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  • Plasma Source Enables High-Volume Manufacturing with EUV Lithography

Photonics Spectra
Sep 2011
Phil Alibrandi, Gigaphoton USA Inc.

As the lithography capital equipment sector moves toward early field deployment of extreme-ultraviolet (EUV) scanners and support, the 13.5-nm EUV energy source continues to be a key factor in the ultimate adoption and economic success of the technology. Given the high price of capital equipment investment, traditional high-volume manufacturing (HVM) measures continue to be relevant and are hurdles to wide-scale acceptance by chip makers.

In 2002, Gigaphoton began drawing upon the resources of Komatsu Ltd. of Hiratsuka, Japan, and its ongoing work with the Extreme Ultraviolet Lithography System Development Association in Tokyo. Early on in this collaboration, laser-produced plasma (LPP) emerged as a preferred method of energy creation. Because of the technique’s scalability, higher efficiency and the inherent spatial freedom of the plasma, the CO2 laser-tin-LPP scheme is a promising candidate for long-term HVM success.

Specific features in development address three obstacles: productivity, reliability and cost of ownership. They include – but are not limited to – prepulse strategy, droplet generation, ease of maintenance/access, tin droplet debris mitigation and collector mirror lifetime.


Figure 1.
Illustration of an EUV chamber: (a) tin droplet generator, (b) prepulse laser port, (c) main laser port, (d) EUV output, (e) collector mirror, (f) plasma ball, (g) ion catcher and (h) superconducting magnets, located outside the chamber. Images courtesy of Gigaphoton USA Inc.


Conversion efficiency and prepulse strategy

In the semiconductor industry’s drive for initial 250-W source power at the 13.5-nm wavelength for HVM, conversion efficiency is the metric used to track energy from the CO2 laser to the EUV scanner. The beginning 5 percent target is only one factor in the equation needed to reach economical production throughput (power).

A conversion efficiency of 3.3 percent is achievable; the key to reaching the 5 percent requirement is a YAG laser prepulse strategy. This is an optimal wavelength for inducing thermal expansion of the tin droplet and for creating a fine mist, exposing more material to the main CO2 laser pulse for maximum in-band energy conversion.


Figure 2.
The graph shows a proof of concept for an optimized prepulse strategy using two independent Gigaphoton Inc. lasers with different wavelengths.


In combination with an optimized droplet size, this prepulse, distinct from the main power laser, allows for improved laser timing control and 100 percent tin ionization. It precludes premature energy loss. Only these conditions create the proper reverse slope leading to the necessary conversion efficiency, with no increase in droplet size or excessive tin debris. To reach ≥5 percent conversion efficiency, a gating item for ≥50-W power, all entrants still have work to do (see Figures 2 and 3).

Droplet generation

Tin droplet size, uniformity and Z-direction location also are significant technical challenges to successful HVM operation.

Unique concepts in nozzle design and material purity in sub-20-µm droplet extraction highlight the importance of cooperating closely with droplet generator subsuppliers. The algorithms and control needed to acquire, track and impact these targets at 100 kHz are nontrivial but well along in development and execution.


Figure 3. Shown over time are shadow graph images of the independent prepulse strategy with resulting minimization of tin fragments. Note the thermal expansion of the droplet, conserving potential energy prior to the main pulse.

Debris mitigation

The unavoidable result of tin ionization is debris created by ions, neutrals and potential tin fragments. Without an effective strategy for avoiding and eliminating debris, the collector mirror quickly becomes contaminated. Its effective lifetime and availability are reduced, and the cost of ownership increases. As little as 1 nm of deposition is estimated to reduce reflectivity by 10 percent, a likely trigger for mirror replacement.

Proper droplet size and an independent prepulse can avoid fragments and maximize ionization. Magnetic ion guiding, now a proven method, is used to mitigate debris. In the EUV chamber’s vacuum environment, ionized tin atoms are guided to the tin catcher by a magnetic field, created by placing superconducting magnets on either side of the chamber (see Figure 4). The magnets are well integrated with the source in terms of both size and safety.


Figure 4.
Tin ions, guided toward the tin catcher, are visible in the magnetic flux, well above and parallel to the collector mirror.


To minimize the deposition of tin neutrals on the mirror, a chemical cleaning process involving a common gas at low pressure is used. Sweeping away debris magnetically, well away from the collector mirror, is not only a low-risk technique but also permits a very small chamber design and a mirror diameter of just 400 mm. There is no need for extra-large vessels or high gas flows to protect the collector mirror, especially in this vacuum area. This type of EUV chamber need stand only about 1.5 m – significantly smaller in dimension and volume than discharge-produced plasma or other LPP designs.

Conclusion

Significant challenges are involved in designing, manufacturing and supporting EUV technology to achieve an economically viable HVM lithography solution per the International Technology Roadmap for Semiconductors, the industry’s 15-year assessment of its technology requirements. Ongoing EUV source development and solutions in prepulse strategy, debris mitigation and chamber size will result in various benefits to end users. These will include more efficient power and wafer output, and higher reliability and uptime, with a resulting lower cost of ownership. The industry requirement to reach 250 W of clean power at intermediate focus will be realized by the extension of main CO2 laser power from current levels to 23 kW and conversion efficiency from ~3 to 5 percent.

Meet the author

Phil Alibrandi is director of sales and international account management for Gigaphoton USA Inc. in Beaverton, Ore.; e-mail: phil.a@gigaphotonusa.com.

Reference and acknowledgment


H. Mizoguchi et al (April 2011). 100W 1st generation laser-produced plasma light source system for HVM EUV lithography. Proc SPIE.

This work was partly supported by the New Energy and Industrial Technology Development Organization in Japan.


GLOSSARY
conversion efficiency
In a pumped laser system, the ratio of output energy to pump energy.
mirror
A smooth, highly polished surface, for reflecting light, that may be plane or curved if wanting to focus and or magnify the image formed by the mirror. The actual reflecting surface is usually a thin coating of silver or aluminum on glass.
plasma
A gas made up of electrons and ions.
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