Mechanism of Particle Deposition on Silicon Surface during Dilute HF Cleans

Abstract

The mechanism of particle deposition onto silicon wafer surfaces during dilute HF cleans is discussed. Direct surface force measurement using an atomic force microscope showed that particle redeposition on a silicon surface is due to the dominant van der Waals interaction between the particle and the wafer surface. The addition of surfactants can affect the clean effectiveness of a dilute HF solution by changing the surface interaction forces between a particle and a wafer. We show that there is a simple correlation between particle deposition and the product of the zeta-potentials of the wafer and particle. This correlation can be explained by introducing the linear superposition approximation to the derivation of electrical double-layer interactions. The addition of surfactant will also decrease dispersion attraction, introduce steric repulsion, and eliminate adhesion force, as indicated by the results of surface force measurements.With the semiconductor industry moving to smaller and smaller dimensions, removing particulate contamination has become extremely critical. 1 Silicon wafer ultra-cleaning is thus a vital area in semiconductor processing. The most widely used cleaning method is the RCA-based wet chemical clean, 2 which removes organic residues, particles, and metal ions on wafer surfaces. An RCA-cleaned wafer has a chemical oxide layer about 1-2 nm in thickness on the surface. The dilute HF clean is employed to remove this layer and to obtain a hydrogen-passivated bare silicon surface, which has high stability against oxidation 3,4 with low density of surface states. 5 However during HF cleaning, particles in the bath tend to redeposit onto the bare silicon surface. 6,7 The mechanism of bare silicon wafer contamination by particles has been discussed previously. 6-8 It was postulated that the electrostatic interaction between particles and wafers played a key role in particle deposition/removal. To simulate a 0.5% HF environment, experiments have been conducted at pH 3.3 using dilute HCl. 6,7 However, the simulated pH value may be higher than the actual value. It was found that the zeta-potential of bare silicon at pH 3.3 was negative ͑about Ϫ23 mV) while most particles were positive at this pH. Therefore, the susceptibility of the bare silicon wafer to contamination by particles was explained based on the attractive electrical interaction between the silicon wafer and particles. In more general discussions on particle deposition Spielman et al. 9 and Riley et al. 10 considered the deposition as diffusion of particles from the bulk solution to the surface under a potential caused by the electrical double-layer interaction and van der Waals attraction. More recently, Vos et al. postulated that a repulsive hydration force exists in the interaction between a silicon surface and a silicon nitride particle. 11 In this paper the mechanism of dilute HF cleaning is studied using an atomic force microscope ͑AFM͒. To our knowledge this is the first instance to quantitatively elucidate the mechanism for DHF clea...

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