Shot noise, LER and quantum efficiency of EUV photoresists

The shot noise, line edge roughness (LER) and quantum efficiency of EUV interaction with seven resists related to EUV-2D (XP98248B) are studied. These resists were identical to EUV-2D except were prepared with seven levels of added base while keeping all other resist variables constant. These seven resists were patterned with EUV lithography, and LER was measured on 100-200 nm dense lines. Similarly, the resists were also imaged using DUV lithography and LER was determined for 300-500 nm dense lines. LER results for both wavelengths were plotted against E-size. Both curves show very similar LER behavior - the resists requiring low doses have poor LER, whereas the resists requiring high doses have good LER. One possible explanation for the observed LER response is that the added base improves LER by reacting with the photogenerated acid to control the lateral spread of acid, leading to better chemical contrast at the line edge. An alternative explanation to the observed relationship between LER and E-size is that shot-noise generated LER decreases as the number of photons absorbed at the line edge increases. We present an analytical model for the influence of shot noise based on Poisson statistics that predicts that the LER is proportional to (E-size)(-1/2). Indeed, both sets of data give straight lines when plotted this way (DUV r(2) = 0.94; EUV r(2) = 0.97). We decided to further evaluate this interpretation by constructing a simulation model for shot noise resulting from exposure and acid diffusion at the mask edge. In order to acquire the data for this model, we used the base titration method developed by Szmanda et al. to determine C-parameters and hence the quantum efficiency for producing photogenerated acid. This information, together with film absorptivity, allows the calculation of number and location of acid molecules generated at the mask edge by assuming a stochastic distribution of individual photons corresponding to the aerial image function. The edge "roughness" of the acid molecule distribution in the film at the mask edge is then simulated as a function of acid diffusion length and compared to the experimental data. In addition, comparisons between of the number of acid molecules generated and photons consumed leads to values of quantum efficiencies for these EUV resists.