifier  14  is constant. The output amplitude of a VFO  13  is controlled by the amplitude modulation signal  25 . The VFO  13  therefore applies an AM/FM signal to the RF amplifier  14 , which provides the power RF signal  16 . Selection of transformations A(t) and A(r) may also be accomplished in accordance with Vona, Jr. et al., U.S. Pat. No. 6,700,092, “Pulsing Intelligent RF Modulation Controller”, Mar. 2, 2004, the specification of which is hereby incorporated by reference.  
         [0039]      FIG. 11  depicts spectral diagrams of various FM signals and a spectrum diagram of a single-tone CW signal  46 . A vertical axis  48  depicts energy of the frequency components of the RF signals. A horizontal axis  50  indicates the frequencies of the RF signal components with respect to a center frequency, f 0 . The spectral diagrams are not to scale and actual spectrums will vary according to the RF signals that are applied to the plasma chamber  18 . A spectrum of a four-frequency FHSS signal is represented by the four vertical lines  52 . A spectrum of a DSS signal is represented by an area  54 , and spectrums of other FM modes are represented by a curve  56 .  
         [0040]      FIG. 12  depicts a method which may be used with the closed loop controllers  36 ,  38 . The method may be operated continuously, such as by an analog controller, or executed periodically by a microcontroller that implements the controller. The method begins at block  58  and determines an impedance match between the matching network  20  and the plasma chamber  18 . The impedance match is directly related to the degree of RF coupling and may therefore be determined from the measurement signal  34 . From block  58  the method proceeds to a decision block  60  and determines whether the impedance match is acceptable. An acceptable impedance match may include one which couples sufficient RF power to the plasma chamber  18  to sustain plasma generation while reflected RF power from the plasma chamber  18  is at a low enough level such as not to damage the RF signal generation system. If the impedance match is acceptable then method returns to block  58  and continues to monitor the impedance match. Returning to decision block  60 —if the impedance match is not acceptable then the method proceeds to block  62  and varies the frequency and/or phase of the RF signal in accordance with the selected FM mode.  
         [0041]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.