ICMCTF2003 Session B4: Ion Beam Technologies

Different coating conditions in an Ion Beam Assisted Deposition (IBAD) process were tested in order to optimize the deposition of alumina films with a thickness gradient ("gradient coatings") for gas sensor applications. Using aluminum-tri-sec-butylate as precursor the alumina gradient coatings were deposited on a gas sensor microarray (GSMA) of high integrity which has been developed at the Forschungszentrum Karlsruhe. The GSMA is based on sputtered SnO₂ or WO₃ thin films, the electrical conductivity of which sensitively depends on the ambient gas composition. Parallel Pt strip electrodes are sputter deposited on top of the metal oxide films thus subdividing the latter into the sensor segments of the GSMA (currently 38 on an area of 4 by 8 mm²). Initially all sensor segments are identical with respect to their gas response. However, they have to be differentiated to respond with different gas selectivity in order to enable gas-recognition with the GSMA. The latter is obtained by the gas characteristic conductivity patterns which are measured at the microarray due to the differences in the gas selectivity of its sensor segments. This differentiation is achieved by coating the GSMA with the above mentioned gradient films. IBAD enables the deposition of gradient coatings when the current density of the ion bombardment is varied across the substrate yielding laterally different deposition rates.

With the aim of optimizing the deposition of the alumina gradient coatings concerning deposition rate, film composition and applicability of the coatings for gas analysis (which requires a high gas permeability) different IBAD coating parameters were varied, e.g. precursor partial pressure and total pressure, the ion energy and the substrate temperature. The gradient layers were examined as prepared and after annealing at 300°C (typical operating temperature of the GSMA) using surface analytical methods, optical investigations and nuclear analysis techniques. Furthermore, gas exposure tests were performed with coated GSMAs in order to evaluate the influence of the gradient coatings on the gas sensing properties.

Preventing the environmental degradation of depleted uranium (DU) weapons systems is important to maximize their shelf life and reliability. The corrosion behavior of uranium alloys is complex, however, making many conventional corrosion prevention solutions less than optimal. There is a need to find a novel corrosion protection scheme for these alloys that is rugged and effective in a variety of environmental conditions. Recent studies using electrochemical techniques in conjunction with advanced surface spectroscopy have led to a greater understanding of the fundamental corrosion mechanisms in such systems, and these new insights allow for the intelligent tailoring of alloying additions, coating compositions, or surface treatments to prevent DU corrosion. Plasma immersion ion processing (PIIP), plasma source ion implantation (PSII), and ion beam assisted deposition (IBAD) are reproducible, environmentally benign methods used to modify surface chemistry, and may provide effective surface treatment to reduce or prevent DU corrosion.

In this study, coupons of DU were implanted with 35 keV nitrogen ions using PSII to a dose of approximately 3 x 10¹⁷ atoms/cm². Both implanted and non-implanted DU coupons were then coated with 2 µm of either IBAD chromium nitride (e-beam evaporated Cr with a nitrogen ion assist) or with diamondlike carbon (PIIP with an acetylene gas precursor). Accelerated electrochemical tests were done using 0.1 M nitric acid to assess the effectiveness of the coatings at preventing DU dissolution, as well as the effectiveness of the nitrogen implantation at reducing pinhole corrosion.

¹This work was supported by the Environmental Quality Basic Research & Development program, managed by the Industrial Ecology Center, Picatinny Arsenal. SUNY-SB support provided through Army Research Office contract DAAD190110799.

Control of cell growth with radio therapy is a well established treatment for a variety of human diseases. Irradiation by brachytherapy spares healthy tissue but requires, that the radiation source is brought in close contact with the target volume (brachy (Greek) = near).

At the end of the recent millennium, radioactive stents were tested in clinical trials to prevent restenosis after minimal invasive treatment of coronary artery disease. A stent is a small expandable stainless steel mesh, which is inserted into the artery as a scaffold. Suitable radiation for such applications is delivered by pure beta emitters, such as phosphorous-32 (E_betamax = 1.7 MeV, T_1/2 = 14.3d).

Radioactive stents were implanted into approximately 300 patients and proved to be effective in suppressing restenosis inside the body of the stent. Unfortunately restenosis at the ends of the stent occurred with a rate as high as with non radioactive stents. However, stents eluting drugs from their surface were proved to be effective in avoiding restenosis, before this so called "edge effect" could be solved.

We have used ion implantation of radioactive atoms to activate the stents. In accordance with results published before, our investigations have shown, that the surface properties of steel are improved by phosphorous implantation. Furthermore ion implantation allows to control the radioactivity on the device quite precisely, which is mandatory for a permanent medical implant¹.

There is a variety of medical applications, where ion implanted P-32 sources can be useful. In animals activated capsular tension rings have been tried successfully to suppress secondary cataract formation after surgical replacement of the lens of the eye². The edge effect observed with radioactive stents, indicates, that moderate radiation increases the healing response of injured tissue. This can be used to improve the outcome of minimal invasive treatment of cerebral aneurysms. The feasibility of the method was tested in animals³.

¹M.-A. Golombeck, S. Heise, K. Schloesser, B. Schuessler, H. Schweickert, to be published in Nucl. Inst. Meth B (2003).

²A.M. Joussen, B. Huppertz, H.R. Koch, N. Kernert, K. Camphausen, K. Schloesser, A.M.H. Foerster, F. E. Kruse, A. Lappas, and B. Kirchhof; Int. J. Radiation Oncology Biol. Phys. 2001;49;817-825.

³J. Raymond, P. Leblanc, A.-C. Desfaits, I. Salazkin, F. Morel, C. Janiki, S. Roorda; Stroke. 2002;33;421-427.

Ion beam techniques as a surface modification and coating technique offer some advantages such as high controllability and versatility, and low process temperature. However, they suffer from a basic limitation, their line-of-sight character. Only the part of an object which is turned towards the ion beam can be treated properly. This is one of the reasons why the application of ion beam techniques to three-dimensional objects is quite limited. If one would like to take advantage of e.g. the low process temprature for ion beam treatment of temperature-sensitive materials, but the object is three-dimensional, special set-ups have to be used.

The present paper describes different apparatus for ion beam sputter coating of rings, cylinders and tubes. They use conical sputter targets which are placed inside the objects. A beam of energetic argon ions impinges onto the target and sputters material onto the inner walls.

Rings, cylinders and tubes of aluminum were coated at temperatures below 200°C with thin amorphous carbon films. Adhesion measurements showed that the films were well adhering to the aluminum substrates. Electrochemical polarization measurements in sodium chloride solution showed that the films offered good corrosion protection due to their low microporosity.

The present study uses a dual ion beam system where one ion beam is used to sputter materials, providing a controlled number of atoms of Ti and Si arriving at the substrate, and the second beam is used to provide noble gas and nitrogen ion assistance with controlled amount of additional energy.

A calculation shows that the TiN crystals have diameters between 4 and 7 nm for Si incorporation levels of 11 to 7 at %, respectively. Transmission Electron Microscopy of ion-assist samples confirm these sizes of TiN crystals. Microhardness measurements show that there is indeed a peak in hardness of about 40 GPa at around 9% Si content. Hardness decreases at higher crystal sizes, but with a lower rate when compared with multilayered structures. We believe that the higher amount of developed interphase surfaces in our nanocrystalline system is responsible for this different behaviour. Preliminary experimental data show that, at substrate temperatures around 800 K, the maximum hardness of the nanocomposite film is higher than 45 GPa. Further study of deposition parameters is continuing to identify the conditions for the formation of ultra-hard layers.

An essential requirement for new applications of ion beam technology is a desired tailoring of the beam profile. For optimization and adjustment, knowlegde of physical beam parameters and plasma processes in the ion source is necessary.

In our experiments the radial distribution of the energy influx by an Ar ion beam (ion beam source: EC/A 125, IOM Leipzig) towards a substrate surface has been determined at different distances from the ion source. For the measurement of the energy influx a special thermal probe has been used [1]. The axial and radial distribution of the deposited energy has been studied in dependence on typical process parameters of the ion source as supplied microwave-power, beam voltage, accelerator voltage, and gas flow, respectively. The experimentally determined radial beam profiles show a characteristic decrease of the energy flux density which is transferred to the substrate during processing.

The profile can also be visualized by the interaction of the ion beam with a micro-disperse particle cloud which has been charged and confined in an additional rf-plasma [2]. This method allows for a faithful image of the extraction grid system of the ion source, too. By knowing the typical beam profile it is possible to decide on the use of the ion beam source in surface treatment, and on the tailoring of desired distributions of a broad beam ion source.

[1] Kersten,H., Deutsch,H., Steffen,H., Kroesen,G.M.W., Hippler,R., Vacuum 63(2001), 385.

[2] Kersten,H., Deutsch,H.,Otte,M., Swinkels,G., Kroesen,G.M.W., Thin Solid Films 377-378(2000), 530.