We have previously demonstrated that photoresist removal rate is enhanced by adding a trace amount of O 2 to the atmosphere in which H radicals are produced from H 2 on a hot metal filament. In this case, not only H radicals but also O and OH radicals are produced. The populations of O and OH radicals are a few hundredth parts of that of H radicals, but these radicals must play important roles. It is not clear which radicals contribute more to the enhancement of the removal rate. We used He/O 2 mixtures in this study, instead of H 2 /O 2 , to produce O radicals without co-producing H and OH to make clear the contribution of O radicals on the removal rate. The removal rate increased slightly with increasing the O 2 additive amount when the filament was unheated. This may be caused by thermal oxidation. On the other hand, the removal rate with a hot filament decreased by the addition of O 2. In short, the removal rate is not enhanced by O radicals. The enhancement in H 2 /O 2 mixtures must only be ascribed to OH radicals.

We have previously demonstrated that photoresist removal rate comparable to oxygen plasma is accomplished by optimizing the removal conditions with H radicals produced on hot metal filament surfaces from H 2 /N 2 mixtures (H 2 :N 2 = 10:90 vol%). N 2 gas was used to dilute the concentration of the H 2 gas and to reduce the risk of explosion. However, it is not clear how the dilution of H 2 by N 2 affects the removal rate. In this paper, we examined the relationship between the removal rate and the H 2 content; the flow rate ratio of H 2 to H 2 +N 2. The removal rate increased with increasing the H 2 content. In addition, the removal rate increased with increasing the substrate temperature according to an Arrhenius equation, when the H 2 content was over 90%. However, below 60%, the removal rate decreased with increasing the temperature over 230 ± 5 °C. Denaturation of photoresist, e.g. hardening and/or crosslinking, may be induced by substrate heating when the H-radical density is low. The removal rate decreases not only by the deficiency of H radicals but also by the denaturation of films in H 2 /N 2 mixed systems.

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