Title of Invention	MEGASONIC PROCESSING APPARATUS WITH FREQUENCY SWEEPING OF THICKNESS MODE TRANSDUCERS
Abstract	A megasonic processing apparatus and method has one or more piezoelectric transducers operating in thickness mode at fundamental resonant frequencies of at least 300 KHz. A generator powers the transducers with a variable-frequency driving signal that varies or sweeps throughout a predetermined sweep frequency range. The generator repeatedly varies or sweeps the frequency of the driving signal through a sweep frequency range that includes the resonant frequencies of all the transducers.

Title of Invention

MEGASONIC PROCESSING APPARATUS WITH FREQUENCY SWEEPING OF THICKNESS MODE TRANSDUCERS

Abstract

A megasonic processing apparatus and method has one or more piezoelectric transducers operating in thickness mode at fundamental resonant frequencies of at least 300 KHz. A generator powers the transducers with a variable-frequency driving signal that varies or sweeps throughout a predetermined sweep frequency range. The generator repeatedly varies or sweeps the frequency of the driving signal through a sweep frequency range that includes the resonant frequencies of all the transducers.

Full Text	MEGASONIC PROCESSING APPARATUS WITH FREQUENCY SWEEPING OF THICKNESS MODS TRANSDUCERS RELATED APPLICATION This application claims priority from co-pending U.S. Provisional Application No. 60/783,213, filed March 17,2006, entitled MEGASONIC PROCESSING APPARATUS WITH FREQUENCY SWEEPING, and U.S. Patent Application entitled MEGASONIC PROCESSING APPARATUS WITH FREQUENCY SWEEPING OF THICKNESS MODE TRANSDUCERS filed March 16, 2007 and invented by J. Michael Goodson. These prior applications are expressly incorporated herein by reference. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates generally to megasonic processing apparatus and associated methods involving one or more piezoelectric transducers operating in thickness mode at megasonic frequencies of at least 300 KHz or higher, and relates more particularly to improving performance by sweeping the frequency of a driving signal throughout a predetermined or programmable frequency range that spans the resonant frequencies of all the transducers. DESCRIPTION OF THE RELEVANT ART Megasonic processing involves generating and using high frequency energy at frequencies above 300 KHz. Many megasonic systems operate at frequencies at or near 1,000 KHz, or one megahertz. Although 1 MHz is the consensus, preferred frequency for many applications, the frequency range goes much higher, with frequencies as high as 10 MHz. Typical uses for megasonic systems include cleaning delicate objects, such as semiconductor wafers and disc drive media. Such a megasonic cleaning process involves placing the objects to be cleaned in a fluid-filled tank, and applying vibrational energy at megaeonic frequencies to a radiating surface or surfaces of the tank. One or more piezoelectric transducers are used to generate the vibrational energy. A generator supplies an alternating current driving signal at the resonant frequency of the transducers. Megasomc transducers operate in thickness mode, where a piezoelectric element is excited by an alternating current driving signal that causes alternating expansion and contraction of the transducer, primarily expanding and contracting the thickness of the transducer. A piezoelectric transducer having a thickness of 0.080 inches has a fundamental, thickness mode, resonant frequency of 1,000 KHz. Megasonic processing has some similarities with ultrasonic processing, which involves lower fundamental frequencies, typically from about 25 KHz to about 192 KHz. Ultrasonic transducers are typically mass-balanced, with inert masses on either side of a piezoelectric element, and have a significant radial component of movement at right angles to the thickness component. One common construction of an ultrasonic transducer is to stack several layers of ring-shaped piezoelectric elements between two masses, and to hold the assembly together with an axial compression bolt. Ultrasonic cleaning is based on cavitation, which is the formation and collapse of bubbles in the fluid. At the frequencies used for megasonic cleaning, significant cavitation does not occur, so die cleaning action is based on another mechanism known as micro-streaming, which is a general flow of detached particles flowing away from the megasonic transducers. This flow consists of planar waves originating at the surface to which the transducers are mounted. The planar nature of these micro-streams affects the distribution of megasonic energy throughout the tank. One way to improve the distribution is to cover a high percentage of the surface area of the tank with transducers. Another but less efficient way is to oscillate or move the parts to be processed throughout die tank so that all surfaces are exposed to sufficiently high megasonic energy. It is known that radial-mode ultrasonic activity in a cleaning tank may benefit from a process of sweeping or varying the frequency of the driving signal. However, there has been an industry-wide belief that you cannot sweep megasonic frequencies because the sound waves are too small and weak for any benefit from sweeping. In addition, it has been thought that there would be no benefit from sweeping megasonic frequencies because of the thickness mode transducers and resultant planar nature of megasonic vibrations and due to the different cleaning mechanisms at work as compared to ultrasonics. SUMMARY OF THE INVENTION The present invention relates to a megasonic processing apparatus and method having one or more piezoelectric transducers (PZT) operating in thickness mode at megasonic frequencies in excess of 300 KHz. A megasonic generator operating at megasonic frequencies drives the transducers with a variable-frequency driving signal that varies or sweeps throughout a predetermined or programmable sweep frequency range. The megasonic generator generates the driving signal at megasonic frequencies to energize the megasonio piezoelectric transducers to cause them to vibrate in thickness mode at their megasonic resonant frequencies. The piezoelectric transducers emit energy at the megaeonic frequencies that can be used for various applications, such as cleaning objects in a fluid-filled tank. The generator repeatedly varies or sweeps the frequency of the driving signal through a sweep frequency range that includes the resonant frequencies of all the megaeonic piezoelectric transducers. Another aspect of the present invention involves grouping the megasonic piezoelectric transducers into groups having similar resonant frequencies, and powering each group with a separate frequency-sweeping driving signal from a generator operating within a sweep frequency Tange that includes the resonant frequencies of the group of associated transducers. This subdivides the overall sweep frequency range into smaller subranges, which may or may not overlap, and reduces the range of each frequency sweep. The effect of grouping transducers is to proportionately increase the amount of time mat any particular transducer is operating at or close to its resonant frequency and thereby improve efficiency. The present invention encompasses a megasonic system that includes one or more piezoelectric transducers and one or more megasonic generators coupled to the transducers for supplying varying-frequency megasonic driving signals at selectable or programmable frequency ranges and sweep rates. When a megasonic process is used, for example, for cleaning silicon wafers or disc drive media, sweeping the driving signal through me resonant frequencies of all the thickness-mode megasonic transducers will equalize the megasonic energy generated by the transducers and will cause the transducers to perform in unison. This results in a more uniform distribution of megasonic energy and improved performance. The same improved megasonic energy uniformity and functionality can also be achieved in liquid processing, non-destructive testing, medical imaging, and other processes using megasonic thickness-mode transducers by sweeping the range of resonant frequencies of the transducers. The frequency sweeping process will also extend the life of the megasonic transducers because it is less stressful to the transducers than operating at a single fixed frequency. The frequency sweeping process also improves the uniformity of megasonic energy throughout the tank or other apparatus because each transducer operates at its resonant frequency during at least part of each frequency sweeping cycle. It is expected feat any application of process using megsonic frequencies will benefit from fee uniform distribution of power created by sweeping fee driving signal through all the transducers' resonant frequencies. A key to optimizing efficiency of a megasonic process is to have uniform energy throughout fee radiating surface being excited wife megasonics. To do this, preferably 80% or more of the area of fee radiating surface is covered by thickness-mode megasonic transducers. Furthermore, each megasonic transducer produces consistent megasonic energy by sweeping fee frequency of fee driving signal through the highest and lowest resonant frequencies of a group of transducers. For best performance, each megasonic transducer needs to be energized substantially the same as other megasonic transducers bonded to the same surface. To achieve this, the driving frequency is swept through fee resonant frequencies of all fee transducers. Sweeping the resonant frequencies of megasonic transducers drives every transducer at its resonant frequency at some point in each cycle. This creates uniformity in transducer performance not previously achieved in the industry. In addition, frequency sweeping of megasonic transducers reduces a "fountain effect" observed wife fixed-frequency megasonic transducers. The fountain effect is thought to be caused by a transducer operating at its resonant frequency wife a fixed frequency driving signal, which produces a significant up-surge of liquid in the tank above feat transducer. Sweeping fee megasonic frequency driving signal ensures that any particular transducer will not be driven continuously at its resonant frequency, feus eliminating fee upsurge associated wife fee fountain effect. Instead, the megasonic energy is uniformly distributed throughout the tank because all transducers are operating efficiently at their resonant frequencies at some point during each sweep cycle. Frequency sweeping is far more dramatic wife megasonic frequencies than ultrasonic frequencies like 40 KHz. Improvements in power distribution of 500 to 700 % have been seen wife megasonic resonant frequency sweeping and (his means substantially better processing. The features and advantages described in fee specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in fee art in view of fee drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an overall perspective view of a megasonic processing system according to the present invention. Figure 2 is a top perspective view of a tank used in the megasonic processing system of the present invention. Figure 3 is a bottom perspective view of the tank. Figure 4 is a side elevation view of the tank. Figure 5 is a bottom view of the tank. Figure 6 is a schematic view of the megasonic processing system and a sectional view of the tank and an attached megasonic transducer with a generator that supplies driving signals to the transducer for creating megasonic vibrations in liquid in the tank. Figure 7 is a graph of frequency versus time of a driving signal used in one embodiment of the present invention. Figure 8 is a graph of frequency versus time of two driving signals used in another embodiment of the present invention in which the sweep period is the same as in Figure 7. Figure 9 is a graph of frequency versus time of two driving signals used in another embodiment of the present invention in which the sweep rate is the same as in Figure 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawings depict various preferred embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. One aspect of the present invention is a megasonic processing apparatus and method having a megasonic generator with a programmable sweep frequency range and a programmable sweep rate. The sweep frequency range is the range of frequencies or bandwith within which the megasonic generator outputs a driving signal to drive one or more megasonic thickness-mode piezoelectric transducers at their resonant frequencies. The sweep rate is the number of times the resonant frequencies are swept per second. The megasonic generator preferably includes a controller or other controlling device with means to allow a user to select or program the sweep frequency range or bandwidth and the sweep rate for the driving signal. The user inputs one or more combinations of sweep frequency range and sweep rate into the memory device of the generator. The generator generates and outputs the driving signal according to the sweep frequency range and sweep rate selected by the user. When used in a cleaning application, for example, the megasonic piezoelectric transducer or transducers may be mounted on the bottom or sides of a tank, or enclosed in an immersible container within the tank.. The sweeping frequency generator may be used to drive megasonic transducers in applications other than cleaning. Preferably, the transducers are piezoelectric crystals or piezoelectric ceramic (also known as PZTs), such as barium titanate or lead zirconate titanate, operating in thickness mode. Using different sweep rates or sweep frequency ranges in the same process may enhance cleaning of some parts because certain frequencies may be more effective than others. A device that sweeps the frequency of the driving signal is incorporated into the megasonic generator that generates the driving signal. The generator includes a user interface that includes one or more input devices, such as knobs, dials, software, keyboard, graphical user interface, network connection, or other input devices, mat permit a user to set a sweep frequency range or bandwidth over which the generator operates and also to set a sweep rate at which the generator sweeps through the programmed range. The controls for user programming the sweep frequency range and sweep rate may be analog or digital. As shown in Figures 1-6, one embodiment of the present invention is a cleaning system 10 mat includes a quartz cleaning tank 12 containing a cleaning liquid or solution 14 and one or more pieces 15 to be cleaned. Megasonic energy is supplied to the cleaning liquid 14 by one or more megasonic frequency transducers 16 affixed to the bottom of the tank 12. Alternatively, megasonic transducers could be affixed to one or more sides of the tank or immersed in the tank. Preferably, the megasonic transducer 16 has a piezoelectric dement (PZT) 18 adhesively bonded or otherwise attached to one side of a silicon carbide plate 20. The other side of the silicon carbide plate 20 is adhesively bonded or otherwise attached to the outside bottom surface of the cleaning tank 12. Preferably, bonding layers 22 between the silicon carbide plate 20 and the tank 12 and between the silicon carbide plate and the piezoelectric element 18 are composed of perforated copper foil and an impedance matching adhesive. Alternatively, the bonding layers may be composed of epoxy or other adhesive used for die bonding semiconductor chips to package substrates. The piezoelectric element can be square, rectangular, or a circular disk, or other shape having uniform thickness. For example, for operation at a nominal frequency of 1,000 KHz, the piezoelectric element 18 would have a thickness of about 0.08 inches, the silicon carbide plate 20 would have a thickness of about 0.19 inches, and the bottom of the quartz tank 12 would have a thickness of about 0.20 inches. Transducer 16 and cleaning system 10 is just one example of a transducer and apparatus that incorporates the present invention. As shown in Figures 3-6, the transducers 16 are preferably rectangular in shape and are arranged parallel to each other. Preferably, the transducers 16 cover a substantial portion of the bottom surface of the tank 12, preferably at least 80%. It is desirable to generate megasonic energy and transfer it to the tank 12 and fluid 14 uniformly throughout the entire area of the surface to which the transducers 16 are attached. Covering a high percentage of the surface area of the tank bottom with transducers ensures that the megasonic energy transferred to the fluid 14 is relatively uniform. As shown in Figure 6, the transducers 16 are driven by a driving signal supplied over electrical wires 24 by a programmable generator 26. The generator 26 is programmed by a user through a user input or interface 28 to set the sweep frequency range or bandwidth and the sweep rate of the driving signal output by the generator. A megasonic frequency piezoelectric transducer operates in thickness mode such that applied voltages cause the transducer to expand and contract in thickness. These expansions and contractions are transmitted through the silicon carbide resonator 20 and tank 12 to the fluid 14 and objects IS in the tank. As shown in Figure 6, these megaeonic-frequency vibrations are primarily horizontal waves 17, provided mat the transducers 16 are on the bottom of the tank 12. The waves propagate upwards and convey particles cleaned or separated from the objects 15 in the tank. This is a processed known at micro-streaming, in which there is a net movement upward, away from the source of megasonic energy. As shown in Figures 1 and 2, the tank has a weir 21 over which excess fluid and particles flow, and a pump 23 and filter 25 to recirculate and clean the fluid. Resonant frequency is generally the frequency where the mechanical and electrical properties of a transducer can most efficiently transmit sound waves. In megaeonic transducers operating in thickness mode, the thickness of the transducer determines the resonant frequency. For example, a transducer that is 0.08 inches thick will have a resonant frequency of about 1,000 KHz. A transducer that is 0.065 inches thick will have a resonant frequency of about 1230 KHz. A transducer that is 0.050 inches thick will have a resonant frequency of about 1600 KHz. The term "resonant frequency" is used herein to mean the lowest, fundamental frequency where the transducer as installed has a natural resonance. As stated above, a piezoelectric transducer having a thickness of 0.080 inches has a fundamental resonant frequency of 1,000 KHz. A tolerance on the thickness of such a transducer has a significant effect on the resonant frequency. A thickness variation of 0.001 inch would cause a resonant frequency variance of 12.5 KHz. Also, the two major surfaces of the transducer should be flat and co-planar, but any variances can also affect the resonant frequency. Even though it is desirable from a performance standpoint for all transducers to have exactly the same resonant frequency, from a manufacturing tolerance standpoint, it is impractical. However, the frequency sweeping of the present invention overcomes this obstacle. One advantage of the present invention is that sweeping the frequency of the driving signal through the resonant frequencies of all the transducers distributes the sound waves equally among the transducers. This makes it possible to have substantially equal megasonic energy throughout the tank. This is important because the thickness-mode transducers produce sound waves that travel vertically from the bottom to the top of the tank with little spreading in lateral directions. The even distribution of megasonic energy can best be achieved by sweeping just outside the highest and lowest resonant frequencies of the transducers. Another advantage of the present invention is that it accommodates tolerances in the resonant frequencies of the transducers. Performance is best if variances of the resonant frequency are minimized. Choosing transducers with exactly the tame resonant frequency will help minimize variances (although at increased cost), but even then there will be some variances from the adhesives or other binder miserable used to mount the transducers because any variation in thickness creates a variation in frequency with thickness mode applications. Sweeping the frequency of the driving signal according to the present invention accommodates such inevitable variations. Still another advantage of the present invention is that it reduces surges of fluid in the tank. Without sweeping the driving signal, the transducers at or closest to the frequency of the driving signal tend to create a powerful upward force that pushes the fluid upward, sometimes as much as two inches above the surface level. Such surface surges are a problem because they cause air to be incorporated into the fluid as it is recirculated, which can interfere with the megasonic process. Surges are also a problem because if the liquid is solvent it will evaporate in the air and can be harmful to the operator and or the people in the area, especially if the fluid is an acid or other hazardous material. Sweeping the driving signal with the present invention reduces these problems. As shown in Figure 7, the generator 26 varies the frequency of the driving signal as a function of time. For example, the frequency of the driving signal may vary linearly in a saw-tooth pattern over a programmed sweep frequency range 30 that includes the resonant frequencies 31 of all the megasonic transducers 16. The sweep frequency range or bandwidth of the generator is programmed by a user and stored in a memory device associated with the generator 26. The rate at which the frequency varies is determined by the sweep rate programmed by the user and stored in the memory device of the generator. The generator can be programmed to vary the frequencies of the driving signal according to other functions or programs and need not be limited to linear functions that form a triangular wave or saw tooth pattern as shown in Figure 7. The variation in frequency can be, for example, sinusoidal, exponential, and other functions. The driving signal itself may be sinusoidal, square, triangular, or other wave shape. The sweep rates need not be the same for sweeping upwards (increasing frequency) aad downwards (decreasing frequency). Preferably, the user can also set the number of periods and can establish rest times when the generator shuts off the driving signal. In a cleaning application, some parts may be best cleaned by a single transducer instead of multiple transducers. In such a configuration, the performance of me transducer can be enhanced by using a programmed software program thai identifies the optimum resonant frequency and sweeps through this frequency within a defined range. For best results, the driving frequency can be swept through a sweep range of 1% or less to ensure mat the resonant frequency of the transducer is being excited repeatedly. A benefit of fee present invention is that it reduces the adverse effects of resonant frequency drifting because the resonant frequency of each transducer is being excited each cycle even if it changes with time, provided that the sweep range or bandwidth is wide enough. Commonly, multiple megasonic transducers 16 are used for a given task or process, in which case it is common to drive all transducers with the same generator and driving signal. Where multiple transducers are used, however, there may not be a single optimum frequency due to performance variations and manufacturing tolerances among the transducers. Production tolerances result in megasonic transducers having resonant frequencies within a 3% to 4% range. For example, at 1000 KHz, a 4% range would be plus or minus 20 KHz from the nominal 1000 KHz, or a range of 980 to 1020 KHz. In such applications, according to the present invention, it is appropriate to repeatedly sweep the frequency of the driving signal to ensure that at least some of the time the transducer 16 is operating at or near its resonant frequency. In order to have each transducer 16 operate at or near its resonant frequency, the generator sweeps through a predetermined sweep frequency range that is designed to reach the lowest and highest resonant frequencies 31 of the group of transducers. The sweeping frequency function of the generator 26 covers that range of variance. The frequency sweeping function can be fixed or it can be programmed to be variable as to speed (sweeps per second) or range (minimum and maximum frequencies). Another aspect of the present invention relates to grouping the megasonic piezoelectric transducers into multiple groups according to their resonant frequencies, and driving each group with a separate variable-frequency driving signal. Transducers with similar resonant frequencies are grouped together to reduce the range of frequencies through which the generator must sweep in order to operate the group of transducers at or near their resonant frequencies. Reducing the frequency range of the sweep increases the time that each transducer operates at or near its resonant frequency. As the range of sweep frequency coverage is reduced, the rate of sweep can be increased to create more activity if required by a particular application, or if the sweep rate remains the same, men the repetition rate is increased. The result is that the megasonic transmission at each transducer's resonant frequency will be greater since the sweep covers a shorter span and the transducer operates for a greater percentage of time at or near its resonant frequency, which increases the efficiency of the megasonic process. This point is illustrated in Figures 7, 8, and 9. In Figure 7, a single generator sweeps the driving signal between minimum and maximum frequencies over a range 30. In Figure 8, two generators are used to cover the same overall range, but each generator coven a subrange 32 that is one-half of the foil range 30. Half of the transducers have resonant frequencies 31' in the upper subrange 32', and the other half of the transducers have resonant frequencies 31" in the lower subrange 32". The number of sweeps per unit time is the same in Figures 7 and 8. In Figure 9, the rate of change of the sweeping frequency is the same as in Figure 7, but the range is cut in half so that twice as many sweeps occur in the same period of time. As an example of grouping, assume that twelve megasonic transducers are used in a process having the following nominal resonant frequencies (in KHz): 1010 1030 1015 1007 1019 1004 1027 1038 1022 1014 1031 1040 These frequencies range from a minimum of 1004 KHz to a maximum of 1040 KHz, for a total range of 36 KHz (±18 KHz) centered at 1022 KHz. Sweeping the frequency of the driving signal to include the resonant frequencies of all twelve transducers would require a total sweep of 36 KHz. These twelve transducers could be divided into two groups, A and B, to reduce the sweep range: Generator A Generator B 1004 1014 1022 1031 1007 1015 1027 1038 1010 1019 1030 1040 The tranaducers driven by Generator A range from 1004 KHz to 1019 KHz, for a total range of 15 KHz (±7.5 KHz) centered at 1011.5 KHz. The transducers' driven by Generator B range from 1022 KHz to 1040, for a total range of 18 KHz (±9 KHz) centered at 1031 KHz. By grouping the transducers according to their resonant frequencies and reducing the sweep range for each sweeping generator, the number of sweeps per unit time can be increased or the sweep rate can be decreased, either of which allows the transducers to be driven at or near their resonant frequencies more often, which enhances the megasonic process. In actual practice, the sweep frequency ranges are set slightly outside the maximum and minimum resonant frequencies for the associated transducers. So, in the example above, the sweep frequency range of Generator A might be set to 1003 to 1020 KHz or 1002 to 1021 KHz and the sweep frequency range of Generator B might be set to 1021 to 1041 KHz or 1020 to 1042 KHz. This ensures that each transducer operates both below and above its resonant frequency in each frequency sweep cycle and also allows for shifts of the resonant frequencies that may occur due to heating or other variables. Transducers can be grouped within an individual system or process or among multiple systems or processes operating simultaneously. For example if there are two teaks with multiple transducers each and bom tanks will be used simultaneously, one can group transducers from the larger universe of all transducers on the two tanks. Groupings may be further selected to produce a more uniform result as the transducers powered by a single generator do not have to be next to each other or used with the same tank to be in the group. Because all transducers work simultaneously, the designer of the transducer layout can focus on maximizing the efficiency of the grouping without regard to where the members of the groups are located. As an example of grouping among multiple, simultaneous processes, assume that the same twelve megasonic transducers set forth in the previous example are located on two different tanks: The twelve transducers of Tanks 1 and 2 are divided into two groups according to resonant frequencies and are driven by Generators A and B as follows (with the tank number shown in parentheses): Generator A drives four transducers of Tank 1 and two transducers of Tank 2. Generator B drives two transducers of Tank 1 and four transducers of Tank 2. Since all trasducers are operating at the same time, this grouping allows the two generators to sweep across smaller ranges. Thus, in cleaning and other processes where multiple tanks or systems are used, the entire population of transducers in multiple tanks or systems caa be combined to create an optimum assortment of frequencies to be grouped together, with each group powered by a different sweeping generator. For example in four processes using four tanks, transducers from any or all of the four tanks may be networked together to achieve the optimum range of frequencies for sweeping. Of course, all processes must be active at the same time for such grouping. Another aspect of the present invention is the construction of the megasonic transducer 16 and its attachment to another structure, such as the bottom of tank 12, using a perforated metal layer and impedance matching adhesive. As shown in Figures 4 and 6, the megasonic transducer 16 preferably has a silicon carbide plate 20 between the piezoelectric element 18 and the surface of the cleaning tank 12 or other structure to which the transducer is attached. The piezoelectric element 18 is bonded to the silicon carbide plate 20, and the assembly is bonded to the tank 12 with bonding layers composed of a perforated metal foil, preferably copper, and an adhesive. The perforated copper (or other metal) foil improves flatness and uniformity of thickness of the bonding layer 22. The perforated copper has a predetermined thickness that allows the adhesive to be evenly distributed, thus avoiding irregularities or non- uniformity of adhesive thickness without using a jig or other stabilizing device. The perforated metal provides a controllable flat structure to maintain uniformity in thickness of the adhesive. The perforated metal also serves as an electrode between the piezoelectric element and the silicon carbide plate. The application of the present invention is not limited to cleaning operations. The same principle of sweeping the acoustical energy for megasonic transducers can be applied to other uses of micro-streeming of megasonic energy, such as non destructive testing, or any other applications using thickness mode transducers having fundamental resonant frequencies of at least 300 KHz. Sweeping megasonic transducers creates greater energy bursts, which create improved and stronger micro-streaming activity which improves the efficiency of micro-streaming cleaning and other uses of micro-streaming. Micro-streaming is a flow of energized liquid created by the release of ultrasonic energy that is too weak to cause cavitation. At frequencies in excess of 300 KHz, cavitations cease to exist but the megasonic frequency energy creates a flow of the liquid. From the above description, it will be apparent that the invention disclosed herein provides a novel and advantageous megasonic processing apparatus and method utilizing a variable frequency driving signal. The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in various other forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. What is claimed is: 1. A megasonic processing apparatus comprising: one or more piezoelectric transducers, each having a fundamental resonant frequency of at least 300 KHz; a tank adapted to contain fluid and one or more parts to be processed, wherein said one or more transducers are adapted for providing vibrations to the tank and its contents; a generator coupled to the transducers for supplying a driving signal at a variable frequency throughout a frequency range that includes the resonant frequencies of all the transducers. 2. An apparatus as recited in claim 1 wherein the generator has an adjustable sweep rate and an adjustable frequency range. 3. An apparatus as recited in claim 2 wherein the sweep rate is in the range of 50 to 1200 sweeps per second. 4. An apparatus as recited in claim 1 wherein the apparatus has at least four transducers and two generators, wherein me transducers are grouped by similar resonant frequencies, and wherein each group of transducers is powered by a separate generator that generates a driving signal having a variable frequency that varies within a frequency range that includes the resonant frequencies of all the transducers of its associated group. 5. An apparatus as recited in claim 1 wherein said piezoelectric transducers operate in thickness mode. 6. A megasonic processing system comprising: two or more tanks, each tank adapted to contain fluid and one or more parte to be processed; one or more piezoelectric transducers coupled to each tank, each transducer having a fundamental resonant frequency of at least 300 KHz;, wherein the transducers are capable of providing vibrations to the tanks and their contents; two or more generators coupled to the transducers for supplying driving signals to the transducers, wherein the transducers are grouped by similar resonant frequencies, end wherein each group of transducers is powered by a separate generator that generates a driving signal having a variable frequency that varies within a frequency range that includes the resonant frequencies of all the transducers of its associated group. 7. A megasonic cleaning apparatus comprising: one or more piezoelectric transducers, each having a fundamental resonant frequency in thickness mode vibration at a frequency of at least 300 KHz; a tank adapted to contain cleaning fluid and one or more parts to be cleaned, said one or more transducers adapted for providing vibrations to cleaning fluid and parts in the tank; a generator coupled to said one or more transducers for supplying a driving signal at a predetermined frequency range and sweep rate, wherein the frequency range includes the resonant frequency of all said one or more transducers, and wherein the generator includes programmable means for defining a sweep frequency range and a sweep rate for the driving signal. 8. A megasonic processing method comprising: providing one or more piezoelectric transducers, each having a fundamental resonant frequency of at least 300 KHz; providing a tank adapted to contain fluid and one or more parts to be processed, wherein said one or more transducers are coupled to the tank and adapted for providing vibrations to the tank and its contents; generating and supplying a driving signal to the transducers, wherein the driving signal has a variable frequency throughout a frequency range that includes the resonant frequencies of all the transducers. A megasonic processing apparatus and method has one or more piezoelectric transducers operating in thickness mode at fundamental resonant frequencies of at least 300 KHz. A generator powers the transducers with a variable-frequency driving signal that varies or sweeps throughout a predetermined sweep frequency range. The generator repeatedly varies or sweeps the frequency of the driving signal through a sweep frequency range that includes the resonant frequencies of all the transducers.

Full Text

MEGASONIC PROCESSING APPARATUS WITH FREQUENCY
SWEEPING OF THICKNESS MODS TRANSDUCERS
RELATED APPLICATION
This application claims priority from co-pending U.S. Provisional Application No.
60/783,213, filed March 17,2006, entitled MEGASONIC PROCESSING APPARATUS
WITH FREQUENCY SWEEPING, and U.S. Patent Application entitled MEGASONIC
PROCESSING APPARATUS WITH FREQUENCY SWEEPING OF THICKNESS
MODE TRANSDUCERS filed March 16, 2007 and invented by J. Michael Goodson.
These prior applications are expressly incorporated herein by reference.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates generally to megasonic processing apparatus and associated
methods involving one or more piezoelectric transducers operating in thickness mode at
megasonic frequencies of at least 300 KHz or higher, and relates more particularly to
improving performance by sweeping the frequency of a driving signal throughout a
predetermined or programmable frequency range that spans the resonant frequencies of
all the transducers.
DESCRIPTION OF THE RELEVANT ART
Megasonic processing involves generating and using high frequency energy at
frequencies above 300 KHz. Many megasonic systems operate at frequencies at or near
1,000 KHz, or one megahertz. Although 1 MHz is the consensus, preferred frequency for
many applications, the frequency range goes much higher, with frequencies as high as 10
MHz. Typical uses for megasonic systems include cleaning delicate objects, such as
semiconductor wafers and disc drive media. Such a megasonic cleaning process involves
placing the objects to be cleaned in a fluid-filled tank, and applying vibrational energy at
megaeonic frequencies to a radiating surface or surfaces of the tank. One or more
piezoelectric transducers are used to generate the vibrational energy. A generator
supplies an alternating current driving signal at the resonant frequency of the transducers.
Megasomc transducers operate in thickness mode, where a piezoelectric element is
excited by an alternating current driving signal that causes alternating expansion and

contraction of the transducer, primarily expanding and contracting the thickness of the
transducer. A piezoelectric transducer having a thickness of 0.080 inches has a
fundamental, thickness mode, resonant frequency of 1,000 KHz.
Megasonic processing has some similarities with ultrasonic processing, which
involves lower fundamental frequencies, typically from about 25 KHz to about 192 KHz.
Ultrasonic transducers are typically mass-balanced, with inert masses on either side of a
piezoelectric element, and have a significant radial component of movement at right
angles to the thickness component. One common construction of an ultrasonic transducer
is to stack several layers of ring-shaped piezoelectric elements between two masses, and
to hold the assembly together with an axial compression bolt. Ultrasonic cleaning is
based on cavitation, which is the formation and collapse of bubbles in the fluid.
At the frequencies used for megasonic cleaning, significant cavitation does not
occur, so die cleaning action is based on another mechanism known as micro-streaming,
which is a general flow of detached particles flowing away from the megasonic
transducers. This flow consists of planar waves originating at the surface to which the
transducers are mounted. The planar nature of these micro-streams affects the
distribution of megasonic energy throughout the tank. One way to improve the
distribution is to cover a high percentage of the surface area of the tank with transducers.
Another but less efficient way is to oscillate or move the parts to be processed throughout
die tank so that all surfaces are exposed to sufficiently high megasonic energy.
It is known that radial-mode ultrasonic activity in a cleaning tank may benefit
from a process of sweeping or varying the frequency of the driving signal. However,
there has been an industry-wide belief that you cannot sweep megasonic frequencies
because the sound waves are too small and weak for any benefit from sweeping. In
addition, it has been thought that there would be no benefit from sweeping megasonic
frequencies because of the thickness mode transducers and resultant planar nature of
megasonic vibrations and due to the different cleaning mechanisms at work as compared
to ultrasonics.
SUMMARY OF THE INVENTION
The present invention relates to a megasonic processing apparatus and method
having one or more piezoelectric transducers (PZT) operating in thickness mode at
megasonic frequencies in excess of 300 KHz. A megasonic generator operating at

megasonic frequencies drives the transducers with a variable-frequency driving signal
that varies or sweeps throughout a predetermined or programmable sweep frequency
range. The megasonic generator generates the driving signal at megasonic frequencies to
energize the megasonio piezoelectric transducers to cause them to vibrate in thickness
mode at their megasonic resonant frequencies. The piezoelectric transducers emit energy
at the megaeonic frequencies that can be used for various applications, such as cleaning
objects in a fluid-filled tank. The generator repeatedly varies or sweeps the frequency of
the driving signal through a sweep frequency range that includes the resonant frequencies
of all the megaeonic piezoelectric transducers.
Another aspect of the present invention involves grouping the megasonic
piezoelectric transducers into groups having similar resonant frequencies, and powering
each group with a separate frequency-sweeping driving signal from a generator operating
within a sweep frequency Tange that includes the resonant frequencies of the group of
associated transducers. This subdivides the overall sweep frequency range into smaller
subranges, which may or may not overlap, and reduces the range of each frequency
sweep. The effect of grouping transducers is to proportionately increase the amount of
time mat any particular transducer is operating at or close to its resonant frequency and
thereby improve efficiency.
The present invention encompasses a megasonic system that includes one or more
piezoelectric transducers and one or more megasonic generators coupled to the
transducers for supplying varying-frequency megasonic driving signals at selectable or
programmable frequency ranges and sweep rates.
When a megasonic process is used, for example, for cleaning silicon wafers or
disc drive media, sweeping the driving signal through me resonant frequencies of all the
thickness-mode megasonic transducers will equalize the megasonic energy generated by
the transducers and will cause the transducers to perform in unison. This results in a
more uniform distribution of megasonic energy and improved performance. The same
improved megasonic energy uniformity and functionality can also be achieved in liquid
processing, non-destructive testing, medical imaging, and other processes using
megasonic thickness-mode transducers by sweeping the range of resonant frequencies of
the transducers. The frequency sweeping process will also extend the life of the
megasonic transducers because it is less stressful to the transducers than operating at a
single fixed frequency. The frequency sweeping process also improves the uniformity of

megasonic energy throughout the tank or other apparatus because each transducer
operates at its resonant frequency during at least part of each frequency sweeping cycle.
It is expected feat any application of process using megsonic frequencies will benefit
from fee uniform distribution of power created by sweeping fee driving signal through all
the transducers' resonant frequencies.
A key to optimizing efficiency of a megasonic process is to have uniform energy
throughout fee radiating surface being excited wife megasonics. To do this, preferably
80% or more of the area of fee radiating surface is covered by thickness-mode megasonic
transducers. Furthermore, each megasonic transducer produces consistent megasonic
energy by sweeping fee frequency of fee driving signal through the highest and lowest
resonant frequencies of a group of transducers.
For best performance, each megasonic transducer needs to be energized
substantially the same as other megasonic transducers bonded to the same surface. To
achieve this, the driving frequency is swept through fee resonant frequencies of all fee
transducers. Sweeping the resonant frequencies of megasonic transducers drives every
transducer at its resonant frequency at some point in each cycle. This creates uniformity
in transducer performance not previously achieved in the industry.
In addition, frequency sweeping of megasonic transducers reduces a "fountain
effect" observed wife fixed-frequency megasonic transducers. The fountain effect is
thought to be caused by a transducer operating at its resonant frequency wife a fixed
frequency driving signal, which produces a significant up-surge of liquid in the tank
above feat transducer. Sweeping fee megasonic frequency driving signal ensures that any
particular transducer will not be driven continuously at its resonant frequency, feus
eliminating fee upsurge associated wife fee fountain effect. Instead, the megasonic
energy is uniformly distributed throughout the tank because all transducers are operating
efficiently at their resonant frequencies at some point during each sweep cycle.
Frequency sweeping is far more dramatic wife megasonic frequencies than
ultrasonic frequencies like 40 KHz. Improvements in power distribution of 500 to 700 %
have been seen wife megasonic resonant frequency sweeping and (his means substantially
better processing.
The features and advantages described in fee specification are not all inclusive,
and particularly, many additional features and advantages will be apparent to one of
ordinary skill in fee art in view of fee drawings, specification and claims hereof.

Moreover, it should be noted that the language used in the specification has been
principally selected for readability and instructional purposes, and may not have been
selected to delineate or circumscribe the inventive subject matter, resort to the claims
being necessary to determine such inventive subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an overall perspective view of a megasonic processing system
according to the present invention.
Figure 2 is a top perspective view of a tank used in the megasonic processing
system of the present invention.
Figure 3 is a bottom perspective view of the tank.
Figure 4 is a side elevation view of the tank.
Figure 5 is a bottom view of the tank.
Figure 6 is a schematic view of the megasonic processing system and a sectional
view of the tank and an attached megasonic transducer with a generator that supplies
driving signals to the transducer for creating megasonic vibrations in liquid in the tank.
Figure 7 is a graph of frequency versus time of a driving signal used in one
embodiment of the present invention.
Figure 8 is a graph of frequency versus time of two driving signals used in another
embodiment of the present invention in which the sweep period is the same as in Figure
7.
Figure 9 is a graph of frequency versus time of two driving signals used in another
embodiment of the present invention in which the sweep rate is the same as in Figure 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The drawings depict various preferred embodiments of the present invention for
purposes of illustration only. One skilled in the art will readily recognize from the
following discussion that alternative embodiments of the structures and methods
illustrated herein may be employed without departing from the principles of the invention
described herein.
One aspect of the present invention is a megasonic processing apparatus and
method having a megasonic generator with a programmable sweep frequency range and a
programmable sweep rate. The sweep frequency range is the range of frequencies or

bandwith within which the megasonic generator outputs a driving signal to drive one or
more megasonic thickness-mode piezoelectric transducers at their resonant frequencies.
The sweep rate is the number of times the resonant frequencies are swept per second.
The megasonic generator preferably includes a controller or other controlling
device with means to allow a user to select or program the sweep frequency range or
bandwidth and the sweep rate for the driving signal. The user inputs one or more
combinations of sweep frequency range and sweep rate into the memory device of the
generator. The generator generates and outputs the driving signal according to the sweep
frequency range and sweep rate selected by the user.
When used in a cleaning application, for example, the megasonic piezoelectric
transducer or transducers may be mounted on the bottom or sides of a tank, or enclosed in
an immersible container within the tank.. The sweeping frequency generator may be used
to drive megasonic transducers in applications other than cleaning. Preferably, the
transducers are piezoelectric crystals or piezoelectric ceramic (also known as PZTs), such
as barium titanate or lead zirconate titanate, operating in thickness mode. Using different
sweep rates or sweep frequency ranges in the same process may enhance cleaning of
some parts because certain frequencies may be more effective than others.
A device that sweeps the frequency of the driving signal is incorporated into the
megasonic generator that generates the driving signal. The generator includes a user
interface that includes one or more input devices, such as knobs, dials, software,
keyboard, graphical user interface, network connection, or other input devices, mat permit
a user to set a sweep frequency range or bandwidth over which the generator operates and
also to set a sweep rate at which the generator sweeps through the programmed range.
The controls for user programming the sweep frequency range and sweep rate may be
analog or digital.
As shown in Figures 1-6, one embodiment of the present invention is a cleaning
system 10 mat includes a quartz cleaning tank 12 containing a cleaning liquid or solution
14 and one or more pieces 15 to be cleaned. Megasonic energy is supplied to the cleaning
liquid 14 by one or more megasonic frequency transducers 16 affixed to the bottom of the
tank 12. Alternatively, megasonic transducers could be affixed to one or more sides of
the tank or immersed in the tank. Preferably, the megasonic transducer 16 has a
piezoelectric dement (PZT) 18 adhesively bonded or otherwise attached to one side of a
silicon carbide plate 20. The other side of the silicon carbide plate 20 is adhesively

bonded or otherwise attached to the outside bottom surface of the cleaning tank 12.
Preferably, bonding layers 22 between the silicon carbide plate 20 and the tank 12 and
between the silicon carbide plate and the piezoelectric element 18 are composed of
perforated copper foil and an impedance matching adhesive. Alternatively, the bonding
layers may be composed of epoxy or other adhesive used for die bonding semiconductor
chips to package substrates.
The piezoelectric element can be square, rectangular, or a circular disk, or other
shape having uniform thickness. For example, for operation at a nominal frequency of
1,000 KHz, the piezoelectric element 18 would have a thickness of about 0.08 inches, the
silicon carbide plate 20 would have a thickness of about 0.19 inches, and the bottom of
the quartz tank 12 would have a thickness of about 0.20 inches. Transducer 16 and
cleaning system 10 is just one example of a transducer and apparatus that incorporates the
present invention.
As shown in Figures 3-6, the transducers 16 are preferably rectangular in shape
and are arranged parallel to each other. Preferably, the transducers 16 cover a substantial
portion of the bottom surface of the tank 12, preferably at least 80%. It is desirable to
generate megasonic energy and transfer it to the tank 12 and fluid 14 uniformly
throughout the entire area of the surface to which the transducers 16 are attached.
Covering a high percentage of the surface area of the tank bottom with transducers
ensures that the megasonic energy transferred to the fluid 14 is relatively uniform.
As shown in Figure 6, the transducers 16 are driven by a driving signal supplied
over electrical wires 24 by a programmable generator 26. The generator 26 is
programmed by a user through a user input or interface 28 to set the sweep frequency
range or bandwidth and the sweep rate of the driving signal output by the generator.
A megasonic frequency piezoelectric transducer operates in thickness mode such
that applied voltages cause the transducer to expand and contract in thickness. These
expansions and contractions are transmitted through the silicon carbide resonator 20 and
tank 12 to the fluid 14 and objects IS in the tank. As shown in Figure 6, these
megaeonic-frequency vibrations are primarily horizontal waves 17, provided mat the
transducers 16 are on the bottom of the tank 12. The waves propagate upwards and
convey particles cleaned or separated from the objects 15 in the tank. This is a processed
known at micro-streaming, in which there is a net movement upward, away from the
source of megasonic energy. As shown in Figures 1 and 2, the tank has a weir 21 over

which excess fluid and particles flow, and a pump 23 and filter 25 to recirculate and clean
the fluid.
Resonant frequency is generally the frequency where the mechanical and
electrical properties of a transducer can most efficiently transmit sound waves. In
megaeonic transducers operating in thickness mode, the thickness of the transducer
determines the resonant frequency. For example, a transducer that is 0.08 inches thick
will have a resonant frequency of about 1,000 KHz. A transducer that is 0.065 inches
thick will have a resonant frequency of about 1230 KHz. A transducer that is 0.050
inches thick will have a resonant frequency of about 1600 KHz. The term "resonant
frequency" is used herein to mean the lowest, fundamental frequency where the
transducer as installed has a natural resonance.
As stated above, a piezoelectric transducer having a thickness of 0.080 inches has
a fundamental resonant frequency of 1,000 KHz. A tolerance on the thickness of such a
transducer has a significant effect on the resonant frequency. A thickness variation of
0.001 inch would cause a resonant frequency variance of 12.5 KHz. Also, the two major
surfaces of the transducer should be flat and co-planar, but any variances can also affect
the resonant frequency. Even though it is desirable from a performance standpoint for all
transducers to have exactly the same resonant frequency, from a manufacturing tolerance
standpoint, it is impractical. However, the frequency sweeping of the present invention
overcomes this obstacle.
One advantage of the present invention is that sweeping the frequency of the
driving signal through the resonant frequencies of all the transducers distributes the sound
waves equally among the transducers. This makes it possible to have substantially equal
megasonic energy throughout the tank. This is important because the thickness-mode
transducers produce sound waves that travel vertically from the bottom to the top of the
tank with little spreading in lateral directions. The even distribution of megasonic energy
can best be achieved by sweeping just outside the highest and lowest resonant frequencies
of the transducers.
Another advantage of the present invention is that it accommodates tolerances in
the resonant frequencies of the transducers. Performance is best if variances of the
resonant frequency are minimized. Choosing transducers with exactly the tame resonant
frequency will help minimize variances (although at increased cost), but even then there
will be some variances from the adhesives or other binder miserable used to mount the

transducers because any variation in thickness creates a variation in frequency with
thickness mode applications. Sweeping the frequency of the driving signal according to
the present invention accommodates such inevitable variations.
Still another advantage of the present invention is that it reduces surges of fluid in
the tank. Without sweeping the driving signal, the transducers at or closest to the
frequency of the driving signal tend to create a powerful upward force that pushes the
fluid upward, sometimes as much as two inches above the surface level. Such surface
surges are a problem because they cause air to be incorporated into the fluid as it is
recirculated, which can interfere with the megasonic process. Surges are also a problem
because if the liquid is solvent it will evaporate in the air and can be harmful to the
operator and or the people in the area, especially if the fluid is an acid or other hazardous
material. Sweeping the driving signal with the present invention reduces these problems.
As shown in Figure 7, the generator 26 varies the frequency of the driving signal
as a function of time. For example, the frequency of the driving signal may vary linearly
in a saw-tooth pattern over a programmed sweep frequency range 30 that includes the
resonant frequencies 31 of all the megasonic transducers 16. The sweep frequency range
or bandwidth of the generator is programmed by a user and stored in a memory device
associated with the generator 26. The rate at which the frequency varies is determined by
the sweep rate programmed by the user and stored in the memory device of the generator.
The generator can be programmed to vary the frequencies of the driving signal according
to other functions or programs and need not be limited to linear functions that form a
triangular wave or saw tooth pattern as shown in Figure 7. The variation in frequency can
be, for example, sinusoidal, exponential, and other functions. The driving signal itself
may be sinusoidal, square, triangular, or other wave shape. The sweep rates need not be
the same for sweeping upwards (increasing frequency) aad downwards (decreasing
frequency). Preferably, the user can also set the number of periods and can establish rest
times when the generator shuts off the driving signal.
In a cleaning application, some parts may be best cleaned by a single transducer
instead of multiple transducers. In such a configuration, the performance of me
transducer can be enhanced by using a programmed software program thai identifies the
optimum resonant frequency and sweeps through this frequency within a defined range.
For best results, the driving frequency can be swept through a sweep range of 1% or less
to ensure mat the resonant frequency of the transducer is being excited repeatedly. A

benefit of fee present invention is that it reduces the adverse effects of resonant frequency
drifting because the resonant frequency of each transducer is being excited each cycle
even if it changes with time, provided that the sweep range or bandwidth is wide enough.
Commonly, multiple megasonic transducers 16 are used for a given task or
process, in which case it is common to drive all transducers with the same generator and
driving signal. Where multiple transducers are used, however, there may not be a single
optimum frequency due to performance variations and manufacturing tolerances among
the transducers. Production tolerances result in megasonic transducers having resonant
frequencies within a 3% to 4% range. For example, at 1000 KHz, a 4% range would be
plus or minus 20 KHz from the nominal 1000 KHz, or a range of 980 to 1020 KHz.
In such applications, according to the present invention, it is appropriate to
repeatedly sweep the frequency of the driving signal to ensure that at least some of the
time the transducer 16 is operating at or near its resonant frequency. In order to have
each transducer 16 operate at or near its resonant frequency, the generator sweeps through
a predetermined sweep frequency range that is designed to reach the lowest and highest
resonant frequencies 31 of the group of transducers. The sweeping frequency function of
the generator 26 covers that range of variance. The frequency sweeping function can be
fixed or it can be programmed to be variable as to speed (sweeps per second) or range
(minimum and maximum frequencies).
Another aspect of the present invention relates to grouping the megasonic
piezoelectric transducers into multiple groups according to their resonant frequencies, and
driving each group with a separate variable-frequency driving signal. Transducers with
similar resonant frequencies are grouped together to reduce the range of frequencies
through which the generator must sweep in order to operate the group of transducers at or
near their resonant frequencies. Reducing the frequency range of the sweep increases the
time that each transducer operates at or near its resonant frequency.
As the range of sweep frequency coverage is reduced, the rate of sweep can be
increased to create more activity if required by a particular application, or if the sweep
rate remains the same, men the repetition rate is increased. The result is that the
megasonic transmission at each transducer's resonant frequency will be greater since the
sweep covers a shorter span and the transducer operates for a greater percentage of time
at or near its resonant frequency, which increases the efficiency of the megasonic process.

This point is illustrated in Figures 7, 8, and 9. In Figure 7, a single generator
sweeps the driving signal between minimum and maximum frequencies over a range 30.
In Figure 8, two generators are used to cover the same overall range, but each generator
coven a subrange 32 that is one-half of the foil range 30. Half of the transducers have
resonant frequencies 31' in the upper subrange 32', and the other half of the transducers
have resonant frequencies 31" in the lower subrange 32". The number of sweeps per
unit time is the same in Figures 7 and 8. In Figure 9, the rate of change of the sweeping
frequency is the same as in Figure 7, but the range is cut in half so that twice as many
sweeps occur in the same period of time.
As an example of grouping, assume that twelve megasonic transducers are used in
a process having the following nominal resonant frequencies (in KHz):
1010 1030 1015 1007
1019 1004 1027 1038
1022 1014 1031 1040
These frequencies range from a minimum of 1004 KHz to a maximum of 1040 KHz, for a
total range of 36 KHz (±18 KHz) centered at 1022 KHz. Sweeping the frequency of the
driving signal to include the resonant frequencies of all twelve transducers would require
a total sweep of 36 KHz.
These twelve transducers could be divided into two groups, A and B, to reduce the
sweep range:
Generator A Generator B
1004 1014 1022 1031
1007 1015 1027 1038
1010 1019 1030 1040
The tranaducers driven by Generator A range from 1004 KHz to 1019 KHz, for a total
range of 15 KHz (±7.5 KHz) centered at 1011.5 KHz. The transducers' driven by
Generator B range from 1022 KHz to 1040, for a total range of 18 KHz (±9 KHz)
centered at 1031 KHz. By grouping the transducers according to their resonant
frequencies and reducing the sweep range for each sweeping generator, the number of
sweeps per unit time can be increased or the sweep rate can be decreased, either of which
allows the transducers to be driven at or near their resonant frequencies more often, which
enhances the megasonic process.

In actual practice, the sweep frequency ranges are set slightly outside the
maximum and minimum resonant frequencies for the associated transducers. So, in the
example above, the sweep frequency range of Generator A might be set to 1003 to 1020
KHz or 1002 to 1021 KHz and the sweep frequency range of Generator B might be set to
1021 to 1041 KHz or 1020 to 1042 KHz. This ensures that each transducer operates both
below and above its resonant frequency in each frequency sweep cycle and also allows
for shifts of the resonant frequencies that may occur due to heating or other variables.
Transducers can be grouped within an individual system or process or among
multiple systems or processes operating simultaneously. For example if there are two
teaks with multiple transducers each and bom tanks will be used simultaneously, one can
group transducers from the larger universe of all transducers on the two tanks. Groupings
may be further selected to produce a more uniform result as the transducers powered by a
single generator do not have to be next to each other or used with the same tank to be in
the group. Because all transducers work simultaneously, the designer of the transducer
layout can focus on maximizing the efficiency of the grouping without regard to where
the members of the groups are located.
As an example of grouping among multiple, simultaneous processes, assume that
the same twelve megasonic transducers set forth in the previous example are located on
two different tanks:

The twelve transducers of Tanks 1 and 2 are divided into two groups according to
resonant frequencies and are driven by Generators A and B as follows (with the tank
number shown in parentheses):

Generator A drives four transducers of Tank 1 and two transducers of Tank 2. Generator
B drives two transducers of Tank 1 and four transducers of Tank 2. Since all trasducers

are operating at the same time, this grouping allows the two generators to sweep across
smaller ranges.
Thus, in cleaning and other processes where multiple tanks or systems are used,
the entire population of transducers in multiple tanks or systems caa be combined to
create an optimum assortment of frequencies to be grouped together, with each group
powered by a different sweeping generator. For example in four processes using four
tanks, transducers from any or all of the four tanks may be networked together to achieve
the optimum range of frequencies for sweeping. Of course, all processes must be active
at the same time for such grouping.
Another aspect of the present invention is the construction of the megasonic
transducer 16 and its attachment to another structure, such as the bottom of tank 12, using
a perforated metal layer and impedance matching adhesive. As shown in Figures 4 and 6,
the megasonic transducer 16 preferably has a silicon carbide plate 20 between the
piezoelectric element 18 and the surface of the cleaning tank 12 or other structure to
which the transducer is attached. The piezoelectric element 18 is bonded to the silicon
carbide plate 20, and the assembly is bonded to the tank 12 with bonding layers composed
of a perforated metal foil, preferably copper, and an adhesive.
The perforated copper (or other metal) foil improves flatness and uniformity of
thickness of the bonding layer 22. The perforated copper has a predetermined thickness
that allows the adhesive to be evenly distributed, thus avoiding irregularities or non-
uniformity of adhesive thickness without using a jig or other stabilizing device. The
perforated metal provides a controllable flat structure to maintain uniformity in thickness
of the adhesive. The perforated metal also serves as an electrode between the
piezoelectric element and the silicon carbide plate.
The application of the present invention is not limited to cleaning operations. The
same principle of sweeping the acoustical energy for megasonic transducers can be
applied to other uses of micro-streeming of megasonic energy, such as non destructive
testing, or any other applications using thickness mode transducers having fundamental
resonant frequencies of at least 300 KHz. Sweeping megasonic transducers creates
greater energy bursts, which create improved and stronger micro-streaming activity which
improves the efficiency of micro-streaming cleaning and other uses of micro-streaming.
Micro-streaming is a flow of energized liquid created by the release of ultrasonic energy

that is too weak to cause cavitation. At frequencies in excess of 300 KHz, cavitations
cease to exist but the megasonic frequency energy creates a flow of the liquid.
From the above description, it will be apparent that the invention disclosed herein
provides a novel and advantageous megasonic processing apparatus and method utilizing
a variable frequency driving signal. The foregoing discussion discloses and describes
merely exemplary methods and embodiments of the present invention. As will be
understood by those familiar with the art, the invention may be embodied in various other
forms without departing from the spirit or essential characteristics thereof. Accordingly,
the disclosure of the present invention is intended to be illustrative, but not limiting, of
the scope of the invention, which is set forth in the following claims.

What is claimed is:
1. A megasonic processing apparatus comprising:
one or more piezoelectric transducers, each having a fundamental resonant
frequency of at least 300 KHz;
a tank adapted to contain fluid and one or more parts to be processed, wherein said
one or more transducers are adapted for providing vibrations to the tank and its contents;
a generator coupled to the transducers for supplying a driving signal at a variable
frequency throughout a frequency range that includes the resonant frequencies of all the
transducers.
2. An apparatus as recited in claim 1 wherein the generator has an adjustable
sweep rate and an adjustable frequency range.
3. An apparatus as recited in claim 2 wherein the sweep rate is in the range of
50 to 1200 sweeps per second.
4. An apparatus as recited in claim 1 wherein the apparatus has at least four
transducers and two generators, wherein me transducers are grouped by similar resonant
frequencies, and wherein each group of transducers is powered by a separate generator
that generates a driving signal having a variable frequency that varies within a frequency
range that includes the resonant frequencies of all the transducers of its associated group.
5. An apparatus as recited in claim 1 wherein said piezoelectric transducers
operate in thickness mode.
6. A megasonic processing system comprising:
two or more tanks, each tank adapted to contain fluid and one or more parte to be
processed;
one or more piezoelectric transducers coupled to each tank, each transducer
having a fundamental resonant frequency of at least 300 KHz;, wherein the transducers
are capable of providing vibrations to the tanks and their contents;

two or more generators coupled to the transducers for supplying driving signals to
the transducers, wherein the transducers are grouped by similar resonant frequencies, end
wherein each group of transducers is powered by a separate generator that generates a
driving signal having a variable frequency that varies within a frequency range that
includes the resonant frequencies of all the transducers of its associated group.
7. A megasonic cleaning apparatus comprising:
one or more piezoelectric transducers, each having a fundamental resonant
frequency in thickness mode vibration at a frequency of at least 300 KHz;
a tank adapted to contain cleaning fluid and one or more parts to be cleaned, said
one or more transducers adapted for providing vibrations to cleaning fluid and parts in the
tank;
a generator coupled to said one or more transducers for supplying a driving signal
at a predetermined frequency range and sweep rate, wherein the frequency range includes
the resonant frequency of all said one or more transducers, and wherein the generator
includes programmable means for defining a sweep frequency range and a sweep rate for
the driving signal.
8. A megasonic processing method comprising:
providing one or more piezoelectric transducers, each having a fundamental
resonant frequency of at least 300 KHz;
providing a tank adapted to contain fluid and one or more parts to be processed,
wherein said one or more transducers are coupled to the tank and adapted for providing
vibrations to the tank and its contents;
generating and supplying a driving signal to the transducers, wherein the driving
signal has a variable frequency throughout a frequency range that includes the resonant
frequencies of all the transducers.

A megasonic processing apparatus and method has one or more piezoelectric transducers operating in thickness mode at fundamental resonant frequencies of at least 300 KHz. A generator powers the transducers with a variable-frequency driving signal that varies or sweeps throughout a predetermined sweep frequency range. The generator repeatedly varies or sweeps the frequency of the driving signal through a sweep frequency range that includes the resonant frequencies of all the transducers.

Documents:

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Patent Number

270662

Indian Patent Application Number

4079/KOLNP/2008

PG Journal Number

02/2016

Publication Date

08-Jan-2016

Grant Date

07-Jan-2016

Date of Filing

08-Oct-2008

Name of Patentee

GOODSON, MICHAEL, J.

Applicant Address

92 ROLLING HILL ROAD, SKILLMAN NJ

Inventors:

#	Inventor's Name	Inventor's Address
1	GOODSON, MICHAEL, J.	92 ROLLING HILL ROAD, SKILLMAN, NJ 08558

PCT International Classification Number

H01L 41/08

PCT International Application Number

PCT/US2007/006885

PCT International Filing date

2007-03-18

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	60/783213	2006-03-17	U.S.A.