Point of Use HF Purification for Silicon Surface Preparation by Ion Exchange

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Abstract
The point of use (POU) HF purification performance of an ion exchange membrane (IEM) is evaluated in this paper. First, no cationic extractable (i.e., Cu, Fe, and Ni) from two IEMs was detected in HF 0.5% which makes these membranes compatible with sub‐ppb grade HF. The IEM purification performance was evaluated with 0.5% HF spiked with 10 ppb of Fe, Ni, and Cu nitrates. The results show that after less than five turnovers through an IEM, the impurity concentration drops below 1 ppb. The decrease rate can be fitted to a model assuming the experimental tanks to be continuously stirred tank reactors and that the impurity concentration after the membrane is a function of the single‐pass purification efficiency of the IEM, the concentration before purification, and the metals desorbed from the IEM. The concentration after purification was investigated up to a cumulative Fe loading of 300 ppb in the 23 liter recirculated loop. It increases linearly vs. cumulative loading and can be explained by the Langmuir theory resulting in a purification efficiency at the equilibrium of close to 99.5% in this loading regime. This suggests that the capacity of the membrane is high enough to ensure an adequate lifetime. Finally, a POU IEM can reduce the Cu concentration in the bath resulting in less Cu outplating. No impact was noticed on particle and organic deposition, on surface roughness, and on 5 nm gate oxide integrity.

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