Publications

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.

Journal of Physics: Conference Series 58, 391-394 (2007).

Morris, O., Hayden, P., Dunne, P., O’Reilly, F., O’Sullivan, G., Sokell, E., Antonsen, E. L., Srivastava, S. N., Thompson, K. C., Ruzic, D. N.

We have performed time of flight (TOF) analysis to determine the intensity of ion distribution from tin based plasma for a range of charged tin ions (Sn¹⁺-Sn¹⁰⁺). A Nd:YAG laser operating at 1064 nm with a full width at half maximum pulse duration of 5.2 ns was used to create the plasma under vacuum with a base pressure of 10⁻⁶ Torr. The plasma formation occurred on a custom made optical system, which could be rotated with respect to the detector so TOF analysis could be preformed at various angles of emission, while maintaining a normal angle of incidence for the laser pulse with respect to the target. The detector used was an energy sector analyser (ESA), which is a well-characterised diagnostic capable of measuring ion energy and discriminating by charge state. Analysis was performed on ions of various charge states, with energy/charge state ratios ranging from 3 keV to 50 eV, for angles of emission from the plasma ranging from 90 to 15 degrees.

J. Microlithography, Microfabrication, and Microsystems, 5(3), 033007(1-11) (2006).

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

A critical issue for EUV lithography (EUVL) is the minimization of collector degradation from intense plasma erosion, debris deposition, and hydrocarbon/oxide contamination. Collector optics reflectivity and lifetime heavily depend on surface chemistry and interactions between fuels and various mirror materials, such as silicon, in addition to high-energy ion and neutral particle erosion effects. As a continuation of our prior investigations of discharge-produced plasma (DPP) and laser-produced plasma (LPP) Xe plasma interactions with collector optics surfaces, the University of Illinois at Urbana–Champaign (UIUC) has analyzed collector samples before and after exposure in a Sn-upgraded Xtreme Technologies EUV source. Sn DPP postexposure characterization includes multiple samples, Si/Mo multilayer film with normal incidence, 200-nm-thick Ru film with grazing incidence, as well as a Gibbsian segregated (GS) Mo-Au alloy developed on silicon using a dc dual-magnetron cosputtering system at UIUC for enhanced surface roughness properties, erosion resistance, and self-healing characteristics to maintain reflectivity over a longer period of mirror lifetime. Surface analysis draws heavily on the expertise of the Center for Microanalysis of Materials at UIUC, and investigates mirror degradation mechanisms by measuring changes in surface roughness and film thickness as well as analysis of deposition of energetic Sn ions, Sn diffusion, and mixing of multilayer. Results from atomic force microscopy (AFM) and auger electron spectroscopy (AES) measurements show exposure effects on surface roughness and contamination. The best estimates of thickness and the resultant erosion measurements are obtained from scanning electron microscopy (SEM). Deposition, diffusion, and mixing effects are analyzed with depth profiles by AES. Material characterization on samples removed after varying exposure times in the XTS source can identify the onset of different degradation mechanisms within each sample. These samples are the first fully characterized materials to be exposed to a Sn-based DPP EUV source. Several valuable lessons are learned. First, hot mirrors exposed to SnCl₄ gas will cause decomposition of the gas and build up a contamination layer on the surface. Second, erosion is mitigated to some extent by the simultaneous deposition of material. Third, and most important, Gibbsian segregation works and a thin Au layer is maintained during exposure, even though overall erosion is taking place. This phenomenon could be very useful in the design of a collector optics surface. In addition, we present Sn DPP collector erosion mechanisms and contamination and provide insight into plasma-facing optics lifetime as high-volume manufacturing (HVM) tool conditions are approached.

J. Microlithography, Microfabrication, and Microsystems, 5(3), 033006(1-11) (2006).

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

The University of Illinois at Urbana-Champaign (UIUC) and several national laboratories are collaborating on an effort to characterize Xe plasma source exposure effects on extreme ultraviolet (EUV) collector optics. A series of mirror samples provided by SEMATECH were exposed for 10 million shots in an Xtreme Technologies XTS 13-35 commercial EUV discharge produced plasma (DPP) source at UIUC and 500,000 shots at the high-power TRW laser produced plasma (LPP) source at Sandia National Laboratory, Livermore (SNLL). Results for both pre- and post-exposure material characterization are presented for samples exposed in both facilities. Surface analysis performed at the Center for Microanalysis of Materials at UIUC investigates mirror degradation mechanisms by measuring changes in surface roughness, texture, and grain sizes as well as analysis of implantation of energetic ions, Xe diffusion, and mixing of multilayers. Materials characterization on samples removed after varying exposure times in the XTS source identify the onset of different degradation mechanisms within each sample over 1 million to 10 million shots. Results for DPP-exposed samples for 10 million shots in the XCEED (Xtreme Commercial EUV Emission Diagnostic) experiment show that samples are eroded and that the surface is roughened with little change to the texture. Atomic force microscopy (AFM) results show an increase in roughness by a factor of 2 to 6 times, with two exceptions. This is confirmed by x-ray reflectivity (XRR) data, which shows similar roughening characteristics and also confirms the smoothening of two samples. Scanning electron microscopy (SEM) pictures showed that erosion is from 5 to 54  nm, depending on the sample material and angle of incidence for debris ions. Finally, microanalysis of the exposed samples indicates that electrode material is implanted at varying depths in the samples. The erosion mechanism is explored using a spherical energy sector analyzer (ESA) to measure fast ion species and their energy spectra. Energy spectra for ions derived from various chamber sources are measured as a function of the argon flow rate and angle from the centerline of the pinch. Results show creation of high-energy ions (up to E=13  keV). Species noted include ions of Xe, Ar (a buffer gas), and various materials from the electrodes and debris tool. The bulk of fast ion ejection from the pinch includes Xe⁺, which maximizes at ~8  keV, followed by Xe²⁺, which maximizes at ~5  keV. Data from samples analysis and ESA measurements combined indicate mechanism and effect for debris-optic interactions and detail the effectiveness of the current debris mitigation schemes.

J. Vac. Sci. Technol., B, 24(3), 1071-1087 (2006).

Ning, Li, Brenner, P. W., Ruzic, D. N.

A zero-order semiempirical model has been developed for chemically enhanced physical vapor deposition (CEPVD), a recently developed hybrid approach to film deposition offering the step coverage of chemical vapor deposition while maintaining film quality similar to films produced by ionized physical vapor deposition (IPVD). CEPVD is done by introducing a chemical precursor to the substrate during IPVD. A synergistic effect between the two processes results in which the high energy ions from IPVD aid in the decomposition of the precursor. The precursor then provides film deposition on surfaces that are not easily impacted by IPVD’s directional ions. The model stems from knowledge of reactive sputtering and plasma-enhanced chemical-vapor deposition processes as well as results acquired from CEPVD experiment. It focuses on the Ta–N material system since Ta/TaN is widely used as a diffusion barrier in Cu damascene processing. Processing parameters are correlated with the target and film surface coverage by Ta, TaN, and organic sites, from which one can predict the operation mode, either metallic or poison, and the film elemental composition. The organic by-products accounting for the detection of carbon on the substrate by Auger electron spectroscopy analysis and poisoning of the target during the processing are categorized into nonvolatile products (OR1) and volatile products (OR2) in a lump-sum assumption. Electron impact, H reduction and ion bombardment are considered as the enhancing mechanisms between the physical and chemical components and included as the reactants of the chemical reactions. Simulation results compare favorably with the experimental data.

J. Nucl. Mater., 350, 101-112 (2006).

Nieto, M., Ruzic, D. N., Olczak, W., Stubbers, R.

Due to its low atomic number, low sputtering yield, high sputtered ion fraction and excellent thermal properties, liquid lithium has been proposed as a potential candidate for advanced plasma-facing components (PFC). Using a liquid material opens the possibility of a continuously flowing, self-regenerating plasma-facing surface with a small residence time. This would allow such component to handle very high heat loads that are expected. There are, however, multiple unanswered questions regarding how such a liquid PFC would interact with the plasma in the reactor. The issue of particle control is critical, and it can be a factor to determine the feasibility of these advanced concepts. Hydrogen and helium are important in this regard: hydrogen transport by a flowing PFC impacts the reactor fuel recycling regime and tritium inventory; helium transport can help quantify ash removal by the flowing PFC. The flowing liquid-metal retention experiment (FLIRE) was built at the University of Illinois to answer some of the questions regarding particle transport by flowing liquid films exposed to plasmas. Experimental results regarding helium transport by a flowing lithium film after irradiation with an energetic He ion beam are presented in this work. Retained fraction values up to 2% were measured for the experimental conditions, and the retention was found to increase linearly with implanted ion energy. A pure diffusion model was used to describe the helium transport by the Li film, and it was found that such model predicts a diffusion coefficient of (2.8 ± 0.6) × 10⁻¹¹ m²/s, based on the experimental retention measurements. Preliminary evidence of long-term trapping of helium will also be presented.

J. Appl. Phys., 99, 063301 (2006).

Antonsen, E. L., Thompson, K. C., Hendricks, M. R., Alman, D. A., Jurczyk, B. E., Ruzic, D. N.

An XTREME Technologies XTS 13-35 extreme ultraviolet (EUV) light source creates a xenon z pinch that generates 13.5  nm light. Due to the near x-ray nature of light at this wavelength, extremely smooth metal mirrors for photon collection must be employed. These are exposed to the source debris. Dissolution of the z-pinch gas column results in high-energy ion and neutral release throughout the chamber that can have adverse effects on mirror surfaces. The XTREME commercial EUV emission diagnostic chamber was designed to maximize diagnostic access to the light and particulate emissions from the z pinch. The principal investigation is characterization of the debris field and the erosive effects on optics present. Light emission from the z pinch is followed by ejection of multiply charged ions and fast neutral particles that make up an erosive flux to chamber surfaces. Attenuation of this erosive flux to optical surfaces is attempted by inclusion of a debris mitigation tool consisting of foil traps and neutral buffer gas flow. Characterization of the z-pinch ejecta is performed with a spherical sector energy analyzer (ESA) that diagnoses fast ion species by energy-to-charge ratio using ion time-of-flight (ITOF) analysis. This is used to evaluate the debris tool’s ability to divert direct fast ions from impact on optic surfaces. The ITOF-ESA is used to characterize both the energy and angular distribution of the direct fast ions. Xe⁺ up to Xe⁺⁴ ions have been characterized along with Ar⁺ (the buffer gas used), W⁺, Mo⁺, Si⁺, Fe⁺, and Ni⁺. Energy spectra for these species from 0.5 up to 13  keV are defined at 20° and 30° from the pinch centerline in the chamber. Results show a drop in ion flux with angular increase. The dominant species is Xe⁺ which peaks around 8  keV. Ion flux measured against buffer gas flow rate suggests that the direct fast ion population is significantly attenuated through increases in buffer gas flow rate. This does not address momentum transfer from scattered ions or fast neutral particles. These results are discussed in the context of other investigations on the effects of total particle flux to normal incidence mirror samples exposed for 1×10⁷ pulses. The samples (Si/Mo multilayer with Ru capping layer, Au, C, Mo, Pd, Ru, and Si) were exposed to the source plasma with 75% argon flow rate in the debris mitigation tool and surface metrology was performed using x-ray photoelectron spectroscopy, atomic force microscopy, x-ray reflectivity, and scanning electron microscopy to analyze erosion effects on mirrors. These results are compared to the measured direct ion debris field.

Microelectronics Engineering, 83, 476-484 (2006).

Thompson, K. C., Antonsen, E. L., Hendricks, M. R., Jurczyk, B. E., Williams, M., Ruzic, D. N.

A commercial EUV light source is currently used in the MS-13 EUV Micro Exposure Tool (MET) produced by Exitech Ltd. The source uses a xenon z-pinch discharge to produce 13.5 nm light intended for use in extreme ultraviolet lithography (EUVL). During operation, an erosive flux of particles is ejected from the pinch plasma, contributing to limitations in the lifetime of nearby collector optics. A diagnostic chamber is presented that permits characterization of the debris fields present, exposure of optical samples, and evaluation of debris mitigation techniques. Available diagnostics include a Faraday cup, a spherical sector energy analyzer (ESA), and a EUV photodiode. This paper details the chamber design and initial results of source characterization. Faraday cup analysis shows that the maximum theoretical ion energy is 53 keV, ESA measurements show the presence of Xe⁺, Xe²⁺, Ar⁺, W⁺, and Mo⁺ ions, and microanalysis of exposed mirror samples is used to show the erosive effects of plasma exposure.

Nucl. Instrum. and Meth. B, 239, 347-355 (2005).

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

Sputtering does not vary strongly with target temperature for most solid materials. Sputtering yield measurements of liquid lithium self-sputtering for energies of 200–1000 eV at oblique incidence, however, show an enhancement near an order of magnitude as the temperature of the liquid lithium target is increased from near its melting point at 473 K up to about 690 K (1.5T_m) after accounting for thermal evaporation. A new model that couples both the near-surface cascade of the hot liquid metal and the effect of multiple interaction mechanisms on the binding of the emitted particle explains this anomalous erosion enhancement with target temperature. The model, has been validated using a new hybrid computational tool named MD-TRIM (molecular dynamics in transport of ions in matter), at 473 K and 653 K and compared to experimental data. MD-TRIM consists of molecular dynamics and binary collision approximation (BCA) codes, TRIM (transport of ions in matter). The MD-TRIM code was designed to aid in understanding erosion enhancement mechanisms of lithium self-bombardment sputtering at low bombarding energies with a rise in target temperature. MD calculations alone do not show the sputtering enhancement with temperature rise due to an inadequate surface potential model.