Extreme Ultraviolet Lithography Tool Research Articles

Aperiodic multilayer masks for M3D mitigation in high- and hyper-NA extreme ultraviolet lithography

As extreme ultraviolet lithography tools with higher image numerical apertures (NAi) are introduced, the range of angles at the multilayer mask stack is also increased. Lithography systems are designed to fulfill the “abbe sine rule,” where NAm is related to NAi by the reduction factor. As a result, increases in NAi will increase NAm. High-NA and hyper-NA systems will be implemented with anamorphic optics, 4× in “X,” and 8× in “Y” to reduce the necessary angles. Even so, hyper-NA masks may see illumination angles as high as 13.4°, up from 10.8° for 0.33 NA. This represents a challenge for maintaining through-angle mask reflection using the current periodic mask multilayer structures. In addition to the reflectance amplitude, the phase of reflected light, which plays an important role in imaging, is also strongly influenced by increasing angles. The propagation of light through each bilayer in the stack imparts a phase shift based on the incidence angle. This is then accumulated over many layers, inducing phase effects, which are unique to each illumination point. This will become especially true for high-NA and hyper-NA mask applications. While adjustments to the multilayer period are sufficient to achieve acceptable reflectance, periodic multilayers may suffer in normalized image log slope (NILS) and image placement error (IPE) metrics as a result of strong oblique multilayer M3D phase effects. In this work, we present a first principles methodology for the systematic reduction of oblique multilayer M3D effects through the use of aperiodic multilayer design. We find that when used for low-k1, hyper-NA patterning NILS is improved by 40+%, while IPE through pitch is similarly improved. Under high-NA conditions, a 5%–25% NILS gain is found alongside the enhanced depth of focus.

Transition from ambipolar to free diffusion in an EUV-induced argon plasma

Extreme Ultraviolet (EUV) optical components used in EUV lithography tools are continuously impacted by an exotic and highly transient type of plasma: EUV-induced plasma. Such an EUV-induced plasma is generated in a repetitive fashion upon sending a pulsed beam of high energy (92 eV) photons through a low-pressure background gas. Although its formation occurs on a time scale of ∼100 ns, it is the plasma's decay dynamics on longer time scales that dictates the fluxes and energy distribution of the produced ions. Therefore, the plasma decay also determines the overall impact on plasma-facing EUV optical components. Enabled by electron density measurements using Microwave Cavity Resonance Spectroscopy at a much higher sensitivity, we clearly show the breakdown of the ambipolar field in an EUV photon-induced plasma below electron densities of ∼2 × 1012 m−3 and the—until now—unidentified transition from ambipolar diffusion-driven decay into a decay regime driven by free diffusion. These results not only further improve the understanding of elementary processes in this type of plasma but also have a significant value for modeling and predicting the stability and lifetime of optical components in EUV lithography.

Predicting radiation-induced carbon contamination of EUV optics

Predictions are made for the radiation-induced carbon contamination threat to ruthenium-coated extreme ultraviolet (EUV) optics for a range of incident EUV intensities, exposure pressures and types of hydrocarbon. A calculational philosophy is developed that acknowledges the ruthenium capping layer may have adsorbed oxygen on it and that the carbon contamination film is partially hydrogenated. The calculations incorporate the Nitta Multisite Adsorption framework, which accounts for the configurational adsorption difficulty encountered by the adsorption of large molecules on surfaces. Contributions from “out-of-band” radiation are included, both in the direct photon-induced dissociation of hydrocarbon molecules and in the out-of-band production of secondary electrons. For the hydrocarbon molecules, n-tetradecane, n-dodecane, n-decane, and benzene, for a range of EUV powers and hydrocarbon pressures, predictions are made for carbon thicknesses, the overall carbon deposition rates, and the relative amounts of contamination produced by primary photon excitation, secondary electrons, and out-of-band radiation. The comparison is made to relevant prior experiments. The model, with no adjustable parameters, provides a good account of prior experiments on n-tetradecane, n-decane, and benzene over the pressure ranges examined by the experiments (∼1 × 10−10 to ∼1 × 10−7 Torr) and over the EUV intensity range 0.001–100 mW/mm2. The level of agreement is within a factor of ∼4 or better, which is consistent with expectations based on the experimental uncertainties. Comparison with prior data for n-decane indicates that the carbon deposit produced by the EUV-induced dissociation of hydrocarbons is substantially hydrogenated. Out-of-band radiation accounts for ∼9%–12% of the overall optic contamination. Secondary electrons account for ∼2% of the overall optic contamination. The results show that the dominant mechanistic cause of the EUV carbon contamination is primary photon absorption by the adsorbed hydrocarbon molecule. The removal of carbon or hydrogen by electron stimulated desorption due to secondary electrons or photon stimulated desorption by primary EUV absorption can be safely ignored as negligible compared to the EUV-induced carbon deposition rate. The results allow comparison with past experiments, provide a framework for conducting future experiments, and predict contamination threats relevant for practical EUV lithography tool operation. The calculations also clarify the underlying physical phenomena at work in the EUV carbon contamination problem.

Reflective EUVL tool design with generalized Gaussian constant mathematics.

This study aims to develop a systematic design procedure for extreme ultraviolet lithography tools. Through analysis using generalized Gaussian constants, relationships between optical properties and requirements can be obtained, and can be used to help ensure that optical system properties required for the tool are upheld during the design process. As verification of the design method, an eight-mirror 0.4 NA tool design has been demonstrated.

Ion energy distributions in highly transient EUV induced plasma in hydrogen

This work reports on the measurements of ion flux composition and ion energy distribution functions (IEDFs) at surfaces in contact with hydrogen plasmas induced by extreme ultraviolet (EUV) radiation. This special type of plasma is gaining interest from industries because of its appearance in extreme ultraviolet lithography tools, where it affects exposed surfaces. The studied plasma is induced in 5 Pa hydrogen gas by irradiating the gas with short (30 ns) pulses of EUV radiation (λ= 10–20 nm). Due to the low duty cycle (10–4), the plasma is highly transient. The composition and IEDF are measured using an energy resolved ion mass spectrometer. The total ion flux consists of H+, H2+, and H3+. H3+ is the dominant ion as a result of the efficient conversion of H2+ to H3+ upon collision with background hydrogen molecules. The IEDFs of H2+ and H3+ appear similar, showing a broad distribution with a cut-off energy at approximately 8 eV. In contrast, the IEDF of H+ shows an energetic tail up to 18 eV. Most probably, the ions in this tail gain their energy during their creation process by photoionization and dissociative electron impact ionization.

Fast resist-activation dosimetry for extreme ultra-violet lithography.

Due to the rather broad band emission spectrum of the extremely hot plasma in its extreme ultra-violet (EUV) source, an EUV lithography scanner also projects out-of-band vacuum- and deep-UV (OoB V/DUV) light on the photoresist on a wafer. As this type of uncontrolled and undesirable light can activate resist chemistry, it will impair the critical dimension uniformity of the patterns, especially across the borders of the fields. Hence, OoB V/DUV quantification technology is required in the pre-production phase. For this reason, the systematic characterization of the EUV-source emission spectrum and the spatial profile of the light as projected on the wafer is indispensable to sustain stable integrated circuit production with EUV lithography. This paper introduces an in-band EUV and OoB V/DUV dosimetry method that is based on enhanced energy sensitivity by resist contrast (EESRC). This dosimetry method is applied in an EUV lithography tool to quantitatively analyze the spatial distribution the resist activation by in-band EUV and OoB V/DUV light, under several exposure conditions. This pragmatic approach can replace the current best-practice of measuring the full spectrum of an EUV light source.

Thermalization of electrons in decaying extreme ultraviolet photons induced low pressure argon plasma

We monitored—in the pressure range: 0.5–15 Pa—the electron temperature in decaying plasmas induced in argon gas by pulsed irradiation with extreme ultraviolet (EUV) photons with wavelengths closely around 13.5 nm. For this purpose, temporal measurements of the space-averaged and electric field weighted electron density after pulsed EUV irradiation are combined with an ambipolar diffusion model of the plasma. Results demonstrate that electrons are thermalized to room temperature before the plasma has fully expanded to the chamber walls for pressures of 3 Pa and higher. At pressures below 3 Pa, the electron temperature was found to be up to 0.1 eV above room temperature which is explained by the fact that plasma expansion is too quick for the electrons to fully thermalize. The comparison between plasma expansion duration towards a surface, plasma decay at a surface and time needed for thermalization and cooling of electrons is essential for designers of EUV lithography tools and EUV sources since the temperature of electrons dictates many fundamental physical processes.

Cleaning of Organic Contamination from EUV Optics Surfaces Using Hydrogen-based Plasmas

The efficiency of optics used in extreme ultraviolet (EUV) range suffers from reflectivity degradation due to oxidation and carbon contamination of mirror surfaces. Therefore, an efficient cleaning procedure should be ascertained in order to facilitate applications of EUV lithography tools. Carbon contamination removal from the mirror surface has been reported in Hydrogen plasmas, and laser-induced Hydrogen plasmas in EUV tools. Here, we performed measurements in a helicon-type RF plasma reactor with hydrogen pressures similar to the ones used in laser-induced plasmas in EUV tools. The method of optical actinometry was used to determine hydrogen atom concentration and the degree of dissociation. The results are compared to those obtained by pressure-rise measurements. Preliminary results of plasma etching of thin organic films (PMMA) were also obtained to assess the cleaning efficiency.

Metrology development for extreme ultraviolet lithography: Flare and out-of-band qualification

Extreme ultraviolet lithography (EUVL) is the leading candidate for lithography beyond the 22 nm half-pitch device manufacturing node. These geometries impose tighter requirements for standard critical dimension metrology and call for new strategies able to quantify and monitor extreme ultraviolet (EUV) specific parameters. In this paper, the approaches to measure two key EUV imaging parameters, namely flare and out-of-band (OoB) radiation, are discussed. EUV sources are known to emit a broad spectrum of wavelengths ranging from EUV to deep ultraviolet (DUV) and beyond. As the DUV can contribute to the photoresist exposure and degrade imaging performance, it is critical to accurately determine the amount of DUV OoB in EUVL exposure tools at the wafer level. In this paper, a methodology using an aluminum-coated reticle to measure the DUV/EUV ratio in resist is discussed. Such a mask is able to provide quantitative in situ information on the scanner DUV content thanks to its ability to transmit DUV and absorb EUV. The experimental OoB results for two EUVL tools are reported and compared with modeling predictions. Flare in EUVL is caused by light scattered by the surface roughness of the optical elements and has a larger impact as compared to optical lithography. As a consequence, a precise and accurate flare metrology is essential to guarantee a proper qualification of the effect, as well as to implement an effective compensation strategy. However, the flare level estimate has been historically based on operator and tool-dependent procedures that are unable to meet the requirements for accuracy and precision dictated by EUVL. A robust in-line approach to flare metrology is developed and qualified. As in the case of OoB, experimental flare results for two EUVL tools are reported. The experimental data are compared to full-chip simulations using the point spread function of the tool’s optical system.

Low Energy Xe+ Ion Beam Machining of the Ultralow Expansion Glass Substrates for Aspherical Extreme Ultraviolet Lithography Projection Optics

Aspherical substrates for extreme ultraviolet lithography (EUVL) optics require an ultrahigh shape accuracy of less than 0.15 nm rms and a high-spatial frequency roughness (HSFR; spatial wavelength: less than 1 µm) of 0.12 nm rms. Generally, the ultra low expansion glass (ULE®) substrate with HSFR of 0.06–0.08 nm rms can be produced by mechanical machining methods. However, it is difficult to obtain the shape accuracy of less than 0.12 nm rms using mechanical machining methods. Therefore, ion beam figuring (IBF) may be adapted to final shape correction of the substrates for the projection optics of EUVL tools. In this study, we investigated the HSFR and machining rate of the ULE® substrate machined by a 0.3–1.0 keV Xe+ ion beam at off normal ion incidence angles and obtained the following results: the HSFRs of the ULE® substrate machined by a 1.0 keV Xe+ ion beam at a ion incidence angle of lower than 30° and a 0.3–0.5 keV Xe+ ion beam at an ion incidence angle of 0–45° are below 0.12 nm rms, which is smaller than the required HSFR specification of EUVL projection optics. From our experimental result and discussion, we concluded that the scan fine beam and tilt target mode smoothing for processing of the ULE® substrate meets the required specification of the HSFR (0.12 nm rms) of hemispherical ULE® substrates of EUVL projection optics.

Design for manufacture to deal with mask-induced critical dimension errors in the extreme ultraviolet

The actual extreme ultraviolet lithography tools will have aberrations around seven times larger than those of the latest ArF lithography tools in wavelength normalized rms. We calculated the influence of aberrations on the size error and pattern shift error using Zernike sensitivity analysis. Mask-induced aberration restricts the specification of aberration. Without periodic additional pattern, the aberration level that can be accepted to form 22 nm dual-gate patterns was <8 m rms. Arranging the periodic additional pattern relaxed the aberration tolerance. With periodic additional pattern, the acceptable aberration level to form 22 nm patterns was below <37 m rms. It is important to make pattern periodicity for the relaxation of the aberration specification.

Extreme ultraviolet multilayer mirror with near-zero IR reflectance

We have developed a multilayer mirror for extreme ultraviolet (EUV) radiation that has low reflectance for IR radiation at 10.6 mum wavelength. The mirror is based on a multilayer coating comprising alternating layers of diamondlike carbon and silicon, for which we demonstrate an EUV reflectance of up to 49.7%. We have made a functional prototype in which the multilayer coating is included as part of an antireflection coating for IR radiation, resulting in reflectance values of 42.5% and 4.4% for EUV and IR, respectively. The mirror can replace a standard Mo/Si mirror in an EUV lithography tool to form an efficient solution for the suppression of unwanted CO(2) laser radiation.

Development of 6DOF Magnetic Levitation Stage for Lithography Tool

In this paper, a detail design of MAGLEV(MAGnetic LEVitation) stage is put into effect based on the concept that was persuaded in 1st paper, ”Development of 6DOF Magnetic Levitation Stage for Lithography Tool -Background and First Report-“, in that the limit of vacuum compatible air bearing was discussed. Because the proof of concept should be quickly required, local design such as material and component was changed from the initial concept. The key specification, control bandwidth, coupling between other axes, vibration transmissibility and positioning accuracy were evaluated and the concept was verified. The possibility of application to the EUVL(Extreme Ultra Violet Lithography tool) reticle stage is obtained by changing the material to high stiffness rate ceramics in control simulation.

Development of 6DOF Magnetic Levitation Stage for Lithography Tool

At first, I explain some key points and difficulty of lithography tool design, issues that should be overcome during the development and requirements for stages in lithography tool in this paper. Then, I have ascertained a limit of static-pressure air bearing with differential pumping mechanism for vacuum application, for instance EUVL(Extreme Ultra Violet Lithography tool) by manufacturing so-called the “vacuum compatible” 1-axis air bearing and verifying the performance by comparing with equivalent model simulation. Finally, one of MAGLEV(MAGnetic LEVitated) stage concepts is proposed instead of common AIRLEV(AIR LEVitated) stage by seriously considering a high throughput for mass production. I simultaneously contribute 2nd paper, “Development of 6DOF Magnetic Levitation Stage for Lithography Tool -Proof Of Concept (Second Report)-“ which is a sequel to this paper.

Ion beam machining of Si layer deposited on Zerodur® substrate

Ion beam figuring is suitable for the final correction of the surface figure error of aspherical substrates using an extreme ultraviolet lithography tool. In ion beam figuring, however, the machined surfaces of substrates become rougher than the unprocessed surfaces. Moreover, the surface is positively charged due to the positive charges of impinging ions. In this experiment, a Si layer was deposited by ion beam sputtering on a Zerodur® substrate with a depth of ∼300nm; then, this was machined by an Ar+ ion beam with energies in the range of 3–10keV. The mid-spatial-frequency roughness of the surface machined to a depth of less than 50nm was comparable to that of an unprocessed surface. The high-spatial-frequency roughness (HSFR) of the unprocessed surface was 0.21nm rms, whereas the average HSFRs of the surface machined up to a depth of 50nm were 0.25, 0.33, 0.39, and 0.59nm root mean square at energies of 3, 5, 7, and 10keV, respectively. The HSFR of the machined surface increased with the ion beam energy.

Plasma sources for EUV lithography exposure tools

The source is an integral part of an extreme ultraviolet lithography (EUVL) tool. Such a source, as well as the EUVL tool, has to fulfil extremely high demands both technical and cost oriented. The EUVL tool operates at a wavelength in the range 13–14 nm, which requires a major re-thinking of state-of-the-art lithography systems operating in the DUV range. The light production mechanism changes from conventional lamps and lasers to relatively high temperature emitting plasmas. The light transport, mainly refractive for DUV, should become reflective for EUV. The source specifications are derived from the customer requirements for the complete tool, which are: throughput, cost of ownership (CoO) and imaging quality. The EUVL system is considered as a follow up of the existing DUV based lithography technology and, while improving the feature resolution, it has to maintain high wafer throughput performance, which is driven by the overall CoO picture. This in turn puts quite high requirements on the collectable in-band power produced by an EUV source. Increased, due to improved feature resolution, critical dimension (CD) control requirements, together with reflective optics restrictions, necessitate pulse-to-pulse repeatability, spatial stability control and repetition rates, which are substantially better than those of current optical systems. All together the following aspects of the source specification will be addressed: the operating wavelength, the EUV power, the hot spot size, the collectable angle, the repetition rate, the pulse-to-pulse repeatability and the debris induced lifetime of components.

Improved emission uniformity from a liquid-jet laser-plasma extreme-ultraviolet source

Many modern compact soft-x-ray and extreme-ultraviolet (EUV) imaging systems operate with small fields of view and therefore benefit from the use of small high-brightness sources. Such systems include water-window microscopes and EUV lithography tools. We show that the photon losses in such systems can be minimized while uniformity of object-plane illumination is maintained by controlled scanning of the source. The improved collection efficiency is demonstrated both theoretically and experimentally for a scanned laser-plasma source compared with static sources. A prospective aerial image microscope and a liquid-xenon-jet laser-plasma source are offered as examples of modern imaging tools that may benefit from such scanning of the source.

High performance resist for EUV lithography

As the semiconductor industry moves to the 32 nm node, it becomes apparent that new lithography technology will be needed. One possibility for fabricating this feature size is extreme ultraviolet (EUV) technology. With the advent of EUV, new high performance resist materials will need to be developed. This study looks at the use of a poly[4-hydroxystyrene- co-2-(4-methoxybutyl)-2-adamantyl methacrylate] system as a possible candidate material for extreme ultraviolet lithography (EUVL). This material was synthesized and evaluated as an EUV chemically amplified resist. This resist system showed reasonable sensitivity and contrast compared to conventional PMMA resist. This resist system exhibits both negative and positive tone behavior at high and low dose, respectively. A negative pattern of 35 nm L/S (70 nm pitch) was printed using the transmission-grating EUV Interference Lithography (EUV-IL) tool [J.K. Chen, F.H. Ko, H.L. Chen, F.C. Chang, Jpn. J. Appl. Phys. 42 (2003) 3838]. A positive pattern of 50 nm (pitch 180 nm) was achieved using the EUVL tool at Sandia National Laboratory [Synchrotron Radiation Center website www.src.wisc.edu & Center for Nanotechnology website www.nanotech.wisc.edu]. Resist patterns were characterized in detail using scanning electron microscopy and atomic force microscopy. Both showed similar characteristics of the pattern lines.

Design and performance of capping layers for extreme-ultraviolet multilayer mirrors.

Multilayer lifetime has emerged as one of the major issues for the commercialization of extreme-ultraviolet lithography (EUVL). We describe the performance of an oxidation-resistant capping layer of Ru atop multilayers that results in a reflectivity above 69% at 13.2 nm, which is suitable for EUVL projection optics and has been tested with accelerated electron-beam and extreme-ultraviolet (EUV) light in a water-vapor environment. Based on accelerated exposure results, we calculated multilayer lifetimes for all reflective mirrors in a typical commercial EUVL tool and concluded that Ru-capped multilayers have approximately 40x longer lifetimes than Si-capped multilayers, which translates to 3 months to many years, depending on the mirror dose.

Stress reduction of Mo/Si multilayer structures

Due to the stringent surface figure requirements for the multilayer-coated optics in an extreme ultraviolet (EUV) projection lithography system make it desirable to minimize deformation due to the multilayer structure stress. However, the stress must be reduced or compensated without reducing the EUV reflectivity, since the reflectivity has a strong impact on the throughput of the EUV lithography tool. In this work, we evaluate techniques for stress reduction and compensation as applied to multilayer Mo/Si structures. The experimental research of multilayer Mo/Si-structures by the methods of X-ray reflectometry and electron microscopy has shown that reflectivity of Mo/Si-structures largely depends on the parameter γ= d Mo/ d and in the spectral region of 13–13.7 nm reaches the maximum value at γ≈0.42–0.44. The measured film stress for Mo/Si films with EUV reflectances near 66–69% at 13.3 nm is approximately −250 MPa (compressive). Variation of the Mo-to-Si ratio can be used to reduce the stress to near-zero levels but at a large loss in EUV reflectance (≈54–57%). Technologically it is easiest to use for a buffer layer a multilayer structure made of the same materials (Mo and Si) as the top, reflecting, structure, but with a different parameter γ≈0.8. The stress in the structures containing antistress buffer sublayers was σ=−6 MPa. The reflectivity values for these structures are quite close, and still the reflection coefficient of the Mo/Si structure deposited on a substrate with an antistress Mo/Si buffer coating was smaller by 0.5–0.7% in absolute values smaller.