Auger electron spectroscopy interface study of evaporated and ion-plated silver films on steel substrates

Thin Solid Films, 105 (1983) 319-323 319

METALLURGICAL AND PROTECTIVE COATINGS

AUGER ELECTRON SPECTROSCOPY INTERFACE STUDY OF E V A P O R A T E D AND I O N - P L A T E D SILVER FILMS ON STEEL SUBSTRATES L. M. WANG*, F. Z. WANG, J. H. ZHANG AND L. H. ZHAI

Department of Metallic Materials Science and Engineering, Beifing Polytechnic University, Beijing (China) P. G. CAO

Central Laboratory, Beijing Electron Tube Factor)', Beijing (China) Received July 13, 1982; accepted January 25, 1983)

Thin silver films were deposited onto steel substrates by either evaporation or ton plating, which included not only the conventional ion plating process with a high substrate bias ( - 3 kV) and a resistance-heated evaporation source but also a process with low bias ( - 5 0 V) and a hollow cathode electron beam source which could provide a higher ionizing rate. The width of the interfaces and the oxygen distribution were analysed by Auger electron spectroscopy combined with the ion etching technique. Also the adhesion of the films was examined by means of a scratch test and a thermal shock test. It was found that for silver on steel, an insoluble pair of materials, only ion plating with a high substrate bias could produce a "pseudodiffusion"-graded interface, and the excellent adhesion of the film to the substrate should be attributed to the cleaned and graded interface.

I. INTRODUCTION

Ion-plated films show excellent adhesion to the substrate regardless of the filmsubstrate combination. The exceptionally good film adherence has been attributed to the cleaning of the substrate and the formation of a graded interface, which reduces damaging stress gradients across the interface caused, for instance, by differences in thermal expansion coefficients1"2. When the film and substrate materials are mutually soluble, the graded interface may form by thermal diffusion. However, when the film and substrate materials are insoluble, diffusion is difficult. In this case, ion plating can still produce a graded interface by a "pseudodiffusion''1 mechanism. This is the major advantage claimed for ion plating. A number of ion-plated graded interfaces have been reported in the l i t e r a t u r e L 3-9. However, it seems to us that only a few Auger electron spectroscopy (AES) studies have been made on the nature of the interface between insoluble materials except for the work on the Ag-Ni interface by Walls e t al. 9 To understand the adhesion mechanisms in ion-plated films better, more information on the interface between insoluble materials is needed. * Present address: Materials ScienceProgram, University of Wisconsin,Madison, WI 53706,U.S.A. 0040-6090/83/$3.00

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In this paper we present the results of a comparative study on the silver-steel interface, an interface between a typical insoluble pair of materials 1°. Silver films were prepared on mild steel (containing 0.3°o C) substrates by means of four different evaporation and ion plating processes. The composition profile of the interfaces was determined with AES combined with the ion sputter etching technique. The widths of the interfaces were compared as well as the oxygen contamination in the interfacial region. Also the adhesion of the films was examined to determine its relationship with the composition profile of the interfaces. 2. EXPERIMENTAL DETAILS

The coating was performed in an oil-diffusion-pumped system. The substrates were well polished and mounted on an electrode which was the cathode of a d.c. supply. The four coating processes used in this study were as follows: (1) evaporation without prior sputter cleaning; (2) evaporation with prior sputter cleaning; (3) conventional d.c. diode ion plating; (4) ion plating with a hollow cathode discharge (HCD). The two ion plating processes also included a sputter cleaning procedure before the plating. During sputter cleaning the chamber was backfilled with argon gas of purity 99.999~o to a pressure of 2 x 10-2 Tort after it had been evacuated to a pressure of less than 2 x 10- 4 Torr; the substrate bias was - 3 kV. The duration of the sputter cleaning was 30 rain in processes (2), (3) and (4). The silver used in the study was better than 99.999')0 pure. In processes (1), (2) and (3), the silver was evaporated from a resistance-heated tantalum boat while the argon pressure was kept constant at 2 x 10 2 Torr. In the evaporation procedure of processes (1) and (2) there was no bias on the substrate, while in that of process (3) the substrate bias remained at - 3 kV after sputter cleaning and the current density at the substrate was 0.2 mA cm z. In process (4), a graphite liner was charged with silver and placed in a water-cooled copper hearth. The silver was evaporated with an HCD, which could provide a high ionizing rate at relatively low pressure and low bias ~l. The H C D gun was situated at an angle of 45 ~' with respect to the water-cooled copper hearth. The hollow cathode consists of a tantalum pipe of inside diameter 3 mm and l mm thick. Argon was introduced through this pipe to sustain the discharge. During evaporation with the HCD, the argon pressure, the substrate bias and the current density were 5 x 10 a Torr, - 50 V and 0.8 mA c m - 2 respectively. The hollow cathode power supply was at approximately 30 V and the current was 30 A. The surface temperature of the specimens was measured with a movable thermocouple as soon as the substrate bias was removed. It was observed that the substrate temperature reached 260 °C after the 30 min sputter cleaning and that no further considerable rise in temperature occurred during the various evaporation procedures, which took only 1 min or less for the specimens used in the AES analysis. The interface analyses were performed in a JAM P-10 Auger spectrometer with a base pressure of 6 x 10 -9 Torr. A primary electron beam energy of 10 keV and a beam current of 0.1 gA were used and the beam diameter was about 50 nm. Composition profiles were obtained by removing successive atomic layers from the surface by ion beam etching. The ion etching used for profiling was performed with a 3 keV ion gun supplied with argon gas of purity 99.999~/0 at a pressure of 6 x l0 5 Torr. The mean etching rate was estimated to be about 10 nm min i. The electron

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and ion beams were made coincident and concentric and thus the effects caused by the crater shape 8 were minimized. It should be noted that the profiles presented in this paper show the peak-to-peak heights of selected Auger peaks plotted against the sputtering time instead of concentrations plotted against depth, so that the results must be regarded as qualitative. However, this is not too serious in a comparative study. The adhesion of the films was examined by means of a scratch test and a thermal shock test. The films prepared for the adhesion test were all 600 nm thick. 3. RESULTS AND DISCUSSION The surface of the steel substrate contained an oxygen contamination layer as shown in Fig. 1. The typical composition profile of the interface prepared by process (1) is shown in Fig. 2. It is obvious that an oxygen peak occurs at the interface. The width of the interface for this particular system should be sharp, but it was found to be approximately 70 nm in Fig. 2. This is probably due to broadening by knock-on effects during the sputter profiling 9' ~2. Ag

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Figure 3 shows an interface between an evaporated film and a sputter-cleaned substrate prepared by process (2). Sputter cleaning of the steel substrate before evaporation was found to reduce the oxygen contamination at the interface effectively. The width of the interface is similar to that prepared by process (1), although the substrate temperature reached 260 °C when the evaporation started after 30 min of sputter cleaning. The composition profile of the interface between an ion-plated film and a substrate prepared by process (3) is shown in Fig. 4. The oxygen contamination at the interface has been eliminated and the width of the interface has been broadened. Figure 5 is a differentiated Auger spectrum recorded from the same point as Fig. 4 after 25 min of sputter etching, which, together with Fig. 4, indicates an interface

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width of above 200 nm. Considering the broadening by knock-on effects during the sputter profiling, we estimate that a "pseudodiffusion"-graded interface of depth about 130 nm was produced by the ion plating process. The formation of such a graded interface cannot be explained in terms of the ion implantation mechanism because the ions and energetic neutrals arrive at the substrate with energies of only several hundred electronvolts as calculated by Teer 2. The most probable mechanism for the formation of this graded interface is the codeposition of evaporated particles of film material and particles of substrate material which are sputtered off the dNldE

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Fig. 5. Differentiated Auger spectrum recorded from the same point as Fig. 4 after 25 min of sputter etching. Fig. 6. Composition profile of the interface between a silver film ion plated with an H C D and a sputtercleaned steel substrate. Daring coating the substrate bias and current density were ---50V and 0.8 m A c m - 2 respectively.

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substrate under bombardment and are returned by gas scattering during ion plating; this mechanism has been well discussed by Teer 2 and Mattox 13. Also the knock-on process could promote the formation of the graded interface. Figure 6 shows the composition profile of an interface prepared by process (4), the HCD ion plating process. The oxygen contamination at the interface has been removed but the width of the interface is similar to that prepared by evaporation processes. This indicates that no graded interface between a silver film and a steel substrate can be produced under a low substrate bias, even though the current density at the substrate is high, which implies a higher ionizing rate. In the scratch adhesion test, the specimen prepared by process (1) showed very poor film adhesion, while no distinct comparison could be made between the specimens prepared by the other three processes. However, after thermal shock by cyclically rapidly heating the specimens in a 500 °C furnace and cooling in water at room temperature, only the specimen prepared by process (3), i.e. ion plating with a high substrate bias ( - 3 kV), showed good adhesion. The excellent adhesion of the film to the substrate may be attributed to the cleaned and graded interface. 4. CONCLUSIONS (1) Ion sputter cleaning before deposition can effectively remove the oxygen contamination on the surface of a steel substrate. (2) During ion plating the substrate bias in the discharge is a very important factor which influences the formation of the "pseudodiffusion"-gradedinterface. For silver on steel, an insoluble pair of materials, only ion plating with a high bias can produce such an interface. (3) Cleaning of the substrate and the formation of a "pseudodiffusion"interface enhance notably the adhesion between a silver film and a steel substrate. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

D.M. Mattox, Monogr. SC-R-65-852, 1965 (Sandia Corporation). D.G. Teer, J. Adhes.,8(1977) 289. D.G. Teer and F. Salem, Thin Solid Films, 45 (1977) 583. D. G. Teer, Proc. ConJl on Ion Plating and Allied Techniques. Edinburgh, June 1977, CEP Consultants, Edinburgh, 1977, p. 13. T. Narusawa and S. Komiya, J. Vac. Sci. Technol., 11 (1974) 312. D.W. Hoffman and R. Nimmagadda, J. Vac. Sci. Technol., 11 (1974) 657. B. Swaroop and I. Adler, J. Vac. Sci. Technol., 10 (1973) 503. C.W.B. Martinson, P. J. Nordlander and S. E. Karlsson, Vacuum, 27 (1977) 119. J.M. Walls, D.D. Hall, D.G. TeerandB. L. Delcea, ThinSolidFilms, 54(1978)303. M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. S. KomiyaandK. Tsuruoka, Jpn. J. AppLPhys.,Suppl. 2, Partl(1974)415. R.S. Nelson, Radiat. Eft., 2 (1969) 47. D . M . Mattox, Proc. 2nd Int. Conf on Ion Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979.