Retardation phenomenon of oxide removal during the formation of dual gate oxide via PR-mask wet etching

PR-mask oxide wet etching process is generally applied for the formation of dual gate oxide (Gox) transistor (TR) with different thickness of gate oxide. Oxide residues, which could not be removed properly with conventional wet etching process by dHF was observed when PR rework process by strip with SPM and SC1 was preceded before oxide wet etching. The root cause on this oxide removal retardation issue was studied by XPS for the analysis of surface element, SEM for the observation of surface morphology and optical spectroscopy for the measurement of thickness of oxide. It was found that PR rework process is main factor for oxide residue, because no unetched oxide layer was observed after dHF etching if there was no PR rework. A model test showed that when NH₄OH component was included during PR rework process, retardation of oxide etching was occurred. The abnormal high content of carbon ingredient on oxide surface after NH₄OH treatment with SC1 or NH₄OH only solution shows that some kind of blocking layer generated upon adsorbed NH₄OH molecules on oxide surface may hinder oxide wet etching by HF. It is postulated that anionic molecules such as PAG (photoacid generator) or anionic surfactant arisen from PR developing process may be able to combine with NH₄OH molecules, forming complex layer by electrostatic interaction. This assumption was clearly verified that no oxide residues was found after dHF etching if ozonated water (O₃ DIW) treatment was applied between PR developing and oxide wet etching step, since O3DIW can remove organic component with high efficiency, resulting the elimination of blocking layer for wet etching process.