Materials Letters 65 (2011) 1092–1094

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Materials Letters

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ZrSiO4 ceramics for microwave integrated circuit applications

Jobin Varghese, Tony Joseph, M.T. Sebastian ⁎Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology (CSIR), Thiruvananthapuram 695019, India

⁎ Corresponding author. Tel.: +91 471 2515294; fax:E-mail address: mailadils@yahoo.com (M.T. Sebastia

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.01.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 November 2010Accepted 12 January 2011Available online 19 January 2011

Keywords:CeramicsDielectricsElectronic materialsMicrostructureFTIRX-ray techniques

The sintering temperature of ZrSiO4 ceramic was optimized by studying the variation of density as a functionof temperature. The dielectric properties were investigated at the radio and microwave frequencies. It hasεr=10.5, tan δ=0.0016 (at 1 MHz), εr=7.4, tan δ=0.0006 (at 5.15 GHz) and τε=225 ppm/°C (at 1 MHz).The ceramic exhibited a negative coefficient of thermal expansion (CTE) of −2.4 ppm/°C in the temperaturerange of 30–800 °C.

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1. Introduction

Microwave Monolithic Integrated Circuits (MMIC) and RadioFrequency Integrated Circuits (RFIC) are the foundation of today'stelecommunication and sensor systems. The downscaling of silicontechnologies as per Moore's law [1] has resulted in device perfor-mance close to the III–V technologies with cut-off frequenciesexceeding 400 GHz. Materials used as microwave substrates inmicroelectronic devices have to fulfill diverse requirements like lowdielectric loss, low relative permittivity, good temperature stability,high thermal conductivity and low coefficient of thermal expansion[2]. Low relative permittivity minimizes capacitive coupling as well assignal delay and low dielectric loss reduces signal attenuation alongwith better device performance [3,4]. The silicates, in general, arepredominantly covalently bonded. This restricts the rattling of atomsresulting in low dielectric loss and low relative permittivity. In thepresent work we report the radio andmicrowave dielectric propertiesas well as thermal expansion and temperature stability of relativepermittivity of ZrSiO4 ceramics.

2. Experimental

The high purity ZrSiO4 powder of particle size 44 μm (99.9%Aldrich Chemical Company, Inc., Milwaukee, WI, USA) was groundwell in an agate mortar and pestle to reduce the particle size to about1 μm. It was then mixed with solution of 4 wt.% of polyvinyl alcohol(PVA) (MW=22000, BDH Lab Suppliers, England). The resulting

mixture was dried and ground. The powder was then pressed intocylindrical disks of 11 mm in diameter and ~1.5 mm thickness byapplying a pressure of about 150 MPa for the radio frequencymeasurements. A rectangular sheet of dimensions 50×50×1 mmwas prepared for microwave measurements by applying the samepressure. These compacts were sintered in the temperature range of1250–1600 °C/4 h. The sintering temperature was optimized for thehighest density and best dielectric properties. The crystal structureand phase purity of the powdered samples were studied by the X-raydiffraction technique using Cu–Kα radiation (Philips X'pert PRO MPDXRD; Philips, Eindhoven, the Netherlands) operated at 40 kV and30 mA. The microstructures of the thermally etched sintered sampleswere studied using scanning electron microscope (JEOL-JSM 5600 LV,Tokyo, Japan). The bulk densities of the polished samples weremeasured using Archimedes method. For IR transmission measure-ments (Shimadzu Nicolet Impact 400X FTIR Spectrometer), thesamples were prepared by mixing ZrSiO4 ceramic powder with KBr(potassium bromide) and pressed into pellet under a pressure of~300 bar. FTIR spectrum was recorded on a Nicolet 400X spectrom-eter with resolution of 2 cm−1, accumulating 200 scans. Thedielectric properties at 1 MHz were measured using a LCR meter(Hioki 3532-50 LCR HiTESTER, Nagano, Japan). The microwavedielectric properties such as relative permittivity and dielectric losswere measured by Split Post Dielectric Resonator (SPDR) method [2]using a Vector Network Analyzer (Agilent 8753 ET, Palo Alto, CA).The variation of relative permittivity of ZrSiO4 at 1 MHz was studiedin the temperature range of 25 to 70 °C. Sintered cylindrical samplesof diameter 11 mm and height 8 mm were used to measure thecoefficient of thermal expansion (CTE) of the ZrSiO4 ceramic using adilatometer (NETZSCH, DIL 402 PC, Germany) in the temperaturerange of 30–800 °C.

Fig. 2. The SEM images of thermally etched ZrSiO4 ceramics sintered at 1550 °C/4 h.

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1093J. Varghese et al. / Materials Letters 65 (2011) 1092–1094

3. Results and discussions

Fig. 1(a) shows the powder X-ray diffraction pattern of ZrSiO4

ceramic, sintered at 1550 °C. All the peaks are in excellent match withICDD file card No. 83-1378 for ZrSiO4, which is an indication of theabsence of any secondary phase. It has a tetragonal structure andbelongs to the I41/amd (141) space group. Fig. 1(b) shows the infraredspectrum of ZrSiO4 ceramics, sintered at 1550 °C. The strongestabsorption peak is observed at ~915 cm−1, which is ascribed to theasymmetric stretching vibration of the Si―O―Si bond [5,6]. The peaksnear 800 and 440 cm−1 are ascribed to bending modes of O―Si―Oand Si―O―Si bonds, respectively [6,7]. The shoulder observed at1020 cm−1 is assigned to the Si―O―Si asymmetric bond stretchingvibration [7]. The band appearing at 615 cm−1 in the spectra can beassociated with the Zr―O bond in the ZrO8 group [7]. The XRD and IRstudies indicate that ZrSiO4 is thermally stable at least up to 1550 °C.Fig. 2 shows the microstructure of ZrSiO4 sintered at 1550 °C/4 h. TheZrSiO4 ceramic has grains of variable sizes in the range of 1–5 μm. Thesintered ZrSiO4 ceramic has 93% density, which is in agreement withthe SEM microstructure.

Fig. 3(a) shows the variation of relative density of ZrSiO4 ceramicas a function of sintering temperature. The relative density increaseswith the sintering temperature and reaches a maximum value of 93%

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ZrSiO4 ceramic sintered at 1550 °C

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Fig. 1. (a) Powder X-ray diffraction pattern of ZrSiO4 sintered at 1550 °C (b) FTIRspectrum of ZrSiO4 sintered at 1550 °C.

at 1550 °C. Further increase in sintering temperature leads to a smalldecrease in the relative density. Fig. 3(b) shows the variation ofrelative permittivity and tan δ of the ZrSiO4 ceramic at 1 MHz as a

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Fig. 3. Variation of (a) the relative density, (b) the relative permittivity and dielectricloss as a function of sintering temperature, and (c) the variation of relative permittivitywith respect to temperature.

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30-200 -3.1200-400 -2.3400-600 -2.2600-800 -2.0

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o *

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Temperature (°C)

CTE (30 - 400 °C) = -2.8 ppm/°C

ZrSiO4 Sintered at 1550 °C

Fig. 4. Variation of linear dimension of ZrSiO4 ceramic as a function of operatingtemperature.

1094 J. Varghese et al. / Materials Letters 65 (2011) 1092–1094

function of sintering temperature. The trend of variation of relativepermittivity is similar to that of relative density. Hence the increase inrelative permittivity with sintering temperature is due to thereduction of the low permittivity phase air having εr=1. ZrSiO4

ceramic sintered at 1550 °C/4 h has a relative permittivity of 10.5. Acorrection for porosity is applied using the equation derived by Pennet al. [8]. The porosity corrected relative permittivity (at 1 MHz) isεr=11.8. The dielectric loss decreases with increase in sinteringtemperature and the minimum value obtained is 0.0016, whensintered at 1550 °C/4 h. The reduction in dielectric loss is attributed tothe enhanced densification. In the microwave frequency εr=7.4 andtan δ=0.0006 (at 5.15 GHz). The porosity corrected relative permit-tivity measured at 5.15 GHz is εr=8.3. The variation in the relativepermittivity with temperature should be small for practical applica-tions. Fig. 3(c) shows the variation of the relative permittivity ofZrSiO4 as a function of temperature. The ZrSiO4 ceramic has aτε=225 ppm/°C in the temperature range of 25 to 70 °C. Silicates aregenerally found to have positive temperature coefficient of permit-tivity (τε) [9,10].

Fig. 4 shows the variation of the linear dimension of the ZrSiO4

ceramic as a function of temperature in the range of 30–800 °C. It isfound to have a negative coefficient of thermal expansion (CTE),−2.5 ppm/°C in the measured temperature range. The values of CTEat different temperature ranges are also given in Fig. 4. It can be seenthat except at very low temperatures, the CTE remains nearly thesame. Generally electronic ceramics possess low positive coefficient ofthermal expansion (CTE) with a value b15 ppm/°C. Most of the

negative CTE ceramics have a common feature of two-coordinate(planar) M―O―M or Si―O―Si linkage in their crystal structure.These linkages are typical in metal oxides such as ZrW2O8-type solidsand zeolites, and are responsible for their negative CTE behavior [11].It is reported that phonons with low energies ~10 meV areresponsible for negative CTE of ZrW2O8 [12]. The two importantphonon modes for metal oxides are longitudinal and transverse asshown in the inset of Fig. 1(b). The longitudinal vibrational modestend to increase the bond length on heating. In contrast, the transversevibration modes can have an opposing effect on the M….M distance.The dominance of transverse modes over the longitudinal modes canbe the reason for the negative CTE of the ZrSiO4 ceramics.

4. Conclusions

The dielectric properties of ZrSiO4 were investigated at the radioand microwave frequencies. At 1 MHz ZrSiO4 has εr=10.5 and tanδ=0.0016, whereas at 5.15 GHz it exhibited εr of 7.4 and tan δ of0.0006. The ceramic has a negative coefficient of thermal expansion of−2.4 ppm/°C in the temperature range of 30–800 °C. The low relativepermittivity, loss tangent, CTE and relatively low temperaturevariation of relative permittivity indicates the possibility of thismaterial for applications as microwave substrate.

Acknowledgements

The authors are grateful to the Defense Research and DevelopmentOrganization and Council of Scientific and Industrial Research in Indiafor the research fellowships. The XRD and SEM sections of NIIST(CSIR)-Thiruvananthapuram are also thankfully acknowledged.

References

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[3] Chung DDL. Materials for electronic packaging. Washington: Butterworth-Heinemann; 1995.

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Ceram Soc 1997;80:1885.[9] Ohsato H, Tsunooka T, Sugiyama T, Kakimoto K, Ogawa H. J Electroceram 2006;17:

445.[10] Tsunooka T, AndrouM, Higashida Y, Sugiura H, Ohsato H. J Eur Ceram Soc 2003;23:

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