Laboratory based space experiments using an ion implanter

Vacuum~volume 44/numbers 3/4/pages 167 to 170/1993

0042-207X/9356.00+.00 © 1993 Pergamon Press Ltd

Printed in Great Britain

Laboratory based space experiments using an ion implanter* J S C McKee and M S Mathur,

Accelerator Centre, Department of Physics, University of Manitoba, Winnipeg,

Manitoba, Canada R3T 2N2

The Accelerator Centre at the University of Manitoba has been active in recent years in the study of materials re/evant to the space industry and in the characterization of such materials. Plans are in place to extend this work on the ion implantation of materials to new areas of research in which more esoteric physical processes are examined on Earth prior to the development of structures for space vehicles or planetary study. This paper consists of two parts, the first a summary of recent research to be published in refereed scientific literature, the second an outline of future plans using existing facilities.

1. Introduction This paper consists of two parts, the first a review of recent research from the Accelerator Centre at the University of Manitoba, the second an outline of future plans using existing facilities.

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2. P a t t i 2.1. Summary. Any space vehicle, whether orbiting the Earth, acting as a permanent space laboratory or travelling freely through the solar system is continually exposed to solar wind. Indeed, bombardment by protons with energies from electronvolts to millions of electronvolts is significant even in the upper atmospheric regions of the Earth's environment (see, for example the proton aurora). Reproducible features of solar wind in the laboratory can be valuable in simulating damage to materials used in space vehicles and in characterizing their surfaces. At the University of Manitoba we have used two facilities, one a 120 kV linear implanter with a high current (up to 5 mA) capability, and the other a 50 MeV proton spiral ridge cyclotron to carry out experiments relating to space conditions. At present only the Narodny implanter is, however, available for material studies. The implanter has been used extensively in conjunction with the technique of inelastic Raman light scattering to investigate changes at material surfaces upon bombardment with hydrogenic ions and other light ions present in the near-Earth environment. 2.2. Polyimide films (Dupont KaptonTM). Polyimide (Kapton) polymer films are used in the fabrication of integrated circuits and dielectric Kapton films L2 are also used as insulating materials applicable at temperatures up to 600 K, making Kapton a desirable material for space applications. Repeat units of a polyimide molecule are shown in Figure 1. The interaction with H + particles can result in the addition of

*Supported in part by NSERC and Manitoba Hydro.

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hydrogen at the sites marked with an asterisk on the figure. These are the oxygen atoms of the ether linkage. The implanted hydrogen ion can attach itself to ether oxygen, forming a water molecule and freeing one of the benzene rings. This could effect the thermal stability of the material and render polyimide somewhat ineffective in its use as a dielectric or insulator. The Raman spectrum of the implanted Kapton surface is given in Figure 2. Bands at 615, 1178, 1584, 1606 and 3062 c m - ~are characteristic vibrations of the benzene molecule, suggesting that some benzene rings have indeed become free to yield a characteristic Raman spectrum. This work is currently under completion. 2.3. Carbon, graphite and carbide surfaces. Carbon and graphite are materials frequently used in hostile environments. In addition the resistance of titanium carbide (TIC) to chemical sputtering has made it an attractive low-Z material for fusion containment devices and space vehicles which are exposed to high particle fluxes. We have stimulated the interaction between a fusion plasma and graphite, carbon and TiC by implanting their sur167

J S C McKee and M S Mathur. Laboratory based space experiments 176000

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faces with energetic H~- ions generated by the linear accelerator. In understanding the processes involved in the implantation and post-implantation behaviour of hydrogen atoms in solids, the assumption is that collisions between the substrate and the hydrogen atoms are the main source of energy loss, and that this transfer of energy results in some damage to the solid. The second assumption is that retention and release are governed by trapping at the damage sites and by recombination of molecules at the surface. The third is the formation of new molecular complexes due to the rearrangement of chemical bonds. Conventional techniques involving low-energy electron beams (like SIMS, Auger, LEED, etc.) cause desorption of these complexes and prove somewhat ineffective. However, unenhanced surface Raman scattering techniques have proved successful in the detection of these complexes. A typical Raman spectrum of the TiC surface implanted with 60 keV H~, and TiC samples from inside the fusion reactor which has been subjected to some 1500 discharges are given in Figures 3 and 4. The CH activity and methane formation as welt as the complexes of Ti and H are characterized. The implantation of a TiC surface with H ÷ ions at energies of tens of kiloelectronvolts not only simulates leakage from a fusion plasma but also the bombardment of a space vehicle by solar wind particles. The remarkable result is that significant amounts of C - H and Ti-H activity on the surface are observed, leading directly to amorphization and embrittlement of the surface concerned. Characterization of the implanted surface by means of surface Raman scattering identifies clearly the carbon and hydrogen complexes formed during implantation. The production of carbon and hydrogen complexes on a TiC surface has been studied in detail and published recently 3. A

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complementary piece of work in which TiC coatings were exposed to plasma spraying and the bombarded surface also analysed by inelastic Raman scattering, yielded similar results, and this collaborative work with INRS-l~nergie, Qurbec, has now also been published 4. It therefore seems clear that H ions from solar wind can also stop and build up in the TiC surface, and this, together with induced diffusion, will result in the formation of molecular complexes which are then absorbed on the TiC surface with eventual amorphization a real possibility. The work on TiC was inspired by earlier studies of a graphite surface implanted with D + (ref 5). The high production of deutcrated methanes in the surfaces studied suggested that if TiC behaved in a similar fashion it might be quite unsuitable as a material either for the first wall in a fusion reactor or a space vehicle component. Credence is provided to this argument by recent studies of the carbon samples from inside a Tokamak fusion reactor. The carbon samples were subjected to some 1500 plasma discharges and their surfaces were analysed by the technique of Raman scattering. The formation of methanes and their adsorption on the surface of carbon samples has been reported 6, casting a doubt on the suitability of these materials in an environment rich in proton flux.

40

2.4. C h a r a c t e r i z a t i o n of implanted silicon, a - S i l l films and optical Z

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fibres. The implantation energies used in these experiments are again typical of bombardment by solar wind and much of the research is therefore directly relevant to aerospace studies. Oxygen bombardment in space, however, is likely to be by atomic oxygen, and the effect of atomic oxygen on composite materials, in particular, is of great interest. We have bombarded silicon

d S C McKee and M S Mathur: Laboratory based space experiments

crystals with molecular oxygen and nitrogen which is broken up on collision with the surface and thereafter presumably behaves similarly to the atomic species. The results are described in ref 7. In particular, the bombardment of p-type silicon with ~60~- and t4N+ results not only in the formation of an amorphous silicon layer but in various complexes of silicon, boron, oxygen, and nitrogen ; boron being the p-type impurity in the silicon. This has been shown by the presence of characteristic Raman bands in the surface spectrum of implanted silicon. a-Si:H thin films, suitable for use as photoconverters, lose some of their hydrogen with time and thus leave behind a significant density of defects. The implantation of H + into such films passifies the defects and improves photoconductivity dramatically8, suggesting a novel method for the damage repair of solar converters. 2.5. Conclusion. It will be seen from the work charted above that the characterization of implanted surfaces through inelastic Raman scattering is useful in the understanding of the effects of exposure of space materials to solar wind. The problem of amorphization of refractory materials may require attention. 3. Part II

Future plans for the space related study of materials fall into two distinct categories : (i) the search for evidence of the synthesis of water in lowenergy proton bombardment of silicates ; and (ii) the investigation of the chemical consequences ofion interactions with frozen systems (ice, NH3, CH4, etc.). 3.1. The search for evidence of the synthesis o f water in lowenergy proton bombardment of silicates. Plasmas interact with the

surfaces of many objects in the solar system. Solar wind bombards the surface of the Moon, Mercury, and the asteroids, on a continuing basis. In many cases the bombardment can cause desorption of material from these surfaces and lead to a change in composition, or it can cause sputtering and even changes in the local plasma. We intend to carry out a program comp-

lementary to a proposed NASA experiment 9, in which a search for evidence of water in certain moon rocks is underway. Our experiment will involve the bombardment of silicates by kiloelectronvolt protons at high fluence and an analysis of the implanted surface. O H - radicals, if present, will be detected by the technique of inelastic Raman scattering. The effectiveness of the inelastic light scattering technique in the characterization of implanted surfaces and the detection of molecular complexes formed during bombardment, has already been demonstrated by us 1°. The Raman active symmetric OH stretching vibration in water molecule occurs at v~ = 3645.6 cm-1 (actually a doublet with components at 3646.1 and 3653.9 cm- 1) and for the O H radical AG 1/2vibrational frequency at 3568.4 cm- 1is also Raman active. Infra-red absorption analysis could also be utilized to complement the Raman data. In particular, it is hoped to implant OH free infrasil (SiO2) or superinfrasil in order to determine whether the production of water in such materials is possible. This experiment will simulate the effect of impact between solar wind and planetary rock in a laboratory setting. 3.2. Investigation o f the chemical consequences of ion interactions with frozen systems (ice, NH3, CH4, etc.). The energy of the

reaction partners in classical chemical reactions between systems in thermal equilibrium can be defined by the Maxwell-Boltzmann distribution. The rate constant of the reactions, with the exception of non-equilibriumthermal reactions induced by accelerated ions of kinetic energy in excess of I eV, can be represented by an Arrhenius equation. In the case of injected ions, the projectiles with their kinetic energy and electronic excitation enter into a collision complex overcoming activation energy barriers, opening endothermic reaction channels and transferring excess energy to the solid lattice. These processes constitute an important part of the chemical consequences of ion implantation and could find application in space science. Space is rich in energetic atoms, ions, molecules, fragments, clusters, and grains from cosmic rays, solar or stellar radiation, shock waves and many secondary processes with energies ranging from a few electronvolts (photodissociation) to some teraelectronvolts (cosmic rays). Nonequilibrium chemical processes, thus, typical for space could

Table 1. Some important sources for energetic ions in space*

Solar wind (1 AU)

Solar flares (1 AU)

T-Tauri winds Low energy galactic cosmic rays

Energy and type of ions

Integral flux (cm- 2 s- ~)

Fluence (cm 2)

1 keV H ÷ (95%) 4 keV He2+ (4%) 10 keV C 6÷ (0.1%) 13 keV O s÷ (0.05%) 20 keV Ne I°+ (0.007%) 25 keV Si 14+ (0.008%)
2-3 x l0 s

4 x 1025

4x 1019

106 101

3 X 1019 1017

I A U = semimajor axis of Earth's orbit around the Sun + 1.496 x 1011 m.

*K Roessler in ref 11. 169

J S C McKee and M S Mathur: Laboratory based space experiments

10 9

CO, etc.) a n d mixtures thereof, a n d the reactions o f secondary k n o c k - o n atoms. This will involve a study by S I M S o f the products f o r m e d w h e n frozen systems are subjected to p r o t o n (H ÷) a n d (He 2÷) b o m b a r d m e n t . T h e energy o f the ions will be varied from kiloelectronvolts to megaelectronvolts. This work c a n be vital to the u n d e r s t a n d i n g of organic synthesis a n d the p r o b ability o f life in space.

2-3 × 108 108 2 X 10 7

References

Table 2. Solar wind fluxes for bodies in solar system* Object in space

Distance to sun (AU)

Mercury P/Halley at perihel Venus Earth Mars Asteroids Jupiter and moons Saturn and moons Uranus Neptune P/Halley at aphel

0.387 0.587 0.723 1.000 1.524 3.5 5.202 9.555 19.21 30.109 ~35

Flux of solar wind ions (cm- 2 s- ]) 2 × 10 9 1.3 x 110 9

10 7 3 X 10 6

7 x 105 3-4 x 105 2 x 105

* K Roessler in ref 11.

result in s u p r a t h e r m a l reactions in solids a n d could f o r m the basis for the f o r m a t i o n o f complex organic m a t t e r by multicentre reactions in the collision cascades. Tables 1 a n d 2 s u m m a r i z e the energies, fluxes a n d fluences o f energetic ions f r o m solar wind a n d flares, T-Tauri winds in the early history o f the solar system a n d cosmic rays : the energetic ions can create a great n u m b e r o f h o t secondary reactions irrespective o f the n a t u r e o f the space object. We are interested in the study o f ion interactions with frozen systems ( H 2 0 , NH3, C H 4 ,

170

K Sato, S Harada, A Saiki, T Kimura, T Okubo and M Mukai, IEEE Trans Parts Hybrids Packaging, 9, 176 (1973). 2A M Wilson, Polyirnides, Vol 2, p 715. Plenum Press, New York (1984). 3C B Kwok, M S Mathur, J S C McKee, H A Marzouk and E B Bradley, J Nucl Phys, 170, 57 (1990). 4 M S Mathur, J S C McKee, G G Ross, H A Marzouk and E B Bradley, Can J Phys, 68, 293 (1990). 5M S Mathur, V P Derenchuk and J S C McKee, Can J Phys, 62, 214 (1984). 6M S Mathur, J S C McKee, G G Ross, H A Marzouk and E B Bradley, Mater Lett, 10, 382 (1991). 7M S Mathur, V P Derenchuk and J S C McKee, Can J Phys, 62, 201 (1984). s C B Kwok, M S Mathur and J S C McKee, Mater Lett, 10, 457 (1991). R E Johnson, Proc 14th Int Confon Atomic Collision in Solids, Salford, Nucl lnstrum Meth, B67, 148 (1992). ~°M S Mathur and J S C McKee, Surface Interface Analysis, in press (1992). ~t E Bussoletti and G Strazzulla (Eds), Solid State Astrophysics, p 197. North-Holland, Amsterdam (1991).