Keeping EUV Cool

It’s been clear for a long time that EUV lithography sources will be fairly inefficient. The laser-produced plasma (LPP) source concept involves heating a droplet of metallic tin with a high-powered laser to produce a plasma. Only a fraction of the energy of the laser will be converted to light, rather than heat, and less than 1% of the light emitted by the plasma will be at EUV wavelengths.

Tatsuo Enami, general manager for the sales division at Gigaphoton, estimates that the company’s planned 500 W EUV production source will require a 40 kW CO2 drive laser; other sources suggest this power estimate may be unduly optimistic. CO2 lasers themselves are no more than about 20% efficient, at best. In addition, a fully-integrated source will require power for gas and heat management, a pre-pulse laser if one is used, containment of the plasma, and so forth. Ultimately, Enami estimates that the total power demand per EUV stepper is likely to be between 500 kW and 1 megawatt, most of which will be exhausted as waste heat.

That’s a lot of waste heat. Jokes about potential “Silicon Glacier” fab clusters in Alaska and Scandinavia aside, at first glance it looks like EUV systems will face serious heat management challenges. As it turns out, though, Peter Csatáry, M+W Group Head for Global Technologies, estimates that EUV sources will add only a few percent to the power and heat discharge requirements of the fab. While cooling requirements vary depending on country and climate, he said, cooling systems are commonly oversized, allowing some margin for such contingencies as new process equipment. He doesn’t expect major changes to overall fab cooling will be needed to accommodate EUV.
However, the lithography area specifically has not been a major heat generator in the past. Typical ArF lasers for DUV lithography produce about 120 W of output power, with a much more efficient input to output ratio. EUV lithography cells are likely to require more cooling water and larger piping.

It’s also important to remember that cooling for the EUV exposure source is a separate issue from thermal management for the exposure module itself. The optics, reticle, and wafer must all be kept under vacuum, and the temperatures of all three elements must be controlled to within a fraction of a degree in order to maintain the desired image resolution. This is a more difficult challenge, and one that even building the fab on a glacier won’t solve.