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Materials Science and Engineering B 161 (2009) 198–201

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

abrication of high thermal-conductive silicon nitride ceramics with lowielectric loss

iroyuki Miyazaki ∗, Yu-ichi Yoshizawa, Kiyoshi Hiraoational Institute of Advanced Industrial Science and Technology (AIST), Anagahora 2266-98, Shimo-shidami, Moriyama-ku, Nagoya 463-8560, Japan

r t i c l e i n f o

rticle history:eceived 30 May 2008eceived in revised form 14 October 2008ccepted 21 November 2008

eywords:ilicon nitrideielectric propertiesicrowave

a b s t r a c t

In order to develop high thermal-conductive silicon nitrides with low dielectric loss, three silicon nitrideswith different compositions were pressurelessly sintered with Yb2O3 and SiO2 as sintering additives andthe effect of both composition and annealing on the microwave dielectric properties were studied. Whenthe Yb2O3/SiO2 ratio in the starting powder increased from 0.33 to 1.3, the thermal conductivity of thesamples increased from 46 W m−1 K−1 to∼100 W m−1 K−1, while the tan ı of the sample at 2 GHz decreasedsignificantly from 11.5 × 10−4 to 1.4 × 10−4. The crystallization of the intergranular glassy phases in theas-sintered specimens was observed for all the samples after thermal treatments at 1300 ◦C for 24 h.However, the changes in tan ı were different depending on the compositions; the tan ı of the samplewith the lowest Yb O /SiO ratio reduced to half, that doubled for the sample with the intermediate

rystallization

ntergranular glassy phasehermal conductivity

2 3 2

Yb2O3/SiO2 ratio and that remained almost constant in the case of the highest Yb2O3/SiO2 ratio. Theseresults suggested that the tan ı of the glassy phases as well as the crystalline phases depend greatly onthe composition and that the tan ı of the glassy phases of a certain composition could be lower than thatof the crystalline phases. It was revealed that selecting the proper composition for silicon nitrides couldcombine both high thermal conductivity and low dielectric loss even though the intergranular glassy

y crys

phase was not completel

. Introduction

Advanced electrical insulators with high thermal conductivityre needed for structural components in the semiconductor pro-ess equipments such as microwave devices since an improvementn thermal conductivity not only prevents the failure due to ther-

al stresses but also assures the even temperature profile on theubstrate which is necessary for the homogeneous quality of siliconafers [1]. Silicon nitride (Si3N4) is one of the attractive materials

or such applications because of its superior mechanical reliabilitys well as good thermal conductivity and high electrical resistance2]. For example, Kitayama et al. [3,4] reported that adding Yb2O3r Y2O3 as the sintering aids was favorable for the enhancement ofhermal conductivity of the hot-pressed Si3N4 and that the increasen molar ratio of Y2O3 to SiO2 up to 1.3 resulted in the high thermalonductivity of 100 W m−1 K−1.

Both permittivity (ε′) and dielectric loss (tan ı) of the ceram-

cs are also important factors for the structural components in

icrowave devices since the dielectric absorption of microwaveower results in inhomogeneous temperature profiles on the sub-trates and can damage the products. There are some researches

∗ Corresponding author. Tel.: +81 52 736 7486; fax: +81 52 736 7405.E-mail address: h-miyazaki@aist.go.jp (H. Miyazaki).

921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2008.11.029

tallized.© 2008 Elsevier B.V. All rights reserved.

on the microwave dielectric performances of sintered Si3N4 as wellas chemical vapor deposited (CVD) Si3N4 [5–13]. Theses literaturehave demonstrated that the dielectric properties varied noticeablydepending on both sintering techniques and sintering additives. Inour previous study, the effect of thermal annealing on the dielectriccharacteristics was investigated using two types of Si3N4 sinteredwith Yb2O3 and SiO2 as sintering additives [14]. It was found thatthe crystallization of the intergranular glassy phase was effectiveto reduce the tan ı. However, the Yb2O3/SiO2 ratios for these sam-ples were as low as 0.3, which was not preferable for high thermalconductivities. Few studies have been conducted on the dielectricperformance of the high thermal-conductive Si3N4. It was reportedthat the crystalline Yb4Si2O7N2 phase appeared in the as-sinteredSi3N4 with high Yb2O3/SiO2 ratio [15,16]. Then it seems possible toproduce high thermal-conductive Si3N4 with low tan ı by increas-ing the Yb2O3/SiO2 ratio, since the thermal conductivity can beincreased at high Yb2O3/SiO2 ratio and the intergranular glassyphase which deteriorates the tan ı of the as-sintered samples canbe diminished by the crystallization of the Yb4Si2O7N2 phase.

In this study, three Si3N4 ceramics with various Yb2O3/SiO2

molar ratios were fabricated and heat-treated subsequently tocrystallize the intergranular glassy phases. The effect of the compo-sitions of the samples on the changes in dielectric properties wasstudied and discussed in conjunction with the phase evolutionsobserved in annealed samples.

H. Miyazaki et al. / Materials Science and Engineering B 161 (2009) 198–201 199

ystem

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Yb2O3/SiO2 ratio and eventually approached 100 W m−1 K−1 whenthe ratio became 1.3, which is consistent with the previous report[4]. Fig. 2 shows XRD patterns of the as-sintered samples. Besides�-Si3N4 major phase, only slight existence of �-Yb2Si2O7 phase was

Fig. 1. Composition of this work’s specimens in the ternary s

. Experimental procedure

The starting powders used in this study were �-Si3N4 powderSN-E10, Ube Industries, Ltd., Japan), SiO2 (1000A, Ube Industries,td., Japan) and Yb2O3 (Nihon Yttrium Co. Ltd., Japan). Three com-ositions with various Yb2O3/SiO2 ratios were chosen as shown inable 1. The oxygen content in the Si3N4 raw powders was taken intoonsideration when calculating the composition. The total amountsf the Yb2O3 and SiO2 were kept at ∼12 mol% to produce the similarolume fraction of the secondary phases. Fig. 1 indicates the posi-ions of the three starting compositions on the Si3N4–Yb2O3–SiO2hase diagram [17]. Each composition lies in the different triangles.Hereafter, a specimen with m mol% of Yb2O3 and n mol% of SiO2 iseferred as mYnS.) The starting powders were mixed by planetaryilling in methanol using Si3N4 balls and a Si3N4 pot for 1 h. The

lurry was dried, and then passed through 60 mesh sieve. The mixedowders were formed into compacts of about �15 mm × 100 mm inize under an isostatic pressure of 450 MPa. The relative density ofhe compacts increased from 50% to 56% with the Yb2O3/SiO2 ratio.hese compacts were set in a BN crucible with BN packing powdernd sintered at 1900 ◦C for 3 h in a 0.9 MPa N2 atmosphere usinggraphite resistance furnace. Heat treatments of the as-sintered

odies were performed subsequently at 1300 ◦C for 24 h using theame equipment, since it has been reported that the devitrifica-ion of intergranular glassy phase took place between 1200 ◦C and400 ◦C for 4–24 h [18–20].

The crystal phases in the specimens were identified by X-rayiffraction (XRD). For the measurements of dielectric properties,pecimens with the dimensions of 1.5 mm × 1.5 mm × 80 mm wererepared from the center part of each sample. Dielectric con-tant (ε′) and dielectric loss (tan ı) were measured at 2 GHz byhe perturbation method using a cavity resonator and a vectoretwork analyzer (HP8722ES). Disk specimens (10 mm in diame-er and 3 mm thick) were also machined from both sintered andnnealed bodies. Both sides of the pellet samples were coated with60 nm thick layer of gold using a sputter coater (SC-701AT, Sanyulectron Co. Ltd., Tokyo), followed by a subsequent graphite spray-oating (Bonny-phite, Nippon Graphite Industries, Ltd., Japan) toncrease the amount of energy absorbed. The thermal diffusivity ofhe samples was measured using the laser-flash method. Thermalonductivity was obtained by the product of the thermal diffusivity,ensity and specific heat of the samples. In this study, a constantalue of the specific heat, ∼0.68 J g−1 K−1 was used for the calcula-

ion [21]. The densities of the as-sintered bodies and the annealedamples were measured using the Archimedes technique using theachined samples. Relative densities were evaluated using theo-

etical density calculated from an arithmetic mean of the startingomposition.

of Si3N4–Yb2O3–SiO2 [17]. (a) 3Y9S, (b) 5Y7S, and (c) 7Y5S.

3. Results

3.1. As-sintered specimens

The relative densities of both 3Y9S and 7Y5S as-sintered andmachined specimens were above 97%, while that of the 5Y7S samplewas as low as 91% (Table 1). The weight losses during the sinteringwere relatively small and similar for all the samples (1.7–2.5%), indi-cating that the total composition of the as-sintered sample mightshift toward Yb2O3 rich side slightly due to the preferential evapo-ration of SiO2. The shrinkage of the 5Y7S sample was lowest amongthe samples, which was the main reason of the poor densifica-tion of the 5Y7S sample. The thermal conductivity of the samplesis shown in Table 2. The thermal conductivity increased with the

Fig. 2. X-ray diffraction patterns of as-sintered Si3N4 samples with differentYb2O3/SiO2 ratios.

200 H. Miyazaki et al. / Materials Science and Engineering B 161 (2009) 198–201

Table 1Powder compositions, both weight losses (�w/w0) and shrinkages (�l/l0) during the sintering and relative densities of both as-sintered and annealed silicon nitride samplessintered with Yb2O3 and SiO2 as sintering additives.

Sample Yb2O3 (mol%) SiO2 (mol%) Yb2O3/SiO2 �w/w0 (%) �l/l0 (%) Relative density (%)a

As-sintered Annealed

3Y9S 3.0 9.0 0.33 2.5 19.2 100 1005Y7S 5.0 7.0 0.71 2.0 15.6 91.3 91.47Y5S 7.0 5.4 1.30 1.7 16.8 97.6 97.2

a Relative density was obtained using the machined sample.

Table 2Thermal conductivity and dielectric constant at 2 GHz of both as-sintered and annealed silicon nitride samples with different Yb2O3/SiO2 ratios.

Sample Yb2O3/SiO2 Thermal conductivity (W m−1 K−1) Dielectric constant

As-sintered Annealed As-sintered Annealed

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ples. The slight change in the dielectric constant indicated that thedielectric constant is not so susceptible to the kind of constitutivephases. By contrast, the tan ı for the 3Y9S sample was diminishedby half to 5.4 × 10−4 by the annealing, whereas that for 5Y7S sam-

Y9S 0.33 46Y7S 0.71 75Y5S 1.30 97

dentified for the sample sintered with 3 mol% Yb2O3 and 9 mol%iO2 (3Y9S sample), which was consistent with the phase diagramFig. 1). The background level around 2� = 30◦ was relatively high,hich implied that the additives formed intergranular glassy phase.

n contrast, distinct Yb4Si2O7N2 peaks appeared in the 5Y7S sam-le and they became stronger in the 7Y5S sample. Table 2 showshe dielectric constant (ε′) of each sample as well. The variation of′ among the samples was relatively small, 6.7–7.6. By contrasts,he dielectric loss (tan ı) decreased significantly from 11.5 × 10−4

o 1.4 × 10−4 with an increase in the Yb2O3/SiO2 ratio (Fig. 3).

.2. Annealed specimens

Fig. 4 shows XRD patterns of the samples after the annealing.he �-Yb2Si2O7 peaks disappeared but both strong �-Yb2Si2O7eaks and weak Yb2SiO5 peaks appeared in the 3Y9S sample byhe annealing. By contrast, the intensity of Yb2SiO5 peaks whichhowed up in the annealed 5Y7S sample was larger than that of-Yb2Si2O7 peaks. Almost the same intensity was sustained for theb4Si2O7N2 phase after the annealing of 5Y7S sample. In the casef 7Y5S sample, the Yb4Si2O7N2 peaks became larger and the small

mount of Yb2SiO5 was observed after the heat treatment. Thus thelassy phase in the as-sintered samples was crystallized by the ther-al annealing for 24 h. All these observed crystalline second phases

uggest that SiO2 content preferentially evaporated, which shiftedhe composition of the samples toward Yb2O3 rich side slightly.

ig. 3. Loss tangent of the Si3N4 samples with different Yb2O3/SiO2 ratios beforend after the annealing.

46 7.6 7.676 6.7 6.698 7.6 7.6

The density, thermal conductivity and the dielectric con-stant of each sample hardly changed after the heat treatment(Tables 1 and 2). The reason of the negligible change in thermalconductivity regardless of the crystallization of the grain-boundaryphase can be explained as follows: Kitayama et al. [22] have shownthat the decrease in thermal conductivity due to the increment ofgrain-boundary phase was relatively small as compared with theeffect of the grain-boundary film thickness. It was reported that thethickness of grain-boundary film was the same for the Yb2O3-fluxedsintered silicon nitride, regardless of the heat treatment history[19]. Therefore, it is rational to expect that the crystallization of glasspocket has little influence on the thermal conductivity of the sam-

Fig. 4. X-ray diffraction patterns of annealed Si3N4 samples with differentYb2O3/SiO2 ratios.

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H. Miyazaki et al. / Materials Scienc

le was doubled from 3.1 × 10−4 to 5.9 × 10−4 (Fig. 3). In the casef 7Y5S sample, the change in tan ı was little (1.4–1.7) × 10−4 andemained minimum tan ı level among the samples.

. Discussion

The intrinsic tan ı value of pure Si3N4 crystal was estimated toe as low as ∼10−4 since the tan ı of the CVD Si3N4 at 35 GHz was× 10−4 [5]. The changes in grain size as well as grain morphol-gy due to the annealing was expected to be negligible since thennealing temperature was not enough to promote the dissolutionnd precipitation of Si3N4 through intergranular liquid phases. Its rational to suppose that the contribution from the Si3N4 grainso the observed large changes in tan ı should be small. Therefore,he fact that the behaviors of tan ı due to the heat treatments wereifferent among the samples is attributable to the microstructuralvolutions of the secondary phases during the annealing.

One of the possible explanations for the complex phenomenons that the discrepancy in tan ı of each constitutive phase maye enormous. The large decrease in tan ı for 3Y9S sample due tohe annealing was associated with the crystallization of the glassyhase to Yb2Si2O7 phase, which is consistent with our previouseport [14]. This suggests that the tan ı of the glassy phase in thes-sintered 3Y9S sample is much larger than that of the Yb2Si2O7hase. By contrast, the tan ı for 5Y7S sample increased with theppearance of Yb2SiO5 phase, indicating that the tan ı of the glassyhase in the as-sintered 5Y7S sample is smaller than that of Yb2SiO5hase. It seems reasonable to expect that the tan ı of both Yb2SiO5nd Yb2Si2O7 phase is almost the same since both annealed sam-les showed the similar tan ı values. Consequently, it is likely thathe tan ı of the glassy phase in the as-sintered 3Y9S sample is mucharger than that of the glassy phase in the as-sintered 5Y7S sample.he glassy phase in the as-sintered 7Y5S sample is also supposedo possess a relatively small tan ı value since the as-sintered 7Y5Sample showed the minimum tan ı.

It is well known that the physical properties of the oxyni-ride glass depend on the composition considerably. For example,ampshire and Pomery [23] reported that the viscosity of the–Si–Al–O–N glass increased by >2 orders of magnitude when 18quivalent percent nitrogen is substituted for oxygen. Clarke and Ho5] reported that tan ı of the hot-pressed Si3N4 was closely relatedo the viscous flow of intergranular glassy phase. They suggestedhe following equation to correlate the tan ı with the viscosity:

an ı ∝ nq2

�(1)

here n is the number of mobile ion, q is the charge of the ion,is the viscosity. Thus, it is possible that the tan ı of the oxyni-

ride glassy phase could be varied significantly by the composition.horp et al. [24] reported that the tan ı of Y–Al–Si oxynitride glassest 1.6 GHz was varied between 14 × 10−3 and 32 × 10−3 by the sub-titution of oxygen by nitrogen. Although the composition of glassyhase in our samples is different from these samples, it is likely thatan ı of Yb–Si oxynitride glass in this study is depending on theomposition noticeably. The ranking of tan ı value for crystallinehases can be also estimated by the same approach. The dielec-ric loss of the annealed 7Y5S sample was smaller than those ofther annealed samples, indicating that the tan ı of the Yb4Si2O7N2rystalline phase is smaller than those of Yb2SiO5 and Yb2Si2O7

hases.

The results in this study revealed that the fabrication of the highhermal-conductive Si3N4 ceramics with low tan ı of ∼10−4 coulde possible without the heat treatment when the composition of

ntergranular glassy phase was controlled properly, which is valu-

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Engineering B 161 (2009) 198–201 201

able from the industrial point of view, since the productivity ofthe top quality silicon nitrides for the microwave applications canbe improved significantly as compared with the previous processusing long-time annealing [14].

5. Conclusions

In order to fabricate high thermal-conductive silicon nitridewith low dielectric loss, three different types of Si3N4 ceramics withvarious Yb2O3/SiO2 ratios were prepared and the effect of the com-position on the microwave dielectric properties at the frequency of2 GHz was studied. The Yb2O3/SiO2 ratio was increased progres-sively from 0.33 to 1.3. The thermal conductivity of the samplereached ∼100 W m−1 K−1 when the Yb2O3/SiO2 ratio increased to1.3. The variation in dielectric constant, ε′ among the as-sinteredsamples was small despite the difference in the composition.The tan ı of the sample with the lowest Yb2O3/SiO2 ratio wasrelatively large, ∼11.5 × 10−4, whereas that of sample with thehighest Yb2O3/SiO2 ratio was much smaller, ∼1.4 × 10−4. The heattreatment of the samples caused the different behaviors in tan ıdepending on the compositions, whereas ε′ experienced little vari-ation. The tan ı of the sample with the lowest Yb2O3/SiO2 ratio wasreduced by half after the annealing, which was accompanied by thecrystallization of intergranular glassy phase into Yb2Si2O7 phase.By contrast, a considerable increase in tan ı was observed for themedium-ratio sample in which Yb2SiO5 phase appeared after theannealing. In the case of the sample with the highest Yb2O3/SiO2ratio, the tan ı remained the minimum value regardless of the fur-ther increase in the intensity of the Yb4Si2O7N2 XRD peaks. Theseresults suggested that tan ı of intergranular glassy phase dependedsignificantly on the composition so that tan ı of the as-sinteredsample could be diminished if the glassy phase has high viscos-ity. It was clarified that the high thermal-conductive Si3N4 withlow dielectric loss could be produced without the heat treatmentby controlling the composition of sintering additives.

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