Influence of surface active agents on bubble activity in high frequency cleaning systems

One of the multiple challenges that the semiconductor industry is facing in maintaining the current scaling trends is the removal of undesired contamination, typically originating from the environment or from fabrication process steps. Cleaning steps are among the most critical since they are repeated several times during IC fabrication. In particular, the critical size of the particles to be removed has decreased, following the general IC aggressive scaling trends, to below 30 nm.

Traditional chemistry-based cleaning solutions suffer from excessive underetching, i.g. the removal of a thin substrate layer. With the current stringent requirements for the removal of nanoparticles, substrate loss must be reduced to a minimum. Therefore, the need for new efficient cleaning technologies and/or for a significant improvement of the existing ones has originated from these issues.

Physical cleaning techniques, such as megasonic cleaning and spray cleaning, have been introduced a few decades ago with a double aim: to aid conventional cleans by enabling the removal of contaminants through mechanical forces and to minimize, at the same time, material loss. Among these techniques, megasonic cleaning makes use of microbubbles which are oscillating in a liquid due to the application of sound waves with frequencies in the megahertz range.

It has been theoretically and experimentally demonstrated that acoustic cavitation and the resulting bubble activity is fundamental for a successful cleaning of semiconductor structures. Different types of cavitation bubble regimes co-exist, which give origin to different physical effects. More stable bubble regimes are responsible for phenomena such as acoustic streaming and cavitation microstreaming, leading to shear stresses at the wafer surface and inducing mechanical forces responsible for cleaning. More violent bubble regimes may cause shock waves and jet streams, phenomena that can clean but are also responsible for local damage to the substrate and to the fragile patterned structures. Therefore, particulate removal by means of megasonic fields presents multiple challenges. On one hand, the physical forces generated in acoustic fields need to be tuned in such a way that they can overcome the adhesion forces keeping the contaminants attached to the substrate. On the other hand, the damaging behavior of these physical forces must be minimized. Since the magnitude of these effects depends mostly on the degree of control that can be obtained over the active bubbles, a deeper insight on the fundamental physical phenomena that accompany the acoustic field is essential to improve and optimize the performance of megasonic cleaning.

This thesis investigates acoustic cavitation in 1MHz sound fields over a broad parameter space, with the specific objective of studying the effects of charge addition and of a lower surface tension on acoustically active bubbles. In this regard, two different surface active agents have been selected: Sodium Dodecyl Sulfate (SDS) and Triton X-100, as representative for anionic and nonionic surfactants, respectively. 

In the first part of this work, continuous acoustic fields are employed. Three main techniques are used: sonoluminescence (SL), cavitation noise (CN) and high speed imaging of acoustic bubbles. SL measurements in liquid with dissolved argon show that a hysteretic behavior in bubble activity can occur. This demonstrates that the ability of bubbles to emit light depends also on their previous cavitation history. Next, surfactant-containing solutions are employed after complete characterization. The effect of lower surface tension by means of a nonionic surfactant (Triton X-100) is addressed, which is followed by the effects of bubble charging by means of SDS solutions. Here, the preferred methodology is based on CN recording, since useful spectra can be obtained at low acoustic powers, while a SL signal is absent at low powers. Cavitation activity is found to be significantly enhanced when employing a lower surface tension solution. More in particular, the onset of bubble activity is shifted towards lower acoustic power densities. This enhancement is reproducible and consistent at different oxygen saturation levels.

Subsequently, the effects of charges on the acoustic bubbles are investigated by using SDS. Surprisingly, cavitation activity is dramatically reduced upon SDS addition, at all O2 concentrations and acoustic powers. This effect can be mainly attributed to the decrease of bubble growth by coalescence. The electrostatic repulsion that sets in between bubbles, due to the adsorption of charged molecules, acts on the coalescence growth pathway. High speed imaging of acoustic bubbles in SDS solutions confirms that bubble coalescence is hindered. To further shed light on the mechanisms, a model is proposed, in which the repulsive electrostatic force is balanced with the attractive Bjerknes force for two equally oscillating bubbles. For small oscillation amplitudes, obtained at very low driving pressures, electrostatic repulsion dominates over the mutual inter-bubble attraction.

Based on the experimental results obtained for lower surface tension solutions in combination with continuous acoustic fields, a model is proposed, which can explain the enhancement of bubble activity according to the parametric instability theory. The analysis shows that decreasing the surface tension leads to a higher instability for acoustic bubbles, which in turn leads to an increased degree of fragmentation and bubble production. In particular, at a lower surface tension, resonant active bubbles are already shape unstable at lower driving pressures, thus implying that, at the typical operating pressures utilized in megasonic cleaning tools, more bubbles should be created.

In the last part of this work, it is demonstrated that the use of pulsed acoustic fields under traveling wave conditions is beneficial for the enhancement of acoustic bubble activity. This enhancement is also encountered for lower surface tension solutions and lower acoustic applied powers. The optimal experimental parameters are found to be in good agreement with the theory of bubble dissolution: longer off-times are needed for increased dissolved gas concentrations and/or lower surface tensions. These findings are further complemented by particle removal efficiency (PRE) and damage tests on contaminated blanket and patterned Si wafers, respectively. An increase in the average particle removal efficiency was achieved at lower surface tensions. Furthermore, particle removal experiments confirmed the link between the intensity of the ultraharmonic peaks and cleaning performance. In conclusion, the addition of a nonionic surfactant to the cleaning liquid has a twofold effect. On one hand, it enhances the bubble activity and the particle removal. On the other hand, for a specific applied acoustic power, lowering the bulk surface tension suppresses damage formation compared to a UPW reference liquid.

The results reported in this work constitute a major effort into pushing the capabilities of megasonic cleaning further for the removal of <100nm particles, while at the same time, limiting its harmful damaging effects .