Critical Process Variables for UV-Ozone Etching of Photoresist

Abstract

The combination of ultraviolet light (UV) and ozone (O3) has been used to remove organic contamination and strip organic materials, such as, photoresist from silicon and gallium arsenide wafers during processing of electronic devices. In this work we examined the effect of key processing variables such as substrate temperature (150-300°C), O3 concentration ([03], 4-77 g m−3), O3/oxygen flow rate (0.5-3.5 L min−1), and UV (185 and 254 nm) irradiation on the etch rate of positive photoresist on silicon wafers. The photoresist etch rate increased up to 28-fold over the substrate temperature range of 150-300°C at constant inlet [O3], flow rate and UV irradiation. At constant substrate temperature and reactant flow rate, the etch rate increased an average of 2-fold with an average 5-fold increase in the inlet [O3]. The etch rate also increased at constant substrate temperature and inlet [O3] with increased reactant flow rate. At 150° and 200°C, UV irradiation enhanced the etch rate of photoresist 2 and 4-fold, respectively, compared to etching experiments where UV irradiation was absent. This suggested that the primary etchant species at these temperatures was atomic oxygen created by photodissociation of O3. At 250° and 300°C, UV irradiation during etching did not significantly enhance the etch rate, which suggested that at these higher temperatures, the primary etchant species was atomic oxygen generated from the thermal dissociation of O3 near the wafer surface.