N. Böwering

Extreme ultraviolet emission spectra of highly ionized xenon and their comparison with model calculations

Xenon spectra involving emission from ion species of Xe7+ to Xe12+ were analyzed for a plasma focus discharge developed for extreme ultraviolet lithography. Low and higher resolution spectra were recorded in the 8–21 nm wavelength region for different operating conditions and different He–Xe gas mixtures. The spectra have been compared with Hartree–Fock calculations. The modeling included the distribution of the various xenon ion levels at a given electron equilibrium temperature and plasma opacity effects. Spectral analysis showed that the 4d–5p transition arrays are fairly well separated in wavelength for the ions Xe8+ to Xe11+. Good agreement between experiment and calculations was obtained for line positions and intensities, in particular, for the wavelength region at around 13.5 nm.

LPP EUV source development for HVM

This paper provides a detailed review of development progress for a laser-produced-plasma (LPP) extreme-ultra-violet (EUV) source with performance goals targeted to meet joint requirements from all leading scanner manufacturers. We present the latest results on drive laser power and efficiency, source fuel, conversion efficiency, debris mitigation techniques, multi-layer-mirror coatings, collector efficiency, mass-limited droplet generation, laser-to-droplet targeting control, and system use and experience. The results from full-scale prototype systems are presented. In addition, several smaller lab-scale experimental systems have also been constructed to test specific physical aspects of the light sources. This report reviews the latest experimental results obtained on these systems with a focus on the topics most critical for a source intended for use in high volume manufacturing (HVM). LPP systems have been developed for light-sources applications to enable EUV scanners for optical imaging of circuit features at nodes of 32 nm and below on the international technology roadmap for semiconductors (ITRS). LPP systems have inherent advantages over alternate source types, such as discharge produced plasmas (DPP), with respect to power scalability, source etendue, collector efficiency, and component lifetime. The capability to scale EUV power with laser repetition rate and pulse energy is shown, as well as the modular architecture for extendability. In addition, experimental results of debris mitigation techniques and witness sample lifetime testing of coated multi-layer-mirrors (MLM) are described and used to support the useful lifetime estimation of a normal incidence collector. A roadmap to meet requirements for production scanners planned well into the next decade is also presented.

EUV discharge light source based on a dense plasma focus operated with positive and negative polarity

The application of a dense plasma focus pinch discharge as a light source for extreme ultraviolet (EUV) lithography is discussed. For operation with xenon gas, the radiation emitted at around 13.5 nm is analysed with temporal, spectral or spatial resolution. We describe and compare the operating characteristics and plasma dynamics of the device when energized at positive and negative polarity of the charging voltage. The thermal load distribution, heat deposition and wear of the electrodes are measured and compared for both configurations. High-repetition rate burst mode data show characteristic transients. Source size data are also obtained when tin powder is used as the target element. More favourable performance characteristics were generally obtained for operation of the pinch discharge at negative polarity. (Some figures in this article are in colour only in the electronic version)

Radiation induced degradation and surface charging of organic thin films in ultraviolet photoemission spectroscopy

Abstract The effect of vacuum ultraviolet radiation on the valence electronic structure of the electroluminescent organic materials p-sexiphenyl (6P) and tris-(8-hydroxy quinoline) aluminum (Alq 3 ) was investigated by ultraviolet photoemission spectroscopy (UPS). The intense radiation of an undulator at the storage ring BESSY II (Berlin) caused a loss of conjugation in 6P, evidenced by a decrease in intensity of delocalized π-orbitals in the UPS spectra. Depending on the degree of film degradation, surface charging was observed for both materials. It is shown that by illuminating the sample with laser light that can be absorbed by the organic/metal substrate system the surface charging could be compensated. Thus, the generation of free charge carriers by optical means appears to be a useful substitute for the use of an electron flood-gun in photoemission experiments, whenever sensitive samples could suffer from irradiation with electrons.

Photoionization of jet‐cooled HI with coherent vacuum ultraviolet radiation: Evidence for Hund’s case (e)

In the range of 83 500–89 200 cm−1, the photoionization spectrum of HI is strongly influenced by autoionization of Rydberg series converging to the higher lying 2Π1/2 component of the spin–orbit split 2Π ionic ground state. The photoelectron yield spectrum, exhibiting the various autoionization processes, has been measured on a rotationally cooled sample (Trot∼13 K), with a spectral resolution of the exciting vacuum ultraviolet radiation of 0.5 cm−1. Using a multichannel quantum‐defect theory treatment explicitly including rotation, we calculate spin–orbit and rotational autoionization fine structure in the spectrum. In order to find a qualitative agreement of theory and experiment, the Rydberg states of HI have to be described in terms of a transition in angular momentum coupling cases, going from Hund’s case (c) to case (e).

Performance and Scaling of a Dense Plasma Focus Light Source for EUV Lithography.

A commercially viable light source for EUV lithography has to meet the large set of requirements of a High Volume Manufacturing (HVM) lithography tool. High optical output power, high-repetition rate, long component lifetime, good source stability, and low debris generation are among the most important parameters. The EUV source, being developed at Cymer, Inc. is a discharge produced plasma source in a dense plasma focus (DPF) configuration. Promising results with Xe as a working gas were demonstrated earlier. To scale the DPF parameters to levels required for HVM our efforts are concentrated on the following areas: (1) thermal engineering of the electrodes utilizing direct water cooling techniques; (2) development of improved pulsed power systems for > 4 kHz operation; (3) study of erosion mechanisms for plasma facing components; (4) development of efficient debris mitigation techniques and debris shields; (5) studies of plasma generation and evolution with emphasis on improving conversion efficiency and source stability; (6) development of EUV metrology techniques and instrumentation for measurements of source size; and (7) development of an optimized collector optic matched to our source parameters. In this paper, we will present results from each of these key areas. The total in-band EUV output energy now approaches 60 mJ/pulse into 2πsr and the conversion efficiency has been increased to near 0.5 %. Routine operation at 4 kHz in burst-mode, and continuous operation at 1 kHz has been demonstrated. Improved at-wavelength source metrology now allows a determination of EUV source size utilizing imaging, and monitoring of key features of the spectrum on a pulse-to-pulse basis. With effective suppression of debris generated from the anode by several orders of magnitude, UV/EUV-catalyzed carbon growth now presents the limit in producing a clean source.

LPP source system development for HVM

Laser produced plasma (LPP) systems have been developed as a viable approach for the EUV scanner light sources to support optical imaging of circuit features at sub-22nm nodes on the ITRS roadmap. This paper provides a review of development progress and productization status for LPP extreme-ultra-violet (EUV) sources with performance goals targeted to meet specific requirements from leading scanner manufacturers. The status of first generation High Volume Manufacturing (HVM) sources in production and at a leading semiconductor device manufacturer is discussed. The EUV power at intermediate focus is discussed and the lastest data are presented. An electricity consumption model is described, and our current product roadmap is shown.

Rotationally resolved single-photon ionization of HCl and DCl

The rotationally resolved photoionization yields of jet‐cooled HCl and DCl have been measured in the energy range between their spin–orbit split 2Π ionic thresholds. For single‐photon excitation, narrow‐band vuv radiation is generated by resonant frequency mixing. The spectra are complex due to an interaction of autoionizing resonances belonging to series converging to different rotational states of the ion core. This is taken into account using the multichannel quantum defect theory in a treatment capable to handle a transition in angular momentum coupling from Hund’s case (c) to case (e). Even for Rydberg orders n≳35 the experimental data cannot be explained using a pure case (e) picture.

Orientation dependence of the sticking probability of NO at Ni(100)

In a molecular beam experiment we have measured the sticking probability of NO at Ni(100) as a function of coverage and molecular orientation. The NO molecules in the 2∏1/2|1/2, 1/2, 1/2> ground state are state selected by use of an electrostatic hexapole. The orientation was produced by a homogeneous electric field. The initial sticking probability is found to be higher for N-end collisions. The asymmetry of the initial sticking probability was found to depend linearly on the degree of molecular orientation. This effect is of the order of 70% with respect to the degree of orientation. The orientation dependence of the sticking probability as a function of coverage shows that the asymmetry is nearly constant up to saturation coverage. These results indicate that a simple picture of the adsorption process including only trapping into a ‘precursor’ state, and neglecting direct chemisorption, cannot explain the adsorption kinetics of NO at Ni(100).

Laser produced plasma light source for EUVL

This paper is devoted to the development of laser produced plasma (LPP) EUV source architecture for advanced lithography applications in high volume manufacturing of integrated circuits. The paper describes the development status of subsystems most critical to the performance to meet scanner manufacturer requirements for power and debris mitigation. Spatial and temporal distributions of the radiation delivered to the illuminator of the scanner are important parameters of the production EUV tool, this paper reports on these parameters measured at the nominal repetition rate of the EUV source. The lifetime of the collector mirror is a critical parameter in the development of extreme ultra-violet LPP lithography sources. Deposition of target material and contaminants as well as sputtering and implantation of incident particles can reduce the reflectivity of the mirror coating substantially over time during exposure even though debris mitigation schemes are being employed. We report on progress of life-test experiments of exposed 1.6sr collectors using a Sn LPP EUV light source. The erosion of MLM coating is caused mostly by the high-energy ions generated from the plasma. In this manuscript the ion distribution measured at small (14 degree) and medium (45 degree) angles to the laser beam are presented. The measurements show that the chosen combination of the CO2 laser and Sn droplet targets is characterized by fairly uniform angular ion energy distribution. The maximum ion energy generated from the plasma is in the range of 3-3.5 keV for all incident angles of the collector. The measured maximum energy of the ions is significantly less than that measured and simulated for plasmas generated by short wavelength lasers (1 μm). The separation of ions with different charge states was observed when a retarding potential was applied to the Faraday Cup detector.