APPLICATIONS EUV LITHOGRAPHY
advantages of LPP and DPP – power scalability and high stability. The company announced in October 2012 stable operation of its EUV light source over a period of half a year at the Belgian research institute Imec.
Maximising power output Phil Alibrandi, director of US sales at EUV source provider Gigaphoton, which bases its technology on LPP, says: ‘There are three main areas that can be fine tuned [with LPP] for increased power output: the frequency of the tin droplets supplied to the chamber, the conversion efficiency (CE) of generating EUV energy, and the CO2
drive laser.’
One key technology required for scaling to higher power is a pre-pulse laser, which Cymer and Gigaphoton use. ‘The conversion efficiency without a pre-pulse laser is relatively low because a lot of the drive laser beam misses the droplet,’ says Farrar, the droplet being around 30µm in diameter and the laser beam only focusing down to around 100µm. The purpose of pre-pulse is to hit the tin droplet with a lower-intensity laser pulse ahead of the main pulse. This increases the size of the droplet so the interaction cross-section with the laser beam is better matched. Gigaphoton uses a YAG
wavelength laser to expand a 20µm diameter tin droplet to around
300µm. It then focuses a 9kW CO2 laser to a spot size of just over 300µm to maximise efficiency. Alibrandi says Gigaphoton’s EUV source has a conversion efficiency of 4.7 per cent, which he says is made possible by using a pre-pulse laser. However, a pre-pulse laser only gets you so far, and to increase the power output further requires a higher power CO2
drive laser.
Gigaphoton calculates a 23kW laser will be needed to reach 250W output power. ‘Ionising each droplet at a rate of 100,000 per second, five per cent conversion efficiency, and a 23kW drive laser should allow 250W output power to be achieved,’ states Alibrandi. The problem, however,
with engineering higher power CO2 lasers, involves back-reflection of the
www.electrooptics.com FEBRUARY 2013 l ELECTRO OPTICS 13
laser energy. ‘We’re working to avoid self-oscillation in the laser using filters and pre-amps and increase the [drive laser] power output,’ says Alibrandi. Gigaphoton has begun testing its 25W EUV source, which it will supply to ASML for integration in its scanners. The collection efficiency – how much EUV light is collected and directed to the intermediate focus – is
also important. That depends on the reflectivity and size of the collector mirror, which has to be protected from any tin debris in the chamber. Tin atoms or ions settling on the mirror reduce its reflectivity. Debris mitigation is a challenge,
as more than a few angstroms of tin will render the mirror inoperative. ‘Replacing the mirror is a lengthy and expensive process,’ says
Alibrandi. ‘These scanners are $100 million tools and it’s important that they perform.’ Gigaphoton’s debris mitigation mechanism uses superconducting magnets to sweep away excess tin and preserve the lifetime mirror. The company is also developing an on-demand droplet generator, which, it claims, will reduce tin debris inside the chamber. Cymer claims it can protect its ➤
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