EUV Light Source Makes Progress

Gigaphoton more than doubles light source power, plans for bigger increases.

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EUV Lithography light source maker, Gigaphoton has developed a laser-produced plasma (LPP) light source prototype model that can produce a maximum of 92 watts. This is more than double the 43 watts produced using a traditional LPP light source.

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Figure 1 conceptual diagram of EUV exposure. Source: Gigaphoton/EUVA

The biggest issue with EUV lithography has been the output power of the light source. This extends the time required to photosensitize a resist and to expose a wafer, limiting throughput in a fab. So despite the promise of EUV, double patterning with ArF laser lithography and double exposure or a single exposure plus self-alignment processing have been the methods that have been predominantly used.

Boosting the power source to 92 watts changes the equation somewhat. According to a report by ASML, 70 watts equals 52 wafers per hour. That compares with 40 wafers per hour with a traditional 43 watt source and 60 wafers per hour at 92 watts.

A 13.5nm X linear light of EUV is generated as follows: Generate plasma by irradiating CO² laser while dripping Sn that has been liquefied at a high temperature. Then EUV light is generated with ionized Sn (See reference material 1). By using smaller-diameter droplets, plasma temperature rises and increases the ionization efficiency.

The 92W source was achieved was by reducing the diameter size from traditional 30um to 20um. That was followed by a two-step method for irradiation by the laser, a method that was developed back in 2008. This method starts with irradiation from a low-output YAG (yttrium aluminum garnet) laser to break droplets into tiny mist forms. Then the EUV light is generated by heating those droplets up with CO2 laser at maximum output. Additionally, this process prevented deterioration by the reflecting mirror located in between EUV light and wafer. Specifically, ionized tin (Sn) was entrapped with lines of magnetic force using a superconducting magnet, preventing Sn from adhering onto the mirror. In case of Sn adhesion onto the mirror, Sn was removed by etching with H² gas.

Gigaphoton plans to increase output power of 150W by the end of 2014 and, finally, an output power of 250W in the future. The development is receiving a support from New Energy and Industrial Technology Development (NEDO).

The Japanese version of this article is located here.

Reference Material 1.