Composite thin-foil bandpass filter for EUV astronomy:titanium-antimony-titanium
Patrick Jelinsky, Christopher Martin, Randy Kimble, Stuart Bowyer, and Gordon Steele
Thin metallic foils of antimony and titanium have been investigated in an attempt to develop an EUV filterwith a bandpass from 350 to 550 A. A composite filter has been developed composed of antimony sand-wiched between two titanium foils. The transmissions of sample composite foils and of pure titanium foilsfrom 130 to 1216 A are presented. The absorption coefficients of antimony and titanium and the effect oftitanium oxide on the transmission are derived. The composite filter has been found to be quite stable andmechanically rugged. Among other uses, the filter shows substantial promise for EUV astronomy.
1. Introduction
One of the last wavelength ranges to be explored as-tronomically is the extreme ultraviolet (EUV) from 100to 1000 A. It was thought initially that EUV observa-tions beyond the solar system would not be feasiblebecause of photoelectric absorption by the interstellarmedium (ISM). Recently, however, many lines of ev-idence, including direct stellar observations in theEUV,1-3 have indicated that at least the local ISMdensity is low, and, therefore, visibility is much greaterthan had been thought previously. An upcomingNASA satellite, the Extreme Ultraviolet Explorer, willtake advantage of this situation to perform an all-skysurvey in several EUV bandpasses.
Bandpass filters are essential to EUV astronomy fortwo reasons: (1) they attenuate the copious geocoronalline emissions in the far UV and EUV (such as H I 1216A, He I 584 A, and He II 304 A), which would reduceseverely the sensitivity of photometric observations; (2)they define photometric regions of the source spectrumwhich provide information about the spectral energydistribution, temperature, and chemical compositionof a source as well as eliminating any possible far-UVemission from the source.
A number of EUV bandpass filters have been devel-oped and flown successfully in space instrumentation.
Gordon Steele is with Luxel Corporation, 515 Tucker Avenue,Friday Harbor, Washington 98520; the other authors are with Uni-versity of California, Space Sciences Laboratory, Berkeley, California94720.
However, there previously existed no filters with abandpass from 350 to 550 A, a wavelength range withsubstantial astronomical potential. This bandpasspossesses several desirable features: It lies at wave-lengths sufficiently removed from the Lyman edge (912A) that visibility through the ISM is still reasonablygood. Nevertheless, in this range, the EUV flux froma number of sources will have some absorption by theinterstellar medium, so the bandpass will provide im-portant information about the local ISM. Furthermore,the bandpass avoids the strongest nighttime diffusebackgrounds in the EUV, the 304- and 584-A lines of HeII and He I, permitting greater sensitivity to be achieved.We report here the results of our filter developmentefforts for this bandpass.
II. Development
The goal of the development effort was a filter satis-fying the following criteria:
(1) The bandpass must have minimum transmissionat the He II 304-A and He I 584-A background lines andhave substantial transmission between these lines;
(2) No large secondary bandpasses can exist outsidethis main band;
(3) Transmission must be low ( 10-5) far from thebandpass, especially at the longer wavelengths, wherethe intense H I 1216-A geocoronal line presents a severecontamination problem;
(4) The filter must be uniform and mechanicallyrugged when fabricated to relatively large sizes (>1-cmdiam); and
(5) The filter must not be unusually susceptible tothe formation of pinholes (usually caused by chemicalreactions with atmospheric moisture), which can in-crease wing transmissions to unacceptable levels.
The most promising materials for study were iden-tified through a search of the literature on the trans-mission properties of thin foils46 and through discus-sions with experts in the fabrication of such foils. An-timony, titanium, and tellurium were the only materialsfound with a suitable primary bandpass. Telluriumwas rejected because it has a large secondary bandpass.Antimony had a secondary bandpass, but it was smallenough to satisfy our criteria. The readily availabledata seemed to indicate that titanium did not have asecondary bandpass. Therefore, the materials selectedfor investigation were antimony and titanium.
Transmission measurements in the literature areusually made on small foils, so it was not known if largefoils could be fabricated with the required uniformity,mechanical strength, and pinhole resistance. In addi-tion, wing transmission data existed only down totransmission levels of 10-3. All these properties,therefore, had to be investigated for antimony and ti-tanium.
Sample filters were fabricated by Luxel Corp. Thestandard procedure is to deposit the metallic foils ontoa glass substrate, which has been coated first with arelease agent soluble in an organic solvent. The foil isthen floated off the substrate in the solvent, cementedto a mesh, and the combination is then cemented to aframe.
There are three conventionally used techniques forthe deposition of metallic foils: evaporation from atungsten filament, evaporation from a heated crucible,and electron-beam bombardment. The fabricationdifficulties encountered in the development effort re-quired experimentation with all three techniques.
Our first attempts were directed at the developmentof antimony foils. We found that these foils cannot bemade using the tungsten filament technique. In thistechnique, the metal evaporates from the hottest partof the filament, while new metal is drawn to the evap-oration point by capillary action. The evaporationtemperature of antimony is sufficiently close to itsmelting point that, as soon as the metal is heated enoughto flow, it flash evaporates, stopping the capillary actionand preventing smooth deposition. This type ofproblem can sometimes be overcome by alloying thematerial with a small amount of another metal. It washoped that germanium would permit smooth depositionwhen alloyed with antimony, but such attempts wereunsuccessful.
We found that these foils could be made using theheated crucible technique. The foils, however, werefound to be extremely moisture sensitive and mottledin appearance. These foils immediately developedpinholes if there was any residual moisture in the or-ganic solvent used to release the foil. Even with sub-stantial care, pinholes formed in all the samples withina few hours of fabrication. The mottled appearance wasbelieved to be due to low nucleation density and highsurface migration of the antimony atoms during filmgrowth. Because of these fundamental shortcomings,we abandoned our efforts to develop pure antimony andalloyed antimony filters.
Our next efforts were directed at developing a tita-nium filter. These foils could be fabricated successfullyby use of an electron-beam technique. Such foils ap-peared to be stable and quite rugged. Unfortunately,however, they had a high secondary bandpass shortwardof 300 A (see discussion in Sec. III) and hence did notsatisfy the development goals.
Our next approach was to try to develop a compositefilter of titanium and antimony. The absorptioncoefficients of both these materials indicated a bandpassin the desired range, which encouraged exploration ofthis combination. In addition, it was hoped that thetitanium would perform as an active nucleating surfaceand a protective overcoat layer when deposited beforeand after an antimony layer in rapid sequence in a singlevacuum. The effort was successful; the resulting filterswere uniform and showed a low susceptibility to pinholeformation during processing.
We manufactured several filters for further study:pure titanium filters of 500 ± 20 A and 1080 + 20 A andantimony-titanium sandwich filters consisting of 1120i 20 A of antimony between two 100 + 20-A layers oftitanium. These filters were 16 mm in diameter.
111. Transmission Measurements
Transmission measurements were performed usinga grazing-incidence monochromator to select line ra-diation from a hollow cathode discharge source7 and aPenning discharge lamp.8 The detector used was achannel electron multiplier. The dynamic range of thedetector was extended using calibrated meshes to at-tenuate the direct monochromator beam. All measuredcount rates were corrected for electronic deadtime, andoff-line background measurements permitted sub-traction of spurious counts due to scattering in themonochromator.
The transmission curves for the titanium and theantimony-titanium sandwich filters are shown in Figs.1 and 2, respectively. These figures present the rawdata, except for correction for the 80% transmissionfactor of the nickel support mesh on which the films aremounted. No transmissions below 6 X 10-6 were found.This limit is probably a result of scattering in the cali-bration chamber rather than a property of the filtersthemselves. Therefore, all measurements below 10-5are plotted as upper limits.
These transmission data have been used to derive theabsorption coefficients of antimony and titanium sep-arately. The separation of absorption coefficients isperformed using the following relationships:
Here T(500) and T(1080) are the measured transmis-sions of the 500- and 1080-A thick titanium foils, T(Sb)is the measured transmission of the sandwich foil, XTiOis the thickness of the titanium oxide layer formed, RTiis the reflectivity of titanium, and ATi, 'Sb, ATiO are the
Fig. 1. Transmission of titanium. Closed circles are for 500 i 20-Athick titanium and open circles are for 1080 ± 20 A. One sigma sta-tistical error bars are shown. The points with no error bars have errorbars smaller than the points. Lines drawn through the points are eye
fits to the data.
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Fig. 3. Absorption coefficient of antimony in an antimony-titaniumcomposite filter (see text). One sigma statistical error bars are shown.
The line drawn through the points is an eye fit to the data.
ao :
ioIn0
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WAVELENGTH (A)
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Fig. 4. Absorption coefficient of titanium. One sigma statisticalerror bars are shown. The line drawn through the points is an eye fit
to the data.
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Fig. 2. Transmission of an antimony-titanium sandwich (100 ± 20-ATi/1120 ± 20-A Sb/100 ± 20-A Ti). One sigma statistical error barsare shown. The points with no error bars have error bars smaller thanthe points. The line drawn through the points is an eye fit to the
data.
Fig. 5. (1-RTi) exp[(gTi- pTio)XTio] plotted vs wavelength. Onesigma statistical error bars are shown. The line drawn through the
points is an eye fit to the data.
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15 April 1983 / Vol. 22, No. 8 / APPLIED OPTICS
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absorption coefficients of titanium, antimony, and ti-tanium oxide, respectively, in microns-l.
The absorption coefficient of antimony is shown inFig. 3. Since all antimony results were derived frommeasurements of composite foils, we caution that theantimony results may be influenced by chemical in-teraction with the titanium. The intermetallic com-pounds Ti 4Sb, TiSb, and TiSb 2 are believed to exist. 9
The absorption coefficient of titanium (derived fromthe pure titanium foils) is shown in Fig. 4. We haveseparated out the effect of the surface layers of titaniumand present this factor [1 - RTi] exp[(LzTi - ATiO)XTiO]Iin Fig. 5.
The titanium transmission curves are a factor of -2lower at 400 A than previously published data.5 Theabsorption coefficient agrees well with Sonntag et al. 10for X < 300 A and confirms the existence of a secondtransmission window below 300 A.
The predominantly antimony filter also showed thesame qualitative transmission behavior as in a previousstudy.5 The calculated absorption coefficient predictsa peak transmission a factor of 2 higher than thatmeasured by Rustgi5 and Toots and Marton. Wehave since manufactured a sandwich filter of differentthickness (200-A Ti/1500-A Sb/200-A Ti); its measuredtransmission was 20%-40% lower near the peak of thebandpass than would be predicted by our above results.In contrast, if we use the antimony absorption coeffi-cient of previous measurements,5"11 the predictedtransmission is 2-3 times lower than measured. Theseresults suggest that there indeed may be some type ofmetallurgical reaction between antimony and titaniumwhich affects transmission. Therefore, our absorptioncoefficient should be used to predict the transmissiononly of an antimony-titanium sandwich filter and notthat of antimony alone.
IV. Long-Term Transmission Stability andMechanical Stability
The transmission characteristics described abovemake the antimony-titanium composite filter well-suited for astronomical EUV observations. Spacequalification of the filter further requires stability overtime against pinholing and gradual oxidation, both ofwhich will affect transmission and mechanical stabilitythrough the vibration levels of a rocket launch.
Pinhole formation in submicron foils occurs at se-lected sites on the filter. The chemical action betweenthe foil and the gaseous environment attacks thesepoints so that a certain amount of time is required topenetrate the foil, and thereafter the reaction increasesthe size of the hole. Increased transmission with agingis due to enlargement of initial pinholes with very fewnew ones forming. Typically, the initial stage showsvery little pinhole transmission, but after this phase, thetransmission increases rapidly to some apparent plateauwith a slow but steady increase thereafter.
The standard test for pinhole formation is to exposethe filter to a 60% relative humidity environment (in airfree of traces of the acid gases) and measure the visible
light transmission as a function of time. As a control,some filters are left in a -1% relative humidity (dessi-cated) environment.
Representative pinholing data are provided by alu-minum, tin, and indium, all of which have been used asfilters in space instrumentation. Previous studies in-dicate that initial visible light transmissions are gen-erally -10-8. After six weeks, transmissions due topinholing are typically 10-6-10-5. The initial stage ofaluminum lasts about one week, after which transmis-sion increases by a factor of 103. Indium and tin, on theother hand, have initial stages lasting -3 days. Thetransmission then increases by a factor of 104.
The antimony-titanium composite filters moisturetested were 1500 A of antimony sandwiched betweentwo 200-A titanium foils. The initial transmission ofeach filter was 4 X 10-9. The data shown in Fig. 6demonstrate that the antimony sandwich filter is quiteimmune to pinhole formation due to atmosphericmoisture. Over the period of the test, no degradationwas observed within the sensitivity range of the pho-tometer (1 X 10-9). For comparative purposes, Fig. 6also includes the degradation which would be typical fora 1500-A aluminum foil similarly exposed to 60% rela-tive humidity.
The long-term transmission stability of the filters waschecked by measuring the transmission one year afterthe initial measurements. During that period, the fil-ters were exposed to the laboratory environment for afew weeks and stored in dry nitrogen the rest of thetime.
The results demonstrate remarkable stability for theantimony-titanium combination. In the bandpass ofthe sandwich filter, the transmission decreased by amaximum amount of 13%. In the wings, the trans-mission decreased by a maximum of 25%. The corre-sponding maximum decreases for the 1080-A titaniumfilter are 12% in the bandpass and 70% in the wings.For the 500-A titanium filter, the results are a 30% de-crease in the bandpass and 18% in the wings.
z0
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169L0
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EXPOSURE (days)
Fig. 6. Transmission of visible light through a 200-A Ti/1500-ASb/200-A Ti filter vs time. Closed circles are measurements of filtersleft in a 60% relative humidity environment. Open circles are mea-surements of filters left in a -1% relative humidity (dessicated) en-vironment. The dashed curve is typical of a 1500-A aluminum filter
Sine sweep (each axis)1 g at 1.5 min/octave from 20 to 2000 Hz
dc simulation (5 min each axis)20 g at 40 Hz
This stability is excellent when compared with thatof previously used EUV filters. Standard filters suchas aluminum, tin, and indium show transmission dropsin the bandpass of 20% in only three months.
The filters at launch are subjected to two types ofmechanical stress: vibrational and acoustic. The ex-tremely high strength-to-weight ratio of the foils andmeshes enables them to withstand very high levels ofvibrational loading. The acoustic stress tends to bemore severe. Often filters are launched in vacuumboxes with the detectors, eliminating the acousticloading.
The 16-mm diam filters used for the transmissionmeasurements were vibration-tested to determine themechanical acceptability of the new materials. Thesesamples are smaller than many filters used in EUV as-tronomy, which may be as large as 40-50 mm in diam-eter. The test was very conservative, however, in tworegards. First, the test was performed in air, so therewas acoustic loading. Second, the filters were notmargined. Margining is a process often employed onfilters for space research in which a bead of compliantepoxy is placed around the edge of the filter and filterframe. This procedure greatly reduces the stress on thefilter by distributing the loads at the interface to theframe.
Vibration testing was performed to the qualificationlevels of random vibration and sine sweep for the SpaceShuttle. In addition, they were subjected to a low fre-quency sine (to simulate DC accelerations). Specifi-cations for all tests are given in Table I.
All filters tested (500-A Ti, 1080-A Ti, and the 100-ATi/1120-A Sb/100-A Ti sandwich) survived the randomvibration and sine sweep with no damage. The sand-wich filter and the 1080-A Ti filter survived the dcsimulation with no damage. The 500-A Ti filter had thefoil detach from the mesh in 10-20 locations during thedc simulation test.
V. Summary
Thin antimony and titanium foils have been fabri-cated and investigated for their usefulness as bandpassfilters in the EUV. A composite filter of antimony-titanium has been developed which provides a well-defined bandpass from 350 to 550 A. This filter is
well-suited to EUV astronomy, as it avoids the brightgeocoronal He I 584-A and He II 304-A lines andstrongly rejects the extremely intense H I 1216-Aline.
The absorption coefficients of antimony and titaniumhave been derived, and the effect of the titanium oxidelayer has been determined. These separated coeffi-cients can be used to predict transmission characteris-tics of filters of different thicknesses.
The antimony-titanium sandwich filter has under-gone extensive testing of pinhole susceptibility, long-term transmission stability, and mechanical strength.It exhibits far less moisture sensitivity and equal to orsuperior transmission stability to previously qualifiedEUV filters. Finally, it is rugged enough to survive aShuttle launch.
This work was supported by NASA grant NAS5-24189. We would like to thank Gail Reichert for helpfuldiscussions.
References1. F. Paresce, Earth Extraterr. Sci. 3, 55 (1977).2. R. F. Malina, S. Bowyer, and G. Basri, Astrophys. J. 262, 717
(1982).3. J. B. Holberg, B. R. Sandel, W. T. Forrester, A. L. Broadfoot, H.
L. Shipman, and D. C. Barny, Astrophys. Lett. 242, L119(1980).
4. J. A. R. Samson, in Techniques of Vacuum Ultraviolet Spec-troscopy (Wiley, New York, 1967).
5. 0. Rustgi, J. Opt. Soc. Am. 55, 630 (1965).6. W. R. Hunter, "The Preparation and Use of Unbacked Metal
Films as Filters in the Extreme Ultraviolet," in Physics of ThinFilms (Academic, New York, 1973), Vol. 7.
7. F. Paresce, S. Kumar, and C. S. Bowyer, Appl. Opt. 10, 1904(1971).
8. D. S. Finley, S. Bowyer, F. Paresce, and R. F. Malina, Appl. Opt.18, 649 (1979).
9. M. Hansen, in Constitution of Binary Alloys (McGraw-Hill, NewYork, 1958).
10. B. Sonntag, R. Haensel, and C. Kunz, Solid State Commun. 7,597(1969).
11. J. Toots and L. Marton, J. Opt. Soc. Am. 59, 1305 (1969).
