AES CHARACTERIZATION OF PROTECTIVE THIN LAYERS ON THE AgNi0.15 CONTACT MATERIAL AFTER ITS TREATMENT IN THE RF PLASMA L Koller Institute for Electronics and Vacuum Technics, Ljubljana, Slovenia and M. Bizjak Iskra - Stikala, Kranj, Slovenia Keywords: electrical contacts, Ag contact materials, silver based contact materials, passivation, passivation thin films, RF plasma, AES analysis, Auger Electron Spectroscopy analysis, low cost implementations Abstract: Weil-known surface protection of Ag-contacts with the thin gold layer is rather expensive. We studied some cheaper types of protective layers. Thickness of the layer must be optional to avoid the corrosion effect of the surroundings and at the same time to insure sufficiently good electric conductivity between the two coated surfaces. Material that is used for the layers must be at least as good electric conductor as the ground material unless the contact characteristics deteriorate. Protection of contact surfaces is therefore a compromise between the increase of the contact stability and the deterioration of the initial contact characteristics. Three different protective layers were deposited on the AgNi0.15 contact material. After their treatment in the radio frequency plasma they were analyzed with the Auger spectrometer. AES karakterizacija nanosov tankih zaščitnih plasti na AgNi0.15 kontaktnem materialu po obdelavi v RF plazmi Ključne besede: kontakti električni, Ag materiali kontaktov srebrni, pasivacija, plasti tanke pasivacijske, RF plazma radiofrekvenčna, AES analize s spektroskopijo elektronsko po AUGER metodi, izvedbe cenene Povzetek: Znana površinska zaščita Ag kontaktov s tankim nanosom zlata je cenovno neugodna. Študirali smo nekatere cenejše pasivacijske plasti, ki jih lahko nanesemo dovolj tanko, da preprečimo korozijski učinek okolice in hkrati ohranimo dovolj veliko električno prevodnost stika med pasiviranima površinama, ki ustreza dobremu električnemu kontaktu. Zaščitna plast na kontaktni površini poslabša kontaktne lastnosti, če ni iz električno bolj ali enako prevodnega materiala kot osnovni kontaktni material. Zato pomeni uvedba pasivacije kontaktnih površin kompromis med povečanjem stabilnosti kontakta in verjetnim poslabšanjem začetnih kontaktnih lastnosti. Tri različne pasivacijske tanke plasti smo nanesli na AgNi0.15 kontaktni material. Nanose tankih plasti po obdelavi v radiofrekvenčni plazmi (RF) smo analizirali s spektrometrom Augerjevih elektronov. 1 Introduction The requirements for the stability of contact characteristics /1,2,3/ of switching devices are fulfilled if the parameters of the electric contact are within the limits that ensure the contact to work properly. The conductivity of the contact spot between the two surfaces should be sufficiently constant and the contact material should be conducting and chosen according to the known contact force. The contact material should not be affected by the surroundings that produce the corrosion layers and cause the deposition of foreign particles and layers. For testing contact material the AgNi0.15 alloy was chosen as a typical representative of the silver based material. Stability of AgNi0.15 contacts is affected primarily by the formation (and its growing with time) of AgaS surface layer in the industrial atmosphere. 2 Experimental Test samples were made of the contact material AgNi0.15 in the form of the miniprofile-strip with the cross section of 2.6x0.3 cm^. 50 mm long parts were cut from the strip. Contact surface was half-cylindrical in shape with the curve radius of 9 mm. Three different types of protective layers deposited on the test contacts were studied: • Passivation by waxing. Test samples were treated in the water solution named Silverbrite /4/. • Passivation by chromizing. Layer was made by anode oxidation according to the receipt /5/. ® Passivation by the solid layer. Solid layer Ti-FTiN was formed by sputtering /6/. All the three types of protective layers were tested for the resistance to sulfating in the K2S solution. Samples were exposed to the wet atmosphere containing H2S (climate test). Afterwards samples were treated by the radio frequency (RF) low pressure plasma /7/: first cleaning of the samples in the hydrogen plasma (1 minute of exposition, 1 mbar),then surface oxidation in the reactive oxygen plasma (0.5 minute, 0.6 mbar). Both procedures were performed in the standard discharge vessel which was a cylindrical glass tube with the diameter of 3 cm. Plasma was created by an inductively coupled RF generator. Hopkins liquid nitrogen cooled trap was used. Temperature of the samples was not measured. After the treatment of all the three protective layers in plasma condition of the surface was examined /8/ with the Auger spectrometer (Physical Electronic Ind. SAM 545 A) with the static primary electron beam with the energy of 3 keV, the beam current of 0.5 /jA, and the beam diameter of about 40 ^m. Etching was performed on the surface area of 10 mm x 10 mm with the two Ar+ ion beams with the energy of 1 keV. The incidence angle of the ion beam was 47°. The etching velocity was about 1.7 nm/min and was calibrated with the standard Ni/Cr multilayer sample. For determining the element concentration (except for the nitrogen concentration where the factor was calculated from the standard sample of stehiometric TiN) the sensitivity factors were taken from the spectroscope manufacturer (PEI) manual. Results of the analysis were presented in the profile diagrams (Fig. 1-6). 3 Results and discussion We found that the thin Silverbrite layer on the surface of the AgNiO.15 sample is resistant to the sulfading with K2S (according to the standard test) and also to the wet atmosphere. Protective substance is attached to the Ag base material only by adhesion therefore after the treatment in hydrogen plasma it can not be detected by the Auger spectrometer any more. Very clean surface with only the traces of oxygen can be seen in Fig. 1. After additional exposition of the sample to the oxygen plasma very uniform (black) silver oxide coating is obtained. Formation of silver oxide was confirmed by the Auger analysis. The concentration of oxygen at the surface of the layer is 13 at.% while on the other side the concentration falls to 6 at.%. Thickness of the layer is more than 25 nm and is very clean (Fig. 2). Passivation layer of chromized AgNi0.15 sample is resistant to K2S and to wet atmosphere containing H2S. Thickness of the coating layers of the samples exposed to hydrogen plasma was less than 1 nm. Coating contains Cr and small concentrations of N, P and Ca on the surface (Fig. 3). After the additional exposition to oxygen plasma traces of carbon could be found on the surface, while the inner part of the layer is oxidized with less than 10 at.% of oxygen (Fig. 4). Chromium is removed by the oxygen plasma. AES analysis of the AgNi0.15 with the sputtered thin TIN layer after the treatment with hydrogen plasma indicates that only the surface of the layer is oxidized (Fig. 5). There are carbon atoms inside the TIN layer. They were built-in during the deposition of TIN. When the same sample is treated also with the oxygen plasma the TiN layer surface was oxidized to the high degree (Fig. 6). It is evident that carbon was removed from the surface region. Estimated thickness of this sample's oxide layer is 5 nm. Nitride layer is so 10 11 12 13 14 15 c O C 8 C o o 100- 80- 60- 40 H 20-J ■ / / -.-Ag 1 —0—0 —X—N —A—Cr ♦ p Kfe^'O-r-r-« . , , 1 , , , 1 , , , 1 , 1 , , , , —