The invention utilizes
harmonics of certain clamped
ultrasound transducers to generate
ultrasound within the liquid of an ultrasonic tank and in a frequency range of between about 100 khz to 350 khz (i.e., “microsonic” frequencies). The application of microsonic frequencies to liquid preferably occurs simultaneously with a sweeping of the microsonic frequency within the
transducer's
harmonic bandwidth to reduce or eliminate (a) standing
waves within the liquid, (b) other resonances, (c)
high energy cavitation implosion, and (d) non-uniform sound fields, each of which is undesirable for cleaning and / or
processing of
semiconductor wafers and other delicate parts. The invention can also drive ultrasonic transducers such that the frequency of applied energy has a
sweep rate within the ultrasonic bandwidth of the transducers; and that
sweep rate is also varied so that the
sweep rate is substantially non-constant during operation. This reduces or eliminates resonances which are created by transducers operating with a
single sweep rate. An
ultrasound generator of the invention sometimes utilizes
amplitude modulation (AM), and the AM frequency is swept over time so as to reduce resonances. AM control is preferably provided by selecting a portion of the rectified power line frequency. In applications which utilize multiple generators, multiple transducers, and one or more tanks, simultaneously, the invention synchronizes the operation of the generators to a common FM
signal to reduce beat frequencies between generators. Each such generator can also be adjusted, through AM, to control the process characteristics within the associated tank. Two or more transducers are sometimes used by the invention, in combination, to broaden the overall bandwidth of acoustical energy applied to the liquid around the primary frequency or one of the
harmonics. The bandwidths of the transducers are made to overlap such that an attached generator can drive the transducers, in combination, to deliver ultrasound to the liquid in a broader bandwidth. In a
single chamber ultrasound
system, two or more generators, each operating or optimized to generate a different range of frequencies, are connected to a
multiplexer; and the desired frequency range is selected, and hence the right generator, according to the
cavitation implosion energy that is desired within the tank
chemistry.