ELECTROSTATICALLY INDUCED PRECIPITATION OF SILVER IN SILVER-IMPREGNATED GLASS ANODICALLY-BONDED TO SILICON

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1 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 26 ELECTROSTATICALLY INDUCED PRECIPITATION OF SILVER IN SILVER-IMPREGNATED GLASS ANODICALLY-BONDED TO SILICON Takahashi M., Ikeuchi K. Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka , Japan ELEKTROSTATICKY INDUKOVANÁ PRECIPITÁCIA STRIEBRA V STRIEBROM IMPREGNOVANOM SKLE ANODICKY SPÁJANOM S KREMÍKOM Takahashi M., Ikeuchi K. Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka , Japonsko Abstrakt Striebro je zaujímavé pre sklárskych odborníkov pre jeho vysokú pohyblivos ť v skle ktorá je porovnateľ ná s pohyblivos ť ou sodíka. V tejto štúdii sa vyrobilo striebrom impregnované sklo z bórsilikátového skla. Prísada striebra bola navrhnutá na prevenciu tvorby defektov v spoji, zapríč inených akumuláciou sodíka na rozhraniach spoja v anodicky spájaných spojoch s aplikáciou napätia v smere opač nom ako je spájanie. Striebro v dôsledku jeho vysokej pohyblivosti, slúžilo na nahradenie sodíka v blízkosti styč nej plochy. Schopnos ť spájania striebrom impregnovaného skla s kremíkom anodickým spájaním bolo ekvivalentné s bežným bórsilikátovým sklom. Aplikáciou opač ného napätia sa rozhranie spoja striebrom impregnovaného skla zafarbilo na sivo a nevznikali žiadne defekty spoja. V týchto spojoch sa zabránilo akumulácii sodíka na jeho rozhraní a namiesto toho sa akumulovalo striebro. Nahromadenie striebra nezapríč inilo defekty spoja, ale skôr vytvorilo precipitáty mikrónovej veľ kosti, korunkovitého tvaru, v skle susediacom s rozhraním spoja. Tieto precipitáty zapríč inili zmenu vo farbe rozhrania spoja. Abstract Silver is of interest to glass scientists due to its high mobility in glass which is comparable to sodium s mobility. In this study, silver-impregnated glass was produced from borosilicate glass. The silver addition was intended to prevent formation of joint defects caused by accumulation of sodium at the joint interfaces in anodically bonded joints with application of a voltage in the direction opposite to that for bonding. The silver, due to its high mobility, served to substitute for sodium near the faying surface. Joinability of the silver-impregnated glass to silicon by anodic bonding was equivalent to that of usual borosilicate glass. With application of the reverse voltage, the joint interface in the joint of silver impregnated glass colored gray, and no joint defects formed there. In those joints accumulation of sodium at the joint interface was inhibited, and silver accumulated there instead. Accumulating silver did not cause joint defects but rather formed micron-size precipitates of corollaceous configuration in the glass adjacent to the joint interface. Those precipitates caused a change in color of the joint interface. Key words: anodic bonding, silicon, glass, silver, ion substitution, prevention of joint defect

2 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 27 Introduction Anodic bonding is a method for bonding metal or semiconductor to glass containing alkali ions by applying a D.C. voltage of 1-1 V between them with the metal side anodic at a bonding temperature of 5-7 K. In this temperature range thermal diffusion of alkali ions in glass is activated, and with application of D.C. voltage, those ions drift toward the cathode side, and an alkali ion depletion layer forms in the glass near the joint surface. This layer has a strong negative charge because of the presence of non-bridging oxygen (O) anions that lose their bonds with the alkali ions. A Coulomb force acting between this charge and the charge appearing on the surface of the anode conductor brings the glass and the conductor into intimate contact, and a permanent bond is achieved by the reaction of oxygen from the glass with elements in the conductor at the interface [1]. The bonding temperature for anodic bonding is relatively low and deformation of glass does not occur in this temperature range [2]. And anodic bonding requires no intermediates such as solder or adhesive. These features make anodic bonding an indispensable method for precise joining in assembly of micromachines or microsensors. The application of a voltage to an anodically bonded joint in the opposite direction to that for bonding (the conductor side is cathode and the glass side is anode) causes degradations of the joint - disjunction of the bond interface, cracks in the glass, discoloration of the bond interface, and so on [3,4,5,6]. These phenomena have harmful effects on the reliability of anodically bonded joints in case the joints receive the reverse voltages in the course of manufacturing or during use. These phenomena were caused by accumulation of alkali ions at the joint interface. Alkali ions drift toward the joint interface and accumulate there with application of the reverse voltage, and cause degradation of glass adjacent to the joint interface [5]. Hence, if accumulation of alkali ions at the joint interface is prevented, formation of joint defect will be prevented at the same time. But anodic bonding requires existence of mobile cations in the glass, and usually alkali ions play this role as described above. In this study, glass whose alkali ion content was substituted by Ag + ions that had high mobility in the glass was prepared, and its joinability to silicon by anodic bonding was examined. Also, changes in the joint interfaces in anodically-bonded joints of those glasses caused by application of the reverse voltage were investigated. Experimental Experimental materials were borosilicate glass (Corning 774), silicon wafers (n type, 1 crystal orientation, low resistivity), and silver (99.9 wt.%). Chemical composition of the glass is shown in Table 1. The glass was provided as disks of 25 mm in diameter and 1. mm in thickness, and silicon wafers were 1 mm in diameter and.5 mm in thickness. Faying surfaces of the glass and silicon wafers were mirror finished. Sodium in the glass near the faying surface was substituted by silver in the following way. The faying surface of the glass disk was coated with a silver layer by vacuum deposition. This glass disk was heated to 563 K in a vacuum, and a positive voltage of 2 V was applied to the silver layer for 6 s while the surface of the glass without the silver layer was grounded. The applied voltage induced an electric field in the glass, and Na + ions in the glass migrated toward the surface without the silver layer. This process is very similar to that of usual anodic bonding. The migration of the Na + ions caused depletion of those ions in a region in the glass adjacent to the glass/silver layer interface. The Ag + ions from the silver layer are attracted by the

3 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 28 electric field in the glass, and penetrate into the glass replacing migrating Na + ions. In this way, a Ag + impregnated layer was formed at the faying surface of the glass. The electric current that passed through the glass transferred a charge of ~18 C/m 2 during application of the voltage. From this value, thickness of the formed Ag + impregnated layer can be estimated. Assuming that Ag + ions replaced all the Na + ions that had been originally contained in the Ag + impregnated layer, an expected thickness of the layer formed with the voltage application of 6 s was ~1 µm. After voltage application the glass disk was furnace cooled, and the remained Ag layer was wiped off. Glass that is impregnated with silver by the treatment described above is hereinafter referred to as 'Ag impregnated glass'. Silicon was anodically-bonded to the Ag impregnated glass. The surface of Ag impregnated glass other than the faying surface (the surface that is NOT impregnated with silver) was coated with conductive paint (Aquadag E: colloidal graphite) to uniform the electric potential on this surface. Silicon plate of 15 mm square was cut from the wafer. The faying surface of the silicon plate was placed on that of the Ag impregnated glass. The silicon plate was connected to the positive terminal of the high-voltage D.C. power source, and the surface of Ag impregnated glass coated with conductive paint was connected to the negative terminal. These joint materials were heated to a bonding temperature (T b ) of 563 K in a vacuum of ~1x1-3 Pa and a bonding voltage (V b ) of 5 V was applied to them for a bonding time (t b ) of 3 s after the temperature was stabilized. After application of bonding voltage the joint was furnace cooled. Joints of silicon to unprocessed borosilicate glass were prepared in a similar way for comparison. In this paper, joints of silicon to Ag impregnated glass are called 'Ag applied joints', and joints of silicon to unprocessed borosilicate glass are called 'unprocessed glass joints'. The reverse voltage was applied to joints obtained in a way similar to the bonding. The surface of the glass that was not joined with the silicon plate was coated with conductive paint and connected to the positive terminal of the high-voltage D.C. power source, and the silicon part of the joint was connected to the negative terminal. The joint was heated to a reverse voltage application temperature (T r ) of 563 K in a vacuum, and a reverse voltage (V r ) of 5 V was applied to the joint. Three different reverse voltage application times (t r ) were adopted, 15 s, 3 s, and 6 s. Effects of the reverse voltage on the joint interfaces were examined by appearance inspection, optical microscopy, and transmission electron microscopy (TEM). In appearance inspection and optical microscopy, the joint interfaces were observed through the glass. Thin foil specimens for TEM were prepared with a HITACHI FB-2A focused ion beam (FIB) system equipped with a micro-sampling system, from areas that cut across the joint interfaces. The TEM observation was performed with JEOL JEM-21 transmission electron microscope equipped with energy dispersion spectroscopy (EDS) apparatus. The acceleration voltage that adopted in observation was 16 kv. Table 1 Chemical composition of the borosilicate glass Results The electric current that passed through the joint during the voltage application for anodic bonding (bonding current) was monitored. In Fig. 1(a) are shown the bonding current in anodic bonding of an Ag applied joint and the charge that was transferred by the current as

4 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 29 functions of the application time of the bonding voltage. The profile of the bonding current for the Ag applied joint was very similar to that for the unprocessed glass joint (shown in Fig. 1(b)). Appearances of bond interfaces in an unprocessed glass joint and that in an Ag applied joint are shown in Fig. 2(a) and Fig. 2(b), respectively. These joints were bonded in the same condition (V b = 5 V, t b = 3 s, and T b = 563 K). In both joints, cohesion was achieved in most area of their joint interfaces. (Small unjoined regions were remained around inclusions at the interfaces. Anodic bonding is a solid-state bonding, and it is sensitive to anything to prevent faying surfaces from contact, for example, inclusions or irregularity of the faying surfaces.) These results reveal that anodic bonding of the Ag impregnated glass to silicon is feasible as well as that of the unprocessed borosilicate glass. Current density, i r / (C/m 2 ) Current density, i r / (C/m 2 ) (a) Ag applied glass Charge density Current density Voltage application time, t b / s (b) Unprocessed glass Charge density Current density Voltage application time, t b / s Fig.1 The electric current passing through the glass and the charge transferred by the current during glass/silicon anodic bonding Charge density, q r / (C/m 2 ) Charge density, q r / (C/m 2 ) Fig.2 Appearance of an unprocessed glass joint (a) and Ag applied joint (b) The reverse voltage was applied to Ag applied joints and an unprocessed glass joint. The current that passed through the joints and the charge transferred by the current during application of the reverse voltage as functions of the voltage application time are shown in Fig. 3

5 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 3 (a) and Fig. 3(b) for the Ag applied joint and unprocessed glass joint, respectively. Total application time of the reverse voltage was 6 s for the Ag applied joint and 3 s for unprocessed glass joint. The difference between profiles of the current that passed through the Ag applied joint and the unprocessed glass joint during application of the reverse voltage was small. But differences were evident between the appearances of joint interfaces in the Ag applied joints and the unprocessed glass joint after application of the reverse voltage, as shown in Fig. 4. In the unprocessed glass joint, there appeared many yellow-brown spots at its joint interface (Fig. 4(a)). In the Ag applied joint receiving the reverse voltage for 15 s, no similar spot was found, but the color of its joint interface became gray (Fig. 4(b)). When the application time of the reverse voltage (t r ) was extended to 3 s, the color of the joint interface became darker (Fig. 4(b)), but with extension of t r from 3 s to 6 s, a change in the color of the joint interface was not apparent (Fig. 4(c)). Yellow-brown spots were not observed in Ag applied joints even after application of the reverse voltage for 6 s. In Fig. 5 are shown the optical micrographs of the joint interfaces in the unprocessed glass joint (Fig. 5(a)) and the Ag applied joint (Fig. 5(b) and (c), the latter is the high magnification image) receiving the reverse voltage for 3 s. In the unprocessed glass joint, areas colored yellow-brown appeared at the joint interface, and from some of those areas clacks started into the glass. These colored area and clacks were observed as yellow-brown spots by appearance inspection. They are typical joint defects that appear in anodically bonded joints receiving the reverse voltage, caused by accumulation of sodium at the joint interface. These joint defects were not found at the joint interface in the Ag applied joint receiving the reverse voltage. Instead, dense black dots were formed there. As seen in Fig. 5(c), the size of the black dots was some micrometers. Formation of these black dots caused the change in color of the joint interface. Current density, i r / (C/m 2 ) Current density, i r / (C/m 2 ) (a) Ag applied glass Charge density Current density Voltage application time, t b / s (b) Unprocessed glass Charge density Current density Voltage application time, t b / s Fig.3 The electric current passing through the glass and the charge transferred by the current during application of a reverse voltage to glass/silicon joints Charge density, q r / (C/m 2 ) Charge density, q r / (C/m 2 )

6 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 31 Fig.4 Appearances of unprocessed glass joint receiving the reverse voltage for 3 s (a) and Ag applied joints receiving the reverse voltage for 15 s (b), 3 s (c), and 6 s (d) Fig.5 Optical micrographs of the joint interfaces in the unprocessed glass joint (a) and the Ag applied joint (b) (c) The Ag applied joint receiving the reverse voltage was observed by TEM in order to identify the black dots. Thin foil specimen for TEM was cut from the area across the joint interface in a direction perpendicular to the interface. In Fig. 6 is shown the microstructure around the joint interface in the Ag applied joint receiving the reverse voltage for 6 s. There was a Ag + ion depletion layer ~5 nm thick in the glass adjacent to the joint interface. This layer was thought to be formed in anodic bonding with drift of Ag + ions toward the cathode. From the boundary between the Ag + ion depletion layer and the rest of the glass, a corollaceous product grew toward the inner part of the glass. The size of the product corresponded to that of the black dots observed at the joint interface in the Ag applied joint by optical microscopy. This product was identified by selected area electron diffraction (SAD) and EDS analyses. Results are shown in Fig. 7. In the bright-field image in Fig. 7(a), two positions are indicated in the glass by letters A and B. Positions A and B were off and on the corollaceous product, respectively. The SAD pattern in Fig. 7(b) was taken from the position A. No distinct diffraction spots or rings were observed in this pattern. This is a halo pattern corresponding to a non-crystalline structure which suggests that the specimen at the position A consisted of glass. The SAD pattern taken from the position B is shown in Fig. 7(c). This pattern is a superposition of a halo pattern like the pattern from the position A and distinct diffraction rings. These diffraction rings were indexed as reflections from silver. And in the dark-field image taken by the reflection indicated by the white circle in Fig. 7(c), bright spots appeared on the corollaceous product. According to the EDS analyses, concentration of Ag at position B was much higher than that at position A (Fig. 7, inset table). These results revealed that, the corollaceous product was a precipitate composed of nanocrystals of silver, and this precipitate was embedded in the glass. When the reverse voltage was applied to the Ag applied joint, Ag + ions in the Ag + impregnated layer drifted toward the joint interface and accumulated there. It is thought that, those ions formed the corollaceous Ag precipitate, but the precipitate appeared to start from the boundary between Ag + ion depletion layer and the rest of the glass, not from the joint interface. The distribution of elements in the

7 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 32 Ag + ion depletion layer in the Ag applied joint receiving the reverse voltage for 6 s was observed by EDS. The results are shown in Fig. 8. No specific object was found at the joint interface. Results from EDS point analyses at the positions A-D that are indicated in the micrograph are shown in the table inset in Fig. 8. The glass area in the micrograph was contained within the Ag + ion depletion layer (as shown in Fig. 6, the depletion layer started from the joint interface and its thickness was ~5 nm), therefore, concentration of Ag at the positions A-C was very low. At the joint interface (position D) a slight segregation of Ag was observed. Most of the Ag + ions that drifted with application of the reverse voltage did not reach the joint interface, but they formed the corollaceous precipitates in the Ag + impregnated layer. The concentration of sodium was very low in this area. In the Ag applied joint, segregation of sodium with application of reverse voltage was effectively inhibited. Fig.6 Microstructure around the joint interface in the Ag applied point Fig.7 Ag precipitate appeared in the Ag applied joint. Bright-field image (a), SAD patterns taken from the positions indicated by letters A (b) and B (c) in a, and dark-field image taken by the reflection indicated by a circle in c (d). Results from EDS analyses at positions indicated by letters A and B in a are shown in inset table in at%

8 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 33 Fig.8 Microstructure around the joint interface in the Ag applied joint. Results from EDS analyses at positions indicated by letters A-D in the bright-field image are shown in inset table in at% Discussion The formation of joint defects in anodically bonded joints with application of reverse voltage was effectively inhibited by substitution of sodium in glass by silver. In Ag applied joints, there were Ag + impregnated layers in the glass near the joint interface. Concentration of Na in this layer was very low When the reverse voltage was applied to the Ag applied joint, Na + ions in the glass drifted toward the joint interface, but they could not traverse the Ag + impregnated layer. The estimated thickness of the Ag + impregnated layer was ~1 µm. In the condition of application of the reverse voltage adopted in this study (applied voltage: 5 V, temperature: 563 K), no joint defect was formed in the Ag applied joint. And after application of the reverse voltage for 6 s, the current passing through the joint diminished and amount of the transferred charge came close to saturation because that the Na + depletion layer grew with application of the reverse voltage in the anodic side of the glass. It shows that a significant increase in Na + ion migration would not occur with application of the reverse voltage for a longer time. And as shown in Fig. 4, increase in application time of the reverse voltage from 3 Note: Apparent concentrations of Na in this layer were detected by EDS analysis as shown in the inset table in Fig. 7. But they might not be true values because quantification of Na in this analysis was affected by the characteristic X-rays of Cu and Ga. The specimens did not contain these elements originally, but their characteristic X-rays appeared in the EDS spectra because the mesh sheet on which the TEM specimen was mounted was made of copper, and the specimen was contaminated with gallium because the gallium ion beam was used in the FIB system that was used for preparation of the TEM specimen. In EDS analysis, quantification of Na was done with Na Kα line (1.41 kev), and the energies of Cu Lα line (.93 kev) and Ga Lα line (1.98 kev) are similar to it. [7] It was difficult to distinguish these lines with the EDS system used in this study because the energy resolution was.163 kev.

9 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) 34 s to 6 s made no obvious change in appearance of the joint interface in Ag applied joints. This result also suggested that the changes in the Ag applied joints were nearly saturated after application of the reverse voltage for 6 s. Hence the Ag + impregnated layer of 1 µm is thought to be enough to prevent formation of joint defects with application of the reverse voltage of 5 V at 563 K. When the voltage or the temperature is changed, it is possible that the amount of ion migration will change, and the necessary thickness of Ag + impregnated layer to prevent joint defects will also change. When the reverse voltage was applied to the Ag applied joint, Ag + ions in the Ag + impregnated layer that drifted toward the joint interface did not accumulate at the joint interface, but formed corollaceous precipitates at the boundary between the Ag + ion depletion layer and the rest of the glass. The mechanism of formation of corollaceous precipitates can be thought as follows. The Ag + depletion layer was poor in carriers of electric current and would act as a high resistivity layer when the reverse voltage was applied. When a local electron current path forms through this layer for some reason, Ag + ions drifting from the Ag + impregnated layer will concentrate there and form a corollaceous precipitate. Once corollaceous precipitates are formed and start growing, coming Ag + ions will be consumed with growth of those precipitates and will not reach the joint interface. The mechanism of formation of local electron current paths is not known, but one possibility is that, Ag + ions coming from the Ag + impregnated layer might penetrate into places of relatively low resistivity in the Ag + depletion layer, and formed bridges of silver through the Ag + depletion layer. In Fig. 9 is shown microstructure around the anodically-bonded Ag applied glass/silicon interface receiving the reverse voltage for 32 s. One of the corollaceous precipitate was accompanied with a root-like structure in the Ag + ion depletion layer. At the position of the root-like structure, a higher concentration of Ag than that in other places in the Ag + ion depletion layer was detected. This root-like structure might be the local electron current path in the Ag + ion depletion layer. Ag-impregnated glass bond interface Ag depletion layer Silicon 1 µm Fig.9 Cross-section of Ag applied glass/silicon joint interface receiving a reverse voltage of 5 V for 32 s. One of corollaceous silver precipitate was accompanied with a root-like structure in the Ag + ion depletion layer (indicated by an arrow) Conclusions In order to prevent the formation of joint defects in anodically bonded joints with application of the reverse voltage, glass whose sodium content was substituted by silver (Ag impregnated glass) was produced from borosilicate glass and anodically bonded to silicon. Reverse voltage was applied to produced joints, and induced changes in the joint interfaces were observed. A summary of the results are listed below:

10 Acta Metallurgica Slovaca, 13, 27, 1 (26-35) Ag impregnated glass had joinability equivalent to usual borosilicate glass by anodic bonding. 2. No joint defect was formed in anodically bonded joints of Ag impregnated glass (Ag applied joints) with application of reverse voltage. Substitution of sodium by silver in the glass near the faying surface prevented accumulation of sodium during the application of reverse voltage. Application of reverse voltage caused formation of joint defects in unprocessed glass joints. 3. In Ag applied joints receiving reverse voltage, Ag + ions accumulated near the joint interface, and they formed silver precipitates that have corollaceous configuration. Acknowledgments This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 24. Literature [1] Wallis G., Pomerantz D. I., J. Appl. Phys., 4 (1969) [2] Hiller K., Hahn R., Kaufmann C., Kurth S., Kehr K., Gessner T.: Proc. SPIE Int. Soc. for Optical Engineering, 3878 (1999) 58. [3] Albaugh K. B., Materials Letters, 4 (1986) 465. [4] Morsy M. A., Ikeuchi K., Ushio M., Abe H.: Mat. Trans. JIM, 37 (1996) [5] Takahashi M., Nishikawa S., Ikeuchi K.: Proc. the 7th Int. Symp. JWS, (21) 851. [6] Visser M. M., Plaza J. A., Wang D. T., Hanneborg A. B., J. Micromechanics and Microengineering, 11 (21) N1. [7] Physics Dictionary ('Butsurigaku Jiten') 3rd edition, eds. Editing Committee for Physics Dictionary, Baifukan (1996), 239. (in Japanese)