Sensitivity of EUV resists to out-of-band radiation

Here we present the relative sensitivity of EUV resists to out of band radiation (OOB), specifically wavelengths in the range 157 - 400 nm. EUV light sources have specifications limiting the allowed energy output in that spectral range yet there is little data supporting the specified values. Filters might be required to meet the spectral purity specifications which will likely have the detrimental effect of reducing the in-band radiation at 13.5 nm and therefore negatively impact the cost of ownership of EUV lithography. To better quantify the effects of OOB we obtained contrast curves and absorbance spectra for several EUV resist platforms at nine exposure wavelengths. The 2007 ITRS Roadmap suggests that resist thicknesses will be near 35 - 65 nm when EUV will be used(1). We found that, in this optically thin regime, resist sensitivity increases with increasing absorbance. The sensitivity decreases dramatically for wavelengths approaching 300 nm, and is negligible for longer wavelengths. The OOB sensitivity of the resists examined can be estimated to within an order of magnitude using the resist absorbance value. For resists with absorbance values on the same order of magnitude, sensitivity is determined by other aspects of the resist formulation. Within the wavelength region explored, the greatest concern is near 160 - 240 nm based on current resist sensitivity characteristics. However, there is a gap in data between 13.5 - 157 nm and there may be other reasons to limit the source output in that wavelength range. The data presented here could be useful in setting or modifying the OOB specifications for EUV tools.