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EUV TOOLS 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) F. T. Chen, Proc. SPIE 5037, pp. 347-357 (2003).. 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 H. Komori ''et al.'', Proc. SPIE 5374, pp. 839-846 (2004). B. A. M. Hansson ''et al.'', Proc. SPIE 4688, pp. 102-109 (2002). and other debris S. N. Srivastava ''et al.'', J. Appl. Phys. 102, 023301 (2007).. This damage associated with the high-energy process of generating EUV radiation has precluded the successful implementation of practical EUV light sources for lithography. EUV ABSORPTION IN MATTER 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 B. L . Henke ''et al.'', J. Appl. Phys. 48, pp. 1852-1866 (1977).. These secondary electrons have energies of a few to tens of eV and travel tens of nanometers inside Photoresist D. J. D. Carter ''et al.'', J. Vac. Sci. & Tech. B 15, pp. 2509-2513 (1997). before initiating the desired chemical reaction. A contributing factor for this rather large distance is the fact that polymers have significant amounts of Free Volume N. Shimizu and H. Sato, 1996 IEEE Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, pp. 787-790 (1996). In a recent actual EUV print test,Y. Ekinci ''et al.'', Microelectronic Engineering, vol. 84, pp. 700-704 (2007). it was found that 30 nm spaces could not be resolved, even though the optical resolution and the photoresist composition were not the limiting factor. The response of matter to EUV radiation can be captured in the following equation: EUV photon energy = 92 eV = Electron binding energy + kinetic energy of the emitted photoelectron where the electron binding energy is typically 7-9 eV for organic materials and 4-5 eV for metals. The photoelectron subsequently causes the emission of secondary electrons through the process of Impact Ionization . Sometimes, an Auger Transition is also possible, resulting in the emission of two electrons with the absorption of a single photon. Strictly speaking, photoelectrons, Auger electrons and secondary electrons are all accompanied by positively charged holes in order to preserve charge neutrality. An electron-hole pair is often referred to as an Exciton . For highly energetic electrons, the electron-hole separation can be quite large and the binding energy is correspondingly low, but at lower energy, the electron and hole can be closer to each other. As the name implies, an exciton is an excited state; only when it disappears as the electron and hole recombine, can stable chemical reaction products form. EUV photoresist images often require resist thicknesses roughly equal to the pitchH. H. Solak ''et al.'', Microel. Eng. 67-68, pp. 56-62 (2003).. 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 ). EUV DAMAGE Like other forms of ionizing radiation, EUV and EUV-generated electrons are a likely source of Device Damage . Damage may result from oxide desorption D. Ercolani ''et al.'', Adv. Funct. Mater. 15, pp. 587-592 (2005). or trapped charge following ionization D. J. DiMaria ''et al.'', J. Appl. Phys. 73, pp. 3367-3384 (1993).. Damage may also occur through indefinite positive charging by the Malter Effect . If free electrons cannot return to neutralize the net positive charge, positive ion desorption H. Akazawa, J. Vac. Sci. & Tech. A 16, pp. 3455-3459 (1998). is the only way to restore neutrality. However, Desorption essentially means the photoresist is degraded during exposure, and furthermore, the desorbed atoms contaminate the optics. EUV DEFECTS 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 photoelectronsN. Koch ''et al.'', Thin Solid Films 391, pp. 81-87 (2001). 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 reactionsJ. Hollenshead and L. Klebanoff, J. Vac. Sci. & Tech. B 24, pp. 118-130 (2006).. A third issue is etching of the resist by oxygenJ. Hollenshead and L. Klebanoff, J. Vac. Sci. & Tech. B 24, pp. 64-82 (2006)., 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 sampleM. H. L. van der Velden ''et al.'', J. Appl. Phys. 100, 073303 (2006).. EUV DEVELOPMENT EUV has been the subject of ongoing research and development by many groups. It is unlikely for the technology to be used even at the 22 Nm node in 2011, given that it is still under development in key areas such as light source, photoresists, and defect inspection. 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. It is also fundamentally impossible for EUV with low resolution enhancement and single patterning to compete with the larger depth of focus from the more established approach of using the 193 nm wavelength with strong resolution enhancement and Double Patterning . REFERENCES |
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