Lithography Source Research Articles

Transformation of polycrystalline tungsten to monocrystalline tungsten W(100) and its potential application in Schottky emitters

The electron sources in electron microscopes and electron lithography machines often consist of small diameter W(100) wires, etched to form a sharp tip. The electron emission is facilitated by the Schottky effect, thus the name Schottky emitter. The authors are investigating the feasibility of arrays of such electron emitters for the use in multibeam lithography. This, however, would require large pieces of W(100) which are not easily available. The concept of dc Joule heating for the crystallographic transformation is utilized to convert polycrystalline tungsten to W(100) for the fabrication of the Schottky emitters. A miniature Schottky emitter fabricated by wire electrical discharge machining was heated by dc and the crystallographic orientation was subsequently determined by electron diffraction. The x-ray diffraction measurement on another sample of tungsten filament, heated by similar method, confirms the above results. The concept will be extended for the fabrication of a prototype multibeam source for parallel electron beam lithography.

Electron sources utilizing thin CsBr coatings

We present experimental results obtained in solid Cu targets coated with ∼18 nm thick CsBr films operating in a reflection mode. The results indicate a factor of 50X increase in quantum efficiency relative to uncoated Cu samples. The CsBr/Cu samples are very robust allowing brief exposures to air without a substantial decrease in quantum efficiency and lifetime. The results are very encouraging from the point of view of reliable high efficiency electron sources for lithography or other applications including free electron lasers.

Ablation dynamics of tin micro-droplet irradiated by double pulse laser used for extreme ultraviolet lithography source

The ablation dynamics of a tin (Sn) micro-droplet by double pulse laser irradiation for the development of an extreme ultraviolet lithography source was investigated. The solid state Sn droplet target with a diameter of 30 µm was irradiated by double laser pulses from a Q-switched Nd : YAG laser and a CO2 laser, and the kinetic behaviour of debris such as Sn atoms, ions and of the dense particles from the droplet was investigated by the laser-induced fluorescence imaging method, Faraday cups and high-speed imaging, respectively. After the pre-pulse irradiation of the Nd : YAG laser, the ions were emitted towards the laser incident direction with an average kinetic energy of 3–6 keV and the dense particle cloud moved in the direction opposite to the laser incident direction with expansion by a reaction force due to the plasma expansion at a speed of approximately 500 m s−1. On the other hand, the Sn atoms were ejected in all directions from the target with a speed as fast as 20 km s−1. The expanding target was subsequently irradiated by the main pulse of the CO2 laser with a delay of 800 ns and the dense cloud almost disappeared due to main-pulse irradiation.

Imaging Diagnostics of Debris from Laser-Produced Tin Plasma with Droplet Target for EUV Light Source

The dynamics of debris from laser-produced tin (Sn) plasma was investigated for a practical extreme ultraviolet (EUV) lithography source. The kinetic behaviors of the Sn atoms and of the dense particles from Sn droplet target irradiated by double pulses from the Nd:YAG laser and the CO2 laser were also investigated by the laser-induced fluorescence imaging method and a high-speed imaging, respectively. After the pre-pulse irradiation of the Nd:YAG laser, the Sn atoms were ejected in all direction from the target with a speed of as fast as 20 km/s and the dense particle cloud expanded by a reaction force due to the plasma expansion with a speed of approximately 500 m/s. The expanding target was subsequently irradiated by the main-pulse of CO2 laser and the dense cloud was almost disappeared by main-pulse irradiation.

Source for extreme ultraviolet lithography by the tabletop storage ring MIRRORCLE

Advances of electron storage rings to beam currents of above 1 A and tabletop sizes make possible the development of a synchrotron-based source for EUV lithography (EUVL) at ~13.5-nm wavelength. The MIRRORCLE storage rings can provide on average 3-A electron beam current, 1-min lifetime, 15-ms radiation damping, and beam size ~3*3 mm2. MIRRORCLE-20SX, MIRRORCLE-6X, and MIRRORCLE-CV4 store electrons with energies of 20 MeV, 6 MeV, and 4 MeV, respectively. These machines can emit EUV from a tiny target, hit by the circulating beam, via transition radiation or diffusive radiation. Using a multilayer microelectromechanical system (MEMS) target allows enhancement and spectral purification of the emitted EUV. Aligning many such MEMS along the electron beam orbit and radiation collection by only one quasi-elliptical EUV mirror can provide EUV satisfying the joint requirements for an EUVL source.

Laser wavelength dependence of extreme ultraviolet light and particle emissions from laser-produced lithium plasmas

Maximum extreme ultraviolet (EUV) conversion efficiencies (CEs) of 2.3% and 1.8% were achieved in planar Li targets by using pulsed 2ω and 1ω Nd:YAG laser irradiation, respectively. In a forced recombination scheme, the total CE can be expected to be about 4%. The maximum kinetic energy of the lithium ion debris was found to be less than 1 keV, indicating that mirror damage caused by lithium ion debris is more easily mitigated by using a magnetic field than for tin ions. These results suggest that a Li target is a reasonable candidate for an EUV lithography source.

Efficient 13.5nm extreme ultraviolet emission from Sn plasma irradiated by a long CO2 laser pulse

The effect of pulse duration on in-band (2% bandwidth) conversion efficiency (CE) from a CO2 laser to 13.5nm extreme ultraviolet (EUV) light was investigated for Sn plasma. It was found that high in-band CE, 2.6%, is consistently obtained using a CO2 laser with pulse durations from 25to110ns. Employing a long pulse, for example, 110ns, in a CO2 laser system used in an EUV lithography source could make the system significantly more efficient, simpler, and cheaper as compared to that using a short pulse of 25ns or shorter.

High-harmonic generation by resonant plasmon field enhancement

High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent extreme-ultraviolet (EUV) light. This nonlinear conversion process requires high pulse intensities, greater than 10(13) W cm(-2), which are not directly attainable using only the output power of a femtosecond oscillator. Chirped-pulse amplification enables the pulse intensity to exceed this threshold by incorporating several regenerative and/or multi-pass amplifier cavities in tandem. Intracavity pulse amplification (designed not to reduce the pulse repetition rate) also requires a long cavity. Here we demonstrate a method of high-harmonic generation that requires no extra cavities. This is achieved by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate. In our experiment, the output beam emitted from a modest femtosecond oscillator (100-kW peak power, 1.3-nJ pulse energy and 10-fs pulse duration) is directly focused onto the nanostructure with a pulse intensity of only 10(11) W cm(-2). The enhancement factor exceeds 20 dB, which is sufficient to produce EUV wavelengths down to 47 nm by injection with an argon gas jet. The method could form the basis for constructing laptop-sized EUV light sources for advanced lithography and high-resolution imaging applications.

Power scaling of an extreme ultraviolet light source for future lithography

For future lithography applications, high-power extreme ultraviolet (EUV) light sources are needed at a central wavelength of 13.5nm within 2% bandwidth. We have demonstrated that from a physics point of view the Philips alpha-prototype source concept is scalable up to the power levels required for high-volume manufacturing (HVM) purposes. Scalability is shown both in frequency, up to 100kHz, and pulse energy, up to 55mJ collectable EUV per pulse, which allows us to find an optimal working point for future HVM sources within a wide parameter space.

Plasma physics and radiation hydrodynamics in developing an extreme ultraviolet light source for lithography

Extreme ultraviolet (EUV) radiation from laser-produced plasma (LPP) has been thoroughly studied for application in mass production of next-generation semiconductor devices. One critical issue for the realization of an LPP-EUV light source for lithography is the conversion efficiency (CE) from incident laser power to EUV radiation of 13.5-nm wavelength (within 2% bandwidth). Another issue is solving the problem of damage caused when debris reaches an EUV collecting mirror. Here, we present an improved power balance model, which can be used for the optimization of laser and target conditions to obtain high CE. An integrated numerical simulation code has been developed for the target design. The code agrees well with experimental results not only for CE but also for detailed EUV spectral structure. We propose a two-pulse irradiation scheme for high CE, and reduced ion debris using a carbon dioxide laser and a droplet or a punch-out target. Using our benchmarked numerical simulation code, we find a possibility to obtain CE up to 6–7%, which is more than twice that achieved to date. We discuss the reduction of ion energy within the two-pulse irradiation scheme. The mitigation of energetic ions by a magnetic field is also discussed, and we conclude that no serious instability occurs due to large ion gyroradius.

Progress of laser fusion in the last 40 years and expected prosperous applications

Inertial confinement fusion has remarkably developed in the last 40 years. In the 21st century, we can expect fusion energy for civilian use. We had performed two types of fusion experiments: High Temperature Demonstration and High Density Demonstration. The former experiment attained a neutron yield 1013 using the LHART target driven by the GEKKO XII laser. The latter achieved the 1000 times normal density using the random phased laser beams which realized the Edward Teller proposal of IQEC 1972. Now, the FIREX project to explore fast ignition is going on. The heating process of energetic electrons as well as ions is a key issue of the fast ignition. Investigation on the extreme condition of plasma in high density and high temperature which are introduced by the PW laser give us a new field of nuclear science. On the way to the final fusion goal, we can expect various fruits in the field of high power laser applications, such as laser-induced nuclear reaction, EUV light source for lithography, nuclear transmutation, laser astrophysics, medical application of particle beam and so on.

Optimum laser-produced plasma for extreme ultraviolet light source

The optimum plasma conditions of extreme ultraviolet (EUV) light source for lithography were experimentally clarified that is optically thin and minimum mass Sn plasma generated from a limited size target. Sn plasma is quite opaque for EUV light of 13.5 nm in wavelength, therefore 13.5 nm light emitted from deep within a Sn plasma is strongly absorbed, thus optically thin plasma production is essential for efficient EUV generation. Contamination of EUV optics caused by debris emanated from laser irradiated Sn targets is a serious problem in the Sn based EUV source system. Target residue around the laser spot is the dominant source of neutral debris, which can be reduced with supplying the minimum mass target containing the minimum number of Sn atoms required for sufficient EUV generation. Spectral purity of generated EUV light is an important requirement to expose clear mask image on a photo-resist film without chromatism. Out-of-band radiation in the vacuum ultraviolet range is dominantly radiated from the laser spot peripheral. Target size must be equal to the laser spot size to suppress the out-of-band radiation.

3D PIC simulation of ion debris mitigation by B-field for LPP-EUV source

In laser produced plasmas (LPP) for EUV light source, the maximum ion kinetic energy sometime exceeds 10 keV. Such fast ion debris from LPP may cause damage on a collecting mirror of the light source. Therefore the mitigation of the fast ions is one of the critical issues that should be overcome for the practical use of LPP as EUV source for lithography. We have investigated both free plasma expansion and plasma expansion in an external magnetic field by using 3D particle simulation. The maximum ion energy in free expansion is essentially determined from the ratio of initial target radius to initial Debye length. Based on this relation we discuss the possibility to reduce the maximum ion energy. We show that the maximum ion energy perpendicular to the magnetic field is also reduced and that the ion radial flight distance becomes less than the gyro diameter estimated from the maximum energy. We also show that generated electric field causes the interchange instability of the plasma, but the instability is limited within a certain radial region. Simulation results indicate that the fast ions can be efficiently exhausted along the magnetic field without causing crucial damage on an EUV collecting mirror with the use of a single magnetic coil.

Extreme ultraviolet light production from a ZaP flow z-pinch xenon plasma

Abstract. A potential alternative extreme ultraviolet EUV light sourcefor lithography is studied. The ZaP z-pinch Flow Z-Pinch experiment isstudying the effect of sheared flow on gross plasma stability. In the ex-periment, hydrogen gas is used to produce plasma with quiescent peri-ods in the magnetic mode activity, which are 2000 times longer thanother plasma concepts for creating EUV light, in a configuration with 300times the volume. Similar results are found with xenon gas. An EUVdetector designed using an AXUV100, silicon/zirconium filtered photodi-ode with an 11- to 18-nm bandpass is used to detect EUV emissionswithin that spectrum and the total power of the emissions. EUV emis-sions in 17.4% of the z-pinch have lasted longer than 16 s, with anaverage power of 550 kW. The total EUV power potential inband at13.5 nm from 17.4% of the z-pinch is calculated to be 140 W at theintermediate focus, with a total 100-cm z-pinch emission potential of800 W at 1 kHz. Based on this information, the ZaP Flow Z-Pinch ex-periment is a promising EUV light source for lithography if properlyscaled and optimized for EUV light production.

Atomic Model and Optimization of EUV Light Source

Extreme ultraviolet (EUV) radiation from laser produced plasmas (LPP) has been theoretically studied as a light source for lithography in mass-production of the next generation semiconductor devices. One of the critical issues for realization of a LPP-EUV light source is the conversion efficiency (CE) from incident laser power to EUV radiation of 13.5nm wavelength (within 2% bandwidth). From an atomic physical point of view, we explain the reason why tin has been chosen as the most suitable radiation material compared with xenon and lithium. We also present a power balance model, which can be used for the optimization of laser and target conditions to obtain high CE. We propose a double-pulse irradiation scheme for high CE using a carbon dioxides laser and a droplet target. Using our benchmarked numerical simulation code, we show a possibility to obtain CE up to 5-7%, much higher presently achieved.

High power fiber laser driver for efficient EUV lithography source with tin-doped water droplet targets

In this paper we report the development of nanosecond-pulsed fiber laser technology for the next generation EUV lithography sources. The demonstrated fiber laser system incorporates large core fibers and arbitrary optical waveform generation, which enables achieving optimum intensities and other critical beam characteristics on a laser-plasma target. Experiment demonstrates efficient EUV generation with conversion efficiency of up to 2.07% for in-band 13.5-nm radiation using mass-limited Sn-doped droplet targets. This result opens a new technological path towards fiber laser based high power EUV sources for high-throughput lithography steppers.

Investigation of the dynamic of an expanding laser plume by a shadowgraphic technique

The new generation of EUV sources for lithography, based on a high current z-pinch, exploits the laser ablation of a Sn target for triggering of a discharge and delivery of working material. The dynamic of the Sn plume expansion during the first 120 ns, which strongly affects the later behavior of z-pinch was investigated by a shadowgraphic technique. Radiation in a spectral range from 18 to 20 nm was used for detection of shadow images of the Sn plume because 20 nm radiation is absorbed by the inner shells of neutrals and first ions. Thus, the probing beam is efficiently absorbed by the species most important in the formation and evolution of z-pinch. Images of the Sn plume were detected at 22 ns, 55 ns, 88 ns and 120 ns delays after the plume ignition. The technique enabled the observation of the dynamic of Sn species expansion within a range of 2 mm from the target surface. A software for the processing the detected images was developed. The estimated total number of ablated Sn neutrals and first ions varied from ∼ 2–4 × 10 14 for intensities of the focused ablating pulse in the range 8 × 10 8–10 10 W/cm 2. The experimentally detected dynamic of Sn plume expansion and total number of ablated Sn species coincide reasonably with simulated data.

Cathode ray tube type electron gun as a source for multibeam electron lithography

The authors have investigated the potential of using a dispenser cathode in space charge limited regime for employment in an electron beam lithography electron source. The space charge limitation guarantees stable and uniform emission even if there are small work function variations or bumps and depressions on the surface. Employment of a dispenser cathode in the space charge limited regime enables high beam currents and splitting of the electron beam into many sub-beams for parallel multibeam electron lithography. In the reported experiment, the electron beam is split into 194 sub-beams. The reduced brightness, defined as current divided by normalized emittance, was measured at different cathode temperatures and extraction potentials for a cathode ray tube type electron source equipped with an I-type dispenser cathode. In the central 25 sub-beams, reduced brightness values of up to 106Am−2sr−1V−1 were observed. Such a high reduced brightness in combination with a high total emission current (up to 20mA) indicates potential application in electron beam lithography systems. In accord with theory, the experimentally observed reduced brightness is directly proportional to the emission current density. It was found, however, that the brightness drops if the emission current density is increased beyond the level where the emitter leaves the space charge limited regime. Within the space charge regime, increasing reduced brightness as a function of increasing current density is found to be caused by a decreasing virtual source size, while the angular current density remains nearly invariant.

Design studies for a high brightness, energetic neutral atom source for proximity lithography

Line-edge roughness due to mask and wafer charging in ion beam proximity lithography can be simply and completely eliminated by neutralizing the ions before they reach the mask. This dramatically increases the usable depth of field and enables high quality imaging below 20nm. Neutralization is accomplished by charge transfer scattering in a high pressure cell, a process that preserves both the energy and direction of the parent ion with remarkable accuracy. In this article, the authors discuss the design of an energetic atom source and characterize an optimized helium ion injector based on a dc-multicusp ion source. The injector provides a crossover at a fixed position in the charge transfer cell over a design energy range from 30to50keV. For 100W discharge power, the full width at half maximum diameter and normalized brightness of the crossover are 95μm and 100A∕m2srV, respectively.

Research and development in short-wave radiation sources for new-generation lithography

Research and development in short-wave radiation sources for new-generation lithography