Extreme ultraviolet lithography

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Extreme Ultraviolet Lithography (also known as EUV or EUVL) is a next-generation lithography technology using the 13.5 nm wavelength. EUV is a significant departure from the deep ultraviolet lithography used today. All matter absorbs EUV radiation. Hence, EUV lithography needs to take place in a vacuum. All the optical elements, including the photomask, must make use of defect-free Mo/Si multilayers which act to reflect light by means of interlayer interference; any one of these mirrors will absorb around 30% of the incident light. This limitation can be avoided in maskless interference lithography systems. However, the latter tools are restricted to producing periodic patterns only.

The pre-production EUV systems being built to date are expected to contain at least two condenser multilayer mirrors, six projection multilayer mirrors, and a multilayer object (mask)^[1]. Since the optics already absorbs 96% of the available EUV light, the ideal EUV source will need to be sufficiently bright. EUV source development has focused on plasmas generated by laser or discharge pulses. The mirror responsible for collecting the light is directly exposed to the plasma and is therefore vulnerable to damage from the high-energy ions^{[2] [3]}. This damage associated with the high-energy process of generating EUV radiation has precluded the successful implementation of practical EUV light sources for lithography.

When an EUV photon is absorbed, photoelectrons and secondary electrons are generated by ionization, much like what happens when X-rays or electron beams are absorbed by matter^[4]. These secondary electrons have energies of a few to tens of eV and travel tens of nanometers inside photoresist^[5] before initiating the desired chemical reaction.

EUV photoresist images often require resist thicknesses roughly equal to the pitch^[6]. This is not only due to EUV absorption causing less light to reach the bottom of the resist but also to forward scattering from the secondary electrons (similar to low-energy electron beam lithography).

Like other forms of ionizing radiation, EUV and EUV-generated electrons are a likely source of device damage. Damage may result from oxide desorption^[7] or trapped charge following ionization^[8].

EUVL faces specific defect issues analogous to those being encountered by immersion lithography. Whereas the immersion-specific defects are due to unoptimized contact between the water and the photoresist, EUV-related defects are attributed to the inherently ionizing energy of EUV radiation. The first issue is positive charging, due to ejection of photoelectrons^[9] freed from the top resist surface by the EUV radiation. This could lead to electrostatic discharge or particle contamination as well as the device damage mentioned above. A second issue is contamination deposition on the resist from ambient or outgassed hydrocarbons, which results from EUV- or electron-driven reactions^[10]. A third issue is etching of the resist by oxygen^[11], argon or other ambient gases, which have been dissociated by the EUV radiation or the electrons generated by EUV. Ambient gases in the lithography chamber may be used for purging and contamination reduction. These gases are ionized by EUV radiation, leading to plasma generation in the vicinity of exposed surfaces, resulting in damage to the multilayer optics and inadvertent exposure of the sample^[12].

EUV is still the subject of ongoing research and development by many groups. The aim is to implement the technology into semiconductor manufacturing in the same way as 193 nm lithography was successfully implemented at the 90 nm node. The difficulties faced stem from the dramatically higher energy of the EUV photon (92 eV for EUV light vs. 6.4 eV for 193 nm light), which underlies the difficulty of damage-free generation and control of EUV light and confining the energy absorption within materials.

[edit] References

1. ^ F. T. Chen, Proc. SPIE 5037, pp. 347-357 (2003).
2. ^ H. Komori et al., Proc. SPIE 5374, pp. 839-846 (2004).
3. ^ B. A. M. Hansson et al., Proc. SPIE 4688, pp. 102-109 (2002).
4. ^ B. L . Henke et al., J. Appl. Phys. 48, pp. 1852-1866 (1977).
5. ^ D. J. D. Carter et al., J. Vac. Sci. & Tech. B 15, pp. 2509-2513 (1997).
6. ^ H. H. Solak et al., Microel. Eng. 67-68, pp. 56-62 (2003).
7. ^ D. Ercolani et. al., Adv. Funct. Mater. 15, pp. 587-592 (2005).
8. ^ D. J. DiMaria et. al., J. Appl. Phys. 73, pp. 3367-3384 (1993).
9. ^ N. Koch et. al., Thin Solid Films 391, pp. 81-87 (2001).
10. ^ J. Hollenshead and L. Klebanoff, J. Vac. Sci. & Tech. B 24, pp. 118-130 (2006).
11. ^ J. Hollenshead and L. Klebanoff, J. Vac. Sci. & Tech. B 24, pp. 64-82 (2006).
12. ^ M. H. L. van der Velden et al., J. Appl. Phys. 100, 073303 (2006).