Study On Photoresist Ashing Process In Beol With Dry Etch Method

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, in BEOL ashing process, because of the introduction of fluorinated gases, will produce a series of problems. 1. Ashing rate was a significant decline of instability. 2. As the F-ion chemical properties of active and cause greater wear and tear of these parts, so that rapid shortening life. 3. Fluorinated gases into a kind of blind prone to defects, such defects would result in yield per wafer reduction of 3% to 5%. That capacity of equipment, process costs and product quality have a greater impact.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 are proposed. Using SEM (scanning electron microscope) of the blind analysis of surface defects, and use the FIB (focused ion beam) defect on the blind analysis of the internal morphology, and then discussed the formation mechanism of blind defects.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. Analysis of the blind defect, the defect occurs only in dense hole area, hole in the barrier layer are pushed into the surface processes and internal cavities. Further study found that added the F-gases(CF₄) during the process there be such defects occur. Base on previous conclusion, we propose to optimize the ashing gas composition and ratio to improve stability and the ashing rate, reduce the machine parts wear and tear and solve blind defects. Specific methods are: the ashing gas composition change from mass O₂+ little CF₄+ little N₂ to mass O₂+ little N₂.To further improve the ashing rate, wafer temperature we will do the appropriate increase.Comparison of validation tests showed that: mass O₂+ little N₂ gas mix can be a stable rate of ash, and can reduce machine parts damage and to solve defects issue. Increase the wafer temperature can greatly improve the ashing rate, thereby enhancing the capacity to dispensing equipment, reduce technology costs.