Aaldert Zijlstra

Efficient Sonochemistry through Microbubbles Generated with Micromachined Surfaces

Sonochemical reactors are used in water treatment, the synthesis of fine chemicals, pharmaceutics and others. The low e ciency of sonoreactors have prevented its massive usage at industrial scales. Controlling the appearance of bubbles in place and time is the most limiting factor. A novel type of sonochemical reactor was designed making use of micro-fabrication techniques to control the nucleation sites of micro-bubbles. The e ciency was increased first by locating the nucleation sites in the most active region of a micro-chamber; additionally the desired chemical e ect was significantly higher at the same powers than when not controlled. Silicon substrates were micromachined with “artificial nucleation sites” or pits, and placed at the bottom of the micro-chamber. The pits entrap gas which, upon ultrasonic excitation, sheds o a stream of microbubbles. The gas content of the pits is not depleted but is replenished by di usion and the emission of microbubbles can continue for hours. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201005533 Published in: Angewandte Chemie International Edition, 49:50, (2010), 9699-9701

Ultrasound artificially nucleated bubbles and their sonochemical radical production

We describe the ejection of bubbles from air-filled pits micromachined on a silicon surface when exposed to ultrasound at a frequency of approximately 200 kHz. As the pressure amplitude is increased the bubbles ejected from the micropits tend to be larger and they interact in complex ways. With more than one pit, there is a threshold pressure beyond which the bubbles follow a trajectory parallel to the substrate surface and converge at the center point of the pit array. We have determined the size distribution of bubbles ejected from one, two and three pits, for three different pressure amplitudes and correlated them with sonochemical OH· radical production. Experimental evidence of shock wave emission from the bubble clusters, deformed bubble shapes and jetting events that might lead to surface erosion are presented. We describe numerical simulations of sonochemical conversion using the empirical bubble size distributions, and compare the calculated values with experimental results.

Localized removal of layers of metal, polymer, or biomaterial by ultrasound cavitation bubbles

We present an ultrasonic device with the ability to locally remove deposited layers from a glass slide in a controlled and rapid manner. The cleaning takes place as the result of cavitating bubbles near the deposited layers and not due to acoustic streaming. The bubbles are ejected from air-filled cavities micromachined in a silicon surface, which, when vibrated ultrasonically at a frequency of 200 kHz, generate a stream of bubbles that travel to the layer deposited on an opposing glass slide. Depending on the pressure amplitude, the bubble clouds ejected from the micropits attain different shapes as a result of complex bubble interaction forces, leading to distinct shapes of the cleaned areas. We have determined the removal rates for several inorganic and organic materials and obtained an improved efficiency in cleaning when compared to conventional cleaning equipment. We also provide values of the force the bubbles are able to exert on an atomic force microscope tip.

Enhancing acoustic cavitation using artificial crevice bubbles

We study the response of pre-defined cavitation nuclei driven continuously in the kHz regime (80, 100 and 200 kHz). The nuclei consist of stabilized gaspockets in cylindrical pits of 30 μm diameter etched in silicon or glass substrates. It is found that above an acoustic pressure threshold the dynamics of the liquid-gas meniscus switches from a stable drum-like vibration to expansion and deformation, frequently resulting in detachment of microbubbles. Just above this threshold small bubbles are continuously and intermittently ejected. At elevated input powers bubble detachment becomes more frequent and cavitation bubble clouds are formed and remain in the vicinity of the pit bubble. Surprisingly, the resulting loss of gas does not lead to deactivation of the pit which can be explained by a rectified gas diffusion process.

Oscillations of a gas pocket on a liquid-covered solid surface

The dynamic response of a gas bubble entrapped in a cavity on the surface of a submerged solid subject to an acoustic field is investigated in the linear approximation. We derive semi-analytical expressions for the resonance frequency, damping, and interface shape of the bubble. For the liquid phase, we consider two limit cases: potential flow and unsteady Stokes flow. The oscillation frequency and interface shape are found to depend on two dimensionless parameters: the ratio of the gas stiffness to the surface tension stiffness, and the Ohnesorge number, representing the relative importance of viscous forces. We perform a parametric study and show, among others, that an increase in the gas pressure or a decrease in the surface tension leads to an increase in the resonance frequency until an asymptotic value is reached.

High Speed Imaging of 1 MHz Driven Microbubbles in Contact with a Rigid Wall

Since the introduction of megasonic cleaning in semiconductor industry a debate has been going on about which physical mechanism is responsible for the removal of particles. Because of the high frequency range it was believed that acoustic cavitation could not occur and cleaning was attributed to phenomena like Eckart and Schlichting streaming or pressure build-up on particles [1,2]. Recently it was shown however, that the removal of nanoparticles is closely related to the presence of acoustic cavitation in megasonic cleaning systems [3]. The dependence of particle removal efficiency on the concentration of dissolved gas and the presence of sonoluminescence are clear (but indirect) indications that the underlying mechanism is related to bubble dynamics. As the requirements for cleaning in semiconductor processing are ever more stringent, it becomes necessary to obtain a thorough understanding of the physical behavior of acoustically driven microbubbles in contact with a solid wall. In particular, the forces exerted thereby which might clean or damage a substrate are of interest. Here, a step in this direction is taken by visualization of both the removal of nanoparticles and the sub-microsecond timescale dynamics of the cavitation bubbles responsible thereof.

On fiber optic probe hydrophone measurements in a cavitating liquid

The measurement of high-pressure signals is often hampered by cavitation activity. The usage of a fiber optic probe hydrophone possesses advantages over other hydrophones, yet when measuring in a cavitating liquid large variations in the signal amplitude are found; in particular when the pressure signal recovers back to positive values. With shadowgraphy the wave propagation and cavity dynamics are imaged and the important contributions of secondary shock waves emitted from collapsing cavitation bubbles are revealed. Interestingly, just adding a small amount of acidic acid reduces the cavitation activity to a large extent. With this treatment an altered primary pressure profile which does not force the cavitation bubbles close to fiber tip into collapse has been found. Thereby, the shot-to-shot variations are greatly reduced.

Impact of Acoustical Reflections on Megasonic Cleaning Performance

Electrical measurements have shown a direct impact of reflection of acoustic waves back into a transducer. Impedance measurements illustrate in specific cases the existence of multiple resonance peaks when reflected acoustic waves are present. Current and voltage measurements have confirmed this result. From these results, one can already conclude that acoustic reflections have a large impact on the operation of a transducer. Furthermore, it is shown that for megasonic cleaning tools with a face-to-face configuration of transducer and wafer, a precise control over the distance (control over the reflections) between the transducer and wafer is very important. Particle Removal Efficiency (PRE) measurements immediately show a major dependence on the position of the wafer. The PRE dependence is directly linked to the forward power consumed by the transducer, which is largely influenced by the position of the wafer or, in other words, by the reflection of acoustic waves.

In‐situ monitoring of megasonic cleaning

Acoustic agitation is used in ultrasonic cleaning to induce a physical force to remove nanoparticles in semiconductor processing. The performance of cleaning tools is commonly quantified ex‐situ by the particle removal efficiency η (i.e. the ratio of the remaining to the original particle concentration). The resulting η ‐wafer maps often reveal spatial nonuniformities related to tool design. The local study of the cleaning dynamics, quantified by the removal frequency (fR[s‐1]) also indicates that the density of cleaning events over a wafer can be strongly non homogeneous. Because the sound frequencies used in industrial tools are in the MHz range, visualization of the cavitating bubbles (resonant size 3 μm) is highly challenging, both in time and in spatial resolution. Considering that the cleaning effect of a cavitating bubble is permanent a method is presented here to determine the acoustic cleaning event size (Aevent[μm2]) and the event flux (φ[μm‐2s‐1]) through visualization techniques. The method co...