Index Introduction The Research at Rijnhuizen Results in 2008 Education, Training, Outreach and Public Information Output Appendix website Rijnhuizen 3.7 | Surface and thin film processes beyond EUV applications Coordinators: Prof. Dr. F. Bijkerk, Drs. E. Louis Funding: FOM Programme 75 - PSI-lab, an integrated laboratory on plasma-surface interaction FOM-Pilot project on ML optics for 4th generation XUV sources Beyond-EUV - Technology Foundation project 10302 Photolytic Salt Formation - Materials Innovation Institute/ASML project 06.244 Next to the comprehensive XMO programme, reported in section 3.6, the nSI department carries out a number of research topics which are related to different applications than EUV photolithography. These are either more distantly aimed to wavelengths beyond the Extreme UV, as in the case of the European FLASH XUV research, or, contrary, to longer wavelengths, namely 193 nanometer. Both areas generate ample opportunities for challenging new physics and chemistry, either photo- or ion-induced at surfaces. These separate projects are funded by miscellaneous programmes, often in conjunction with industry or academic partners.  3.7.1 XFEL optics pilot project: Study of damage mechanisms. To determine the feasibility of the use of multilayer systems in optical schemes at XUV & X-ray Free Electron Lasers, the fundamental mechanism of the damage of the Mo/Si multilayers at 13.5 nm was studied in detail. The experimental data analysis together with thermodynamical modeling shows that the responsible damage process is a destruction of the layer structure due to ultrafast atomic diffusion and silicide formation, a newly discovered process on this timescale.  The studied Mo/Si multilayers, irradiated with intense femtosecond XUV radiation at the FLASH facility (Hamburg, Germany), consist of fifty 7.0 nm thick bi-layers (Mo and Si). When exposed, the sequence of events on the multilayer is the following: according to the model, the radiation energy is absorbed in the thin volume close to the sample surface. Consequently, the temperature of the sample increases. For radiation fluences above the damage threshold some of the top a-Si layers are melted. Due to the heat diffusion the depth at which a‑Si is melted increases in a few nanoseconds. The maximum depth is reached when the whole top volume of the sample is heated uniformly up to a-Si melting temperature. The solid a-Si layer below is acting as a thermal insulating buffer (because of relatively low thermal conductivity of a-Si) and slows cooling of the surface. The temperature of the top layers stays at melting point for several nanoseconds. In this short time, the atomic diffusion of Mo into the melted Si layers takes place. The atomic diffusion in liquid silicon is much faster (~10 orders of magnitude) than in solid silicon. Despite its very short duration it is fast enough for the complete penetration of Si layers by Mo. This results in the formation of molybdenum silicides and their recrystalisation. The sample looses sharp interfaces between Mo and a-Si and effectively, its layer structure is destroyed. In addition, a crater is formed due to compaction caused by the higher density of the silicides in comparison to the mean density of Mo/Si bi-layers. The multilayer reflectance during the fs exposure is not affected. The multilayer damage mechanism described above – ultrafast atomic diffusion – was observed for the first time and is qualitatively different from the processes known up to now. The silicide-formation has been observed before, but only as the result of steady state thermal annealing, and it is characterized by a low threshold temperature (~325 °C) and a time scale of many hours. In case of fs intense XUV irradiation, the damage occurs at a much higher threshold temperature (~1000 °C) and at the time scale of nanoseconds – several orders of magnitude shorter. These differences play a significant role in the design of the robust optics for short wavelength Free Electron Lasers. Figure 3.29 STEM pictures of the Mo/Si multilayer sample irradiated with an intense fs XUV pulse. The left picture shows the border between undamaged (bottom, left) and damaged (top, right) regions. The reduction of contrast between layers is caused by silicide formation. The pictures on the right show both regions at higher magnification. The one at the bottom shows the undamaged region, with sharp interfaces between a-Si and Mo layers. The one at the top shows the damaged region, with almost uniformly distributed polycrystalic silicides and some Mo-islands.  3.7.2 Investigations on short period multilayers Anticipating further development of lithography, using even shorter wavelengths than the current 13.5 nm, and similar systems for X-ray Free Electron Lasers (X-FELs), new reflective multilayer optics are considered. These may include combinations of B or B4C and metals such as La, Th and U. Experimental B4C/La combinations are observed to yield chemical reactivity and LaB6 and LaC2 interlayer formation at the interfaces, limiting optical contrast and reflection. We applied N2+ plasma treatment to form BN:C and LaN at the interfaces, preventing interlayer formation and increasing optical contrast. The multilayers were studied with reflectometry, angle resolved photo-electron spectroscopy, cross-sectional-TEM, and SAED. We observed that B4C is more readily nitridated to BN:C by N2+ bombardment during or after growth than La is to LaN. Nitridation of La apparently yields an equilibrium that involves La(N2) di-nitrogen complexes. The loosely bound N2 at the La/B4C interface substrate partially diffuses into the adlayer and covers the surface, resulting in surfactant mediated growth. Subsequent nitridation of the adlayer is observed, yielding nitridated interfaces of improved optical contrast without LaB6 and LaC2 interlayer formation. In effect, we enhanced the multilayer’s optical reflection and bandwidth in experimental 75 period multilayers by a factor of up to 1.3. These results are of high relevance to applications in e.g. X-ray free electron lasers, next generation photolithography, soft x-ray spectroscopy, fluorescence analysis and imaging. Figure 3.30 Schematic representation of the interface chemistry in the La/B4C multilayer systems: the standard (left) and passivated (right) case. The undesirable chemical reaction between the pure La and B4C layers is stopped in the second case due to the formation of a LaN and BN barrier layer. 3.7.3 Photolytic salt formation at oxide surfaces During routine operation of photolithographic equipment, optical elements may gradually become contaminated. This might be caused by the presence of trace impurities in the environment close to the optical surface. Although the relevant impurities and the chemical nature of the contaminant formed have been identified in some cases, the precise mechanism leading to the onset of the process is still unclear. In an attempt to rectify this, surface science studies under Ultra-High Vacuum (UHV) conditions are being carried out. The work aims to characterize the reaction processes concerned at the molecular level and is motivated to provide information on ways to suppress the effect. The adsorption behavior of the gas molecules of interest (SO2, NH3 and H2O) was mapped, and a laser irradiation system was set-up and tested. The primary technique employed was Temperature-Programmed Desorption (TPD), commonly employed to probe the strength of the adsorbate-surface interactions. It consists of applying a constant heating ramp to the surface and detecting the desorbed species (on the basis of mass-to-charge (m/z) ratio) by a Quadrupole Mass Spectrometer (QMS), as depicted in the inset of Figure 3.31. TPD profiles of SO2 (m/z=64) desorbing from a quartz (0001) surface following exposure (at 93 K) to sub-monolayer coverage of SO2 and NH3, are shown in Figure 3.31. Two dosing orders were applied: SO2 first (black curve) and NH3 first (red curve). A new and more stable desorption feature is observed at 192 K in the TPD spectra of SO2 upon co-adsorption with NH3, indicating a stronger interaction at the surface. This can be seen by comparison with the TPD curve of singly-dosed SO2 (dotted curve). Of the coverage range shown here, there is no obvious difference as a result of the order in which gases are introduced to the surface. The appearance of a new more stable complex is consistent with a strong chemical interaction between the SO2 and NH3 molecules. The chemical interaction found here is considered a key finding in explaining the contamination observed at lens surfaces of lithography equipment and further verifications are scheduled. Figure 3.31: TPD-spectra of SO2 (m/z 64) from a quartz(0001) surface after exposure at 93 K. The dotted curve depicts the TPD of singly-adsorbed SO2 (0.2 L). The solid curves are for the SO2-NH3 co-adsorbate system, with the order of dosing varied. The heating rate was 2 Ks-1. The higher desorption temperature of SO2 from the co-adsorbates systems indicates a more strongly bound configuration at the surface. The inset depicts the basics principle of the TPD technique – applying a constant temperature ramp to a surface results in desorption of species that are detected by means of a QMS. 3.7.4 Hyper-thermal nitrogen interaction with Ag(111) surfaces This project aims to investigate fundamental aspects of particle-surface interactions. Studying such interactions under highly-defined and well-controlled conditions (ultra-high vacuum (UHV): ambient pressure less than 10-9 mbar) allows us to derive very specific and detailed information on the physical and chemical processes that occur at the atomic scale. Apart from improving our understanding of specific systems, these experiments allow the validation and improvement of computational models. The primary particle source in the current set-up is an expanding thermal plasma (cascaded arc), which is attached to a UHV vessel via a differentially-pumped beamline. The source can be used to produce high intensity beams of hyper-thermal (1-10 eV) neutrals including atoms and molecules, which are then directed onto well-defined sample surfaces. Experimentally, this is one of the more difficult-to-attain energy regimes. In addition, chemical reaction on a surface sometimes involves radicals that are the result of molecular dissociation on the surface. Using the current beam, we have pre-dissociated molecules and are thus able to directly investigate the radical-surface interaction. Figure 3.32: Scattered distributions of Ar and N2 from a clean Ag(111) surface. The inset shows a magnification of the enhanced N2 signal along the sample normal, which is due to N atoms from the incident beam recombining at the surface. Work in 2008 has concentrated on the mixed molecular and atomic nitrogen beams generated in the cascaded arc plasma source. The interaction of hyper-thermal nitrogen with a Ag(111) surface was probed. Scattering results for N2 from the Ag(111) surface are shown in Figure 3.32. For comparison, the results for scattering of Ar with a similar kinetic energy are also shown. The scattered distributions peak at an outgoing angle around 60° (specular) for both N2 and Ar. In addition, there is an increase in the N2 angular distribution in intensity at an outgoing angle of 0° (normal to the surface), which is absent from the Ar distribution. This is attributed to ‘instantaneous’ recombination of the impinging N atoms from the incident beam on the surface. N2 molecules formed by surface recombination have a strong preference to desorb along the surface normal direction.