Feasibility study of defects in 157-nm resist process

In 157-nm resists, fluorine atom is added into the polymers in order to improve the transmittance at the exposure wavelength. It is considered that the fluorine atom increases the hydrophobicity of the resists and that the number of defects during development process increases. We evaluated the development defects focusing on the missing contact holes on 157-nm resists. We used different types of 157-nm resists based on such as monocyclic fluoropolymer type, tetrafluoroethylene-norbornene type, and siloxane type. A KrF and ArF resist were also studied for comparison. We measured the number of the missing contact holes for 2500nm 1:2 hole pattern exposed using a KrF stepper. The number of the missing contact holes is higher for the 157-nm resists than that for the KrF or ArF resist. The contact angle of de-ionized water on the resist film of the 157-nm resists was found to be higher than that of the KrF and ArF resists. We examined the number of missing contact holes for various types of resist polymer and the blocking level, and found that the large contact angle, which corresponds to the high hydrophobicity of the resist film, induces the large number of the missing contact holes. The origin of the missing contact holes is the micro bubbles in the developer solution induced by the dissolved nitrogen. For the 157-nm resists, the wettability between resist surface and developer solution is insufficient due to the high hydrophobicity of the resists. Therefore, the micro bubbles are not easily released from the resist surface, and the number of the micro bubbles increases. In order to reduce the number of the missing contact holes, we tried the surfactant-containing developer solution and also tried the nitrogen-degassed developer solution. In both developers, we found that the number of the missing contact holes is reduced by less than half.