Extreme ultraviolet reflection efficiencies of diamond-turnedaluminum, polished nickel, and evaporated gold surfaces

Roger F. Malina and Webster Cash

We report measurements of reflection efficiencies from 44 A to 1048 A for samples of polished nickel, dia-mond-turned aluminum, and various thicknesses of gold evaporated onto the nickel and aluminum samples.The reflection efficiencies are presented for grazing angles from 5 to 75°. For wavelengths longer than-100 A, the gold-coated nickel surface provides the highest efficiencies, while for wavelengths shorter than100 A the nickel is superior. The optimal thickness of gold is found to be -500 A. The performance of graz-ing incidence optics using nickel and aluminum substrates for an evaporated gold surface is discussed.

1. Introduction

Observations of extreme ultraviolet (EUV) radiation(100-1000 A) from sources outside the solar system haveopened up a new field of astronomy,' The first EUVsource discovered was a white dwarf with a surfacetemperature -105 K, detected between 100 A and 300A.2 More recently a source visible at 500 A has beenreported,3 showing that astronomical observations willbe possible throughout the EUV. The brightest knownEUV source is only 10-6, as bright as the sun as seenfrom the earth. Thus instruments must be orders ofmagnitude more sensitive than those currently used forsolar EUV astronomy. This requires large diameteroptics with large effective areas (300 cm2) and withimaging capability to allow sources to be separated fromthe diffuse background. In addition, such telescopesare required to operate at grazing incidence to providesufficient throughput for wavelengths smaller than 400A.

We have recently fabricated two 38-cm diam grazingincidence Wolter-Schwarzschild type II telescopes forEUV astronomy.4 The metal mirrors were figuredusing diamond-turning5 and polished with diamondabrasives.6 As part of this effort we are investigatingthe reflection efficiencies of various materials in an ef-fort to optimize the fabrication of the telescopes andimprove their performance in the EUV. Publishedexperimental data on reflectivities exist only for re-

The authors are with University of California, Space SciencesLaboratory, Berkeley, California 94720.

Received 6 March 1978.0003-6935/78/1015-3309$0.50/0.© 1978 Optical Society of America.

stricted wavelength bands and limited angles of inci-dence. In particular, the large grazing angles used inEUV optics (up to 300) have not been investigated in thesoft x-ray studies, 7-' 0 and at the longer wavelengths onlynormal incidence behavior has been extensively stud-ied."1-13 Although available optical constants' 4 can beused to predict reflectivities and some theoretical in-terpolations exist, 516 such methods do not alwaysprovide reliable predictions of the reflectivity of acompleted mirror, as the surface roughness and com-position can have a large influence on the reflectivitiesachieved. In this paper we present measured reflec-tivities for flat samples of aluminum, nickel, andevaporated gold, which were fabricated by methodsappropriate for EUV telescopes. The measurementshave been made throughout the EUV and over the en-tire range of relevant grazing angles.

II. Sample Fabrication

A. Aluminum

The aluminum sample was a 6.2-cm (2.5-in.) diamdisk of aluminum 6061-T6. The sample was machinedby the diamond-turning method5 by the MetrologyGroup of the Lawrence Livermore Laboratories. Thesurface roughness of such flat samples is typically250-500 A peak to valley.' 7 The aluminum sample wasnot polished. In Fig. 1 we show a photograph of thesurface viewed through a Zeiss microscope with a No-marski phase control. The regularly spaced tool marksare visible, attesting to the need for polishing to achievelow scatter. Grain boundaries and surface pitting arealso evident. Reflectance measurements were carriedout normal to the machining grooves, as would be foundin a mirror fabricated by this process.

No special precautions were taken for the storage ofthe samples other than keeping them in a dust free

15 October 1978 / Vol. 17, No. 20 / APPLIED OPTICS 3309

Fig. 1. The diamond-turned aluminum surface: X750 magnificationthrough a Zeiss microscope with Nomarski phase control.

in roughness was evident between the different thick-ness coats. The gross surface irregularities of the nickelappear to be replicated by the gold coating.

As discussed by deKorte and Laine,'8 Nomarskiphotographs can be evaluated to provide an indicationof the soft x-ray and EUV performance. We have usedthem primarily to record the appearance of the surface;it is in general difficult to obtain comparable photo-graphs of the concave surface of the telescopes them-selves.

Ill. Experimental Method

The experimental setup is shown in Fig. 3 and con-sists of an EUV source, grazing incidence monochro-mator and reflectivity facility. The EUV source wasa hollow cathode discharge source,19 which was used forwavelengths longer than 200 A; strong emission linesfrom neon, argon, and helium were selected. For

dry-nitrogen atmosphere. The accumulated exposureto a normal laboratory atmosphere exceeded severalmonths, a situation which would be expected for theactual optical elements. This would allow the alumi-num sample to degrade with the formation of the oxidelayer.'3

B. Nickel

The electroless nickel sample was formed by platinga beryllium disk with 7.5 gm of Kanigen. The samplewas polished by the Optics Fabrication Group at theLawrence Livermore Laboratories. The polishingtechniques is based on the use of diamond powder car-ried in silicone oil. Grit size was started at 1 Azm andreduced to 0.25 Am before completion. The pitch lapwas a Gulgoz 64 Swiss pitch with a beeswax and fineairplane silk coating. The surface finish achieved onthe sample was estimated at 20-25 A rms on the basisof Nomarski photographs. Surface quality approachingthat on the sample was achieved on the completedmirrors described by Lampton et al. 4 after an estimated2.5-5-Atm thickness was removed in the polishing pro-cess. In Fig. 2 we show a micrograph of the samplesurface (after gold coating) which is comparable in ap-pearance to those presented by deKorte and Laine.'8Polishing streaks can be seen, but the nickel surface isfree of pitting, apart from localized areas.

C. Gold

The aluminum and nickel samples described abovewere cleaned by immersion for 1-3 min in a freon-vapordegreaser. They were then coated with five strips ofevaporated gold. The deposition rate was -100 A/minand carried out at pressures below 5 X 10-5 Torr. Thelayer thicknesses were 270 A, 490 A, 990 A, and 1930 A,assuming the deposited gold has densities characteristicof the bulk metal. The surfaces were examined with themicrographs. In Fig. 2 we show an area of the coatednickel sample isolated because it showed a small areaof poor gold adhesion. No increase in roughness overthe uncoated samples could be found, and no increase

Fig. 2. Photograph of the polished nickel surface with a 990-Aevaporated gold coat. Area of poor gold adhesion selected to show

presence of gold coat.

VACUUM CHAMBER MONOCHROMATOR

- EUV SOURCE

Fig. 3. Schematic of the experimental setup showing the EUVsource, grazing incidence monochromator, mirror sample on rotatable

table, and ranicon imaging detector on a swing arm.

3310 APPLIED OPTICS / Vol. 17, No. 20 / 15 October 1978

wavelengths shorter than 200 A a Henke x-ray tube20

was used with available lines at 44.7 A (C), 67.6 A (B),83.4 A (), 104.3 A (P), 135.5 A (Si), and 171.4 A (Al).When necessary, an exit filter of Parylene was used toreduce the scattered bremsstrahlung backgroundemitted by the source. Measurements were made bysubtracting from the count rate in the emission line thebackground at an adjacent wavelength, which hadtypically only 1-10% of the line intensity.

The EUV source illuminates the entrance slit of aMcPherson 2.2-m grazing incidence monochromator,with a 300-line/mm aluminum coated grating set at 870.The beam from the exit slit was further restricted by a1-mm diaphragm to reduce scattered light. The exitbeam had a divergence of 1 min of arc producing a beamsize of 1 mm on the reflectivity sample. The polariza-tion of the monochromatic beam has been measuredfrom 200-700 A using the technique described by Ra-binovitch et al.21 and has been found to be less than 5%.At the other wavelengths studied, polarization effectsalso appear to be small since, as discussed below, ourresults are consistent to 10% with those reported byother authors for unpolarized light.

Each mirror sample was mounted on a rotary tableand reflected the beam onto a microchannel plate(MCP) detector with a resistive anode readout22 (ran-icon) mounted on a separate arm. The rotation axis ofthe sample was parallel to the spectrometer slit and alsoto the grating grooves. The grazing angle was moni-tored to an accuracy of 0.20. The 2-D image raniconoutput was displayed on an oscilloscope screen so thatthe reflected beam location and appearance could bemonitored. The amplifier discriminators were set toinclude >95% of the counts. The detector arm wasrotated so that the reflected beam always fell on thesame point on the detector surface, thus avoiding anyerrors due to quantum efficiency variations across theMCP. The geometry of the optical path was such thatthe beam always struck the MCP at normal incidence,avoiding the variations in quantum efficiency with in-cidence angle.23 The detector arm could also be rotateduntil it intercepted the incoming beam directly withoutreflection from the sample. All reflection measure-ments were thus made by obtaining the intensities,corrected for the background, of the unreflected andreflected beams. A calibrated channel electron mul-tiplier24 was used to monitor beam stability. Totalcounts were accumulated wherever possible until 1%Poisson statistics were obtained. Typical relative errors(systematic and random) were estimated from mea-surement repeatability and were found to be 3% or lessfor the higher reflectance efficiency points and -5% forlow reflectance points and for all Henke source points.The ranicon had a size of 75 X 25 mm2 , intercepting allphotons scattered by less than 2.50, and hence ourmeasurements do not indicate the amount of smallangle scattering. However, the image size was moni-tored, and more than 90% of the counts were within thecentral 1-min of arc image, and no degradation of theimage of the monochromator exit slit was noted even atthe smallest grazing angles.

IV. Results

The results are presented graphically in Figs. 4-10.In Figs. 4 and 5 the reflectance as a function of grazingangle is shown for each of the materials at a selection ofthe wavelengths used. The monotonic increase incritical angle with increasing wavelengths up to 1000 Ais apparent. In Figs. 7-10 we show for the selectedgrazing angles of 50, 120, 240, and 750 the variation ofreflectance vs .vavelength. In Figs. 4-10 we report theresults for the 490-A gold coat which had the best re-flectivity. The data presented can be used to derive theoptical constants via the Fresnel equations,15 but in-stead, we present the results in this graphical fashionas we believe they are more useful in the design of theoptical systems. The graph for 750 (Fig. 10) indicatesclearly the need for a switch to grazing incidence forwavelengths shorter than -400 A. At grazing incidencethe aluminum is inferior, while the gold and nickel arecomparable between 400 A and 1000 A. Between 100A and 400 A the gold reflectance is superior, but thenickel is favored for wavelengths shorter than -70 A.

V. Discussion

The reflectance of gold has been studied extensively.Several groups have reported measurements from 7 Ato 190.3 A, with some measurements up to 400 grazingangle.7 -9132 5 Our measurements below 190 A areconsistent with those previously reported. This indi-cates that our adopted fabrication method results inreflectances as high as those achieved for materialsdeposited on glass substrates, normally used in reflec-tivity studies. Canfield et al. 12 report normal incidencereflectances and optical constants for gold from 300 Ato 2000 A. Our results at 750 are in agreement withtheir results.

The superiority of the nickel over the gold for the softx-ray band is in agreement with the theoretical ex-trapolations and experimental results given by Vaianaet al. 16 However, our data do not allow us to investigatein detail the reflectance variations expected for gold inthe vicinity of the absorption edge near 100 A.714 Inaddition smooth features, which may arise from therapidly varying optical constants between 400 A and 600,14 are not evident.

We found that the thickness of the gold layer had anoticeable effect. The 250-A thick layer had a some-what low reflectance (20%); similarly the 2000-A layerreflectance was lower. This may be due either to in-terference effects or to increased surface roughness notdiscernible in the micrograph. A layer of gold -500-1000 A thick appears to have the highest reflectance ofthose studied. A degradation with increasing thicknessof the coat has also been reported for x-ray energies bySeward,2 6 indicating that thin coats of -500 A are mostsuitable.

The performance of the aluminum is inferior to eitherthe gold or nickel except at the shallowest grazing anglesand at the longer wavelengths. The reflectances atnormal incidence are consistent with those expected for

15 October 1978 / Vol. 17, No. 20 / APPLIED OPTICS 3311

aluminum with a heavy oxide coat.13'2 5 At grazing in-cidence, however, the diamond tool marks degrade theperformance significantly, indicating that polishing isrequired.

We have found that the results reported here for theflats can also be achieved in the large concave surfacesof EUV telescopes. Two telescopes have been fabri-cated using the techniques used on these flats. Thefirst, an f/4 system described by Lampton et al.,4 wasfabricated by polishing a diamond-turned aluminumsubstrate. The aluminum was polished by conventional

1.0

0.8

C 0.6z

-J 0.4Li-

r 0.2

0.8- o/

GRAZING ANGLE 5 _06 j_W 0.4

a: 0.2

I I I I i I , 1 1 1 | 1 1

200 400 600 800 1000WAVELENGTH (ANGSTROMS)

Fig. 7. Measured reflectance as a function of wavelength at selected

incidence angles for the nickel (filled circles), gold (crosses), andaluminum (triangles).

zId

J

w

GRAZING ANGLE (Degrees)

Fig. 4. Measured reflectance from the nickel sample as a functionof incidence angle at each wavelength as indicated.

Li

I-

Lii

Lii

zI-

UJ

LLU-w

0.8

0.6

0.4

0.2

50

GRAZING ANGLE (Degrees)

Fig. 5. Reflectance of the gold sample (490 A thick).

1.0 I II I I II'

- ~~~~Al1Q8

Lo 0.6C)z_

UiC)1

GRAZING ANGLE (Degrees)

Fig. 6. Reflectance of the aluminum sample.

0.8LiiC)z

< 0.6I-_

J 0.4adI-IwU_ QL2

200 400 600 800

WAVELENGTH (ANGSTROMS)

Fig. 8. Grazing angle 12°.

I I I I , I , I I I IT I I 'I I 2GRAZING ANGLE = 25-

,I . I , I . I I I .1 _200 400 600 800 1000

WAVELENGTH (ANGSTROMS)

Fig. 9. Grazing angle 250.

§ I § , I, I I, II I I I I

GRAZING ANGLE 75 _

200 400 600 800WAVELENGTH (ANGSTROMS)

Fig. 10. Grazing angle 75°.

techniques,6 but the results were not entirely satisfac-tory. It was found that the gold-coated aluminumsurface showed a severely degraded reflectivity short-ward of 250 A.

The second telescope, an f/10 system also describedin Lampton et al.,4 was fabricated by polishing a nickelsurface coated over the diamond-turned substrate. The

3312 APPLIED OPTICS / Vol. 17, No. 20 / 15 October 1978

1000

nickel surface after gold coating was found to have areflectance between 113 A and 1000 A, which was 75-90% of that achieved on the flat.

The Rayleigh criterion requires that surface irregu-larities be smaller than 0.12 X/sinO, i.e., for 0 = 10°, X =100 A, irregularities should not exceed -70 A. HenceEUV optics can be fabricated with much less stringentrequirements than those needed for the soft x-ray re-gion. -The dominant loss in the image quality of theEUV telescopes arises not in the surface finish butrather from longer wavelength irregularities that maybe introduced in the diamond turning or in the polishingof very concave surfaces. We have, in this paper, shownthat the present fabrication techniques for metal mir-rors provide surfaces with close to optimal reflectivitiesin the EUV.

We acknowledge the support and encouragement ofS. Bowyer and thank J. Bryan and P. Baker of Law-rence Livermore Laboratory for providing the samplesand G. Steers of the Lawrence Berkeley Laboratory, forcarrying out the gold coating. This research was sup-ported by NASA grant NGR 05-003-450 and NAS5-24189.

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Robert J. Potter of Xerox Corporation, Reproduction Technology, Stamford, Connecticut

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