Category: Journal Articles

J. of Nuclear Materials, 378, 105-109 (2008).

Jawarski, M. A., Laua, C. Y., Urbansky, D. L., Malfa, M. B., Gray, T. K., Neumann, M. J., Ruzic, D. N.

Recent experiments with liquid lithium in magnetically confined plasmas have demonstrated improved plasma performance. These results have led researchers working on the National Spherical Torus Experiment to consider using a porous molybdenum foam and liquid lithium composite as a future liquid lithium divertor. In order to better understand the properties of this composite material, a small experimental apparatus was constructed at the University of Illinois to test lithium wetting uptake into the porous material. We report here results of the wetting behavior of a porous molybdenum foam with liquid lithium. Based on these observations, a simple model was used to estimate the thermal properties of the lithium infused porous material. Finally, the results of water-bath cleaning tests of the porous metal after lithium exposure are shown.

Appl. Opt. 47, 2443-2451 (2008).

Qiu, H., Srivastava, S. N., Thompson, K. C., Neumann, M. J., Ruzic, D. N.

Successful implementation of extreme ultraviolet (EUV) lithography depends on research and progress toward minimizing collector optics degradation from intense plasma erosion and debris deposition. Thus studying the surface degradation process and implementing innovative methods, which could enhance the surface chemistry causing the mirrors to suffer less damage, is crucial for this technology development. A Mo-Au Gibbsian segregation (GS) alloy is deposited on Si using a dc dual-magnetron cosputtering system and the damage is investigated as a result of time dependent exposure in an EUV source. A thin Au segregating layer is maintained through segregation during exposure, even though overall erosion in the Mo-Au sample is taking place in the bulk. The reflective material, Mo, underneath the segregating layer is protected by this sacrificial layer, which is lost due to preferential sputtering. In addition to theoretical work, experimental results are presented on the effectiveness of the GS alloys to be used as potential EUV collector optics material.

J. Vac. Sci. Technol., 26(3), 389-398 (2008).

Shin, H., Srivastava, S. N., Ruzic, D. N.

Tin (Sn) has the advantage of delivering higher conversion efficiency compared to other fuel materials (e.g., Xe or Li) in an extreme ultraviolet (EUV) source, a necessary component for the leading next generation lithography. However, the use of a condensable fuel in a lithography system leads to some additional challenges for maintaining a satisfactory lifetime of the collector optics. A critical issue leading to decreased mirror lifetime is the buildup of debris on the surface of the primary mirror that comes from the use of Sn in either gas discharge produced plasma (GDPP) or laser produced plasma (LPP). This leads to a decreased reflectivity from the added material thickness and increased surface roughness that contributes to scattering. Inductively coupled plasma reactive ion etching with halide ions is one potential solution to this problem. This article presents results for etch rate and selectivity of Sn over SiO₂ and Ru. The Sn etch rate in a chlorine plasma is found to be much higher (of the order of hundreds of nm/min) than the etch rate of other materials. A thermally evaporated Sn on Ru sample was prepared and cleaned using an inductively coupled plasma etching method. Cleaning was confirmed using several material characterization techniques. Furthermore, a collector mock-up shell was then constructed and etching was performed on Sn samples prepared in a Sn EUV source using an optimized etching recipe. The sample surface before and after cleaning was analyzed by atomic force microscopy, x-ray photoelectron spectroscopy, and Auger electron spectroscopy. The results show the dependence of etch rate on the location of Sn samples placed on the collector mock-up shell.

J. Applied Physics, 103, 044901-4 (2008).

Castano, D., Aghazarian, M., Ruzic, D. N.

Preliminary evidence of enhanced etching of rhenium by XeF₂ under the influence of an electric field (3.36  GV/m) is presented. Scanning electron microscope photographs of sharp rhenium tips show etching of at least 0.40  µm ±0.07 in 32  min at the point of maximum electric field, indicating a field enhanced etching rate of 13  nm/min ±2. A control experiment shows a maximum spontaneous etching of rhenium by XeF₂ of 0.1  µm ±0.07 in 30  min, indicating a maximum possible spontaneous etching rate of rhenium by XeF₂ of 3  nm/min ±2. The spontaneous rate of tungsten by XeF₂ reported in the literature is 0.2  nm/min.

Physical Review B, 76, 205434 (2007).

Allain, J. P., Coventry, M. D., Ruzic, D. N.

The lithium sputtering yield from lithium and tin-lithium surfaces in the liquid state under bombardment by low-energy, singly charged particles as a function of target temperature is measured by using the Ion-surface Interaction Experiment facility. Total erosion exceeds that expected from conventional collisional sputtering after accounting for lithium evaporation for temperatures between 200 and 400  °C. Lithium surfaces treated with high-fluence D atoms are bombarded by H⁺, D⁺, He⁺, and Li⁺ at energies between 200 and 1000  eV and 45° incidence. Erosion measurements account for temperature-dependent evaporation. For example, 700  eV He⁺ particles bombarding the D-treated liquid Li surface at room temperature result in a sputter yield of 0.12 Li/ion and at temperatures ~2.0T_m (where T_m is the melting temperature of the sample), a yield near and above unity. The enhancement of lithium sputtering is observed to be a strong function of temperature and moderately on particle energy. Bombardment of a low-vapor-pressure lithium alloy (0.8 Sn-Li), used for comparison, also results in nonlinear rise of lithium erosion as a function of temperature. Measurements on both pure liquid Li and the alloy indicate a weak dependence with surface temperature of the secondary ion-induced secondary ion emission. Treatment of liquid Li surfaces with D, yields reduced sputtering under He⁺ impact by a factor of 5–6 when measured at room temperature due to preferential sputtering effects.

J. of Applied Physics, 102, 023301 (2007).

Srivastava, S. N., Thompson, K. C., Antonsen, E. L., Qiu, H., Spencer, J. B., Papke, D., Ruzic, D. N.

Next generation lithography to fabricate smaller and faster chips will use extreme ultraviolet (EUV) light sources with emission at 13.5  nm. A challenging problem in the development of this technology is the lifetime of collector optics. Mirror surfaces are subjected to harsh debris fluxes of plasma in the form of ions, neutrals, and other radiation, which can damage the surface and degrade reflectivity. This manuscript presents the measurement of debris ion fluxes and energies in absolute units from Xe and Sn EUV sources using a spherical sector ion energy analyzer. Experimentally measured erosion on Xe exposed samples is in good agreement with predicted erosion. This result allows prediction of erosion using measured ion fluxes in experiment. Collector optic lifetime is then calculated for Xe and Sn sources without debris mitigation. Lifetime is predicted as 6  h for Xe EUV sources and 34  h for Sn EUV sources. This result allows calculation of expected collector optic lifetimes, which can be an important tool in optimizing source operation for high volume manufacturing.

Journal of Micro/nanolithography Mems and Moems, 6:2, 23005 (2007).

Neumann, M. J., Defrees, R. A., Qiu, H., Ruzic, D. N., Khodykin, O., Ershov, A., Bristol, R. L.

One of the critical issues within extreme ultraviolet lithography is mirror lifetime and the degradation due to debris from the pinch. This research investigated and showed the efficacy of using a helium secondary plasma and heat for removal of Li debris from collecting on the surface of collector optics. A He helicon plasma, which minimizes self-biasing and sputtering, has good extreme ultraviolet (EUV) photon wavelength transmission and preferential sputtering of lithium compared to other collector optics material. Through the combined use of heating and a He secondary plasma, EUV collector sample surface roughness and surface composition was able to be maintained near as-received status. The use of the He secondary plasma while the collector optics sample is exposed to Li debris shows promise as an in situ cleaning process for collector optics and can extend the lifetime of collector optics.

J. Nuclear Materials, 363-365, 1032-1036 (2007).

Gray, T. K., Jaworski, M. A., Ruzic, D. N.

The ELM simulating plasma gun (ESP-gun) has been developed to study the effects of transient, blob-like plasmas on the plasma facing components of TOKAMAKs. ESP-gun utilizes a RF helicon plasma to pre-ionize a plasma column underneath a conical, θ-pinch coil, which is used to compress and eject plasmas. Measurements have been made of the existing RF plasma and the subsequent compressed plasma. A copper target was placed downstream of the θ-pinch, and its temperature rise was measured with respect to time. For modest argon plasmas, n_e 10¹⁸ m⁻³ and T_e 100 eV, the target temperature was observed to have an equivalent heat loading of up to 90 kJ/m². Given that the plasma density and temperature are low, it is believed that the target heat loading will scale linearly with plasma density such that plasmas of 10²⁰–10²¹ m⁻³ would reach target heat loading in excess of 1 MJ/m². A zero dimensional thermal model will be presented to estimate the expected target heat loading.

IEEE Transactions on Plasma Science, 35 (3), 606-613 (2007).

Ruzic, D. N., Thompson, K., Jurczyk, B., Antonsen, E. L., Srivastava, S. N., Spencer, J.

This paper studies the expanding plasma dynamics of ions produced from a 5J Z-pinch xenon light source used for extreme ultraviolet (EUV) lithography. Fast ion debris produced in such plasmas cause damage to the collector mirror surface. Because of the great degree of erosion and the change in surface roughness properties, the reflectivity of EUV light at 13.5 nm drops drastically. Reducing ion energies and stopping the ion flux are a potential solution toward the success of EUV lithography. Ion energies are measured in kiloelectronvolt range using a spherical sector electrostatic energy analyzer. Preliminary computational work indicates that the observed high energies of ions are probably resulting from Coulomb explosion initiated by pinch instability. Mixed fuel experiments are performed using a mixture of Xe, N₂, and H₂. The average energy of the expelled Xe ions is significantly decreased if the mobile lighter gas species are present in the main fuel. The magnitude of the Xe ion signal is reduced as well. This reduction in the quantity of heavy ions and their energy could greatly extend the lifetime of the collector optics used in EUV lithography.

J. Microlithography, Microfabrication, and Microsystems, 6(1), 013006 (2007).

Alman, D. A., Qiu, H., Spila, T., Thompson, K. C., Antonsen, E. L., Jurczyk, B. E., Ruzic, D. N.

Extreme ultraviolet (EUV) light sources with efficient emission at 13.5  nm are needed for next-generation lithography. A critical consideration in the development of such a source is the lifetime of collector optics. These experiments expose optics to a large flux of energetic particles coming from the expansion of the pulsed-plasma EUV source to investigate mirror damage due to erosion, layer mixing, and ion implantation. The debris ion spectra are analyzed using a spherical sector energy analyzer (ESA) showing ion energies of 2  to  13  keV, including Xe⁺-Xe⁺⁴, Ar⁺, W⁺, Mo⁺, Fe⁺, Ni⁺, and Si⁺. Microanalysis is performed on samples exposed to 10 million pulses, including atomic force microscopy (AFM), showing increased roughness for most exposed samples. Notably, a Mo–Au Gibbsean segregated alloy showed surface smoothing over this time frame, suggesting that the segregation worked in situ. TRIM predictions for ion implantation are consistent with ion debris measurements from the ESA. Finally, time exposures of samples from 2, 20, and 40 million pulses show an initial roughening with smoothing of the exposed samples at longer time frames. Constant erosion is demonstrated with the SEM. These analyses give an experimental account of the effects of the ion debris field on optic samples exposed to the EUV source.