Extreme Ultraviolet Projection Lithography Research Articles

Optimization Design Method of a 6‐DOF Micromanipulation Mechanism for Extreme Ultraviolet Projection Lithography Objective Lens

Optimization Design Method of a 6‐DOF Micromanipulation Mechanism for Extreme Ultraviolet Projection Lithography Objective Lens

Design of an extreme ultraviolet lithography projection objective with a grouping design method through forward and reverse real ray tracing.

As the logic node gets more and more advanced, the performance of extreme ultraviolet (EUV) objective projection is required to be higher and higher in a large field of view. It is known that a good initial structure can greatly reduce the dependence on the experience of optical designers. In this paper, a grouping design method through forward and reverse real ray tracing is proposed to design the aspheric initial structure for the EUV objective system. The system is first divided into three groups, and each spherical group is designed separately. Then, the three groups are connected as a whole spherical initial objective system. Through forward and reverse real ray tracing, each spherical group is recalculated to an aspheric structure in turn. Finally, an iterative process is applied to improve the performance of the aspheric initial structure. The aspheric initial structure calculated by this method can be taken as a good starting point for further optimization. As verification of the design method, a six-aspheric-mirror EUV lithography objective with a numerical aperture of 0.33 has been designed, whose root mean square (RMS) wavefront error is less than 0.2 nm and distortion is less than 0.1 nm.

Localized and cascading secondary electron generation as causes of stochastic defects in extreme ultraviolet projection lithography (Erratum)

The <i>Journal of Micro/Nanopatterning, Materials, and Metrology</i> (JM3) publishes peer-reviewed papers on the core enabling technologies that address the patterning needs of the electronics industry.

High-Precision Measurement Method of Polarization Aberrations for Large Numerical Aperture (NA=0.55) Variable-Magnification Extreme Ultraviolet Lithography Projection Objective

High-Precision Measurement Method of Polarization Aberrations for Large Numerical Aperture (NA=0.55) Variable-Magnification Extreme Ultraviolet Lithography Projection Objective

Design of an illumination system for high numerical aperture anamorphic extreme ultraviolet projection lithography.

The illumination system design for high numerical aperture (NA) anamorphic objectives is a key challenge for extreme ultraviolet lithography. In this paper, a reverse design method of the off-axis mixed-conic-surface-type relay system and an automatic arrangement method of field facets are proposed to design a high NA anamorphic illumination system. The two off-axis relay mirrors are fitted into different conic surfaces based on the conjugation of the mask plane and field facet and that of the illumination exit pupil and pupil facet. To eliminate ray obscuration between neighboring field facets, the field facets are automatically arranged according to the distances that are determined by the relative tilt angles of neighboring field facets under the current illumination mode. The proposed methods are applied in the design of an illumination system matching the NA0.60 anamorphic objective. Simulation results show that the uniformity of the scanning energy distribution can reach 99% on the mask plane under different illumination modes.

Localized and cascading secondary electron generation as causes of stochastic defects in extreme ultraviolet projection lithography

Projection lithography using extreme ultraviolet (EUV) light at 13-nm wavelength is expected to achieve production of integrated circuits below 10 nm design-rules. In pursuit of further miniaturization, however, stochastic pattern defect problems have arisen. Here, we discuss the possible impact of spatially inhomogeneous secondary electron (SE) generation on stochastic defects. Two mechanisms are investigated: (1) accidental connections of photon shot noises enhanced by densely localized SE generation and (2) cascading SE generation along photoelectron trajectory traveling from pattern edge into a dark region. Since such defect probabilities are extremely low (typically 10 − 4 to ∼10 − 12), results of Monte Carlo simulation based on classical optical image and electron scattering simulations are converted into probability functions for densities of physical/chemical events such as photon absorption, SE generation, and elementary reaction in chemically amplified resists. Probabilities of pattern formation and of defect generation are modeled using these functions. Results of performance optimization using a multiobjective genetic algorithm show higher stochastic defects probability in EUV than in conventional deep-UV exposure due to larger spatial inhomogeneity in reaction density and existence of SE generation strings. Defect probabilities are strongly dependent on absolute pattern sizes in the two mechanisms, regardless of the resolution capability of imaging systems. Guidelines for suppressing stochastic defects are suggested, such as homogenization of reaction density, material composition for increasing scattering cross-section, and suppression of pattern edge fluctuation.

Protection of extreme ultraviolet lithography masks. I. Thermophoretic protection factors at low pressure for diffusing nanoscale particles

Model calculations are presented for thermophoretic protection of an extreme ultraviolet (EUV) mask placed face down in an EUV mask inspection tool. The protection factors, defined as the ratio of challenge particles to deposited particles, are calculated for a variety of test conditions (temperature gradient, gas type, particle density, and particle position) for a reticle bathed in clean gas from a facing showerhead. Thermophoretic protection (in combination with gravity) provides robust protection for particle sizes greater than ∼20 nm. However, for particle sizes less than ∼20 nm, protection falters quickly and is severely degraded for highly diffusing 10 nm particles that are of concern for mask contamination. Estimates are made for the required level of particle protection in both EUV mask inspection and EUV projection lithography. When compared with these estimates for the required protection, it is clear that thermophoresis alone cannot successfully defend against particles smaller than ∼20 nm, and must be augmented or replaced by another approach. Initial calculations are presented that suggest a cross-cutting gas flow, in combination with thermophoresis and a face-down mask orientation, can successfully protect mask surfaces from particle deposition for particles with 10 nm diameter or greater, motivating more detailed calculations of flow-based mask protection in a companion paper Part II.

A model for multilayer analysis in a coated extreme ultra-violet lithography projection system

Reflection-enhancing multilayer coating is one of the key technologies in Extreme Ultra-Violet Lithography (EUVL). The typical thickness of the Mo/Si multilayer coatings generally adopted in EUVL is about 300nm, which is much greater than the operating wavelength of 13.5nm. The EUV is reflected completely back to the vacuum before punching the substrate. This changes the actual reflective surface by dozens of waves and creates extra aberrations. In this study, an equivalent working surface model for multilayer analysis based on the energy conservation principle was developed. Under the premise of the same energy modulation function for real and model film systems, each multilayer film coated optical element with complex energy propagation is transformed into a single surface. Optical design software can be used to assess these virtual surface in terms of estimation and optimization. Finally, the model was applied to a same Schwarzschild system but with different coating solutions. And the best solution was confirmed in which the coating-induced aberrations could be compensated for by image defocus.

New technique for aberration diagnostics and alignment of an extreme ultraviolet Schwarzschild objective

Schwarzschild objectives are widely used in the extreme ultraviolet (EUV)/soft X-ray spectral region both as reduction and magnification optics, e.g. for small-field projection lithography and microscopy, respectively. When using a Schwarzschild objective as a micro-exposure tool (MET) at high spatial resolution (half-pitch≤0.1μm), in addition to the tight requirements on the design and surface figure for the single optics, also an accurate alignment between the two mirrors is needed to reach the planned spatial imaging detail. Ideally, at-wavelength alignment should be done in order to overcome limitations due to diffractive effects. While this can be easily performed on synchrotron beam lines, it becomes time expensive (and components consuming) on low-power laboratory plasma sources. In this work we propose and test a new technique to align a EUV Schwarzschild objective by means of ultraviolet light. The aligned objective allowed the attainment of lithographic patterning with edge response of 90nm, as part of the laboratory-scale MET for EUV projection lithography realized at the ENEA Frascati Research Centre.

Mitigation of ion and particulate emission from laser-produced plasmas used for extreme ultraviolet lithography

While developing a laboratory-scale micro-exposure tool for extreme ultraviolet (EUV) projection lithography which uses a laser-produced plasma emitting EUV pulsed radiation, we faced the problem of suppressing the various debris (ions, neutrals, particulate, clusters, droplets) emitted by the plasma target. The suppression of debris is a crucial task in the frame of EUV projection lithography, mainly because debris seriously limit both lifetime and performance of the expensive optics and filters put close to the plasma source. In this paper we present the experimental measurements of main debris characteristics (velocity, size, charge, momentum, spectral energy, spatial distribution). Then, we present the operating results of a patented debris mitigation systems (DMS) specifically designed to suppress debris with the measured characteristics. We achieved reduction factors ∼800 for atoms and nm-size clusters, and ∼1600 for particles larger than 500nm. These results are at the forefront in this field. The excellent performance of our DMS was a breakthrough to achieve a 90-nm patterning on commercial resists by our micro-exposure tool EUV projection lithography.

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.

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

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

Ar+ ion beam machining of the carbon thin layer deposited on the Zerodur® substrate for EUVL projection optics

In order to overcome roughening and charging of the Zerodur® substrate for extreme ultra violet lithography (EUVL) optics during 0.5–10keV Ar+ ion beam machining, we had already proposed a method in which a thin Si layer was deposited on the Zerodur® substrate and figuring the deposited Si layer. In this experiment, in order to get smoother surface than Si layer, carbon which has smaller atomic size than that of Si was deposited on the Zerodur® substrate. Then, we investigated ion energy and machined depth dependence of the surface roughness of the carbon layer machined by Ar+ ion beam in the range of 0.3–3keV and obtained the following results; surface roughness of the carbon layer machined by 1.0keV Ar+ ion beam to the machined depth of 50nm becomes 0.08nm rms and this value is smaller than that (0.12nm rms) of Si. Therefore, the proposed processing method can be applicable for the figuring of substrates for EUVL projection optics.

极紫外投影光刻掩模衍射简化模型的研究

极紫外投影光刻掩模衍射简化模型的研究

极紫外投影光刻掩模衍射简化模型的研究

极紫外投影光刻掩模衍射简化模型的研究

极紫外投影光刻掩模衍射简化模型的研究

极紫外投影光刻掩模衍射简化模型的研究

Fully model-based methodology for simultaneous correction of extreme ultraviolet mask shadowing and proximity effects

Extreme ultraviolet (EUV) lithography is a promising candidate for high-volume manufacturing at the 22-nm half-pitch node and beyond. EUV projection lithography systems need to rely on reflective optical elements and masks with oblique illumination for image formation. It leads to undesired effects such as pattern shift and horizontal-to-vertical critical dimension bias, which are generally reported as shadowing. Rule-based approaches proposed to compensate for shadowing include changing mask topography, introducing mask defocus, and biasing patterns differently at different slit positions. However, the electromagnetic interaction between the incident light and the mask topography with complicated geometric patterns, such as optical diffraction, not only causes shadowing but also induces proximity effects. This phenomenon cannot be easily taken into account by rule-based corrections and thus imposes a challenge on a partially model-based correction flow, the so-called combination of rule- and model-based corrections. A fully model-based correction flow, which integrates an in-house optical proximity correction algorithm with rigorous three-dimensional mask simulation, is proposed to simultaneously compensate for shadowing and proximity effects. Simulation results for practical circuit layouts indicate that the fully model-based correction flow significantly outperforms the partially model-based one in terms of correction accuracy, while the total run time is slightly increased.

Two stage ion beam figuring and smoothening method for shape error correction of ULE® substrates of extreme ultraviolet lithography projection optics: Evaluation of high-spatial frequency roughness

The ULE® substrates used in projection optics of extreme ultraviolet lithography (EUVL) tools are mechanically prefinished with shape accuracy of several nanometer rms (specification: under 0.15 nm rms) with high-spatial frequency roughness (HSFR) (spatial wavelength: under 1 μm) of 0.06 nm rms. The ion beam figuring is used for the final shape error correction of the substrates at low spatial wavelength of greater than 1 mm using high energy (5–10 keV) Ar+ ion beam with 1 mm beam diameter. However, the surface roughness values of the ULE® substrates when machined by 5 and 10 keV Ar+ ion beams result in 0.15 and 0.17 nm rms, respectively; also it is to be noted that these values are greater than the required 0.15 nm rms specification of HFSR. Therefore, the authors developed a method where low energy (0.3 and 0.5 keV) ion beams are used for smoothening the surface of ULE® substrates, previously treated with high energy (5 and 10 keV) ion beams for figuring. By this two-stage operation of ion beam figuring and followed by smoothening, ULE® substrates with HSFR of 0.1 nm rms were obtained. Therefore, the authors conclude that the method presented here is one of the most effective methods for the figure error correction of ULE® substrate and will be useful in EUVL projection optics.

Laser–plasma–source debris-related investigations: an aspect of the ENEA micro-exposure tool

Debris mitigation is still a critical issue for the development of extreme ultraviolet projection lithography (EUVL). Here, we describe the analysis of the efficiency of the debris-mitigation system (DMS) presently in use in the laser–plasma–source micro-exposure tool (MET) operating at the Frascati ENEA Research Center. Basic to such an analysis is a code specifically developed for processing the images of debris-flux-exposed glass slides. The code stands out as a tool for further plasma debris-related analyses.

Protection and reduction of surface oxidation of Mo∕Si multilayers for extreme ultraviolet lithography projection optics by control of hydrocarbon gas atmosphere

To protect the surface oxidation of Mo∕Si multilayer films by extreme ultraviolet (EUV) irradiation under a vacuum atmosphere with residual water, two experiments were carried out. One consisted of examining the oxidation protection effect for isopropyl alcohol (IPA) and n-decane gases. The reflectivity change of the Ru-capped multilayer film by EUV irradiation was investigated under a vacuum atmosphere with residual water vapor at a pressure of 1.3×10−5Pa, and, in addition, each hydrocarbon (HC) gas was introduced by changing its pressure. A protective effect against oxidation was observed in both gases when introduced at a pressure in the order of 10−6Pa. For IPA, no remarkable decrease in the reflectivity was observed even when the introductory pressure was raised to the order of 10−4Pa. However, for n-decane, the reflectivity decreased remarkably when pressure in the order of 10−5Pa was introduced. The other experiment consisted of examining the reduction effect of the oxidized surface by EUV irradiation when introducing HC gas. Ru- and Si-capped multilayers were once oxidized by EUV irradiation under a water vapor atmosphere. However, for the Ru-capped multilayer, the reflectivity was recovered when EUV was irradiated in the presence of ethanol gas at a pressure of 3.8×10−5Pa. The oxide layer of the Ru cap was reduced at this time.