Study On The Improvement Of Ashing Rate Of Photoresist Ashing Process With CF4 Gas

In modern VLSI fabrication, fluorine gas are usually added into photoresist ashing process to remove the residue of crust formed after the implantation with higher energy. However, after that, it can be found that its ashing rate is much smaller and its diffuser's lifetime is very shorter than that of asher with pure oxygen plasma, and these finally result in the decrease of throughput and increase of process cost.In this paper, XPS (X-ray Photoelectron Spectroscopy) are used to analysis the surface composition of the diffuser used in the asher with fluorine and oxygen thoroughly, and then the mechanism of the ashing rate's decrease are discussed. After that, the physical chemical model about the surface fluorine passivation layer forming and a detailed solution on ashing rate holding are proposed.The study shows that the surface coverage layer on the diffuser contacted with F-O is the thickest and mainly porous aluminum hydro-oxide which is the byproducts of the reaction between fluorine and aluminum oxide on the diffuser's surface. For O-F contacted gas diffuser, because of attack from fluorine, the surface structure is porous, which makes beneath metal uncovered and easily reacts with radical oxygen to form aluminum oxide. It happens repeatedly to cause the consumption of oxygen and leads to the dropping of ash rate.Base on previous conclusion, a pretreatment process on the new gas diffuser is designed to improve the fluorine erosion-resistance of the diffuser in F-O ashing process, and then decrease the loss on ashing rate. After the aluminum surface are oxidized in pure oxygen, a photo resist layer are coated on it to protect the oxide layer by means of conventional ashing process, then a fluorine passivation layer are formed in gases with oxygen and fluorine in low temperature to prevent from the erosion of fluorine. By means of a series of comparable experiments, it shows that this pretreatment process on new gas diffuser can effectively increase the throughput of the asher with fluorine since it ashing rate is kept. And at the same time the process cost can be greatly saved due to the extension on the diffuser's lifetime.