Unraveling strength and mechanical properties of CNT-based EUV pellicle

Abstract

EUV lithography has been introduced in semiconductor fabrication and maximizing yield and throughput is extremely important. One key enabler is the use of a high-transmission pellicle to hold particles out of the focal plane and thereby minimize their impact on imaging. Imec initiated the development of a promising pellicle based on a network of carbon nanotubes (CNT). This CNT membrane offers the advantage of very high EUV transmission (> 95 %) and durability compatible with the EUV scanner power roadmap. Moreover, wafer printing with a CNT pelliclized mask on ASML's EUV scanner at imec has been successfully demonstrated with good printing performance.Since the CNT pellicle is only a few tens of nanometers thick and suspended over an area of tens of centimeters, a major challenge of the pellicle is to control and optimize its mechanical stability and robustness when used in the EUV scanner. The pellicle rupture probability depends on a multitude of parameters, including pressure changes during mask loading and unloading, thermal expansion during exposure, initial stress/strain variations over the large pellicle, membrane degradation in the hydrogen plasma environment, and thickness of the pellicle.In this paper, the mechanical pellicle characterization as a function of the pressure changes for different CNT membranes is presented. The characterization is based on small-size sample evaluation using a bulge test method. By applying controlled plasma to such samples, it was possible to characterize the membranes not only as freshly fabricated but also after exposure to EUV scanner-like conditions. Additionally, the parameters obtained from small samples could be correlated to the actual movement during scanner manipulation. These measurements enable a fundamental understanding of CNT membranes and how they will behave in an industrial environment.