POINT OF USE HF PURIFICATION FOR SILICON SURFACE PREPARATION BY ION-EXCHANGE

The point of use (POU) HF purification performance of an ion exchange membrane (IEM) is evaluated in this paper. First, no cationic extracta ble (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: puri fication 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 tu rnovers through an IEM, the impurity concentration drops below 1 ppb. The decrease rate can be fitted to a model assuming the experimental t anks to be continuously stirred tank reactors and that the impurity co ncentration after the membrane is a function of the single-pass purifi cation efficiency of the IEM, the concentration before purification, a nd the metals desorbed from the IEM. The concentration after purificat ion was investigated up to a cumulative Fe loading of 300 ppb in the 2 3 liter recirculated loop. It increases linearly vs. cumulative loadin g and can be explained by the Langmuir theory resulting in a purificat ion efficiency at the equilibrium of close to 99.5% in this loading re gime. This suggests that the capacity of the membrane is high enough t o ensure an adequate lifetime. Finally, a POU IEM can reduce the Cu co ncentration 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.