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In attenuated phase-shift mask technique for window pattern formation, the amount of mask bias has to be optimized, not only to obtain wider focus margin but also to avoid resist film thickness loss at side-lobe position. In general, larger mask bias is necessary to preclude side- lobe printing, but depth of focus decreases with increasing mask bias. In this paper, surface insoluble layer was applied to prevent this side- lobe printing, instead of larger mask bias addition. At first, the pattern formation capability of 0.35 micrometers window was investigated with an attenuated phase-shift mask of 8% transmittance in i-line lithography (NA equals 0.6, (sigma) equals 0.3, i-line stepper; 1 micrometers thick novolac type resist). Surface insoluble layer was formed by alkaline developer (2.38 wt% tetramethylammonium hydroxide TMAH) treatment followed by water rinse before i-line exposure. After more than 1 min treatment, deep resist film dimple due to side lobe was suppressed almost perfectly, even in small mask bias (<EQ 0.05 micrometers ) condition. As a result, the depth of focus of 0.35 micrometers window increased to 2.0 micrometers (mask bias, 0.05 micrometers ) from 1.4 micrometers (mask bias, 0.1 micrometers ). Next, this technique was also applied to KrF excimer laser lithography to improve the process margin of 0.25 micrometers window formation. A 5% transmittance attenuated phase-shift mask and the combination of KrF excimer laser stepper with 0.5 NA and 0.3 (sigma) and 0.7 micrometers thick chemically amplified positive resist were used. It was found that surface insoluble layer can be formed by the same TMAH alkaline treatment in KrF chemical amplified resist. As a result, the focus margin off 0.25 micrometers window was improved to 2.0 approximately equals 2.5 micrometers (mask bias, 0.0 micrometers ) from 1.5 approximately equals 2.0 micrometers (mask bias, 0.03 micrometers ). In conclusion, we can select more suitable mask bias by means of this surface insoluble layer formation technique, indicting that wider (approximately equals 2.0 micrometers ) depth of focus of window pattern is achieved in both i-line and KrF excimer laser lithography.
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