Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-08-09
2001-09-11
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06288476
ABSTRACT:
BACKGROUND OF THE INVENTION
Ultrasonic systems for processing and cleaning parts are widely used by industry. Such systems typically include (a) a tank to hold the process chemistry such as cleaning solution, (b) an ultrasound generator, and (c) one or more transducers connected to the tank and the generator to deliver ultrasound energy to the process chemistry. These systems are generally adequate for low frequency operation, i.e., where the energy applied to the chemistry is around 20 khz. However, prior art ultrasound processing equipment has important technology limitations when operating at high frequencies and high power; and delicate parts such as disk drives for the computer industry require high frequency, high power ultrasound in order to effectively process components without damage. In one failure mode, for example, prior art transducers are known to fail when subjected to extended periods of operation, especially at high frequency and high power. In addition, prior art transducers are generally non-linear with respect to power output as a function of drive frequency. Therefore, prior art ultrasonic processing systems sometimes include costly electronics to compensate for such non-linearities.
There are other problems. For example, certain manufacturers require that a particular generator be matched to a particular tank since that combination is measured and known to provide particular process characteristics. However, this is cumbersome to an end user who cannot swap one generator for another in the event of a failure. More importantly, though, end users are not able to effectively monitor whether the system has degraded. Typically, for example, end users become aware of failure modes only after parts are damaged or destroyed within the process. There is a need, therefore, of monitoring systems which monitor processes in real-time.
It is, accordingly, one object of the invention to provide systems, apparatus and methods for delivering high frequency, high power ultrasound energy to process chemistries. Another object of the invention is to provide generators and systems which enable multi-frequency operation, selectively and without undue difficulty. Still another object of the invention is to provide improved transducer designs which increase system reliability and which improve power delivery. Yet another object of the invention is to provide systems, apparatus and methods for monitoring ultrasound processes in real-time or as a quality control (“QC”) step.
SUMMARY OF THE INVENTION
As used herein, “ultrasound” and “ultrasonic” generally refer to acoustic disturbances in a frequency range above about eighteen kilohertz and which extend upwards to over two megahertz. “Lower frequency” ultrasound, or “low frequency” ultrasound mean ultrasound between about 18 khz and 90 khz. “Megasonics” or “megasonic” refer to acoustic disturbances between 600 khz and 2 Mhz. As discussed above, the prior art has manufactured “low frequency” and “megasonic” ultrasound systems. Typical prior art low frequency systems, for example, operate at 25 khz, 40 khz, and as high as 90 khz. Typical prior art megasonic systems operate between 600 khz and 1 Mhz. Certain aspects of the invention apply to low frequency ultrasound and to megasonics. However, certain aspects of the invention apply to ultrasound in the 100 khz to 350 khz region, a frequency range which is sometimes denoted herein as “microsonics.”
As used herein, “resonant transducer” means a transducer operated at a frequency or in a range of frequencies that correspond to a one-half wavelength (&lgr;) of sound in the transducer stack. “Harmonic transducer” means a transducer operated at a frequency or in a range of frequencies that correspond to 1&lgr;, 1.5&lgr;, 2&lgr; or 2.5&lgr; of sound, and so on, in the transducer stack. “Bandwidth” means the range of frequencies in a resonant or harmonic region of a transducer over which the acoustic power output of a transducer remains between 50% and 100% of the maximum value.
As used herein, a “delicate part” refers to those parts which are undergoing a manufacture, process, or cleaning operation within liquid subjected to ultrasonic energy. By way of example, one delicate part is a semiconductor wafer which has extremely small features and which is easily damaged by cavitation implosion. A delicate part often defines components in the computer industry, including disk drives, semiconductor components, and the like.
As used herein, “khz” refers to kilohertz and a frequency magnitude of one thousand hertz. “MHz” refers to megahertz and a frequency magnitude of one million hertz.
As used herein, “sweep rate” or “sweep frequency” refer to the rate or frequency at which a generator and transducer's frequency is varied. That is, it is generally undesirable to operate an ultrasonic transducer at a fixed, single frequency because of the resonances created at that frequency. Therefore, an ultrasonic generator can sweep (i.e., linearly change) the operational frequency through some or all of the available frequencies within the transducer's bandwidth at a “sweep rate.” Accordingly, particular frequencies have only short duration during the sweep cycle (i.e., the time period for sweeping the ultrasound frequency through a range of frequencies within the bandwidth). “Sweep the sweep rate” or “double sweeping” or “dual sweep” refer to an operation of changing the sweep rate as a function of time. In accord with the invention, “sweeping the sweep rate” generally refers to the operation of sweeping (i.e., linearly changing) the sweep rate so as to reduce or eliminate resonances generated at the sweep frequency.
In one aspect, the invention provides ultrasound transducer apparatus. In the apparatus, at least one ceramic drive element is sandwiched between a front driver and a backplate. The drive element has electrical contacts or electrodes mounted on either face and is responsive to voltages applied to the contacts or electrodes so as to produce ultrasound energy. A connecting element—e.g., a bolt—connects the back plate to the front driver and compresses the drive element therebetween. In accord with the invention, the front driver and/or the backplate are shaped so that the apparatus produces substantially uniform power as a function of frequency over a range of frequencies. In another aspect, the shape of the driver and/or backplate are selected so as to provide a varying power function as a function of frequency.
In another aspect, a multi-frequency ultrasound generator is provided. In one aspect, the generator includes a constant power output circuit with means for switching the center frequency of the output signal selectively. The switching means operates such that little or no intermediate frequencies are output during transition between one center frequency and another.
Another multi-frequency generator of the invention includes two or more circuits which independently create ultrasound frequencies. By way of example, one circuit can generate 40 khz ultrasound energy; while another circuit can generate 104 khz energy. A switching network connects the plurality of circuits such that the generator is shut down and relay switching takes place in a zero voltage condition. As above, therefore, the switching occurs such that little or no intermediate frequencies are output during transition between one center frequency and another.
In still another aspect, a two stage ultrasonic processing system is provided. The system includes (a) one or more transducers with a defined ultrasound bandwidth defined by an upper frequency and a lower frequency. The system further includes (b) a frequency generator for driving the transducers from the upper frequency to the lower frequency over a selected or variable time period and (c) a process tank connected with the transducers so as to generate ultrasound energy within the tank at frequencies defined by the generator. During a given cycle, the generator drives the transducers from the upper frequency to the lower frequency. Once the lower frequency is
Budd Mark O.
McDermott & Will & Emery
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