Sensitivity of EUV resists to out-of-band radiation

Abstract

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.Keywords: Extreme Ultraviolet Lithography, EUVL, Resist, Sensitivity, out-of-band, spectral purity INTRODUCTIONEUV Lithography is being developed as a potential successor to 193 nm lithography for printing the smallest microprocessor features 2 . Resists developed for EUV are typically based on modifications to materials developed for other lithographic exposures, specifically 248 nm, 193 nm, and e-beam 3 . Therefore, EUV resists are sensitive to radiation other than 13.5 nm, as has been demonstrated [4][5][6] . EUV sources produce light over a wide spectral range [7][8][9][10] . The multilayer coated (MLC) mirrors used in the optics are effective at narrowing the EUV spectral output to peak at 13.5 nm with a bandwidth of ±2%. However, the mirrors also reflect other wavelengths. If OOB light produced by the source reaches the wafer plane, and the resist is sensitive to that light, then contrast will be reduced resulting in degraded imaging performance 6 . The purpose of this study is to characterize the sensitivity of typical EUV resists to OOB radiation. Mbanao et al. measured the sensitivity of 4 resists in the spectral range 190-650 nm 5 . The study presented here extends the spectral range down to 157 nm and includes a wider breadth of resist types with data collected on 13 different materials. Moreover, all resists in this study were 60 nm thick (which is expected to be relevant when EUV is i...

High Sensitivity Resists for EUV Lithography: A Review of Material Design Strategies and Performance Results

“…These PAGs exhibited decreased Deep UV (DUV) absorption by the incorporation of insensitive cations and they were considered in resist formulations based both on blended PAG and polymer-bound PAG. The concept and merit of OoB insensitivity was confirmed by the obtained DUV and EUV sensitivity [49][50][51][52].…”

Out of band radiation effects on resist patterning

“…Some of the EUV resists that are currently being utilized for patterning studies have been shown to be many times more sensitive to specific wavelengths in the 150-300nm DUV/UV region as compared to the sensitivity at 13.5nm [4]. This is of significant concern since the tin plasma based EUV light sources are known to produce quantifiable emission in this region along with the required source bandwidth.…”

Assessing out-of-band flare effects at the wafer level for EUV lithography

“…9 Depending on the design the number of mirrors utilized for projection and imaging may vary and estimates for a 10 and 11 mirror system are given in table 1. Depending on the resist platforms utilized, the absorption of OOB wavelengths will vary 6 and this may lead to considerable reduction in patterning fidelity. Traditionally, flare from OOB is treated as a uniform background (DC) problem.…”