Sascha Brose

Diffraction-assisted extreme ultraviolet proximity lithography for fabrication of nanophotonic arrays

In this article, the possibilities and limitations of proximity lithography with extreme ultraviolet (EUV) radiation are explored theoretically and experimentally. Utilizing partially coherent EUV radiation with a wavelength of 10.88 nm from a Xe/Ar discharge plasma EUV source, proximity patterning of various nanoantenna arrays has been performed. The experimental results are compared with the results of numerical scalar diffraction simulations, and it is shown that proximity printing in the Fresnel diffraction mode can enable production of high-resolution features even with lower resolution masks, successfully demonstrating sub-30 nm edge resolution in the resist. The potential of the method is explored by simulation of the patterning through circular and triangular apertures as well as through bowtie antenna patterns, with the results suggesting that precise control of the proximity gap and the exposure dose together with simulation-supported mask design optimizations may allow for a wide variety of high-resolution structures to be printed through relatively simple transmission masks. The method is especially suited for high-performance manufacturing of submicrometer sized nanophotonic arrays.

Line image sensors for spectroscopic applications in the extreme ultraviolet

The spectral range of extreme ultraviolet radiation (XUV or EUV) is an active area of research incorporating many scientific fields such as microscopy, lithography or reflectometry. During the last decade, a lot of effort has been put into transferring many of the known techniques developed at linear accelerators into the laboratory using discharge-produced plasmas (DPPs) or laser-produced plasmas (LPPs) as an alternative light source. In particular, the semiconductor industry is in need of on-site tools in the shorter wavelength range for production and inspection of structured surfaces with nanometer resolution. Here traditional charge coupled device (CCD) image sensors are inapplicable as detectors because of the strong absorption of XUV by matter prohibiting any generation of electron–hole pairs inside a deep lying p–n junction. As a solution, two-dimensional backthinned CCDs are available in the market offering high sensitivity to XUV light. Although for many applications a one-dimensional line scanning image sensor would be sufficient, they are non-existent for XUV. It is only lately that manufacturers have started to adopt the principle of backthinning to CCD line sensors to enhance sensitivity in the long wavelength UV range (>200 nm). Here we show that generally these compact sensors offer good quantum efficiencies in the XUV which make them a candidate for many spectroscopic applications and future industrial inline inspection tools for which costly two-dimensional CCDs are oversized. We have successfully implemented a compact sensor device into a laboratory XUV spectrometer and reflectometer. Our measurements compare the quantum efficiency of a state-of-the-art XUV array CCD to a phosphor-coated line sensor and a new backthinned line sensor. Additionally, we show recorded spectra from a laboratory DPP source to demonstrate the potential of a wide range of applications.

Extreme ultraviolet proximity lithography for fast, flexible and parallel fabrication of infrared antennas

We present a method for fabrication of large arrays of nano-antennas using extreme-ultraviolet (EUV) illumination. A discharge-produced plasma source generating EUV radiation around 10.88 nm wavelength is used for the illumination of a photoresist via a mask in a proximity printing setup. The method of metallic nanoantennas fabrication utilizes a bilayer photoresist and employs a lift-off process. The impact of Fresnel-diffraction of EUV light in the mask on a shape of the nanostructures has been investigated. It is shown how by the use of the same rectangular apertures in the transmission mask, antennas of various shapes can be fabricated. Using Fourier transform infrared spectroscopy, spectra of antennas reflectivity were measured and compared to FDTD simulations demonstrating good agreement.

Achromatic Talbot lithography with partially coherent extreme ultraviolet radiation: process window analysis

Abstract. The main purpose of this work is the experimental determination of the process window for achromatic Talbot lithography with partially coherent extreme ultraviolet (EUV) radiation. This work has been performed using the EUV laboratory exposure tool. It consists of a discharge produced plasma source with a direct beam path to a phase-shifting transmission mask, avoiding losses due to additional optical components, the photoresist-coated wafer, and a positioning system for each component. Both the source and the mask are optimized for 11-nm wavelength. The process window has been identified by a systematic analysis of several exposure series. The optimization of exposure parameters resulted in 50-nm half-pitch of the wafer features using a transmission mask with a rectangular dot array of 70-nm half-pitch. The depth of field is found to be 20  μm, and it can be extended by spatial filtering. The exposure dose and mask–wafer distance are varied around their optimal values to estimate the process window, using defectivity of the pattern as a control parameter.

Tabletop coherent diffraction imaging with a discharge plasma EUV source

We present a coherent diffraction imaging (CDI) experiment using a high-frequency discharge plasma based, extreme ultraviolet (EUV) source. By using different illumination geometries, we generated EUV beams witha varying degree of spatial coherence, which were used to produce far field diffraction patterns from test objects. We then successfully reconstructed an illumination wavefront defined by a circular aperture. The present work explores the feasibility of compact tabletop CDI using a discharge plasma EUV source emerged from the technology development of EUV lithography, which can potentially find application in nanoscience and metrology.

Enabling laboratory EUV research with a compact exposure tool

In this work we present the capabilities of the designed and realized extreme ultraviolet laboratory exposure tool (EUVLET) which has been developed at the RWTH-Aachen, Chair for the Technology of Optical Systems (TOS), in cooperation with the Fraunhofer Institute for Laser Technology (ILT) and Bruker ASC GmbH. Main purpose of this laboratory setup is the direct application in research facilities and companies with small batch production, where the fabrication of high resolution periodic arrays over large areas is required. The setup can also be utilized for resist characterization and evaluation of its pre- and post-exposure processing. The tool utilizes a partially coherent discharge produced plasma (DPP) source and minimizes the number of other critical components to a transmission grating, the photoresist coated wafer and the positioning system for wafer and grating and utilizes the Talbot lithography approach. To identify the limits of this approach first each component is analyzed and optimized separately and relations between these components are identified. The EUV source has been optimized to achieve the best values for spatial and temporal coherence. Phase-shifting and amplitude transmission gratings have been fabricated and exposed. Several commercially available electron beam resists and one EUV resist have been characterized by open frame exposures to determine their contrast under EUV radiation. Cold development procedure has been performed to further increase the resist contrast. By analyzing the exposure results it can be demonstrated that only a 1:1 copy of the mask structure can be fully resolved by the utilization of amplitude masks. The utilized phase-shift masks offer higher 1st order diffraction efficiency and allow a demagnification of the mask structure in the achromatic Talbot plane.

Deposition and characterization of B4C/CeO2 multilayers at 6.x nm extreme ultraviolet wavelengths

New multilayers of boron carbide/cerium dioxide (B4C/CeO2) combination on silicon (Si) substrate are manufactured to represent reflective-optics candidates for future lithography at 6.x nm wavelength. This is one of only a few attempts to make multilayers of this kind. Combination of several innovative experiments enables detailed study of optical properties, structural properties, and interface profiles of the multilayers in order to open up a room for further optimization of the manufacturing process. The interface profile is visualized by high-angle annular dark-field imaging which provides highly sensitive contrast to atomic number. Synchrotron based at-wavelength extreme ultraviolet (EUV) reflectance measurements near the boron (B) absorption edge allow derivation of optical parameters with high sensitivity to local atom interactions. X-ray reflectivity measurements at Cu-Kalpha (8 keV) determine the period of multilayers with high in-depth resolution. By combining these measurements and choosing rob...

Lensless proximity EUV lithography with a xenon gas discharge plasma radiation

The possibilities and limitations of proximity and interference lithography under extreme ultraviolet (EUV) radiation are explored. Utilizing partially coherent EUV radiation at central wavelength of 10.9 nm from a Xe gas discharge plasma source, it is shown that proximity printing in the Fresnel diffraction regime can produce the high-resolution features even with low-resolution masks, and also yield sub-30 nm edge resolution in the resist. The scalability limit within non-paraxial case has been also studied. The effect of the diffraction behind the transmission mask is evaluated with respect to the achievable resolution. The finite-difference time-domain simulation of the diffraction patterns behind the Ni/Nb-based transmission mask is performed varying the pitch size. The results demonstrate that the method can be used to produce patterns with resolution down to 7.5 nm half-pitch with 2:1 mask demagnification utilizing achromatic Talbot effect with transverse electric (TE)-polarized light.

Actinic laboratory EUV tools for mask and pellicle metrology (Conference Presentation)

The growing industrial need for at-wavelength metrology of crucial elements in the EUV production chain, such as EUV masks and pellicles, can only be answered by cost-effective stand-alone tools. Especially the scaling to higher average operating powers of future EUV scanners is a task requiring multiple tools that can investigate not only critical degradation under increased EUV intensities, but also give information about underlying physical mechanisms of degradation and the imposed structural and chemical changes. At RWTH Aachen University, prototype tools are developed that address this challenge. The realized high-intensity EUV exposure tool, with a tightly focused sub-100 µm EUV spot, provides a unique opportunity to investigate the influence of EUV intensities on the level of the 250 W EUV scanner generation and beyond. With its pulse-to-pulse measurement system, the exposure dose in the tool is controlled with sub-mW/mm2 precision and it is independent from the temporal stability of the source. The influence of these exposures on the specimen can directly be characterized by the versatile laboratory EUV spectrometer. The tool can perform spectroscopic measurements of reflectance and transmittance under variable incidence angles and operates spectroscopically in the important wavelength range from 9 nm to 17 nm. In reflectance mode, the incidence angle of illumination is adjustable from 5° to 12° grazing with sub-0.1° resolution, which allows extraction of optical constants for reflectance measurements. It uses an EUV-radiation source based on a gas-discharge plasma, whose instabilities are accounted for by permanent reference monitoring during measurements. This in combination with the use of a calibrated reference sample allows to determine reflectivities with an accuracy <1 %. An additional transmission mode for sufficiently thin membranes allows for an independent determination of absorption coefficients of mono- and multi-layered membranes. The developed analysis algorithms allow not only to obtain best-fit layer parameters, but also to analyze the resulting precision and cross-correlations of parameters. In this contribution, exemplary use-cases of the tools will be presented. The application of the described methods to resist and pellicle investigations and to contamination studies will be introduced and discussed.

Holographic masks for computational proximity lithography with EUV radiation

Nowadays, EUV projection lithography has been proven effective for high-volume manufacturing of microchips. In parallel, high-resolution nanopatterning has been demonstrated utilizing interference lithography [1]. However, the former suffers from the complexity of projection optics, and the latter is limited to periodic structures. The presented approach is free of imaging optics and moreover allows for printing arbitrary (non-periodic) structures. Taking advantage of iterative designing of synthetic holograms, the described idea enables creating dedicated optical structure that can be applied for proximity lithography with EUV radiation. The method does not require a sophisticated optical system but necessitates numerical computation of a holographic mask, which gives desired intensity distribution at wafer. It is an inverse problem: for known intensity distribution at the wafer a design of holographic mask has to be inferred. The light field distribution in the plane of the mask can be calculated using phase retrieval methods based on Gerchberg-Saxton algorithm. The process can be described as iterative propagation of light field between mask and wafer planes at which certain constrains are applied: limited number of phase levels, minimal element size on the mask due to the fabrication process, correlation between absorption and phase-shifts and also the resist response. Due to the appropriate optical properties, a photoresist has been chosen as phase shifting material allowing for patterning of arbitrary mask structures. For the realization of the holographic phase shifting mask we used two phase-shifting levels. The fabrication process of the designed mask and experimental results of its characterization are also presented and discussed.