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This guest editorial introduces and summarizes the Special Section on EUV Lithography for the 3-nm Node and Beyond

Extreme ultraviolet lithography (EUVL) is the leading technology being considered for printing circuits at the 32-nm node and below in a high-volume manufacturing (HVM) environment fab. In EUVL, a 13.5-nm-radiation wavelength generated by an EUV source is used to print circuits. Because light radiation is strongly absorbed at this wavelength, the entire EUVL scanner system must be in a vacuum environment, and all optics must be reflective, not refractive. Based on the HVM requirements of 100-wafer∕h throughput and other system requirements for optics, resist sensitivity, and overhead (Table 1.1), a power requirement of 115 W has been specified for HVM EUVL scanners. Besides power, EUV sources must meet additional specifications. The production-level requirements in Table 1.1 have been jointly agreed upon by major scanner manufacturers. Discharge-produced plasma (DPP) and laser-produced plasma (LPP) are the leading technologies for generating high-power EUV radiation at 13.5 nm. In both technologies, hot plasma of ≈20–50 eV of the chosen fuel material is generated, which produces EUV radiation. In DPP, magnetic pinching of low-temperature plasma generates the high-temperature plasma. In LPP, the target material is heated by a laser pulse to generate high-temperature plasma. Xenon, tin, and lithium are the fuel materials of choice for EUV sources. The cost-effective implementation of EUVL in HVM presents many technical challenges, of which the EUV source power has remained the greatest one until recently. In the fall of 2004, significant progress in EUV source power was reported at the EUVL Symposium in Miyazaki, Japan, making source power a lesser concern. The current challenges for implementing EUVL in HVM are listed in Table 1.2. Today worldwide, more than eight suppliers and consortia are working to develop high-power EUV sources for EUVL. In addition, some suppliers are working to develop low-power EUV sources that are finding applications in metrology to support EUVL. This chapter presents the status of high-power EUV source technology and summarizes the technical challenges that must be overcome to meet the specifications for high-power EUV sources in HVM.

This chapter gives an overview of LPP EUV source development work at Northrop Grumman Corporation (NGC). The chapter covers development of the laser module, xenon target, and overall system. The volume editor (V. Bakshi) prepared this chapter as a summary of information provided to him by NGC. Lasers for LPP EUV sources are expected to produce tens of kilowatts of high-pulse-rate, high-pulse-energy, short-pulse-width, near-diffraction-limited output. Such lasers will be focused onto a condensed jet of cryogenic xenon or tin targets to produce a plasma with sufficient temperature to generate EUV radiation. For the generation of the EUV-producing plasma, pulse widths of around 10 ns and pulse energies in the range of 0.5 to 1 J are required. High beam quality and low pointing error are required to maintain constant high intensity on the EUV source target so that the radiated EUV power and consequent exposure doses on the semiconductor wafer are uniform. Depending on the choice of target material, eventually pulse rates of at least 7500 Hz and laser powers of 10–30 kW will be required to ensure the required power collection at the intermediate focus (IF). In 1999, NGC constructed a 1700-W diode-pumped Nd:YAG phase-conjugated master oscillator-power amplifier (MOPA) laser, designated EUV-Alpha, which was used in a lithography testbed at Sandia Labs in Livermore (see Chapter 24 for further description). Later NGC built an EUV-Beta laser (Fig. 25.1) that produced 4500 W and was operated at NGC's EUV source development facility at Cutting Edge Optronics (CEO). The Beta laser, a modular design for better maintainability, was twice as efficient and had two-thirds the footprint of the Alpha laser. For this laser, NGC selected a MOPA architecture (Fig. 25.2) using stimulated Brillouin scattering (SBS) phase conjugation to compensate for aberrations, figure error, and thermal distortions in the Nd:YAG gain media. The output of a custom 12-W master oscillator (MO) was split in two with a polarizer and directed to two amplifier trains. Each amplifier train consisted of two diode-pumped zigzag slab amplifiers, image relay telescopes, shaping optics, and an SBS cell. After round trips through the two slab amplifiers, the two MO beams were brought to their full 750-W power in each train, and then polarization-combined for a total of 1500 W. In the Beta laser, there were three such 1500-W modules, which yielded a system total of 4500 W at 7500 Hz.

The development of a source bright enough for EUVL, possessing the required long-term stability within a reasonable cost, is forcing plasma physicists to address issues associated with dense plasmas that have up until now received scant attention. With both discharge plasmas and laser plasmas, addressing these issues has stimulated many new ideas and concepts for source design and measures associated with them. Although these new developments are currently directed toward satisfying the needs of next-generation lithography (NGL), it is likely they will also open new fields of application in the years ahead. Early on in the development of EUVL sources, when the required source power (2π sr, 2% bandwidth) was ∼10 W, we speculated that the so-called debris problem would be the most difficult to overcome. It was for this reason that we proposed the notion of the mass-limited target for the laser plasma source. In this scenario, the material supplied to the source should consist only of a mass sufficient to provide the minimum required number of atomic radiators.

This chapter presents a summary of plasma-surface interactions in electrode and condenser-optic materials in plasma pinch sources for EUV light generation, with special emphasis on DPPs. In DPP EUV devices, electrodes at the source are exposed to short (10–20-ns) high-intensity plasmas, leading to a variety of erosion mechanisms. Erosion of the electrodes is dictated by the dynamics of the plasma pinch for configurations such as the dense plasma focus (DPF), Z pinch, and capillary. The transient discharge deposits 1–2 J∕cm2 per pulse on electrode surfaces. Large heat flux is deposited at corners and edges, leading to enhanced erosion. Understanding of how particular materials respond to these conditions is part of the rigorous design of DPP electrode systems. Erosion mechanisms can include physical sputtering, current-induced macroscopic erosion, melt formation, and droplet and particulate ejection. Erosion at the surface is also governed by the dynamics of how a plasma can generate a vapor cloud, leading to a self-shielding effect, which results in ultimate protection of the surface bombarded. Determining which will dominate—either microscopic erosion mechanisms such as physical sputtering, or macroscopic mechanisms such as melt formation and droplet ejection—remains an open question in DPP electrode design. This is because such mechanisms are inherently dependent on the pinch dynamics and operation of the source. In addition to plasma-surface interactions in electrodes, for condenser optics, especially collector optics, erosion is due to fast ions and neutrals born in the plasma pinch, leading mainly to physical sputtering and other bombardment-induced mechanisms. If the surface is composed of more than one species, which is mostly the case, then radiation-induced and thermally activated effects govern the behavior of the surface and govern lifetime levels of the exposed material. Exposure includes debris from electrodes, high-energy ions and neutrals, highly charged ions (HCIs), background impurities, photon radiation [13.5-nm and out-of-band (OOB)], and redeposited eroded mirror material. Figure 35.1 presents an overview of DPP plasma-surface interaction modeling that can be complemented by materials testing. It shows the transient plasma region (region 1) to the left and the quiescent expanding plasma region (region 3) to the right where the collector optics is located. Region 2 comprises a number of debris mitigation schemes that can also be modeled and experimentally tested. A number of modeling simulators can couple regions 1–3 in a self-consistent manner. This can be useful in designing mirrors that are compatible with the debris mitigation schemes selected as well as the EUV fuel used at the source. This is especially important if alternative EUV radiator fuels are selected, such as Sn or Li. Designs at the source with alternative radiator fuels can be studied and their effect on mirrors assessed.

EUV lithography (EUVL) is the leading technology, beyond 193-nm-based optical lithography, for printing circuits at the 32-nm node and below in a high volume manufacturing (HVM) environment fab. In EUVL, a 13.5-nm radiation wavelength generated by an EUV source is used to print circuits. Because light is strongly absorbed at this wavelength, the entire EUVL scanner system must be in a vacuum environment, and all optics must be reflective instead of refractive. Based on the HVM requirements of 100 wafers per hour (WPH) throughput and other system requirements for optics, resist sensitivity, and overhead (see Sec. 3.2.3), a power requirement of 115 to 180 W, based on resist sensitivity, has been specified for HVM EUVL scanners. Besides power, EUV sources must meet additional specifications (Sec. 3.2.2). The production-level requirements for EUV sources (Table 3.1) have been jointly agreed upon by major EUVL scanner manufacturers. Table 3.2 shows the change in EUV source requirements over time. Discharge-produced plasma (DPP) and laser-produced plasma (LPP) are the leading technologies for generating high-power EUV radiation at 13.5 nm. In both technologies, hot plasma of ~30 eV of the chosen fuel material is generated, which produces EUV radiation. In DPP, magnetic pinching of low-temperature plasma generates the high-temperature plasma. In LPP, the target material is heated by a laser pulse to generate the high-temperature plasma. Today, tin (Sn) is the leading fuel for HVM-level EUV sources, with xenon (Xe) still being used to meet the lower power requirements of metrology sources, microexposure tools (METs), and alpha-level scanners.

Exposure of collector mirrors facing the hot, dense pinch plasma in plasma-based EUV light sources remains one of the highest critical issues of source component lifetime and commercial feasibility of EUV lithography technology. Studies at Argonne have focused on understanding the underlying mechanisms that hinder collector mirror performance under Sn exposure and developing methods to mitigate them. Both Sn ion irradiation and thermal evaporation exposes candidate mirrors tested (i.e., Ru, Rh and Pd) in the experimental facility known as IMPACT (Interaction of Materials with charged Particles and Components Testing). Studies have led to an understanding of how Sn energetic ions compared to Sn thermal atoms affect three main surface properties of the collector mirror: 1) surface chemical state, 2) surface structure and 3) surface morphology. All these properties are crucial in understanding how collector mirrors will respond to Sn-based EUV source operation. This is primarily due to the correlation of how variation in these properties affects the reflectivity of photons in the EUV spectral range of interest (in-band 13.5-nm). This paper discusses the first property and its impact on 13.5-nm reflectivity. Investigation in the IMPACT experiment has focused on Sn thermal and energetic particle exposure on collector mirrors (Ru, Pd and Rh) and its effect on mirror performance as a function of incident thermal flux, incident ion flux, incident angle and temperature. This is possible by a new state-of-the-art in-situ EUV reflectometry system that measures real time relative EUV reflectivity at 15-degree incidence and 13.5-nm during Sn exposure. These results are then compared to at-wavelength EUV reflectivity measurements using the newly upgraded NIST-SURF facility. Sn energetic ions at 1- keV and fluxes of about 1013 cm-2s-1 are used in conjunction with a moderate flux Sn evaporative source delivering Sn fluences ranging from 1015-1017 cm-2. The temperature of the mirror sample is locally varied between 25 and 200 C with the chemical state of the surface simultaneously monitored using X-ray photoelectron spectroscopy, and lowenergy ion scattering spectroscopy. Results demonstrate the balance between energetic and thermal Sn has on the total Sn surface fraction during exposure and its effect on the structural and reflective properties of the mirror surface.

Extreme ultraviolet lithography (EUVL) is the leading technology for patterning at the 32 nm technology node and be-yond. EUVL light at 13.5 nm is used to print circuits. This light is produced by heating fuel (Xe, Sn) in EUV sources to a very high temperature by using either magnetic compression or laser irradiation. Today EUV source power remains the number one concern for implementation of EUVL in high volume manufacturing. Over the last few years, much pro-gress has been made in EUV source performance and availability. Today, alpha level high power (~10 W) EUV sources have been integrated in alpha level EUVL scanners. Medium and low power EUV sources are used for in-house metrol-ogy and performance studies on EUV mask blanks, EUV masks, photoresists, and optical elements. These compact dis-charge sources with medium power in the range of 10-100 mW/sr/2% bandwidth and low power EUV tubes are being used by various RD today, these low power EUV sources are instrumental in allowing in-house R&D projects. In this paper, the latest status of high power EUV sources, low and medium power metrology sources, and some of their appli-cations are described.

High power lasers (1-10 ns pulse width, 10-20 kW) are needed for laser-produced plasma (LPP)-based EUV sources to support extreme ultraviolet (EUV) lithography. The requirements and status of high power laser technology will be reviewed.

Soon after the first explorations of the potential of EUVL, it became obvious that the wafer throughput of future EUVL systems, and thus the EUV source brightness, was a key factor in the industrial value of this new imaging technology. The need for plasma-based light sources of moderate temperature, though strongly confined in space and critically optimized for emission at 13.5 nm, implied the use of suitable diagnostics, well adjusted for the particular application. Especially in the early days of EUVL development, a variety of source diagnostics were being used by the different source developers, each having backgrounds in different research and technology areas. These diagnostics ranged from uncalibrated grating spectrographs and luminescent detectors to broadband detection systems, such as filtered calorimeters and photocathodes, each detector obviously having its own characteristics. The measurement units applied varied correspondingly, ranging from J·eV−1·J1−1 to percent. From the data collected with these devices, extrapolations were then made to the most relevant quantity, namely, the true EUV power within the bandwidth and the radiation acceptance angle of the total EUVL system. In order to allow a direct comparison of the performance of the different candidate EUV sources, in 2000 ASML sponsored a source metrology activity by FOM and Philips Research for which a portable, absolutely calibrated set of diagnostics was used. The approach was to assess each of the different sources with one single diagnostics set, circumventing any uncertainty introduced by the specific properties or calibration of the various metrology devices. In addition, the measurement procedure was standardized. Collecting data from sources on different locations worldwide, the activity became known as Flying Circus, or FC for short, and initially resulted in a full benchmark characterization of five candidate EUV light sources and the development of a common diagnostic standard. The follow-up project, Flying Circus 2 (FC2), sponsored by International SEMATECH, had the broader scope of further assessing the progress of EUV source performance, calibrating EUV source metrology equipment from different EUV source suppliers, and continuing the development of globally accepted EUV source measurement procedures. Nowadays, more then a dozen sources are characterized in this way, and the FC-type diagnostic has found permanent use at the majority of the source development laboratories. FC diagnostics are commercially offered by Scientec Engineering (www.scientec.nl). The general acceptance of the method and the continued program of on-site calibration using the portable FC equipment at the different source labs, as well as further-developed FC hardware, considerably assisted in the standardization of source performance representation.

We describe a source of 13.5 nm radiation, based on multi-kHz laser-plasmas created from tin-bearing micro-droplets that has a high probability of satisfying the requirements for EUVL, the next generation lithography for computer chip fabrication.

Summary form only given. Both laser and discharge produced plasma such as Z-pinch devices are being used as a light source for EUV lithography. A key challenge for discharge produced plasma (DPP) devices is achieving sufficient brightness to support the throughput requirements of high-volume manufacturing (HVM) lithography exposure tools. An integrated model for the description of hydrodynamics and optical processes in a DPP device has been developed and integrated into the HEIGHTS-EUV computer simulation package. Model development consisted of three main tasks: plasma evolution and MHD processes; detailed photon radiation transport, and physics of plasma/electrode interactions in DPP devices. Plasma flows have multidimensional character in pinch systems. Advanced numerical methods for the description of magnetic compression and diffusion in a cylindrical geometry are used in the HEIGHTS package. Radiation transport of both continuum and lines is taken into account with detailed spectral profiles in the EUV region. Radiation transport is solved using two different methods. Discharges using xenon and tin gases are simulated and compared. Benchmarking of EUV signals and plasma parameters in both laser and Z-pinch devices is presented

The 13 nm emission that results from laser plasmas created from tin targets, results from a milliard of transitions occurring in many ions of tin (Sn6+-Sn13+). Understanding the energy manifolds within these multiple states will further our ability to manipulate energy into the narrow emission band demanded by EUV Lithography. A combined experimental theoretical program is underway to measure and interpret the detailed EUV emission spectra from laser plasmas suitable for EUVL, particularly mass-limited droplet laser plasmas. We employ high resolution spectroscopy in the 2 - 60 nm wavelength regions to characterize the emission from the plasma. This is interpreted with the aid of combined hydrodynamic/ radiation transport computer models. The results of this study will have impact on the in-band EUV conversion efficiency, estimation of the out-of-band short-wavelength emission, and in the development of electron temperature plasma diagnostics.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The EUVL collector mirror reflectivity degradation can be measured as erosion of the mirror surface caused by the high energy ion emissions. Characterizing the ion emission permits the analysis of the mechanisms of erosion and provides the capability to reduce the high energy ion emission which directly reduces the erosion rate. The degradation can also be measured as deposition of particulate debris on the mirror surface. The debris particles have sizes of only a few nanometers. We have demonstrated that the use of electrostatic repeller fields mitigates large fraction of the particle transfer. Our microscopic tin-doped droplet target is a mass-limited target and is designed to limit the flux of uncharged particulate matter emanating from the target, with the eventual objective of only generating charged material. The latter then may be inhibited from degrading EUV optics with the use of electrostatic repeller fields and other mitigation schemes. We present tin-doped droplet target ion emission characteristics in terms of ion energy distribution obtained using our ion spectrometer. Extensive studies on particle generation by controlling plasma conditions and the repeller field effect on individual ion species and particles is also described.

Detailed understanding of the complex UTA emission from Xe and Sn laser plasmas is imperative to the development of efficient 13.5 nm sources for EUVL. We are developing a comprehensive theoretical modeling approach to these sources, utilizing state-of-the-art hydrodynamic and radiation transport plasma codes. These models are specifically applied to Xe and Sn-doped microscopic droplet targets laser-plasmas irradiated with nanosecond laser pulses. The plasma expansion models are compared to experimental determinations of the plasma electron density distributions. The output of the radiation transport code is used to interpret details of the spectral emission measured from these plasmas over a broad range of parameters.

Light sources based on laser plasmas using tin as target material are known to provide high conversion efficiency of laser power to emission in the 13.5 nm spectral region. In addition, laser plasmas produced from microscopic droplet targets enable the utilization of the mass-limited concept which minimizes the effect of target debris produced from the laser plasma interaction. By combining the mass-limited target concept and tin as the choice of target material, we are developing an extreme-ultraviolet (EUV) light source that can supply high power while remaining essentially debris-free. This source uses tin-doped microscopic droplet liquid targets that are generated at high-repetition rates (>30 kHz), which allows convenient upward power scaling when coupled with a high averaged-power laser. Detailed studies of the radiation from this source have been made using a precision Nd:YAG laser. Broad parametric studies of the conversion efficiency along with in-band spectroscopy of this EUV source have been performed. The parametric dependence of conversion efficiency is established based on measurements made by the Flying Circus diagnostic tool and a calibrated high-resolution flat-field spectrometer. These measurements have been independently confirmed by the Flying Circus 2 team.

Discharge produced plasma (DPP) devices are being used as a light source for Extreme Ultraviolet (EUV) Lithography. A key challenge for DPP is achieving sufficient brightness to support the throughput requirements of exposure tools for high-volume manufacturing lithography. An integrated model is being developed to simulate the environment of the EUV source and optimize the output of the source. The model describes the hydrodynamic and optical processes that occur in DPP devices. It takes into account plasma evolution and magnetohydrodynamic processes as well as detailed photon radiation transport. The total variation diminishing scheme in the Lax-Friedrich formulation for the description of magnetic compression and diffusion in a cylindrical geometry is used. Several models are being developed for opacity calculations: a collisional radiation equilibrium model, a self-consistent field model with Auger processes, and a non-stationary kinetic model. Radiation transport for both continuum and lines with detailed spectral profiles is taken into account. The developed models are being integrated into the HEIGHTS-EUV computer simulation package. Preliminary results of a numerical simulation of xenon gas hydrodynamics and EUV radiation output are presented for various plasma conditions.

Small plasma-pinch devices operating at a gas mixture of Xe and He with a frequency of 5-10 kHz and pulsed energy of 1-100 J are very promising sources of EUV radiation for lithography. A key issue in design of EUV sources is erosion of the pinch facing material under the hot Xe plasma and electric currents. Material erosion limits the lifetime of device components and thereby reduces the economical feasibility of these devices. Selection of high-resistant materials is critically important for development of future commercial EUV sources. Experiments are being carried out at plasma gun facilities in well-diagnosed and controlled conditions. The plasma gun is applied as a source of pulsed energetic Xe plasma capable of generating Xe plasma streams with a velocity 4 10 6 - 4 10 7 cm/s and duration of the plasma pulse 10-40 microseconds. Xenon plasma stream velocity of 4-10 10 6 is sufficient to obtain plasma temperture of 30-50 eV, i.e., typical for pinch EUV devices. The formation of plasma could makes possible to study erosion and surface damage induced by particles and radiation of Xe plasma at these temperatures. Initial results of material testing by Xe plasma particles are presented. Samples of copper and tungsten, which are currently being used as electrode materials in pinch devices, were exposed to multiple irradiations by pulsed energetic Xe plasma. Material erosion and surface damages are analyzed. Future results will permit identification of the erosion mechanisms induced by Xe plasma particles, plasma radiation, and electric currents and their contributions to the net material erosion. The experimental data are being used for validation of numerical models developed in the HEIGHTS-EUV package for evaluation of material erosion in EUV sources.

This thesis presents a statistical method to identify the test escapes. Test often acquires parametric measurements as a function of logical state of a chip. The usual method of classifying chips as pass or fail is to compare each state measurement to a test limit. Subtle manufacturing defects are escaping the test limits due to process variations in deep sub-micron technologies which results in mixing of healthy and faulty parametric test measurements. This thesis identifies the chips with subtle defects by using rank order of the parametric measurements. A hypothesis is developed that a defect is likely to disturb the defect-free ranking, whereas a shift caused by process variations will not affect the rank. The hypothesis does not depend on a-priori knowledge of a defect-free ranking of parametric measurements. This thesis introduces a modified Estimation Maximization (EM) algorithm to separate the healthy and faulty tau components calculated from parametric responses of die pairs on a wafer. The modified EM uses generalized beta distributions to model the two components of tau mixture distribution. The modified EM estimates the faulty probability of each die on a wafer. The sensitivity of the modified EM is evaluated using Monte Carlo simulations. The modified EM is applied on production product A. An average 30% reduction in DPPM (defective parts per million) is observed in Product A across all lots.

It has been previously suggested1 that in general the modified semiempirical formula (SEM) gives better predictions of electron‐impact width of ionized atom lines as compared to the semiempirical formula (SE). However, in the case of Si III, the predictions of SE seem to be better. The average ratio of measured and theoretical values (R) for SEM is 0.,67, while that for SE is 1.06. Studies of C III,1 which has a similar electronic configuration, also show similar results (R=1.29 for SEM and R=1.21 for SE). The electron‐impact widths for Si‐III transitions2 have been calculated by using the semiempirical formulation.1 Resuls are given in Table 1. Experiments are under way to measure the electron‐impact width and transition probability of Si III transitions.

Physical models are developed to investigate the following conditions relevant to discharge-produced plasma (DPP) devices under development for extreme ultraviolet (EUV) lithography: gaseous jet propagation in the chamber, removal of neutral particles with a gaseous jet, and deviation of charged particles with a magnetic field. Several geometries of the mitigation systems are considered for removing debris during the EUV lithographic process. The design of a mitigation system is proposed and simulated with the computer models. The behavior of Xe, Li, and Sn debris in Ar and He jets is simulated by using the high energy interaction with general heterogeneous target systems (HEIGHTS) integrated package. Final energy and local distributions are calculated using experimental debris data from current EUV facilities.

Mirror erosion by high energy ion emission from extreme UV light sources is one of the main factors contributing to EUVL collector mirror reflectivity degradation. We are measuring ion energy distributions at the mirror distance from the plasma utilizing three different ion diagnostics for the case of tin-doped microscopic droplet laser plasmas. Typical ion energy distributions measured by an electrostatic spectrometer are described. From the ion energy distributions, an estimate of mirror erosion is obtained. The effectiveness of electrostatic field mitigation is evaluated for the EUVL source requirement.

A modified form of the Saha equation for a two-temperature plasma is presented, which accounts for the underpopulation of the excited states due to deviation from local thermodynamic equilibrium.

This paper deals with an experimental investigation of the conditions for local thermodynamic equilibrium (LTE) to exist in an atmospheric argon plasma jet. We have measured the emission coefficient of seven Ar I transition and the line shape of the hydrogen H β transition. After transforming the side-on data into radial space, the population densities of the excited neutral argon atoms and the electron density are determined as functions of the radius of the plasma jet. It is found that LTE does not exist below an electron density of 6 x 10 22 m -3 . The two-temperature Saha equation is shown to give non-physical results for gas temperatures in our electron-density range of 2–6 x 10 22 m -3 . We have also shown that diffusion effects are important under our experimental conditions.

New tables are presented for fitting experimental data to the Voigt function. We used a 16,000-point line profile to generate new tables and have found up to 28% difference with existing tables.

The Stark widths of seven Ar I transitions are reported. Axial line shape data from an atmospheric d.c. argon plasma jet were Abel-inverted to obtain radial line shapes. The electron density was determined from Stark-width measurements of the hydrogen H β transition. In the electron-density region of ⩽6 × 10 22 m -3 , the experimental Ar I Stark widths are fitted to a linear dependence on the electron density. Values of Stark widths extrapolated to other electron densities are compared with measurements reported in the literature on the 4 s -4 p array. Experimental values are up to 45% smaller than those predicted by Griem's theory of Stark broadening. We report experimental values of the electron-impact parameter W . Uncertainties in our results are ⩽ 10%.

The effect of sinusoidal power variations on emissivity and line-shape measurements of Ar I transitions and the Hβ transition are examined, both by a computer simulation and with experiments. The average over substantial power variation (20–50%) for both total emissivity and transition half-width data is shown to agree, within data-processing errors, with results obtained from pure d.c. operation.

Measurements are presented which allow determination of temperature fluctuations in the plasma column of a non-transferred d.c. argon plasma. Temperatures, computed from spectral line emission data, show short-term fluctuations of up to 10%.

In recent years the generation of high harmonic radiation (HHG) in laser produced plasmas has been reported. One of the noteworthy features of this process is the enhancement of harmonics that coincide with resonances in the spectrum of atoms or ions in the plasma [1]. We report on the potential of new, wide resonances in the 40 nm region of a range of elements suitable for the enhanced HHG of the 19th or 21st harmonic of a Ti:Sapphire laser. The laser has a central wavelength of 800 nm, pulse energy of 30 mJ and pulse duration of 30 fs. The resonances were observed using the dual-laser photoabsorption technique using a 1 m normal incidence spectrometer which is sensitive in the 20-200 nm region [2]. Using the Time Dependent Local Density Approximation (TDLDA) the resonance cross sections were calculated for comparison with experimental results. These wavelengths are of interest for surface studies in materials science [3]. References [1] R.A Ganeev, Harmonic generation in laser- produced plasmas containing atoms, ions and clusters: a review, Journal of Modern Optics 59:5, 409-439 [2] D.A. LIBERMANI, A.Zangwill “A Relativistic Program For Optical Response In Atoms Using A Time Dependent Local Density Approximation.” Computer Physics Communications 32 (1984) 75-82 North-Holland, Amsterdam [3] T. Higashiguchi et al Characteristics of extreme ultraviolet emission from a discharge- produced potassium plasma for surface morphology application APPLIED PHYSICS LETTERS 96, 131505 2010