A Multilayer Low Pass Filter Fabricated by Ferrite and Ceramic Cofiring System

A Multilayer Low Pass Filter Fabricated by Ferrite and Ceramic Cofiring System

Based on LTCC Technology

Yuanxun Li1, Yingli Liu1, Huaiwu Zhang1, Likun Han2, Zongbao Yang3

1State Key Laboratory of Electronic Thin Film and Integrated Devices

University of Electronic Science and Technology of China Chengdu, 610054, China

2Institute of Astronautics and Aeronautics

University of Electronic Science and Technology of China Chengdu, 610054, China

3Integrated microcircuit company of Anhui Province, Hefei, 230088, Anhui, China

Abstract

LTCC (Low Temperature Co-fired Ceramics) has been become the key technology of packaging for the integrated of passive components due to its higher performance of thermal sink, reliability and plays an important role in increasing higher frequency, decreasing the loss, minimize the volume, etc. This paper mainly focuses on the design and fabrication of low pass filter based on ferrite and ceramic cofiring system by LTCC technology. Firstly, the match conditions of co-firing characteristics for ferrite and ceramic cofiring system were carefully studied by TMA measurement and the excellent shrinkage controlling was obtained. The low-pass filters with cut-off frequency at 120MHz were fabricated using Co/Ti doped barium ferrite and ULF140 material as the dielectrics to further validate the circuit model and the samples were tested with high consistence with the simulated data.

I. Introduction

With the current explosive growth of communication technologies and a wide variety of applications being found for high density packaging of electronic components, LTCC (Low Temperature Cofired Ceramics) with materials that possess different characteristics have been rapidly developed, due to its greater multifunctionality, sub-miniaturization and higher performances [1-3]. Especially, the multilayer ceramics based on ferrites and ceramics are absorbing more and more interest for their excellent magnetic and electric properties [3-7]. For example, as Figure 1 illustrates the ferrite/ceramic co-firing system for the LTCC applications. In Fig. 1 (a), an embedded LTCC MCM (multichip module) can be constructed. The ferrite tape layers could be used for embedded inductors and transformers and the high dielectric constant tape layer for embedded capacitors. Fig. 1 (b) describes the integration of passive components for high density packaging.

Whereas the considerable attentions for their applications have been paid to, the sintering problems in these multilayer materials have not yet been thoroughly resolved [8-9]. And the co-fired mismatch of these ferrites and ceramics is often serious. This has greatly affected the reliability and electronic properties of the multilayer devices.

In this paper, we proposed a manufacturing method of low pass filters (LPFs) based on the ferrite and ceramic cofiring system by LTCC technology. By employing the proposed design method, the multilayer chip LPFs configuration can be made more compact and flexible. The designed chip-type LPFs were realized by implementing the multilayer chip inductors and capacitors. The inductors and capacitors were implemented using Co/Ti doped M type ferrites and ceramic material respectively. The dielectric constant and loss tangent of the ceramics were chosen to be 14 and 0.0015. The permeability and cut-off frequency of the ferrites were 13.5 and beyond 1GHz. The designed LPFs were fabricated by 20um thickness films and constructed by LTCC technology with excellent consistency between simulation and fabrication with the merits of simple structures, sub-miniaturization and low cost for wide applications.

(a) (b)

Fig. 1 The MCM module (a) and integration of passive

components (b) constructed by ferrite/ceramic co-firing

system

II. Circuit and Stimulation

Circuit Model

The LPFs were constructed by traditional three-order Butterworth filter configuration based on the lumped-element L-C filter circuit. The typical circuit for LPFs was shown in Fig. 2 which is composed of inductors and capacitors by circuit stimulation.

Fig. 2 The circuit model for low pass filters Component Implementation

The physical prototype for LPFs was shown in Fig. 3. After optimized, the characteristics of the filters simulated can

be achieved and presented in Fig. 4.

Fig. 3 The physical prototype for band-pass filters

(a) (b)

Fig. 4 The stimulated results of low-pass filters by HFSS software with the cut-off frequency at 120MHz (a) for the parameter S 21 center frequency at 1.8GHz, (b) for the

parameter S 11

From the response of three dimension configurations in Fig. 4., a cut off frequency at 120MHz,more than 20dB of the attenuation above 360MHz frequency band can be achieved, which will satisfy the performances of LPFs.

Fig. 5 shows the graphics of silk screen and relative sizes for one unit which will guarantee the good connection for each layer and formation of capacitors and inductors with the packaging size of 0805.

(a)

(b)

Fig. 5 Silk screen (a), (b) for the fabrication of LPFs by LTCC technology with the packaging size of 0805

III. Experimental

For LTCC process technology, the compatibility of

different materials with respect to shrinkage, thermal expansion coefficient and chemical compatibility must be considered. And the different shrinkage rate of the fired specimens during co-firing make the materials distort, which cause deviation of the designed component. A. Casting

After careful casting process, the diaphragm with good quality of ceramic and ferrite were shown in Fig. 6.

(a) (b)

Fig. 6 The diaphragms made by ULF 140 ceramics (a) and M

type barium ferrites (b)

B. Ferrite and Ceramic Cofiring Behavior

Fig. 7 shows that the sintering behaviors of the barium ferrites and ceramics. The shrinkage profiles of ferrites gives an outstanding resemblance with that of the ceramics, which demonstrates a good co-firing matching condition between these two materials.

Fig. 7 The densification characteristics of the ferrite and

ceramic

Fig. 8 gives back-scatter morphology near the interface between ferrites and ceramics in a multilayer sample prepared by tape casting and sintered at 920°C for 6h. Alternate heterogeneous layers are in good connection, with no evidence of cracks or delimitations. Both ferrite and ceramic layer show a dense microstructure and the corresponding grains grow normally.

Fig. 8 The cross-sectional microstructure of a bi-layer sample

C. The Manufacture of LPFs by LTCC Technology

Based on the ferrite and ceramic cofiring system, the laminated LPFs samples were fabricated by the conventional LTCC technology process with the outline dimension of 2.0mm×1.2mm×0.9mm shown in Fig. 9.

Fig. 9 Experimental prototype for the samples

D. The Measurements of the Performances for LPFs Samples

The measurements were carried on by Agilent 8722ES and the collected data was then calibrated to the desired reference plane by the thru-reflect line (TRL) technique through carefully designed calibration standards embedded in the same LTCC tie. The measured responses of the filters are shown Fig. 10. From Fig. 10, it can be clearly seen that the measured results agree with the simulated data basically. The cut-off frequency is about 130MHz (3dB) and the attenuation beyond 30dB (360MHz~1GHz).Thus, a good coincidence between simulated and measured data is observed. The difference between the measurement and the stimulation is that the cut-off frequency is a little larger than the designed. It is believed that the deviation comes from the additional inductive and capacitive parasitic effects. On the other hand, the process of LTCC technology also brings many errors which will introduce the manufacturing inaccuracy. Firstly, during the co-firing, the organic solvent in the silver conductor cannot dispel all air bubbles, which cause the value of inductance and capacitance smaller and make the self-resonance frequency larger. Secondly, the effective value of the component cannot be controlled accurately due to the difficulties of tiny manipulation problems including the printing and laminating.

(a) (b)

Fig. 10 The testing results of low pass filter with cut-off

frequency at 120MHz (a) for S21 parameter varied with

frequency (b) for S11 parameter varied with frequency

Conclusions

The structure analysis of LTCC-based passive components

is reported for the design of a small multilayer chip LPFs

based on the ferrite and ceramic cofiring system. The LPFs

fabricated by LTCC process has small size

(2.0mm×1.2mm×0.9mm).The cut-off frequency for LPFs

samples is about 130MHz and the attenuation beyond 30dB

(360MHz~1GHz) respectively. The testing results are in a

good agreement with the simulated data which will be helpful

for the manufacture of high integrated components.

Acknowledgments

This work was supported by the Youth Fund of Sichuan

Province under Grant No 08ZQ026-013, the Foundation for

Innovative Research Groups of the NSFC under Grant No.

60721001, the Youth Fund of University of Electronic

Science and Technology of China under Grant No.

L08010301JX0725.

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