Laser Produced Plasma Light Source Research Articles

High-irradiance illumination system for transmission extreme ultraviolet microscopy

A high-irradiance illumination system with normal-incidence multilayer mirrors is developed. The system consists of two concave Mo/Si multilayer mirrors with aspherical substrates that provide critical illumination by imaging a laser-produced plasma (LPP) light source at a wavelength of 13.6 nm with a magnification of almost unity. By effectively correcting spherical aberration, a raytracing computation indicates that the irradiance on the sample can be increased by about four times compared to that for a conventional system with spherical mirrors. We experimentally confirmed the enhancement by combining a LPP light source with aspherical mirrors fabricated by magneto rheological finishing.

Non-axisymmetric droplet irradiation effects on ion and extreme ultraviolet light emission of laser-produced plasma light sources

Extreme ultraviolet (EUV) light emission and ion emissions in the lateral direction generated by Nd:YAG irradiated droplet targets were studied experimentally under non-axisymmetric droplet irradiation. EUV energy monitors and ion electrostatic probes were positioned symmetrically at fixed positions around the plasma. Bulk ion kinetic energy Ekin and bulk ion density ni were characterized for angles of γ=31° to γ=90° from the laser axis in a low-pressure nitrogen atmosphere of 1×10−2mbar. The EUV light emission reached a maximum level, with an increase of 5%, at γ=60° through a shift of the droplet by around 18% of the droplet diameter. The ion bulk kinetic energies at 31° and 75° also increased by 9% and 15%, respectively, with a small increase in ion density of around 3%. Intentional offset of the droplet target can, therefore, potentially result in higher EUV light output for non-axisymmetric mirror configurations, at the expense of a higher ion load but a decreased droplet fragment load. The ion kinetic energy showed a strongly anisotropic expansion behavior when offsetting the droplet relative to the laser focus, with kinetic energies increasing up to 60% in the lateral direction by offsetting the droplet by more than 0.5 droplet diameters for a lateral range of 31°–90°. The ion density showed a more isotropic change with droplet offset, while maintaining a forward-peaked distribution.

Increase of the emission of laser-produced plasmas under N2 gas atmosphere in the 2.9–6 nm region

The wavelength region between the K-edges of carbon and oxygen is termed as the water window (λ = 2.3–4.5 nm) and is considered to be suitable for the observation of nanometer-scale structures composed of light elements in an aqueous solution because of both the short wavelength and transparency of the light. The nanometer-scale structures in an aqueous solution fluctuate owing to Brownian motion. Therefore, the observation of the nanometer-scale structures can be achieved by one-shot exposure of the light in the water window. To observe the nanometer-scale structures by the one-shot exposure, the light source of the microscope working in the water window should exhibit a high light-flux. One of the light sources used for the one-shot exposure is the laser-produced plasma (LPP) light source. The conversion efficiency of the LPP light source in the water window is 1% or less. Therefore, a high intensity laser or improvement of the conversion efficiency is required for the one-shot observation. In this study, an emission-intensity increase in the LPP light source is observed in the wavelength region from 2.9 nm to 6 nm under a N2 gas atmosphere. The intensity value, calibrated by the gas absorbance, was increased up to 5 times in proportion to the N2 pressure, which was increased from 0 Pa to 400 Pa. Experimental conditions demonstrate that the conversion efficiency of the LPP light source is improved by the introduction of gas.

Light sources for high-volume manufacturing EUV lithography: technology, performance, and power scaling

Abstract Extreme ultraviolet (EUV) lithography is expected to succeed in 193-nm immersion multi-patterning technology for sub-10-nm critical layer patterning. In order to be successful, EUV lithography has to demonstrate that it can satisfy the industry requirements in the following critical areas: power, dose stability, etendue, spectral content, and lifetime. Currently, development of second-generation laser-produced plasma (LPP) light sources for the ASML’s NXE:3300B EUV scanner is complete, and first units are installed and operational at chipmaker customers. We describe different aspects and performance characteristics of the sources, dose stability results, power scaling, and availability data for EUV sources and also report new development results.

Spectral emission properties of a laser-produced plasma light source in the sub-200 nm range for wafer inspection applications

The spectral emission properties of a droplet-based laser-produced plasma are investigated in the vacuum ultraviolet (VUV) range. Measurements are performed with a spectrograph that operates from 30 to 180 nm with a spectral resolution of 0.1 nm. The emission spectra are recorded for different metal droplet targets, namely tin, indium, and gallium. Measurements were performed at different pressure levels of the background gas. Several characteristic emission lines are observed. The spectra are also calibrated in intensity in terms of spectral radiance to allow absolute emission power estimations from the light source in the VUV region. The presented experimental results are relevant for alternative light sources that would be needed for future wafer inspection tools. In addition, the experimental results help to determine the out-of-band radiation emission of a tin-based extreme ultraviolet (EUV) source. By tuning the type of fuel, the laser energies, and the background gas, the laser-produced plasma light source shows good capabilities to be operated as a light source that covers a spectral emission range from the EUV to the sub-200 nm range.

Experimental study on laser produced tin droplet plasma extreme ultraviolet light source

Extreme ultraviolet lithography (EUVL), which uses the extreme ultraviolet radiation at a wavelength of 13.5 nm, is the leading candidate of next generation lithography addressing not only the 10 nm half-pitch nodes, but several nodes beyond that. Among all the methods for getting EUV radiation, laser-produced plasma (LPP) light source is the most promising EUV light source because of its high conversion efficiency (CE), large collect angle and low debris output. In this paper, pulsed TEA-CO2 laser and Nd:YAG laser are used to irradiate tin droplets to obtain plasma EUV emission, and the properties of EUV radiation from the plasma are studied. Results show that the EUV emission spectra induced by Nd:YAG laser have an obvious blueshift as compared with those by CO2 laser. In addition, the LPP sources are point light sources, so that the angular distribution of EUV emission from LPP can be described by Lambertian distribution.

Laser-produced plasma light source for extreme-ultraviolet lithography applications

High-brightness extreme-ultraviolet light sources are required for mask inspections and metrology, including mask blank inspection, actinic pattern inspection, and aerial image measurement system to improve yield and lower cost of ownership. Laser-produced plasma (LPP) light sources have the highest potential to achieve the brightness requirements for all the range of mask inspection tools currently foreseen. High brightness of LPP sources (100 to 1000 W/mm2 sr) is the result of a smaller source size ( ∼ 0.1 mm) than that of competing technologies. Since brightness is inversely proportional to the area of the source, smaller source size corresponds with greater brightness and hence greater inspection throughput. At the Laboratory for Energy Conversion of ETH Zurich, a fully operational continuous-running multi-kHz LPP light source has been developed over the last five years and is now undergoing system optimization. Adlyte, a spin-off of ETH Zurich, is working with industry leaders to commercialize this LPP source. Individual subsystem configuration and the physical boundary conditions and limitations that affect power, brightness, stability, and lifetime management are discussed. This integrated system produces a measured brightness of 259 W/mm2 sr. Outlook for the future growth and integration of the source in high-volume manufacturing tools is then discussed.

Lifetime Calculations on Collector Optics from Laser Plasma Extreme Ultraviolet Sources with Minimum Mass

An ion flux and its kinetic energy spectrum are obtained using a self similar spherically symmetric fluid model of expansion of a collisionless plasma into vacuum. According to the ion flux and energy distribution, the collector optical lifetime is estimated by knowledge of the sputtering yield of conventional Mo/Si multilayer coatings for the CO2 and Nd:YAG pulsed-laser produced plasmas based on the minimum mass tin droplet target without debris mitigation. The results show that the longer wavelength of the CO2 laser produced plasma light source is more suitable for extreme ultraviolet lithography than Nd:YAG laser in respect of fast ion debris induced sputtering damage to the collector mirror.

The effect of excitation wavelength on dynamics of laser-produced tin plasma

We investigated the effect of the excitation wavelength on the density evolution of laser-produced tin plasmas, both experimentally and numerically. For producing plasmas, Sn targets were excited with either 10.6 μm CO2 laser or 1.06 μm Nd:yttrium aluminum garnet laser; both are considered to be potential excitation lasers for extreme ultraviolet lithography laser-produced plasma light sources. The electron density of the plasma during the isothermal expansion regime was estimated using an interferometric technique. The Stark broadening of isolated singly-ionized emission was employed for deducing the density during the plasma adiabatic expansion regime. Our results indicate that the excitation source wavelength determines the initial density of the plasma, as well the plume expansion dynamics. Numerical simulation using HEIGHTS simulation package agrees well with the experimentally measured density profile.

The effect of laser wavelength on emission and particle dynamics of Sn plasma

We investigated the effects of laser wavelength on the atomic, ionic, and radiative emission from laser-produced tin plasmas. For generating plasmas, planar tin targets were excited using either high intensity neodymium-doped yttrium aluminum garnet (Nd:YAG, 1.06 μm) or carbon dioxide (CO2, 10.6 μm) laser pulses; both are considered to be potential excitation lasers for an extreme ultraviolet (EUV) lithography laser-produced plasma light source. Various diagnostic tools were utilized for investigating ionic, neutral, and radiative emission from Sn plasmas including Faraday cup, witness plate in conjunction with x-ray photoelectron spectroscopy (XPS), EUV, and visible emission spectroscopy and photography. Atomic and ionic analysis showed that the amount of debris emitted by the Nd:YAG generated plasmas was considerably higher than the CO2 laser-produced plasmas. The angular distributions of both atomic and ionic debris were found to be more forward-centric for the 1.06 μm generated plasma while being much more uniform for the 10.6 μm heated plasma. EUV and visible emission images of the plasma also showed a forward-centric appearance for 1.06 μm heated plasmas. The strength of excited neutral emission was considerably lower for the case of the 10.6 μm plasma while the kinetic energies of ions debris were found to be much higher for CO2 generated plasmas. Surface analysis of the craters created by the lasers showed that the mass ablation rate is 3.6 times higher for Nd:YAG laser generated plasmas compared to CO2 generated plasmas at maximum EUV emission.

High throughput and wide field of view EUV microscope for blur-free one-shot imaging of living organisms

We present and demonstrate the use of an extreme ultraviolet (EUV) microscope that was developed in-house. Images are acquired using Bragg reflection multilayer optics and a laser-produced plasma light source. The upper-limit spatial resolution of the EUV microscope is 130 nm with a 10 ns exposure time and 250 x 250 microm(2) field of view. Resolution is superior to that of visible microscopes with the same size of field of view, and the exposure time is short enough to observe fine structures in-vivo. Observation of the cerebral cortex of a mouse is demonstrated.

Laser-Produced Plasma EUV Light Source for Microlithography

The current status of the laser produced plasma light source development for EUV lithography is presented. Our key technology is based on an RF-excited CO2 driver laser combined with a Sn droplet target, because this approach enables cost-effective high-conversion efficiency. The RF-excited CO2 laser has a MOPA (master oscillator power amplifier) configuration. It currently generates an average output power of 2.6kW at 130kHz repetition rate. Due to negligible wavefront distortion of the laser beam during amplification, a laser focus of 100μm (FWHM) has been obtained. The Sn target and magnetic ion mitigation development as well as an evaluation of the collector mirror lifetime are presented.

10 kHz 40 W Ti:sapphire regenerative ring amplifier

A cryogenically cooled Ti:sapphire regenerative ring amplifier was developed as a laser for generating a laser-produced plasma light source. With a 10 kHz 180 W pump laser, the amplifier output is 40 W before compression and 26 W after compression. We believe it to be the current highest average-power output from a single stage Ti:sapphire amplifier. The effective focal length of the thermal lens is measured to be 2.2 m at 100 K for 180 W of pump power. With a 1 m focal length lens placed in the resonator, the effect of a thermal lens on the resonator mode is suppressed. High conversion efficiency is achieved for the whole pumping power range without any additional measures for thermal compensation.

Thin film filters for an EUV multilayer mirror optics with a laser produced plasma light source

For rejection of unwanted background component in the reflection spectrum of EUV multilayer mirrors, thin film transmission filters of Mo, Zr, and Be were evaluated with a laser produced plasma EUV source and an imaging plate detector. It is found that in addition to a standard Zr filter, a Mo filter has good rejection characteristics. In particular, for multilayer mirror use at a large angle of incidence such as the polarizing angles around 45°, Mo shows better rejection characteristics than Zr. It is also confirmed that Be transmits non-ignorable amount of UV background.

Laser-Produced Plasma Light Source Development for Extreme Ultraviolet Lithography

We present recent results of our laser-produced plasma light source development for next-generation lithography. The plasma target of the extreme ultraviolet (EUV) source system is a liquid xenon jet and the driver laser is a 600 W Nd:YAG laser operating at a repetition rate of 10 kHz. A EUV output power of 2.2 W at 13.5 nm (2% bandwidth, 2π sr) having a stability of 0.72% (1σ, 50-pulse moving average) has been achieved. Related to future collector mirror lifetime considerations, fast ions from the laser-produced plasma have been characterized by time-of-flight (TOF) measurements. Using a low repetition rate 8-ns, 100-mJ Nd:YAG laser Xe+ to Xe6+ ions were observed with Xe2+ being the main charge state. In addition, the effects of fast ions on Mo/Si multilayer mirrors have been studied using a Xe ion gun. Ion sputtering of the multilayer structure is the main damage mechanism but layer boundary mixing and surface roughness increase are also observed.

Laser Produced Plasma Light Sources. High Average Power Laser Produced Plasma EUV Light Sources.

This paper reviews the research and development of the high average power, extreme ultraviolet light source based on laser produced plasma by EUVA. The technology is based on a liquid Xe micro jet, high repetition rate short pulse Nd:YAG laser, and various diagnostics for plasma optimization are described.

Laser-produced plasma light source for extreme ultraviolet lithography

Pulsed Nd:YAG lasers have been developed to achieve high peak power and high pulse repetition rate. These systems are being used as drivers for laser-produced plasmas which efficiently convert the 1064-nm laser output to extreme ultraviolet (EUV) light at 13.5 nm for future microlithography systems. The requirements for laser-produced plasma EUV light sources and their integration in lithography tools for high-volume manufacturing are reviewed to establish the key design issues for high-power lasers and plasma targets. Xenon has been identified as a leading target material to realize the goals of intense EUV emission and clean operation. Recent progress in high-power diode-pumped Nd:YAG lasers and xenon targets for EUV generation is reviewed, showing that laser-produced plasma sources meet the needs for current EUV lithography development tools. Future directions to meet EUV source requirements for high-volume manufacturing tools are discussed.

The spectrum and term system of S VIII

The spectrum of seven times ionized sulfur, S VIII, has been observed in the wavelength range 50-1200 Å. Both beam-foil and laser-produced plasma light sources have been used. 71 new lines representing the transitions 2p5-2p43d, 2p43s-3p, 3p-3d, 3d-4f, 4f-5g and 5g-6h have been identified and 100 energy levels, of which 46 are new, are now accurately established. The spectral analysis is supported by isoelectronic comparisons along the F I-sequence and by parametric least-squares fits of Slater integrals to the observed levels. The ionization potential in S VIII is determined to 2 651 899 ± 60 cm−1 and estimated ionization energies are derived for the isoelectronic elements between F I and Ar X.

Time-resolved measurements of short-wavelength fluorescence from x-ray-excited ions

We demonstrate a novel technique for time-resolved spectroscopic studies of highly excited ions. The technique uses a laser-produced plasma as a short-pulse, soft-x-ray light source with a high repetition rate. A Nd:YAG laser with a pulse duration of 90 psec, a pulse energy of 70 microJ, and repetition rate of 10(4) pulses per second is focused onto a rotating metal target. Soft x rays from the resulting plasma photoionize a gas surrounding the target, and fluorescence from the gas is detected by using a spectrometer and a high-speed photodetector. Using the technique of time-correlated photon counting, we determined the radiative lifetime and collisional quenching rate of the Xe III 5s(0)5p(6)(1)S(0) state by observing its fluorescence at 108.9 nm. A time resolution of better than 400 psec was obtained. We also measured relative Auger decay yields of a core hole state in xenon using a higher-energy laser-produced plasma light source at a lower repetition rate.

Laser produced plasma light sources for high resolution XUV and VUV spectroscopy

Light from a laser produced plasma light source has been observed with the highest possible spectral resolution currently available in the 4–120 nm region. Plasma radiations observed from W, Yb, and Hf targets were found to be continuous with very few line emissions. Examples of highly resolved absorption spectra of He I and Ar I obtained using the plasma light source are presented.