Thick film materials have proved to possess economic processing and functional advantages over other technologies in high-volume production of miniaturized ...
J O U R N A L O F M AT E R I A L S S C I E N C E : M AT E R I A L S I N E L E C T RO N I C S 1 1 ( 2 0 0 0 ) 6 6 7 ± 6 7 4
A study on sintering and microstructure development of fritless silver thick ®lm conductors SUNIT RANE*, VIJAYA PURI Thick & Thin Film Device Lab, Department of Physics, Shivaji University, Kolhapur-416004, India DINESH AMALNERKAR Centre for Materials for Electronics Technology, Panchawati, Pune-411008, India Thick ®lm materials have proved to possess economic processing and functional advantages over other technologies in high-volume production of miniaturized circuits. The development of a low-cost fritless silver paste with different binder composition is described in the present work. The effect of composition and ®ring temperature (700±900 C) on the microstructure is also reported. The sheet resistivity and surface morphology of all the indigenously formulated Ag thick ®lm pastes are compared with the imported (ESL) paste. The results suggest that the thick ®lms can be ®red at 700 C instead of the conventional 900 C ®ring temperature. # 2000 Kluwer Academic Publishers
1. Introduction
Thick ®lm technology is an established method for ef®cient manufacture of hybrid microelectronic circuitry. The steady growth of thick ®lm technology has been due to the continual ¯ow of new microelectronic circuit needs and the ability to develop materials to accommodate them. The future growth of thick ®lm hybrids depends on improved material technology, and silver has to be established as a high reliability option since it has an enormous economic advantage compared to standard gold/platinum conductors. However, the solubility of silver in molten tin-lead solder and the tendency of silver ion migration under the in¯uence of an electrical ®eld and moisture are serious problems associated with silverbased pastes. Such problems can be circumvented (to acceptable levels) by proper circuit design and avoiding large voltage difference between adjacent conductors or by using protective over-glazes or by alloying with 20± 40% of palladium. Our recent studies [1±4] and those of others [5±7] have shown that thick ®lms can also be used as metallization for microwave purposes. A range of standard thick ®lm pastes is available for the metallization for ceramics. Glass frits or different oxides, which are added to thick ®lm conductor pastes, are responsible for the adhesion of metal ®lm on conventional alumina substrates via formation of vitreous bonds. The advantage of high thermal conductivity will be obviously affected by the glass phase acting as an interlocking layer. Lin and Wang [8] studied the structural evolution of printed silver thick ®lms during sintering to understand the surface properties of powders on thick ®lm ®ring.
The present work was undertaken to gain a better understanding of the sintering mechanisms related to fritless silver thick ®lm conductors and to control their microstructure development. A basic understanding of microstructure development is expected to improve the performance and reliability of hybrid circuits especially in the microwave range of frequencies. The microwave properties were reported in our previous paper [9]. With the goal of ®nding metallization systems with low cost as well as microwave properties comparable to ESL (USA) paste, a glass frit free thick ®lm paste was developed.
2. Experimental
The silver based thick ®lm pastes (SBR3, SBR5 and SBR4, SBR6) having a composition of metal : inorganic:organic (80 : 10 : 10 and 83 : 7 : 10 by wt %) were formulated. Two different inorganic binder compositions were used for the paste formulation. In the ®rst case, PbO, Al2 O3 , TiO2 and Bi2 O3 were used while only Bi2 O3 was used in the second case. Silver powder and oxide powders were uniformly mixed with an appropriate amount of ethyl cellulose and butyl carbitol acetate in an agate mortar. The microstripline of width 635 mm was screen printed using 200 (number of threads per inch) mesh screen on 96% alumina substrate and ®red in a conventional way at three different peak ®ring temperatures (i.e., 700, 800 and 900 C). The average ®red ®lm thickness of the thick ®lm was found to be in the range 10±15 mm. The physical characterization (i.e., crystal structure, microstructure, microstipline, etc. of
*Present address: Centre for Materials for Electronics Technology, Panchawati, Pune-411008, India.
0957±4522
# 2000 Kluwer Academic Publishers
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the samples was carried out by using X-ray diffractometry and scanning electron microscopy. X-ray diffractograms were recorded with a Philips diffractometer (Model PW-1710) using CuKa radiation with a Ni ®lter
l 0.1542 nm. Scanning electron micrographs and energy dispersive analysis by X-rays (EDAX) measurements were taken with a Philips SEM/EDAX (Model XL 30). The d.c. resistivity of the different formulations was also measured using the four-probe method.
3. Results
The data of sheet resistivity of the thick ®lms is given in Table I. From the table, it is seen that the sheet resistivity is high for SBR3 and SBR5 paste composition as compared to other compositions. Also, it is observed that there are major changes in the sheet resistivity value for all three ®ring temperatures. Since the same diffraction pattern is observed for all paste compositions, only one typical pattern is reported in Fig. 1. XRD examination revealed that the ®lms are polycrystalline, since it shows sharp peaks corresponding to silver. The ®lms have (1 1 1) (2 0 0), (2 2 0) and (3 1 1) re¯ecting planes since the major contributing material of the paste was Ag, and the XRD results showed the expected dominance of the silver peak, especially the (1 1 1) plane. The observed d values of silver are in good agreement with standard d values taken from the ASTM diffraction data ®le and are independent of the ®ring temperature. Compositionally our indigenous pastes SBR5 and SBR6 compare very well with the ESL (USA) paste except that our paste does not contain Mn which is present in ESL paste as seen in the EDAX data. SBR3 and SBR4 also show similar peaks of Ag as the ESL paste. The data of line width and edge de®nition as measured by optical microscopy (Nikon) is given in Table II. From the table, it is evident that the fabricated width is, in general, larger than the designed width of the line. The structure patterns of the microstripline are shown in Fig. 2. The patterns of SBR3 and SBR6 are poor compared to those of SBR4, SBR5. The ESL pattern seems to be the best amongst the thick ®lm circuits. The SEM micrographs of the un®red ®lms (shown in Fig. 3) indicate a granular structure. Fig. 4 shows the SEM patterns of thick ®lms ®red at 700 C. Grain growth is observed for all the ®lms. However, the uniform granular structure is noticed in the SBR5 paste composition. The T A B L E I Sheet resistivity of the thick ®lms Paste composition
Sheet resistivity (mO cm)
Firing temperature ( C)
700
800
900
SBR3 SBR4 SBR5 SBR6 ESL*
0.072 0.067 0.070 0.066 0.066
0.072 0.069 0.071 0.068 0.067
0.072 0.067 0.061 0.061 0.061
*
Measured by our own set up.
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Figure 1 X-ray diffraction pattern of silver thick ®lm.
neck formation is observed in all the indigenous (SBR series) composition. The ESL paste did not show much change in grains/particle size on sintering. Similarly, the SEM patterns of thick ®lms ®red at 800 C and 900 C are shown in Fig. 5 and Fig. 6, respectively. The SBR3 composition shows a continuous formation of grains with the distinct appearance of grain boundaries for the ®lms ®red at 800 C while ®lms ®red at 900 C did not show either the presence of grain boundaries or the occurrence of continuity in the ®lm. The SBR4 composition shows the formation of a continuous ®lm but the grain boundaries are not clearly observed for both ®ring temperatures (800 and 900 C). SBR5 and SBR6 compositions show clearly visible grain boundaries with some voids but the grain size is larger than the ®lms ®red at 900 C. The uniform distribution of grains occurred in ®lms ®red at 900 C.
4. Discussion
As seen from Table I, the thick ®lm composition shows a higher sheet resistivity than its bulk value, but it is not drastically different indicating that there is lack of compactness in these ®lms, which is attributable to isolation of metal particles by the bonding medium. The increase in sheet resistivity can also be attributed to a certain extent to the inorganic additives but its signi®cant fraction might be associated with the speci®c preparative method leading to the formation of a porous complex system with a random distribution of metal particles and oxide species [4]. Lin and Wang [8] reported that there is a steep decrease in resistivity when the sintering temperature is raised from 450 to 600 C and subsequently there is a gradual decrease in resistivity as sintering temperature is raised to 800 C. The differences between the line width might be due to chamfering, use of approximate equations for calculations of parameters, errors in the mask preparation and differences in paste velocity. The X-ray studies of the ESL and the indigenous (SBR series) silver pastes indicate almost the same order of particle size. However, the SEM examination clearly reveals relatively smaller particles for the ESL paste in comparison to the SBR series pastes. The particle size difference causes the difference in line patterns. Also, a spreading of the ®lm is seen at the edges which might be responsible for the greater line width. Large periodic variations caused by the mesh of the mask can also be noticed [4].
T A B L E I I Data of the designed line width, actual line width and line/edge de®nition of microstipline Firing temperature ( C)
Designed width (mm)
700 800 900
635 635 635
Actual width (mm)
Line/edge de®nition (mm)
SBR3
SBR4
SBR5
SBR6
ESL
SBR3
SBR4
SBR5
SBR6
ESL
675 610 687
578 620 736
592 670 720
560 695 741
650 600 670
44 40 62
41 26 32
31 32 28
35 39 29
28 44 40
(a)
(b)
(c)
(d)
(e) Figure 2 Structure pattern of silver thick ®lm microstripline: (a) SBR3 paste, (b) SBR4 paste, (c) SBR5 paste, and (d) SBR6 paste and (e) ESL paste.
The components of the thick ®lm are blended together in the desired ratio and milled to achieve deagglomeration and homogenization without affecting the morphological characteristics of the functional powders. Lin and Wang [8] reported that a homogeneous dispersion of particles provides a large interfacial area for glass softening which, in turn, enhances the granular transformation. Surface irregularities and large particle
size distribution promote uneven sintering during the ®ring stages, which ultimately leads to poorly de®ned conducting layers and hence inferior edge de®nition and higher microwave losses [4]. The sintering process develops the microstructure of thick ®lm conductors, but the situation is signi®cantly different from normal powder metallurgical process in which the sintering begins from an extremely low-density compact. There is 669
(a)
(b)
(c)
(d)
(e) Figure 3 SEM patterns of un®red silver thick ®lms: (a) SBR3 paste, (b) SBR4 paste, (c) SBR5 paste, (d) SBR6 paste and (e) ESL paste.
a very low or almost negligible compacting pressure exerted during screen-printing. Blisters are caused by release of gas from within the ®lm after metal on the surface has sintered to a high density. Two main sources of gas are oxidation of residual carbonate materials from the organic screening agent and chemical reactions occurring between the bonding agent in the thick ®lm ink and the substrate. The ®nal ®lm density and hence electrical resistivity are intrinsically related to the density of the metal compact, which exists even after removal of organic constituents. All the thick ®lm formulations (SBR series) made in our laboratory contain Bi2 O3 in addition to Al2 O3 , TiO2 and PbO in SBR3, SBR4 and only Bi2 O3 in SBR5 and SBR6 pastes. Since these are fritless compositions, the bonding between the metal (Ag) and the alumina may be of the reactive type where the Bi2 O3 reacts with alumina to form a Bi2 Al4 O9 layer at the interface 670
between the ®lm and the substrate [10]. There is a coexistence of physical adhesion by wetting of the melt and chemical adhesion by the newly formed layer. Taylor et al. [11] suggested a layer of solid solution with composition Bix Al2 ÿ x O3 present at the interface of the ®lm and substrate. Ogawa et al. [12] have suggested that a Bi2 O3 Al2 O3 eutectic can be formed and also a transient liquid which might help in the adhesion of metal. Bi2 O3 might also act as a ¯uxing agent. Borland and Sinta [13] have reported CuAlO2 type spinel phases present at the boundary when copper oxide is used as the binder. Smetana and Reicher [14] also reported the formation of spinels on reaction with alumina. They claimed that the process improves metallization. Rane and Puri [9] reported that the transmittance, re¯ectance and loss characteristics of all ®ve formulations are similar even though the binder used in the ESL paste is different from our indigenous (laboratory) formulations.
(a)
(b)
(c)
(d)
(e) Figure 4 SEM patterns of silver thick ®lms ®red at 700 C: (a) SBR3 paste, (b) SBR4 paste, (c) SBR5 paste, (d) SBR6 paste and (e) ESL paste.
The EDAX result shows that Mn is present in the ESL composition. SBR3 and SBR4 pastes have PbO, Al2 O3 and TiO2 in addition to Bi2 O3 . The function of Al2 O3 and TiO2 might give refractory to stability [15]. Since the surface resistivity, line de®nition and surface morphology are not very different between the various compositions, Al2 O3 and TiO2 might be just assisting in the bonding and not interfering in the other properties or the paste. According to Vest [16], the Al2 O3 particles physically impede the upper conductor ®lm from moving down to the lower conductor ®lm. TiO2 particles act as nucleation sites. Chen and Wae [17] have suggested that TiO2 reacts with Bi2 O3 to form Bi2 Ti2 O7 at temperature as low as 600 C. This forms an interfacial layer with alumina. Titanium also introduces segregation at the interface between Ti in the ®lm and the substrate [18]. Adalbing et al. [19] reported the addition of Ti results in an interfacial reaction yielding TiN and Ti3 CuAlN0:7 as the bonding
phase. They also reported that the high adhesion strength obtained is related to the development of Ti-enriched layers as the interface between the metallization and the ceramic. The metallic phase itself is expected to bond to the ceramic substrate after ®ring in fritless thick ®lm metallization. Peytour et al. [20] reported that titaniumcontaining alloys react rapidly with various ceramic substrates to form intermediate or transitional phases at the interface. The reaction between Ti and Al2 O3 is considered in both active brazing and diffusion bonding. Lin and Wang [8] reported that the ®lms ®red at 600 C show a granular structure irrespective of the modi®cations in the silver powders and further increase in ®ring temperature which resulted in grain growth yielding denser ®lms. The uniform grain growth observed in sample SBR5 (Figs 4c, 5c, and 6c) leads to almost the same line de®nition observed for samples ®red at the three temperatures (Table II). The line de®nition data also 671
(a)
(b)
(c)
(d)
(e) Figure 5 SEM patterns of silver thick ®lms ®red at 800 C: (a) SBR3 paste, (b) SBR4 paste, (c) SBR5 paste, (d) SBR6 paste and (e) ESL paste.
shows a higher value for the SBR3 sample, which according to SEM data does not show continuous ®lm and also grain boundaries. When a dried ®lm is ®red at higher temperature, the organic vehicle is removed and softened (or molten) binder instead of the organic vehicle mainly exerts the 672
capillary force. The capillary force resulting from the softened binder is much stronger than that from the organic vehicle, which results in intense mass transport at higher temperatures. The softened binder activates granular transformation. The more dispersed the inorganic part, the more substantial is the capillary effect.
(a)
(b)
(c)
(d)
(e) Figure 6 SEM patterns of silver thick ®lms ®red at 900 C: (a) SBR3 paste, (b) SBR4 paste, (c) SBR5 paste, (d) SBR6 paste and (e) ESL paste.
This, in turn, lowers the temperature required for obtaining a dense ®red ®lm. When the ®ring temperature is increased, mass transport became more signi®cant and leads to grain growth.
5. Conclusion
Our results indicate that, in combination with suitable binders, silver can be a viable cost effective alternative for the thick ®lm hybrid circuits. Since the fritless Ag
thick ®lm ®red at 700 C shows comparable properties to those ®red at 800 C and 900 C, it is felt that a substantial advantage in energy savings and increased furnace life is inherent in ®ring temperatures below the typical temperature of 900 C. The grain growth is observed at a ®ring temperature of 700 C. A further increase in the ®ring temperature resulted in grain growth and yielding denser ®red ®lms. The thick ®lms with Bi2 O3 as a binder show a uniform granular structure compared to ®lms of Al2 O3 , TiO2 and PbO with Bi2 O3 as 673
a binder. The sheet resistivity of all the compositions is in the range of 0.061±0.072 mO for all the ®ring temperatures. The ®lms containing only Bi2 O3, as a binder show the lowest sheet resistivity at a 900 C ®ring temperature. It is felt that our indigenous thick ®lm pastes can be an alternative source to the imported pastes. Further research to optimize the other parameters of the fritless thick ®lm pastes is underway.
5. 6. 7. 8. 9. 10. 11.
Acknowledgment
One of us (Sunit Rane) gratefully acknowledges the Department of Science and Technology, India for providing ®nancial assistance for the work and Vijaya Puri gratefully acknowledges the University Grants Commission, India for the award of Research Scientist B. Also, the authors are gratefully to C-MET, Pune for providing the SEM characterization.
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Received 17 April and accepted 2 August 2000
