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Low Earth orbit effects on indium tin oxide and polyester and comparison with laboratory simulations
Surface and Coatings Technology, 62 (1993) 499—503
499
Low Earth orbit effects on indium tin oxide and polyester and comparison with laboratory simulations R. A. Synowicki, Jeffrey S. Hale, N. J. lanno and John A. Woollam Centerfor Microelectronic and Optical Materials Research, and Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511 (USA)
Paul D. Hambourger Department of Physics, Cleveland State University, Cleveland, OH 44115 (USA)
Abstract Laboratory simulation of the low Earth orbit (LEO) environment using oxygen plasma ashers are discussed. Their effectiveness as space simulators are compared with LEO through analysis of indium tin oxide (ITO) thin films and bulk polyester exposed to both environments. Spectrophotometry and atomic force microscopy have been used to characterize optical and microstructural changes as a result ofexposure to the simulated (oxygen plasma asher) and the actual space environment aboard shuttle flight STS-46. Results show that the low Earth orbit space environment is much harsher than the plasma asher on the optical properties of ITO as well as the surface roughness of polyester. On space-exposed samples, a significant shift in the ITO absorption edge is seen for fluences of 2 but not on films exposed in the asher. The surface roughness of polyester exposed in the asher increases by a 2factor x 1020 cm that ofpolyester exposed in space increases by a factor of 20 for the same atomic oxygen fluence. The directional of atoms 5.5, while nature and higher kinetic energy of atomic oxygen in LEO serves to erode polyester more than in the asher. The different results obtained in the asher for both ITO and polyester bring into question the suitability of using plasma ashers as space simulators for these materials.
1. Introduction The low Earth orbit (LEO) environment proves to be extremely harsh and corrosive for spacecraft designed for long operational lifetimes, such as the proposed space station Freedom [1, 2]. Molecular oxygen is broken down into atomic oxygen (AO) by solar UV radiation, The effect of AO on materials results in oxidation, erosion and texturing of surfaces. Changes in surface properties can and do cause detrimental effects for space power systems and optical surfaces, which can greatly reduce the operational lifetime of spacecraft deployed into this environment [3]. The LEO environment is commonly simulated through the use of semiconductor plasma etching systems using air or oxygen as the process gas [4, 5]. These “ashers” have come into wide use because of their simplicity, high plasma density and low cost. However, the use of ashers as space simulators is often questioned owing to the low kinetic energy of incident particles (approximately 0.5 eV, while an LEO corresponds to approximately 4.5 eV), the omnidirectional nature of the reactive species striking the surface, and contamination from SiO 2. In addition to these, the presence of many different reactive species (many in excited states) in the asher plasma makes it a complicated environment corn-
0257—8972/93/56.00
pared with LEO. The result is that, rather than having an exact simulation, one uses a simple rule of thumb that, if a material will survive asher exposure for a short fraction of operational lifetime, it will also withstand extended exposure in LEO. This assumption may not be true for all materials. Thus the suitability of ashers as LEO simulators for a particular material must be considered on a case-by-case basis. This paper examines the differences between LEO and the simulated space environment of oxygen plasma ashers for two of several materials under consideration for space use, namely indium tin oxide (ITO) and polyester. Significant differences between asher and space environments are shown by optical and microstructural data. The results indicate that the asher environment is far less severe than actual LEO, indicating that the use of ashers for accurate space simulation is questionable for these materials. 2. Experimental details Data were obtained for three different thicknesses of ITO (In2 _~Sn~O3) [6—9]deposited onto clear polyester substrates by Courtaulds Performance Films Corporation. The nominal film thicknesses were approxi-
©
1993
—
Elsevier Sequoia. All rights reserved
500
R. A. Synowicki et al.
/
LEO effects on ITO and polyester
mately 10, 25 and 100 nm and were patterned into long narrow rectangles (nominally 0.4 cm x 2 cm) by marking the ITO films with resist and etching with H2 SO4. This patterning was done to facilitate four-point probe electrical resistivity measurements on the samples. This procedure resulted in exposure of the bare polyester substrate to AO as well as the ITO films. One set of these films was prepared for launch aboard the space shuttle Atlantis on the Evaluation of Oxygen Interaction with Materials payload (EOIM III). The samples were exposed to a ram (normal to sample 20 overapproximately 40 h exposure to 2anx LEO surface) 2, AOaccumulated fluence of 10 atoms at 128 cm nautical miles above the Earth. A second set was exposed in the simulated LEO environment of an oxygen plasma asher, in which the base pressure was 50 mTorr. The chamber was backfihled with oxygen gas to a pressure of 100 mTorr. This air+ oxygen mixture was then excited to a plasma state by applying 50 W r.f. power at 13.56 MHz. The dimensions of the excited volume are 25 cm long by 4 cm diameter. AO flux was determined by mass loss of Kapton polyimide exposed to this plasma environment. The average AO flux in the asher was approximately 1.5 x 1020 atoms cm2 h~, allowing simulation of EOIM III exposure in 1.33 h. Samples exposed in the asher were also ashed to much higher fluences to simulate long-term effects of LEO. All films were characterized by spectrophotometry using a UV—visible--near-IR spectrophotometer. Optical absorbance was measured before and after exposure to LEO or asher environments. The surface microstructure of the films was studied by atomic force microscopy (AFM) with a stand-alone head.
~ ~ 20
_______________________________________________________ 920 200
~
Fig. I. Absorbance changes for ITO (25 nm)/polyester exposed on payload EOIM III to an AO fluence of 2 x 1020 atoms cm2:—--, exposed to 2 x 1020 atoms cm2 ———, masked flight sample ground control sample 1; — — —, ground control sample 2. Note that the three control samples are nearly identical while the exposed sample shows a marked increase in absorbance.
~
40
/f~ 20
~oo
~oc
500
600
700
500
000
‘coo
1V~2~LN0H,
Fig. 2. Absorbance of ITO (25 nm)/polyester exposed in a plasma asher to a fluence of 2 x 1020 atoms cm2 showing no change in optical absorbance for fluences equal to that of EOIM III:—, no
3. Results and discussion
ashing;
3.1. Indium tin oxide thin films Figure 1 shows the optical absorbance of ITO 25 nm thick exposed aboard EOIM III. Four samples are shown on the graph. The full curve is for the sample exposed to AO in orbit. The “masked” sample was placed beneath the exposed flight sample and served as a flight control, receiving no significant AO fluence. The “ground control” samples were kept safe in laboratories during the flight and were not exposed to AO. The graph shows a significant change in absorbance below a wavelength of 450 nm. Exposure to 2 x 1020 0 atoms cm2 in space most probably changes the defect chemistry of ITO, and the optical properties are sensitive to the oxygen concentration [3]. This increased absorbance is characteristic of visible “darkening” of ITO films exposed to AO. Figure 2 shows an identical sample exposed in the asher
environment to an identical fluence. No significant change in absorption is noted in this case. This trend continues for longer ashing times and higher fluences. Figure 3 shows the absorbance of ITO 100 nrn thick exposed in the asher to a fluence of 1021 atoms cm2, showing no change in the optical absorbance. Also, the mean roughness of the ITO films on polyester was 9.8 nm. Exposure of the ITO films to the asher environment showed no change in surface roughness from the mean value. The ITO films exposed on orbit showed an increased roughness by factors of 1.9, 1.5 and 0.79 for film thicknesses of 10 nm, 25 nm and 100 nm respectively.
ashed, 2 x
1020
atoms cm2.
3.2. Polyester Exposure of the substrate to LEO and asher environments resulted in significant erosion of the polyester.
R. A. Synowicki et al.
/
LEO effects on ITO and polyester
mass loss) identical with that shown in Fig. 5 (2 x 1020 atoms cm2). The surface roughness increased by a factor of 20 over that of the unashed samples! Table 1 shows the roughness changes of polyester more clearly. Note that the space-exposed samples are three to five times rougher than the asher-exposed samples. The samples exposed in the asher to a fluence of 1021 atoms cm2 are a factor of only 2—2.4 times rougher than those exposed in space to 2 x 1020 atoms cm2 the visible changes were much less pronounced than for the space-exposed samples.
90 80 70 L
50 40
2~ 1 0
~~.300
,
400
5CC
600 700 WAVE.EN0~H,nm
800
501
900
000
Fig. 3. Absorbance of ITO (100 nm)/polyester exposed in a plasma asher to a fluence of 1 x 1021 atoms cm2:—, no ashing’ ashed, 1x 2. This sample was ashed to a fluence five times that 102i of EOIM atoms III cmand still shows no change in optical absorbance.
The normally transparent polyester substrate, when exposed to the LEO, appeared white to the naked eye, having no specular reflectance. AFM images of the polyester show greatly increased surface roughening due to LEO exposure, causing the visible optical changes. Figure 4 shows a surface of the unashed polyester substrate for reference. The mean roughness of unashed polyester samples averaged to 13.1 nm r.m.s. Figure 5 shows the polyester after exposure to 2 x 1020 atoms cm2 in the plasma asher. Exposure in the asher caused an increase in the mean surface roughness by a factor of 5.5. Finally, Fig. 6 shows the surface plot of the LEO-exposed polyester aboard EOIM III. This sample was exposed to a fluence (as determined by Kapton
The increased roughness of the space-exposed polyester can be attributed to two important factors. First, the AO fluence in LEO is directional, whereas the asher provides reactive species from all directtons. Second, the average kinetic energy of ram AO atoms in LEO is approximately 4.5 eV, compared with a mean energy of 0.5 eV in the asher environment. .
.
.
4. Conclusion Identical samples of ITO and polyester were exposed to both LEO and ground-based simulations using an oxygen plasma asher, revealing large differences in effects on the optical properties of ITO and surface roughness of polyester. The changing optical properties of ITO most likely result from a change of stoichiometry, namely the defect chemistry. AFM roughness studies of polyester before and after exposure to both environments show large differences. For space-exposed polyester, the surface roughnesses were approximately three to five times greater than asher-exposed samples. This is to be some-
500.0 n~
250.0
flM
0.0 ~
100 JiM
Fig. 4. AFM surface plot of typical unashed polyester substrate. The r.m.s. surface roughness for the unashed polyester is 13.1 nm.
502
R. A. Synowicki et a!.
/
LEO effects on ITO and polyester
JiM
0.8
JiM
z C C
0.0J.rM 100
0 C N
75
C C
50 25
0
0
25
50
75
0 100 PM
Fig. 5. AFM surface plot ofpolyester substrate ashed to a fluence of 2 x 1020 atoms cm -2 in a plasma asher. In the asher environment, reactive species impinge on the surface from all directions. Asher-exposed polyester shows an increase in roughness of 5.5 times compared with that of unashed polyester.
~
JiM
0 7
JiM
0.0
PM
100 JiM
2. Note the roughness of the spaceFig. 6. AFM surface plot of polyester substrate exposed aboard EOIM III to a fluence of 2 x 1020 atoms cm exposed samples is three to five times that of samples ashed in the laboratory and 20 times that of unashed samples. The greater roughnesses of the space-exposed samples are due to the directional nature and kinetic energy of AO in the LEO.
what expected, since the orbital environment provides directional AO attack at a higher kinetic energy. The use of ashers is commonly justified by noting that the asher environment is harsher than LEO owing to the higher fluxes and wider variety of reactive species present in the plasma. Our data indicate that the opposite is true, and the use of ashers may need to be
reconsidered in favor of other LEO simulation methods for these materials. Acknowledgment The authors would like to acknowledge the ElectroPhysics Branch of the Lewis Research Center, National
R. A. Synowicki et a!.
/
LEO effects on ITO and polyester
503
TABLE 1. Comparison of r.m.s. surface roughness for polyester substrate material exposed in space and asher environments
was also supp ~rted by NASA Cooperative Agreement NCC-3- 19
Exposure type
Oxygen fluence 2) (atoms cm
R.m.s. surface roughness (nm)
Ratio of roughness to 13.1 nma
References
None
0
Asher
2x
Space
2 x 1020
16.7 11.0 12.51 12.24 84.9 60 259 272 537 637
1.27 0.84 0.95 093 6.48 4.58 19.75 20.74 41.0 48.6
Asher
1020
x
2i 1f~
The average r.m.s. surface roughness ofunashed samples was 13.1 nm. Asher exposure to a fluence of 2 x 1020 atoms cm2 results in a roughness increase of approximately 5.5 times. Space exposure on EOIM III to the same fluence results in a rougbness increase of 20 times. The difference is due to the directional nature of LEO AO with a higher kinetic energy than in the asher. aUnashed r.m.s. roughness, 13.1 nm.
Aeronautics and Space Administration (NASA), for support of this work under Grant NAG-3-95. This work
1 US Standard Atmosphere, 1976, US Government Printing Office, Washington, DC, 1976. 2 D. A. Gulino, Space station solar concentrator materials research, Adv. Mater. Manuf. Proc., 3 (2) (1988) 261—277. 3 L. L. Leger, Oxygen atom reaction with shuttle materials at orbital altitudes, NASA Tech. Memo. TM-58246, 1982 (National Aeronautics and Space Administration). 4 5. K. Rutledge, B. A. Banks, F. DiFilippo, J. Brady, J. T. Dever and D. Hotes, An evaluation of candidate oxidation resistant materials for space applications in LEO, NASA Tech. Memo TM-100122, November 1986 (National Aeronautics and Space Administration). 5 R. L. Parsons and D. A. Gulino, Effect of an oxygen plasma on uncoated thin aluminum reflecting films, NASA Tech. Memo. TM89882, 1987 (National Aeronautics and Space Administration). 6 J. L. Vossen, Physics of transparent conducting films, Thin Films, 9 (1) (1977). 7 K. L. Chopra, S. Major and D. K. Pandya, Transparent conductors—a status review, Thin Solid Films, 102 (1) (1983). 8 I. Hamberg and C. G. Grandvist, Evaporated Sn-doped In 2O3 films: basic properties and applications to energy efficient windows, 9 5. Ray, R. Banerjee, N.
BaSU,
J. App!. Phys., 54 (1983) 3479.
A. K. Batabyal and A. K. Barua,
499
Low Earth orbit effects on indium tin oxide and polyester and comparison with laboratory simulations R. A. Synowicki, Jeffrey S. Hale, N. J. lanno and John A. Woollam Centerfor Microelectronic and Optical Materials Research, and Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511 (USA)
Paul D. Hambourger Department of Physics, Cleveland State University, Cleveland, OH 44115 (USA)
Abstract Laboratory simulation of the low Earth orbit (LEO) environment using oxygen plasma ashers are discussed. Their effectiveness as space simulators are compared with LEO through analysis of indium tin oxide (ITO) thin films and bulk polyester exposed to both environments. Spectrophotometry and atomic force microscopy have been used to characterize optical and microstructural changes as a result ofexposure to the simulated (oxygen plasma asher) and the actual space environment aboard shuttle flight STS-46. Results show that the low Earth orbit space environment is much harsher than the plasma asher on the optical properties of ITO as well as the surface roughness of polyester. On space-exposed samples, a significant shift in the ITO absorption edge is seen for fluences of 2 but not on films exposed in the asher. The surface roughness of polyester exposed in the asher increases by a 2factor x 1020 cm that ofpolyester exposed in space increases by a factor of 20 for the same atomic oxygen fluence. The directional of atoms 5.5, while nature and higher kinetic energy of atomic oxygen in LEO serves to erode polyester more than in the asher. The different results obtained in the asher for both ITO and polyester bring into question the suitability of using plasma ashers as space simulators for these materials.
1. Introduction The low Earth orbit (LEO) environment proves to be extremely harsh and corrosive for spacecraft designed for long operational lifetimes, such as the proposed space station Freedom [1, 2]. Molecular oxygen is broken down into atomic oxygen (AO) by solar UV radiation, The effect of AO on materials results in oxidation, erosion and texturing of surfaces. Changes in surface properties can and do cause detrimental effects for space power systems and optical surfaces, which can greatly reduce the operational lifetime of spacecraft deployed into this environment [3]. The LEO environment is commonly simulated through the use of semiconductor plasma etching systems using air or oxygen as the process gas [4, 5]. These “ashers” have come into wide use because of their simplicity, high plasma density and low cost. However, the use of ashers as space simulators is often questioned owing to the low kinetic energy of incident particles (approximately 0.5 eV, while an LEO corresponds to approximately 4.5 eV), the omnidirectional nature of the reactive species striking the surface, and contamination from SiO 2. In addition to these, the presence of many different reactive species (many in excited states) in the asher plasma makes it a complicated environment corn-
0257—8972/93/56.00
pared with LEO. The result is that, rather than having an exact simulation, one uses a simple rule of thumb that, if a material will survive asher exposure for a short fraction of operational lifetime, it will also withstand extended exposure in LEO. This assumption may not be true for all materials. Thus the suitability of ashers as LEO simulators for a particular material must be considered on a case-by-case basis. This paper examines the differences between LEO and the simulated space environment of oxygen plasma ashers for two of several materials under consideration for space use, namely indium tin oxide (ITO) and polyester. Significant differences between asher and space environments are shown by optical and microstructural data. The results indicate that the asher environment is far less severe than actual LEO, indicating that the use of ashers for accurate space simulation is questionable for these materials. 2. Experimental details Data were obtained for three different thicknesses of ITO (In2 _~Sn~O3) [6—9]deposited onto clear polyester substrates by Courtaulds Performance Films Corporation. The nominal film thicknesses were approxi-
©
1993
—
Elsevier Sequoia. All rights reserved
500
R. A. Synowicki et al.
/
LEO effects on ITO and polyester
mately 10, 25 and 100 nm and were patterned into long narrow rectangles (nominally 0.4 cm x 2 cm) by marking the ITO films with resist and etching with H2 SO4. This patterning was done to facilitate four-point probe electrical resistivity measurements on the samples. This procedure resulted in exposure of the bare polyester substrate to AO as well as the ITO films. One set of these films was prepared for launch aboard the space shuttle Atlantis on the Evaluation of Oxygen Interaction with Materials payload (EOIM III). The samples were exposed to a ram (normal to sample 20 overapproximately 40 h exposure to 2anx LEO surface) 2, AOaccumulated fluence of 10 atoms at 128 cm nautical miles above the Earth. A second set was exposed in the simulated LEO environment of an oxygen plasma asher, in which the base pressure was 50 mTorr. The chamber was backfihled with oxygen gas to a pressure of 100 mTorr. This air+ oxygen mixture was then excited to a plasma state by applying 50 W r.f. power at 13.56 MHz. The dimensions of the excited volume are 25 cm long by 4 cm diameter. AO flux was determined by mass loss of Kapton polyimide exposed to this plasma environment. The average AO flux in the asher was approximately 1.5 x 1020 atoms cm2 h~, allowing simulation of EOIM III exposure in 1.33 h. Samples exposed in the asher were also ashed to much higher fluences to simulate long-term effects of LEO. All films were characterized by spectrophotometry using a UV—visible--near-IR spectrophotometer. Optical absorbance was measured before and after exposure to LEO or asher environments. The surface microstructure of the films was studied by atomic force microscopy (AFM) with a stand-alone head.
~ ~ 20
_______________________________________________________ 920 200
~
Fig. I. Absorbance changes for ITO (25 nm)/polyester exposed on payload EOIM III to an AO fluence of 2 x 1020 atoms cm2:—--, exposed to 2 x 1020 atoms cm2 ———, masked flight sample ground control sample 1; — — —, ground control sample 2. Note that the three control samples are nearly identical while the exposed sample shows a marked increase in absorbance.
~
40
/f~ 20
~oo
~oc
500
600
700
500
000
‘coo
1V~2~LN0H,
Fig. 2. Absorbance of ITO (25 nm)/polyester exposed in a plasma asher to a fluence of 2 x 1020 atoms cm2 showing no change in optical absorbance for fluences equal to that of EOIM III:—, no
3. Results and discussion
ashing;
3.1. Indium tin oxide thin films Figure 1 shows the optical absorbance of ITO 25 nm thick exposed aboard EOIM III. Four samples are shown on the graph. The full curve is for the sample exposed to AO in orbit. The “masked” sample was placed beneath the exposed flight sample and served as a flight control, receiving no significant AO fluence. The “ground control” samples were kept safe in laboratories during the flight and were not exposed to AO. The graph shows a significant change in absorbance below a wavelength of 450 nm. Exposure to 2 x 1020 0 atoms cm2 in space most probably changes the defect chemistry of ITO, and the optical properties are sensitive to the oxygen concentration [3]. This increased absorbance is characteristic of visible “darkening” of ITO films exposed to AO. Figure 2 shows an identical sample exposed in the asher
environment to an identical fluence. No significant change in absorption is noted in this case. This trend continues for longer ashing times and higher fluences. Figure 3 shows the absorbance of ITO 100 nrn thick exposed in the asher to a fluence of 1021 atoms cm2, showing no change in the optical absorbance. Also, the mean roughness of the ITO films on polyester was 9.8 nm. Exposure of the ITO films to the asher environment showed no change in surface roughness from the mean value. The ITO films exposed on orbit showed an increased roughness by factors of 1.9, 1.5 and 0.79 for film thicknesses of 10 nm, 25 nm and 100 nm respectively.
ashed, 2 x
1020
atoms cm2.
3.2. Polyester Exposure of the substrate to LEO and asher environments resulted in significant erosion of the polyester.
R. A. Synowicki et al.
/
LEO effects on ITO and polyester
mass loss) identical with that shown in Fig. 5 (2 x 1020 atoms cm2). The surface roughness increased by a factor of 20 over that of the unashed samples! Table 1 shows the roughness changes of polyester more clearly. Note that the space-exposed samples are three to five times rougher than the asher-exposed samples. The samples exposed in the asher to a fluence of 1021 atoms cm2 are a factor of only 2—2.4 times rougher than those exposed in space to 2 x 1020 atoms cm2 the visible changes were much less pronounced than for the space-exposed samples.
90 80 70 L
50 40
2~ 1 0
~~.300
,
400
5CC
600 700 WAVE.EN0~H,nm
800
501
900
000
Fig. 3. Absorbance of ITO (100 nm)/polyester exposed in a plasma asher to a fluence of 1 x 1021 atoms cm2:—, no ashing’ ashed, 1x 2. This sample was ashed to a fluence five times that 102i of EOIM atoms III cmand still shows no change in optical absorbance.
The normally transparent polyester substrate, when exposed to the LEO, appeared white to the naked eye, having no specular reflectance. AFM images of the polyester show greatly increased surface roughening due to LEO exposure, causing the visible optical changes. Figure 4 shows a surface of the unashed polyester substrate for reference. The mean roughness of unashed polyester samples averaged to 13.1 nm r.m.s. Figure 5 shows the polyester after exposure to 2 x 1020 atoms cm2 in the plasma asher. Exposure in the asher caused an increase in the mean surface roughness by a factor of 5.5. Finally, Fig. 6 shows the surface plot of the LEO-exposed polyester aboard EOIM III. This sample was exposed to a fluence (as determined by Kapton
The increased roughness of the space-exposed polyester can be attributed to two important factors. First, the AO fluence in LEO is directional, whereas the asher provides reactive species from all directtons. Second, the average kinetic energy of ram AO atoms in LEO is approximately 4.5 eV, compared with a mean energy of 0.5 eV in the asher environment. .
.
.
4. Conclusion Identical samples of ITO and polyester were exposed to both LEO and ground-based simulations using an oxygen plasma asher, revealing large differences in effects on the optical properties of ITO and surface roughness of polyester. The changing optical properties of ITO most likely result from a change of stoichiometry, namely the defect chemistry. AFM roughness studies of polyester before and after exposure to both environments show large differences. For space-exposed polyester, the surface roughnesses were approximately three to five times greater than asher-exposed samples. This is to be some-
500.0 n~
250.0
flM
0.0 ~
100 JiM
Fig. 4. AFM surface plot of typical unashed polyester substrate. The r.m.s. surface roughness for the unashed polyester is 13.1 nm.
502
R. A. Synowicki et a!.
/
LEO effects on ITO and polyester
JiM
0.8
JiM
z C C
0.0J.rM 100
0 C N
75
C C
50 25
0
0
25
50
75
0 100 PM
Fig. 5. AFM surface plot ofpolyester substrate ashed to a fluence of 2 x 1020 atoms cm -2 in a plasma asher. In the asher environment, reactive species impinge on the surface from all directions. Asher-exposed polyester shows an increase in roughness of 5.5 times compared with that of unashed polyester.
~
JiM
0 7
JiM
0.0
PM
100 JiM
2. Note the roughness of the spaceFig. 6. AFM surface plot of polyester substrate exposed aboard EOIM III to a fluence of 2 x 1020 atoms cm exposed samples is three to five times that of samples ashed in the laboratory and 20 times that of unashed samples. The greater roughnesses of the space-exposed samples are due to the directional nature and kinetic energy of AO in the LEO.
what expected, since the orbital environment provides directional AO attack at a higher kinetic energy. The use of ashers is commonly justified by noting that the asher environment is harsher than LEO owing to the higher fluxes and wider variety of reactive species present in the plasma. Our data indicate that the opposite is true, and the use of ashers may need to be
reconsidered in favor of other LEO simulation methods for these materials. Acknowledgment The authors would like to acknowledge the ElectroPhysics Branch of the Lewis Research Center, National
R. A. Synowicki et a!.
/
LEO effects on ITO and polyester
503
TABLE 1. Comparison of r.m.s. surface roughness for polyester substrate material exposed in space and asher environments
was also supp ~rted by NASA Cooperative Agreement NCC-3- 19
Exposure type
Oxygen fluence 2) (atoms cm
R.m.s. surface roughness (nm)
Ratio of roughness to 13.1 nma
References
None
0
Asher
2x
Space
2 x 1020
16.7 11.0 12.51 12.24 84.9 60 259 272 537 637
1.27 0.84 0.95 093 6.48 4.58 19.75 20.74 41.0 48.6
Asher
1020
x
2i 1f~
The average r.m.s. surface roughness ofunashed samples was 13.1 nm. Asher exposure to a fluence of 2 x 1020 atoms cm2 results in a roughness increase of approximately 5.5 times. Space exposure on EOIM III to the same fluence results in a rougbness increase of 20 times. The difference is due to the directional nature of LEO AO with a higher kinetic energy than in the asher. aUnashed r.m.s. roughness, 13.1 nm.
Aeronautics and Space Administration (NASA), for support of this work under Grant NAG-3-95. This work
1 US Standard Atmosphere, 1976, US Government Printing Office, Washington, DC, 1976. 2 D. A. Gulino, Space station solar concentrator materials research, Adv. Mater. Manuf. Proc., 3 (2) (1988) 261—277. 3 L. L. Leger, Oxygen atom reaction with shuttle materials at orbital altitudes, NASA Tech. Memo. TM-58246, 1982 (National Aeronautics and Space Administration). 4 5. K. Rutledge, B. A. Banks, F. DiFilippo, J. Brady, J. T. Dever and D. Hotes, An evaluation of candidate oxidation resistant materials for space applications in LEO, NASA Tech. Memo TM-100122, November 1986 (National Aeronautics and Space Administration). 5 R. L. Parsons and D. A. Gulino, Effect of an oxygen plasma on uncoated thin aluminum reflecting films, NASA Tech. Memo. TM89882, 1987 (National Aeronautics and Space Administration). 6 J. L. Vossen, Physics of transparent conducting films, Thin Films, 9 (1) (1977). 7 K. L. Chopra, S. Major and D. K. Pandya, Transparent conductors—a status review, Thin Solid Films, 102 (1) (1983). 8 I. Hamberg and C. G. Grandvist, Evaporated Sn-doped In 2O3 films: basic properties and applications to energy efficient windows, 9 5. Ray, R. Banerjee, N.
BaSU,
J. App!. Phys., 54 (1983) 3479.
A. K. Batabyal and A. K. Barua,