March 4, 2011 — Gigaphoton released the latest developments in its EUV source program at this week’s SPIE Advanced Lithography conference. The company reported achieving a conversion efficiency (CE) of 3.3% with tin droplets <20µm in diameter (see figure) with its plasma-based laser produced plasma (LPP).

Click to Enlarge

Figure. Conversion efficiency vs. droplet size for tin plasma-based LPP EUV source. SOURCE: Gigaphoton

Click to EnlargeIn a podcast interview at the conference, Kenji Takahisa, manager in Gigaphoton’s management planning department, explained the physics behind the performance. Listen to the interview: Download (iPhone/iPod users) or Play Now

The key is the use of a short wavelength pre-pulse to irradiate the tin droplet. The small focus size of the pre-pulse laser penetrates the tin droplets, creating optimal conditions (i.e., an ideal "fine mist"). When the main laser irradiates the target droplet, a "perfect mist" is distributed in the focal plane of the CO2 main laser; the mist is transformed into vapor and is ionized. Any remaining debris, which is also ionized, can then be guided by a magnetic mitigation system to a tin "catcher" away from the collector mirror. Such mitigation ensures a longer life of the collector mirror. Takahisa also explains the experimental data shown in the figure in the interview.

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