Effect of Surfactants on Cavitation and Cleaning Efficiency in Single Wafer Megasonic System

Abstract

Megasonic cleaning is widely used in the semiconductor process. Various forces have been proposed that are responsible for particle removal, including forces due to acoustic pressure gradients, acoustic streaming, and fine cavitation. In particular, transient cavitation is an unstable cavitation that has a large kinetic energy and affects particle removal efficiency (PRE) and pattern features. Adding a surfactant influences bubble interfaces and thus can effectively control cavitation behavior. In this study, the adsorption behavior of surfactant at the bubble-solution interfaces and particle removal efficiency were evaluated according to the surfactant type and surfactant concentration. Sodium dodecyl sulfate (SDS) and Triton X-100 were used as surfactants. For the evaluation of PRE, 100nm Si3N4 particles were contaminated to a 2x2 cm Si wafer coupon (pre-treated with SC1). Pattern damage tests were performed with poly-Si patterns (widths of 70nm). Both PRE and pattern damage evaluations were performed using a single wafer megasonic cleaning system (GF1, Akrion, USA). Optical microscope (LV-100D, Nikon, Japan) and FE-SEM (MIRA3 model, TEXCAN, Czech) were used to count the particles and observe the degree of pattern damage, respectively. In order to visualize the bubble cavitation, a high-speed camera (VEO710, Phantom, USA) was used. Adding a low concentration of SDS has a great effect on PRE enhancement. However, it was decreased significantly at higher concentrations. In the case of Triton X-100, the PRE increased and maintained a high PRE with increasing concentration. Unlike the PRE results, the pattern damage gradually decreased with increasing concentrations of both SDS and Triton X-100. It was found that the dynamic surface tension of the SDS surfactant caused different effects on cavitation bubble size with increasing concentration and consequently affected PRE and pattern damage. In the case of the Triton X-100, the strong acoustic flow was induced by active bubble cavitation and it was efficient for particle removal.