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Degradation of far ultraviolet reflectance of aluminum films exposed to atomic oxygen. In-orbit coating application
1 March 1996
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
124 (1996) 208-215
Degradation of far ultraviolet reflectance of aluminum films exposed to atomic oxygen. In-orbit coating application Juan Ignacio Larruquert, Jo& Antonio Mhndez, Jo& Antonio Aznhrez Institute de Ffsica Aplicada. C.S.I.C.. USerram
121, 28006-Madrid, Spain
Received 9 August 1995; revised 13 October 1995: accepted
revised version 26 October 1995
Abstract The drop in reflectance at the interval 82.6-174.4 nm of ultra high vacuum prepared aluminum coatings when exposed to controlled doses of atomic oxygen with average energy 0.17 eV has been measured by the first time. We show that atomic oxygen produces a much stronger effect on the far ultraviolet (FUV) reflectance of aluminum that molecular oxygen: for the same relative drop in reflectance of 40% at 82.6 nm, the required exposure to molecular oxygen is about 5000 times larger than to atomic oxygen. These measurements allowed us to calculate a higher limit of the useful lifetime of an aluminum mirror placed in a low earth orbit (LEO, 200 to 700 km altitude; oxygen atoms with energy of 5 eV). This limit was found to be as short as a few hours. To avoid the oxidation by atomic oxygen a high altitude orbit should be used. A more practical alternative may be found by placing the aluminum mirror behind a wakeshield device, a well known proposal, in a LEO. In this case, the flux of oxygen atoms impinging on the mirror would be strongly reduced and consequently the lifetime of the mirror greatly increased. on the shielded mirror would range from 0.07 to 1.8 eV. As the energy of oxygen atoms in our experiment is within that interval of energies, a plausible estimate of the lifetime expected for an in-orbit aluminum coating behind a wakeshield can be derived from our experimental measurements. For a wakeshield operating as a free flyer at a 300 km altitude orbit an aluminum mirror would suffer a negligible drop in its FUV reflectance after a time as long as 20 years, which is a stimulating prospective. We have calculated that the energy of oxygen atoms impinging
1. Introduction The 90 to 120 nm spectral region in the far ultraviolet
(FUV) contains several spectral lines of extraordinary interest to advance the knowledge of galaxies, stars, intergalactic clouds, etc. [ I]. The richness of this interval was demonstrated by the Copernicus satellite in the 70’s, but later missions able to adequately exploit these possibilities have not been developed yet: Lyman Project, an ESA/NASA initiative, was cancelled at the end of the SO’s, and the still alive NASA project for a Far Ultraviolet Spectroscopic Explorer (FUSE) has received recently a considerable reduction to its financial budget [ 21. The Astro- 1 Observatory, launched in 0030-40 I8/96/$12.00
0 1996 Elsevier Science B.V. All rights reserved
SDlOO30-4018(95)00679-6
December 1990, was able to perform limited observations in the above internal during a 9-day flight on board the Columbia shuttle. Observations were made by means of an optical system based on a telescope mirror and a diffraction grating coated with OS and Ir, respectively, with FUV reflectivities about 20% [ 31. The non-existence of transparent materials in this interval, the low efficiency of reflecting materials and the absorption of FUV radiation in the atmosphere, have rendered difficult the exploration of this spectral region up to the present. Pure unoxidized aluminum is the material with the highest reflectance, about 90%, in the 90 to 120 nm spectral region, but unfortunately aluminum quickly oxidizes even under a high vacuum
J.I. Lurruquert
et al. /Optics
Communications
environment and the thin oxide film grown on its surface strongly absorbs the FUV radiation for wavelengths below 200 nm. The protective action of LiF and MgF, overcoatings on aluminum films surface is only able to extend the useful wavelength range down to 105 or 1 15 nm, their respectively cutoff limits. The development of a space technology around the shuttle orbiter permitted to bring back the proposal of Hass and Hunter [ 41 to coat FUV optical elements with aluminum in a high enough orbit so keeping the aluminum film unoxidized in the space vacuum. This possibility has been considered in Lyman [ l] and FUSE [ 5 1 projects. Nevertheless, as stated by Osantowski et al. in Ref. [ 5 1, this requires totally new technology and space flight demonstration before consideration for any specific application. Some efforts have been addressed very recently with participation of the authors to space demonstration of deposition technology ‘. However, new experiments have to be done and a more advanced knowledge is needed about the reaction of evaporated aluminum films with residual gaseous species in orbit. ’ The Discovery Space Shuttle transported in the STS-64 flight of September 1994 the EDMO (Experiment for the Deposition of materials in Orbit) payload, a joint development of the Spanish firm CRISA and the Spanish Consejo Superior de lnvestigaciones Cienmicas. within the In-orbit Technology Demonstration Programme of the European Space Agency. Inside a GAS canister with motorized door, EDMO achieved to prepare thin films of aluminium. gold and silicon under the shuttle orbit high vacuum.
124 (1996) 208-215
209
Some studies in the laboratory on FUV reflectance drop of aluminum mirrors exposed to molecular oxygen have been already performed [ 6-81, but similar data on exposure to atomic oxygen, the most abundant and active species in low earth orbits (LEO), are lacking. We present here the first data of FUV reflectance drop of aluminum thin films evaporated under ultra high vacuum (UHV) conditions after the films were exposed to controlled doses of atomic oxygen. An estimate of the serviceable lifetime of an aluminum mirror for the FUV placed in a LEO is also calculated.
2. Experimental
equipment
The FUV reflectometer used in this experiment has been described elsewhere [ 8,9]. A sample oxidation chamber (SOC) was developed for the present experiment. It is an UHV chamber pumped by ion and Ti sublimation pumps, which is attached to the reflectometer through a gate valve. The SOC is provided with a 450 W Oxford Applied Research MPD2 1 atomic oxygen source. A transfer manipulator transports the aluminum sample alternately between the reflectometer, where deposition and reflectance measurements are performed, and the SOC, where the sample is exposed to controlled doses of atomic oxygen. A sketch of the equipment is given in Fig. 1. The atomic oxygen source supplies a beam of mixed atomic plus molecular oxy-
PMP3
Fig. I. Scheme of the components of the experimental equipment: C: capillary; L: lamp; M: monochromator: G: diffraction grating; ENS: entrance slit: EXS: exit slit; MC: modulation chamber; CB: chopper blade; S: aperture stop; DRC: deposition and reflectometry chamber; C,, C? and C,: channel electron multipliers; MP, MP2, MP3: sample manipulators; SH: sample holder: P,, PZ and P1: small conductance pipes; SOC: sample oxidation chamber; AOS: atomic oxygen source: STC: sample transfer chamber; GV: gate valve.
210
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gen. Locquet and Mgchler [lo] determined a proportion of 30 atoms per 100 molecules in the beam in an equivalent source; this proportion will be assumed here. 90% of oxygen atoms entering the SOC are thermalised, the other 10% have kinetic energies ranging from 2 to 5 eV [ I I 1. According to our measurements using optical pyrometry, the temperature of the discharge tube, where molecules are being dissociated, is 1300 K. An ionization gauge not facing the beam measured the total pressure in the oxidation chamber. A quadrupole mass spectrometer directly facing the oxygen beam qualitatively monitored the oxygen dissociation process through detection of mass 16. As a procedure to control the stability of the beam source, the signal given by an optical detector that measures the radiation emitted by the discharge [lo] was kept constant throughout the exposure of the aluminum films to oxygen.
3. Experimental
results
The flux of oxygen molecules impinging on the aluminum surface is obtained from the pressure measured by the ion gauge: Q’(02) =pS/k,TA
,
(1)
where p is the pressure measured by the ion gauge, S = I 100 1/s is the pumping speed, kB the Boltzmann constant and T=300 K the chamber temperature. In Eq. ( I ) we assume that there is no recombination of atoms into molecules in the vacuum chamber and that no oxygen atom reach the ion gauge. The atoms and molecules are emitted from the source through 5 holes of 0.3 mm diameter giving rise to a beam opened with a semiangle of 15” [ 111. The aluminum sample is placed 15 cm away the atom source exit. At this distance the beam section is about A = 50 cm’. According to the aforementioned proportion of 30 atoms per 100 molecules we obtain for the flux of atoms impinging on the aluminum surface: @( 0 atoms) = 0.3 X @( 02) = 0.3pSlkTA
.
(2)
We measured the reflectance of aluminum films in the interval 82.6-174.4 nm at 10” near normal incidence angle. Samples A, B, and C were 180 nm thick aluminum films deposited on supersmooth substrates. Samples D and E were 180 nm thick aluminum films
10
124 (1996) 208-215
1
80
100
120
Wavelength
140
noxidmsd
Al
160
180
(nm)
Fig. 2. Reflectance of an unoxidized aluminum film and the aluminum film exposed to increasing fluences of atomic oxygen, expressed in atoms X cm-‘. versus the wavelength.
deposited on previously oxidized aluminum films. Aluminum was deposited and maintained in UHV conditions. The evaporation rate was about 3 rim/s.. Base pressure was 3 X lo- I0 mbars. During deposition the pressure increased to lo-’ mbars but in a few seconds after deposition a total pressure of lo-” mbars was retrieved. Immediately after deposition we measured in situ the reflectance of samples A to E. Afterwards, the films were exposed to increasing doses of atomic oxygen and the reflectance of the resulting oxidized films was also measured. The longest exposure time was 40 minutes. Samples were placed facing the oxygen atom beam at room temperature. No measurement was made of the sample temperature during exposure to the beam. In Fig. 2 we show the reflectance of sample A versus the wavelength for several doses of atomic oxygen. The drop in reflectance is maximum at shorter wavelengths and continuously decreases towards longer wavelengths. In Fig. 3 we represent the reflectance (normalized to the unoxidized aluminum reflectance) of samples A to E together versus the atomic oxygen fluence (integrated oxygen flux) plotted on a logarithmic scale for different wavelengths. It can be seen that a linear relationship between the reflectance and the decimal logarithm of the fluence is a good fit to the experimental data. Higher reactivities of oxygen atoms against molecules have been reported in the literature on metals such
J.1. Larnryuert et ul. /Optics
Communications
as MO [ 121, Si and Ge [ 131, but as far as we know similar investigations on bare aluminum films have not been reported. We have compared the efficiency of atomic and molecular oxygen to produce a decrease in the FUV reflectance of aluminum films by exposing to molecular oxygen two new 180 nm thick aluminum samples F and G. prepared as described above. The atomic oxygen source was switched on as for atomic oxygen exposure, but the source was switched off a
’ O1
few seconds before starting the exposure. In that way samples F and G were exposed to molecular oxygen in thermal conditions as similar as possible to those for atomic oxygen exposure, although oxygen molecules will have fairly lower energy than atoms because the source is switched off. As the difference in energies is small it should not have a strong influence on their relative oxidizing efficiencies. Also in the case of atomic oxygen exposure the temperature of the sample
“‘1
.
(a)
21 I
124 (1996) 208-215
(b)
. Molecules
0.8
0.8 -
0.6
0.6 2 2
04
0.2
0.2 1
\o ,
0 0 lEt13
“,l”q
,
lE+14
(,,,,
‘1
(
lE+15
Oxygen
,,,,,,
q
00
7Tlllql
lE+16
lE+17
Fluence
lE+lB
lEtI
(cme2
)
I E+13
lEt20
1 lE+14
’
rnrlll lE+15
Oxygen
’ “““9 lE+16
lE+17
Fluence
lE+18
(cmm2
’I
lE+19
IE+20
)
Molecules
000 lE+13
lE+14
lE+15
Oxygen Fig. 3.
(a)-( g ),
lE+16
Drop in reflectance.
and molecular oxygen.
lEc17
Fluence
lE+18
IE+19
(cm-*
normalized
lE+13
lE+20
to the reflectance
lE+14
lEc15
Oxygen
) of the unoxidized
IE+16
lE+17
Fluence
lE+18
(cm-’
IE+19
lE+ZO
)
film, of aluminum coatings versus the fluence of atomic
J.I. L~wruyurrt
et al. /Optics
Communications
124 (1996) 208-215
bo
Molecules
Molecules 0 0
Atoms
Atoms
0
\
\
0 3 lEt13
, ,,“,,! , ,,,,,1 , ,111 “1 r ,,,,,,9 , ,,,,,‘1 , (),,,,1 ( T lE+14
1E+15
Oxygen
lE+16
lEf17
Fluence
lE+lB
(cm-’
lE+19
lE+ZO
lE+13
lE+14
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lEt15
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lEt16
lE+17
Fluence
lE+18
lE+lS
lE+ZO
(cm -2 )
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075m_ 1E+13
lE+14
lE+15
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lE+16
lEc17
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lE+18
lE+19
lE+ZO
(cm -a )
Fig. 3. Continued might have somewhat increased due to irradiation from the atomic oxygen source. In Fig. 3 we also represent the reflectance of samples F and G versus the molecular oxygen fluence calculated through relation ( 1). In general the effect of oxygen atoms on aluminum FUV reflectance is stronger than that of oxygen molecules and the difference between the drops in reflectance for the two species increases greatly for long fluences. Let EA and E, be the doses of atoms and molecules, respectively. that produce the samerelativedrop in reflectance
at a particular wavelength. The ratio p( EA, A) = EM/ E, expresses how many oxygen molecules are necessary to produce the same drop in reflectance as one oxygen atom for a certain value of EA and A. p increases monotonically with the exposure. For low values of E,, p takes a value of about five that does not significantly change within the spectral region covered, even though the drop in reflectance reduces when increasing the wavelength. For an intermediate exposure I$ = 2.5 X 10” atoms Xcme2, p takes a constant value of about
J.I. Lorruqrrert
et al. /Optics
5000 in the spectral interval 82.6-104.8 nm. For the exposure PA and for a wavelength higher than or equal to 120.0 nm, p increases progressively with wavelength so that the l& necessary to produce the same drop than EA is much larger than the maximum dose 3.9X 10” molecules X cm -’ applied on our experiment.
4. Calculation
of the lifetime of Al coatings in LEO
The results presented in Section 3 allow us to estimate the effect of atomic oxygen in the outer atmosphere on the FUV reflectance of an aluminum coating supposedly deposited in-orbit. Only LEO are going to be considered in the calculation. The velocity of a LEO orbiter is much higher than the thermal speed of the residual atmospheric species. Hence, the orbiter observes a strong anisotropy in the directional flux of species. which impinge on the orbiter with an energy of about 5 eV. At orbit altitudes from 200 to 700 km the number density of oxygen atoms monotonically decreases, the average values ranging from IO”’ to 2X lOi cm--’ [ 141. The flux that impinges on the orbiter is the product of the number density by the orbiter velocity. Comparing the flux so obtained with our experimental figures of drop in reflectance versus Huence, an aluminum mirror located in a 700 km altitude orbit and facing the advance sense would suffer a 40% relative drop in reflectance at 82.6 nm after less than 5 hours, at 104.8 nm after less than 10 hours and at 120 nm after less than 40 hours. The calculation has not taken into account that the in-orbit oxygen atoms are much more energetic than those in our experimental equipment. The influence of the energy of the oxygen atoms on the reaction with a material strongly depends on the material nature, as it has been shown, e.g., for Pt. OS and SIC [ 15-I 81. The actual FUV drop in reflectance of bare aluminum deposited in orbit and exposed to energetic atoms can be reasonably expected to be equal to or larger than the drop obtained at our laboratory with thermal energy atoms. This means that the extremely short lifetime obtained for an aluminum film is even a higher limit for a real case in a LEO. This result would impose the use of higher altitude orbits in order to reduce the atomic oxygen flux impinging on an aluminum mirror. Nevertheless, LEO could still be useful by applying the wakeshield concept [ 191. The wakeshield or molecular shield is a system to block the
Communications
124 (1996) 208-215
213
flow of atmospheric gas due to the orbiter velocity, thereby hypothetically achieving a large reduction in pressure at that altitude. Since the orbital velocity in a LEO (about 7.6 km/s) is much larger than the Maxwellian speed of the atmospheric atoms (about 1 km/ s) there is virtually no probability for an atmospheric atom or molecule to enter an instrument compartment from the hemisphere on the wake side. opposite the flight direction [20]. Naumann [21] calculated the flux of the ambient atmosphere that can overtake the wake region from the rear part of the orbiter and obtained an equivalent pressure of oxygen atoms as low as lo-j3 mbars. The major source of oxygen atoms that can reach the surface on the wake side is backscattering of ambient oxygen atoms by collisions with thermal molecules in the induced atmosphere of the orbiter. The thermal molecules cannot be backscattered toward the surface of interest, but the ambient molecules, as oxygen atoms, can be, provided the thermal molecule has a mass higher than the ambient molecule. Naumann considered two cases. In the first one, the wakeshield is attached to a space shuttle; the outgassing of the shuttle would limit the capabilities of the wake shield. In the second one, the wakeshield operates a a free flyer, where the only source of thermal molecules would be the outgassing from the shield itself, so that the wakeshield would achieve its ultimate capabilities. Naumann calculated the atomic oxygen fluxes that could reach the volume behind a 5-m diameter wakeshield attached to a shuttle orbiter or operating as a free flyer, in both cases in a 300 km altitude orbit. If the Chemical Beam Epitaxy process molecules considered in Naumann’s calculations are discounted in our case, we obtain the range 1.1 X 10” to 5.8X IO” cm-‘s-l (equivalent pressure: 3.1 X 10p"' to 1.6~ lo-” mbars) for the shuttle attached mode, and the range 7.2X 10” to 3.7 X 10J cm-’ s-’ (equivalent pressure: 1.9~ IO-” to 9.9X lo-” mbars) for the free flyer mode. Despite the high kinetic energy of the oxygen atoms incident upon the orbiter (5 eV) , the energy of an oxygen atom backscattered by collision with the induced atmosphere (mainly H,O and in a lesser extent N, and 0,) is much lower. Assuming elastic collisions, an oxygen atom that would collide with a thermal water molecule in the shuttle attached mode would conserve an energy that depends on the angle of direction change, namely 0.017 eV for 180” (impinging on the wake volume from the rear part), 0.29 eV for 90” (impinging
214
J.I. Larruquert et al. /Optics
Cwnmunicarions
normal to the orbit direction) and 2.77 eV for 45”. We calculated the average energy of the oxygen atoms impinging on the wake after colliding with water molecules for the shuttle-attached mode by using the directional flux plotted in Fig. 6 of Ref. [ 211 (for a 30” minimum acceptance angle - cutoff angle - ). We obtained for a rear facing surface (only attainable by oxygen atoms changing their direction more than 90”) a value of 0.07 eV and for a side facing surface (parallel to the orbit, attainable by oxygen atoms changing direction more than 30”) a value of 1.8 eV. A suitable design of the wakeshield geometry may increase the cutoff angle, so reducing the average energy of the impinging atoms. In our equipment the average energy of the thermal atoms is 0.17 eV, related to a 1300 K emission temperature, that would simulate quite well the average energy of a high cutoff angle wakeshield. Hence, a plausible estimate of the lifetime expected for an inorbit aluminum coating behind a wakeshield can be derived from our experimental measurements. According to the above values of the flux for a 300 km altitude orbit calculated by Naumann, an aluminum coating behind a wakeshield in the shuttle-attached mode would suffer a 40% relative drop in reflectance at 82.6 nm in a time as short as 1 hour. A higher altitude orbit would be necessary to enlarge the mirror lifetime. A much more stimulating result is obtained in the free flyer mode. In this case an aluminum mirror in a 300 km altitude orbit would suffer a negligible drop in FUV reflectance after a time as long as 20 years.
5. Conclusions Data on the drop in reflectance in the 82.6-174.4 nm interval of bare aluminum films prepared in UHV conditions and exposed in situ to atomic oxygen are presented for the first time in the literature. The drop in reflectance is shown to be maximum at shorter wavelengths. continuously decreasing towards longer wavelengths. We also measured the reflectance drop of aluminum films exposed to molecular oxygen to compare the effect of both species on the aluminum FUV reflectance and obtained that the effect of oxygen atoms on aluminum reflectance was stronger than that of oxygen molecules and that the difference between the two species increased greatly for long fluences. The comparison was expressed by the ratio p of the molecular
124 (1996) 208-215
oxygen fluence to the atomic oxygen fluence, both fluences for the same drop in reflectance at a particular wavelength. p increased from about five for low fluences up to about 5000 for an intermediate oxygen fluence of 2.5 X 10” atoms cm ~ ‘, which corresponded to a relative drop in reflectance of 40% at 82.6 nm. For this fluence p remained constant in the spectral interval 82.6-104.8 nm and progressively increased for wavelengths longer than or equal to 120.0 nm. The above measurements of the drop in FUV reflectance of aluminum versus the atomic oxygen fluence were used to estimate the life of a mirror for the FUV aluminized in a LEO and exposed to the ambient atomic oxygen. A 40% relative reflectance drop at 82.6 nm would be attained in an extremely short period of 5 hours in an orbit as high as 700 km altitude for the mirror facing the advance sense. As the energy of the ambient oxygen atoms (5 eV) is higher than that of the atoms in our experiment (0.17 eV), the above short lifetime is even a higher limit of the actual case. The fluence of atomic oxygen on an aluminum coating placed in a LEO could be largely reduced, and correspondingly the life of a highly reflecting aluminum mirror for the FUV greatly increased, by placing the aluminum coating behind a wakeshield. In addition the average energy of oxygen atoms backscattered towards the mirror by collisions with molecules outgassed from the orbiter, mainly water, was calculated to vary from 0.07 eV for a rear facing surface to 1.8 eV for a side facing surface, considerably lower than the 5 eV in the orbit. The average energy of atoms impinging on the mirror could be further reduced by means of a suitable design of the wakeshield geometry. Hence, our experiments are a good approach to describe the actual evolution of an aluminum mirror behind a wakeshield at a LEO. Such an aluminum mirror in a 300 km altitude orbit operating as a free flyer would suffer a negligible drop in the FUV reflectance after a time as long as 20 years, which is a stimulating prospective.
Acknowledgements
The technical assistance of Mr. Jose M. SanchezOrejuela is gratefully acknowledged. This work was performed under financial support Nr. ESP93-0324 from the National Programme for Space Research through Spanish CICYT.
J.I. L~rrrryuerfet al. /Optics Communications 124 (1996) 208-215
References
] I ] L. Lyman, Assessment Study, European Space Agency SCI(85) 4 (1985) I. I? 1F. Flam. Space Science News. Science 265 ( 1994) 180 1. ( 3 1A.F. Davidsen, Science 259 ( 1993) 327. ]J I G. Hass and W.R. Hunter. Appl. Optics 6 ( 1967) 2097. )5 1J.F. Osantowski. P.M. Davila and T.T. Saha, Opt. Eng. 25 (1986) 1039. 16 1W.M. Burton. Proc. SPIE Vol. 445 ( 1984) 340. 17 ] J.S. Edmends. C.N. Malde and S.J.B. Con’igan, Vacuum 40 (1990) 471. [ 8 1J.I. Larruquert, J.A. Mendez and J.A. Azmirez, Appl. Optics 33 (1994) 3518. 19 I J.1. Larruqueit, J.A. Mendez and J.A. Azniirez, Appl. Optics 34 ( 1995) 4892; J.A. Aznirez, J.I. Larruquert and J.A. Mendez, Review of Scientific Instruments, to be published.
215
[ IO] J.P. Locquet and E. Miichler, J. Vat. Sci. Technol. A IO ( 1992) 3100. [ I I ] Information given by the manufacturer. [ 121 D.E. Rosner and H.D. Allendorf, J. Chem. Phys. 40 (1964) 3441. [ 131 R.J. Madix and A.A. Susu, Surf. Sci. 20 ( 1970) 377. [ 141 G.W. Sjolander and L. Bare&. 18th Intern. SAMPE Techn. Conference, 1986, p. 722. 1151T.R. Gull, H. Herzig, J.F. Osantowski and A.R. Toft, Appl. Optics 24 (1985) 2660. [ 161 H. Herzig, A.R. Toft and C.M. Fleetwood, Jr., Appl. Optics 32 (1993) 1798. [ 171 J.S. Goela. M.A. Pickering, R.L. Taylor, B.W. Murray and A. Lompado, Appl. Optics 30 ( 199 1) 3 166. 1IS] J.F. Seely, GE. Holland, W.R. Hunter, R.P. McCoy, K.F. Dymond and M. Corson, Appl. Optics 32 (1993) 1805. 1191 A. Ignatiev andC.W. Chu, Metall. Trans. A, 19A ( 1988) 2639. ] 201 A.N. Bunner. Optical Coating in Space, Perkin-Elmer ER-59 I report number, 1983, and references therein. [ 211 R.J. Naumann, J. Vat. Sci. Technol. A 7 ( 1989) 90.
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
124 (1996) 208-215
Degradation of far ultraviolet reflectance of aluminum films exposed to atomic oxygen. In-orbit coating application Juan Ignacio Larruquert, Jo& Antonio Mhndez, Jo& Antonio Aznhrez Institute de Ffsica Aplicada. C.S.I.C.. USerram
121, 28006-Madrid, Spain
Received 9 August 1995; revised 13 October 1995: accepted
revised version 26 October 1995
Abstract The drop in reflectance at the interval 82.6-174.4 nm of ultra high vacuum prepared aluminum coatings when exposed to controlled doses of atomic oxygen with average energy 0.17 eV has been measured by the first time. We show that atomic oxygen produces a much stronger effect on the far ultraviolet (FUV) reflectance of aluminum that molecular oxygen: for the same relative drop in reflectance of 40% at 82.6 nm, the required exposure to molecular oxygen is about 5000 times larger than to atomic oxygen. These measurements allowed us to calculate a higher limit of the useful lifetime of an aluminum mirror placed in a low earth orbit (LEO, 200 to 700 km altitude; oxygen atoms with energy of 5 eV). This limit was found to be as short as a few hours. To avoid the oxidation by atomic oxygen a high altitude orbit should be used. A more practical alternative may be found by placing the aluminum mirror behind a wakeshield device, a well known proposal, in a LEO. In this case, the flux of oxygen atoms impinging on the mirror would be strongly reduced and consequently the lifetime of the mirror greatly increased. on the shielded mirror would range from 0.07 to 1.8 eV. As the energy of oxygen atoms in our experiment is within that interval of energies, a plausible estimate of the lifetime expected for an in-orbit aluminum coating behind a wakeshield can be derived from our experimental measurements. For a wakeshield operating as a free flyer at a 300 km altitude orbit an aluminum mirror would suffer a negligible drop in its FUV reflectance after a time as long as 20 years, which is a stimulating prospective. We have calculated that the energy of oxygen atoms impinging
1. Introduction The 90 to 120 nm spectral region in the far ultraviolet
(FUV) contains several spectral lines of extraordinary interest to advance the knowledge of galaxies, stars, intergalactic clouds, etc. [ I]. The richness of this interval was demonstrated by the Copernicus satellite in the 70’s, but later missions able to adequately exploit these possibilities have not been developed yet: Lyman Project, an ESA/NASA initiative, was cancelled at the end of the SO’s, and the still alive NASA project for a Far Ultraviolet Spectroscopic Explorer (FUSE) has received recently a considerable reduction to its financial budget [ 21. The Astro- 1 Observatory, launched in 0030-40 I8/96/$12.00
0 1996 Elsevier Science B.V. All rights reserved
SDlOO30-4018(95)00679-6
December 1990, was able to perform limited observations in the above internal during a 9-day flight on board the Columbia shuttle. Observations were made by means of an optical system based on a telescope mirror and a diffraction grating coated with OS and Ir, respectively, with FUV reflectivities about 20% [ 31. The non-existence of transparent materials in this interval, the low efficiency of reflecting materials and the absorption of FUV radiation in the atmosphere, have rendered difficult the exploration of this spectral region up to the present. Pure unoxidized aluminum is the material with the highest reflectance, about 90%, in the 90 to 120 nm spectral region, but unfortunately aluminum quickly oxidizes even under a high vacuum
J.I. Lurruquert
et al. /Optics
Communications
environment and the thin oxide film grown on its surface strongly absorbs the FUV radiation for wavelengths below 200 nm. The protective action of LiF and MgF, overcoatings on aluminum films surface is only able to extend the useful wavelength range down to 105 or 1 15 nm, their respectively cutoff limits. The development of a space technology around the shuttle orbiter permitted to bring back the proposal of Hass and Hunter [ 41 to coat FUV optical elements with aluminum in a high enough orbit so keeping the aluminum film unoxidized in the space vacuum. This possibility has been considered in Lyman [ l] and FUSE [ 5 1 projects. Nevertheless, as stated by Osantowski et al. in Ref. [ 5 1, this requires totally new technology and space flight demonstration before consideration for any specific application. Some efforts have been addressed very recently with participation of the authors to space demonstration of deposition technology ‘. However, new experiments have to be done and a more advanced knowledge is needed about the reaction of evaporated aluminum films with residual gaseous species in orbit. ’ The Discovery Space Shuttle transported in the STS-64 flight of September 1994 the EDMO (Experiment for the Deposition of materials in Orbit) payload, a joint development of the Spanish firm CRISA and the Spanish Consejo Superior de lnvestigaciones Cienmicas. within the In-orbit Technology Demonstration Programme of the European Space Agency. Inside a GAS canister with motorized door, EDMO achieved to prepare thin films of aluminium. gold and silicon under the shuttle orbit high vacuum.
124 (1996) 208-215
209
Some studies in the laboratory on FUV reflectance drop of aluminum mirrors exposed to molecular oxygen have been already performed [ 6-81, but similar data on exposure to atomic oxygen, the most abundant and active species in low earth orbits (LEO), are lacking. We present here the first data of FUV reflectance drop of aluminum thin films evaporated under ultra high vacuum (UHV) conditions after the films were exposed to controlled doses of atomic oxygen. An estimate of the serviceable lifetime of an aluminum mirror for the FUV placed in a LEO is also calculated.
2. Experimental
equipment
The FUV reflectometer used in this experiment has been described elsewhere [ 8,9]. A sample oxidation chamber (SOC) was developed for the present experiment. It is an UHV chamber pumped by ion and Ti sublimation pumps, which is attached to the reflectometer through a gate valve. The SOC is provided with a 450 W Oxford Applied Research MPD2 1 atomic oxygen source. A transfer manipulator transports the aluminum sample alternately between the reflectometer, where deposition and reflectance measurements are performed, and the SOC, where the sample is exposed to controlled doses of atomic oxygen. A sketch of the equipment is given in Fig. 1. The atomic oxygen source supplies a beam of mixed atomic plus molecular oxy-
PMP3
Fig. I. Scheme of the components of the experimental equipment: C: capillary; L: lamp; M: monochromator: G: diffraction grating; ENS: entrance slit: EXS: exit slit; MC: modulation chamber; CB: chopper blade; S: aperture stop; DRC: deposition and reflectometry chamber; C,, C? and C,: channel electron multipliers; MP, MP2, MP3: sample manipulators; SH: sample holder: P,, PZ and P1: small conductance pipes; SOC: sample oxidation chamber; AOS: atomic oxygen source: STC: sample transfer chamber; GV: gate valve.
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gen. Locquet and Mgchler [lo] determined a proportion of 30 atoms per 100 molecules in the beam in an equivalent source; this proportion will be assumed here. 90% of oxygen atoms entering the SOC are thermalised, the other 10% have kinetic energies ranging from 2 to 5 eV [ I I 1. According to our measurements using optical pyrometry, the temperature of the discharge tube, where molecules are being dissociated, is 1300 K. An ionization gauge not facing the beam measured the total pressure in the oxidation chamber. A quadrupole mass spectrometer directly facing the oxygen beam qualitatively monitored the oxygen dissociation process through detection of mass 16. As a procedure to control the stability of the beam source, the signal given by an optical detector that measures the radiation emitted by the discharge [lo] was kept constant throughout the exposure of the aluminum films to oxygen.
3. Experimental
results
The flux of oxygen molecules impinging on the aluminum surface is obtained from the pressure measured by the ion gauge: Q’(02) =pS/k,TA
,
(1)
where p is the pressure measured by the ion gauge, S = I 100 1/s is the pumping speed, kB the Boltzmann constant and T=300 K the chamber temperature. In Eq. ( I ) we assume that there is no recombination of atoms into molecules in the vacuum chamber and that no oxygen atom reach the ion gauge. The atoms and molecules are emitted from the source through 5 holes of 0.3 mm diameter giving rise to a beam opened with a semiangle of 15” [ 111. The aluminum sample is placed 15 cm away the atom source exit. At this distance the beam section is about A = 50 cm’. According to the aforementioned proportion of 30 atoms per 100 molecules we obtain for the flux of atoms impinging on the aluminum surface: @( 0 atoms) = 0.3 X @( 02) = 0.3pSlkTA
.
(2)
We measured the reflectance of aluminum films in the interval 82.6-174.4 nm at 10” near normal incidence angle. Samples A, B, and C were 180 nm thick aluminum films deposited on supersmooth substrates. Samples D and E were 180 nm thick aluminum films
10
124 (1996) 208-215
1
80
100
120
Wavelength
140
noxidmsd
Al
160
180
(nm)
Fig. 2. Reflectance of an unoxidized aluminum film and the aluminum film exposed to increasing fluences of atomic oxygen, expressed in atoms X cm-‘. versus the wavelength.
deposited on previously oxidized aluminum films. Aluminum was deposited and maintained in UHV conditions. The evaporation rate was about 3 rim/s.. Base pressure was 3 X lo- I0 mbars. During deposition the pressure increased to lo-’ mbars but in a few seconds after deposition a total pressure of lo-” mbars was retrieved. Immediately after deposition we measured in situ the reflectance of samples A to E. Afterwards, the films were exposed to increasing doses of atomic oxygen and the reflectance of the resulting oxidized films was also measured. The longest exposure time was 40 minutes. Samples were placed facing the oxygen atom beam at room temperature. No measurement was made of the sample temperature during exposure to the beam. In Fig. 2 we show the reflectance of sample A versus the wavelength for several doses of atomic oxygen. The drop in reflectance is maximum at shorter wavelengths and continuously decreases towards longer wavelengths. In Fig. 3 we represent the reflectance (normalized to the unoxidized aluminum reflectance) of samples A to E together versus the atomic oxygen fluence (integrated oxygen flux) plotted on a logarithmic scale for different wavelengths. It can be seen that a linear relationship between the reflectance and the decimal logarithm of the fluence is a good fit to the experimental data. Higher reactivities of oxygen atoms against molecules have been reported in the literature on metals such
J.1. Larnryuert et ul. /Optics
Communications
as MO [ 121, Si and Ge [ 131, but as far as we know similar investigations on bare aluminum films have not been reported. We have compared the efficiency of atomic and molecular oxygen to produce a decrease in the FUV reflectance of aluminum films by exposing to molecular oxygen two new 180 nm thick aluminum samples F and G. prepared as described above. The atomic oxygen source was switched on as for atomic oxygen exposure, but the source was switched off a
’ O1
few seconds before starting the exposure. In that way samples F and G were exposed to molecular oxygen in thermal conditions as similar as possible to those for atomic oxygen exposure, although oxygen molecules will have fairly lower energy than atoms because the source is switched off. As the difference in energies is small it should not have a strong influence on their relative oxidizing efficiencies. Also in the case of atomic oxygen exposure the temperature of the sample
“‘1
.
(a)
21 I
124 (1996) 208-215
(b)
. Molecules
0.8
0.8 -
0.6
0.6 2 2
04
0.2
0.2 1
\o ,
0 0 lEt13
“,l”q
,
lE+14
(,,,,
‘1
(
lE+15
Oxygen
,,,,,,
q
00
7Tlllql
lE+16
lE+17
Fluence
lE+lB
lEtI
(cme2
)
I E+13
lEt20
1 lE+14
’
rnrlll lE+15
Oxygen
’ “““9 lE+16
lE+17
Fluence
lE+18
(cmm2
’I
lE+19
IE+20
)
Molecules
000 lE+13
lE+14
lE+15
Oxygen Fig. 3.
(a)-( g ),
lE+16
Drop in reflectance.
and molecular oxygen.
lEc17
Fluence
lE+18
IE+19
(cm-*
normalized
lE+13
lE+20
to the reflectance
lE+14
lEc15
Oxygen
) of the unoxidized
IE+16
lE+17
Fluence
lE+18
(cm-’
IE+19
lE+ZO
)
film, of aluminum coatings versus the fluence of atomic
J.I. L~wruyurrt
et al. /Optics
Communications
124 (1996) 208-215
bo
Molecules
Molecules 0 0
Atoms
Atoms
0
\
\
0 3 lEt13
, ,,“,,! , ,,,,,1 , ,111 “1 r ,,,,,,9 , ,,,,,‘1 , (),,,,1 ( T lE+14
1E+15
Oxygen
lE+16
lEf17
Fluence
lE+lB
(cm-’
lE+19
lE+ZO
lE+13
lE+14
)
lEt15
Oxygen
lEt16
lE+17
Fluence
lE+18
lE+lS
lE+ZO
(cm -2 )
Molecules
075m_ 1E+13
lE+14
lE+15
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lE+16
lEc17
Fluence
lE+18
lE+19
lE+ZO
(cm -a )
Fig. 3. Continued might have somewhat increased due to irradiation from the atomic oxygen source. In Fig. 3 we also represent the reflectance of samples F and G versus the molecular oxygen fluence calculated through relation ( 1). In general the effect of oxygen atoms on aluminum FUV reflectance is stronger than that of oxygen molecules and the difference between the drops in reflectance for the two species increases greatly for long fluences. Let EA and E, be the doses of atoms and molecules, respectively. that produce the samerelativedrop in reflectance
at a particular wavelength. The ratio p( EA, A) = EM/ E, expresses how many oxygen molecules are necessary to produce the same drop in reflectance as one oxygen atom for a certain value of EA and A. p increases monotonically with the exposure. For low values of E,, p takes a value of about five that does not significantly change within the spectral region covered, even though the drop in reflectance reduces when increasing the wavelength. For an intermediate exposure I$ = 2.5 X 10” atoms Xcme2, p takes a constant value of about
J.I. Lorruqrrert
et al. /Optics
5000 in the spectral interval 82.6-104.8 nm. For the exposure PA and for a wavelength higher than or equal to 120.0 nm, p increases progressively with wavelength so that the l& necessary to produce the same drop than EA is much larger than the maximum dose 3.9X 10” molecules X cm -’ applied on our experiment.
4. Calculation
of the lifetime of Al coatings in LEO
The results presented in Section 3 allow us to estimate the effect of atomic oxygen in the outer atmosphere on the FUV reflectance of an aluminum coating supposedly deposited in-orbit. Only LEO are going to be considered in the calculation. The velocity of a LEO orbiter is much higher than the thermal speed of the residual atmospheric species. Hence, the orbiter observes a strong anisotropy in the directional flux of species. which impinge on the orbiter with an energy of about 5 eV. At orbit altitudes from 200 to 700 km the number density of oxygen atoms monotonically decreases, the average values ranging from IO”’ to 2X lOi cm--’ [ 141. The flux that impinges on the orbiter is the product of the number density by the orbiter velocity. Comparing the flux so obtained with our experimental figures of drop in reflectance versus Huence, an aluminum mirror located in a 700 km altitude orbit and facing the advance sense would suffer a 40% relative drop in reflectance at 82.6 nm after less than 5 hours, at 104.8 nm after less than 10 hours and at 120 nm after less than 40 hours. The calculation has not taken into account that the in-orbit oxygen atoms are much more energetic than those in our experimental equipment. The influence of the energy of the oxygen atoms on the reaction with a material strongly depends on the material nature, as it has been shown, e.g., for Pt. OS and SIC [ 15-I 81. The actual FUV drop in reflectance of bare aluminum deposited in orbit and exposed to energetic atoms can be reasonably expected to be equal to or larger than the drop obtained at our laboratory with thermal energy atoms. This means that the extremely short lifetime obtained for an aluminum film is even a higher limit for a real case in a LEO. This result would impose the use of higher altitude orbits in order to reduce the atomic oxygen flux impinging on an aluminum mirror. Nevertheless, LEO could still be useful by applying the wakeshield concept [ 191. The wakeshield or molecular shield is a system to block the
Communications
124 (1996) 208-215
213
flow of atmospheric gas due to the orbiter velocity, thereby hypothetically achieving a large reduction in pressure at that altitude. Since the orbital velocity in a LEO (about 7.6 km/s) is much larger than the Maxwellian speed of the atmospheric atoms (about 1 km/ s) there is virtually no probability for an atmospheric atom or molecule to enter an instrument compartment from the hemisphere on the wake side. opposite the flight direction [20]. Naumann [21] calculated the flux of the ambient atmosphere that can overtake the wake region from the rear part of the orbiter and obtained an equivalent pressure of oxygen atoms as low as lo-j3 mbars. The major source of oxygen atoms that can reach the surface on the wake side is backscattering of ambient oxygen atoms by collisions with thermal molecules in the induced atmosphere of the orbiter. The thermal molecules cannot be backscattered toward the surface of interest, but the ambient molecules, as oxygen atoms, can be, provided the thermal molecule has a mass higher than the ambient molecule. Naumann considered two cases. In the first one, the wakeshield is attached to a space shuttle; the outgassing of the shuttle would limit the capabilities of the wake shield. In the second one, the wakeshield operates a a free flyer, where the only source of thermal molecules would be the outgassing from the shield itself, so that the wakeshield would achieve its ultimate capabilities. Naumann calculated the atomic oxygen fluxes that could reach the volume behind a 5-m diameter wakeshield attached to a shuttle orbiter or operating as a free flyer, in both cases in a 300 km altitude orbit. If the Chemical Beam Epitaxy process molecules considered in Naumann’s calculations are discounted in our case, we obtain the range 1.1 X 10” to 5.8X IO” cm-‘s-l (equivalent pressure: 3.1 X 10p"' to 1.6~ lo-” mbars) for the shuttle attached mode, and the range 7.2X 10” to 3.7 X 10J cm-’ s-’ (equivalent pressure: 1.9~ IO-” to 9.9X lo-” mbars) for the free flyer mode. Despite the high kinetic energy of the oxygen atoms incident upon the orbiter (5 eV) , the energy of an oxygen atom backscattered by collision with the induced atmosphere (mainly H,O and in a lesser extent N, and 0,) is much lower. Assuming elastic collisions, an oxygen atom that would collide with a thermal water molecule in the shuttle attached mode would conserve an energy that depends on the angle of direction change, namely 0.017 eV for 180” (impinging on the wake volume from the rear part), 0.29 eV for 90” (impinging
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J.I. Larruquert et al. /Optics
Cwnmunicarions
normal to the orbit direction) and 2.77 eV for 45”. We calculated the average energy of the oxygen atoms impinging on the wake after colliding with water molecules for the shuttle-attached mode by using the directional flux plotted in Fig. 6 of Ref. [ 211 (for a 30” minimum acceptance angle - cutoff angle - ). We obtained for a rear facing surface (only attainable by oxygen atoms changing their direction more than 90”) a value of 0.07 eV and for a side facing surface (parallel to the orbit, attainable by oxygen atoms changing direction more than 30”) a value of 1.8 eV. A suitable design of the wakeshield geometry may increase the cutoff angle, so reducing the average energy of the impinging atoms. In our equipment the average energy of the thermal atoms is 0.17 eV, related to a 1300 K emission temperature, that would simulate quite well the average energy of a high cutoff angle wakeshield. Hence, a plausible estimate of the lifetime expected for an inorbit aluminum coating behind a wakeshield can be derived from our experimental measurements. According to the above values of the flux for a 300 km altitude orbit calculated by Naumann, an aluminum coating behind a wakeshield in the shuttle-attached mode would suffer a 40% relative drop in reflectance at 82.6 nm in a time as short as 1 hour. A higher altitude orbit would be necessary to enlarge the mirror lifetime. A much more stimulating result is obtained in the free flyer mode. In this case an aluminum mirror in a 300 km altitude orbit would suffer a negligible drop in FUV reflectance after a time as long as 20 years.
5. Conclusions Data on the drop in reflectance in the 82.6-174.4 nm interval of bare aluminum films prepared in UHV conditions and exposed in situ to atomic oxygen are presented for the first time in the literature. The drop in reflectance is shown to be maximum at shorter wavelengths. continuously decreasing towards longer wavelengths. We also measured the reflectance drop of aluminum films exposed to molecular oxygen to compare the effect of both species on the aluminum FUV reflectance and obtained that the effect of oxygen atoms on aluminum reflectance was stronger than that of oxygen molecules and that the difference between the two species increased greatly for long fluences. The comparison was expressed by the ratio p of the molecular
124 (1996) 208-215
oxygen fluence to the atomic oxygen fluence, both fluences for the same drop in reflectance at a particular wavelength. p increased from about five for low fluences up to about 5000 for an intermediate oxygen fluence of 2.5 X 10” atoms cm ~ ‘, which corresponded to a relative drop in reflectance of 40% at 82.6 nm. For this fluence p remained constant in the spectral interval 82.6-104.8 nm and progressively increased for wavelengths longer than or equal to 120.0 nm. The above measurements of the drop in FUV reflectance of aluminum versus the atomic oxygen fluence were used to estimate the life of a mirror for the FUV aluminized in a LEO and exposed to the ambient atomic oxygen. A 40% relative reflectance drop at 82.6 nm would be attained in an extremely short period of 5 hours in an orbit as high as 700 km altitude for the mirror facing the advance sense. As the energy of the ambient oxygen atoms (5 eV) is higher than that of the atoms in our experiment (0.17 eV), the above short lifetime is even a higher limit of the actual case. The fluence of atomic oxygen on an aluminum coating placed in a LEO could be largely reduced, and correspondingly the life of a highly reflecting aluminum mirror for the FUV greatly increased, by placing the aluminum coating behind a wakeshield. In addition the average energy of oxygen atoms backscattered towards the mirror by collisions with molecules outgassed from the orbiter, mainly water, was calculated to vary from 0.07 eV for a rear facing surface to 1.8 eV for a side facing surface, considerably lower than the 5 eV in the orbit. The average energy of atoms impinging on the mirror could be further reduced by means of a suitable design of the wakeshield geometry. Hence, our experiments are a good approach to describe the actual evolution of an aluminum mirror behind a wakeshield at a LEO. Such an aluminum mirror in a 300 km altitude orbit operating as a free flyer would suffer a negligible drop in the FUV reflectance after a time as long as 20 years, which is a stimulating prospective.
Acknowledgements
The technical assistance of Mr. Jose M. SanchezOrejuela is gratefully acknowledged. This work was performed under financial support Nr. ESP93-0324 from the National Programme for Space Research through Spanish CICYT.
J.I. L~rrrryuerfet al. /Optics Communications 124 (1996) 208-215
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