CLEANING OF THIN PASSIVATION LAYERS ON THE Ag CONTACT MATERIAL WITH VACUUM OUTGASSING L Kollerl, m. Bizjak2, B. Pračeks 11nstitute for Electronics and Vacuum Techniques, Ljubljana, Slovenia 2|skra - Stikala, Kranj, Slovenia 3ITP0, Ljubljana, Slovenia Keywords: electrical contacts, material for electrical contacts, surface treating, surface protection, passivation tfiin films, vacuum outgassing Abstract; Metallic silver is relatively soft material and a good electric conductor. Therefore it is very suitable material for electric contacts. However, because silver is not resistant to the atmosphere containing H2S, SO2 and CI2, the surface of the silver contact material must be protected. Well known surface protection with the thin gold layer is too expensive to be economical. In our study we considered some cheaper passivation thin layers which could prevent the corrosive effects of the environment and at the same time assure good electric contact. We chose three different types of passivation thin layers. The first one was deposited by waxing in the Silverbrite solution, for the second layer chromizing was used while the titanium nitride layer was deposited by sputtering. The contact material used was AgNiO,15. Beside electrical properties of the material its clean surface is very important to achieve good electric contact and low level of contact resistance. In the high vacuum outgassing equipment designed and developed in our laboratory samples of silver contact material were outgassed for 48 hours in high vacuum (1x10"^ mbar) at 200"C. Analysis of the outgassed substances with the quadrupole mass spectrometer showed that the outgassing rates of all the three passivation layers were low while the composition depended on the type of the layer. Then the thin layers were analyzed with the Auger electron spectroscopy, v Čiščenje tankih pasivacijskih prevlek na Ag kontaktnem materialu z vakuumskim razplinjevanjem Ključne besede: kontakti električni, materiali kontaktov električnih, obdelava površinska, zaščita površin, plasti tanke pasivacijske, razplinjevanje vakuumsko Izvleček: Srebro je dobra električno prevodna in sorazmerno mehka kovina. Zaradi tega je po elektromehanskih lastnostih primerna za električne kontakte. Ker pa je neobstojno v atmosferi s primesmi H2S, SO2 in CI2, je treba kontaktni material površinsko zaščititi. Izbrali smo tri pasivacijske zaščitne prevleke (prva je bila nanesena z voskanjem v raztopini SILVERBRITE, druga s kromatiranjem, tretja, titannitridna, pa je bila napršena) in jih nanesli na kontaktni material AgNiO,15. Za dober električni kontakt in nizko kontaktno upornost je poleg električnih lastnosti zelo pomembna tudi čista površina kontaktnega materiala. V laboratorijski razplinjevalni napravi, razviti doma, smo vzorce pasiviranega srebrnega kontaktnega materiala razplinjevali v visokem vakuumu 1 x10"'mbar pri temperaturi 200°C. Analiza razplinjenih substanc je pokazala, daje velikost razplinjevanja vseh treh zaščitnih prevlek majhna, koncentracija plinskih nečistoč pa je odvisna od zaščitne plasti same. Nanose tankih zaščitnih prevlek po visokovakuumskem razplinjevanju smo analizirali še s spektrometrom Augerjevih elektronov. 1 Introduction Recently the properties of the outgassed materials /1 -3/ hermetically encapsulated into the electronic components attracted considerable attention. One of the main reasons for the failures of the electronic components is the surface contamination /4-7/ of the electric contacts. Contamination film increases the contact resistance and deteriorates the reliability of an electronic component. The most common contamination films are the oxides formed during the terma! diffusion and outgassing processes. Besides oxide films the corrosion products and particles resulting from wearing are the main sources of contamination. The growth of industrialization together with the air pollution encounter the problem of the contact sulfating. The specific resistance of AgsS ranges from 10^ Qcm to 10® ncm at the room temperature (silver sulfide decomposes at about 300°C) so the sulfating of contacts increases the contact resistance. Contacts may be protected by thin gold plating but there are cheaper passivation coatings, too. We studied the outgassing properties of three possible useful protective thin layers on the contact surface of silver (AgNi0.15) contact material in order to decrease the influence of the surrounding atmosphere. All three layers were outgassed in high vacuum at the increased temperature. The mass spectrometer was used to de- termine the composition of the outgassed products. Purity and the composition of the protective layers were determined by the Auger spectroscopy. 2 Experimental To protect the silver based contact material AgNi0.15 three different types of thin layers were used: • Passivation by waxing. Test samples were treated in the water solution named Silverbrite /8/. • Passivation by chromizing. Layers were made by anode oxidation according to the receipt /9/. • Passivation by solid layer. Solid layer Ti/TiN was formed by sputtering /10/. All the passivation thin layers were outgassed in high vacuum 1 x10"^ mbar atthe temperature 200°C48 hours in the vacuum outgassing comora/11/. Products of the outgassing procedure were analysed with the quadrupole mass spectrometer (masses from 1 to 100) TRAN-SPECTOR GAS ANALYSIS SYSTEM - MODEL C100, F LEYBOLD Inficon Inc. Mass spectra were taken twice: after half an hour and after 48 hours of outgassing. Purity and composition of three surface layers after the outgassing were determined by the Auger spectros- copy (AES). AES spectrometer/12/ (PEI, SAM, Model 545A) with the static primary electron beam with the energy of 3 keV, the beam current of 0.5 ^^A, and the beam diameter of about 40 ^m was used for the analysis. 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 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. 3 Results and discussion The composition of gasses evaporated out of the heated surface of samples made of the passivated contact material AgNiO.15 is presented in mass spectra in Figures 1 to 6. Mass spectra of the outgassed products from the silver contact material AgNiO.15 coated with the thin passivation layer in the Silverbrite solution are presented in Fig.1 after 30 minutes and in Fig. 2 after 48 hours of outgassing, respectively. Spectrum in Fig. 2 shows that after 48 hours of outgassing at 200°C and at the total pressure 2x10'^ mbar nearly all the eva-porable contaminants and hydrogen Hs are removed. The same conclusion can be drawn for the outgassing of cromated contact material (Figures 3 and 4). When the contact material AgNi0.15 was passivated by the solid layer Ti/TiN (mass spectra in Figures 5 and 6) greater amount of contaminants than in the first two cases is noticed on the surface before the beginning of the outgassing. After the 48 hours of outgassing at 200°C the contaminants are not removed completely. Atthe same time the strong hydrogen peak is observed. During the procedure of sputtering considerable amount of molecules are built in the protective layer. Also, when the procedure of sputtering is finished the I II I II I I II 21 28 m/8 Figure 3. Mass spectrum of the outgassed products from the cromated silver contact material AgNiO. 15 after 30 minutes of ougassing (4x10^^ mbar, 25°C). 22 12 HJ' . I 21 27^^29 39 38 " m/s 51 Figure I.Mass spectrum of the outgassed products from the silver contact material AgNiO. 15 waxed in the Silverbrite solution after 30 minutes of ougassing (2x10^'^ mbar, 25°C). H 16 12 18 -i_ I I I 28 _I 31 il m/« Figure 4. Mass spectrum of the outgassed products from the cromated silver contact material AgNiO. 15 after 48 hours of ougassing (1.8x10-^ mbar, 200°C). e 3 « 2 1 15 " It 16 18 27 1.1 51 m/t 18 27 28 38 42 -L-lXl-H tl 21 Figure 2. Mass spectrum of the outgassed products from the silver contact material AgNiO. 15 waxed in the Silverbrite solution after 48 hours of ougassing (2x10-^ mbar, 200°C). Figure 5. Mass spectrum of the outgassed products from the sputtered silver contact material AgNiO. 15 with the titanium nitride Ti/TIN after 30 minutes of ougassing (8x10'^ mbar, 25°C). Jfil Figure 6. Mass spectrum of the outgassed products from tfie sputtered silver contact matehai AgNiO. 15 witti the titanium nitride Ti/TiN after 48 hours of ougassing (4x10''^ mbar, 200°C). c s £= O ü 100- 80- 60- 40- 20- 4 5 6 7 8 9 10 11 12 13 14 15 Sputter Time (min) Figure 9. Profile diagram of the sputtered silver contact material AgNiO.15 with the titanium nitride Ti/TiN after the outgassing. c o c d o o 100- 80- 60- 40- 20- 11 12 13 14 15 Sputter Time (min) Figure 7. Profile diagram of the silver contact material AgNiO. 15 v\/axed in the Silverbrite solution after the outgassing. 100- c 0 1 c s C o ü 3 4 5 6 7 8 9 10 11 12 13 14 15 Sputter Time (min) Figure 8. Profile diagram of the cromated silver contact material AgNiO.15 after the outgassing. the waxed silver contact material AgNiO. 15 (Fig, 7) after the outgassirig shows the extremely clean surface with just the traces of oxygen. Profile diagram of the cromated silver contact material AgNiO.15 (Fig. 8) afterthe outgassing shows the passivation layer which is less than 1 nm thick and consists mainly of chromium with small concentrations of nitrogen, phosphorus and calcium on the surface. The surface is oxidized to a great deal while there is no oxygen inside the layer. Profile diagram of the silver contact material AgNiO.15 passi-vated by the Ti/TiN solid layer (Fig. 9) after the outgassing shows that titan nitride layer is slightly oxidized only at the surface. Inside the layer the carbon incorporated during the formation of the layer is detected. 4 Conclusions Outgassing properties of three conducting passivation thin layers on the silver contact material AgNiO.15 are studied. First passivation layer was deposited by waxing in the Silverbrite solution, the next one by chromiz-ing using the electrochemical procedure, while thethird one (Ti/TiN) was deposited by sputtering. Outgassing was performed in high vacuum (1x10^^ mbar, 200°C, 48 hours), the mass spectrometer was used to determine the composition of the outgassed products. Thin passivated protective layers after the outgassing was studied with the use of the Auger spectroscopy. Our studies show that all the three passivation thin layers considered are suitable for protection of silver contact material. The layers produced are very clean with low outgassing rate and are not expensive. protective layer is rather active so that larger amount of gas can be absorbed. Contamination of the layer depends very much on the technology of the procedure, Results of the AES analysis as three profile diagrams are presented in Figures 7, 8 and 9, Profile diagrams of 5 References /1/ M Wutz, H Adam, W Walcher, in Theory and Practice of Vacuum Technology, Friedr. Vieweg & Sohn, Braunschweig/Wiesbaden, 1989. /2/ L Koller, M Jenko, S Spruk, B Praček, S Vrhovec. Vacuum. 1995, 46, 827. /3/ L Koller, S Vrhovec, M Jenko, Kovine, zlitine, tehnologije, 1995, 29, 515. /4/ R Holm, in Electric Contacts Handbook, Springer, New York, 1967. /5/ M Bizjak, L Koller. K Požun, J Leskovšek, ICEC'98 Offenbach, VDE-Veriag 1998, 47. /6/ SKChawla, JK Payer, J. Electrochem. 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