SUB-MICROMETER ORDER CORROSION OF SILVER BY SULFUR VAPOR IN AIR STUDIED BY MEANS OF QUARTZ CRYSTAL MICROBALANCE

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1 16th International Corrosion Congress September 19-24,, Beijing, China SUB-MICROMETER ORDER CORROSION OF SILVER BY SULFUR VAPOR IN AIR STUDIED BY MEANS OF QUARTZ CRYSTAL MICROBALANCE *Jun ichi SAKAI 1, Lisa AOKI 1, Keishin OHSAKA 1 Yuichi ISHIKAWA 2 1 Waseda University, Bldg 6-7, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169-8, JAPAN 2 Yokohama National University, 79-1 Tokiwadai, Hodogaya-ku, Yokohama 2-81, JAPAN Abstract The purpose of this study is to clarify the effect of temperature (T = -7 ), relative humidity (RH = -%) and distance between sulfur source and silver surface (l = 1-12 cm) on sub-micron order corrosion of silver during exposure to sulfur vapor. Quartz Crystal Microbalance (QCM) technique was employed to continuously and reproducibly detect extremely small mass changes during corrosion of silver by sulfur vapor. Corrosion progressed linearly with time during all the tests. No significant dependence on RH was observed. The corrosion rate increased as temperature rose. This corresponds to the temperature dependence of sulfur vapor pressure. The corrosion rate decreased markedly with increasing distance of up to 6 ~ 8 cm, and beyond that the corrosion rate reached a constant value. The results were in good agreement with the calculated profile of the amount of the arrived sulfur vapor from the solid sulfur source using Jost s cylindrical diffusion model. This suggests that the diffusion of sulfur vapor to the silver controls initial sulfide formation of silver. Keywords: Silver, Sulfur, Diffusion, QCM, Electronics 1. Introduction Corrosion has recently become a more significant factor in the reliability of electronic devices and components, because of design requirement of higher device density and faster signal processing and expanding application areas, thus resulting in smaller devices with ever finer feature sizes and exposure to more severe uncontrolled environment. Silver is extensively used for interconnection in electronic components because of its superior electrical conductivity. Although silver is one of noble metals and resistant to corrosion in water and oxygen, it loses its nobility in contact with sulfur compound in air and forms black sulfide layer. Even with a thin sulfide layer formation it may lead to a fatal damage to the electronic circuit. Although there have been several papers examining corrosion rates of silver in sulfur containing environments 1)-3). In the application to electronics the effect of relative humidity, temperature and configurations on corrosion must be addressed and clarified. The purpose of this study is to clarify the effect of temperature, relative humidity (RH) and distance between sulfur source and silver surface on sub-micron order corrosion of silver during exposure to sulfur vapor through continuous and reproducible monitoring by QCM technique. 2. Experimental We used circular AT-cut quartz crystals with 2 mm diameter from Hokuto Denko Co. Ltd., nominal frequency 6MHz. They were coated on both sides by vacuum deposition of chromium

2 (thickness approximately nm) and silver (thickness. m). On the side that was in contact with sulfur vapor during corrosion tests, the coated area was 1.33 cm 2. The crystals were attached to a holder that contains an oscillator circuit. The frequency was measured with a universal counter and a PC stored and handled the frequency data. From the observed frequency changes, we derived mass changes by use of Sauerbrey equation 4) 2 1/2 f = -C m /A= -(2f m) /A (µ ρ) (1) where f is the frequency change, C (Hzcm 2 /g), mass sensitivity coefficient that is independent of the coated material and given as C = 2f /( x ) 1/2, f, resonance frequency, elastic constant and density of quartz crystal, respectively, m, mass change, A, exposure area. A mass change of 24.6 ng/cm 2 leads to a frequency change of 2 Hz. Upon identification of the corrosion product formed as silver sulfide, the mass change was then converted to thickness change by assuming the theoretical density of silver sulfide, 7.4 g/cm 3. The apparatus for sulfur vapor exposure tests is a glass bell jar of three liters with a diameter of sixteen cm. The QCM holder was suspended from the top of the bell jar through a non-sulfur hardened rubber plug and its height from the sulfur source placed on the bottom of the bell jar can be adjusted from one cm to twelve cm. Sulfur vapors were evolved from a large excess of sulfur flowers spread across a glass dish. The vapor pressure of sulfur at is approximately 2.6 mpa, 2 mpa at and 2 mpa at 7. A saturated salt solution and distilled pure water, each contained in a ml beaker was placed at the bottom of the bell jar for RH control. The salts used were lithium bromide and sodium bromide. The RH over the lithium bromide used at - 7 was approximately %, while the RHs over the sodium bromide were %, % and 6%, respectively when used at, and 7. Temperature of the exposure tests was controlled by placing the bell jar into a laboratory oven. The temperature of the oven was ±3, ±3 and 7±3. The temperature and RH inside the bell jar were also measured by a digital thermometer and hygrometer. The corrosion films were analyzed for the morphology and elements present on a Hitachi Scanning Electron Microscope with an energy disperse X-ray spectrometer. The corrosion film thickness was checked by a Nihon-Denshi Auger Electron Spectrometer as well as by the cathodic reduction technique. The identification of the corrosion products was made by a Rigaku Denki X-ray diffractometer RINT RAPID. Corrosion products were removed from the QCM in a form of powder. X-ray diffraction was performed using a copper target at KV and ma. 3. Results Because of the large heat capacity of the glass bell jar containing the exposure setup, the stability of the temperature was required for a considerable time when the bell jar was placed into the constant temperature oven from the ambient. This caused the instability of the resonance frequency of the QCM. After stabilization of temperature being time taken depending on the test temperature, that is longer at higher temperature, approximately a few hours at 7, the resonance frequency of QCM decreased linearly with time. Examples of the corrosion film thickness increase with time calculated from equation (1) and the density of Ag 2 S as 7.4 [g/cm 3 ] are given in Fig. 1 for exposure tests at with the distance between the specimen and sulfur vapor of seven cm at different RHs. All the corrosion products in the exposure tests were identified by XRD as silver sulfide (monoclinic, d=2.6).

3 Here the time zero is taken as the time when the temperature inside the bell jar became stabilized upon introduction into the oven. As can be seen in Fig. 1, corrosion progresses linearly with time during the time span exposed. Relative humidity does not appear to have a significant effect on the corrosion. At other test temperatures a similar effect of RH was observed. In contrast, temperature shows a drastic effect on corrosion progress. As seen in Fig. 2, at 7 corrosion progresses also linearly with time, but very rapidly and within eight hours the QCM failed to operate because of a nearly complete loss of silver film, while at corrosion progresses slowly but still linearly with time. This agrees with the previous observation of corrosion of silver in sulfide forming environments 1)-3) and in the field exposure ). From this linear relationship between corrosion and time, we calculated the average corrosion rate of silver at different temperatures and RHs. Figure 3 summarizes the result. The drastic effect of temperature on corrosion can be explained by the fact that number of sulfur molecules available in the exposure environment is determined by the vapor pressure of sulfur flower. This indicates the significance of availability of sulfur molecules for corrosion. The effect of distance from the sulfur source to the silver specimen on corrosion is shown in Fig. 4. The corrosion rate exponentially decreases with increasing distance up to eight cm and beyond that the corrosion rate stays almost the same by a further increase in distance. 4. Discussion As pointed out above, the present result of linear corrosion film growth shows a good agreement with the previous investigations on corrosion of silver in sulfur containing environment. In addition the corrosion rate itself shows a remarkably good agreement with the previous measurements using sulfur flower as a sulfur source 1), 3), even though both the exposure setups and methods of corrosion rate measurement (cathodic reduction after long time exposure tests of hours) were completely different. It can be concluded that the QCM provides a quick and reproducible corrosion rate measurement. The effect of RHs on corrosion rate was not evident as similar to the previous study 3). The independence of the corrosion rate on relative humidity indicates that free sulfur directly reacts with silver and moisture does not play a major role as in most cases of corrosion of electronic materials. On the other hand, the effect of temperature and distance between the specimen and the sulfur source was very remarkable as seen from Figs. 3 and 4. These and the observed linearity of the corrosion rate at the conditions of the present study suggest that the process of corrosion is controlled by the arrival of sulfur molecule at the silver surface, similar to the previous study 3). The rate of arrival of sulfur molecules to the silver surface is governed by diffusion of sulfur molecule in air. In a steady state of diffusion of vapor in the cylinder the differential equation was solved by Jost 6). The number of sulfur arrived at the silver surface S [molecules] is given as S = D q t Ce / l ( 2 ) where t:exposure time [s] D:the diffusion coefficient for sulfur molecule [m 2 /s] q: the cross section of the cylinder [m 2 ] Ce:the equilibrium concentration of sulfur vapor just above the sulfur flower determined by the vapor pressure at the temperature [molecules / m 3 ]. The diffusion coefficient of sulfur molecule in air can be estimated from the kinetic theory of gases. Upon assumption of every molecules of sulfur reached the silver being instantly reacted with silver, the number of sulfur atoms arrived at the silver surface per unit time (S/t) would yield the corrosion rate. The calculated profile of the corrosion rate ( ) agrees well with the measured corrosion rate ( )as shown in Fig. 4. This suggests that the diffusion of sulfur vapor to

4 the silver controls initial sulfide formation of silver.. Conclusion The present study demonstrates that extremely small mass changes during corrosion of silver by sulfur vapor can be followed continuously and reproducibly with Quartz Crystal Microbalance (QCM) technique. From the linearity in the corrosion rate and marked dependence of the corrosion rate on both temperature and distance, the arrival of sulfur molecules at the silver surface by diffusion is concluded to determine the corrosion rate. Acknowledgements The work forms a part of a project, which is supported by Suga Weathering Technology Foundation. References [1] W. H. Abbott, IEEE Trans. Parts, Hybrids, Packag., 1974, PHP-, 24. [2] W. H. Abbott, IEEE Trans. Parts, Hybrids, Packag., 1969, PHP-, 16. [3] B. T. Reagor, J. D. Sinclair, J. Electrochem. Soc., 1981, 128, 71. [4] G. Sauerbrey, Z. Phys., 199, 1, 6. [] R. Schubert, B. Meagler, C. C. Chang, J. Electrochem. Soc., 199, 142, 317. [6] W. Jost, Diffusion in Solids, Liquids, Gases, Academic Press, New York, 197, 9.

5 Silver Sulfide Film Thickness [nm] %RH %RH %RH 1 Exposure Time [hr] Silver Sulfide Thickness [nm] (%RH) (%RH) (6%RH) Exposure Time [hr] Fig. 1 Examples of Corrosion film (silver sulfide) growth behavior as a function of relative humidity (T=, Distance between sulfur source and silver specimen l= 7 cm) Fig. 2 Examples of Corrosion film (silver sulfide) growth behavior as a function of temperature (RH= -6%, Distance between sulfur source and silver specimen l= 7 cm) Corrosion Rate [nm/hr] Rerative Hum idity Humidity [%] [%] 7 S1 Tem Temperature [ ] [ ] Corrosion Rate [nm/hr] 2 1 計算値 : caluculated 系列 : measured 2 1 Distance [cm] Fig. 3 Corrosion rates of silver as a function of temperature and relative humidity (Distance between sulfur source and silver specimen l= 7 cm) Fig. 4 Relationship between the measured corrosion rate (shown as ) and the distance between sulfur source and silver specimen (T=, RH= %)