N. N. Tsybin

Multilayer Zr/Si filters for EUV lithography and for radiation source metrology

The technique for fabrication of thin film filters with high mechanical strength, capable of withstanding the prolonged heat load of 1 W/cm2, has been developed. Freestanding multilayer Zr/Si filters of size 20×150 mm2 with high transparency of 76% at wavelength λ = 13 nm were manufactured for EUV lithography tool. We have also developed and fabricated various designs of filters (freestanding or mesh supported) with lower transparency of 40-50% for experiments with intensive EUV sources. The tests of differential pressure withstandability and heat-resistance of filter samples were fulfilled. In order to model the influence on the filter of intensive radiation of the lithography source we have tested Zr/Si film samples by the Joule heating in vacuum at residual pressure of 10-8 Torr. The testing consisted in continuous heating of Zr/Si films at the electrical power per area unit from 0.5 W/cm2 to 6 W/cm2 during long period of time (up to 2 months). The influence of the long-term heat load on the transparency of samples at λ = 13 nm and within wavelength region 0.3 - 2 μm was investigated.

Influence of annealing on the structural and optical properties of thin multilayer EUV filters containing Zr, Mo, and silicides of these metals

New freestanding multilayers consisting of Zr, Mo and silicides have been fabricated and studied as spectral purity filters for the future generation of projection EUV lithography tools with a wavelength of operation 13 nm. The optical properties of multilayers were measured in EUV, visible and IR spectral ranges. Developed filters combine transparency of 70% at λ = 13 nm with high efficiency of suppression of out-of-band radiation (2-3 orders of magnitude from UV to IR regions). The reflectivity of filters at λ = 10.6 μm, where the spectral intensity of background radiation of laser plasma EUV source is at a maximum, achieves 88%. We have tested the withstandability of new multilayer structures to long-term (3 - 7 hours) vacuum heating by CO2-laser radiation at the power, absorbed per area unit of the specimen, of 8 W/cm2. The annealed samples were studied by secondary ion mass-spectroscopy and optical measurements were also performed. The most promising structure Zr/ZrSi2 coated with MoSi2 layers showed smaller decrease in transparency at λ = 13 nm (from 73% to 68%) than other tested compositions. Freestanding filters of 160 mm in diameter with transparency of 65% at wavelength λ = 13 nm were manufactured for EUV lithography tool.

Multilayer thin-film filters of extreme ultraviolet and soft X-ray spectral regions

Multilayer thin-film filters from different materi al pairs with spectral windows within the wavelength range 7n < 60 nm were developed. Optical characteristics of samples in the extreme ultraviolet, visible, and IR spectrum ranges were studied. Ultrathin freestanding Zr/Si large-aperture filters with transparencies up to 76% at 7n = 13 nm were manufactured for projection lithography test benches; variations in their properties during long-term thermal loads were studied.

Thin film multilayer filters for solar EUV telescopes

Al, with a passband in the wavelength range of 17-60 nm, and Zr, with a passband in the wavelength range of 6.5-17 nm, thin films on a support grid or support membrane are frequently used as UV, visible, and near-IR blocking filters in solar observatories. Although they possess acceptable optical performance, these filters also have some shortcomings such as low mechanical strength and low resistance to oxidation. These shortcomings hinder meeting the requirements for filters of future telescopes. We propose multilayer thin film filters on the basis of Al, Zr, and other materials with improved characteristics. It was demonstrated that stretched multilayer films on a support grid with a mesh size up to 5 mm can withstand vibration loads occurring during spacecraft launch. A large mesh size is preferable for filters of high-resolution solar telescopes, since it allows image distortion caused by light diffraction on the support grid to be avoided. We have investigated the thermal stability of Al/Si and Zr/Si multilayers assuming their possible application as filters in the Intergelioprobe project, in which the observation of coronal plasma will take place close to the Sun. Zr/Si films show high thermal stability and may be used as blocking filters in the wavelength range of 12.5-17 nm. Al/Si films show lower thermal stability: a significant decrease in the films transmission in the EUV spectral range and an increase in the visible spectrum have been observed. We suppose that the low thermal stability of Al/Si films restricts their application in the Intergelioprobe project. Thus, there is a lack of filters for the wavelength range of λ>17  nm. Be/Si and Cr/Si filters have been proposed for the wavelength range near 30.4 nm. Although these filters have lower transparency than Al/Si, they are superior in thermal stability. Multilayer Sc/Al filters with relatively high transmission at a wavelength of 58.4 nm (HeI line) and simultaneously sufficient rejection in the wavelength range near 30.4 nm (HeII line) have been fabricated. They are planned to be used in the project KORTES, whose telescopes will have an EUV channel at 58.4 nm.

Current status and development prospects for multilayer X-ray optics at the Institute for Physics of Microstructures, Russian Academy of Sciences

A real opportunity for applying traditional optical methods to soft X-ray and extreme UV (ultraviolet) radiation bands has appeared thanks to recent successes in the area of multilayer-mirror deposition and procedures for fabricating supersmooth and highly precise substrates of mirrors. The implementation of this opportunity opens up fundamentally new prospectss in the nanodiagnostics of substances, micro- and nanoelectronics, microbiology, solar astronomy and other applications. The main directions in multilayer X-ray optics developed at the Institute for the Physics of Microstructures, Russian Academy of Sciences, are presented and the aspects of the use thereof in science and technology are considered. The main problems arising during the fabrication of multilayer interference structures for the soft X-ray and extreme UV bands are discussed. The main results obtained recently in the scope of each direction of investigation are presented. Plans for the future development of these directions are discussed.

A double-stream Xe:He jet plasma emission in the vicinity of 6.7 nm

We present the results of investigations of extreme ultraviolet (EUV) light emission in the range from 5 to 10 nm. The light source was a pulsed “double-stream” Xe:He gas jet target irradiated by a laser beam with a power density of ∼1011 W/cm2. The radiation spectra were measured with a Czerny-Turner monochromator with a plane diffraction grating. The conversion efficiency of the laser energy into EUV radiation caused by Xe+14…+16 ion emission in the range of 6–8 nm was measured using a calibrated power meter. The conversion efficiency of the laser radiation into EUV in the vicinity of 6.7 nm was (2.17 ± 0.13)% in a 1 nm spectral band. In the spectral band of the real optical system (0.7% for La/B multilayer mirrors) emitted into the half-space, it was (0.1 ± 0.006)%. The results of this study provide an impetus for further research on laser plasma sources for maskless EUV lithography at a wavelength of 6.7 nm.

Mo-based EUV multilayer filters with enhanced thermal stability

New Mo-based multilayer structures are proposed, and sample free-standing filters 160 mm in diameter with a transparency of 71% at a wavelength of λ = 13.5 nm are described. The enhanced thermo-oxidative stability of filters with Mo layers is demonstrated as opposed to Zr-based films. It is shown in particular that Mo-based filters can withstand vacuum heat annealing at 800°C for three hours without any optical and structural changes.

Evolution of elemental distribution in free-standing structures of Zr/ZrSi2 with MoSi2 and ZrSi2 protective coatings under annealing

An analysis of the in-depth distribution of elements in annealed samples of multilayer Zr/ZrSi2 EUV filters with protective layers of MoSi2 and ZrSi2 was performed by secondary ion mass spectrometry (SIMS). The vacuum annealing of the samples was performed at temperatures of up to 720–950°C for 3–7 h and at a residual pressure of ∼10−7 torr. It was demonstrated that MoSi2 coatings effectively inhibit the accumulation of oxygen by the heated film, which is the process that determines the optical degradation of the filter.

Laboratory reflectometer for the investigation of optical elements in a wavelength range of 5 – 50 nm: description and testing results

We describe a laboratory reflectometer developed at the IPM RAS for precision measurements of spectral and angular dependences of the reflection and transmission coefficients of optical elements in a wavelength range of 5–50 nm. The radiation is monochromatised using a high-resolution Czerny–Turner spectrometer with a plane diffraction grating and two spherical collimating mirrors. A toroidal mirror focuses the probe monochromatic beam on a sample. The X-ray source is a highly ionised plasma produced in the interaction of a high-power laser beam with a solid target at an intensity of 1011–1012 W cm–2. To stabilise the emission characteristics, the target executes translatory and rotary motions in such a way that every pulse irradiates a new spot. The short-focus lens is protected from contamination by erosion products with the use of a specially designed electromagnetic system. The samples under study are mounted on a goniometer is accommodated in a dedicated chamber, which provides five degrees of freedom for samples up to 500 mm in diameter and two degrees of freedom for a detector. The sample mass may range up to 10 kg. The X-ray radiation is recorded with a detector composed of a CsI photocathode and two microchannel plates. A similar detector monitors the probe beam intensity. The spectral reflectometer resolution is equal to 0.030 nm with the use of ruled gratings with a density of 900 lines mm–1 (spectral range: 5–20 nm) and to 0.067 nm for holographic gratings with a density of 400 lines mm–1 (spectral range: 10–50 nm). We analyse the contribution of higher diffraction orders to the probe signal intensity and the ways of taking it into account in the measurements. Examples are given which serve to illustrate the reflectometer application to the study of multilayer mirrors and filters.

SIMS study of annealing effect on element distribution in free-standing Al/Si and Zr/ZrSi2 multilayer films

The results of secondary ion mass spectrometry (SIMS) depth profile analysis of free-standing Al/Si and Zr/ZrSi2 multilayer films by a TOF.SIMS-5 instrument are reported for the first time. A prestudy of elemental composition in Zr and ZrSi2 single layers and Zr/ZrSi2 multilayer films deposited on Si substrates with sublayers revealed enhanced diffusion of carbon in Zr layers. Ion sputtering and charge neutralization conditions were determined to provide fixed position of free-standing thin films during analysis. An elemental composition of 100-nm-thick Zr/ZrSi2 multilayer structures with Mo and B4C protective layers was measured prior and after laser annealing.