High Throughput Maskless Lithography: Low Voltage versus HighVoltage

摘要

The beam energy is a driving design parameter for electron beam lithography systems. To be able to compare thedifferences of low kV (5 kV) and high kV (100 kV) for a high-throughput system the limitations of both types of systemsare evaluated. First the effect on the CD uniformity and throughput is analyzed. For any shot noise limited system thedose that is needed to obtain a required CD uniformity can be calculated. This dose depends on the total spot size and theefficiency of the electrons in the resist. For a smaller spot less dose is required than for a large spot. The current in asingle beam is also determined by the spot size. A larger spot has more current. With these parameters an optimization ofthe required dose, spot size and single beam current can be made. It is found that although for high kV it is easier tocreate a small spot with a high current the low resist-exposure efficiency of the high-energy electrons limits thethroughput, because the required dose is large. It is also found that for 10 wafers per hour multiple lenses or columns arerequired. For practical reasons (a high kV lens cannot be made as small as a low kV lens) there is a clear preference forthe use of low energy in high-throughput systems. Another aspect that is crucial in the lithography process is the overlay.One of the main differences between high and low energy systems is the power that is dissipated in the wafer and theresulting error due to expansion. It is found that for both energies wafer heating is an issue, but for low kV there seem tobe solutions, while for high kV the problem is 30 times bigger.