Rates and mechanisms of optic contamination in the EUV engineering test stand

Abstract

The EUV Engineering Test Stand (ETS) is a full-field, alpha-class Extreme Ultraviolet Lithography (EUVL) tool that has demonstrated the printing of 70 nm resolution scanned images. The tool employs Mo/Si multilayer-coated optics that reflect EUV radiation (13.4nm / 92.5eV) with ~67% peak reflectance per optic. For good reflectivity, many (≥40) Mo/Si layers must be present. Consequently, processes such as plasma-induced multilayer erosion, which reduces the number of bilayer pairs on plasma-facing optics, need to be understood. In addition, since most materials readily absorb EUV photons, it is important to prevent contamination of mirror surfaces with EUV absorbing material. Contamination can occur by EUV photons or secondary electrons "cracking" hydrocarbons or other species adsorbed on the optical surfaces.The first ETS condenser component, referred to as C1, faces the plasma and is coated with Mo/Si multilayers. Data collected from Mo/Si witness plates placed at the C1 position indicate erosion, using the Xe Laser Produced Plasma (LPP) spray jet, of 1 bilayer per ~15 million shots. In the spray jet studies, erosion was found to depend sensitively on the composition of the residual background environment. Addition of low levels, ~7x10 -7 Torr, of H 2 O to the 1 mTorr Xe background produced oxidation of the Si cap, and significantly slowed spray jet-induced erosion. Operation of the plasma changed the environment in the Illuminator Chamber from oxidizing to carbonizing, thereby changing the nature of the contamination found environment at the C3 optic which does not view the plasma directly (and therefore does not erode). The change in environment is attributed to plasma-induced outgassing of fluorocarbons in the Illuminator. Due to the non-zero conductance between the Illuminator and Main Chambers, fluorocarbons were also found in the Main Chamber during Xe LPP operation. RGA data are presented that document this effect. In the presence of such outgassing, Carbon deposition rates were measured for the C3, and P.O. Box optics. For C3, a C deposition rate of 3 Å / 10 million shots was found, while for the PO Box, a C deposition rate of 0.02 Å / 10 million shots was found from the data. All data was acquired with no attempt to mitigate C deposition with gas-phase additives such as O 2 .

Multi-Watt cavity for 266 nm light in vacuum

“…While this increases the longevity of the mirrors, it cannot prevent their degrading in the presence of UV radiation [4]: Even after baking and pumping to p = 5 × 10 −10 mbar, the cavity spectrum is degrading. To mend this, we would have to remove all hydrocarbons from the vacuum setup [11,32]. However, as the purpose of the cavity is to provide an intense laser grating for matter-wave diffraction of large organic molecules [37,38], this is not an option.…”

“…Laser-induced degradation of optical components in vacuum is a common challenge faced in sealed [1][2][3] and space-based laser systems [4][5][6][7][8], lithography [9][10][11][12], spectroscopy [13][14][15][16][17][18], and quantum optics [19][20][21][22]. Often this can be traced back to residual hydrocarbons that photochemically react with the optical components and deteriorate their properties [3,4,23].…”

“…Constantly flushing the optics with oxygen prevents degradation entirely [2,18] and high power UV-cavities in a segmented vacuum chamber have been demonstrated [15]. Alternatively, removing organic contamination entirely is a very successful route to ensure long-term stability [11,32,33].…”

Magnetic field ion mitigation for EUV light sources

“…For example, multilayer deterioration of a collector mirror by plasma induced erosion has been observed in a laser-produced plasma EUV source using a Xe jet target. 4,5 It is therefore important to characterize the fast ions from the plasma to investigate necessary mitigation techniques. Ion energy distribution measurements have been reported by several laboratories 5,6,7 including our group.…”

Ageing of optical components under laser irradiation at 532nm

“…The main consequence is a degradation of the components optical properties used in UV-lithography tools [01][02] [03] . Contamination is also reported in tests concerning the lifetime of optics in vacuum systems as synchrotrons or Free Electron Laser [04][05] [06] .…”