Cavitation is the occurrence of empty voids in liquids. Being a statistical phenomenon, its position, duration and strength is difficult to predict. Two classical areas of cavitation are hydrodynamic and acoustic cavitation.
Hydrodynamic cavitation occurs in regions with low suction pressure, e.g. in inducers of rocket engines and turbo machinery, and is typically unavoidable. For turbo machinery, a safe operating region has to be found as cavitation occurring close to a rigid boundary can lead to its erosion.
Acoustic cavitation refers to induced cavitation for the purpose of ultrasonic cleaning. An acoustic field induces small bubbles in a liquid to radial oscillations. When the bubbles oscillate at a boundary they create sufficient shear stress to remove contamination from the surface and transport the dirt particulate into the bulk flow.
There is interest, for example in the semiconductor industry, in a sensor that can measure the amount of cavitation activity on a surface to control and optimize cleaning of silicon wafers, while mitigating damaging of delicate structures fabricated in the silicon wafers. Several known cavitation sensors are discussed below.
U.S. Pat. No. 7,210,354 discloses a system having a conductivity sensor, photo sensor, and thermocouple (B2) to measure radical production, light emission and heat generated by cavitation. However, the disclosed system is unable to indicate if the bubbles are creating a flow on the surface to be cleaned.
U.S. Pat. No. 6,497,140 discloses a cavitation sensor based on the measurement of acoustic signatures from oscillating bubbles within an acoustic chamber. The cavitation sensor has a size in the centimeter range. Such a size may lead to gaseous bodies becoming entrapped, thereby disturbing an acoustic field generated or leading to additional cavitation nuclei. Further, as the disclosed sensor needs to be flushed with liquid, it can only measure the pressure in the bulk liquid and not close to a surface. Further, this sensor does not detect the flow created but only the acoustic signals emitted from the oscillating bubbles. Further, this signature is a complex superposition of the emission of many bubbles, thus it is a complex inverse problem to deduce from this signal any meaningful physical quantity bubbles may have on surfaces.
U.S. Pat. No. 7,057,973 discloses a sensor that measures light emitted from sonoluminescence of a sampling liquid in which bubbles collapse. A photomultiplier detects the light from the sampling liquid which is either flush with a light-tight sensor housing or contains a fixed amount of liquid in a closed volume. The detection of cavitation is thus in an indirect manner, so the sensor does not detect whether the bubbles are creating a flow on the surface to be cleaned. Due to the use of photomultipliers, the cost is high.
U.S. Pat. No. 6,450,184 discloses piezoelectric discs assembled on a substrate and connected to a read-out unit. Piezoelectric sensors are sensitive to force acting on their surface and experience displacement from, for example, bubbles oscillating on the surface. However, an acoustic wave or the force from a liquid accelerated by the bubble will also lead to a signal output from the sensors. Thus, it is difficult to determine whether a signal from the sensor is caused from bubble emitted sound at some distance from the surface or from a bubble oscillating on the surface. The latter is the main contributor to surface cleaning, although the size of the sensor provides for the probability that signal arises from the former. The sensor size also does not enable easy integration with the object under probe.
U.S. Pat. No. 7,111,517 discloses cavitation sensing in wafer cleaning tools. The cavitation sensors are pressure sensing elements on a substrate to measure the activity of bubbles on the substrate. Similar to the piezoelectric sensors of U.S. Pat. No. 6,450,184, the pressures sensing elements experience displacement when force is applied to them. Thus, the focus is on the measurement of pressures, rather than flow strength.
There is thus a need to provide a cavitation sensor that detects cavitation occurring close to the sensor surface and is unaffected by cavitation that occurs at another portion of the liquid that the cavitation sensor is placed in.