Barium Neodymium Titanium Borate Glass-Based High k Dielectrics

Seung Min Lee, Yeon Hwa Jo, Hyo Eun Kim, Bhaskar C. Mohanty, and Yong Soo Cho†

Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea

A nonconventional high k glass based on BaO·Nd2O3·-TiO2·B2O3, which is combined with typical fillers of Al2O3 and

BaTiO3, has been investigated for the purpose of generating

potential k (dielectric constant) ~20 and k ~ 40 dielectric mate-

rials as a result of densification at 850°C. Crystallization, den-sification, and dielectric properties depended strongly on the

type and content of the fillers. Glass itself was crystallized

mainly with BaTi(BO3)2. The Al2O3 filler did not seem to be

directly involved in the crystallization process as only nonAl-containing crystalline phases, such as NdBO3, Nd2Ti4O11,

and BaNd2Ti5O14, were observed. On the contrary, the Ba-

TiO3 filler resulted in the crystallization of two phases, BaTi(BO3)2 and BaNd2Ti5O14. As an example of a promising

dielectric candidate, a composite sample consisting of the glass

and 30 wt% Al2O3 exhibited k ~ 22 and tan d ~ 0.009 with

nearly full densification at 850°C.

I. Introduction

I N decades, ceramic-based multilayer substrates have beenused in commercial electronic components, modules, and

devices requiring highly integrated packaging within a limitedvolume.1–4 The low temperature co-fired ceramics (LTCC)technology has been one of the most competitive solutionsthat can satisfy high signal transmission speed, high wiringdensity, high volumetric efficiency, and excellent reliability.5–8

Furthermore, passive components, including capacitors canbe embedded into the LTCC substrates through the co-firingprocess with low cost metals, for example, silver and copper,below 950°C.9–11 For the complete densification at the lowtemperatures, the use of a considerable amount of glass hav-ing a low softening temperature is evitable even in the designof potential high dielectric constant (k) materials for co-fire-able embedded capacitors.

There have been very rare studies in developing low soften-ing glass having k > 20 for the embedded capacitor applica-tions in LTCC.12 Probably, the incorporation of Ti into glassis an easy choice in increasing dielectric constant but alsoaccompanies unexpected crystallization during the subsequentfiring process.13 This work introduces an unreported bariumneodymium titanium borate (BaO·Nd2O3·TiO2·B2O3)-basedhigh k glass as a main component of the potential LTCCmaterials. The glass is assumed to be highly crystallizable.Two common choices of ceramic filler, that is, Al2O3 and Ba-TiO3, have been chosen. Al2O3 is known as the most commonfiller for generating LTCC materials targeting a low dielectricconstant below ~8.5 over the extended frequency range up toa few tens of GHz.14,15 On the other hand, BaTiO3 is an eas-ily available choice for generating higher k materials even

though BaTiO3 itself is not appropriate for applications atmicrowave frequency of greater than GHz.

In this work, crystallization behavior of the glass com-bined with Al2O3 and BaTiO3 are mainly studied with theprogress of various crystalline phases according to the typeand content of the fillers. Correlations of the crystallinephases with observed densification and dielectric propertiesare followed as a critical factor in understanding enhancedperformance of the dielectric materials.

II. Experimental Procedure

A barium neodymium titanium borate glass of 20BaO·15N-d2O3·35TiO2·30B2O3 in mol% was prepared by the conven-tional melting and quenching procedure. High purity rawmaterials, BaCO3 (99%, Aldrich, Milwaukee, WI), Nd2O3

(99%, Aldrich), TiO2 (99%, Aldrich), and B2O3 (99.98%,Aldrich), were initially mixed by hand shaking and melted at1400°C for 1 h in a Pt crucible. The homogeneous melt wasquenched into a water bath. The quenched glass was pulver-ized to produce frit having an average particle size of ~3 lmby rigorous milling using yttria-stablilized zirconia (YSZ)balls in ethanol for ~20 h and by drying completely at 120°C.

As a next step, two different types of typical ceramic fillers,such as Al2O3 (99.7%, Aldrich, ~1.4 lm in average particlesize) and BaTiO3 (99.9%, Aldrich, ~2.2 lm), were selected tomake composites consisting of glass and ceramic filler. Thecontent of ceramic filler relative to the glass was fixed as 30, 40,and 50 wt%. The sample ID was designated by following thetype and content of filler. For example, the AL40 sample indi-cates a sample consisting of 60 wt% glass and 40 wt% Al2O3.Each combination was admixed by ball milling in ethanol for~18 h and drying at 120°C. The dried mixture was sieved andpressed uniaxially at ~80 MPa to produce pellets of ~12 mm indiameter. The pressed specimens were sintered at 850°C for30 min at a heating rate of 5°C/min in a box furnace.

Phase analysis was performed by an X-ray diffractometer(Rigaku B/Max-2500/PC, Tokyo, Japan) using CuKa radia-tion. Fracture microstructure of the sintered samples wasexamined by scanning electron microscopy (SEM, HitachiS-4200; Nissei Sanyo, Tokyo, Japan). Differential thermalanalysis (DTA, Setram TG/DTA-92, Calurie, France) to exam-ine crystallization was conducted at a heating rate of 10°C/min. Densification behavior was investigated up to 900°C at afixed heating rate of 5°C/min by thermo mechanical analysis(TMA) using a dilatometer (Dilatonic Tokyo Industry, Tokyo,Japan). Dielectric properties were measured using an imped-ance analyzer (HP4194A) at a fixed frequency of 100 kHz.

III. Results and Discussion

(1) Crystallization BehaviorFigure 1(a) shows the typical DTA curves of only glass,AL40, and BT40 samples, which can be specified with distin-guishable exothermic peaks corresponding to crystallization.The glass of 20BaO·15Nd2O3·35TiO2·30B2O3 itself seems tobe crystallizable during firing. The crystallization of pure glasshappens in the range of 770°C–820°C with a peak crystalliza-tion temperature (Tc,peak) of ~795°C. The phase crystallized

A. Feteira—contributing editor

Manuscript No. 29922. Received June 25, 2011; approved October 17, 2011.This work was supported by Korea Energy Management Corporation (KEMCO)

of the Ministry of Knowledge Economy and partially, by the Defense Nano-Technol-ogy Application Center.

†Author to whom correspondence should be addressed. e-mail: ycho@yonsei.ac.kr

1

J. Am. Ceram. Soc., 1–4 (2011)

DOI: 10.1111/j.1551-2916.2011.04966.x

© 2011 The American Ceramic Society

Journal

was found to be mainly BaTi(BO3)2 with BaNd2Ti5O14 as aminor phase as shown in the XRD pattern of the glass firedat 850°C in Fig. 1(b). The glass was further characterized viaa separate DTA run as having a glass transition temperature(Tg) of ~630°C and a softening temperature (Ts) of ~650°C.

Figure 1(a) clearly shows that the addition of filler influ-ences the behavior of crystallization as much lowered crystal-lization temperatures are observed for the AL40 and BT40samples. The BaTiO3 seems to induce much lower crystalliza-tion temperatures than Al2O3. Figure 2 shows the plots ofcrystallization onset temperature (Tc,onset) and crystallizationpeak temperature (Tc,peak) as a function of relative content ofeach filler for the samples fired at 850°C, which are obtainedfrom the DTA runs. It is clear that crystallization tempera-ture depends on the content of filler, indicating that certainconstituents in filler may affect the process of crystallization.The effect of the content of fillers on crystallization was notidentical. For example, increasing the content of Al2O3 from30 to 50 wt% tended to raise slightly Tc,peak from ~760°C to~764°C, while increasing the content of BaTiO3 lowered Tc,

peak from ~748°C to ~738°C.Figure 3 shows the XRD patterns of the samples with dif-

ferent contents of Al2O3 and BaTiO3. Several unexpectedcrystalline phases were found depending on the type of filler.These phases are believed to be crystallized over the tempera-ture range above 700°C, which was defined from the DTAexothermic peaks. As reported previously in the study of sim-ilar glasses, it is very hard to find separate exothermic peakof each phase in the DTA curve.16 The AL samples wereobserved to consist of crystalline phases, NdBO3, Nd2Ti4O11,and BaNd2Ti5O14, in the order of the higher intensity phase,along with Al2O3 phase added as filler [Fig. 3(a)]. Note thatthe BaNd2Ti5O14 crystalline phase was present as a minorphase in the pure glass. It is interesting to note that Al fromAl2O3 has not been involved in any crystalline phase as nocrystalline phase incorporating Al is observed. The role of

Al2O3 filler may be facilitating crystallization at earlier tem-peratures, and at the same time, inducing third crystallinephases, such as NdBO3 and Nd2Ti4O11.

In the BT samples, on the other hand, the crystalline phasesof BaNd2Ti5O14 (as a predominant phase) and BaTi(BO3)2were found with the distinct BaTiO3 filler phase [Fig. 3(b)].These two crystalline phases of BaNd2Ti5O14 and BaTi(BO3)2were also found in the crystallized glass [Fig. 1(b)]. Therefore,the BaTiO3 filler did not seem to bring any third crystallinephase. The BaTiO3 filler may provide sufficient sources of Baand Ti for the crystallization during firing.

Relative evolution of each phase with increase in the con-tent of filler was quantitatively studied by plotting the ratioof the highest peak intensity of each phase to that of the ref-erence phase.17 Figure 4(a) shows the variations in the inten-sity of each crystalline phase, NdBO3, Nd2Ti4O11, orBaNd2Ti5O14, relative to the intensity of Al2O3 (100) peakwith increase in the content of Al2O3 filler. Regardless of theAl2O3 content, the order of relative peak intensity for thethree phases of NdBO3, Nd2Ti4O11, and BaNd2Ti5O14, wasnot changed. The relative peak intensities of all these phasestend to decrease significantly with increase in the content ofAl2O3. This tendency is reasonable as the relative peak inten-sity of Al2O3 becomes stronger with the higher content ofthe Al2O3 filler. Note that Al2O3 was not involved directly inthe process of crystallization as evidenced in the XRD pat-tern of Fig. 3(a).

Figure 4(b) represents the variations in the relative con-tents of crystallized phases at each filler level, which wereobtained by comparing the strongest intensity peak of eachphase relative to the peak summation of all existing crystal-line phases (not including Al2O3). For example, the relativecontent of NdBO3 was obtained by the following equation,

Relative ratio of NdBO3 ¼ INdBO3

INdBO3þ INd2Ti4O11

þ IBaNd2Ti5O14

Fig. 1. (a) DTA curves of the glass itself, AL40 and BT40 samples,and (b) XRD pattern of the pure glass fired at 850°C.

Fig. 2. Plots of crystallization onset temperature and crystallizationpeak temperature as a function of (a) Al2O3 and (b) BaTiO3 fillercontent.

Fig. 3. X-ray diffraction patterns of the samples containingdifferent contents of (a) Al2O3 and (b) BaTiO3 filler.

Fig. 4. Variations of peak intensity of each crystalline phase relativeto (a) the intensity of Al2O3 filler and (b) the summation of peakintensities of all crystallized phases, as a function of Al2O3 content.

2 Journal of the American Ceramic Society—Lee et al.

where INdBO3is the intensity of the strongest (111) peak of the

NdBO3 phase, as an example. The ratio indicates the relativecontent of the corresponding phase, irrespective of the filler.These plots clearly indicate that the relative content of eachphase does not depend on the content of Al2O3. Accordingly,it suggests that increasing the content of Al2O3 does not influ-ence selectively the relative crystallization of the phases.

The same procedure of comparing the strongest peakintensity of each phase with the reference intensity wasrepeated for the BT samples as shown in Fig. 5. As expectedfrom the case of Al2O3 filler, the crystallized phases ofBaNd2Ti5O14 and BaTi(BO3)2 tend to decrease with increasein the BaTiO3 content [Fig. 5(a)]. The relative contents ofthese two crystalline phases of BaTi(BO3)2 and BaNd2Ti5O14

were kept consistent with increase in BaTiO3 [Fig. 5(b)].These trends in Figs. 5(a) and (b) suggest that BaTiO3 doesnot affect the relative crystallization of the crystalline phasesas similar to the case of Al2O3.

(2) Densification and Dielectric PropertiesDensification behavior of the AL and BT samples are illus-trated in the TMA curves of Fig. 6. The curves show thecharacteristics of a typical path that can be highlighted withits abrupt decrease corresponding to the progress of densifi-cation driven by a low softening glass, and then with a sud-den stop of densification at a higher temperature. There areslight differences in the onset densification temperature anddensification routes, depending on the filler type and content.Particularly, the BT samples showed earlier paths to densifi-cation, compared to the AL samples. As expected, a lowercontent of filler showed slightly earlier densification and alarger dimensional change. In other words, a higher contentof glass must allow full densification with a higher value ofshrinkage at earlier temperatures. For example, the densifica-

tion of AL30 sample started at ~631°C and then ended at~692°C, which are lower than ~639°C and ~706°C, respec-tively, for the AL50 sample.

Figure 7 shows variations in the actual values of bulk den-sity and linear x–y shrinkage for the AL and BT samplesfired at 850°C, according to the content of filler. Both shrink-age and density values were found to decrease with increasein the relative content of the fillers. Figure 8 shows theselected fracture surfaces of the corresponding samples.Microstructural characteristics tend to match well with thetendency of shrinkage and density variations in each case.Densified microstructure, specifically in the AL30 and BT30samples, were found to consist of various crystalline phasesthat are well-dispersed in the residual glass matrix. The sam-ples having the lower content of filler (or the higher contentof glass) showed the better characteristics of microstructure,that is, less porosity. In general, the degree of densificationdepends on the softening temperature of glass in a compositeof glass and refractory ceramic.13,18,19 Earlier softening ofglass is preferred for full densification at lower temperatures,while densification is influenced by the temperature of crys-tallization. Earlier densification prior to extensive crystalliza-tion is typically preferred as densification is difficult toproceed once significant crystallization happens.10,16 TheBT50 sample demonstrated more residual enclosed pores onthe surface compared to the AL50 sample (Fig. 8). As mea-sured, the porosity of ~1% for the AL30 and BT30 samplesincreased to ~9% for AL50 and ~16% for BT50. The exten-sive earlier crystallization in the case of BT samples mayhamper full densification if there is no sufficient glass pres-ent.

However, such discussion on the relation of crystallizationto densification behavior could not be exactly correlated

Fig. 5. Variations of peak intensity of each crystalline phaserelative to (a) the intensity of BaTiO3 filler and (b) the summation ofpeak intensities of all crystallized phases, as a function of BaTiO3

content.

Fig. 6. Densification behavior of the (a) AL and (b) BT samples.

Fig. 7. Variations of fired density and linear shrinkage for the (a)AL and (b) BT samples fired at 850°C.

Fig. 8. Microstructures of fracture surfaces of the fired samplescontaining different contents of Al2O3 and BaTiO3 filler.

Barium Neodymium Titanium Borate Glass-Based High k Dielectrics 3

when the crystallization temperature obtained from DTAand the shrinkage offset temperature from TMA are com-pared in this study. For example, the BT40 sample demon-strated the shrinkage offset temperature of ~670°C [Fig. 6(b)]even before the crystallization onset temperature of ~730°C[Fig. 1(a)]. This temperature discrepancy is believed to comemainly from the different heating rates used for the measure-ments of TMA and DTA. The TMA was conducted at 5°C/min whereas DTA at 10°C/min. In particular, crystallizationtemperature obtained from DTA varies significantly withheating rate. A higher heating rate generally tends to show ahigher crystallization temperature. Our previous work,14 asan example, showed the crystallization peak temperature of~880°C at 3°C/min and ~935°C at 7°C/min in the DTA runof a highly crystallizable 15ZnO·25Nd2O5·60B2O3 glass com-bined with 50 wt% alumina. In this regard, similar to otherhighly crystallizable glasses, the densification of glass studiedherein is also assumed to be affected by crystallization.

Figure 9 shows the variations in dielectric constant andloss measured at 100 kHz with increase in the content of fil-ler. Dielectric constant depended on the relative content ofthe fillers, as expected, but it showed the opposite cases.Increasing the content of Al2O3 tends to decrease dielectricconstant gradually from ~22 to ~9.5, which is reasonablewhen considered the gradual decrease of fired density withdecrease in the content of glass and the low dielectric con-stant (k ~ 10) of Al2O3 itself. As the relative contents of crys-talline phases, NdBO3, Nd2Ti4O11, and BaNd2Ti5O14, areconsistent regardless of Al2O3 content [Fig. 4(b)], dielectricconstant must depend mainly on the level of densificationand the content of Al2O3 filler.

On the other hand, the increase of BaTiO3 led to theincrease of dielectric constant from ~39 to ~48 at 100 kHzeven with the accompanying decrease of density. The increaseof dielectric constant seems to be associated with the relativecontents of crystalline phases present at each filler level. Aswell as the larger contribution of the BaTiO3 filler itself, theBaNd2Ti5O14 phase having k ~ 77.620 is likely responsible forthe increase of dielectric constant. The dielectric loss valueswere very stable over the compositional ranges regardless ofthe type of filler, which can be defined as having <0.01. Eventhough the loss values are quite low, the gradual decrease ofthe loss was clearly found with increase in the content ofeither filler. It is interesting to note the decreasing tendencybecause a higher porosity generally induces a higher dielectricloss.14 It is assumed that dielectric loss of the current mate-rial system is more sensitive to the crystalline phases at eachfiller level than the level of porosity. A larger decrease of theloss in the case of Al2O3 may also indicate the influence oflow loss Al2O3 itself.

IV. Conclusions

The nonconventional borate glass of 20BaO·15Nd2O3·35TiO2·30B2O3 was clearly evidenced as a promising candi-

date for the high k LTCC applications. Incorporation of twotypical fillers, Al2O3 and BaTiO3, through the intimate mix-ing with the glass successfully generated k ~ 20 and k ~ 40materials at 850°C, respectively. The Al2O3 filler led to someunexpected crystalline phase of NdBO3 and Nd2Ti4O11,along with BaNd2Ti5O14 that was observed in the crystallizedpure glass. On the other hand, the BaTiO3 filler did not cre-ate third crystalline phase, but influenced crystallizationstrongly by inducing earlier crystallization. The type andcontent of the fillers were understood as critical factors incontrolling densification and final dielectric properties as dif-ferent interactions between the glass and filler were induced,especially in the process of crystallization.

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Fig. 9. Dielectric constant and loss tangent of the (a) AL and (b)BT samples fired at 850°C.

4 Journal of the American Ceramic Society—Lee et al.