Modification of surface properties by dynamic recoil mixing

Materials Science and Engineering, 69 (1985) 67-73

67

Modification of Surface Properties by Dynamic Recoil Mixing* J. S. COLLIGON

Thin Film and Surface Research Centre, University of Salford, Salford M5 4 WT (Gt. Britain) (Received September 17, 1984)

ABSTRACT

In dynamic recoil mixing ( D R M ) a layer o f constant o p t i m u m thickness o f a material A is maintained on a substrate o f material B whilst the surface o f A is bombarded with energetic particles. Under the correct conditions the irradiating particles deposit their m a x i m u m energy at the interface between A and B, causing considerable mixing o f these species. Thus new combinations o f materials can be created or well-bonded layers of A on B can be formed. A n apparatus for producing D R M layers is described and experimental methods for looking at the layers formed and determining o p t i m u m parameters for D R M are presented. The results for gold and antimony films are reported and compared with those predicted by a simple computer model of the process. Specific applications o f the technique are discussed including the formation o f hard layers and wear-resistant surfaces (silicon nitride on steel), electrochemical sensors (gold on titanium), catalytic layers (platinum on titanium) and thermojunctions (antimony on bismuth). S o m e preliminary results on the bonding o f D R M layers are given and other applications o f the technique described.

1. INTRODUCTION When a thin film of material A covers a substrate S and is bombarded with ions of energy such that their most probable range is of the same order as the thickness of A, considerable mixing of atoms A and S occurs near the film-substrate boundary. This phenomenon has been investigated by several research *Paper presented at the International Conference on Surface Modification of Metals by Ion Beams,

Heidelberg, F.R.G., September 17-21, 1984.

0025-5416/85/$3.30

groups [1-9] and is referred to as ion beam mixing. Considerable atomic mixing can occur under these conditions but, because of sputtering of the top material A, the process is limited; eventually all film A is removed and the mixed layer itself begins to be eroded. To prevent this erosion a technique known as dynamic recoil mixing (DRM) was developed by the present author and coworkers [ 1 0 , 1 1 ] . In this m e t h o d the film A is maintained in dynamic equilibrium by laying down atoms of A at the same rate as they are sputtered away. The film can now be maintained at a constant o p t i m u m thickness indefinitely. As a result the ion energy can be deposited at the interface continuously, leading to extensive atomic mixing of atoms A and S. In m a n y cases the result is the creation of a narrow region across which a gradual transition occurs with the concentration of A falling from 100% to 0% and that of S increasing from 0% to 100%. Such a film-substrate structure often produces an extremely good bond between the two materials so that the layer A is particularly durable. This is especially useful in situations where an ultrathin surface coating is all that is required, e.g. in sensing electrodes for electrochemical reactions, for well-bonded ohmic contacts in semiconductor technology or for hard coatings on engineers' gauges. If thicker layers are required, the DRM process may be followed by non-equilibrium DRM where the film is allowed to build up. Alternatively, after DRM, thicker films of A can simply be sputtered on top of the mixed layer. The apparatus developed to study DRM is illustrated schematically in Fig. 1. A full description has been given elsewhere [11] but, briefly, the apparatus produces two ion beams, an intense I keV, 1 mA beam for sputtering A and a second 10-40 keV, 10 pA beam for recoil implanting A into S. In most © Elsevier Sequoia/Printed in The Netherlands

68

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cases, both beams can be of argon ions but other species can sometimes be used to advantage. A crystal oscillator can be placed in the substrate position to monitor deposition and resputtering rates so that conditions can be established to maintain, as nearly as posible, a constant thickness of film. In fact the oscillator monitors constant mass but this is usually acceptable. Recently, other research groups have begun to study similar types of system [12, 13] where the properties of films produced under so-called "ion-beam-assisted" deposition are investigated. It is the purpose of this review, however, to summarize the results obtained using the original DRM equipment. First we shall look at experiments carried out to determine the optimum parameters for the recoil mixing process. Next a range of applications already studied will be reviewed and, finally, the advantages of the m e t h o d over established surface modification techniques and latest developments will be discussed. 2. OPTIMIZATION OF PARAMETERS A recent c o m p u t e r simulation model of Karpuzov e t al. [14] allows a prediction of the number of particles transmission sputtered through a thin film of constant thickness. This is n o t the exact equivalent to the situation existing during DRM where the thin film sits on a substrate. Nevertheless, the model will predict recoil yields to a first approximation, especially if the substrate atoms are lighter than those of the film, and it can be usefully applied to show h o w this recoil yield

varies with fluence and film thickness. In the model, which is based on the MARLOWE computer code of Robinson and Torrens [15] and H o u and Robinson [ 16 ], the ion-induced collision cascade is considered to develop according to a binary collision approximation with a correction for nearly simultaneous interactions. Inelastic effects are considered and, to simulate amorphous targets, directional correlations are destroyed by rotating the target before each collision. An experiment was devised to compare measured recoil yields with those predicted by the c o m p u t e r simulation and this has been reported in detail elsewhere [17]. The experimental technique used either small-angle emergence Rutherford backscattering of scattered 2 MeV He + ions or conventional Rutherford backscattering following a chemical etch to remove the non-mixed surface film. In the first set of experiments the low energy edge of the implant species was observed to change as a result of mixing. The degree of mixing or recoil yield could be estimated by matching this edge to an error function and noting the change in spectra before and after DRM or by looking at the broadening of the whole Rutherford backscattering peak of the implant species. In the second series of experiments the number of implant species recorded b y Rutherford backscattering after etching was taken as the number that were recoil mixed. The results for antimony on silicon and gold on silicon are summarized in Fig. 2 which shows the effect of using different film thicknesses for DRM with 10 keV Ar ÷ ions. The full curves represent the theoretical prediction which is seen to agree fairly well with experimental results. Clearly the o p t i m u m thickness of the film is a b o u t 11 nm for antimony and should be a b o u t 4 nm for gold, if such a thin film were continuous. It is of practical interest to note that the recoil ratio (atoms injected per incident ion) exceeds unity so that the process occurs at a rate comparable with ion implantation. F o r recoil yield as a function of ion fluence there is for antimony a linear response over the range of fluences studied (1015-1017 ions cm-2). However, the results for gold are complicated by a secondary effect; the silicon becomes mobile and at higher fluences (greater than 3 X 1015 ions cm -2) starts to grow into

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the gold layer. These types of problem are to be expected in situations where there is a high deposition energy density and high level of mixing and have been seen elsewhere for gold on silicon [ 18]. Certainly, careful calibration experiments should be carried out on any new system and with any new combination of species. The Rutherford backscattering technique has been used in all our major studies and has often shown up artefacts and inconsistencies in the DRM process, allowing modifications in technique to be made which are essential to the eventual production of the desired mixed layers under optimum conditions.

3. APPLICATIONS OF DYNAMIC RECOIL MIXING

3.1. Production o f hard surfaces The production of hard layers with low friction and low wear rates is particularly important in engineering. Often tolerances on components are such that, whilst the improvem e n t in these properties is desirable, the change in dimensions accompanying largescale coating is unacceptable. For example in engineers' gauges, where a standard caliper

spacing or a thickness gauge is required, conventional coating is normally out of the question. A further problem can be the tendency for the coating to flake off. Ion implantation is already used for nitriding [19] and is very effective where the material being coated forms a suitable nitride having good mechanical properties. Often this is not possible and the addition of an ultrathin well-bonded hard layer is the only alternative. Such layers have been produced by DRM and evaluated by applying conventional microhardness, friction and wear tests [20]. The film material chosen for this work was silicon. This was sputtered onto steel and DRM conditions were established with nitrogen ions forming both the sputtering and the recoil ion beam. The Knoop hardness coefficient was shown to increase from 410 to 640 kgf mm -2 after a DRM fluence of 1016 N + ions cm -2 and from 410 to 920 kgf m m -2 after 10 z7 N + ions cm -2. Even at these high fluences the hardness continues to increase with no evidence of saturation. Pin and disc measurements of wear of these same samples showed an improvement of a factor of more than 3 in wear resistance over this same fluence range, again without saturation being seen. The coefficient of friction was reduced by a factor of 2 in this same fluence interval. In order to make a comparison with conventional ion implantation, samples of steel were bombarded with 5 X 1017 N + ions cm -a. Whilst some marginal improvements in hardness, wear and friction properties occurred, they were considerably less than those shown by the DRM samples. Rutherford backscattering analysis of DRM samples showed the presence of silicon and nitrogen in the steel in the correct proportions for Si3N4 but, with such thin layers, it is difficult to prove conclusively that the layer had the usual stoichiometry of bulk silicon nitride. Another earlier study [21] of the formation of an N i - F e layer by DRM (using Ar + ions this time) also showed improved microhardness properties but this work was not extended to look at the effect of nitrogen bombardment.

3.2. Electrochemical applications 3.2. Ultrathin electrochemical sensing electrodes Electrodes to monitor electrochemical reactions rates (e.g. oxygen levels in solutions)

70 play an important role in the modern electrochemical industry. They are often made from expensive materials and, frequently, a deterioration in performance occurs over a period of time, necessitating closure of plant whilst the electrodes are cleaned. The disposable electrode would offer many advantages, such an electrode being constructed of a robust support coated with an ultrathin layer of the precious metal. Conventional evaporated films, sputtered films and ion-plated films have been considered but lack durability and deteriorate quickly in the chemical solutions. For this reason, novel electrodes were prepared using DRM [22]. The desired surface material in this study was gold. Substrates were of various kinds, including glass, silica, FEP plastic and titanium. The DRM process was followed by sputtering further layers of gold onto the surface until the resistance was sufficiently low. This was usually achieved when the gold layer was 2 0 - 5 0 nm in thickness. In all cases the performance of the electrodes was found to be similar to that for pure gold. Figure 3 shows some recent singlesweep voltammograms for a gold film on titanium compared with those for a pure gold electrode. The position of the first peak indicates the reduction of oxygen and, the lower the voltage E, the more efficient is the sys-

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71

tern, since less power will be used to monitor the process. Curves have been displaced vertically in Fig. 3 to assist in the comparison. The numerals on the curves represent the numbers of weeks of operation of these electrodes and it is clear that the new DRM electrode (Fig. 3(b), 0) starts by responding at a lower voltage than new pure gold (Fig. 3(a), 0) but the response changes to coincide with that for pure gold as the surfaces become poisoned. The durability of these films is excellent and far better than those produced by sputtering which only survived several days in solution.

3.2.2. Electrocatalytic layers One of the most important materials for electrocatalysis is platinum. In this study the

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DRM technique has been used to produce thin platinum layers on titanium and the properties of the electrode so formed have been studied by observing the cyclic voltammogram. Figure 4 shows the results for a DRM layer which has had further platinum sputtered onto it. The response is clearly similar to that for pure platinum (see inset) and, again, lifetime tests have shown that these films survive for considerable periods (so far in excess of several months).

3.3. Thermoelectric junctions DRM provides the challenging possibility of producing novel alloys with special thermoelectric properties. Contact with the sample to be monitored can be particularly good and

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thermal inertia and conductivity can be minimized by using very thin mixed layers. Benenson et al. [13] have studied the response of an S b - T e alloy formed by ion beam mixing and found values for the Seebeck coefficient approaching 50 pV °C-1. Our own study has been on the S b - B i combination. A thick layer of bismuth was sputtered onto glass to form a substrate. A n t i m o n y was then introduced by DRM to form the Sb-Bi junction. Figure 5 shows the variation in e.m.f, with the temperature for samples treated with various fluences of the 10 keV Ar ÷ ion beam used in the DRM process. A maximum response of 39 #V °C -1 was found to occur at a fluence of (1-2) × 1016 ions cm -2. The response of such junctions has been found to be reproducible and hysteresis effects are small, i.e. the response for rising temperatures is almost the same as that for falling temperatures. At higher fluences the response again decreases, as shown in Fig. 6. Also the reverse characteristic (changing hot and cold junctions) is identical.

3.4. Adhesion o f films Whilst the durability of films produced by DRM in chemical environments has been established, the mechanical bonding of such films is also of interest. So far this programme of study has only considered gold films on

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silica and glass but, as the bonding of gold to these materials is not generally easy to attain, they provide a good test for the DRM process. The procedure has been to produce a gold-silica (or gold-glass) layer by DRM, to sputter a further layer of gold onto this mixed layer, to use a special Loctite cement to attach steel rods and to pull these in a tensiletesting machine. A standard surface-cleaning procedure has been followed in all cases. The results show a remarkable improvement in failure stress as the 10 keV Ar + ion fluence increases. Sputtered films fail at stresses of the order of 2 kgf cm -2 whereas films produced by DRM using 10 keV Ar ÷ ions at 1017 ions cm -2 have stresses ranging from 45 to 85 kgf cm -2. The scatter in the data is caused by non-reproducibility of the bonding of the glue and alternative techniques for testing are being developed.

4. C O N C L U D I N G R E M A R K S

This review has primarily concentrated on the applications of DRM to the modification of metals. There are several other exciting possibilities under investigation where the technique is being used on plastics and on semiconductor materials. For plastics, metallic layers can be well bonded to the plastic. This is a particular advantage of DRM since the technique relies on placing a top layer of metal onto the plastic before b o m b a r d m e n t and this serves to conduct away charge and heat. In semiconductors, shallow implanted layers have been produced [23] and also silicide layers have been formed. However, the work so far has involved treatment of relatively small areas. The next stage of scaling up the system to treat surfaces 10 cm in diameter and to produce patterned implants by raster scanning the recoil beam is well under way. A higher energy (40 keV) recoil facility has been added. This will allow DRM to be performed using thicker films; the gold situation, for which the computer model predicted an o p t i m u m thickness of 4 nm, was unsatisfactory since such gold layers are not continuous. This can n o w be remedied because with 40 keV Ar + ions the optimum thickness will be a b o u t 16 nm. As to the future, the problems of treating large areas should merely be a matter of

73 scaling-up, i.e. an engineering p r o b l e m . It is likely t h a t investigations here and in o t h e r l a b o r a t o r i e s will p o i n t the w a y to an u n d e r standing o f the i m p o r t a n t p a r a m e t e r s relating t o a t o m i c m i x i n g so t h a t c o n v e n t i o n a l s p u t t e r coating, plasma processing or ion-plating rigs can t h e n be a d a p t e d t o provide an environm e n t in w h i c h the initial D R M c o n d i t i o n s can be m a i n t a i n e d b e f o r e films are allowed t o build up in thickness. O n c e this is achieved, D R M will u n d o u b t e d l y b e c o m e the m a j o r c o a t i n g or surface m o d i f i c a t i o n process in large-scale engineering applications since it has the f o l l o w i n g attributes. (1) Relatively c h e a p e q u i p m e n t is required. (2) A wide range o f materials can be used. (3) I n s u l a t o r s can be c o a t e d .

ACKNOWLEDGMENTS I have d r a w n o n material p r o d u c e d in j o i n t research p r o g r a m m e s involving m a n y colleagues a n d students. I a m m o s t grateful to all these p e o p l e for their helpful c o m m e n t s and advice in p r e p a r i n g this review. I am also grateful t o the E l e c t r i c i t y Research Council for providing the e l e c t r o c h e m i c a l d a t a for Fig. 4. T h e s u p p o r t o f the Science and E n g i n e e r i n g R e s e a r c h C o u n c i l is gratefully acknowledged,

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23 N. A. A. Hussein, H. Kheyrandish, A. E. Hill and J. S. Colligon, Proc. 6th Int. Conf. on Thin Films, Stockholm, August 13-1 7, 1984, in Thin Solid Films, to be published.