The liquid-xenon-jet laser-plasma source is one of the
extreme-ultraviolet (EUV) source technologies under development
for EUV lithography. This paper discuss the basic, demanding,
requirements of a source for EUV lithography including high
in-band EUV power, absence of mirror contamination and high
stability. It is further discussed how the liquid-xenon-jet can
meet these requirements, and specifically how the ability to
operate the plasma far from any mechanical details such as the
nozzle will facilitate high power operation with low resulting
mirror degradation. Furthermore, a new laser-to-EUV conversion
efficiency result of 0.55%/(2%BW 2pisr) at lambda=13.45 nm is presented together with a detailed
description of the method for calibrated EUV-power measurement.
The Engineering Test Stand (ETS) is an EUV lithography tool designed to demonstrate full-field EUV imaging and provide data required to accelerate production-tool development. Early lithographic results and progress on continuing functional upgrades are presented and discussed. In the ETS a source of 13.4 nm radiation is provided by a laser plasma source in which a Nd:YAG laser beam is focused onto a xenon- cluster target. A condenser system, comprised of multilayer-coated and grazing incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. The resulting EUV illumination at the reticle and pupil has been measured and meets requirements for acquisition of first images. Tool setup experiments have been completed using a developmental projection system with (lambda) /14 wavefront error (WFE), while the assembly and alignment of the final projection system with (lambda) /24 WFE progresses in parallel. These experiments included identification of best focus at the central field point and characterization of imaging performance in static imaging mode. A small amount of astigmatism was observed and corrected in situ, as is routinely done in advanced optical lithographic tools. Pitch and roll corrections were made to achieve focus throughout the arc-shaped field of view. Scan parameters were identified by printing dense features with varying amounts of magnification and skew correction. Through-focus scanned imaging results, showing 100 nm isolated and dense features, will be presented. Phase 2 implementation goals for the ETS will also be discussed.
SAGEM, through its REOSC product line, is participating since November 1999 to PREUVE, the French EUV initiative, and work within this program especially in the field of EUV illumination and projection optics. After a short description of the PREUVE main lines of activity, we will detail our contributions to this program and work progress. This is mainly focused on basic EUV optics fabrication technology in order to ensure the fabrication of the entire optics assembly of an EUV micro exposure tool.
The absence of obstructions is an essential requirement for reflective ring-field projection systems to be used in EUV lithography. However, due to the large number of variables and constraints involved, choosing unobstructed starting configurations for subsequent optimization is a nontrivial issue in EUV system design. For this purpose we have developed a systematic method based on paraxial obstruction analysis. Despite the fact that in the parameter space of all possible system configurations unobstructed domains are very small, we can identify the unobstructed regions very effectively. The paraxial results are validated through comparison with ray tracing. We can convert even most of the paraxial solutions that lead to ray failure into ray-traceable starting points for optimization. We also introduce a new classification method based on the relative arrangement of the mirrors in a EUV system - a feature which in typical situations is unaffected by optimization. We present some new possible design forms for EUV imaging systems belonging to different classes and show the remarkable flexibility of the obscuration borders.
We have recently installed a new sample chamber at the NIST/DARPA EUV Reflectometry Facility at the National Institute of Standards and Technology. The chamber replaces a much smaller system on Beamline 7 at the Synchrotron Ultraviolet Radiation Facility that was commissioned almost ten years ago as a dedicated facility for the measurement of normal-incidence extreme ultraviolet optics used in lithography development, astronomy, and plasma physics. The previous measurement chamber was limited to optics less than 10 cm in diameter, and, thus, we were not able to measure many of the optics used in modern telescopes and extreme ultraviolet lithography steppers. The new chamber was designed to accommodate optics up to 36 cm in diameter and weighing up to 40 kg, and with modifications will be able to measure optics up to 50 cm in diameter. It has three translation and three rotation axes for the sample and two rotation axes for the detector, with an accuracy of better than 0.5 mm in translation and 0.04o in rotation. We will describe design considerations, performance of the positioning mechanisms, and initial reflectivity measurements of both curved and flat surfaces.
Extreme-ultraviolet phase-shifting point diffraction interferometer (PS/PDI) was studied by using the NewSUBARU undulator radiation. The wave-front error of a Schwarzchild test optics was measured. Since this is a common path PDI technique, optics pre-alignment is very important to receive enough power at the second pinhole. We carried out this pre-alignment by using the same common path PS/PDI system but by using a He-Ne laser. A temporal wave-front error attained by pre-alignment was 4.4 nm rms. We then studied band width requirement to carry out this PS/PDI in EUV. We found that the wavelength ((lambda) ) dependency of grating diffraction angle plays an important role in phase matching at the CCD camera location, although significant optical path difference exists at the edge of the fringe field. A 1 micrometers square double window experiment was carried out with (lambda) /(Delta) (lambda) is congruent to 30, and straight fringes were observed throughout the CCD field. A PDI experiment using larger pinholes compared with nominal sizes was also conducted, and various factors, which were posed onto the experimental results, were investigated.
Soft x-ray emission spectroscopy is a well established technique, which has seen increased interest in recent years due to the development of high brilliance synchrotron radiation facilities. Emission spectra are obtained when a hole is created by impinging photons, and an electron from the valence band decays into the core hole emitting its excess energy radiatively. The fluorescence yield is intrinsically low in the range of soft x-rays, usually less than about 0.5%, since the most probable decay channels are radiationless channels like Coster Kronig and Auger processes. Emission is over all space, but only a restricted solid angle can be collected and this further reduces the efficiency of the instrumentation. Moreover, the emitted photons must be energy selected, and this involves the use of soft x-ray monochromators with good resolving power. We designed a photon spectrometer to be located in the experimental chamber of one of ELETTRA's beamlines (BACH) to perform inelastic X-ray scattering measurement in the energy range 20-1200 eV with relatively high resolving power (above 2000 in most of the range). The spectrometer is based on two interchangeable mechanically ruled spherical VLS gratings with blaze profile of the grooves. The diffracted photons are collected by a back illuminated CCD of 1300X1340 pixels, each of 20X20 micrometers 2 dimension. The two interchangeable gratings are inserted in or removed from the beam by manual translation and are kept fixed during the measurement. The energy scan (or selection) is made by a mono-dimensional translation of the CCD. To cover the whole energy range, including the zero order light, only a small movement (about 7 cm of linear translation) of the detector is necessary. No detector rotation or bi-dimensional displacement is needed. Since the size of the CCD is quite large, it is possible to collect, in a single shot, a very large photon energy window, about 20-30% of the full energy range of each grating. The optical layout and the expected performances are reported.
We have developed a new stress control technique in which modified molybdenum (Mo)/silicon (Si) multilayers are deposited by ion-beam sputtering, together with a method of sub-multilayering of each Mo layer into a trilayer of Mo/ruthenium (Ru)/Mo and a method of ion-beam polishing (IBP) after the deposition of each Mo layer. We fabricated conventional and stress-controlled Mo/Si multilayer coatings using an ion-beam sputtering system and measured the internal stress of these coatings. The conventional Mo/Si multilayers had a compressive stress of approximately - 450 MPa, while the stress-controlled multilayers had a tensile stress of +13 MPa. The modified Mo/Si multilayers had the same reflectivity as and a slightly larger bandwidth than the conventional ones. For the purpose of investigating the mechanism of stress control, we have developed an in situ stress monitoring system using an electrostatic capacitance. For single-layer Mo films, tensile stress was observed in the early stage of film growth. In the case of the conventional Mo/Si multilayers, partial stress was tensile during the deposition of Mo layers, but became compressive during the deposition of Si layers. In the case of the stress-controlled multilayers, it was observed that IBP of the surface of each Mo layer suppressed the generation of compressive stress in the ensuing Si layers.
Commercial EUV lithographic systems require multilayers with higher reflectance and better stability then that published to date. This work represents our effort to meet these specifications. Interface-engineered Mo/Si multilayers with 70% reflectance at 13.5 nm wavelength (peak width of 0.545 nm) and 71% at 12.7 nm wavelength (peak width of 0.49 nm) were developed. These results were achieved with 50 bilayers. These new multilayers consist of Mo and Si layers separated by thin boron carbide layers. Depositing boron carbide on interfaces leads to reduction in silicide formation of the Mo-on-Si interfaces. Bilayer contraction is reduced by 30% implying that there is less intermixing of Mo and Si to form silicide. As a result the Mo-on-Si interfaces are sharper in interface-engineered multilayers than in standard Mo/Si multilayers. The optimum boron carbide thicknesses have been determined and appear to be different for Mo-on-Si and Si-on-Mo interfaces. The best results were obtained with 0.4 nm thick boron carbide layer for the Mo-on-Si interface and 0.25 nm thick boron carbide layer for the Si-on-Mo interface. Increase in reflectance is consistent with multilayers with sharper and smoother interfaces. A significant improvement in oxidation resistance of EUV multilayers has been achieved with ruthenium terminated Mo/Si multilayers. The best capping layer design consists of a Ru layer separated from the last Si layer by a boron carbide diffusion barrier. This design achieves high reflectance and the best oxidation resistance in a water vapor (i.e. oxidation) environment. Electron beam exposures of 4.5 hours in the presence of 5x10-7 torr water vapor partial pressure show no measurable reflectance loss and no increase in the oxide thickness of Ru terminated multilayers. Longer exposures in different environments are necessary to test lifetime stability of many years.
Experimental results on the realization of Mo/Si multilayer mirrors for EUV applications are presented. The multilayers have been deposited using RF-magnetron sputtering. The characterization of single layers and multilayers has been performed using different physical techniques. The reflectivity of multilayer mirrors optimised for 13 and 19 nm radiation has been measured and compared to simulation.
Cr/Sc multilayers have been grown on Si substrates using DC magnetron sputtering. The multilayers are intended as condenser mirrors in a soft x-ray microscope operating at the wavelength 3.374 nm. They were designed for normal reflection of the first and second order with multilayer periods of 1.692 nm and 3.381 nm, and layer thickness ratios of 0.471 and 0.237, respectively. At-wavelength soft x-ray reflectivity measurements were carried out using a reflectometer with a compact soft x-ray laser-plasma source. The multilayers were irradiated during growth with Ar ions, varying both in energy (9-113 eV) and flux, in order to stimulate the ad-atom mobility and improve the interface flatness. It was found that to obtain a maximum soft x-ray reflectivity with a low flux (Cr=0.76, Sc=2.5) of Ar ions a rather high energy of 53 eV was required. Such energy also caused intermixing of the layers. By the use of a solenoid surrounding the substrate, the arriving ion-to-metal flux ratio could be increased 10 times and the ion energy could be decreased. A high flux (Cr=7.1, Sc=23.1) of low energy (9 eV) Ar ions founded the most favorable growth condition in order to limit the intermixing with a subsistent surface flatness.
Extreme ultraviolet lithography requires vacuum conditions in the optical train. In order to maintain sufficient energy throughput, reflection reduction of multilayer mirrors due to contamination has to be minimized. We report on oxidation and carbonization experiments on MoSi mirrors under exposure with EUV radiation from a synchrotron. To mimic the effects of EUV radiation we also exposed samples using an electron gun. The oxidation rate was found to be ~0.015 nm/h per mW/mm2 of EUV radiation under vacuum conditions that are typical for a high throughput EUVL system, I.e. 10-6 mbar H2O. This oxidation can to a large extend be suppressed by using smart gas blend strategies during exposure, e.g. using ethanol. A deposition rate of 0.25 nm/h was found when the hydrocarbon pressure of Fomblin was reduced to 10(superscript -9 mbar. We demonstrate that carbonization can be suppressed by admitting oxygen during electron gun exposure.
Degradation of EUV optics during irradiation is a crucial topic as regards lifetime and performance in EUV lithography. To simulate irradiation conditions for future lithography tools, PTB (the German national metrology institute) operates two dedicated beamlines at the electron storage ring BESSY II. Both, undispersed undulator radiation from an EUV optimized undulator as well as focused and filtered bending magnet radiation can be used. Both beamlines provide EUV radiation with power densities of several mW / mm2. A dedicated irradiation chamber with sample load lock and differential pumping allows components such as substrates, multilayer mirrors or filters to be exposed to EUV radiation under different vacuum conditions. At the same laboratory, high-accuracy EUV reflectometry can be performed for proximate assessment of the resulting performance.
Bufferlayer and caplayer engineering strategies are getting progressively important for improving crucial properties of EUVL multilayer optics and EUV reflection masks. While bufferlayers modifying the contact between the reflecting interference multilayer and the superpolished substrate aim for a partial smoothing of the residual substrate roughness or for a mitigation of local substrate defects as well as for multilayer film stress relaxation, surface caplayers are capable to enhance the multilayer reflectivity of EUV mirrors and masks. We present experimental results on various bufferlayer systems (singlelayer and multilayer) applied to different substrate materials (ULE, Zerodur, silicon). The bufferlayers have been deposited by e-beam evaporation and ion-polishing techniques at UHV conditions and substrates with and without bufferlayer have been coated with standard Mo/Si multilayers (50 doublelayers, d-spacing 6.8 nm) in the same deposition run. The samples have been analyzed exsitu by means of AFM, TEM, X-ray scattering and reflection and normal-incidence EUV reflectance measurements. We have found significant improvement (+ 0.7 %) of the Mo/Si multilayer EUV reflectivity for some bufferlayer systems applied to substrates with 0.2 - 0.3 nm r.m.s. high spatial frequency roughness (HSFR), while no effect was found on superpolished substrates exhibiting 0.1 nm r.m.s HSFR. The effect of caplayer modification applied to Mo/Si multilayers has been examined regarding EUV reflectivity. The native siliconoxide layer on top of the Mo/Si multilayer coatings has been replaced by an ultrathin (2 nm) chemically inert caplayer. We have found a 1 % - 1.5 % improvement in EUV peak reflectance for one cap material applied to several Mo/Si multilayer in comparison to a native siliconoxide cap.
X-rays have tremendous potential for imaging at the highest angular resolution. The high surface brightness of many x-ray sources will reveal angular scales heretofore thought unreachable. The short wavelengths make instrumentation compact and baselines short. We discuss how practical x-ray interferometers can be built for astronomy using existing technology. We describe the Maxim Pathfinder and Maxim missions which will achieve 100 and 0.1 micro-arcsecond imaging respectively. The science to be tackled with resolution of up to one million times that of HST will be outlined, with emphasis on eventually imaging the event horizon of a black hole.
We describe HERMES (High Energy Remote-Sensing of Mercury's Surface), a novel X-ray imaging spectrometer for potential accommodation in the Mercury Planetary Orbiter (MPO) component of ESA's BepiColombo mission to Mercury. The instrument combines recently developed micro channel plate optics with large-format compound semiconductor imaging arrays. MCP optics offer the distinct advantage of a large collecting area coupled to arcminute angular resolution in a light-weight package and short focal length. Measurements on a prototype optic indicate it should be possible to achieve an angular resolution below 1 arcmin over a fov of 1 degree(s). Energy resolution of 270 eV FWHM at 5.9 keV has been achieved at room temperature for a prototype GaAs array. We estimate that HERMES will detect ~2000 x-ray fluorescent photons s-1 from the surface of Mercury during solar quiet conditions at the pericenter of the orbit. The maximum expected surface spatial resolution from this altitude is ~200m and the fov 40 km2. Over the orbiter's 2 year mission life, HERMES will provide the first very high resolution compositional maps of any planetary surface.
Soft x-ray scanning transmission x-ray microscopy allows one to image dry and wet environmental science, biological, polymer, and geochemical specimens on a nanoscale. Recent advances in instrumentation at the X-1A beamline at the National Synchrotron Light Source at Brookhaven National Laboratory are described. Recent results on Nomarski differential phase contrast and first results on investigations at the oxygen K edge and iron L edge of hydrous ferric oxide transformations are presented.
A new experimental station for soft x-ray microscopy is under construction at BACH beamline, at Elettra Synchrotron Radiation Facility (Italy). This station will be devoted both to scanning transmission x-ray microscopy (STXM) and photoemission microscopy (SPEM), with spatial resolution of about 50 nm. A Fresnel Zone Plate (FZP) will provide the micro-focusing of the beam delivered by the monochromator of BACH. The photon beam features are high resolving power (30000-5000 in the 40-1500 eV range), high flux (more than 1011 photons/s after the exit slit) and the possibility to select the light polarization. The experimental chamber will host several photon and electron detectors which should provide spatially resolved information of the bulk and surface composition. The expected acquisition times are of the order of the seconds for STXM and less than 1 minute for SPEM. The branch line hosting this station will start from the exit slit of the BACH monochromator. A toroidal mirror will focus the exit slit-spot on a pinhole which will be the source for the following FZP. With a 10x10 micrometers 2 pinhole it will be possible to obtain a spot of about 50x50 nm2 with enough flux (from 108 to 109 ph/sec) to perform microscopic experiments with polarized radiation. In this paper we present the optical scheme of the instrument as well as the foreseen performances in terms of resolution and flux.
In this contribution, we present a novel technique for converting the specimens phase information into strong image contrast, the differential interference contrast x-ray microscopy (X-DIC). In the used setup, X-DIC operation was accomplished by a zone plate doublet (ZPD), i. e. two zone plates on both sides of the same substrate, laterally shifted by about one outermost zone width. In order to be able to manufacture such ZPDs, new e-beam and nanofabrication techniques have been developed. Once a ZPD has been successfully produced, it is - despite almost all other phase sensitive methods - as easy to use as a single zone plate, without any alignment difficulty or further requirements to the coherence of the illumination. The tremendous contrast enhancement was demonstrated at the microscopy beamline ID2 at ESRF in Grenoble for test objects and biological samples. It could also be shown that ZPDs allows for full field X-DIC imaging as well as for DIC scanning transmission x-ray microscopy. Though the first experiments were carried out at 4 keV photon energy, X-DIC can be adapted to any photon energy where ZPDs with appropriate parameters can be designed and manufactured.
Applications of multilayer mirrors for extreme ultraviolet lithography (EUVL) require not only a high normal incidence reflectivity but also a long lifetime and minimal residual stress. A serious problem of Mo-Si multilayers is the structural instability in the case of localized absorption of in- and outband radiation from the EUV source followed by the degradation of the multilayer. A number os solutions have been envisaged in the past, including the use of compound materials (MoSi2Si) as well as the use of C barrier layers. We focused our interest on two Si-based systems: Mo/Si and Mo2C/Si multilayer mirrors. The mirrors were designed for normal incidence reflectivity at about 13 nm wavelength and were deposited by dc magnetron sputtering. Maximum normal incidence reflectivities of 68.4%12.8 nm for Mo/Si multilayer mirrors and 66.8%12.8 nm for Mo2C/Si have been achieved. Investigating the thermal stability of the multilayers in the temperature range from 300 degree(s)C to 500 degree(s)C we found that the reflectivity of Mo/Si mirrors is drastically decreasing after annealing above 300 degree(s)C, whereas the Mo2C/Si multilayers show a superior stability up to 400 degree(s)C...500 degree(s)C. Another problem of EUV multilayer mirrors is the large residual compressive stress (-400 to - 500 MPa), which causes undesirable distortion of the substrate figure. The reduction of residual stress of Mo/Si and Mo2C/Si multilayers with annealing has been investigated. Using a slow thermal annealing (1 degree(s)C/min), it is possible to reduce the stress from -520 MPa to zero by heating the Mo/Si samples up to 310 degree(s)C. However, this results ina reflectivity drop of about 3...4%. On the other hand one can reduce the stress of a Mo2C/Si multilayer from -490 MPa to zero by annealing without a considerable reflectivity drop.