Low Temperature Co-fired Ceramics Systems Research Articles

Low-permittivity BaCuSi4O10-based dielectric Ceramics: An available solution to connect low temperature cofired ceramic technology and millimeter-wave communications

Low temperature cofired ceramic (LTCC) technology can serve for next-generation millimeter-wave electronic products to bring the advantages of integration, miniaturization and excellent high-frequency performance. However, challenges such as the performance of LTCC powders, quality of green tapes and Ag co-firing compatibility still need to be addressed for the application of LTCC technology. In this work, a systematic investigation from powder modification, LTCC process to filter design were carried out based on novel BaCuSi4O10-based dielectric ceramics. Multiple effects in terms of suitable sintering temperature (840 ℃ suit for Ag co-firing), low-permittivity (εr ∼ 5.7) and good thermal stability (temperature coefficient of resonant frequency ∼ − 27 ppm/℃, thermal expansion coefficient ∼3.9 ppm/℃) were achieved by introducing Li-B-Si glass and LiF composite additive. The origin of dielectric response of this LTCC system were discussed using the FIR reflectivity spectrum and THz-TDS. After optimizing the tape casting process for the LTCC powder, green tapes with uniform thickness, smooth surface and no cracking were obtained. Furthermore, the bandpass and lowpass filters exhibited a high level of filtering performance own to excellent dielectric properties and low surface roughness of the LTCC. Consequently, the BaCuSi4O10-based dielectric ceramic is an attractive candidate as LTCC technology expands into millimeter-wave communications.

Wide process latitude, Pb-free low temperature co-fired ceramic (LTCC)

Low temperature co-fired ceramics, LTCC, are an established material platform for making high-quality, high-performance, and high-reliability electronic devices; however, systems exhibiting a sufficiently wide processing window have traditionally employed Pb-containing glasses. Micromax™ GreenTape™ LF95C has been introduced as a Pb-free LTCC system with a host of attractive physical, thermal, and electronic properties including repeatable shrinkage, dielectric loss of <0.005 at 10 GHz, refire stability, and a full Ag-based metallization system. The ceramic densifies by glass viscous flow which provides the ability to co-fire over a wide process window. The high-conductivity Ag metallization, low Df, and repeatable shrinkage and Dk make LF95C an excellent material platform for producing high-reliability electronic devices while advancing sustainability goals and working to satisfy the spirit of the REACH and RoHS initiatives.

Exploring Dielectric Constant and Dissipation Factor of LTCC Using Machine Learning.

Low-temperature co-fired ceramics (LTCCs) have been attracting attention due to rapid advances in wireless telecommunications. Low-dielectric-constant (Dk) and low-dissipation-factor (Df) LTCCs enable a low propagation delay and high signal quality. However, the wide ranges of glass, ceramic filler compositions, and processing features in fabricating LTCC make property modulating difficult via experimental trial-and-error approaches. In this study, we explored Dk and Df values of LTCCs using a machine learning method with a Gaussian kernel ridge regression model. A principal component analysis and k-means methods were initially performed to visually analyze data clustering and to reduce the dimension complexity. Model assessments, by using a five-fold cross-validation, residual analysis, and randomized test, suggest that the proposed Dk and Df models had some predictive ability, that the model selection was appropriate, and that the fittings were not just numerical due to a rather small data set. A cross-plot analysis and property contour plot were performed for the purpose of exploring potential LTCCs for real applications with Dk and Df values less than 10 and 2 × 10−3, respectively, at an operating frequency of 1 GHz. The proposed machine learning models can potentially be utilized to accelerate the design of technology-related LTCC systems.

The Effect of Processing Parameters on the Physical and Microwave Electrical Properties of Low Temperature Co-fired Ceramics

Low Temperature Co-fired Ceramics (LTCC) represent a multilayer electronic packaging technology with the ability to incorporate many electrical components such as conductive, resistive, and dielectric materials, into a single monolithic package. Example devices would include capacitors, inductors, resistors, antennae, transformers, hybrid circuits, etc. In general, the LTCC package consists of a glass/ceramic dielectric and compatible conductive inks (Au, Ag, etc.). The dielectrics and conductors are patterned/laminated to form the desired 3D structure and then co-fired to complete densification. The resulting monolithic block is mechanically robust with a high density of (often buried) electrical components. Typically, LTCC systems have found implementation in circuitry utilizing the RF and microwave frequency ranges. In general, the material design and processing parameters, and knowledge of how these affect the final mechanical and electrical properties of the LTCC package are needed for successful circuit design. The electronic performance of an LTCC dielectric/conductor system (Ferro L8) was investigated as a function of processing parameters (peak fire temperature, temperature ramp rate, furnace air flow, etc.). Comparisons were also made between multiple manufacturing sites as the mentioned processing parameters vary slightly from site to site even with nominally identical set points due to differences in furnace design, fab layout, etc. Material properties of fired parts such as effective permittivity and electrical losses were measured both on bare dielectric using resonant cavity methods and on the dielectric/conductor system as a whole using microstrip transmission line methods to frequencies as high as 50 GHz. Correlations were made between electrical performance and models based on material properties. The effects of varied processing parameters and processing sites were also studied and used to help establish process understanding and improvement with respect to high frequency electrical performance. As an example, optimization of electrical performance could be achieved by adjusting the nominal furnace temperature set point at a specific site where losses were slightly high even with identical processing parameters to those at another manufacturing site.

In situ shrinkage measurement during pressure‐assisted sintering of low temperature co‐fired ceramic panels

AbstractShrinkage measurements of miniaturized low temperature co‐fired ceramics (LTCC) samples under load typically lead to collapsing of the samples, which hampers the characterization of shrinkage up to full densification. In this paper, a measurement setup is presented, which allows for in situ shrinkage measurements of practical, large LTCC panels during pressure‐assisted sintering in a sintering press. The shrinkage behavior of two commercial LTCC systems (GreenTape 951 and Ceramtape GC) has been measured under loads of up to 1 MPa. No crushing of the specimens was observed and reproducible characterization of shrinkage up to full densification has been performed. Based on comparisons to thermomechanical analyzer measurements in this and other studies, it was found that the in situ approach is much better suited for shrinkage characterization of LTCC under load. Reproducibility and accuracy of the method are discussed and practical as well as more academic applications are proposed.

Influence of various multilayer LTCC systems on dielectric properties’ stability in GHz frequency range

Purpose The purpose of this paper is to create multilayer substrate (composite) from various low temperature co-fired ceramic (LTCC) substrates by their mutual combinations and to analyse influence of these multilayer substrates on dielectric properties in GHz frequency range. Design/methodology/approach GreenTape 951, GreenTape 9K7 and Murata LFC were used to create compound multilayer substrates that include three layers: middle layer is from Murata LFC, and both upper and bottom layers are either from GreenTape 951 or GreenTape 9K7. Shrinkage in all x-, y- and z-axes of all substrates including multilayer substrates were analysed, and influence of different shrinkage on dielectric properties was examined by microstrip ring resonators applied on all mentioned of substrates. Findings The middle layer of Murata LFC has significant influence on shrinkage value of composites which has a good repeatability and minimalizes problems with design of multilayer LTCC devices. Impact of middle layer from Murata LFC on dielectric constant is not significant, but on the other hand Q factor (loss tangent) of these composites is increased according to inhomogeneity between single LTCC layers, especially at frequency around 6 GHz. Originality/value The novelty of this work lies in creating multilayers systems from different types of LTCC substrates to find combination with the most suitable physical and dielectric properties for various purposes in GHz range applications.

Low temperature sintering behavior of La-Co substituted M-type strontium hexaferrites for use in microwave LTCC technology

The La-Co substituted Sr1-xLaxFe12-xCoxO19 (x=0-0.5) ferrites with appropriate Bi2O3 additive were prepared by conventional sintering method and microwave sintering method at low sintering temperatures compatible with LTCC (low temperature co-fired ceramics) systems, and their sintering behavior was chiefly investigated, including the crystal structure, saturation magnetization Ms, magnetic anisotropy field Ha, intrinsic coercivity Hci, and Curie temperature TC. Experiment results clearly showed that the pure M-type crystal phase was successfully obtained when the La-Co substitution amount x did not exceed 0.3. However, the single M-type phase structure transformed to multiphase structure with further increased x, where the M-type phase coexisted with the non-magnetic phase such as α-Fe2O3 phase, La2O3 phase, and LaCoO3 phase. Appropriate La-Co substitution improved the Ms (>62 emu/g), Ha (>1400 kA/m), and Hci (>320 kA/m) for the ferrites with x varying from 0.1 to 0.3, but the TC decreased with increasing substitution amount. Moreover, the microwave sintered ferrites could provide larger Hci and similar Ms compared with the conventional sintered ferrites.

Microwave sintered Sr1−xLaxFe12−xCoxO19 (x = 0–0.5) ferrites for use in low temperature co-fired ceramics technology

The La3+ and Co2+ substituted Sr1−xLaxFe12−xCoxO19 (x = 0–0.5) ferrites with Bi2O3 sintering aid were prepared by microwave sintering method at a low sintering temperature of 870 °C compatible with low temperature co-fired ceramics (LTCC) systems. Detailed effect of La–Co substitution amount on their crystal structure and magnetic properties were mainly investigated. The results show that the pure M-type crystal phase is successfully obtained for the ferrites with La–Co substitution amount x not higher than 0.3. With further increased substitution amount, the multiphase structure is formed, where the non-magnetic α-Fe2O3 phase and LaCoO3 phase coexist with the M-type phase. Appropriate La–Co substitution improves the magnetic properties of the ferrites, including the saturation magnetization Ms, effective magnetic anisotropy constant Keff, magnetic anisotropy field Ha, and intrinsic coercivity Hci. Besides, the Curie temperature TC of the ferrites exhibits a rapid decrease when the La–Co substitution amount exceeds 0.3. It is suggested that the La3+–Co2+ ions can partially substitute the Sr2+–Fe3+ ions for the ferrites, leading to enhanced magnetic properties. Typically, the obtained Sr1−xLaxFe12−xCoxO19 (x = 0.2–0.3) ferrites can provide improved Ms above 64 emu/g, Ha above 1510 kA/m, and Hci above 330 kA/m as low temperature sintered M-type hexaferrites, which are promising for use in future design and realisation of self-biased circulators in LTCC systems.

Effect of La–CO substitution on the crystal structure and magnetic properties of low temperature sintered Sr1−xLaxFe12−xCoxO19 (x=0–0.5) ferrites

The La–Co substituted Sr1−xLaxFe12−xCoxO19 (x=0–0.5) ferrites with appropriate Bi2O3 additive were prepared at a low sintering temperature of 890°C compatible with LTCC (low temperature co-fired ceramics) systems, and the effect of La–Co substitution on their crystal structure and magnetic properties was investigated. The results show that the pure M-type phase is successfully obtained when the La–Co substitution amount x does not exceed 0.3. However, the single M-type phase structure transforms to multiphase structure with further increased x, where the α-Fe2O3 phase and La2O3 phase coexist with the M-type phase. Moreover, the saturation magnetization Ms, magnetic anisotropy field Ha, intrinsic coercivity Hci, and Curie temperature TC of the ferrites depend on the La–Co substitution amount strongly, which are suggested to be determined by the partially substitution of La3+–Co2+ ions for Sr2+–Fe3+ ions with x not higher than 0.3. It is found that the obtained Sr1−xLaxFe12−xCoxO19 (x=0.2 and 0.3) ferrites can provide improved magnetic properties (Ms>62emu/g, Ha>1400kA/m, and Hci>320kA/m) as low temperature sintered M-type hexaferrites for microwave LTCC applications.

Effect of La–Co substitution on the crystal structure and magnetic properties of hot press sintered Sr1−xLaxFe12−xCoxO19 (x=0−0.5) ferrites for use in LTCC technology

The La3+ and Co2+ substituted Sr1−xLaxFe12−xCoxO19 (x=0−0.5) ferrites with Bi2O3 additive were prepared by hot press sintering method at a low sintering temperature of 870°C compatible with LTCC (low temperature co-fired ceramics) systems, and their crystal structure and magnetic properties were mainly investigated. The results show that the pure M-type crystal phase is successfully obtained for the ferrites with La–Co substitution amount x not higher than 0.3. When the substitution amount further increases to 0.4 and 0.5, the α-Fe2O3 phase and La2O3 phase are found to coexist with the M-type phase, thus the multiphase structure is formed. Besides, the saturation magnetization Ms, intrinsic coercivity Hci, and Curie temperature TC of the ferrites are strongly correlated with the La–Co substitution amount. Enhanced Ms and Hci are obtained when the substitution amount varies from 0.2 to 0.4, which can exceed 3600Gs and 4000Oe, respectively. However, the TC decreases rapidly when the substitution amount exceeds 0.3, and a value under 410°C is observed. It is suggested that the La3+–Co2+ ions can partially substitute the Sr2+–Fe3+ ions for the hot press sintered ferrites at low sintering temperatures, leading to improved Ms and Hci for use in microwave LTCC ferrite devices.

High-Performance and High-Data-Rate Quasi-Coaxial LTCC Vertical Interconnect Transitions for Multichip Modules and System-on-Package Applications

A new design of stripline transition structures and flip-chip interconnects for high-speed digital communication systems implemented in low-temperature cofired ceramic (LTCC) substrates is presented. Simplified fabrication, suitability for LTCC machining, suitability for integration with other components, and connection to integrated stripline or microstrip interconnects for LTCC multichip modules and system on package make this approach well suited for miniaturized, advanced broadband, and highly integrated multichip ceramic modules. The transition provides excellent signal integrity at high-speed digital data rates up to 28 Gbits/s. Full-wave simulations and experimental results demonstrate a cost-effective solution for a wide frequency range from dc to 30 GHz and beyond. Signal integrity and high-speed digital data rate performances are verified through eye diagram and time-domain reflectometry and time-domain transmissometry measurements over a 10-cm long stripline.

A Biocompatible Low Temperature Co‐fired Ceramic Substrate for Biosensors

Low temperature co‐fired ceramics (LTCC) are widely utilized in the fabrication of compact, three dimensional, and highly integrated microelectronic components. Typical applications include military and mobile electronics applications requiring custom, hermetic, and/or high reliability packaging. These characteristics make LTCC attractive for use in microfluidic and biomedical application. However, commercial LTCC systems are not designed for biomedical applications and have unknown cytocompatibility. In the current work, an LTCC tape has been developed starting with materials of known composition and biocompatibility, a commercially available lime silicate glass and alumina. Tapes achieve high density for sintering temperatures compatible with LTCC processing (850 < T < 1000°C). The LTCC also exhibits low dielectric constant (K = 9.3) and dielectric loss (tan d < 0.005). A commercial gold electrode paste has also been co‐fired with the LTCC, with no delamination, cracks nor camber observed. In vitro biocompatibility was subsequently evaluated using human umbilical vein endothelial cells (HUVECs). HUVECs are observed to attach and spread on fibronectin‐coated LTCC substrates, and also in the leachate obtained by soaking LTCC in cell medium for 7 days. These results establish that the developed LTCC material is biocompatible and suitable for in vitro applications utilizing live cells.

Temperature‐Compensated Bandpass Filters in Low Temperature Co‐Fired Ceramic

For low temperature co‐fired ceramic (LTCC) modules targeting radio frequency applications, a near 0 ppm/°C temperature coefficient of resonant frequency (τf) ensures temperature stability of embedded filters. The base dielectrics of most commercial LTCC systems have a τf in the range −50 to −80 ppm/°C. By integrating co‐fireable compensating dielectrics with an opposite τf to that of the host dielectric, we demonstrated multilayer stripline (SL) bandpass filters with a τf approaching 0 ppm/°C using DuPont 951 LTCC. The effective dielectric constant of the SL in multilayer LTCC with dissimilar layered dielectrics was calculated using a variational method with transmission‐line technique.

Electroplating and Electroless Plating Process Development for DuPont™ GreenTape™ 9K7 LTCC

Low Temperature Co-fired Ceramic (LTCC) technology provides an attractive packaging platform for microwave and millimeter wave circuits and systems due to its unique properties. Generally, thick film gold or silver conductors are used as metallizations on LTCC substrates along with occasional use of copper thick films. This paper reports methods and results of extensive process development experiments DuPont Microcircuit Materials has undertaken to establish a commercially viable plating process for the market leading DuPont™ GreenTape™ 9K7 LTCC system. Both Electroplating and Electroless plating processes are investigated in this work. These techniques provide certain advantages when used in isolation or in combination with standard thick film metallizations, helping to extend their applicability. Electroplating of copper on LTCC provides a means of using copper as the external conductor without having to use complicated firing processes in oxygen-free atmosphere as required for copper thick film. This approach leads to a much more cost effective approach if copper is required as the external metal. Electroless Nickel/Gold plating (ENIG) of both silver and copper (electroplated and/or thick film) provides an industry standard, highly reliable, robust surface finish. Such surface finish enables easy integration of both soldering and wire bonding processes.

Biocompatible low temperature co-fired ceramic for biosensors

Low temperature co-fired ceramic (LTCC) electronic packaging materials are applied for their ease of fabrication, three dimensional features and integration of multifunctional component, such as optical and electrical functions. For these reasons LTCC is attractive for biomedical microfluidics and Lab-on-a-Chip systems. However, commercial LTCC systems, optimized for microelectrics applications, are not designed for biomedical applications, and have unknown cytocompatibility. In the current work, LTCC has been developed starting with materials of known composition and biocompatibility. The developed LTCC, fabricated from a lime silicate glass and pure alumina, exhibits low sintering temperature (<1000°C) and high density. Alumina reacts with the glass and forms anorthite type crystalline phase CaAl2Si2O8 at temperature 900°C. A commercial gold electrode paste has also been co-fired with the LTCC, with no delamination, cracks nor camber observed. In-vitro biocompatibility of LTCC has been evaluated using human umbilical vein endothelial cells (HUVEC). The HUVECs attach and spread on the surface of the LTCC substrates, and also in the leachate obtained by soaking LTCC in cell media for seven days. The cell density and percentage of live cells on LTCC surface are comparative with those of control. Results indicate the developed LTCC materials are biocompatible and suitable for biomedical applications.

Localized Temperature Stability in Low‐Temperature Cofired Ceramics

Low‐temperature cofired ceramic (LTCC) is a multilayer 3D packaging, interconnection, and integration technology. For LTCC modules targeting radio and microwave frequency (RF and MW) applications, a low or near 0 ppm/°C temperature coefficient of resonant frequency (τf) ensures temperature stability of embedded resonator and filter functions. The base dielectrics of most commercial LTCC systems have a τf in the range −50 to −80 ppm/°C. This study explored a method to achieve a zero τf on stripline (SL) resonators by locally cofiring, in a multilayer LTCC structure, compensating dielectrics (CD) with an opposite τf to that of the host dielectric. The formulation, synthesis, dielectric properties, and microstructure of SrTiO3 (STO)‐based low‐fire τf CD are presented. Chemical interactions and physical compatibility between the compensating and the host LTCC dielectrics are investigated for cofireability. The dependence of τf compensation on the wt% of STO, the printed thickness, and the location of the CD in multilayer LTCC are discussed. The most effective τf compensation is achieved by integrating CD next to the resonator lines, and can be explained by the concentration of electromagnetic energy via total internal reflection of electromagnetic waves inside the CD layer.

Plating reliability and high frequency testing of DuPont™ GreenTape™ 9K7 LTCC

Low Temperature Co-fired Ceramic (LTCC) with low dielectric loss is suitable for use on microwave and millimeter wave circuits. Thick film gold or silver conductors are used as metallization on LTCC substrates. Gold prices are increasing at a rapid rate, so efforts were made to lower the dependency on gold by substituting silver with Ni/Au surface finishes as the top conductor. The external thick film silver conductors were plated using a standard Electroless Nickel Immersion Gold (ENIG) process. The Ni/Au surface finishes provides substantial improvement to fretting corrosion, environmental protection, contact resistance, wire bond strengths, solder ability and solder joint reliability. The reliability testing of DuPont™ GreenTape™ 9K7 LTCC with Ni/Au surface finishes is being conducted. The reliability test methods & conditions were chosen from well-established industry standard test protocols. This paper reports the reliability and high frequency testing results on the ENIG plated GreenTape™ 9K7 LTCC system.

A Low‐Loss Fully Embedded Stripline Parallel Coupled BPF for Applications using the 60 GHz Band

This article presents a low‐loss fully embedded bandpass filter (BPF) using low‐temperature co‐fired ceramic (LTCC) for multilayer System‐in‐Package (SiP) and Multi‐Chip‐Module (MCM) applications, for example, wireless applications for the unlicensed 60 GHz band. Critical coupling gaps of conventional bandpass filters in this frequency range, which are not suitable for screen printing, could be eased by introducing coupling plates in adjacent layers. The four‐layer BPF is designed for Du Pont's 9k7 LTCC system, and occupies a substrate area of 5.6 × 2.1 × 0.42 mm3 including transitions and a shielding via fence. The filter, including the two grounded coplanar waveguide transmission line (CPWg) to stripline transitions, has a measured insertion loss of 1.5 dB at the center frequency 58 GHz, and a return loss of less than −10 dB over the 11 GHz 3 dB passband.

Localized Temperature Stability in Multilayer LTCC

Low temperature cofired ceramic (LTCC) is a multilayer 3D packaging, interconnection, and integration technology. One of the advantages of LTCC is the ability it affords to integrate passive components via the cofiring processes. For LTCC modules with embedded resonator functions targeting high frequency applications, the temperature coefficient of resonant frequency (τf) is a critical parameter. The base dielectrics of commercial LTCC systems have a τf in the range of −50 ppm/°C to −80 ppm/°C. This study explores a method to achieve zero or near zero τf embedded resonators by incorporating τf compensating materials locally into a multilayer LTCC structure. Chemical interactions and physical compatibility between the τf modifiers and the host LTCC dielectrics are investigated. A stripline (SL) ring resonator with near zero τf is demonstrated in a nonzero τf commercial LTCC.

Novel Low‐Temperature Cofired Ceramic System Containing Plate‐like Filler

A new LTCC (low‐temperature cofired ceramic) system containing Al2O3platelets instead of conventional Al2O3particles as a filler material was investigated. As the Al2O3platelets were arranged parallel to the casting direction during tape casting and had preferred orientation in the green tape, the linear shrinkage of the green body took place mainly in the direction perpendicular to the platelet planes with just a little shrinkage in the horizontal without any external force during sintering. The intended anisotropic shrinkage of the new LTCC system made the constrained sintering of 10‐mm‐thick ceramic body possible, which decreased the sintering shrinkage tolerance in thex–ydirections of the 10‐mm‐thick ceramic body from 0.5% to 0.05%. The new LTCC system showed equivalent electrical properties and remarkable physical properties in comparison with a conventional LTCC system.