ISSN 10628738, Bulletin of the Russian Academy of Sciences: Physics, 2010, Vol. 74, No. 1, pp. 46–49. © Allerton Press, Inc., 2010. Original Russian Text © B.A. Volodin, S.A. Gusev, M.N. Drozdov, S.Yu. Zuev, E.B. Klyuenkov, A.Ya. Lopatin, V.I. Luchin, A.E. Pestov, N.N. Salashchenko, N.N. Tsybin, N.I. Chkhalo, 2010, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2010, Vol. 74, No. 1, pp. 53–57.
Multilayer ThinFilm Filters of Extreme Ultraviolet and Soft Xray Spectral Regions B. A. Volodin, S. A. Gusev, M. N. Drozdov, S. Yu. Zuev, E. B. Klyuenkov, A. Ya. Lopatin, V. I. Luchin, A. E. Pestov, N. N. Salashchenko, N. N. Tsybin, and N. I. Chkhalo Institute for the Physics of Microstructures, Russian Academy of Sciences, Nizhni Novgorod, 603950 Russia email:
[email protected]nnov.ru Abstract—Multilayer thinfilm filters from different material pairs with spectral windows within the wave length range λ < 60 nm were developed. Optical characteristics of samples in the extreme ultraviolet, visible, and IR spectrum ranges were studied. Ultrathin freestanding Zr/Si largeaperture filters with transparencies up to 76% at λ = 13 nm were manufactured for projection lithography test benches; variations in their prop erties during longterm thermal loads were studied. DOI: 10.3103/S1062873810010120
ical and vibrational loads. Requirements as to the fil ter’s ability to block longwave background radiation also become more rigorous, leading to strict limita tions on the size and number of throughandthrough micropores, the latter being manufacturing defects. Transmittance in the visible light range for the receiv ing filters of orbital devices should not exceed ∼10–6.
INTRODUCTION When working with sources of extreme ultraviolet (EUV) and soft Xray (SXR) radiation, submicron metallic film and coatings are used as filters suppress ing background radiation in the UV, visible, and IR spectral regions. Together with multilayer Xray mir rors, thinfilm transparent filters are used in spectral devices and imaging optics to diagnose laboratory plasma and measure the characteristics of roentgen optical elements in circuits of projection EUV lithog raphy and in orbital equipment designed for solar observations in the SXR and EUV ranges. B.A. Volodin, S.A. Gusev, M.N. Drozdov, S.Yu. Zuev, E.B. Klyuenkov, A.Ya. Lopatin, V.I. Luchin, A.E. Pestov, N.N. Salashchenko, N.N. Tsybin, N.I. Chkhalo. Depending on the purpose of thinfilm filters, the requirements for their properties and quality can differ drastically. Studies of laboratory radiation sources require, as a rule, smallaperture filters with transmit tance in an operating range on the order of 40–50%. This transparency corresponds to metallic film thick nesses of hundreds of nanometers and attenuation of background UV and visible radiation for such samples of no less than several orders of magnitude. Samples prepared using the technique of vacuum deposition with subsequent separation from a substrate exhibit acceptable mechanical strength that can be addition ally improved by pasting the film onto a supporting grid [1–3]. The requirement of high reliability takes center stage when developing thinfilm filters for space exploration of the Sun in the EUV spectral region. The necessary margin of safety is in this case achieved by using a rigid support grid, optimizing the manufactur ing process and filter parameters, and carefully select ing the samples and testing their resistance to mechan
When discussing the prospects for using thinfilm filters in the process of projection lithography in the λ = 13 nm wavelength range with a plasma radiation source, extremely high transparency (up to 70–80%) at the operating wavelength and maximum duration of operation at high thermal loads are required. If the power of the source in the passband of the multilayer mirrors in the imaging system is estimated at ~100 W, the power of the background radiation incident on the receiving filter will produce several kilowatts. In case of such high incident power, even a largesize filter (~100 cm2) will heat up to several hundreds of degrees, which can drastically activate oxidation of the film material with residual gases. Thus, the problem is to develop a largeaperture ultrathin thermostable filter less than 100 nm thick without a supporting grid. The present paper reviews the potentialities of dif ferent techniques applied by the IPM RAS for manu facturing and testing thinfilm elements intended for different purposes, with different composition and design. As far as their structure is concerned, they are themselves multilayer films made from alternating pairs of materials such as Zr/Si, Mo/C, Cr/Sc, Zr/Al, Al/Si, and a number of others. Spectral windows of the developed filters cover different sections of the λ < 60 nm range. 46
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Examples of filter variants Sample transmittance, % Zr/Si structure d1 = 2.8 nm, d2 = 1.4 nm, N = 43 d1 = 2.8 nm, d2 = 1.4 nm, N = 50 d1 = 2.8 nm, d2 = 1.4 nm, N = 50 d1 = 1.6 nm, d2 = 0.6 nm, N = 25 d1 = 3.1 nm, d2 = 1.2 nm, N = 37
λ = 13 nm
λ = 633 nm
50 43 43 76 51
4 × 10–4 8 × 10–5 8 × 10–5 2.2 1 × 10–3
Variant Without grid, D = 36 mm Without grid, D = 100 mm 33 × 38 мм2, rigid grid, adhesive Without grid, 20 × 140 mm2 D = 30 mm, grid, galvanic
Note: d1, d2—Zr and Si thickness over the period, respectively; N—number of periods; D—filter aperture.
METHODS OF SAMPLE MANUFACTURING AND TESTING Thinfilm samples are manufactured using the technique of magnetron deposition. Setups with up to six magnetrons in the vacuum chamber with different targets are used. This allows a multilayer twomaterial structure, a sublayer (required to separate the structure from the substrate), and, if necessary, some additional layers to be formed on a silicon plate over one cycle. To obtain a freestanding structure, a thin film is carried over after sublayer dissolution in the process of liquid etching onto a holder of required size. Apart from absorption film filters, this technique allows elements with improved periodic structure and interlayer rough ness (i.e., transmission phase shifters for the SXR range) to be developed [4, 5]. If a largeaperture filter is to separate volumes with different pressure levels, we face the problem of pro ducing a structure reinforced with a supporting grid. In this case, an adhesion layer and a copper coating are deposited onto a multilayer film, and an image of a grid with a 1.6 mm cell and 80% geometrical transmit tance is then lithographically formed on the coating. Up to thicknesses of 15–20 µm, it is grown by the gal vanic deposition of copper. The lithography and gal vanic deposition are performed by the ReperNN company (Nizhni Novgorod). During the subsequent etching, the multilayer film is separated from the sub strate together with the grid and is fixed on the holder. A special filter design in which a rigid support grid is fixed with epoxy adhesive to a stretched multilayer film has been developed for solar astronomy devices. Filter characteristics in the SXR and EUV ranges are measured using the channels of a gratingtype grazing spectrometer–monochromator and large aperture reflectometer with Xray mirrors [6]. Both devices are equipped with an Xray tube as the radia tion source. The mirror reflectometer makes it possi ble to precisely determine sample transparency at the operating wavelength of the used mirrors, while the spectrometer equipped with a diffraction grating allows measurements with high spectral resolution and scanning over the spectrum within the limits of the emission lines of the used anodes. The degree of back
ground radiation attenuation by the filters is estimated in the present study on basis of the results from sample transmittance measurements at the wavelength of a heliumneodymium laser (λ = 633 nm). Smallangle reflectometry in the hard Xray range (λ = 0.154 nm) is used to control the multilayer structure parameters, i.e., the period d and the ratio of material thicknesses in the period. When necessary, thinfilm structures are studied using the techniques of electron microscopy and sec ondaryion mass spectroscopy (SIMS), taking advan tage of the potentialities of the JEOL 2000 microscope and the TOF.SIMS5 complex. In particular, the crys talline structure and elemental composition of the fil ters for projection EUV lithography are studied. A spe cial technique in which the thermal load of EUV source radiation on a filter is simulated by heating a freestanding film with DC electric current or current pulses 100 ns in duration with frequencies of up to 50 Hz has been proposed for longterm filter testing. FILTERS FOR PLASMA EXPERIMENTS AND SOLAR ASTRONOMY When developing thinfilm filters, the source data determining the choice of layer materials and thick nesses are the sample transmittance in the passband or at specified wavelengths. For the spectrum region near λ = 13 nm (which is promising from the point of view of projection EUV lithography), we manufactured and tested samples of Zr/Si, Nb/Si, and Mo/Si multilayer structures [3]. The table presents data for the several manufactured versions of the Zr/Si filter, which was characterized by optimal transparency [7, 8]. Filters with wider spectral windows in the SXR and EUV ranges were developed using other materials. The transmittance characteristics of the manufactured samples are shown in Fig. 1. A focusing spectrometer based on the Gamosh scheme [9] was equipped with a freestanding multilayer Cr/Sc filter (dCr = 1.5 nm, dSc = 1.6 nm, N = 63). The filter was transparent in the spectral region of the “water window” (λ = 2.3– 4.4 nm) and suppressed radiation in the visible, UV, and nearIR ranges by no less than 6 orders of magni
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VOLODIN et al. T, % Cr/Sc Zr/Si
60 Mo/C
Zr/Al 40
20
0
5
10
15
20
25 λ, nm
Fig. 1. Spectral transmittance of manufactured filters in the SXR and EUV ranges. The marks denote the measured values; the dashed line shows the characteristic depen dence for the Zr/Si structure.
tude. Freestanding filters based on Mo/C structures (dMo = 2.1 nm, dC = 0.6 nm, N = 60) and Zr/Al (dZr = 5.1 nm, dAl = 3.6 nm, N = 11) with passbands in the range of λ < 22 nm were developed and manufactured for experiments on detecting EUV radiation generated during the reflection of a laser pulse at a nonlinear plasma wave (a relativistic mirror) [10]. At λ = 633 nm, the transmittance of the samples is 5 × 10–6 and 1.2 × 10–4, respectively. We manufactured thinfilm structures of increased strength for use as receiving filters in the CORONAS PHOTON satellite’s equipment [11]. Largeaperture filters were made of separate rectangular elements with 33 × 38 mm sides that form a rigid grid with a fixed multilayer structure. The grid is 0.5 mm thick and its geometrical transparency is 80%, each cell being 1.7 mm in size. In addition to the Zr/Si structures with the parameters listed in table, multilayer Al/Si filters (dAl = 3.2 nm, dSi = 1.5 nm, N = 65) of the same com position were manufactured for orbital devices operat ing in the 29–32 nm range. The Al/Si structure had a wide passband in the wavelength range of 17.1 nm < λ < 60 nm, while the measured sample transparency was T = 25% at λ = 30.4 nm and T = 39% at λ = 17.1 nm. ULTRATHIN FILTERS FOR PROJECTION EUV LITHOGRAPHY
Fig. 2. Photo of an ultrathin Zr/Si filter with operating transmittance T = 76%.
R, Ω
T(λ = 633 nm), % 12
72 10
1
8
64
6 2
56
4 2
48 20
60
100
140
180 Time, h
0
Fig. 3. Time dependences of transparency at λ = 633 nm (1, right Yaxis) and electrical resistance of a sample (2, left axis) heated in vacuum at power density q = 1.5 W cm–2. Transmittance of the tested film at a wavelength of 13 nm fell from 76 to 56% during the test.
A distinguishing feature of the filters mounted in the prototypes of industrial lithography schemes of the 13 nm range is the exceptional thinness of the films. The possibility of producing a freestanding largeaper ture Zr/Si structure with transparency of up to 76% was experimentally demonstrated in [8]. In particular, bananashaped samples with a characteristic size of 20 × 140 mm were obtained (Fig. 2). To determine the critical level of the filter thermal loads, model experiments in which Zr/Si film samples were heated with electric current were performed in a lithographic setup. The sample to be tested was a free standing multilayer structure that covered a rectangu lar opening (8 × 8 mm) with a quartz substrate 0.5 mm thick and 28 × 22 mm in size. Film contact pads with two pairs of the electrical contacts used to measure film resistance were attached to both sides. Heating took place in a chamber evacuated to a pressure of ∼10–8 Torr, while electrical resistance and sample transparency at wavelength λ = 633 nm were continu ously measured (Fig. 3). Upon completion of the heating, structure transmittance at λ = 13 nm was measured. It was shown that the allowable level of power released per one unit of the Zr/Si film at which the filter can operate in the scheme for long periods of time (up to 1000 h) without changing its optical prop erties is limited by 1 W cm–2. Based on the data from sample Xray fluorescent analysis, the reduction in transparency at wavelength λ = 13 nm is attributable to
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8 6
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(1 W cm–2) at which filter optical properties start to deteriorate was found. At present, multilayer struc tures with better thermal load stability are being sought. At the same time, the possibility of manufac turing samples with larger apertures are being consid ered. In particular, multilayer Zrbased filters 160 mm in diameter with transparency T = 65% at λ = 13 nm have already been manufactured.
2 4 2 0
200
400
600
800
1000 Time, s
Fig. 4. Increase (based on SIMS data) in the oxygen con tent of a Zrbased filter annealed in vacuum for 7 hours at q = 2.4 W cm–2; 1 is the initial sample, and 2 is the annealed sample. The Xaxis measures the intensity of oxygen ion detection; the Yaxis measures the time of ion film etching. The measured drop in filter operating trans parency was from 65 to 56%.
oxidation of the thin film material by residual oxygen. This conclusion is confirmed by the SIMS results shown in Fig. 4. CONCLUSIONS Techniques for developing and testing multilayer filters of soft Xray and extreme ultraviolet ranges were developed. On their basis, a set of filters with spectral windows covering the spectral region of λ < 60 nm were developed and studied. Variations in the structure and the methods of film fixing make it possible to use the products in multipurpose devices, ranging from laboratory spectrometers to the telescopes of space observatories. Freestanding largeaperture filters with transparencies close to 80% for projection nanolithog raphy at a wavelength of 13 nm were developed. Long term tests of the samples at high thermal loads were performed. The limit of the acceptable power
ACKNOWLEDGEMENTS The study was supported by the Russian Founda tion for Basic Research, project nos. 070200992, 09 0201473 and 090200389. REFERENCES 1. Mitrofanov, A.V. and Tokarchuk, D.N., Nucl. Instr. Meth. A, 1989, vol. 282, p. 546. 2. Powell, F.R. and Johnson, T.A., Proc. SPIE, 2001, vol. 4343, p. 585. 3. Andreev, S.S., Zuev, S.Yu., Klyuenkov, E.B., et al., Po verkhnost’. Rentgen., Sinkhrotron. i Neitron. Issled., 2003, no. 2, p. 6. 4. Andreev, S.S., Bibishkin, M.S., Kimura, H., et al., Izv. Akad. Nauk, Ser. Fiz., 2004, vol. 68, no. 4, p. 565. 5. Andreev, S.S., Bibishkin, M.S., Chkhalo, N.I., et al., Nucl. Instr. Meth. A, 2005, vol. 543, p. 340. 6. Bibishkin, M.S., Chehonadskih, D.P., Chkhalo, N.I., et al., Proc. SPIE, 2003, vol. 5401, p. 8. 7. Bibishkin, M.S., Zuev, S.Yu., Klimov, A.Yu., et al., Materialy Simpoziuma “Nanofizika i nanoelektronika” (Proc. Symp. “Nanophysics and Nanoelectronics”), Nizhni Novgorod: IFM RAN, 2005, vol. 5, p. 497. 8. Bibishkin, M.S., Chkhalo, N.I., Gusev, S.A., et al., Proc. SPIE, 2008, vol. 7025, p. 702502. 9. Borozdin, Yu.E., Kazakov, E.D., Luchin, V.I., et al., Pis’ma v Zh. Eksperim. Teoret. Fiz., 2008, vol. 87, no. 1, p. 33 [JETP Lett., (Engl. Transl.), 2008, vol. 87, no. 1, p. 27]. 10. Ragozin, E.N., Pirozhkov, A.S., Yogo, A., et al., Rev. Sci. Instrum., 2006, vol. 77, p. 123302. 11. Kuzin, S.V., Zhitnik, I.A., Bugaenko, O.I., et al., Izv. Akad. Nauk, Ser. Fiz., 2005, vol. 69, no. 2, p. 191.
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