CD uniformity improvement for EUV resists process: EUV resolution enhancement layer

Abstract

Extreme ultra violet (EUV) resists have been developed to be able to print sub-30nm L/S features with EUV alpha DEMO tool (ADT) having 0.25NA. However, a lithographic performance of EUV resist is not comparable to that of DUV resist. At same process constant (k1), the imaging capability of EUV resist is poor than that of DUV resists. The most critical issues are line width roughness (LWR) and critical dimension (CD) variation across a field. Although there are many studies to improve the LWR of EUV resist, the issue on CD variation across a field is not much explored, because the problem can be detected at full field exposure. In this paper, sources of the CD variation across a field are mainly investigated, and solutions to improve the CD uniformity are explored. Out of band (OOB) radiation and its reflectivity at REticle MAsking (REMA) unit of scanner or absorber of mask is regarded as one of the sources which aggravates imaging quality of EUV resist. In addition, the optical density of black border at EUV wavelength is also known to have an impact on this CD variation. Although the exact spectrum of OOB radiation is not open, LASER produced plasma (LPP) type source and discharge produced plasma (DPP) type source are believed to have the OOB radiation. Therefore, to improve pattern fidelity and LWR of EUV resist, the mitigation of OOB radiation impact is required. It is found that the resist sensitivity to DUV compared to EUV is important, and this property affects on CD uniformity. Furthermore, new material which can mitigate the OOB radiation impact is developed. This material is applied as an additional layer on conventional EUV resist film, and shows no intermixing. Process window is not changed by applying this layer. The filtering ability of OOB radiation is explored. LWR and pattern fidelity are much improved by applying this material to EUV process.