We have observed that increasing the thickness of the resist can improve the critical dimension uniformity (CDU) in electron beam lithography. This is our first experimental demonstration that increasing the acid generation in the resist by incident electrons is a pathway to reduce the effect of shot noise on CDU. The measurements were made with our Quadra raster shaped beam lithography system. The resist was REAP 200, a chemically amplified resist. The thicknesses were 200, 300 and 600 nm. The phenomenon is consistent with our model prediction that there would be a reduction of the shot-noise-induced CDU as the number of acid molecules generated in the chemically amplified resist increased with the resist thickness. We used the model to estimate the acid generation efficiency and the resist blur. We have also observed deviations from this trend in the thick resist (600 nm) suggesting complexity that may not be explained by the model. We are continuing our investigation to confirm these preliminary results.
We have used our Quadra lithography system to evaluate the shot-noise-induced critical dimension uniformity (CDU). We found that at the isofocal dose, the shot-noise-induced CDU is directly proportional to the edge blur, and hence the rate of CD changes with dose. This emphasizes the importance of minimizing beam blur of the system. We used a phenomenological model to analyze our experimental data. The model included the counting statistics of the incident electrons and that of the electron induced chemistry. With the proper parameters, this model matches the experimental observations well. It also predicts the limit of the improvements and suggests guides for the optimization of the lithographic process.
We have made measurements with our Quadra raster shaped beam lithography system to evaluate the shot-noise-induced critical dimension uniformity (CDU). We found that at the isofocal dose, the shot-noise-induced CDU is directly proportional to the edge blur, and is linear with the rate of CD change with dose. Here we propose a phenomenological model which permits an experimentalist to relate the CDU to controllable lithographic parameters. The model considers both the counting statistics of the incident electrons and the noise from the electron induced chemistry. The model suggested that the shot-noise induced CDU may be minimized by reducing the beam blur, forward scattering, through the optimization of the resist smoothing distance and maximizing the number of acid molecules created in the resist by an incident electron. With the proper parameters, this model matches the experimental observations well. It also predicts the limit of the improvements and suggests guides for future resist development. Shot-noise induced line edge roughness is also discussed.
A raster multibeam lithography tool is in Etec’s roadmap to meet the stringent requirements of sub 100 nm mask fabrication. The tool leverages the long experience obtained with the ALTA laser pattern generators and the high resolution capabilities of e-beam lithography. A photocathode controlled by acousto-optic modulated 257nm laser beams is utilized to generate 32 electron beams. The beams are accelerated at 50 KV in an electron column, demagnified and focussed on the mask or wafer substrate. The performance of the photocathode and other system components will be presented together with preliminary lithographic patterning.