Scanning probe microscopy study of the tolerance of patterned EUV masks to megasonic cleaning

摘要

EUV lithography (EUVL) is considered the most attractive solution for semiconductor device manufacturing beyond the 22nm half-pitch node. In EUVL, one of the greatest challenges is the lack of a pellicle, which makes EUV masks prone to particle contamination. Therefore, mask cleaning plays an important role in keeping masks clean during both fabrication in the mask shop and usage in the wafer fab. According to the International Technology Roadmap for Semiconductors (ITRS), in 2013 mask cleaning processes should remove all defects larger than 25nm without damaging 78nm and smaller patterns for the 23nm Flash half-pitch node [1]. In addition to contamination concerns, EUV masks introduce new materials and a multilayer structure that is different from the Cr on glass used in traditional optical masks. Physical forces applied by megasonic cleaning to remove particles on an optical mask could damage EUV mask patterns. Thus, it is important to determine the magnitude of the physical forces that can break absorber patterns (TaN or TaBN) from the surface of a Ru-capped MoSi multilayer film. The adhesion of particles of interest to the Ru-capped multilayer should also be measured. In the complex structure of an EUV mask, adhesion forces of particles on the top surface are modified by the different layers beneath the Ru. Hence, it is crucial to directly measure the force required to remove particles and break absorber patterns on EUV mask surfaces to determine the process window for applicable cleaning forces. We used scanning probe microscopy (SPM) to quantify these forces. The SPM probe was precisely controlled to remove particles and break patterns on Ru-capped EUV mask blanks. While being manipulated, the deflection signals of the probe were monitored and then converted to forces using a simple beam model. In this paper, we present the measured breakage forces for absorber patterns as a function of their size and compare them with removal forces for 50nm and 100nm SiO_2 and polystyrenelatex (PSL) particles. Based on these data and our analysis, we will demonstrate a process window for physical force that can successfully clean EUV masks beyond the 16nm half-pitch node.