It is important to quantify the cavitation energy in all applications ranging from ultrasonic cleaners to cell disruptors. A cavitation meter measures the energy intensity and frequency within ultrasonic and megasonic cleaners, including probes with side-mounted sensors that can be placed within the megasonic jet streams and single wafer cleaners, as well as those resembling a beaker to quantify the energy emanating from the ultrasonic horns used for cell disruption and homogenizing.

Ultrasonic and megasonic cleaners are used in varied applications to clean substrate surfaces. In a typical assembly, a cleaning system includes a tank that holds a fluid medium such as an aqueous solution, which generally includes additives such as surfactants and detergents that enhance the cleaning performance of the system. Lately, more distinctive means of delivery have been utilized, particularly for single wafer applications. In one example, megasonics is diverted into a stream of fluid that impacts the substrate surface. In another, megasonics imparts directly on a film of fluid no more than a few millimeters thick on the wafer surface. Ultrasonics can also be delivered via an ultrasonic horn, which is a popular method not for cleaning but for cell disruption, emulsification, and homogenizing of biological matter. In both cleaning and cell disruption applications, it is the phenomenon of cavitation that drives the actions.

FUNDAMENTALS OF CAVITATION
The term “ultrasonic” represents sonic waves having a wave frequency above approximately 20 kHz and includes both the traditional ultrasonic cleaning spectrum which extends in frequency from approximately 20 kHz to 500 kHz, and the more recently used megasonic cleaning spectrum which extends in frequency from about 0.5 MHz to about 5 MHz. The device used for cell disruption has traditionally been the ultrasonic horn. This device works at a fixed frequency, normally between 20 and 50 kHz, and is designed to be resonant in the longitudinal mode of vibration.

In a typical ultrasonic cleaner, a transducer mounted on the bottom generates high frequency vibrations in the cleaning tank in response to an electrical signal input. Once generated, the transducer vibrations propagate through the fluid medium in the cleaning tank until they reach the substrate to be cleaned. Cavitation bubbles are formed and grow when a liquid is put in significant state of tension. Liquids, though unable to support shear stresses, can support compressive stresses, and for short periods, tensile stresses.² The acoustic pressure wave undergoes a compression and rarefaction cycle, and the pressure in the liquid becomes a negative during the rarefaction portion of the cycle. When the negative pressure falls below the vapor pressure of the fluid medium, the ultrasonic wave can cause voids or cavitation bubbles to form in the fluid medium.

Coleman et al showed a remarkable photograph (Figure 1) of a bubble collapsing near a boundary.³ Once the cavitation is generated, a cavitation bubble may undergo two different kinds of radial oscillations. One may oscillate nonlinearly during many cycles of the acoustic wave, termed “stable cavitation.” The other may grow rapidly and collapse (i.e. implode) violently in one or two acoustic cycles, termed “transient cavitation.” ⁴ During bubble implosion, surrounding fluid quickly flows to fill the void created by the collapsing bubble. This flow results in an intense shock wave which is uniquely suited to substrate surface cleaning. Specifically, bubble implosions that occur near or at the substrate surface will generate shock waves that can dislodge contaminants and other soils from the substrate surface. When the bubble collapses, pressure up to 20,000 psi and a “high local temperature, possibly in the order of 5,000K,”⁵ are achieved.