Impact of EUVL mask buffer and absorber material properties on mask quality and performance

Abstract

The success of extreme ultraviolet lithography (EUVL) mask fabrication process depends on the correct choice of EUVL mask buffer and absorber materials. There are four criterion to take into account for the right buffer and absorber materials for EUVL masks. The first is the material choice impact on the mask quality such as critical dimension (CD) control, defect control, registration control, cleaning durability. The second is the impact on the mask lithography performance such as imaging contrast, shadowing effect, focus shift effect, thermal effect. The third is the impact on the mask inspection and repair, such as repair etch rate and selectivity to the Mo/Si multiplayer (ML) and optical inspection contrast of the absorber to the buffer layer as well as that of the absorber to the ML. The last is the impact on the lithography integration, such as mask stress due to buffer and absorber versus mask chucking, mask conductivity versus mask protection scheme. In the past, most attention on buffer and absorber material consideration has been paid to the processing side (as long as the absorber materials have a relatively high EUV absorption). The mask performance difference with different buffer and absorber combinations has not been fully explored. In this paper, the impact of buffer and absorber material properties on all these aspects will be discussed with special emphasis of the material property impact to the mask lithographic performance. Detailed simulation studies of EUVL mask performance with different combinations of buffer and absorber materials will be presented. It, however, will be shown that buffer and absorber material properties such as the real and imaginary part of the index of refraction coefficient, play an important role in mask lithographic performance such as the image contrast, the shadowing effect, and the focus shift effect, especially at the small design rules, such as 30nm generations. The comparison of different buffer and absorber material properties and the corresponding mask performances will allow us to understand the tradeoffs between the choice of different buffer and absorber combinations so that the most promising buffer and absorber materials can be identified and corresponding mask fabrication process can be developed.