Diffusivity of silver ions in the low temperature co-fired ceramic (LTCC)substrates
Chi-Shiung Hsi •Yung-Ren Chen •Hsing-I Hsiang
Received:13November 2010/Accepted:7February 2011/Published online:16February 2011ÓSpringer Science+Business Media,LLC 2011
Abstract Diffusion of silver inner-electrode occurred during sintering of commercial low temperature co-fired glass ceramic substrate made the dielectric surface become light yellow.The samples added with silicon oxide (SiO 2)powder,however,maintained white color.Silicon-oxide powder was used to modified the sintering behavior and inhibit the silver ions diffusion for the LTCC ceramics.The alumina particles in the LTCC substrates could be regarded as the diffusion barrier of silver ions.The activation energy for silver ions diffusion in the LTCC substrates was 101kJ/mol.When 5wt%SiO 2powder was added into the LTCC substrate,the diffusion activation energy of silver ions became 145kJ/mol.At sintering temperature of 1180K,the diffusion coefficient of silver ion in the LTCC ceramic substrates with and without additional SiO 2were 8.88910-13cm 2/s and 1.08910-12cm 2/s,respectively.
Introduction
In recent years,low temperature co-fired ceramic (LTCC)substrates integrated with passive devices have been used extensively for high-density packaging module [1–3]and high-performance wireless components [4].High-conduc-tivity metals,such as Cu and Ag have been widely used as inner-electrode materials in the LTCC components and
modules.Since it can be sintered without atmosphere control,Ag metal and its alloy have been considered as high effective inner-electrode materials in the LTCC manufacturing.Adhesion and shrinkage match between the electrode and substrate,camber behavior of the sintered modules,and diffusion of silver ion during sintering are three major issues for the metallization of LTCC compo-nents and modules.Camber behavior of multilayer struc-ture co-firing with Ag paste has been intensive studied by Jean and co-workers [5,6].Diffusion of Ag ions in the high-glass content LTCC substrates was considered to influence of reliability and performance of the LTCC modules on the subjects of increasing leakage current and decreasing insulation resistance [7].Although migration of silver ions in various electronic components and hybrid electronics were investigated early [8,9],the diffusion of silver ions in the high-glass content LTCC substrate was seldom reported.Reaction kinetic and mechanism between silver electrodes and ceramic-filled glass substrates [10],silver diffusion and microstructure development in the LTCC system [11]were previously investigated.
reaction diffusion
In this study,diffusion kinetics of silver ions in a commercial LTCC substrate was investigated.Silica powder was used as an additive for the LTCC substrate in order to inhibit the diffusion of Ag ions in the substrates.
Experimental procedure
LTCC substrates were prepared using commercial powder of ceramic-filled glass (alumina-filled Si–Ba-B–Al-Ti–Ca glass)and the same powder added with 5wt%reagent-grade amorphous silica.The powders mixed with organic binder (B-73305,Ferro,San Narcos,CA,USA)at 50:50ratio were ball-milled using high-purity alumina balls
C.-S.Hsi ÁY.-R.Chen
Department of Materials Science and Engineering,
National United University,1Lein-Da Road,Kung-Ching Li,MiaoLi 36003,Taiwan
H.-I.Hsiang (&)
Department of Resources Engineering,
National Cheng Kung University,Tainan 70101,Taiwan e-mail:ku.edu.tw
J Mater Sci (2011)46:4695–4700DOI 10.1007/s10853-011-5377-z
before tape-cast to desired thickness.Silver conducting paste(Ag8985,Shoei Co.,Japan)was printed on the green tape and dried at130°C for30min.The printed green tapes were laminated at60°C under pressure of20MPa for3min in an isostatic pressing chamber.The laminated sample,then,was diced as chips with size of  2.249 1.3691.1mm3.The diced samples were heated to450°C with a10°C/h heating rate for binder burnout before co-firing was conducted at temperatures between1050and 1180vK for15–120min by a heating rate of4°C/min. The samples were cut perpendicular to substrate surface along the longitude of the conducting layer.Cross-section samples were then polished using diamondfilms to1l m surface roughness before etching with diluted3% HF?HCl solution.Analysis of microstructure was per-formed with scanning electron microscopy(SEM,JEOL 5600,Tokyo,Japan).Inter-diffusion between Ag and LTCC substrate was examined using energy dispersive X-ray spectroscopy(EDS,6587,Oxford,England)equip-ped with SEM.Crystalline phases of the co-fired resistors
were determined by X-ray diffractometry(XRD)using X-2000(Scintag,CA,USA).Raman spectrum meas
ure-ments were performed at room temperature with a resolu-tion of about0.5cm-1,and the signals were recorded by a Jobin–Yvon LabRAM HR micro-Raman spectrometer equipped with a liquid-nitrogen-cooled CCD.The40mW output of the514.5-nm Ar?ion laser was used as the excitation source.
Results and discussions
Diffusion of silver inner-electrode occurred during sinter-ing of LTCC substrate made the dielectric surface become light brown,the left side samples shown in Fig.1. It appears that adding SiO2to the LTCC substrate,the right side samples shown in Fig.1,has effectively reduced silver diffusion into the LTCC substrate,leading to its surface maintained white in color.
Single layer LTCC tape without SiO2,sintered at 1150K for30min,had shrinkage of23,23,and12%on the length,width and thick directions.When5wt%SiO2 was added to the LTCC substrate,the shrinkage of the LTCC single layer tape on the length and width direction reduced to20%,its shrinkage on the perpendicular direc-tion increased to14%.In addition of SiO2increased the viscosity of the glassy phase in the materials and also increased the sintering temperature of LTCC materials [12].Higher viscosity of glassy phase resulted from the addition of SiO2made the substrate had lower shrinkage in the planar directions(x and y directions),and increased the shrinkage in the thickness(z)direction.The single layer LTCC tapes with and without SiO2addition had similar volume shrinkage of about47%.
During sintering,the multilayer LTCC chips had dif-ferent shrinkage ratios with different directions,as listed in Table1.The shrinkage on the z direction had largest shrinkage among three directions of the chips with and without SiO2addition.Owing to constraint sintering resulted from the difference of the shrinkage behavior between the inner-silver electrode and LTCC materials during sintering,the chip’s shrinkage on the x and y directions were about half of the single layer LTCC tape, and hence made the shrinkage of the chip on the z direction was larger than that of the LTCC single layer tape.When the chips were sintered at temperatures between1050and 1180K,the shrinkages of the multilayer chips without SiO2addition on the x and y directions were around 10.5–13.5and the shrinkage on the z direction was about 17–18%.When5wt%SiO2was added to the LTCC tape, the chips’shrinkages on the x and y directions were between7and11%,which were lower than the chips without SiO2addition.The shrinkage of the SiO2added chips on the z direction was about double values of that of the chips without SiO2addition,which had shrinkage around31–33.5%.The volumes of chips without SiO2 added shrunk about33–37%after sintering,the chips with SiO2added,however,had volume decreasing41–45%. From the microstructures observation results,the dielectric layer in the chip without SiO2addition had higher porosity than the dielectric layer in the SiO2added chip,as shown in Fig.2.
In the previous investigation[12],LTCC material with SiO2addition had two shrinkage stages,and the tempera-ture difference betweenfirst and second shrinkage
stage Fig.1Morphologies of silver metalized LTCC chips with(right side)and without(left side)SiO2addition,the chips were aligned in y direction
was about 100K.There was only one stage shrinkage behavior of the LTCC material without SiO 2addition.The pores in the SiO 2added chips remained open after first stage shrinkage,the pore continuously shrunk before sec-ond stage shrinkage occurred.The constrained sintering effects in the SiO 2added chips were more evident than that in the chips without SiO 2addition,which resulted in the x and y direction shrinkages of former chip were smaller than the latter.Therefore,the SiO 2added chip
s continu-ously shrunk in z direction after first stage shrinkage,leading to the shrinkage in z direction was much higher than that of LTCC chips without adding SiO 2.When the chips were sintered at temperature between 1050and 1150K,the shrinkages for the chips with or without SiO 2additions increased with increasing the sintering tempera-ture or time.The SiO 2added chips reached second stage shrinkage when they were sintered at temperature higher than 1050K.After sintering at temperature of 1180K,the
chips with or without SiO 2addition had lower shrinkage than those were sintered at 1150K,which may be because of the dissolution of alumina in the glass.
Alumina was the only crystalline phase presented in the X-ray diffraction pattern for LTCC tape sintered at 1180K (Chen and Hsi unpublished paper).Dark and gray areas were observed from the SEM micrographs,dark areas were pointed by ‘‘A’’and grey areas were pointed by ‘‘B’’in Fig.2.The dark areas in the microstructure were identified as high-aluminum contented phases under EDS analysis,where were considered as the locations of alumina phase.The gray areas contained Al 2O 3,SiO 2,BaO,and CaO as major compositions.The gray area in the chips without SiO 2addition had higher Al 2O 3and CaO contents than those of the SiO 2added chips.
Silver atoms were detected from the grey areas in the chips with or without SiO 2added,their contents increased with increasing sintering temperature,as shown in Fig.3.The gray areas in the chips without SiO 2addition had silver content in the range of 2.76–3.36wt%.The silver contents in the gray phases of SiO 2added chips,however,were lower than 1.59wt%.Figure 4illustrates Raman spectra collected from chips with and without SiO 2addition,and LTCC sintered tape sintered at 1100K for 30min.Three bands at 445,510,and 605cm -1were clearly observed for all the samples,however,an additional band at 240cm -1was solely found at the chip without SiO 2added.The band at 240cm -1related to the Ag–O phonons,which was of similar frequency in the Raman spectrum of d s (AgO 2)of the distorted square-planar ‘Ag III O 4’unit of AgO structure [13,14].Silver ion in the chip without SiO 2addition formed Ag–O bond in the glassy phase of the chip.The light brown appeared on the surface of the chip without SiO 2addition,therefore,was because of the formation of Ag–O bonds in the chip.When the silver content was higher than 2.5wt%as detected from the sintered chips without the addition of SiO 2,the Ag–O bonds formed and the chips turned light brown color.When the silver con-tents were lower than 1.5wt%as those measured from SiO 2added chips,Ag–O bond was not found in the chips and it appeared as white color.
Silver concentration profiles in the LTCC dielectric layer perpendicular to the direction of sliver electrod
e layer of chips without SiO 2addition sintered at 1050,1100,1150,and 1180K for 30min are shown in Fig.5a.The diffusion process can be assessed as undimentional diffu-sion into a semi-infinite medium from a constant surface concentration of silver,Cs (Ag).The concentration of silver ions in LTCC substrates were calculated by the Fick’s second law,as the follows:
C Ag ðx ;t Þ Cs ðAg Þ¼1Àerf x .2
D Ag t ÀÁ0:5h i
ð1
Þ
Fig.2Microstructures of LTCC chips a without and b with SiO 2additions sintered at temperature 1150K for 30min
where C Ag(x,t)is the concentration of silver ions measured at distance x from the interface of sliver layer for a given period of time,Cs(Ag)is equal to100%for silver electrode, and D Ag is the diffusion coefficient of silver ion,the erf in Eq.1stands for error function[15].The value of C Ag(x,t)/ Cs(Ag)ratio was directly calculated from the EPMA mea-surement of sample.The diffusivities of silver ions in the LTCC materials were calculated from the silver concen-tration profiles shown in Fig.6by Eq.1.It is found that the average diffusivities of silver ions were3.01910-13cm2/ s at1050K,  4.19910-13cm2/s at1100K,8.719 10-13cm2/s at1150K,and  1.08910-12cm2/s at 1180K.These results were higher than previous report [10],which might be because of the different glass type used and alumina content in this study.There was only 5wt%alumina content for the LTCC material used in the previous report,but the alumina content for the LTCC material used in this study was*40%.Alumina grains can be considered as diffusion barriers for silver ions in the LTCC chip,the true diffusion path of silver ions were longer than that directly measured from the cross-section of the chips.Calculated from the silver concentration profile of the SiO2added dielectric layer,as indicated in Fig.5b, the diffusivities of silver io
ns in the chip were 9.87910-14cm2/s at1050K,  2.59910-13cm2/s at
Table1Shrinkage of LTCC chips in x(length),y(width), and z(thickness)directions during sintering Sintering temperature(K)Sintering time(min)Shrinkage(%)
Without SiO2With5wt%SiO2
Length Width Thickness Length Width Thickness
10501512.4910.8617.039.617.6732.51
3012.7611.2417.319.897.9832.82
6012.8111.5717.5710.298.1633.03
12013.0611.8118.0110.588.3733.42 11001512.7411.0317.319.867.9132.7
3012.8711.3617.52108.0733.09
6013.0911.7817.8910.378.2833.37
12013.2912.0118.1110.668.5733.68 11501512.8111.4717.489.908.1532.7
3012.9411.5917.6910.248.2332.94
6013.3511.7917.9410.598.4933.28
12013.6712.1918.3210.788.7733.54 11801512.7511.1917.249.417.9432.34
3012.5110.9816.979.017.932.07
6012.2110.6616.398.767.7231.76
12011.9310.4916.098.487.24
31.39
1100K,  4.87910-13cm 2/s at 1150K,and 8.88910-13cm 2/s at 1180K.The diffusivities of silver ions in the SiO 2added LTCC dielectric layers were about one order lower than those in the LTCC dielectric layers without SiO 2additions.
Summarizing the diffusion coefficients for all tempera-tures investigated,an Arrhenius plot was obtained as shown in Fig.6.The activation energy (E)of silver ion in the LTCC dielectric material can be calculated by the Arrhenius equation:D ¼D 0exp ðÀQ d =RT Þ
ð2Þ
where D 0is a temperature-independent pre-exponential (m 2/s),Q d is the activation energy for diffusion,R is the gas constant,8.31J/mol-K.The diffusion activation energy of silver ions in the LTCC dielectric layer of the chip determined from the slope of Arrhenius plot was 101kJ/mol.The diffusion activation energy of silver ions in the SiO 2added LTCC dielectric layer was 145kJ/mol.The diffusion activation energies of silver ions measured in this study were much higher than those of silver ions in glasses [10],which may be because of high-alumina content (*40wt%)in the LTCC materials used in this study.The increase in the viscosity of glassy phase in the LTCC dielectric material because of the addition of SiO 2would be considered as the major factor to decrease the diffusivity of silver ions,and to increase the activation energy of silver ions in the LTCC dielectric material.
Conclusions
In this study,the authors demonstrate an easy method to decrease the diffusivity of silver ions in LTCC dielectric materials by adding 5wt%SiO 2to the dielectric raw materials.This process effectively improved the coloring LTCC chips by decreasing the silver ions content in the glassy phase of dielectric.The increase in the viscosity of glassy phase in the LTCC dielectric material because of the addition of SiO 2may decrease the diffusivity of silver ions,and increase the activation energy of silver ions in the LTCC dielectric material.Alumina grains in the LTCC dielectric layer can behave as diffusion barriers of silver ions in the dielectric layer,which reduced the diffusivity and increased the diffusion acti
vation energy of silver ions in the dielectric layer.
References
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2.Dalaney K,Barrett J,Barton J,Doyle R (1999)IEEE Trans Adv Packaging
22:78
Fig.5Silver-concentration profiles of dielectric measuring from Ag/LTCC interface of a Chips without SiO 2addition and b chips with 5wt%SiO 2addition.The samples were sintered at temperature of 1050,1100,1150,and 1180K for 30
min
Fig.6Logarithm of Ag diffusivity (ln(D Ag ))as a function of reciprocal temperature (1/T )

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