Engineering optical constants for broadband single layer anti-reflection coatings

Single layer anti-reflection coatings are often assumed to be limited to minimize reflection in a narrow bandwidth around a specific wavelength. In this paper we show that there are in principle no fundamental restrictions for single layer anti-reflection coatings to exhibit broadband suppression. We present theoretically derived design rules for an anti-reflection coating for the spectral range of 100−400 nm, applied here on top of a molybdenum-silicon multilayer mirror as commonly used in extreme-ultraviolet lithography. The design rules for optimal suppression are found to be strongly dependent on the thickness and optical constants of the coating. As a proof of principle, thin films were deposited that exhibit optical constants close to the design rules for suppression around 300 nm. In order to minimize EUV absorption losses, we used silicon based compounds which are relatively transparent to EUV radiation. SixCyNz layers were deposited by electron beam co-deposition of silicon and carbon, with N+ ion implantation during growth. The thin films were analyzed with variable angle spectroscopic ellipsometry to characterize the optical constants. The results show that the optical constants of SixCyNz films can be engineered to match those specified by the design rules. This work illustrates that a single layer anti-reflection coating can be used to achieve broadband suppression using appropriately tailored optical constants.