Efficient Extreme Ultraviolet Research Articles

Efficient extreme ultraviolet emission by multiple laser pulses

We demonstrated an efficient extreme ultraviolet (EUV) source at a wavelength of 13.5 nm using spatially separated multiple solid-state-laser pulse irradiation. The maximum conversion efficiency (CE) achieved was 3.8% for ±30° oblique laser pulse injection, which was about twice as high as that for single laser pulse irradiation of 1.7%, with an EUV source size of about 100 μm for two spatially separated laser pulses with a total laser energy of 500 mJ at a laser intensity of 2×1011 W/cm2. In addition, we achieved an EUV CE of 4.7% for ±60° oblique laser pulse injection, which was one of the highest values ever reported, in the case of a 1-μm solid-state laser-produced planar Sn target plasma by multiple laser pulse irradiation. This result suggests that multiple laser-pulse irradiation at high repetition rate operation could credibly provide the next technology for future high-power EUV sources and exposure tools toward future EUV technology nodes.

Experimental study and modeling of extreme ultraviolet 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers.

Multilayer coated diffraction gratings are crucial components for extreme ultraviolet (EUV) applications such as spectroscopy or spectro-imaging. However, for high groove density, the smoothening of the grating surface profile with multilayer deposition remains a limitation that requires further investigation. In this paper, we report on the design, characterization, and modeling of 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers for EUV radiation. Two types of gratings with different groove depths are compared. Multilayer coatings were designed using a genetic algorithm to maximize the first-order diffraction efficiency in the 17-21 and 19-23nm wavelength ranges at normal incidence. Periodic and aperiodic multilayers with different numbers of layers were deposited by magnetron sputtering on the two types of fused silica gratings, and the grating groove profile evolution was measured by atomic force microscopy and cross-section transmission electron microscopy. The first-order diffraction efficiency was measured in the EUV at 5° incidence using monochromatic synchrotron radiation and modeled using the rigorous coupled-wave analysis method. The simulation models refined by using the Debye-Waller factor to account for the multilayer interfacial roughness show good agreement with experimental data. The results reported in this study will allow for designing efficient EUV multilayer gratings for high-resolution spectro-imaging instruments.

Optimization of extreme ultra-violet light emitted from the CO2 laser-irradiated tin plasmas using 2D radiation hydrodynamic simulations.

We studied Extreme Ultra-Violet (EUV) emission characteristics of the 13.5 nm wavelength from CO2 laser-irradiated pre-formed tin plasmas using 2D radiation hydrodynamic simulations. Our results indicate that when a CO2 laser irradiates pre-formed tin plasma, the heated plasma expands towards the surrounding plasma, steepening the density at the ablation front and lowering the density near the laser axis due to the transverse motion of the plasma. Consequently, the laser absorption fraction decreases, and the contribution to EUV output from the ablation front becomes dominant over that from the low-density plasmas. We estimated that an EUV conversion efficiency of 10% from laser to EUV emission could be achieved with a larger laser spot size, shortened laser pulse width, and longer pre-formed plasma density scale length. Our results offer one optimizing solution to achieve an efficient and powerful EUV light source for the next-generation semiconductors.

Extreme ultraviolet light from a tin plasma driven by a 2-µm-wavelength laser.

An experimental study of laser-produced plasmas is performed by irradiating a planar tin target by laser pulses, of 4.8 ns duration, produced from a KTP-based 2-µm-wavelength master oscillator power amplifier. Comparative spectroscopic investigations are performed for plasmas driven by 1-µm- and 2-µm-wavelength pulsed lasers, over a wide range of laser intensities spanning 0.5 - 5 × 1011 W/cm 2. Similar extreme ultraviolet (EUV) spectra in the 5.5-25.5 nm wavelength range and underlying plasma ionicities are obtained when the intensity ratio is kept fixed at I1µm/I2µm = 2.4(7). Crucially, the conversion efficiency (CE) of 2-µm-laser energy into radiation within a 2% bandwidth centered at 13.5 nm relevant for industrial applications is found to be a factor of two larger, at a 60 degree observation angle, than in the case of the denser 1-µm-laser-driven plasma. Our findings regarding the scaling of the optimum laser intensity for efficient EUV generation and CE with drive laser wavelength are extended to other laser wavelengths using available literature data.

Tuning laser wavelength and pulse duration to improve the conversion efficiency and performance of EUV sources for nanolithography

The optimum conditions for efficient extreme ultraviolet (EUV) sources in laser-produced plasma (LPP) depend on the ideal combination of several parameters such as laser wavelength, intensity, spot size, and pulse duration. These parameters need to be optimized in relation to the actual target size and geometry. The laser wavelength can play a major role in this regard. While target related effects are not significant in the case of a Nd:YAG laser, the achievable conversion efficiency (CE) of EUV sources produced by a CO2 laser from small Sn droplets can be several times lower compared to that from a flat Sn target. Improving the efficiency and performance of EUV sources requires detailed investigation of various laser/target combinations using comprehensive integrated models. We simulated and characterized in detail plasmas produced from the Sn droplet irradiated by lasers with various wavelengths. The optimum laser intensity with respect to the laser and target sizes was determined for a range of laser wavelengths from 1 μm to 10 μm. This work was inspired by the recent developments of lasers with a 2 μm wavelength. The laser pulse duration is another important parameter in the optimization of LPP for EUV sources. Our comprehensive HEIGHTS modeling showed that increasing the pulse duration of the 2 μm laser leads to an increase in the CE of EUV sources. Our results also indicate that a significant reduction in the kinetic energies of ions generated from LPP can be achieved by optimizing the laser pulse duration.

On-Wafer FinFET-Based EUV/eBeam Detector Arrays for Advanced Lithography Processes

A novel microdetector array (MDA) for monitoring electron beam (eBeam) and extreme ultraviolet (EUV) lithography processes in 5 nm and beyond FinFET technology is first-time presented. This on-wafer detector array consists of high-density sensing cells which are fully compatible with standard FinFET CMOS processes. Fin coupling structures and energy-sensing pads are first applied in an ultrasmall detector for realizing efficient eBeam and EUV photon detection. In advanced lithography process, eBeam or EUV level projected on the wafer can be precisely recorded on the on-wafer MDA without power or batteries. The distributions and variations on the beam intensities collected by MDA can be electrically measured in real time or inline through wafer level test after eBeam or EUV exposures. The proposed MDA is expected to provide real-time feedback for the optimization and stable maintenance of advanced photolithography processed critical to the development nanometer CMOS technologies.

Stagnation layer development from two colliding Sn plasmas for an efficient EUV source

The collision of two laser-produced plasmas (LPP), individually generated from the opposing faces of a tin wedge, was studied experimentally and through the HEIGHTS computer simulation package to explore onset and development of the stagnation layer and extreme ultraviolet (EUV) photoemission. The colliding Sn plasma offers promise for a powerful source of 13.5 nm light, the industry-identified wavelength being targeted for commercial EUV lithography light sources. A second laser pulse could efficiently couple with the material accumulated along the collision midplane to produce EUV with high power. Imaging of the LPP configuration is accomplished using an ICCD camera while electron temperature and density of the stagnation layer were determined using spectroscopic methods—both were verified using HEIGHTS which was also used to show the potential of an EUV source with 700 W power at the intermediate focus for efficient high-volume manufacturing.

Low temperature plasmas induced in SF6 by extreme ultraviolet (EUV) pulses

In this work, a comparative study of extreme ultraviolet (EUV) induced low temperature SF6-based plasmas, created using two different irradiation systems, was performed. Both systems utilized laser-produced plasma (LPP) EUV sources. The essential difference between the systems concerned the formation of the driving EUV beam. The first one contained an efficient ellipsoidal EUV collector allowing for focusing of the EUV radiation at a large distance from the LPP source. The spectrum of focused radiation was limited to the long-wavelength part of the total LPP emission, λ > 8 nm, due to the reflective properties of the collector. The second system did not contain any EUV collector. The gas to be ionized was injected in the vicinity of the LPP, at a distance of the order of 10 mm. In both systems, energies of the driving photons were high enough for dissociative ionization of the SF6 molecules and ionization of atoms or even singly charged ions. Plasmas, created due to these processes, were investigated by spectral measurements in the EUV, ultraviolet (UV), and visible (VIS) spectral ranges. These low temperature plasmas were employed for preliminary experiments concerning surface treatment. The formation of pronounced nanostructures on the silicon surface after plasma treatment was demonstrated.

KW-class picosecond thin-disc prepulse laser Perla for efficient EUV generation

The technology for extreme ultraviolet (EUV) lithography sources is maturing. Laser produced plasma (LPP) sources with usable power >100 W have been used in high-volume manufacturing (HVM) applications, and 250-W sources are expected to be introduced in HVM soon. However, a further increase of power and cleanness may benefit a powerful picosecond (ps) laser in the near-infrared and wavelength converted spectral region. The HiLASE Centre has been working in thin-disc laser technology and has demonstrated a 0.5-kW platform Perla-C based on a very compact Yb:YAG regenerative amplifier. 100-kHz ps operation has been achieved with a fundamental spatial mode and excellent long-term pointing and energy stability. It is reported on a thin-disc-based ps Yb:YAG solid-state laser technology platform Perla developed in the Czech Republic and the present performance of delivering >4 mJ, <2-ps pulses at a 100-kHz repetition rate with the potential to be upgraded to 1 kW of average power and 1-MHz pulse repetition rate. The ps laser extendibility is important for kW-class LPP sources and controlled free electron laser EUV sources in 10-kW power region.

Near-resonant four-wave mixing of attosecond extreme-ultraviolet pulses with near-infrared pulses in neon: Detection of electronic coherences

Coherent narrow-band extreme-ultraviolet (EUV) light is generated by a near-resonant four-wave mixing (FWM) process between attosecond pulse trains and near-infrared pulses in neon gas. The near-resonant FWM process involves one vacuum-ultraviolet (VUV) photon and two near-infrared (NIR) photons and produces new higher-energy frequency components corresponding to the $ns/nd$ to ground-state $(2{s}^{2}2{p}^{6})$ transitions in the neon atom. The EUV emission exhibits small angular divergence (2 mrad) and monotonically increasing intensity over a pressure range of 0.5--16 Torr, suggesting phase matching in the production of the narrow-bandwidth coherent EUV light. In addition, time-resolved scans of the NIR nonlinear mixing process reveal the detection of a persistent, ultrafast bound electronic wave packet based on a coherent superposition initiated by the VUV pulse in the neon atoms. This FWM process using attosecond pulses offers a means for both efficient narrow-band EUV source generation and time-resolved investigations of ultrafast dynamics.

Polarization contrast of nanoscale waveguides in high harmonic imaging

The optical polarization response of a structured material is one of its most significant properties, carrying information about microscopic anisotropies as well as chiral features and spin orientations. Polarization analysis is therefore a key element of imaging and spectroscopy techniques throughout the entire spectrum. In the case of extreme ultraviolet (EUV) radiation, however, both the preparation and detection of well-defined polarization states remain challenging. As a result, polarization-sensitive EUV microscopy based on table-top sources has not yet been realized, despite its great potential, for example, in nanoscale magnetic imaging. Here, we demonstrate polarization contrast in coherent diffractive imaging using high harmonic radiation and investigate the polarization properties of nanoscale transmission waveguides. We quantify the achievable polarization extinction ratio for different waveguide geometries and wavelengths. Our results demonstrate the utility of slab waveguides for efficient EUV polarization control and illustrate the importance of considering polarization contrast in the imaging of nanoscale structures.

Optimizing conversion efficiency and reducing ion energy in a laser-produced Gd plasma

We have demonstrated an efficient extreme ultraviolet (EUV) source at 6.7 nm by irradiating Gd targets with 0.8 and 1.06 μm laser pulses of 140 fs to 10 ns duration. Maximum conversion efficiency of 0.4% was observed within a 0.6% bandwidth. A Faraday cup observed ion yield and time of flight signals for ions from plasmas generated by each laser. Ion kinetic energy was lower for shorter pulse durations, which yielded higher electron temperatures required for efficient EUV emission, due to higher laser intensity. Picosecond laser pulses were found to be the best suited to 6.7 nm EUV source generation.

Extreme ultraviolet source at 6.7 nm based on a low-density plasma

We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2% BW), which is 1.6 times larger than the 0.33% (0.6% BW) CE produced from a solid density target. Enhancement of the EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.

Combined effects of prepulsing and target geometry on efficient extreme ultraviolet production from laser produced plasma experiments and modeling

Laser produced plasmas (LPPs) are currently a promising source of an efficient extreme ultraviolet (EUV) photon source production for advanced lithography. Optimum laser pulse parameters with adjusted wavelength, energy, and duration for a simple planar or spherical tin target can provide 2% to 3% conversion efficiency (CE) in laboratory experiments. Additional effects such as targets with complex geometry and tin-doped targets using prepulsing of laser beams can significantly increase CE. Recent studies showed that such improvements in an LPP system are due to reduction in laser energy losses by decreasing photons transmission (higher harmonic of Nd:yttrium-aluminum-garnet laser) or photons reflection (for CO2 laser). Optimization of target heating using prepulses or ablating low-density and nanoporous tin oxide can further improve LLP sources by creating more efficient plasma plumes and, as a result, increase CE, the most important parameter for EUV sources. We investigated the combined effects of prepulsing with various parameters and different target geometries on EUV conversion efficiency and on energetic ions production. The much higher reflectivity of CO2 laser from a tin target leads to two possible ways for system improvement using prepulses with shorter laser wavelengths or using more complex targets geometries with special grooves as developed previously by the authors.

Efficient laser-produced plasma extreme ultraviolet sources using grooved Sn targets

An efficient extreme ultraviolet (EUV) generation method has been developed with the use of a CO2 laser-produced plasma from a grooved target. A ∼5% conversion efficiency from laser to 13.5 nm photons was obtained with the use of grooves in a tin target or by repeated laser pulse shots at the same target position. Modeling studies proved that the groove target controls the hydrodynamic expansion of the plasma leading to confinement which prevents the plasma escaping from the EUV production zone.

Effects of plasma spatial profile on conversion efficiency of laser-produced plasma sources for EUV lithography

Extreme ultraviolet (EUV) lithography devices that use laser-produced plasma (LPP), discharge-produced plasma (DPP), and hybrid devices need to be optimized to achieve sufficient brightness with minimum debris generation to support the throughput requirements of high-volume manufacturing lithography exposure tools with a long lifetime. Source performance, debris mitigation, and reflector system are all critical to efficient EUV collection and component lifetime. Enhanced integrated models continue to be developed using the High Energy Interaction with General Heterogeneous Target Systems (HEIGHTS) computer package to simulate EUV photon emission, debris generation, and transport in both single and multiple laser beam interaction systems with various targets. A new Center for Materials under Extreme Environments (CMUXE) was recently established to benchmark HEIGHTS models for various EUV-related issues. The models being developed and enhanced were used to study the effect of plasma hydrodynamics evolution on the EUV radiation emission for planar and spherical geometry of a tin target and explain the higher conversion efficiency of a planar target in comparison to a spherical target. HEIGHTS can study multiple laser beams, various target geometries, and pre-pulses to optimize EUV photon production. Recent CMUXE and other experimental results are in good agreement with HEIGHTS simulation.

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.

Development of an Efficient EUV Source by Use of a Liquid Tin Oxide Micro-Target

We report on a low-debris, efficient extreme ultraviolet (EUV) source by use of a liquid micro-target containing low-concentration tin oxide nano-particles. Conversion efficiency from a laser pulse energy to an in-band EUV energy (2% bandwidth (BW) and 2πIsr solid angle) depended on various experimental parameters, such as tin oxide concentration, laser intensity, and pulse separation time between dual laser pulses. By using a low concentration (6%) colloidal target together with dual laser pulses under the optimum separation time, we observed the EUV conversion efficiency of 1.2% with undetectable debris.

Potential of discharge-based lithium plasma as an extreme ultraviolet source

Extreme ultraviolet (EUV) discharge-based lamps for EUV lithography need to generate extremely high power in the spectrum band of 13.5±0.135nm. A model was developed to investigate the wavelength-integrated Lyman-α lines light outputs in hydrogen-like lithium ion. The analysis reveals that the commonly observed low conversion efficiency is largely due to a transient nature of Z discharge-based plasma and that a magnetically confined lithium plasma is an efficient EUV source even at low electron temperature. Calculation shows necessary confinement time that raises the conversion efficiency up to half the spectral efficiency.

Debris characteristics of a laser-produced tin plasma for extreme ultraviolet source

We measured debris characteristics of a tin (Sn) plasma produced by a 10-ns infrared Nd:YAG laser. A maximum kinetic energy of 7keV of tin ions was observed. Such suprathermal tin ions emitted from a solid planar target consisted of singly and doubly ionized tin ions. Both suprathermal ions and neutral fragments emitted from a target showed the angular distributions of cos4θ which were narrower than the 13.5-nm extreme ultraviolet (EUV) emission distribution of cos0.5θ. These measurements would give important information on debris mitigation for efficient EUV sources in the next generation lithography.