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 c ontamination, typically originating from the environment or from fabrica tion process steps. Cleaning steps are among the most critical since the y are repeated several times during IC fabrication. In particular, the c ritical 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 und eretching, 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 clea ning technologies and/or for a significant improvement of the existing o nes has originated from these issues. Physical cleaning techniques, such as megasonic cleaning and spray clean ing, have been introduced a few decades ago with a double aim: to aid co nventional cleans by enabling the removal of contaminants through mechan ical forces and to minimize, at the same time, material loss. Among thes e techniques, megasonic cleaning makes use of microbubbles which are osc illating in a liquid due to the application of sound waves with frequenc ies in the megahertz range. It has been theoretically and experimentally demonstrated that acoustic cavitation and the resulting bubble activity is fundamental for a succes sful cleaning of semiconductor structures. Different types of cavitation bubble regimes co-exist, which give origin to different physical effect s. More stable bubble regimes are responsible for phenomena such as acou stic streaming and cavitation microstreaming, leading to shear stresses at the wafer surface and inducing mechanical forces responsible for clea ning. More violent bubble regimes may cause shock waves and jet streams, phenomena that can clean but are also responsible for local damage to t he substrate and to the fragile patterned structures. Therefore, particu late 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 keepin g the contaminants attached to the substrate. On the other hand, the dam aging behavior of these physical forces must be minimized. Since the mag nitude of these effects depends mostly on the degree of control that can be obtained over the active bubbles, a deeper insight on the fundamenta l physical phenomena that accompany the acoustic field is essential to i mprove 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 effe cts of charge addition and of a lower surface tension on acoustically ac tive bubbles. In this regard, two different surface active agents have b een selected: Sodium Dodecyl Sulfate (SDS) and Triton X-100, as represen tative 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 liq uid with dissolved argon show that a hysteretic behavior in bubble activ ity can occur. This demonstrates that the ability of bubbles to emit lig ht depends also on their previous cavitation history. Next, surfactant-c ontaining solutions are employed after complete characterization. The ef fect 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 power s, while a SL signal is absent at low powers. Cavitation activity is fou nd to be significantly enhanced when employing a lower surface tension s olution. More in particular, the onset of bubble activity is shifted tow ards lower acoustic power densities. This enhancement is reproducible an d consistent at different oxygen saturation levels. Subsequently, the effects of charges on the acoustic bubbles are investi gated by using SDS. Surprisingly, cavitation activity is dramatically re duced upon SDS addition, at all O2 concentrations and acoustic powers. T his effect can be mainly attributed to the decrease of bubble growth by coalescence. The electrostatic repulsion that sets in between bubbles, d ue to the adsorption of charged molecules, acts on the coalescence growt h pathway. High speed imaging of acoustic bubbles in SDS solutions confi rms that bubble coalescence is hindered. To further shed light on the me chanisms, a model is proposed, in which the repulsive electrostatic forc e is balanced with the attractive Bjerknes force for two equally oscilla ting bubbles. For small oscillation amplitudes, obtained at very low dri ving pressures, electrostatic repulsion dominates over the mutual inter- bubble attraction. Based on the experimental results obtained for lower surface tension sol utions in combination with continuous acoustic fields, a model is propos ed, which can explain the enhancement of bubble activity according to th e parametric instability theory. The analysis shows that decreasing the surface tension leads to a higher instability for acoustic bubbles, whic h in turn leads to an increased degree of fragmentation and bubble produ ction. In particular, at a lower surface tension, resonant active bubble s are already shape unstable at lower driving pressures, thus implying t hat, at the typical operating pressures utilized in megasonic cleaning t ools, 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 e nhancement of acoustic bubble activity. This enhancement is also encount ered for lower surface tension solutions and lower acoustic applied powe rs. The optimal experimental parameters are found to be in good agreemen t with the theory of bubble dissolution: longer off-times are needed for increased dissolved gas concentrations and/or lower surface tensions. T hese findings are further complemented by particle removal efficiency (P RE) and damage tests on contaminated blanket and patterned Si wafers, re spectively. An increase in the average particle removal efficiency was a chieved at lower surface tensions. Furthermore, particle removal experim ents confirmed the link between the intensity of the ultraharmonic peaks and cleaning performance. In conclusion, the addition of a nonionic sur factant to the cleaning liquid has a twofold effect. On one hand, it enh ances the bubble activity and the particle removal. On the other hand, f or a specific applied acoustic power, lowering the bulk surface tension suppresses damage formation compared to a UPW reference liquid.< br> 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 dam aging effects .

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