Multilayer resonators and a bandpass filter fabricated from a novel low-temperature co-fired ceramic

Journal of Electronic Materials, Mar 2002 by Jantunen, H, Leppavuori, S, Turunen, A, Uusimaki, A

The main objective of this study was to make components from a novel low-- loss, low-temperature Co-fired ceramic (LTCC) dielectric, which was also compatible with a high-conductivity silver paste. The multilayer-component fabrication procedure is presented together with a composition for a tape-casting slurry, choice of conductor paste, and LTCC process parameters. A good Q factor, >100 at 2 GHz, using the novel material system has been achieved for (gamma)/2 resonators operating in the frequency range 1.7-3.7 GHz. An excellent frequency response for a 2 GHz bandpass filter has also been achieved; the insertion losses in the passband were less than -2 dB (bandwidth 60 MHz) and the attenuation more than 25 dB in the stopband located 190 MHz higher.

Key words: Delectric material, LTCC, multilayer component, Q factor, resonator, bandpass filter

(Received June 15, 2001; accepted November 14, 2001)

INTRODUCTION

The increasing demands of the telecommunications industry to miniaturize radio frequency (RF) components and to further develop integration are being met by low-temperature Co-fired ceramic (LTCC) technology.1,2 The advantages offered by ceramics to reduce the size of RF components have been exploited for several decades. The most familiar example is the dielectric resonator filter made from ceramics, which requires firing at high temperatures, typically ~1350 deg C. Popular ceramics for these applications are based on the Ba-Nd-Ti-O or Zr-Sn-- Ti-O systems. After the ceramic shape is fired, it is patterned with either silver or copper pastes fired on in a separate operation.5 Another approach has been to exploit high-temperature Co-fired ceramic technology when the firing temperature of the dielectric necessitates the use of the refractory metals tungsten and molybdenum for the electrodes.2,6 Although these technologies are highly successful for the construction of RF devices and packages,3,6-10 none is suited to the fabrication of complex multilayer structures incorporating highly conductive metals, such as low melting-point silver or oxidizable copper. Recently, the development of the LTCC technology has enabled such structures to be made.

In LTCC-based multilayer technology, the ceramic sheets are formed by tape casting from which the required shape, together with the vias, is punched out. The vias are filled, and the required patterns are usually screen printed on using a conductive paste. The sheets are then stacked and laminated by appropriate heating and pressing. The laminate is then fired. This technology demands that the materials meet the following requirements. First, the ceramic must have dielectric properties suited to the intended application, namely, suitable permittivity, Er, low dissipation factor (DF) (or high fa factor), and, usually, a temperature coefficient of the resonant frequency, (tau)^sub f^ ~ 0 ppm/K.2,11 Second, the ceramic should sinter to full density below the melting points of the conductive metals." In the case of silver and copper paste, the ceramic sintering temperature would need to be below approximately 950 deg C. Third, and very importantly, the conductive pastes and ceramics must be compatible at the sintering temperature so that there is good ceramic-metal adhesion but not excessive penetration of the ceramic by the metal.1,13

The last decade has seen the appearance of many commercial LTCC material systems produced e.g. by DuPont, Ferro, and Heraeus with compatible conductive and resistive pastes. The electrical and thermomechanical properties have been well reported.11,14,15 This development has accelerated their exploitation, and, although RF filters have already been constructed,1,16,11 improved ceramics having lower losses are still required.

In our earlier studies,18,19 a novel LTCC material based on the MgCaTiO^sub 3^-ZnO-SiO^sub 2^-B^sub 2^O^sub 3^ system was described. To optimize the composition, a dry-pressing compaction method for sample preparation was employed and the dielectric and thermomechanical properties investigated. It was also found that this composition could be made in a straightforward way simply by mixing all the oxides and sintering without any prior glass-preparation route.2

ACKNOWLEDGEMENT

One of the authors (H.J.) acknowledges the financial support of Tauno T/inning Foundation and Nokia Foundation.

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