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Filtering Capacitor with Bi-Zn-Oxide Dielectric Embedded in a

LTCC Substrate

Jean-Pierre Ganne, Michel Paté, Richard Lebourgeois, THALES Research & Technology, Palaiseau, France Jean-Pierre Bertinet, Eddie Leleux, Jean-Pierre Cazenave, THALES Microelectronics, Châteaubourg, France

Edda Mueller, Franz Bechtold, Via-Electronic, Hermsdorf, Germany Introduction

Among different substrate technologies, LTCC (Low Temperature Co-fired Ceramic), which offers performance suitable for RF and Microwave applications, features unique capabilities with regard to passive integration. This paper reports the evaluation of a new developed dielectric material and related process used to build RF capacitor embedded in a LTCC multi-layer substrate. Technical Approach

Several solutions are used to build integrated capacitors within LTCC multilayer substrate [1]. The first option consists in printing locally a high-K paste on the standard tape (a). In the second approach, the standard LTCC tape itself is used as the dielectric of the capacitor. With this technique, thinner tapes and several layers can be stacked to increase the capacitance density [5]. Due to the poor thickness accuracy of the printing process, solution (a) is not suitable to build high tolerance capacitors and is therefore not relevant for filtering applications. Due to the better thickness control of the tape casting process, option (b) can offer better capacitance accuracy. Anyhow, the low dielectric constant of the standard LTCC materials (7.8 for DuPont 951) limits the capacitance density to 2 to 3.5pF/mm²/per layer depending on the tape thickness. This approach needs anyhow a specific material featuring a good compatibility with the standard LTCC material. Evaluation of mixed-dielectric structures to build buried capacitors in LTCC have been reported [2], [3], [4] mentioning dielectric permittivity in the 20 to 150 range. So far, anyhow, only a few high-K tape materials are commercially available from conventional LTCC material suppliers. We decided to develop and study a new material suitable for high frequency applications featuring low dielectric losses (Tanδ < 10 –3), medium permittivity in the 80-100 range and limited dielectric constant drift over temperature (less than 250ppm/°C). Development of the new high-K tape

For RF filter applications, we need a material with “Type I” dielectric properties: high dielectric constant (k = 100), low dielectric losses (Tand < 10 –3) and dielectric constant stable enough versus temperature (less than 250 ppm/K). There are many such materials, but they are generally sintered at high temperature (T > 1100°C), and consequently not suitable for integration in a LTCC multilayer substrate. After a rather extensive review of literature, we chose to work on the phase diagram Bi2O3-ZnO-Nb2O5. Several publications show there are compositions with suitable dielectric properties, and low sintering temperature [6-8]. BZN dielectric materials were produced by the standard ceramic process. In the first step, a dielectric powder is produced by high temperature reaction (calcination) from powder raw materials of pure oxides Bi2O3, ZnO and Nb2O5. The calcination reaction was studied through Differential Thermal Analysis (DTA) and Thermo-Gravimetric Analysis (TGA) and the purity of the resulting BZN phase was characterized through X-Ray Diffraction. Dielectric powder is then pressed or tape-cast in order to produce “green” samples. These green samples are then sintered at a temperature generally higher than calcining temperature (875°C in order to follow the standard LTCC process). Sintering was studied through Thermo-Mechanical Analysis (TMA). Samples of the powder were pressed in shape of cylinders and sintered at 875°C during 2 hours. The permittivity and dielectric losses were measured using the dielectric resonator method. We obtained the following results: Permittivity : 77.8 at 2.9 GHz Dielectric losses : 1.10-3 at 2.6 GHz. Thermal Coefficient: 230 ppm/°C at 1 kHz Studies on BZN tapes and multilayers stacks with BZN tapes BZN powders were mixed with solvents, plasticizers, deflocculents to prepare a slurry, which was cast into tapes through the doctor-blade technique. The mixing process of the different constituents of the slurry, their relative proportions, and the casting conditions were studied through viscosity measurements, in order to get the best

IMAPS ATW on Integrated/Embedded Passives San Jose November 2007 1

mechanical properties of the tapes. The behaviour of the tapes during firing cycles were optimized through thermal analyses (TGA, DTA and TMA). Preliminary Studies of Multilayer stacks and co-firing: Three kinds of multilayer “small” stacks (between 1cm*1cm and 5cm*5cm) of “green tapes” were prepared by pressing and co-fired: - Only BZN tapes (10 Layers) - Mixed stacks with alternate BZN tapes and DuPont LTCC DP951 dielectric tapes (10 layers) - Mixed stacks (3 DP951/1 BZN/1 DP951/1 BZN/3 DP951). The smaller samples were co-fired in the dilatometer. The resulting curve is shown Figure 1. For “pure BZN” stacks, sintering starts at 700°C and ends at 950°C. Shrinkage is 25%. For composite BZN/Alumina stacks, sintering starts at 700°C and ends before 900°C.

s t ac k B ZN o n ly s t ac k B ZN / D u p o n t a lt e rn an c e S t ac k B ZN / D u p o n t 3 -1 -1 -1 -3

Tem p s / s 0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0

Tem p erat u re °C

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

1 0 0 0 %

-2 8

-2 6

-2 4

-2 2

-2 0

-1 8

-1 6

-1 4

-1 2

-1 0

-8

-6

-4

-2

0

Figure 1 Co-firing curves on multilayer stacks composed of BZN and Dupont tapes

Starting from these observations, we optimised BZN/DP951 multilayer stacks for co-firing in the standard LTCC process. We performed Scanning Electron Microscopy on fractures of co-fired stacks DP951/BZN/DP951, in order to observe the inner microstructure, and the interface between layers of different materials. The result is shown on Figure 2. The light grey band is BZN, the dark grey zone on its both sides is Du Pont 951 dielectric material. The thicknesses are about 65 microns for the BZN layer, 300 microns for the DP951. The interface is quite smooth, a thin diffusion zone of about 2 microns can be observed at the strongest enhancement (picture on the right, x 2500).

Figure 2 Micrographs of co-fired stacks DP951/BZN/DP951 at different scales


After proving the feasibility of co-fired BZN/DP 951 stacks we made test structures in a LTCC 4-inch development line, in order to study possible deformations and inter-diffusions between different materials in a representative LTCC multilayer including interconnection levels with DP 951 tapes, filtering capacitors levels with BZN, with printed internal electrodes or lines, and vias filled with silver inks. We found that strongly dissymmetric stacks suffered deformation and warpage, while symmetric ones were perfectly flat. The cause is strong stresses occurring during co-firing different materials which present different sintering curves. A polished cross section of a test structure was observed and analysed through Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The SEM image shows the different layers, DP951 in dark grey, BZN and silver electrodes or vias in light grey Figure 3. The interfaces are well-defined, every material seems to keep its own integrity, there is no interdiffusion , delamination , or chemical reaction visible. EDS images and pointed analyses (not shown) confirm these results. Line scan EDS analyses were made through the different zones of materials: DP 951 layer, BZN layer, silver via, DP 951, selecting the most intense EDS peak for every chemical element. Figure 4 shows the line scan intensities for the different elements: Oxygen (O), Zinc (Zn), Niobium (Nb), Bismuth (Bi), Silver (Ag), through the different zones of materials. The vertical dotted lines represent the limits between the different materials, determined by SEM. The elements are present where expected. There is very little inter-diffusion, the interfaces are quite sharp, showing very limited reactions between the different materials.

Figure 3 LTCC with BZN layers, Ag lines and vias

Figure 4 EDS Analysis

DPT

BZN

Ag (via) DPT

O Zn Nb Bi Ag

120 µm

80 µmDPT

BZN

Ag (via) DPT

O Zn Nb Bi Ag

120 µm

80 µm

LTCC Process adaptation

Starting from the conventional LTCC process, all the manufacturing steps were adapted to the new material as well as the mixed dielectric structure. A particular emphasis was dedicated to the sintering operation to allow the realization of flat substrates with the so-called free sintering option. Regarding the shrinkage in x & y of the substrate after co-firing, it was demonstrated that 951/BZN sandwiches such as 2 x 951A2 / 1 x BZN / 2 x 951A2 allow to keep the nominal shrinkage of the 951 dielectric (i.e. 12.7%). Test vehicles design and realization

A specific test vehicle (TV2) was designed to perform the electrical characterization of the integrated capacitors. This test vehicle includes in total 42 capacitors featuring various electrode dimensions and configurations. Figure 5 shows the typical vertical structure of an integrated parallel capacitor designed with a single BZN layer. The top electrode width and length vary from 550µm to 1650 µm while its resulting area varies from 0.30 to 1.8mm². The bottom electrode is 100µm larger than the top one (50µm on both sides) to reduce the effect of possible misalignment between electrodes on capacitance variation. To allow RF measurement, the top electrode of each


buried capacitor is connected via a micro-strip line to coplanar access pads printed on the top surface of the LTCC substrate. A set of RF calibration structures is also included in the test vehicle design.

Top electrode

Bottom electrode

RF ground

Ground

Top electrodeconnection

High-K BZN layerStandard Klayers

Figure 5 : z structure of an integrated capacitor

Several panels of this specific test vehicle were manufactured for technology evaluation and RF characterization purpose. A picture of a test vehicle sample is given in Figure 6 showing its topside with the RF access pads.

Figure 6 : picture of a test vehicle sample Figure 7 : cross section of a capacitor

The flatness of the test substrate was verified. The optimized sintering profile allows to obtain satisfactory results, the overall camber being lower than 0.3% of the panel diagonal. Several cross-sections of the test vehicle were realized to check the integrity of the multilayer structure. The picture in Figure 7 shows a typical cross-section of an integrated capacitor. No voids or delamination occurs at the high-K BZN / 951 interface. The average value of the High K BZN dielectric thickness is 62µm (between electrodes). Building mixed dielectric multilayer may degrade the mechanical performance. The flexural strength and the Young modulus of the substrate was therefore verified, performing 3-point bending test on mixed structures as well as on pure 951 stacks. The two configurations did not show any significant difference giving average values of 250MPa for flexural strength and 100GPa for Young modulus. RF characterization

As previously mentioned, all the embedded capacitor test structures on test vehicles are connected to the top surface of the LTCC panel via the same interface made of a 50-ohm micro-strip lines terminated with a 50-ohm coplanar pad structure. Capacitor elements were measured from 50MHz to 6GHz with coplanar microwave probes (Ground-Signal-Ground, 350µm pitch, 40GHz).


The Vector Network Analyzer (VNA) was calibrated using the SOLT calibration kit included on the test panels to shift the electrical reference plane from the coplanar pads to the capacitor element. The S-parameters obtained from the VNA was processed via a microwave circuit simulation software [9]. The different capacitors test structures featuring various electrode size and shape were measured on several LTCC test panels. Table 1 gives the synthesis of the overall results.

Electrode area range 0.30 to 1.8mm²

Capacitance range @ 300MHz 5 - 30pF

Average Capacitance density 15 pF/mm²

Q factor range @ 300MHz 40 - 60

Resonance frequency range 2 – 3.5 GHz

Table 1 : synthesis of C measurements

Assembly and environmental evaluation

The compatibility of the developed material and process with assembly and packaging techniques was evaluated performing wire-bonding trials, flip chip assembly experiments as well as building BGA structures. A second test vehicle was designed for this purpose and two batches were manufactured : a first one with the high-K BZN layer and a second one without high-K BZN layer as the reference (conventional LTCC). All the assembly trials we performed did not point out any drawback related to the high-K BZN technology with regard to the different process aspects. As an example, Figure 8 shows a cross section of a flip chip assembly structure on a high-K BZN/ DP951 test panel.

chip size : 10 x 10 mm² I/O's : 184, 200µm pitch via diameter : 100µm

Si C hip

LTCC via

DP951 tape

High - K tape

So lder bumps

Figure 8 : cross-section of a flip chip assembly structure

To evaluate the reliability of the developed integrated capacitor technology, several TV2 test panels were submitted to extensive environmental trials. Three groups were defined to go through the following tests :

High temperature storage : 1000 hours @ 150°C Damp heat test : 1000 hours @ 85°C, 85% RH Thermal shocks (2-chamber test) : 500 cycles –55°C /+125°C

Each capacitor was measured before and after environmental tests as well as at intermediate steps. No failure occurred and no significant drift of the capacitance values was observed along the different tests. Conclusion

A new high K tape was developed to allow the integration of filtering capacitors in LTCC substrate. This material is well adapted to the DuPont 951 base material system and inner silver conductors. Providing some specific process adaptations it is fully compatible with the conventional LTCC manufacturing flow and does not affect the assembly and packaging capabilities of the standard technology. In addition, a good reliability of the mixed-dielectric structures was demonstrated through high temperature storage, damp heat test as well as thermal shocks. The new material provides a nominal capacitance density of 15pF/mm² with a single layer compared to 2pF/mm² with the standard LTCC tape. Capacitor test structures from 4pF to 30pF were produced and characterized in the 300MHz to 10GHz frequency range showing satisfactory properties for use for RF


applications up to 2GHz. Finally, adapted electrical simulation models were defined providing a satisfactory agreement with the RF measurements. Acknowledgements

The presented RTD activities were performed in the frame of the PIDEA PACIFIC BOAT project (EUREKA mainframe) and the Fanimat Nano-Shape project. The authors would like to thank the PIDEA organization as well as the French Ministry of Industry (DGE), the German Ministry of Education and Research (BMBF) and the Projektträger Jülich Organization for their support. References [1] Jens Müller, Daniel Josip, "Integrated Capacitors using LTCC", MicroTech 2002, January 29-30 2002 Crowne Plaza Manchester UK [2] Thomas Bartnitzek, Edda Müller, Raymond van Dijk, "LTCC Phase shifter modules for RF-MEMS-switch integration", IMAPS CICMT, April 10 - 13, 2005Baltimore [3] M. Lahti, K. Kautio, E. Juntunen and P. Karioja, "High-power module integrated in LTCC package", 0003rd EMRS DTC Technical Conference – Edinburgh 2006 [4] V. Sunappan, P. L. Vadiveloo, L. L. Wai, W. Fan, and C. W. Lu, "Processing and electrical characterization of co-sintered composite glass Ceramics", 2006 Electronics Packaging Technology Conference [5] Jean-Pierre Cazenave, Jacky Cerisier, Seung Kyu Choi, Laurent Boyer, Pascal Henquenet, John Cocker, Colin Pickering, Carl Wang, Mike Barker, Christopher RS Needes, "Improved RF Circuit Performance with an Enhanced and Expanded LTCC System", 14th European Microelectronics and Packaging Conference & Exhibition Friedrishshafen, Germany, 23-25 June 2003 [6] Donhang Liu, Yi Liu, Shui-Q. Huang, Xi Yao, “Phase Structure and Dielectric Properties of Bi2O3-ZnO-Nb2O5-Based Dielectric Ceramics”, J. Am. Ceram. Soc., 76 [8] 2129-32 (1993). [7] Xiao-Li Wang, Hong Wang, Xi Yao, “Structures, Phase Transformations, and Dielectrics Properties of Pyrochlores Containing Bismuth”, J. Am. Ceram. Soc., 80 [10] 2745-48 (1997). [8] Man F.Yan, Hung C.Ling, Warren W. Rhodes, “Low Firing, Temperature-stable Dielectric Compositions Based on Bismuth Nickel Zinc Niobates”, J. Am. Ceram. Soc., 73 [4] 1106-107 (1990). [9] Jean-Pierre Bertinet, Eddie Leleux, Jean-Pierre Cazenave, Jean-Pierre Ganne, Michel Paté, Richard Lebourgeois, Edda Mueller, Franz Bechtold, "Filtering Capacitor Embedded in LTCC Substrate for RF and Microwave Applications", Microwave Journal , November 2007.


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