Status of EUV Resist and Process Development at IC Manufacture to Implement EUV Lithography to 2X DRAM and Beyond

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), and several resists start to meet the target for 2X node DRAM application in resolution and sensitivity. However, an overall performance of EUV resists is still not comparable to that of DUV resists. At the same process condition having same process constant (k1), the imaging capability of EUV resists is poorer 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. 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. The resolution limit of current EUV resists is also investigated to see the extendibility of it. At an exposure condition which readily resolves 16nm half pitch (HP) resolution, 20nm resolution is hardly achieved even with a champion resist. To overcome this barrier, more efforts on development of alternative resists are required from today.