Laser and optical developments of a modular laser-plasma source for EUV lithography

The laser system is by far the most expensive part of a laser-produced plasma (LPP) EUV source. Thus LPP source designers have not only to optimize the technical performances of the lasers but also cost, efficiency, reliability, redundancy and industrial availability of their approach. Based on the typical conversion efficiencies of present LPP source targets (CE ~ 1- 3%), the laser system will have to deliver an average power of at least 20 kW. This can hardly be obtained with only few laser chains. Starting from this consideration, the EXULITE consortium has first proposed the concept of a spatially multiplexed and modular LPP source using 10 to 20 identical, moderate power laser units which are simultaneously focused on the same spot. This approach increases system redundancy and reduces the development cost of the laser. We will present an original technical approach which allows both to manage the multi-beam attack of the target using a closed-loop control and to collect a maximum of EUV power. The grazing incidence EUV collector is designed such that optimized vacuum pumping and debris mitigation can be obtained. Finally, we can show that modular LPP sources can satisfy the severe EUV dose stability that is commonly requested.