Investigating the Degradation of EUV Transmittance of an EUV Pellicle Membrane

The extreme ultraviolet (EUV) pellicle is a freestanding membrane that protects EUV masks from particle contamination during EUV exposure. Although a high EUV transmittance of the pellicle is required to minimize the loss of throughput, the degradation of EUV transmittance during the extended exposure of the pellicle has been recently reported. This may adversely affect the throughput of the lithography process. However, the cause of this phenomenon has not yet been clarified. Therefore, we investigated the cause of the degradation in the EUV transmittance by observing the compositional change when the Ru/SiN_x pellicle composite was heated in an emulated EUV scanner environment. The Ru thin film that was deposited at high pressure had more void networks but was not oxidized, whereas the SiN_x thin film was oxidized after heating. This was because the void network in the Ru thin film served as a preferential diffusion path for oxygen and caused oxidation of the SiN_x thin film. It was confirmed that the degradation of the EUV transmittance was due to the oxidation of SiN_x. The results verified the effect of diffusivity in the thin film due to the void network on oxidation and EUV transmittance.