Patent Publication Number: US-7909934-B2

Title: Megasonic cleaning efficiency using auto-tuning of a RF generator at constant maximum efficiency

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of claims priority from U.S. patent application Ser. No. 10/360,322 filed on Feb. 6, 2003 and issued as U.S. Pat. No. 6,995,067 on Feb. 7, 2006, and entitled “Improved Megasonic Cleaning Efficiency Using Auto-Tuning of an RF Generator at Constant Maximum Efficiency,” which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to systems and methods of tuning an RF generator, and more particularly, to methods and systems for automatically tuning an RF generator for a substrate cleaning system. 
     2. Description of the Related Art 
     The use of acoustic energy is a highly advanced, non-contact, cleaning technology for removing small-particles from substrates such as semiconductor wafers in various states of fabrication, flat panel displays, micro-electro-mechanical systems (MEMS), micro-opto-electro-mechanical systems (MOEMS), and the like. The cleaning process typically involves the propagation of acoustic energy through a liquid medium to remove particles from, and clean, a surface of a substrate. The megasonic energy is typically propagated in a frequency range of about 700 kHz (0.7 Megahertz (MHz)) to about 1.0 MHz, inclusive. The liquid medium can be deionized water or any one or more of several substrate cleaning chemicals and combinations thereof. The propagation of acoustic energy through a liquid medium achieves non-contact substrate cleaning chiefly through the formation and collapse of bubbles from dissolved gases in the liquid medium, herein referred to as cavitation, microstreaming, and chemical reaction enhancement when chemicals are used as the liquid medium through improved mass transport, or providing activation energy to facilitate the chemical reactions. 
       FIG. 1A  is a diagram of a typical batch substrate cleaning system  10 .  FIG. 1B  is a top view of the batch substrate cleaning system  10 . A tank  11  is filled with a cleaning solution  16  such as deionized water or other substrate cleaning chemicals. A substrate carrier  12 , typically a cassette of substrates, holds a batch of substrates  14  to be cleaned. One or more transducers  18 A,  18 B,  18 C generate the emitted acoustic energy  15  that is propagated through the cleaning solution  16 . The relative location and distance between the substrates  14  and the transducers  18 A,  18 B and  18 C are typically approximately constant from one batch of substrates  14  to another through use of locating fixtures  19 A,  19 B that contact and locate the carrier  12 . 
     The emitted energy  15 , with or without appropriate chemistry to control particle re-adhesion, achieves substrate cleaning through cavitation, acoustic streaming, and enhanced mass transport if cleaning chemicals are used. A batch substrate cleaning process typically requires lengthy processing times, and also can consume excessive volumes of cleaning chemicals  16 . Additionally, consistency and substrate-to-substrate control are difficult to achieve. Such conditions as “shadowing” and “hot spots” are common in batch, and other, substrate megasonic processes. Shadowing occurs due to reflection and/or constructive and destructive interference of emitted energy  15 , and is compounded with the additional substrate surface area of multiple substrates  14 , walls of the process tank etc. The occurrence of hot spots, primarily the result of constructive interference due to the use of multiple transducers and to reflection, can also increase with additional multiple-substrate surface areas. These issues problems are typically addressed by depending on the averaging effects of the multiple reflections of the acoustic energy on the substrate, which can lead to a lower average power to the substrate surfaces. To compensate for the lower average power, and provide effective cleaning and particle removal, power to the transducers is increased, thereby increasing the emitted energy  15  and increasing cavitation and acoustic streaming, which thereby increases the cleaning effectiveness. Additionally, pulsing the multiple transducer arrays  18 A,  18 B and  18 C is used (i.e. providing a duty cycle such as turning the transducers on for 20 ms, and then off for 10 ms. The transducers  18 A,  18 B and  18 C can also be operated out of phase (e.g., activated sequentially) to reduce compound reflections and interference. 
       FIG. 1C  is a prior art, schematic  30  of an RF supply to supply one or more of the transducers  18 A,  18 B,  18 C. An adjustable voltage controlled oscillator (VCO)  32  outputs a signal  33 , at a selected frequency, to an RF generator  34 . The RF generator  34  amplifies the signal  33  to produce a signal  35  with an increased power. The signal  35  is output to the transducer  18 B. A power sensor  36  monitors the signal  35 . The transducer  18 B outputs emitted energy  15 . 
     The precise impedance of the transducer  18 B can vary depending on many variables such as the number, size and spacing of substrates  14  in the carrier  12  and the distance between the substrates  14  and the transducer  18 B. The precise impedance of the transducer  18 B can also vary as the transducer  18 B ages through repeated usage. By way of example, if signals  33 ,  35  have a frequency of about 1 MHz, the wavelength is about 1.5 mm (0.060 inches) in a deionized water medium such as the cleaning solution  16 . As a result, referring again to  FIG. 1A , if the location of the substrates  14  and carrier  12  is off by as little as about 0.5 mm (0.020 inches) or even less, the impedance of the transducer  18 B can vary substantially. 
     Adjusting the frequency of the VCO can adjust the impedance of the transducer  18 B by varying the frequency and therefore the wavelength of the signals  33 ,  35  and the emitted energy  15 . Typically, a carrier  12  that is loaded with substrates  14  is placed in the tank  11  and the VCO  32  is adjusted to change the frequency of the signals  33 ,  35  and the emitted energy  15  until the impedance of the transducer  18 B is matched, as indicated by a minimum value of a reflected signal  38  that is detected by the power meter  36 . Once the VCO  32  has been adjusted to achieve the minimum reflected signal  38 , the VCO  32  is typically not adjusted again unless significant repairs or maintenance are performed on the substrate cleaning system  10 . 
     When the transducer  18 B impedance is not matched, a portion of the emitted energy  17  (i.e., waves) emitted from the transducer  18 B is reflected back toward the transducer  18 B. On the surface of the transducer  18 B, the reflected energy  17  can interfere with the emitted energy  15  causing constructive and destructive interference. The destructive interference reduces the effective cleaning power of the emitted energy  15  because a portion of the emitted energy  15  is effectively cancelled out by the reflected energy  17 . As a result, the RF generator  34  efficiency is reduced. 
     The constructive interference can cause excess energy that can cause hot spots on the surfaces of the substrates  14  being cleaned. The hot spots can exceed an energy threshold of the substrates  14  and can damage the substrates  14 .  FIG. 1D  is a typical transducer  18 B.  FIG. 1E  is a graph  100  of the energy distribution across the transducer  18 B. Curve  102  is a curve of the energy emitted across the transducer  18 B in the x-axis. Curve  104  is a curve of the energy emitted across the transducer  18 B in the y-axis. Curve  120  is a curve of the composite energy emitted across the transducer  18 B in both the x-axis and the y-axis. The composite energy emitted across the transducer  18 B in both the x-axis and the y-axis typically can vary between curve  120  and curve  122  as the known variations (e.g., location of the substrates, aging of the transducer, and wobble of a rotating substrate relative to the transducer etc.) cause the impedance of the transducer  18 B to vary. A threshold energy level T is the damage threshold to the substrate(s)  14 . Typically, the maximum power of the RF signal  35  and the resulting emitted energy  15  output by the transducer  18 B is reduced to a level such that the maximum constructive interference results in a peak magnitude (i.e., peaks in curve  120 ) of less than the energy threshold T of the substrates  14  so as to prevent damage to the substrate  14 . However, the reduced power of the RF signal  35  and the emitted energy  15  increases the cleaning process time required to achieve the desired cleaning result. In some instances, the reduced power of the signal  35  and the emitted energy  15  is insufficient to remove the some of the targeted particles from the substrates  14 . As shown, the effective emitted energy can vary to a much lower level (represented by valleys in curve  122 ) such that the effectiveness of the cleaning process is severely impacted because the effective energy is so low (about 3) and therefore results in an energy window that extends from about 3 to about 17 as shown on the energy scale. 
     The transducer  18 B is typically a piezoelectric device such as a crystal. The constructive and destructive interference caused by the reflected energy  17  can also impart a force to the surface of the transducer  18 B sufficient to cause the transducer  18 B to produce a corresponding reflected signal  38 . The power sensor  36  can detect the reflected signal  38  that is reflected from the transducer  18 B toward the RF generator  34 . The reflected signal  38  can constructively or destructively interfere with the signal  35  output from the RF generator  34  to further reduce the efficiency of the RF generator  34 . 
     In view of the foregoing, there is a need for an improved megasonic cleaning system that provides increased efficiency of the RF generator and a reduced energy window of the emitted acoustic energy and reduces the probability of substrate damage. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the present invention fills these needs by providing a dynamically cleaning system that uses an automatically tuned RF generator that is constantly tuned to the maintain resonance of the transducer and the emitted energy from the transducer. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, computer readable media, or a device. Several inventive embodiments of the present invention are described below. 
     One embodiment includes a method of cleaning a substrate that includes applying an RF signal at a frequency f to a transducer. The transducer being oriented toward the substrate such that the transducer emits an acoustic energy at the frequency f toward the substrate. The substrate is moved relative to the transducer. The RF signal is dynamically adjusted to maintain a resonance of the acoustic energy. 
     Dynamically adjusting frequency f can include automatically adjusting the frequency f for each cycle of the RF signal. 
     Moving the substrate relative to the transducer can include rotating the substrate. 
     The substrate can also be submerged in a cleaning solution. The cleaning solution can be deionized water. The cleaning solution can include one or more of a plurality of cleaning chemicals. 
     Dynamically adjusting the RF signal to maintain the resonance of the acoustic energy can include maintaining a constant voltage of the RF signal applied to the transducer. 
     An RF generator can apply the RF signal to the transducer and maintaining a constant voltage of the RF signal applied to the transducer can include measuring a first voltage of the RF signal, comparing the first voltage to a desired set point voltage, and inputting a control signal to a variable DC power supply so as to adjust an output voltage of the variable DC power supply, the variable DC power supply supplying DC power to the RF generator. 
     Dynamically adjusting the RF signal to maintain the resonance of the acoustic energy can include dynamically adjusting a frequency f of the RF signal applied to the transducer. 
     An RF generator can apply the RF signal to the transducer and dynamically adjusting the frequency f of the RF signal applied to the transducer can include measuring a supply voltage applied to the RF generator, measuring a peak voltage across an output amplifier included in the RF generator, producing a frequency control signal when the peak voltage is not equal to a selected ratio of the supply voltage, and applying the frequency control signal to a frequency control input of an oscillator that generates the RF signal. 
     An RF generator can apply the RF signal to the transducer and dynamically adjusting the frequency f of the RF signal applied to the transducer can include inputting an RF input signal from an oscillator the RF generator and amplifying the RF signal in the RF generator. A first phase of an input voltage of the RF input signal is measured, a second phase of a voltage of the RF signal output from the RF generator is measured. A frequency control signal is produced when the first phase is not equal to the second phase. The frequency control signal is applied to a frequency control input of the oscillator. 
     Another embodiment includes a system for cleaning a substrate includes a cleaning chamber that includes a transducer and a substrate. The transducer being oriented toward the substrate. A variable distance d separates the transducer and the substrate. The system also includes a dynamically adjustable RF generator that has an output coupled to the transducer and a feedback circuit coupled to a control input of the adjustable RF generator. 
     The substrate can be rotated. The distance d can vary about ½ wavelength of an RF signal output from the RF generator as the substrate is rotated. 
     The dynamically adjustable RF generator can include a variable DC power supply having a control input and a DC output coupled to the RF generator. The feedback circuit can include a first comparator that includes a first input coupled to a set point control signal, a second input coupled to the RF generator RF output, and a control signal output coupled to the control input of the adjustable RF generator. The control input includes a voltage control input on the variable DC power supply. 
     The dynamically adjustable RF generator can include an oscillator, an output amplifier coupled to the oscillator output and a load network. The oscillator has a control signal input and an RF signal output. The load network coupled between an output of the output amplifier and the output of the RF generator. The feedback circuit can include a peak voltage detector, and a second comparator. The peak voltage detector can be coupled across the output amplifier. The second comparator includes a third input coupled to an output of the variable DC power supply, a fourth input coupled to an output of the peak voltage detector, and a second comparator output coupled to the control input of the adjustable RF generator. The control input can include the oscillator control signal input. 
     The dynamically adjustable RF generator can include an oscillator and an RF generator input coupled to the oscillator RF signal output. The oscillator having a frequency control input and an RF signal output. The feedback circuit can include a voltage phase detector. The voltage phase detector can include a first phase input coupled to the RF signal output of the oscillator, a second phase input coupled to the RF generator output, and a frequency control signal output coupled to the control input of the adjustable RF generator. The control input can include the oscillator frequency control voltage input. 
     The dynamically adjustable RF generator can include a supply voltage source, an oscillator having a control signal input and an RF signal output, an output amplifier coupled to the oscillator output, a load network coupled between an output of the output amplifier and the output of the RF generator. The feedback circuit can include a peak voltage detector coupled across the output amplifier, and a comparator circuit. The comparator circuit can include a first input coupled to the supply voltage source, a second input coupled to an output of the peak voltage detector, and a comparator output coupled to the control input of the adjustable RF generator. The control input can include the oscillator control signal input. 
     The dynamically adjustable RF generator can include an oscillator, an RF generator input coupled to the oscillator RF signal output, and the feedback circuit can include a voltage phase detector. The oscillator has a frequency control input and an RF signal output. The voltage phase detector that includes a first phase input coupled to the RF signal output of the oscillator, a second phase input coupled to the RF generator output, and a frequency control signal output coupled to the control input of the adjustable RF generator. The control input can include the oscillator frequency control voltage input. 
     The transducer can include two or more transducers. The dynamically adjustable RF generator can include two or more dynamically adjustable RF generators each having a respective output coupled to one of the two or more transducers. 
     The transducer can include a first transducer oriented toward an active surface of the substrate and a second transducer oriented toward a non-active side of the substrate. 
     The present invention provides the advantage of significantly reduced cleaning processing time because the higher power acoustic energy can be used without damaging the substrate being cleaned. 
     The present invention also reduces the number of substrates damaged due to excess acoustic energy being applied to the substrate. 
     The auto-tuned RF generator also automatically adjusts for process-induced impedance changes such as different cleaning chemistries, different locations of the substrate, different process tank configurations etc, thereby providing a more flexible and robust cleaning process. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, and like reference numerals designate like structural elements. 
         FIG. 1A  is a diagram of a typical batch substrate cleaning system. 
         FIG. 1B  is a top view of the batch substrate cleaning system. 
         FIG. 1C  is a prior art, schematic of an RF supply to supply one or more of the transducers. 
         FIG. 1D  is a typical transducer  18 B. 
         FIG. 1E  is a graph of the energy distribution across the transducer. 
         FIGS. 2A and 2B  show a dynamic, single substrate cleaning system, in accordance with one embodiment of the present invention. 
         FIG. 2C  is a flowchart of the method operations of an auto-tuning RF generator system used in a megasonic cleaning system, such as described in  FIGS. 2A and 2B  above, in accordance with one embodiment of the present invention. 
         FIG. 3  is a block diagram of an auto-tuning RF generator system in accordance with one embodiment of the present invention. 
         FIG. 4  is a flowchart of the method operations of the auto-tuning RF generator system while the RF generator is supplying an RF signal to the transducer, in accordance with one embodiment of the present invention. 
         FIG. 5A  is a schematic diagram of the peak V ds  detector in accordance with one embodiment of the present invention. 
         FIG. 5  B is a graph of waveforms of the peak voltage (V ds ) detected by the peak voltage detector, in accordance with one embodiment of the present invention. 
         FIG. 6  is a block diagram of an auto-tuning RF generator system according to one embodiment of the present invention. 
         FIG. 7  is a flowchart of the method operations of the auto-tuning RF generator system according to one embodiment of the present invention. 
         FIGS. 8A-8C  show graphs of three examples of the relationships between phase P 1  and phase P 2  in accordance with one embodiment of the present invention. 
         FIG. 9  is a block diagram of an auto-tuning RF generator system according to one embodiment of the present invention. 
         FIG. 10  is a flowchart of the method operations of the auto-tuning RF generator system, in accordance with one embodiment of the present invention. 
         FIG. 11  is a diagram of a megasonic module in accordance with one embodiment of the present invention. 
         FIG. 12  is a graph of the energy distribution across the transducer in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Several exemplary embodiments for an acoustic energy cleaning system that automatically adjusts RF signal applied to the transducer for maximum efficiency will now be described. It will be apparent to those skilled in the art that the present invention may be practiced without some or all of the specific details set forth herein. 
     As described above, it is very important to increase the cleaning effectiveness, efficiencies and throughput rate of substrate cleaning systems, while reducing probability of damage to the substrate. These requirements are exacerbated by the continuously shrinking device sizes and the fact that many cleaning systems are evolving to single substrate cleaning systems. 
       FIGS. 2A and 2B  show a dynamic, single substrate cleaning system  200 , in accordance with one embodiment of the present invention.  FIG. 2A  shows a side view of the dynamic, single substrate cleaning system  200 .  FIG. 2B  shows a top view of the dynamic, single substrate cleaning system  200 . The substrate  202  is immersed in cleaning solution  204  contained within a cleaning chamber  206 . The cleaning solution  204  can be deionized water (DI water) or other cleaning chemistries that are well known in the art and combinations thereof. 
     The substrate  202  is substantially circular and is held by three or more edge rollers  208 A,  208 B,  208 C (or similar edge holding devices) so that the substrate  202  can be rotated (e.g., in direction  209 A) as the cleaning process is applied to the substrate  202 . One or more of the three edge rollers  208 A,  208 B, and  208 C can be driven (e.g. in direction  209 B) so as to rotate the substrate  202  in direction  209 A. The substrate  202  can be rotated at a rate of up to about 500 RPM. 
     A transducer  210  is also included as part of the cleaning chamber  206 . The transducer  210  can be a piezoelectric device such as a crystal that can convert an RF signal  220  to acoustic energy  214  emitted into the cleaning solution  204 . The transducer  210  can be composed of piezoelectric material such as piezoelectric ceramic, lead zirconium tintanate, piezoelectric quartz, gallium phosphate wherein the piezoelectric material is bonded to a resonator such as ceramic, silicon carbide, stainless steel or aluminum, or quartz. 
     As shown in  FIG. 2B , the transducer  210  can be significantly smaller than the substrate  202 . Smaller transducers can be manufactured more inexpensively and can also offer improved control over the smaller area of the substrate  202  that the emitted energy  214  emitted from the smaller transducer  210  impacts. The active surface  218  (i.e., the surface having the active devices thereon) of the substrate  202  is typically facing the transducer  210 . However, in some embodiments the active surface  218  can be on the side of the substrate  202  opposite the transducer  210 . 
     The three edge rollers  208 A,  208 B,  208 C hold the substrate  202  approximately a fixed distance d 1  from the transducer  210  as the substrate  202  rotates past the transducer  210 . Distance d 1  can be within a range of only a few millimeters to up to about 100 mm or more. The distance d 1  is selected as a distance that matches the impedance of the transducer  210 . In one embodiment the distance d 1  is selected as a resonant distance for the frequency of the emitted energy  214 . Alternatively, the frequency of the emitted energy  214  can be selected so that the distance d 1  is a resonant distance. In either embodiment, at resonance, the minimum reflected energy  216  is reflected from the substrate  202  back toward the transducer  210 . As described above, the reflected energy  216  can interfere with the emitted energy  214  which can decrease the power efficiency of the RF signal  220  and can cause decreased cleaning effectiveness (e.g., interference patterns) on the substrate  210 . 
     However, the substrate  202  can “wobble” somewhat such that the distance between the substrate  202  and the transducer  210  can vary between the first distance d 1  to a second distance d 2  as the substrate  202  rotates past the transducer  210 . The difference between the first distance d 1  and the second distance d 2  can be up to about 0.5 mm (0.020 inches) or even greater. While improved edge rollers  208 A,  208 B,  208 C and other similar technologies may be able to hold the substrate  202  a more consistent distance d 1  from the transducer  210 , the improved edge rollers cannot guarantee an absolute constant distance d 1  and therefore variations in the distance d 1  can still occur. Further, the distance between the substrate  202  and the transducer  210  can vary for other reasons as well (e.g. placement of the substrate  202  within the edge rollers  208 A,  208 B,  208 C, etc.). As will be described in more detail below, the variation in the distance between the substrate  202  and the transducer  210  can severely impact performance and efficiency of the cleaning system  200 . 
     The transducer  210  is coupled to an RF generator  212 .  FIG. 2C  is a flowchart of the method operations  250  of an auto-tuning RF generator system used in a megasonic cleaning system  200 , such as described in  FIGS. 2A and 2B  above, in accordance with one embodiment of the present invention. In operation  255 , the RF generator provides the RF signal  220  to the transducer  210 . The RF signal  220  can have a frequency of between about 400 kHz to about 2 MHz but is typically between about 700 kHz to about 1 MHz. The wavelength of the high frequency acoustic energy  214  emitted from the transducer  210  is about 1.5 mm (0.060 inches) in length, in the cleaning solution  204 . 
     In operation  260 , the distance to the target (e.g., substrate  202 ) varies as the target is moved, relative to the transducer  210 . As the distance d 1  varies the amount of reflected energy  216  also varies because the emitted energy  214  is not always in resonance when the distance d 1  changes (i.e. the impedance of the transducer  210  is mismatched). In operation  270 , the RF generator  212  is automatically and dynamically tuned so that the RF signal  220  is constantly tuned to correct for any impedance mismatches as the distance d 1  changes. 
     Because a wavelength of the emitted energy  214  is about 1.5 mm (0.060 inches), a movement of only 0.50 mm (0.020 inches) can cause a significant impedance variation resulting in, for example, as much as a 50% variation in voltage and power varying between about 25% and 100%. Without an auto-tuning RF generator to compensate for the variations in d 1 , the peak energy level of the emitted energy  214  must be reduced to a low enough value that the energy absorbing ability of the substrate  202  (energy threshold) is not exceeded so as to prevent the peak emitted energy  214  from damaging the substrate  202 . 
     The auto-tuning RF generator  212  can be automatically tuned to compensate for the variations in the distance d 1  through varying approaches. In one embodiment, a peak voltage is detected so as to maintain the RF generator  212  at an impedance optimized frequency of the RF signal  220 . In another embodiment, the phase of the voltage is maintained so as to produce an impedance optimized frequency of the RF signal  220 . In yet another embodiment, the power supply can be adjusted to impedance optimize the RF signal  220 . The various embodiments can also be used in combination within a single auto-tuning RF generator system. 
       FIG. 3  is a block diagram of an auto-tuning RF generator system  300  according to one embodiment of the present invention. The auto-tuning RF generator  302  provides a feedback control signal to the voltage controlled oscillator (VCO)  306  so as to adjust the frequency of a VCO RF signal  310  output from the VCO  306 . The VCO  306  can also be included as part of the RF generator  302 . A DC power supply  312  is included and provides DC power for the amplification of the VCO RF signal  310  in the RF generator  302 . The auto-tuning RF generator  302  includes an inductor  314  in the input portion of the RF generator  302 . One or more amplifiers  320  that amplify the VCO RF signal  310  are also included in the RF generator  302 . 
     In one embodiment, the amplifier  320  is a CMOS and the VCO RF signal  310  is applied to a gate G. A drain D is coupled to DC bias rail  322  and a source S is coupled to a ground potential rail  324 . A peak voltage drain to source (peak V ds ) detector  326  is coupled across the drain D and source S terminals of the amplifier  320  so as to capture the peak voltage drain to source of the amplifier  320 . 
     The output of the amplifier  320  is coupled to an input of a class-E load network  330 . The class-E load network  330  is a common device well known in the art for performing large-scale impedance matching functions between an RF source (i.e., RF generator  302 ) and an RF load (i.e. transducer  332 ). The class-E load network  330  typically includes a LC network. An output of the class-E load network  330  is coupled to an input to the transducer  332 . 
       FIG. 4  is a flowchart of the method operations  400  of the auto-tuning RF generator system  300  while the RF generator  302  is supplying an RF signal  220  to the transducer  332 , in accordance with one embodiment of the present invention. In operation  405 , the DC supply voltage is measured or detected by a comparator device  340 . A voltage divider network  342  can also be included to scale or reduce the amplitude of the respective voltage coupled to the comparator device  340  from the DC power supply  312  to a level useable by the comparator device  340 . Proportional, differential and integral controls can also be included in the comparator device  340  so that the rate and amount of change in the control signal can be selected. 
     In operation  410 , the peak V ds  is detected by the peak V ds  detector  326  and applied to a second input of the comparator device  340 . The peak V ds  detector  326  can also include circuitry to scale or reduce the amplitude of the voltage coupled to the comparator device  340  from the peak V ds  detector  326  to a level useable by the comparator device  340 . 
     By way of example, the DC power supply  312  may output 200 VDC and the comparator device  340  is capable of comparing a 5 VDC signal, therefore the voltage divider network  342  can scale DC power supply voltage from 200 VDC to a voltage of 5 VDC that represents 200 VDC in the comparator device  340 . Similarly, the peak V ds  detector  326  can also include scaling devices such as a voltage divider network so that the actual peak V ds  voltage applied to the comparator device  340  is about 5 VDC. 
     In operation  415 , the comparator device  340  compares the peak V ds  and the DC supply voltage from the DC power supply  312 . If the DC supply voltage is a desired ratio of the peak V ds , then no correction signal is output from the comparator device and the method operations continue in operation  405  above. 
     Alternatively, if the DC supply voltage is not a desired ratio of the peak V ds , then the method operations continue in operation  420 . In operation  420 , a corresponding correction signal is output from the comparator device  340  to the VCO  306  to adjust the frequency of the VCO output signal  310  and the method operations continue in operation  405  above. The correction signal can adjust the frequency of the VCO RF signal  310  to a higher or lower frequency as required. 
     The desired ratio of the DC supply voltage to the peak V ds , is dependant upon the particular values of the various components in the RF generator  302  and the transducer  332  and the system that may include the RF generator  302  and the transducer  332 , such as the substrate cleaning system  200  of  FIG. 2  above. In one embodiment, the desired ratio is within a range of about 3:1 to about 6:1, where the peak V ds  is a larger voltage than the DC supply voltage. In one embodiment the desired ratio is about 4:1 and more specifically about 3.6:1 where the peak V ds  is about equal to about a 3.6 multiple of the DC supply voltage. 
       FIG. 5A  is a schematic diagram of the peak V ds  detector  326  in accordance with one embodiment of the present invention. Serially connected capacitors  502 ,  504  are coupled across the drain D and source S of the amplifier  320 . A diode  506  is coupled in parallel with capacitor  504 . In operation, capacitor  502  couples the peak V ds  of each cycle of the amplified RF signal to capacitor  504 . Capacitor  504  stores the peak V ds  for each cycle of the amplified RF signal that is output from the amplifier  320 . Diode  506  captures the peak V ds  and couples the peak V ds  to the comparator device  340  via the peak V ds  terminal. 
       FIG. 5B  is a graph  550  of waveforms of the peak voltage (V ds ) detected by the peak voltage detector  326 , in accordance with one embodiment of the present invention. When the amplifier device  320  is conducting, the peak voltage detector  326  does not detect much voltage because there is little voltage drop across the amplifier  320 . When the amplifier stops conducting, then the current stored in the inductors and capacitors of the RF generator  302  and load network  330  is discharged, resulting in a voltage waveform  552 ,  554 ,  556  as detected by the peak voltage detector  326 . The amplifier  320  is designed such that as the voltage across the amplifier  320  (V ds ) drops to zero, the amplifier  320  begins to conduct thus creating a tuned amplification circuit. The tuned amplification circuit is affected by any changes in resonance of the transducer  332  (e.g., any movement of the substrate  202  relative to the transducer  332 ), which are reflected through the load network  330  to change the detected waveform  552 ,  554 ,  556 . When in resonance, the amplifier  320  acts as a well tuned class-E amplifier and the waveform  554  occurs. When off resonance, the transducer  332  can have either capacitive or inductive reactance resulting in added capacitive or inductive reactance, which detunes the class-E load network  330 . The detuned class-E load network  330  results in either waveform  552  or  556 , having either a too high peak voltage V 1  or too low peak voltage V 3 . 
     Through experimentation and calculation, it has been found that the peak voltage (V ds ) is a function of the resonance of the transducer  332  and the peak V ds  compared to the applied DC bias voltage has a resonant ratio that is a function of the components of the RF generator circuit  302 . For example, in a typical RF generator, the ratio is about 4:1 peak voltage as compared to the DC bias voltage from the DC power supply, or restated, a peak V ds  of about 4 multiples of the bias voltage from the DC power supply  312  indicates that the transducer  332  is in resonance. 
       FIG. 6  is a block diagram of an auto-tuning RF generator system  600  according to one embodiment of the present invention. A phase P 1  of the voltage of the RF signal  310  output from the VCO  306  is compared to a phase P 2  of the voltage of the input to the transducer  332 . If the voltage phases P 1  and P 2  do not match, a correction signal is applied to the frequency control input of the VCO  306 . The RF generator system  600  includes an RF generator  602 . The RF generator  602  can be any type of RF generator known in the art. A phase detector  604  includes two inputs  606 ,  608 . The first and second inputs  606 ,  608  can also include respective scaling circuits  610 ,  612  (e.g., voltage divider networks) that can scale the detected signals (e.g. phase P 1  and phase P 2 ) to a level useable by the phase detector  604 . The phase detector  604  can be any type of phase detector known in the art that can detect and compare the phases of the respective input voltage signals. Prior art phase detectors compared the phases of the voltage and current of the output RF signal  220 . Testing has shown that comparing the voltage phases P 1  and P 2  can be accomplished more simply and easily and provide the needed signal for adjusting the VCO  306  accordingly. 
       FIG. 7  is a flowchart of the method operations of the auto-tuning RF generator system  600  according to one embodiment of the present invention. In operation  705 , an input RF signal  310  from the VCO  306  is applied to the RF generator  602  and the RF generator  602  amplifies the input RF signal  310  and couples the amplified RF signal  220  to the transducer  332 . 
     In operation  710 , the first input  606  couples a first phase (P 1 ) of the voltage of the RF signal  310  output from the VCO  306  to the phase detector  604 . In operation  715  the second input  608  couples a second phase (P 2 ) of the voltage of the signal input to the transducer  332  to the phase detector  604 . 
     In operation  720 , the phase detector compares phase P 1  and phase P 2  to determine if the phase P 1  matches phase P 2 .  FIGS. 8A-8C  show graphs of three examples of the relationships between phases P 1  and P 2 , in accordance with one embodiment of the present invention. In  FIG. 8A , graph  800  shows phase P 1  leads phase P 2  (e.g., phase P 1  peaks at time T 1  and phase P 2  peaks at a subsequent time T 2 ). This indicates that the impedance of the transducer  332  is not matched and that the transducer  332  is applying a reflected signal  222  into the RF generator  602 . 
     In  FIG. 8B , graph  820  shows phase P 1  lags phase P 2  (e.g., phase P 2  peaks at time T 1  and phase P 1  peaks at a subsequent time T 2 ). This indicates that the impedance of the transducer  332  is not matched and that the transducer  332  is again applying a reflected signal  222  into the RF generator  602 . The reflected signal  222  output by the transducer  332  can be constructively or destructively interfering with the signal  220  output from the RF generator  602 . 
     In  FIG. 8C , graph  850  shows phase P 1  is equal to phase P 2  (e.g., both phase P 1  and phase P 2  peak at time T 1 ). This indicates that the impedance of the transducer  332  is matched and that the transducer  332  is not applying any reflected signal into the RF generator  602 . 
     If, in operation  720 , phase P 1  and phase P 2  are equal, then the method operations continue (repeat) at operation  705 . If, however, in operation  720  phase P 1  and phase P 2  are not equal, then the method operations continue in operation  730 . In operation  730 , an appropriate control signal is applied to the frequency control input of the VCO  306  to adjust the frequency of the RF signal  310  accordingly, and the method operations continue (repeat) at operation  705 . The control signal applied to the frequency control input of the VCO  306  can adjust the frequency to a higher frequency in response to a condition where phase P 1  leads phase P 2 . Alternatively, the control signal applied to the frequency control input of the VCO  306  can adjust the frequency to a lower frequency in response to a condition where phase P 1  lags phase P 2 . 
     The auto-tuning RF generator system  600  can also include a control amplifier  620  that can scale the control signal output by the phase detector  604  to the correct signal level to control the VCO  306 . The control amplifier  620  can also include a set point input so the control amplifier  620  can combine the set point input and the control signal input from the phase detector. In this manner a VCO RF signal  310  can be selected by the set point and then the control signal output by the phase detector  604  can automatically adjust the selected set point. 
     The systems and methods described in  FIGS. 3 through 8C  above can automatically tune the RF generators  302 ,  602  at a very high correction rate (e.g., at each cycle of the input RF signal  310  can cause a subsequent correction in the frequency of the RF signal  310  and the output RF signal  220 ). As a result, the frequency of the input RF signal  310  can be corrected, for example, multiple times during each revolution of the substrate  202  and thereby providing much more precise control of the acoustic energy  214  applied to the substrate  202 . 
     By way of example, if the substrate  202  is being rotated 60 RPM (i.e. 1 revolution per second) and the RF signal  310  is about 1 MHz, then the frequency of the RF signal  310  can be adjusted about one million times per second (i.e., once per microsecond) during each rotation of the substrate  202 . This increased control of the acoustic energy  214  applied to the substrate  202  means that the average energy can be very close to the minimum energy valley and the maximum energy peak of the emitted energy  214 . Therefore a higher average energy can be applied to the substrate  202 , which thereby allows a significantly reduced cleaning process time and improved cleaning effectiveness. 
       FIG. 9  is a block diagram of an auto-tuning RF generator system  900  according to one embodiment of the present invention. The system includes a VCO  306  that is coupled to an input of an RF generator  602 . A variable DC power supply  902  is coupled to the RF generator  602  and provides DC power for the RF generator to amplify the RF signal  310  from the VCO  306 . The output of the RF generator  602  is coupled to the transducer  332 . 
     Typical prior art acoustic energy cleaning systems focus on maintaining a constant net power input to the transducer  332  (i.e., forward power of RF signal  220  less reflected power of reflected signal  222 ). Through experimentation, it has been found that if the voltage of the RF signal  220  is maintained as a constant voltage, then the amplitude of the emitter energy  214  output from the transducer  332  is substantially constant. Further, maintaining the voltage of the RF signal  220  at a constant level, below the energy threshold limit of the substrate  202  protects the substrate from damage while also allowing a maximum acoustic energy  214  to be applied to the substrate  202 . 
       FIG. 10  is a flowchart of the method operations of the auto-tuning RF generator system  900 , in accordance with one embodiment of the present invention. In operation  1005 , the RF generator  602  outputs an RF signal to the transducer  332 . In operation  1010 , a voltage of the RF signal output to the transducer  332  is measured and coupled to a comparator  904 . 
     In operation  1015 , the comparator  904  compares the voltage of the RF signal output from the RF generator  602  to a desired set point voltage. If the output voltage is equal to the desired set point voltage, the method operations continue at operation  1010 . Alternatively, if the output voltage is not equal to the set point voltage, the method operations continue in operation  1030 . 
     In operation  1030 , the comparator  904  outputs a control signal to a control input on the variable DC power supply  902 . By way of example, if the output voltage is too high (i.e., greater than the desired set point voltage), then the control signal will reduce the DC voltage output from the variable DC power supply  902  thereby reducing the gain of the amplification that occurs within the RF generator  602 , thereby reducing the amplitude of the RF signal output by the RF generator  602 . Proportional, differential and integral controls can also be included in the comparator  904  so that the rate and amount of change in the control signal can be selected. 
     A scaling circuit  906  can also be included to scale the voltage output from the RF generator  602  to a level more easily compared to the set point signal. By way of example, the scaling circuit  906  can scale a 200 V RF signal to 5 V for comparison to a 5 V set point signal. The scaling circuit  906  can include a voltage divider. The scaling circuit  906  can also include a rectifier to rectify the voltage of RF signal  220  output from the RF generator  602  to a DC voltage for comparison to a DC set point signal. 
     As described above the methods described in  FIGS. 3 through 8C  above can automatically tune the RF generators  302 ,  602  at a very high correction rate (e.g., once per a few cycles of the RF signal  310 ). Conversely, the system and method described in  FIGS. 9 and 10  can also automatically tune the RF generator  602  but at a slightly slower rate than as described in  FIGS. 3 through 8C  but yet still faster than the likely changes in impedance of the transducer  332  due to the motion of the substrate  202 . The system and method described in  FIGS. 9 and 10  is somewhat slower due in part to the hysteresis included in the variable DC power supply  902 . 
     The system and method described in  FIGS. 9 and 10  can be used in combination with one or more of the systems and methods described in  FIGS. 3 through 8C  above. As such, the system and method described in  FIGS. 9 and 10  can used to provide a very broad range of tuning the RF generator to the dynamic resonance of the transducer  332 , while the systems and methods described in  FIGS. 3 through 8C  above can be used to provide very fine control and adjustment of the tuning the RF generator. 
       FIG. 11  is a diagram of a megasonic module  1100  in accordance with one embodiment of the present invention. The megasonic module  1100  can be a megasonic module, such as a the material described in commonly owned U.S. patent application Ser. No. 10/259,023, entitled “Megasonic Substrate Processing Module” which was filed on Sep. 26, 2002, which is incorporated by reference herein, in its entirety, for all purposes. 
     The megasonic module  1100  includes a substrate processing tank  1102  (hereinafter referred to as tank  1102 ), and a tank lid  1104  (hereinafter referred to as lid  1104 ). A lid megasonic transducer  1108  and a tank megasonic transducer  1106  are positioned on lid  1104  and in tank  1102 , respectively, and provide megasonic energy for simultaneously processing an active and a backside surface of a substrate  1110 . A substrate  1110  is positioned in drive wheels  1112 , and secured in position with substrate stabilizing arm/wheel  1114 . In one embodiment, the substrate stabilizing arm/wheel  1114  is positioned with an actuator  1120  and a positioning rod  1122  to open and close the stabilizing arm/wheel  1114  to receive, secure, and release a substrate  1110  to be processed in the megasonic module  1100 . The lid  1104  can be positioned in an open or a closed position with a actuator system (not shown) that raises and lowers lid  1104  while the tank  1102  remains stationary. Alternatively the tank  1102  can be moved to mate with the lid  1104 . 
     In one embodiment, substrate stabilizing arm/wheel  1114  is configured to secure and support substrate  1110  in a horizontal orientation for processing, and to allow rotation of substrate  1110 . In other embodiments, substrate processing is performed with substrate  1110  in a vertical orientation. Drive wheels  1112  contact a peripheral edge of substrate  1110  and rotate substrate  1110  during processing. Substrate stabilizing arm/wheel  1114  can include a freely spinning wheel to allow for substrate  1110  rotation while supporting substrate  1110  in a horizontal orientation. 
     Once the substrate  1110  is placed in the tank  1102 , the tank  1102  is then filled with processing fluid including deionized (DI) water, or processing chemicals as desired. Once the closed megasonic module  1100  is filled with desired processing fluid, and substrate  1110  is immersed therein, megasonic processing of substrate  1110  is accomplished by tank megasonic transducer  1106  directing megasonic energy against the surface of substrate  1110  facing the tank megasonic transducer  1106 , and by lid megasonic transducer  1108  directing megasonic energy against the surface of substrate  1110  facing the lid megasonic transducer  1108 . With substrate  1110  submerged in processing chemicals, drive wheels  1112  rotate substrate  1110  to ensure complete and uniform processing across the entire surface of both the active and backside surfaces of substrate  1110 . In one embodiment, drive motor  1116  is provided to drive the drive wheels  1112  via a mechanical coupling  1118  (e.g., drive belt, gears, sprocket and chain, etc.). 
     An auto-tuning RF generator system as described in  FIGS. 3-10  above can be coupled to one or both of the lid transducer  1108  and tank transducer  1106  so that the respective transducers  1108 ,  1106  are constantly and automatically tuned for the dynamic impedance of the respective transducers  1108 ,  1106  as the substrate  1110  is rotated. 
       FIG. 12  is a graph  1200  of the energy distribution across the transducer in accordance with one embodiment of the present invention. In comparison with the prior art energy window shown by curves  120  and  122 , an auto-tuning RF generator can result in a much narrower energy window  1202  between curve  1210  and curve  1212 . Since the energy window  1202  is much narrower, then the energy window can be shifted upward closer to the energy threshold T of the substrate and thereby provide a more effective acoustic energy cleaning process. 
     As used herein in connection with the description of the invention, the term “about” means+/−10%. By way of example, the phrase “about 250” indicates a range of between 225 and 275. It will be further appreciated that the instructions represented by the operations in  FIGS. 4 ,  7  and  10  are not required to be performed in the order illustrated, and that all the processing represented by the operations may not be necessary to practice the invention. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present 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 equivalents of the appended claims.