Low-temperature Co-fired Ceramic Materials Research Articles

Fabrication of cavities in low loss LTCC materials for microwave applications

A method of buried cavity fabrication in low loss DP951 and new DP9K7 LTCC (low-temperature co-fired ceramic) materials is described in this paper. Laser micromachining and method based on sacrificial volume material (SVM) are studied. Cavities are fabricated in LTCC materials using two different SVMs—cetyl alcohol and carbon tape. The influence of laser system parameters on cutting quality of the LTCC materials is studied. Moreover, thermal properties of the LTCCs and used SVMs are analyzed using combined thermo-gravimetric analysis, differential thermal analysis and differential thermo-gravimetry. Geometries of the LTCC test structures fabricated using different SVMs are analyzed using a scanning electron microscope and x-ray tomography. Energy dispersive spectroscopy and surface wettability measurements are used to analyze changes in LTCC materials atomic composition after co-firing with SVMs.

Biocompatibility Evaluation of Human Umbilical Vein Endothelial Cells Directly onto Low-Temperature Co-fired Ceramic Materials for Microfluidic Applications

Expansion of Low-Temperature Co-fired Ceramic materials into microfluidic systems technology has many beneficial applications due to their ability to combine complex three dimensional structures with optical, fluidic, electrical functions. Evaluations of the biocompatibility of these Low-Temperature Co-fired Ceramic materials are vital for expanding into biomedical research. The few biocompatibility studies on Low-Temperature Co-fired Ceramics generally show negative cellular response to thick film pastes used in generating the electronic circuitry patterns. In this study, biocompatibility of Human Umbilical Vein Endothelial Cells was examined on Heraeus's Low-Temperature Co-fired Ceramic tape and two of their conductive pastes. The biocompatibility was assessed by monitoring cellular attachment and viability up to three days. This study examines the idea of leachates being detrimental to cells due to a study that suggests the possibility of harmful leachates. Results indicate difficulty in initial attachment of Human Umbilical Vein Endothelial Cells to sintered Low-Temperature Co-fired Ceramic tapes, but no hindrance of cellular attachment and growth onto the two conductive pastes. Outcomes also demonstrate that possible harmful leachates from Low-Temperature Co-fired Ceramic materials don't thwart cellular attachment and growth for up to three days of cell culturing. These results provide a basis for biological devices using Low-Temperature Co-fired Ceramic materials.

Embedded Ferrite LTCC Inductors

This paper presents for the first time one realization of simple planar inductor realized inside the stack of ferrite LTCC (Low Temperature Co-fired Ceramic) tapes. Presented inductor is one layer square spiral fabricated in the LTCC technology. In order to point out benefits of implementation of ferrite material on inductor inductance, the same inductor geometry was fabricated between two dielectric LTCC tapes. Commercially available LTCC material (both ferrite and dielectric) were implemented for the realization of proposed inductors. Designed structures were characterized and obtained experimental results show that even implementation of a very thin layer of ferrite material around inductor lines drastically increases its inductance. For the same inductor design that occupies the same chip area the inductance enhancement over 11 times is achieved. In addition this enhancement is followed with maintenance of good performance of the inductor.

The Effect of Phase Composition on the Mechanical and Thermal Properties of LTCC Material

Low-temperature co-fired ceramic (LTCC) is an important material in the production of ceramic multilayer structures. Large and complex multilayer structures are usually fired at higher temperatures and/or longer firing times compared to the relatively thin LTCC tapes. The firing conditions of LTCC determine the phase composition and the microstructure, which both influence the physical characteristics, such as the mechanical and thermal properties. In this work the effect of the phase composition on the biaxial flexural strength and the temperature coefficient of expansion of the DuPont 951 LTCC is presented. The samples were fired at different temperatures and times to obtain different phase compositions. The phase composition, especially the mass fraction of anorthite, was correlated with the biaxial flexural strength and the thermal coefficient of expansion (TCE). A very long firing time, i.e., 100h at 800 °C, yields, apart from crystalline anorthite, the cristobalite phase. The anorthite that crystalizes from the glass phase in a dense LTCC material changes the crack-propagation mechanism and improves the biaxial flexural strength of the material. The major change in the biaxial flexural strength is observed when the anorthite phase appeared. The increasing mass fraction of anorthite does improve the biaxial flexural strength less drastically. With the increasing mass fraction of the anorthite, decreases of the TCE of the material and a small decrease of the density of the LTCC material occur.

Microstructural evolution and electrical properties of base-metal electroded BaTi4O9 materials with B-Si-Ba-Zn-O glass system

Barium titanate-based microwave dielectrics usually require relatively high temperatures to sinter, which prevents the use of base metals such as copper for electrodes. In this work, BaTi(4)O(9) microwave dielectric ceramics co-fired with copper electrodes are made possible by adding B-Si-Ba- Zn-O glass to induce liquid-phase sintering at sufficiently low temperature and in reduced atmosphere. The microstructures and electric properties of the BaTi(4)O(9) ceramics thus obtained are carefully examined and studied. Proper glass composition may significantly facilitate mass transportation in the low-temperature co-fired ceramic (LTCC) material, resulting in better densification without serious degradation of electric properties. Although the B2O3/SiO2 ratio enhances the glass mobility during sintering, the BaO/ZnO ratio contributes to the chemical affinity of glass to BaTi(4)O(9) ceramics. In addition, various Ba-Ti-O phases with different Ba/Ti ratios may be found in the specimen through the X-ray diffraction patterns when the BaO/ZnO ratio is varied. If the BaO/ZnO ratio is high and the glass flows easily in the material, the Ba(4)Ti(13)O(30) phase is formed. If the BaO/ZnO ratio is low and the glass flows easily in the material, the BaTi(6)O(13) phase appears. We find that glass-induced Ba(4)Ti(13)O(30) transformation may significantly decrease Qxf values in the BT4-BSBZ materials. Therefore, the appropriate glass composition must be selected to ensure the phase stability of dielectrics to achieve the best performance possible.

Optimized cavities for microwave applications using the new low loss LTCC material Du Pont 9k7

This paper presents two different novel methods to manufacture optimized cavity structures, which are suitable for multilayer System-in-Package (SiP) and Multi-Chip-Module (MCM) applications, in the new low temperature cofired ceramic (LTCC) material Du Pont 9k7. In contrast to standard manufacturing, the improved DP 9k7 cavities show almost perfectly orthogonal cross sections and straight edges, achieved by applying special cavity inlays or cast silicone. Ball-wedge bond wires and wedge-wedge ribbons are implemented directly to the cavity rim successfully. Therefore, these cavities are suitable for very short bond wires in complex RF systems and the introduced techniques are very promising for future applications using DP 9k7.

Towards rational design of low-temperature co-fired ceramic (LTCC) materials

High performance low temperature co-fired ceramic (LTCC) dielectrics is highly desired for next generation information technology. The rational design is a key issue for the development of new LTCC materials. In comparison to the design of conventional electroceramics, more attention should be paid on the formation process of the material structure for that of LTCC, in addition to the physical properties, due to the special requirement in fabrication processing. In this paper, sintering mechanism of three types of LTCC materials, i.e., glass-ceramics, glass ceramic composite, and glass bonded ceramics, as well as important factors of their dielectric properties are discussed and summarized, and the design strategies for LTCC dielectrics, based on new matrix materials with much lower sintering temperature or higher quality, are proposed. As an example for rational design, oxyfluoride glass-ceramic based dielectrics, a new class of LTCC materials with low ɛr, is analyzed.

Fabrication of membranes and microchannels in low-temperature co-fired ceramic (LTCC) substrate using novel water-based sacrificial carbon pastes

In this work, 3D structuration of LTCC (low-temperature co-fired ceramic) for microfluidics was studied, using two novel sacrificial carbon paste compositions. These pastes are based on graphite with a water-soluble vehicle consisting of polyvinylpyrrolidone binder (PVP) dissolved in propylene glycol (PG), which is not aggressive to green LTCC material. Both examined pastes differ slightly in binder content and added plasticizer, glycerol (G) or trimethylolpropane (TMP). The thermal properties of the sacrificial carbon pastes have been examined using combined thermo-gravimetric analysis (TGA), differential thermal analysis (DTA) and differential thermo-gravimetry (DTG). The sacrificial carbon pastes have been applied to the fabrication of membranes and microchannels in LTCC substrate. A comparison of the obtained features has been made using X-ray tomography and optical profile measurements. Moreover, changes in the composition of the fired LTCC material after co-firing have been studied using X-ray photoelectron spectroscopy (XPS) and surface wettability measurements.

Magnetic DC-Properties of LTCC-Ferrite Material and Their Temperature Dependence

Low temperature co-fired ceramic (LTCC) technology allows to create 3-D ceramic devices from tapes with different electrical and magnetic properties. Recently ferrimagnetic materials became available as LTCC tapes. To magnetically characterise the ferritic LTCC material we have realised ring samples using ESL 40012 LTCC-tapes. The ring core fabrication process and the sample preparation are described in detail. The quasi-static B(H) curve has been measured by inductive methods, and its dependence on temperature between room temperature and the Curie temperature is shown.

Complex Permeability Changes of Ferritic LTCC Samples With Variation of Sintering Temperatures

The complex permeability of commercially available ferrite LTCC (Low Temperature Cofired Ceramics) tapes in dependence of peak sintering temperature have been measured and calculated in the high-frequency range (300 kHz-1.2 GHz). For the first time this dependence is determine at five peak sintering temperatures up to 1300°C. Permeability values have been calculated from S-parameters measured with a vector network analyzer and a short ended coaxial sample holder into which ferritic LTCC toroids have been placed. The toroids were realized by stacking and laminating 62 layers of green tape and then sintered at peak temperatures of 885°C, 1000° C, 1100°C, 1200°C and 1300°C, respectively. Obtained complex permeability values for these toroidal LTCC samples are presented. The maximum real part of permeability values increase up to 930 (at 1100°C firing peak temperature) and show a shift to lower frequency values. At higher sintering temperatures (1200° C and 1300°C) real part of permeability of ferrite samples starts to decrease with further shifting to lower frequency values. In order to determine variation of grain size caused with increasing of sintering temperature and its influence on material complex permeability, an microscopic inspection of samples was performed. Also, using the model of the complex permeability spectra, dispersion parameters of ferrite LTCC material has been determined for various sintering temperatures.

Phase evolution and enhanced dielectric properties of BaO·Nd2O3·TiO2·B2O3 glass-based dielectrics containing CaCu3Ti4O12

CaCu3Ti4O12 (CCTO) has been studied as a filler for the potential high k LTCC (low temperature co-fired ceramics) materials based on a nonconventional BaO·Nd2O3·TiO2·B2O3 glass in terms of firing temperature and CCTO content. The material system was proven to be high crystallizable as the resultant crystalline phases of BaTi(BO3)2, BaNd2Ti5O14 and NdBO3 were observed. The relative content of the crystalline phases were strongly dependant on firing temperature and CCTO content. Interestingly, the 30wt% CCTO sample showed early densification at 610–670°C and then unexpected expansion after 850°C. The 40–60wt% CCTO samples showed the unexpected secondary densification behavior over the temperature range of 850–950°C. As a promising example, a combination consisting of the glass and 40wt% CCTO exhibited k ∼55 and tanδ ∼0.07, which is very rare at this low sintering temperature of 850°C. These materials system were found to be chemically very stable especially in the H2SO4 acid solution.

Synthesis, sintering and optical properties of CaMoO4: A promising scheelite LTCC and photoluminescent material

AbstractThe synthesis of nanocrystalline calcium molybdate (CaMoO4) through an autoigniting combustion technique is reported in this paper. The structural characterization of the as‐prepared nanocrystallites were done by X‐ray diffraction (XRD), Fourier transform Raman, and Fourier transform infrared (IR) spectroscopy and the morphological studies using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The studies reveal that the as‐prepared powder itself was phase pure with tetragonal structure and of particle size 25 nm. The sample was sintered at a relatively low temperature of 775 °C to a high density of ∼95% for the first time, without the use of any sintering aid. The optical bandgap energy calculated from the ultraviolet–visible absorption spectrum for the as‐prepared and annealed sample was 3.72 and 3.99 eV, respectively. The photoluminescence spectra of the sample showed an intense emission in the green region (528 nm). The dielectric constant and loss factor of the sample at 5 MHz was found to be 11.00 and 6.40 × 10−3 at room temperature. The temperature coefficient of dielectric constant was −95.04 pp/°C. These observations reveal that nanostructured CaMoO4 is a promising scheelite low‐temperature co‐fired ceramic (LTCC) and also an excellent luminescent material.

Sintering Behavior, Properties, and Applications of Co‐Fired Piezoelectric/Low Temperature Co‐Fired Ceramic (PZT‐SKN/LTCC) Multilayer Ceramics

Materials and processing conditions have been developed allowing co‐firing of fluxed PZT‐SKN materials with commercial low temperature co‐fired ceramic (LTCC) tapes. Previously, Pb(Zr0.53, Ti0.47)O3–Sr(K0.25, Nb0.75)O3 (PZT‐SKN) ceramics fluxed with 1 wt% LiBiO2 and 1 wt% CuO addition were shown to sinter to high density at 900°C for 1 h, with a large d33 piezoelectric coefficient of ~415 pm/V. Currently, the master sintering curve (MSC) approach has been used to study the densification behaviors of fluxed PZT‐SKN and LTCC tapes. Different sintering mechanisms for fluxed PZT‐SKN ceramics and LTCC materials are confirmed by analyzing the apparent activation energy (Qa). Using knowledge gained from MSC results, an optimized sintering profile was developed. Multilayer PZT‐SKN/HL2000 (HeraLock™ Tape, Heraeus) stacks co‐fired at 900°C for 0.5 h maintain large piezoelectric coefficient (high field d33 > 340 pm/V). EDS analysis reveal limited interdiffusion of Pb from PZT‐SKN layers in LTCC and the appearance of Al, Ca, and Si in the PZT‐SKN near the PZT‐SKN/LTCC interface. Further, elemental interdiffusion was not detected at the center of piezoelectric layer in PZT‐SKN/LTCC multilayer ceramics and no subsequent reduction in piezoelectric coefficient d33 was observed. Finally, a piezoelectric microbalance with mass sensitivity of 150 kHz/mg was fabricated using the materials and methods developed.

Low-pressure, thermo-compressive lamination

Lamination of green ceramic tapes is one of the most important technological processes in multilayer ceramic technology. Lamination affects the quality of all 3D structures (e.g., channels, chambers, membranes, etc.). Novel chemical methods of lamination reduce the deformation of 3D structures. However, these methods are useless in the fabrication of thin membranes and structures with thick-film electronic components or electric vias. Therefore, thermo-compressive lamination is still the best solution for the lamination of green ceramic tapes. Low-pressure thermo-compressive lamination with an insert material is presented in this paper. The influence of pressure and Low Temperature Cofired Ceramics (LTCC) material on the compressibility and shrinkage of LTCC, as well as the influence of the insert material on deflection and distortion of the membranes are presented and discussed in this paper.

Energieverbrauch und CO2-Emissionen im Lebenszyklus von LTCC-Substraten basierend auf Kleinserienfertigung

Life cycle inventory (LCI) data of three low-temperature co-fired ceramics (LTCC) materials of a small batch production were determined and compared. The main focus was on the measurement and calculation of the energy consumption and the green house gas emissions of the manufacturing steps from the green tape to the co-fired substrate, and on the transport. The co-firing process was performed with two different furnace types and turned out to have by far the highest environmental impact.

Preparation and properties of crystallizable Glass/Al2O3 composites for LTCC material

The investigated low temperature Co fired ceramics(LTCC) composite of 60wt% CaO-Al2O3-B2O3-SiO2 glass and 40wt% α-Al2O3 as a filler is a non-reactive system, which is a critical part of the low temperature Co fired ceramics process. Through a study on densification process, the phase transformation and microstructure can be revealed. Its composites typically consist of CaO-Al2O3-B2O3-SiO2 glass and α-Al2O3 powders of average particle size (D50=3.49 μm). The sintering behavior, phase evaluation, sintered morphology, and microwave dielectric properties were investigated. In the fire range of 800 to 900 °C, the composites were crystallized after completion of densification. It is found that the composites start to densify at 825 °C, simultaneously, the dielectric constant (ɛr) reaches its maximum. With increasing heat-treatment temperatures, due to the loose microstructure of the material, tanδ increases slightly. The last of the sintered samples were identified as partly Anorthite at 850 °C. At that temperature it has ɛr of 7.9 and tanδ less than 1×10−3, and can be used as a promising LTCC material.

A novel LTCC differentially Fed UWB antenna for the 60 GHz band

In this paper a novel differentially fed Ultra-Wide Band (UWB) antenna in low-temperature co-fired ceramics (LTCC) technology to be used in the 60 GHz band for integrated RF front-ends is presented. The antenna is based on the aperture stacked patch fed via H-shaped aperture to achieve more than 10 GHz operational bandwidth. The antenna is fed by a parallel-wire transmission line which enables the antenna to be directly integrated with differential Monolithic Microwave Integrated Circuits (MMICs). To alleviate influence of the surface waves (efficiently excited in LTCC material due to its high dielectric constant) on the antenna radiation and realize uni-directional radiation patterns, a dedicated shield is added to the antenna. The measured results of the shielded antenna showed that the antenna has an operational bandwidth from 51 GHz to over 65 GHz, the gain is about 3.5–8 dBi, and −5 dB beamwidth is about ±30°. The measurement results also demonstrated that the shield indeed improves the antenna impedance bandwidth, gain, and radiation patterns substantially.

Three-Dimensional Surface in LTCC for a MM-Wave Antenna

Low loss LTCC (Low Temperature Cofired Ceramics) materials have already demonstrated their virtues for high density packaging of mm-wave modules and their capability to handle the inherent requirements of heat dissipation. A 60 GHz substrate integrated waveguide fed steerable antenna array is one example of the driving applications for new challenges in the LTCC process. This antenna is suitable for transmit and receive in the 60 GHz WPAN frequency band [1]. Integrating antenna array and mm-wave front-end in one module is a consequent way to avoid amplitude and phase uncertainties associated with connectors and cables. A compact module also helps to reduce losses in the signal path by providing short interconnects. This is important for mm-Wave systems, particularly for the receiver. The antenna design presented here requires a three-dimensional surface structure with features like small and precise cavities and grooves. Different methods to fabricate cavities in LTCC modules will be discussed and a new method which proved suitable for the demanding application will be presented in detail.

Bulk and In-Circuit Dielectric Characterization of LTCC Tape Systems Through Millimeter Wave Frequency Range

Low Temperature Co-fired Ceramic (LTCC) material systems offer a highly versatile microwave and millimeter wave packaging platform. Extremely low microwave loss, excellent control of dielectric constant, uniform dielectric thickness, non-existent water absorption leading to very high hermeticity, ability to support multilayer structure leading to 3-dimensional packaging, ability to embed passive functions within the tape layers, availability of a wide range of metallizations, etc. are some of the key advantages of LTCC for microwave packaging. One of the important parameters which needs to be determined at the very early stages of circuit designs are the dielectric properties - dielectric constant and loss tangent, both of which are functions of frequency. These properties need to be known accurately over the entire frequency range of operation for the circuit. For LTCC based designs, the use of dielectric constant of bulk material can lead to deviations between the performance expected at the design stage and for the fabricated circuit. Such deviations are a significant concern for broadband circuits as well as for circuits with sharp resonant behavior such as filters. One of the significant sources of deviation between bulk LTCC and “in-circuit” dielectric constant is the nature of the thick film metallizations used in LTCC technology. Work described here is a comprehensive characterization of three DuPont™ GreenTape™ LTCC systems 951, 943, and 9K7 - in the frequency range 10 to 70 GHz. Both bulk and “in-circuit” dielectric properties with silver and gold metallizations are studied to quantify the deviations in dielectric properties. A Fabry-Perot open resonator technique is used for the bulk characterization while printed ring resonators are used for the in-circuit characterization. This comprehensive characterization will provide key design data for LTCC designers in the 10 – 70 GHz frequency range.

Introducing DuPont™ GreenTape™ 9K5 Low Dielectric Constant, Low Temperature Co-Fired Ceramic (LTCC) Tape System

Low Temperature Co-fired Ceramic (LTCC) material systems have been successfully used in microwave and millimeter wave systems for several years. LTCC has very low dielectric loss, high reliability due to inherent hermeticity; high interconnect density, multilayer processing capability leading to true 3D packaging, and better cost-performance balance. While the medium range dielectric constants (7.00 – 8.00) offered by current tape systems have advantages, it is generally difficult to realize high speed systems and efficient antennas on LTCC, especially at millimeter wave frequencies. The difficulty arises from the reduced signal propagation velocity in high-speed applications, and lower radiation efficiency for antennas, both due to higher dielectric constant. To enable and extend applications of LTCC technology to these subsystems, DuPont has developed a new low dielectric constant LTCC system – DuPont™ GreenTape™ 9K5 - which has a dielectric constant of 5.80 (at 10 GHz) that is compatible with the commercial DuPont™ GreenTape™ 9K7 LTCC System. This is achieved without compromising excellent microwave loss properties of the 9KX GreenTape™ platform. These materials systems enable high-speed, high reliability applications while also realizing efficient antennas on LTCC. This paper presents initial characterization of the new DuPont™ GreenTape™ 9K5 LTCC system consisting of low K dielectric tape, gold and silver conductors to evaluate the effects of chemistry, processing conditions, processing latitude, microstructure, and microwave performance. Test coupons with various transmission and resonating structures are designed, fabricated, and tested for the evaluation of transmission losses and dielectric properties. Stability of the material system over multiple re-fire steps is also examined