J O U R N A L O F M A T E R I A L S S C I E N C E 3 7 (2 0 0 2 ) 169 – 175
Effect of metal oxide precursor on sintering
shrinkage, microstructure evolution and
electrical properties of silver-based pastes
SAN-YUAN CHEN, CHING-HUAN CHOU
Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu, Taiwan, Republic of China
E-mail: [email protected] SYH-YUH CHENG
Materials Research Laboratories, Industrial Technology Research Institution, Chutung, Taiwan, Republic of China
Since the shrinkage behavior of silver-based films has been correlated with the
characteristics of oxide additives used, the relative role of metal oxides and metal-organic precursors in sintering shrinkage and microstructure evolution of silver films was
investigated and compared in this work. Films with an oxide powder additive exhibit two-stage shrinkage behavior in contrast to one-stage continuous shrinkage, which occurs in silver films with metal-organic precursors, added. Furthermore, metal-organic
precursors are less effective than metal oxide powders in reducing shrinkage of
silver-based films. That can be reasonably explained that metal-organic precursors can be effectively decorated around silver powder to inhibit the sintering densification. The Zr-based organic precursor among the metal-organic precursors exhibits optimal retardation in sintering densification of silver film, which is probably interrelated to refractory characteristics of ZrO2. The unique conductivity and grain growth of silver film
with 1.0 wt% tungsten-organic precursor added was possibly due to the partial dissolution
of W into Ag. C 2002 Kluwer Academic Publishers
1. Introduction
Ceramic dielectric resonators are used as bandpass or bandstop filters and oscillator stabilizer devices in the microwave frequency range of 1 to 12 GHz. These de-vices are becoming increasingly important with the continued development of microwave integrated cir-cuitry [1–3]. In order to reduce the size of microwave devices in communication systems, the dielectric com-ponents must also be miniaturized. Recently, multi-layer devices have been developed to increase volume efficiency [4–6]. In multilayer structure, the sintering temperature of microwave dielectric ceramics was nec-essary to be lowered in order to cofire with low loss conductors such as silver and gold.
A principal problem in multilayer microwave ce-ramic manufacture originates from the firing of the elec-trode and dielectric films simultaneously. During firing of multilayer ceramics made with low-firing dielectrics, the electrode film begins to densify. This is due to the presence of large amounts of silver in the film. Silver has a melting point of about 961◦C in an inert atmo-sphere. Dielectrics, which reach complete densification at around 1100◦C typically, begin to sinter and shrink at about 800◦C to 1000◦C. The mismatch between the sin-tering shrinkage of the electrode and dielectric induces stresses [7–9]. It is known that the addition of metal
oxides or glass to the conductor composition can move the sintering shrinkage of the conductive metal powders to higher temperatures and thereby relieve the stresses formed due to differential electrode/dielectric shrink-age [10–12]. However, discontinuous grain growth and Ostwald ripening often take place due to inhomoge-neous paricle size distribution. A more effective means of alleviating the mismatch of the sintering shrinkage of the electrode with the dielectric is needed.
A new thick film conductor composition suitable for use for electrodes in multilayer capacitors has been de-veloped [13, 14]. The electrode pastes comprise finely divided particles of electrically conductive metal and an organometallic compound, which is insoluble in the electrically conductive metal in a solution of poly-meric binder and volatizable solvent. In some cases, metal-organic compounds of Ru, Rh, Ir or Pt were used to produce better shrinkage matches [15, 16]. How-ever, an onging challenge for future developments in high-performance multilayer microwave ceramics is to reduce the internal electrode cost. Therefore, the de-velopment of low-cost metal-based precursors such as Zn, Cu, W, Ni, Zr becomes more important. How-ever, little report was focused on the study of these low-cost metal-organic precursors on sintering shrink-age and microstructure evolution of silver-based films.
Therefore, in this paper, first, a variety of metal-organic precursors were selected to compare its role in shrink-age mismatch and microstructure evolution of silver-based pastes. For comparison, metal oxides were also used in the present work. The effect of metal-organic precursors on the conductivity of sintered silver-based films is also investigated in this paper. A possible mech-anism will be proposed to explain the shrinkage differ-ence of the silver films with metal-organic precursors and metal oxides added.
2. Experimental procedure
High-purity (more than 99.5%) TiO2and BaCO3were used to prepare Ba2Ti9O20ceramics by a conventional powder-processing technique. After the mixture pow-der was calcined at 1150◦C, oxide-based glass was added. The resultant powder was then used to fabricate the dielectric tape by doctor-blade processing.
Spherical silver powder with an average parti-cle size of 1.2 µm was used as the conductive powder in the electrode paste. The metal-organic pre-cursors (abbreviated as MO) used are tungsten isopropoxide (Alfa, designated as MO-W), zinc ethylhexanoate (Alfa, designated as MO-Zn), iron 2-ethylhexanoate (Alfa, designated as MO-Fe), and zir-conium 2-ethylhexanoate (Alfa, designated as MO-Zr). The metal oxide content of the metal-organic precursors used in the electrode paste constituted 0.5–2.0 wt% of total silver powder. Ethyl cellulose dissolved in a mix-ture of 6 wt% beta-terpineol and 74 wt% butyl carbitol acetate was used as an organic medium. In this exper-iment, the solid content of 75 wt% was used except otherwise noted. Metal oxides including Fe2O3, ZnO, ZrO2and WO3were also used for comparison.
The additives were first stirred into the pastes and then the mixture was roll-milled to ensure maximum dispersion of the additive. Most thick film composi-tions were applied to the dielectric green tape by means of screen-printing. Subsequently, the screen-printed silver-based pastes were sintered in air at various tem-peratures for 0.5 h. A two-layer structure with silver-based film in between dielectrics was also used to in-vestigate the cross-section microstructure. The thermal mechanical analysis (TMA) was further used to study the effect of the additives on the sintering shrinkage of the silver-based films. The silver paste was first cast on an alumina substrate and dried at 150◦C until all paste solvent was removed. Subsequently, the coated-paste substrate was covered with another alumina plate to construct a sandwich structure for the measurement of shrinkage as a function of temperature. Samples of the fired parts were cross-sectioned after mounting the parts in an epoxy potting compound and then polished to expose the inner structure. The microstructure of the sintered samples was observed using scanning elec-tron microscopy (SEM, Hitachi S4000). The conduc-tive properties of silver-based films at high frequencies were evaluated using conductors patterned on 0.025” thick, 99% alumina substrates with the T-patterns on one surface of the substrate and the return plane on the opposite surface [17]. The transmission response of the T-pattern resonator was measured using a Hewlett
Packard 8722D Microwave Network Analyzer Sys-tem. Measurements of the conductor width, thickness, length, and surface roughness were used to calculate the conductivity of conductors.
3. Results
3.1. Shrinkage of silver film and dielectric tape
Glass-added Ba2Ti9O20based compositions were used to fabricate dielectric tape for multilayer microwave. A porous structure composed of fine particle distribu-tion was observed (see Fig. 1), which indicates that little densification occurred for the sample sintered at 800◦C. On the other hand, for pure silver powder, the microstructure change of silver film on the dielectric tape was strongly influenced by the firing tempera-tures as shown in Fig. 2. The shapes of most of the particles become irregular at 400◦C (Fig. 2a), and the irregular-shaped particles grow rapidly into large ones at 600◦C (Fig. 2b) due to the coalescence of silver parti-cles [18]. With increasing soaking time, the silver films become dense and more grain growth takes place. The black regions in the photographs are voids and become fewer. After the silver film was sintered at 800◦C for 0.5 h, a fully dense microstructure, shown in Fig. 2c, exhibiting a regular array of steps was observed. Ali et al. [19] reported that as the silver films were deposited on Si(100) substrate and then annealed at 300◦C, a (100) oriented silver film was observed. With respect to silver symmetry, it was found that (100) was usually a lower energy plane in FCC structure. Therefore, the formation of layered step structure results mainly from the lower energy plane and the fast migration of silver atoms.
The shrinkage curve of the silver-based films, mea-sured by the TMA with a heating rate of 5◦C/min., was used to calculate the percent shrinkage relative to the unfired dried film thickness. Fig. 3 shows the pure sil-ver paste starts to shrink around 400◦C. Subsequently, the shrinkage becomes sluggish with sintering tempera-ture over 600◦C and ends around 800◦C. The shrinkage amount was about 18% and 23% at 600◦C and 800◦C, respectively. On the other hand, the starting tempera-ture of shrinkage for dielectric ceramic tape was about
Figure 1 SEM micrograph of dielectric ceramic sintered at 800◦C for 0.5 h.
Figure 2 SEM surface microstructure of silver films sintered at
(a) 400◦C, (b) 600◦C and (c) 800◦C for 0.5 h.
750–800◦C, much higher than that of silver paste. At 800◦C, the tape shrank about 3.9%, which were approx-imately 22% of the final total shrinkage.
3.2. Additive of metal oxide
A variety of metal oxide powders were used to study its effect on the sintering behavior of silver-based films. As shown in Fig. 3, some interesting features can be no-ticed. First, the TMA curves present two-stage shrink-age. Second, the addition of metal oxides does show effective retardation on the sintering densification of silver film. Third, the ZrO2additive presents the opti-mal retardation on the sintering shrinkage.
Figure 3 Thermal shrinkage characteristics of silver films with metal
oxide added at a heating rate of 5◦C/min.
Depending on the addition of metal oxides, the mi-crostructure development of silver-based films was widely varied as shown in Fig. 4. For comparison, both 600◦C and 800◦C firing temperatures were employed in this work. The microstructure observation shows that WO3-added silver film presents similar structural change to pure silver film. At 600◦C, particle-necking and partial densification occur. As the sintering temper-ature rises to 800◦C, the flat terraces with array of par-allel steps were observed (Fig. 4b). Similar microstruc-ture evolution was also observed for Fe2O3-added sil-ver film. On the other hand, the SEM micrographs in Fig. 4c show the ZnO particles were widely distributed throughout the silver grains as the film was fired at 600◦C. At a higher temperature of 800◦C, it was found that ZnO-added silver film presents a completely dif-ferent microstructure from that of pure silver film. The flat grains with parallel steps were not observed. As the ZrO2 powder was added to silver films, it was found that at 600◦C, Fig. 4d, necking formation took place in between Ag particles. With raising the temperature up to 800◦C, it exhibited a porous microstructure and in-homogeneous sintering densification occurred in some aggregated small grains.
3.3. Additive of metal-organic precursor
For comparison, the metal-organic precursors were added to silver powder to study its effect on the sin-tering shrinkage of silver films. As shown in Fig. 5, the shrinkage curves present a smooth and continuous change with increasing sintering temperature instead of two-stage curves as observed in the Fig. 3 for oxide additives. The shrinkage behavior of silver film with MO-W added was very similar to that of pure silver film. Furthermore, the addition of MO-W apparently promotes sintering instead of retardation in that it pro-duced more shrinkage than the sample with no metal-organic additives in the temperature range up to 800◦C. A larger final total shrinkage was produced (29%) as compared to that of pure silver film.
For silver film with MO-Zn added, it was observed that little shrinkage occurs before 600◦C. However, as the sintering temperature increases over 600◦C, the silver-based film shrinks rapidly with temperature. The final total shrinkage was about 23%. In the case of
Figure 4 Microstructure changes in silver films added with 1 wt% (a) WO3, (b) Fe2O3, (c) ZnO and (d) ZrO2by sintering at 600◦C (left) and 800◦C
(right) for 0.5 h.
MO-Zr, although the sintering shrinkage of the silver film increases with increasing temperature, no appar-ent shrinkage was observed before 600◦C. Furthermore, less total shrinkage occurs in the temperature range up to 800◦C, resulting in less sintering shrinkage mismatch with the dielectric. In addition, the shrinkage amount of the silver-based films was also dependent on the added amount of MO-Zr. As shown in Fig. 6, the thermal
shrinkage was performed for both dielectric tape and 1.5 wt% MO-Zr-added silver film from room tempera-ture heated to 950◦C with a ramp rate of 5◦C/min and then held for 2 hrs. The sintering mismatch was much reduced between the dielectric and MO-Zr added-silver film.
Fig. 7 shows the surface micrographs for both silver films with 1wt% MO-W and MO-Zr added,
Figure 5 Thermal shrinkage characteristics of silver films with
metal-organic precursors added at a heating rate of 5◦C/min.
Figure 6 Compatible thermal shrinkage of 1.5 wt% MO-Zr-added silver
film and dielectric tape.
respectively. The microstructure for the former silver-based film not only exhibits flat with array of parallel steps but also shows larger grain size (Fig. 7a). The grain size in MO-W-added silver film fired at 600◦C was about 12 µm, larger than that in pure silver film (∼7 µm). On the other hand, for the MO-Zr added sil-ver film, a porous microstructure consisting of small grains with little sintering densification was observed in Fig. 7b.
3.4. Conductivity of silver-based films
The conductivity of internal electrode on the microwave properties of dielectric ceramics at high frequencies plays an important role. In order to evaluate the effect of metal oxides or metal-organic precursors on the con-ductivity of silver-based films, a “T-pattern” microstrip resonator was used. As observed in Fig. 8 for the silver films added with 1 wt% metal oxides or metal-organic precursors (MO), the conductivity of most silver-based films except MO-W added was apparently reduced.
4. Discussion
4.1. Retardation of sintering shrinkage
The amount of shrinkage in the temperature range from room temperature to 800◦C is the critical factor in
Figure 7 Surface microstructure of silver films added with 1 wt%
(a) MO-W and (b) MO-Zr sintered at 600◦C for 0.5 h.
Figure 8 Conductivity comparison of silver films with a variety of metal
oxides and metal-organic precursors added.
forming delaminations in multilayer ceramic capacitors since the silver film begins to density, yet the dielectric has not begun to sinter and shrink. So, any remaining shrinkage in the silver film occurring above 800◦C is of less consequence since this is the range in which the di-electric also shrinks. As the silver films were added with a variety of metal oxides, different sintering shrinkage was obtained (Fig. 3). It was believed that the sintering behavior was dependent on the characteristics of metal oxide, i.e., melting point of metal oxide. For example,
Figure 9 Surface microstructure of silver films with 1.5 wt% MO-Zr additive changes with sintering temperatures at (a) 400◦C, (b) 600◦C, (c) 800◦C, and (d) 950◦C for 0.5 h.
ZrO2additive exhibits a remarkable inhibition on sin-tering shrinkage of silver films in comparison to other metal oxides investigated in this work. It was strongly believed that the observed behavior in ZrO2-added sil-ver film was correlated with its higher refractory char-acter.
Further comparison of the sintering shrinkage be-tween Figs 3 and 5 revealed that metal oxide-added silver films exhibit a two-stage sintering shrinkage, which is completely different from the one-stage con-tinuous shrinkage in the silver films added with metal-organic precursors. For the silver films with metal oxide added, the first-stage shrinkage can be thought due to the sintering densification of aggregate silver powders because the metal oxide powder can not be uniformly distributed over all the silver particles, as observed in Fig. 4c. Therefore, some local inhomogeneous densifi-cation and necking between silver powder takes place at the lower temperatures, i.e., 400–600◦C. As increas-ing temperature, the aggregate silver particles coalesce and grow together in this second-stage at 600–800◦C.
On the other hand, for the silver films added with metal-organic precursors, the shrinkage behavior can be elucidated from the microstructure evolution with 1.5 wt% MO-Zr-added silver films (Fig. 9). A possi-ble mechanism for one-stage sintering shrinkage could be explained as follows. As the silver powder was mixed with MO-Zr precursor, the silver powder could be coated with a uniform layer of Zr-precursor. Ac-cording to the report of Chu et al. [20], the MO-Zr
film will be decomposed into an armophous structure at 400–450◦C and crystallization occurs at∼500◦C with nano-sized ZrO2powder being formed. It stands to rea-son that as fired at 400◦C, the metal-organic-doped sil-ver films were more porous (Fig. 9a) prior to appre-ciable sintering (as expected, given the relatively high amount of organic to be removed) than the metal-oxide-doped films. Indeed, the convex shape of the MO-Zr (or MO-Fe, MO-Zn) doped specimens in Fig. 5 is sim-ilar to the initial convex shape seen (e.g., at<600◦C) in the shrinkage curves of the ZrO2, Fe2O3, and ZnO doped films in Fig. 3. At 600◦C, Fig. 9b shows some of silver particles aggregate along with nano-sized ZrO2 particles (marked with the arrows) distributed around or interleaved between silver powders. With increasing the sintering temperature to 800◦C, although the grain coalescence of silver particles constantly takes place, a porous structure resulted as one can see in Fig. 9c, which indicates that the sintering densification of sil-ver powder was apparently inhibited. At 950◦C, a dense microstructure with smaller grain size (∼3 µm) was ob-tained (Fig. 9d) because the grain growth was obviously hampered as compared with that of silver powder with no MO-Zr. Consequently, the shrinkage mismatch can be apparently reduced.
4.2. Role of additives in conductivity
Generally, it was believed that as additives were added to silver, irrespective of whether these additives are
metals, glass or other metal oxides, the conductivity would be diminished. However, our results show that it was reverse for silver film with MO-W added. The conductivity measured for MO-W-added silver film ex-hibits slightly higher than pure silver film. The unique phenomenon observable can be reasonably explained as follows. As well known, W has the characteristics of variable valences. As the MO-W added silver film was sintered at 600◦C. A dense microstructure with larger grain size can be obtained (Fig. 7a) in comparison with silver films with no additive and WO3-added. Also there exists an obvious contrast across the grain boundaries, which further indicates a possibly partial dissolution of W into Ag. Leung et al. reported that the addition of 35–62 vol% of tungsten could increase the welding ten-dency of silver-tungsten [21]. These might imply that silver-tungsten tends to form a solid solution. There-fore, a unique microstructure and conductivity behavior was detected in MO-W added silver film.
5. Conclusions
1. The addition of metal oxide powders shows inferior to metal-organic precursors in reducing shrinkage of silver-based electrode.
2. The use of oxide powder additive shows two-stage shrinkage behavior in contrast to one-stage continuous shrinkage occurred in silver films with metal-organic precursors added.
3. The metal-organic precursors except for MO-W can be effectively decorated around silver powder and in-hibits the sintering densification.
4. The shrinkage mismatch between silver-based films and dielectric ceramic can be much reduced with the addition of MO-Zr.
5. The unique conductivity and grain growth of silver film with 1 wt% MO-W added was possibly due to the partial dissolution of W into Ag.
Acknowledgment
The author would like to thank the financial support from the National Science Council of the Republic of China under Contract No. NSC88-2213-E-009-131.
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Received 24 July 2000 and accepted 8 August 2001
