Extreme Ultraviolet Lithography Optics Research Articles

Design of a high transmission illumination optics for anamorphic EUV lithography optics using deep reinforcement learning.

The design of the illumination optics for high numerical aperture (NA) anamorphic extreme ultraviolet (EUV) projection optics is a critical challenge to EUV lithography in advanced technology node. However, the EUV illumination optics design using conventional methods have flaws in illumination efficiency and illumination uniformity due to the limitations of relay configuration and matching method that can only consider one factor affecting illumination uniformity. One-mirror configuration can improve illumination efficiency by reducing the number of mirrors. Deep reinforcement learning (RL) can solve the limitations by considering multiple factors simultaneously. In this paper, a design method for a high transmission relay system and a matching method for double facets using deep RL are proposed to design NA 0.55 anamorphic EUV illumination optics. The one-mirror relay system is designed by first calculating its coaxial spherical initial configuration using matrix optics; then the one off-axis relay mirror, which is tilted and decentered to eliminate ray obscuration, is fitted into a conic surface. To satisfy the requirements of multiple factors that affect illumination uniformity, the assignment relationships between the field facets and the pupil facets are determined using deep RL under a certain illumination mode. Simulation results show that the illumination efficiency is 26.24%, and the illumination uniformity can reach 99% on the mask under different illumination modes.

碳污染清洗工艺对极紫外光刻光学元件反射率的影响

碳污染清洗工艺对极紫外光刻光学元件反射率的影响

Aberration properties of odd-order surfaces in optical design

We theoretically represent the effectiveness of the odd-order surface for optical designs via an aberration theory. The theory employs an expression of the odd-order surface with aberration coefficients, which are derived by power-series expansion of wavefront aberration with respect to the coordinates of optical surfaces. This expression allows us to understand the aberration characteristics of odd-order surfaces. By applying this aberration theory to the design of extreme ultraviolet lithography optics, we show that odd-order surfaces are effective in reducing higher-order aberrations with fewer coefficients than even-order surfaces do.

Fabrication of high aspect ratio tungsten nanostructures on ultrathin c-Si membranes for extreme UV applications

In this work, we demonstrate a full process for fabricating high aspect ratio diffraction optics for extreme ultraviolet lithography. The transmissive optics consists in nanometer scale tungsten patterns standing on flat, ultrathin (100 nm) and highly transparent (>85% at 13.5 nm) silicon membranes (diameter of 1 mm). These tungsten patterns were achieved using an innovative pseudo-Bosch etching process based on an inductively coupled plasma ignited in a mixture of SF6 and C4F8. Circular ultra-thin Si membranes were fabricated through a state-of-the-art method using direct-bonding with thermal difference. The silicon membranes were sputter-coated with a few hundred nanometers (100–300 nm) of stress-controlled tungsten and a very thin layer of chromium. Nanoscale features were written in a thin resist layer by electron beam lithography and transferred onto tungsten by plasma etching of both the chromium hard mask and the tungsten layer. This etching process results in highly anisotropic tungsten features at room temperature. The homogeneity and the aspect ratio of the advanced pattern transfer on the membranes were characterized with scanning electron microscopy after focus ion beam milling. An aspect ratio of about 6 for 35 nm size pattern is successfully obtained on a 1 mm diameter 100 nm thick Si membrane. The whole fabrication process is fully compatible with standard industrial semiconductor technology.

Transportation of hydrogen radicals for cleaning extreme ultraviolet lithography optics

Hydrogen radical cleaning using the hot-wire (HW) method is considered the future technology for the optical system of extreme ultraviolet lithography (EUVL). However, the optical system of EUVL has a low heat resistance. Therefore, it is necessary to reduce the radiant heat from the HW method. It is crucial to separate the hydrogen radical source from the cleaning site and to control the transportation of hydrogen radicals. However, the inactivation of hydrogen radicals on the surface of a transportation tube poses the problem hydrogen radical inactivation. In this work, we examined the inner surface materials of transportation tubes in order to improve the transportation efficiency of hydrogen radicals. It was found that the transportation efficiency of hydrogen radicals was in the following order: silicon carbon nitride (SiCN) film-coated=quartz>Au-coated>stainless steel (SUS)=Inconel. Furthermore, the sample surface temperature could be lowered from 160°C to 70°C by using a transportation tube.

Nonlinear Behavior of Decrease in Reflectivity of Multilayer Mirrors for Extreme Ultraviolet Lithography Optics by High-Flux Extreme Ultraviolet Irradiation in Various Vacuum Environments

Nonlinear Behavior of Decrease in Reflectivity of Multilayer Mirrors for Extreme Ultraviolet Lithography Optics by High-Flux Extreme Ultraviolet Irradiation in Various Vacuum Environments

Nonlinear Behavior of Decrease in Reflectivity of Multilayer Mirrors for Extreme Ultraviolet Lithography Optics by High-Flux Extreme Ultraviolet Irradiation in Various Vacuum Environments

To estimate the optics lifetime of an extreme ultraviolet lithography (EUVL) system with a contaminated mirror surface, it is indispensable to carry out an acceleration test using high-flux EUV irradiation. In this study, Mo/Si multilayer mirrors were irradiated in various vacuum environments using a contamination evaluation system with an undulator light source, and changes in their reflectivity were evaluated. The dependence of the decrease in reflectivity per unit dose on the EUV irradiance was observed in high-flux irradiation experiments in vacuum with water vapor or n-decane gas introduction at pressures lower than about 10-5 Pa. The irradiance dependence disappeared when the pressures were increased for both water vapor and decane gas. A reflectivity distribution with a doughnut shape, namely, high at the irradiation center, low in the surrounding area, and high in the non-irradiated area, was observed in the irradiation experiments during the simultaneous introduction of water vapor and decane gas. These results can be generally explained by the mechanism of the supply-controlled limit.

EUV Lithography optics contamination and lifetime studies

In this paper we discuss surface phenomena leading to contamination of multilayer optics designed for Extreme Ultraviolet (EUV) lithography. Experimental data supported by calculations indicate dramatic influence of resonance structure of EUV mirror on the secondary electron yield. These low energy electrons play an important role in radiation chemistry at the mirror surface. We also discuss the dependence on the partial pressure of admitted gas of equilibrium surface coverage and contamination rate under EUV irradiation.

Development progress of optics for extreme ultraviolet lithography at Nikon

The full-field extreme ultraviolet (EUV) exposure tool named EUV1 is integrated and exposure experiments are started with a numerical aperture of the projection optics of 0.25, and conventional partial coherent illumination with a coherence factor of 0.8. 32-nm elbow patterns are resolved in a full arc field in static exposure. In a central area, 25-nm line-and-space patterns are resolved. In scanning exposure, 32-nm line-and-space patterns are successfully exposed on a full wafer. Wavefront error of the projection optics is improved to 0.4-nm rms. Flare impact on imaging is clarified, dependent on flare evaluation using the Kirk test. Resolution enhancement technology (RET) fly-eye mirrors and reflection-type spectral purity filters (SPFs) are investigated to increase throughput. High-NA projection optics design is also reviewed.

Decomposition of catechol and carbonaceous residues on TiO2(110): A model system for cleaning of extreme ultraviolet lithography optics

High energy photons used to expose photoresists in extreme ultraviolet lithography (92eV, 13.5nm) photoexcite electrons from Mo∕Si multilayer mirror surfaces. Photoemitted electrons participate in the formation of carbonaceous residues on the mirror surface significantly affecting the mirror reflectivity. We explore mitigation strategies utilizing TiO2(110) as a model for the capping layer. Two carbon containing surfaces are examined; an ordered catechol monolayer and a carbonaceous layer. Excimer laser sources (XeF and KrF) coupled with oxidizing gas backgrounds (NO and O2) are shown to be effective for the photocatalytic removal of carbon. Utilizing x-ray photoemission spectroscopy and scanning tunneling microscopy carbon removal is shown to proceed through oxidation of the overlayer.

Evaluation of surface roughness of ULE® substrates machined by Ar+ ion beam

Ion beam figuring is suitable for the final correction of the surface figure error of aspherical substrates in an extreme ultraviolet lithography optics. This paper reports the investigation of the...

Highly coherent injection-seeded 132 nm tabletop soft x-ray laser

We report a dramatic improvement of the spatial coherence and beam divergence (0.66 mrad) of a 13.2 nm wavelength Ni-like Cd tabletop laser by injection seeding the soft x-ray laser amplifier with high-harmonics pulses generated in a Ne gas jet. This phase coherent laser is an attractive light source for at-wavelength interferometry of extreme ultraviolet lithography optics and other applications.

Evaluation of surface roughness of ULE® substrates machined by Ar+ ion beam

Ion beam figuring is suitable for the final correction of the surface figure error of aspherical substrates in an extreme ultraviolet lithography optics. This paper reports the investigation of the surface roughness of ULE® substrate machined by an Ar+ ion beam of 3–10keV. For an Ar+ ion beam in the range of 3–10keV, the mid-spatial frequency roughness of the surfaces machined to a depth of 50nm was comparable to the surface roughness of an unprocessed one. The high-spatial frequency roughness of the unprocessed surface was 0.08nm rms; whereas, the HSFRs of the surface of the substrate at Ar+ ion beam of 3, 5, 7, and 10keV were 0.10, 0.14, 0.16, and 0.17nm rms, respectively.

In situ x-ray absorption near-edge structure analysis for extreme ultraviolet lithography projection optics contamination

A contamination evaluation system for extreme ultraviolet (EUV) lithography projection optics was developed in the NewSUBARU SR facility, in which in situ surface analysis and elemental concentration mapping were carried out with the use of the x-ray absorption near-edge structure (XANES) method. For concentration mapping, the linearity between the x-ray absorption intensity and contamination thickness was confirmed by examining standard samples. The problem of quantitativity, which was caused by the antagonistic reaction of carbon deposition and surface oxidation on the mirror surface, was successfully solved by taking the ratio of intensities of the upper and lower sides of the absorption edge. Very useful information for protecting the surface of EUV lithography optics was obtained when the in situ XANES analysis was applied to the experiment of EUV irradiation with the introduction of ethanol gas to the vacuum atmosphere.

Transient Thermal and Structural Deformation and Its Impact on Optical Performance of Projection Optics for Extreme Ultraviolet Lithography

For extreme ultraviolet lithography (EUVL) targeting 45 nm technology generation and beyond, thermal absorption is the main source of multilayer-coated mirror structural deformation, which will finally cause degradation of lithographic performance. It becomes necessary to consider thermal absorption issue in the design phase of volume production tool. Utilizing several commercial and in-house software packages, we have performed a complete transient thermo-mechanical analysis of such effects and its impact on optical performance for volume production of EUVL. This article presents the simulation results based on wafer throughput model of 100 300 mm wafers per hour for the 32 nm technology generation.

Oxidation and Reduction of Ultrathin Nanocrystalline Ru Films on Silicon: Model System for Ru-Capped Extreme Ultraviolet Lithography Optics

Ultrathin ruthenium films are promising capping layers protecting extreme ultraviolet lithography optics against carbon growth and oxidation. The structure and reactivity of 2, 5, and 7 nm thick nanocrystalline Ru films on Si (serving as a model system for Ru capping layers) were studied by multiple techniques including scanning electron microscopy, X-ray diffraction, X-ray reflectivity, and X-ray photoelectron spectroscopy. The structural analysis indicated dense and flat Ru films, consisting of preferentially (0001)-oriented grains. High resolution core-level Ru 3d5/2 and O 1s spectroscopy studies have revealed that these Ru films exposed to O2 ambient are resistant to oxidation up to ∼470 K similar to the oxidation of single-crystalline Ru(0001) surfaces. The reduction of the oxide in the H2 ambient is highly effective and proceeds already below 370 K.

The role of ambient hydrocarbon species to reduce oxidation in Ru capping layers for EUVL optics mirrors

The behavior of a Ru capping layer on extreme-ultra-violet lithography (EUVL) optics mirrors was investigated under the influence of ambient hydrocarbon species which was rather similar to real exposure tool conditions. The temporal average EUV intensity was 140 mW/mm 2 at an irradiation center. The multilayer EUV reflectivity at the center was kept near 99% of the initial value throughout irradiation of 2250 J/mm 2. Surface analysis showed slight oxidation at the center. The reflectivity dropped to 96% of the initial value in the peripheral region where carbon deposition was observed. These results are analyzed by considering mirror irradiation conditions in future exposure tools.

Cleaning of extreme ultraviolet lithography optics and masks using 13.5nm and 172nm radiation

Under extreme ultraviolet (EUV) exposure, the surfaces of the imaging optics and mask, which are coated with a Mo∕Si multilayer, become contaminated with organic compounds. Thus, an efficient method of removing carbon contamination from masks and the imaging optics is required. Then, we propose two methods as a removal method of contamination: one is in situ cleaning method without heating a sample by synchrotron radiation irradiation that is mainly targeted at the imaging optics, and another is cleaning method without heating a sample using by 172nm light irradiation that is targeted at an EUV lithography finished mask. For in situ cleaning so called online cleaning, the contamination removal rate is 0.24nm∕min in the condition of O2-rich-vacuum environment at a pressure of 5.0×10−2Pa and an electron beam current of 130mA of 1.0GeV electron storage ring. For offline cleaning using 172nm light, the contamination removal rate is 2nm∕min in the O2-rich-vacuum environment at the pressure of 2×10−3Pa. Both two methods restore the reflectivity of a Mo∕Si multilayer to its original level without causing any surface damage. The effectiveness of both in situ and offline contamination removal are confirmed.

The role of temporal coherence in imaging with extreme ultraviolet lithography optics

Being based on reflective elements, extreme ultraviolet (EUV) lithography optics can support much larger spectral bandwidths then do modern refractive lithography optics. Moreover, EUV source power limitations dictate the use of the full bandwidth supported by the reflective EUV multilayers. A typical four mirror EUV optic supports an operating bandwidth of approximately 360 pm. This large bandwidth, combined with the off-axis nature of EUV lithography optics leading to significant pathlength differences across the field, raises the issue of temporal coherence effects on the imaging properties of EUV systems. Here we present analysis and computer simulation of the temporal coherence effect in EUV lithography optics. The analysis shows that under typical lithographic operating conditions, temporal coherence effects can still be safely ignored. However, non-negligible effects could arise in cases of higher-coherence imaging (small partial coherence factor, σ).

Thermal and structural deformation and its impact on optical performance of projection optics for extreme ultraviolet lithography

Thermal and structural deformation of mirrors becomes an important consideration for extreme ultraviolet lithography (EUVL) when targeting features of 50 nm and below. Such deformation will contribute to the critical dimension control and decrease the performance of the designed system. Multilayer coated mirrors typically absorb 35%–40% of the in-band radiation, which results in the deformation of mirror figures and degrades the performance of the optics. We have employed several software packages to study these effects. This article presents the results for the EUVL production tool with wafer throughput of eighty 300 mm wafers per hour for the 35 nm technology generation.