Co-fired Ceramic Research Articles

Preparation of high-performance Bi-YG ferrite based on LTCC technology and theoretical analysis of its sintering mechanism

In the preparation process of Microwave Multi-Chip Module (MMCM), it is necessary to simultaneously sinter both microwave medium (ferrite) and silver electrodes. Hence, this necessitates keeping the sintering temperature below the melting point of silver (961 °C). An excessively low sintering temperature would significantly impair the electromagnetic properties of the ferrite material. To address this concern, this study synthesizes Bi-YIG (Y3-xBixFe5O12, x = 0–1.0) ferrites with optimized characteristics, including low ferromagnetic resonance line-width (ΔH) and high saturation magnetization (Ms), at 945 °C using Low Temperature Co-fired Ceramic (LTCC) technology. The mechanism of low-temperature liquid phase sintering, facilitated by a fluxing agent, was investigated using a theoretical model. The addition of Bi2O3 fluxing agent was found to reduce the reaction activation energy, disrupt the chemical reaction equilibrium, and promote low-temperature liquid phase sintering, thus lowering the critical sintering temperature below the melting point of silver. Furthermore, optimal bismuth content was identified to regulate the dynamic balance between grain growth and boundary diffusion, suppressing abnormal grain growth and enhancing the electromagnetic properties of the ferrite. This exploration, leveraging LTCC technology, may offer valuable insights for enhancing the overall performance of MMCMs.

Effects of LiF additives, sintering method on the crystal structure, and microwave dielectric properties of Li4CaSr(SiO4)2 ceramic for low temperature co-fired ceramic application

Two series of LiF-doped Li4CaSr(SiO4)2 (LCSS) ceramics with different LiF contents were synthesized by SSR and RS methods. The formation of single-phase LCSS + x wt% LiF (x = 0–4, SSR) (LCSSL04) and LCSS + y wt% LiF (y = 0–8, RS) (LCSSF08)ceramics with an orthorhombic structure (Pbcm space group) was confirmed by the Rietveld refinement of their XRD data. The addition of LiF improved the relative density and microwave dielectric properties of LCSS + x wt% LiF (0 <x ≤ 2, SSR) and LCSS + y wt% LiF (0 <y ≤ 3, RS). The LCSS +2 wt% LiF (SSR) ceramic exhibited good properties, presenting a dielectric permittivity (εr) of 7.03, quality factor (Q × f) of 62,195 GHz (at 11.6 GHz), and temperature coefficient of resonant frequency (τf) of −76 ppm/°C. Further, the LCSS +3 wt% LiF (RS) ceramic exhibited outstanding performances, with an εr of 6.1, Q × f of 103,086 GHz (at 15.8 GHz), and τf of −67 ppm/°C. The chemical compatibility with silver suggests that both of them are potential candidates for low temperature co-fired ceramic (LTCC) devices.

Machinability improvement in micro milling AlN after laser chemical milling

Processing microchannels inside laminated aluminum nitride high-temperature co-fired ceramics (AlN HTCC) packaging, a typical difficult-to-cut ceramic, can effectively solve the heat-dissipation problem of integrated chips used in smart skin. In order to improve the processing efficiency and quality of AlN, the machinability of AlN after laser chemical milling (LCM) was studied through the milling force, machined surface quality, surface defects, formation mechanism, and tool wear. This study established a milling force model that can predict the milling forces of AlN and analyses the reasons for the improvements in the milling force based on experimental data and predicted data. The results from the model and experiments demonstrated that the milling force of the laser chemical milling assisted micro milling (LCAMM) decreased by 85%–90% and 85%–95%, respectively, due to the amount of removal of a single edge was more uniform and the actual inclination angle increased during the cutting process in LCAMM. Moreover, the machined surface quality improved by 65%–76% after LCM because of less tool wear, fewer downward-propagating cracks generated during each feed, and the surface removal mode transformed from intergranular fracture to transgranular fracture, which effectively reducing tool wear and improving tool life. Finally, when feed per tooth and depth of cut were 0.4 μm/z and 5 μm, the optimal machined surface quality was obtained, with a roughness of 64.6 nm Therefore, milling after LCM can improve the machinability of AlN and providing a feasibility for the high-quality and efficient machining of microchannels.

Morphologies of Reactive Nanolayer Stacks Sputtered on Ceramic Low‐Temperature Cofired Ceramic Substrates Having a Micrometer‐Scale Surface Roughness

The deposition of reactive multilayer systems (RMSs) is investigated on low‐temperature cofired ceramic (LTCC) substrates having different surface morphologies. In this study, the morphologies of RMS layers that are deposited on glass‐ceramic LTCC substrates are analyzed. Different surface morphologies are prepared through pretreatments of the LTCC surface. The considered surfaces encompass an untreated natural LTCC surface, a modified sintered LTCC surface by laser ablation, a surface with a deposited metallization layer, and finally, a surface with an additional solder layer put on the deposited metallization layer. The different pretreatments lead to significant differences in the roughness of the LTCC substrates, resulting in different reaction velocities and peak temperatures on the various surface morphologies after the RMS's reaction. As a result, different grades of structural integrity (liftoff, crack formation) between the reacted RMS layer and the LTCC are observed.

Antenna Array Design Based on Low-Temperature Co-Fired Ceramics.

With the continuous development of wireless communication technology, the frequency band of 6G communication systems is moving towards higher frequencies such as millimeter waves and terahertz. In such high-frequency situations, wireless transmission requires antenna modules to be provided with characteristics of miniaturization, high integration, and high gain, which presents new challenges to the development of antenna technology. In this article, a 4 × 4 antenna array using multilayered low-temperature co-fired ceramic is proposed, operating in W-band, with a feeding network based on substrate-integrated waveguide, and an antenna element formed through the combination of a substrate-integrated cavity and surface parasitic patches, which guaranteed the array to possess the advantages of high integration and high gain. Combined with the substrate-integrated waveguide to a rectangular waveguide transition structure designed in the early stage, a physical array with a standard metal rectangular waveguide interface was fabricated and tested. The test results show that the gain of the antenna array is higher than 18 dBi from 88 to 98 GHz, with a maximum of 20.4 dBi.

Low insertion loss interposer approach for RF applications based on commercial LTCC tape, alkaline-free glass and printed metallization

Materials with high RF performance are in increasing demand due to the needs of modern telecommunications. Glass and low-temperature co-fired ceramics (LTCCs) are both interesting candidates for the design of complex signal transmission systems. Glass can be processed in large panels, and has suitable structure dimensions for silicon processing, but its multilayer capability is limited. LTCC is a mature technology for complex multilayer assemblies, but the structure dimensions and conductor line resolution are restricted by the need to use screen printing technology. The approach presented here combines the advantages of both technological domains: a thin glass sheet is bonded with no additional material to a LTCC multilayer ceramic and sintered to form a tight joint. Metallization of the glass is created using printable pastes and electroplating. Two fabrication routes are compared: etching of printed thick film metal layers, and semi-additive structuring. The results of RF measurements show a low attenuation per unit length for both types of metallization, indicating that our approach is a promising one for the integration of heterogeneous systems of RF transmission modules.

Dielectric Properties of Materials Used for Microwave-Based NOx Gas Dosimeters.

Nitrogen oxides (NOx), primarily generated from combustion processes, pose significant health and environmental risks. To improve the coordination of measures against excessive NOx emissions, it is necessary to effectively monitor ambient NOx concentrations, which requires the development of precise and cost-efficient detection methods. This study focuses on developing a microwave- or radio frequency (RF)-based gas dosimeter for NOx detection and addresses the optimization of the dosimeter design by examining the dielectric properties of LTCC-based (Low-Temperature Co-fired Ceramics) sensor substrates and barium-based NOx storage materials. The measurements taken utilizing the Microwave Cavity Perturbation (MCP) method revealed that these materials exhibit more pronounced changes in dielectric losses when storing NOx at elevated temperatures. Consequently, operating such a dosimeter at high temperatures (above 300 °C) is recommended to maximize the sensor signal. To evaluate their high-temperature applicability, LTCC substrates were analyzed by measuring their dielectric losses at temperatures up to 600 °C. In terms of NOx storage materials, coating barium on high-surface-area alumina resolved issues related to limited NOx adsorption in pure barium carbonate powders. Additionally, the adsorption of both NO and NO2 was enabled by the application of a platinum catalyst. The change in dielectric losses, which provides the main signal for an RF-based gas dosimeter, only depends on the stored amount of NOx and not on the specific type of nitrogen oxide. Although the change in dielectric losses increases with the temperature, the maximum storage capacity of the material decreases significantly. In addition, at temperatures above 350 °C, NOx is mostly weakly bound, so it will desorb in the absence of NOx. Therefore, in the future development of a reliable RF-based NOx dosimeter, the trade-off between the sensor signal strength and adsorption behavior must be addressed.

Processing strategy and composite regulation on dielectric performance in Li2O–Al2O3–B2O3 dielectric systems using SrTiO3

AbstractIn this paper, we investigated the effect of processing parameters and composite regulation on the structure and property relationship of lightweight low‐permittivity Li2O–Al2O3–B2O3 dielectric system. Li2BAlO4 ceramic and (1 − x)Li2BAlO4–xSrTiO3 ceramics were synthesized using a conventional solid‐state reaction method. Thermodynamic analysis, sintering temperature, phase formation, crystal structure, and microwave dielectric properties were investigated. Thermal analysis revealed a melting temperature of 847°C, suggesting a comparatively low temperature for the sintering of Li2BAlO4 ceramics. Li2BAlO4 crystalized into a monoclinic structure with P21/c space group, with [Al2B2O8] rings connected by [BO3] triangles and [AlO4] tetrahedra through Al–O–Al bridges. Dense Li2BAlO4 ceramics with a relative density of ∼97.7% were sintered at 750°C and had optimal microwave dielectric properties with relative permittivity (εr) of 5.13, quality factor (Q × f) of 22 610 GHz, and the temperature coefficient of resonance frequency (τf) of −112.5 ppm/°C. The 0.88Li2BAlO4–0.12SrTiO3 composite ceramic has excellent microwave dielectric properties with εr = 8.83, Q × f = 14 000 GHz, τf = 4.6 ppm/°C. The ceramics also demonstrated an inert behavior with silver electrodes proving its application in low‐temperature cofired ceramic (LTCC) technology.

Thermal characterization of ceramic pills at millimeter waves with a TE01δ cavity resonator

The progress in 5G/6G technology prompts a demand for new materials operating at millimeter waves that can be exploited as substrates for electronic devices or antennas. In particular, a rapid development of millimeter wave band communication technology demands ceramic materials, which could be applied in low temperature cofired ceramic (LTCC) techniques. This kind of ceramics should possess a low dielectric constant (εr), a low loss tangent (tanδ) and a near-zero temperature coefficient. In the view of above, there is an urgent need for accurate and reliable material microwave characterization techniques that can operate at frequencies well above 15 GHz. Due to the fact that the majority of LTCC ceramic materials can be produced in a cylindrical shape, a suitable characterization method exploits a cylindrical resonator operating at a TE01δ mode, which is dedicated to the characterization of low-loss materials also as a function of temperature.

Millimeter‐wave end‐fire dual‐polarized frequency scanning antenna using LTCC technology

AbstractThis letter proposes a millimeter‐wave dual‐polarized (DP) end‐fire frequency scanning antenna utilizing low temperature co‐fired ceramic (LTCC) technology. The antenna features DP radiating elements, each comprising a quasi‐Yagi antenna for horizontal polarization (H‐pol) and a pyramidal horn antenna for vertical polarization (V‐pol). These elements are excited via a mode‐composited transmission line (MCTL), integrating substrate integrated coaxial line (TEM mode) with substrate integrated waveguide (TE10 mode). To mitigate phase imbalances between polarizations during beam scanning, a delay line is embedded in each quasi‐Yagi element, ensuring consistent scanning ranges for the two polarizations. Furthermore, the implementation of amplitude weighting in the feeding networks contributes to the enhancement of antenna efficiency. A 1 × 8‐element prototype was fabricated and its performance is assessed. The results show closely matched beam scanning ranges for the two polarizations (±28° for H‐pol and −30° to +34° for V‐pol) across the 26–29 GHz frequency band, with a measured maximum peak gain exceeding 9.2 dBi for both polarizations. The proposed design emerges as a promising candidate for compact mmW frequency scanning front‐end antenna systems.

Effect of quenching-free method on the microstructure and properties of low-temperature sintered glass-ceramic composites

The Bi2O3–B2O3–SiO2–ZnO–Al2O3 (BBSZA) glass/Al2O3 composites are prepared by quenching and quenching-free methods and conducted a comparative analysis of their microstructures and properties. The results reveal that the BBSZA/Al2O3 composite obtained by quenching free method has smaller pores and superior performance. Specifically, at a BBSZA content of 50 wt%, the composite exhibits a thermal conductivity of 5.352 W/m·K and a flexural strength of 193.54 MPa, which are 1.38 times and 2.08 times those obtained using the quenching method, respectively. XRD and TEM analyses, along with parallel experiments using other glasses, suggest that presintering crystallization may be the prerequisite for achieving denser composites via the quenching-free method. BBSZA glass reacts with Al2O3 at the presintering process to generate a great amount of ZnAl2O4 crystals, which enhances grain boundary energy and increases sintering density. This work provides insights into the low-cost and high-performance development of low temperature co-fired ceramics (LTCC).

Controllable sintering behaviors of Al2O3 green tapes and co-firing compatibility of Al2O3/YSZ ceramics

As a typical insulating material, Al2O3 serves as the signal isolation device in multi-layer ceramic NOx sensors. However, the application of Al2O3-based insulating materials is limited by the co-firing compatibility between Al2O3 and yttria-stabilized zirconia (YSZ). In the present study, Al2O3 green tapes with different contents of γ-Al2O3 and SiO2–B2O3 (Si–B) are developed to promote the integration of Al2O3-based insulating material and YSZ. The sintering shrinkage of the obtained Al2O3 green tapes is greatly influenced by the ratio of γ-Al2O3 to α-Al2O3 at 1450 °C, e.g., the maximum shrinkage reaches 17.34% when the γ-Al2O3 to α-Al2O3 ratio is 1:2. The addition of Si–B greatly increases the low density of Al2O3 tapes due to the pores generated during the firing process of the ultrafine γ-Al2O3 powders. The sintering density increases to 97.1% when the Si–B content is 15 wt. %, and the sintering shrinkage reaches a relatively higher value of 17.25%. The obtained Al2O3-based insulating materials show a greatly co-firing matching performance with the YSZ green tapes. This work will contribute to further investigation of ceramic co-firing studies.

Structure evolution and τf influence mechanism of Bi1–xHoxVO4 microwave dielectric ceramics for LTCC applications

Bi1–xHoxVO4 (0.1 ≤ x ≤ 0.9) ceramics were prepared via a solid-state reaction method, and all the ceramics could be well densified in the 920–980 °C range. The ceramics with 0.1 ≤ x < 0.4 were composed of both monoclinic scheelite (M) and tetragonal zircon (T) phases, and a single M phase could be obtained in the range of x ≥ 0.4. The measured εr decreased from 58.9 (x = 0.1) to 14.7 (x = 0.9), so do the calculated values (εr(C−M) = 34.3–12.1), and the main reason for εr > εr(C−M) was the rattling of Ho3+ in the dodecahedron. Two points with zero τf appeared in Bi1–xHoxVO4 (0 ≤ x ≤ 1) ceramics, and the best microwave dielectric properties with εr = 16.6, Q × f = 18,400 GHz (f = 10.69 GHz), and τf = +3.29 ppm/°C were obtained in the Bi0.2Ho0.8VO4 ceramic. The change in temperature coefficient of ionic polarizability (ταm) caused by the rattling effect of cations is the physical essence that affects τf. Therefore, the rattling effect can be used as an effective mechanism to regulate τf in low-εr materials. Furthermore, there was no chemical reaction between Bi1–xHoxVO4 and Ag electrode, which indicates potential applications in low-temperature co-fired ceramic (LTCC) technology.

Development of LTCC IR-Emitter and Its Packaging

In this paper, thick film-based IR source is fabricated using multilayer Low Temperature Co-fired Ceramic (LTCC) Technology. The proposed emitter is developed using screen printing of platinum material on LTCC substrates. The highly uniform meander shaped structure is fabricated with the size of 3.5 mm × 3.5 mm. Electrical, optical and thermal characterization of the fabricated device are carried out. Device temperature reaches 600 °C at 6.5 V. Optical characterization of the developed device shows the spectral range in the mid-IR region with power consumption of ∼3 W. In-house indigenised package is developed using glass-metal seal technique for packaging of developed IR source. Different windows viz., quartz, LiF and CaF2 are used for packaging. Developed IR source demonstrate the potential to meet the performance, size and cost requirements for various applications. The developed LTCC based IR source has planar and simple structure with high temperature stable lead-free interconnects.

Thermal Sintering Matchability between Low‐Temperature Cofired Ceramic Substrate and Silver Pastes and the Effect of CuO on Silver Diffusion Inhibition

Low‐temperature cofired ceramic (LTCC) materials are a promising material for heat dissipation substrates due to their excellent thermal conductivity and thermal expansion coefficient similar to that of silicon. However, the difficulties in surface metallization techniques limit its application. This work uses Bi–B–Si–Zn–Al glass/ceramic composite as the substrate and Bi–B–Si–Zn–Al glass powder as the inorganic binder for silver pastes. The silver thick film with a glass content of 3 wt% is sintered at 800 °C for 30 min, and the samples have excellent electrical conductivity (surface square resistance of 0.29 mΩ £−1) and mechanical properties (interface bonding force of 17.56 MPa). In addition, the introduction of CuO into the glass phase can inhibit the silver diffusion phenomenon during the sintering process and improve the conductive performance of the material. The results are beneficial for the preparation and application stability of LTCC devices.

Application of Reactive Bonding Methods on LTCC Substrates

This paper discusses the application of reactive bonding for the area of Low Temperature Cofired Ceramics (LTCC) assemblies. The goal is to reduce the thermal-mechanical stresses during soldering by transferring heat only locally to the solder joints without heating the entire component. Such a reactive multilayer system (RMS) consists of alternating nanolayers (10-300 nm) of at least two metal components which produce an exothermal reaction after ignition. Although the deposition of an RMS is established on silicon substrates for the use in micro-electromechanical systems (MEMS), it is very challenging to create them on LTCC substrates. One of the main obstacles is to overcome all issues connected with the significant roughness, becasue it is not an optimum territory to deposit nanolayers. In this paper, different methods like chemical mechanical polishing (CMP) and laser ablation, to modify the surface morphology, are presented. A direct relation between the morphology and the exothermal reaction can be observed. In addition, 3-D Computational Fluid Dynamics (CFD) simulations were conducted to analyze the process in more detail. These simulations make use of a shoebox model with different layers and an adjustable user-defined function for the heat release of the RMS to adapt the reaction front velocity and the combustion temperature to the experimental values.

LTCC-based Ceramic Substrates for Identification of Trustworthy Electronics

The technology of low temperature cofired ceramics (LTCC) is already established in the production of reliable and robust electronic assemblies (e.g., multi-chip modules, system-in-package) as well as in safety-critical areas. With the incorporation of additional features based on surface profiles modular protection functions and barriers will be developed as a contribution to trustworthy electronics. Both obvious and hidden structures are investigated in a modular way, which in combination ensures a one-to-one tamper-proof identification.

Microstructure and dielectric properties of low-temperature sintered MgO-based ceramics at millimeter wave and terahertz frequencies

Low-temperature co-fired ceramics (LTCC) applied in millimeter/microwave and terahertz frequencies (5G/6G) have attracted a lot of attention recently. In this study, MgO-based dielectric ceramics were successfully sintered at 950∘C with the sintering aids: x wt.% of LiF fluoride ([Formula: see text], 4, 6, 8, 10) and 0.5[Formula: see text]wt.% of BBSZ (Bi2O3–B2O3–SiO2–ZnO) glass. BBSZ glass was introduced as another sintering aid to facilitate the sintering and densification. Crystalline structure and micro-morphology were investigated and analyzed. Dielectric properties ([Formula: see text], [Formula: see text], [Formula: see text]) at millimeter/microwave and terahertz wave frequencies were also studied. The ionic characteristics of Mg–O bond ([Formula: see text]), the lattice energy (U) and the bond energy (E) were calculated and analyzed. It is suggested that the optimal [Formula: see text], where [Formula: see text], [Formula: see text][Formula: see text]GHz (@12[Formula: see text]GHz) and [Formula: see text][Formula: see text]ppm/∘C at millimeter/microwave range. When the frequency was up to terahertz (1.0[Formula: see text]THz), the [Formula: see text] values were 8.8–9.35 and the tan[Formula: see text] were [Formula: see text]–[Formula: see text]. The experimental results indicated that the low-temperature sintered MgO-based ceramics have potential for millimeter/microwave and terahertz communication applications.

Advanced Dielectric Resonator Antenna Technology for 5G and 6G Applications.

We review dielectric resonator antenna (DRA) designs. This review examines recent advancements across several categories, specifically focusing on their applicability in array configurations for millimeter-wave (mmW) bands, particularly in the context of 5G and beyond 5G applications. Notably, the off-chip DRA designs, including in-substrate and compact DRAs, have gained prominence in recent years. This surge in popularity can be attributed to the rapid development of cost-effective multilayer laminate manufacturing techniques, such as printed circuit boards (PCBs) and low-temperature co-fired ceramic (LTCC). Furthermore, there is a growing demand for DRAs with beam-steering, dual-band functions, and on-chip alignment availability, as they offer versatile alternatives to traditional lossy printed antennas. DRAs exhibit distinct advantages of lower conductive losses and greater flexibility in shapes and materials. We discuss and compare the performances of different DRA designs, considering their material usage, manufacturing feasibility, overall performance, and applications. By exploring the pros and cons of these diverse DRA designs, this review provides valuable insights for researchers in the field.

A Compact Low-profile 5G Millimeter-wave Circularly Polarized Antenna Based on LTCC

In this paper, a circularly polarized millimeter-wave L-shaped dipole antenna based on low temperature cofired ceramic (LTCC) technology is proposed, which realizes compact size and low-profile performance. The designed antenna consists of radiation patches and the grounded coplanar waveguide-substrate integrated waveguide (GCPW-SIW) feeding structure, which connects each other by two via holes. The radiation patches include a pair of L-shaped patches and four parasitic patches. The simulated results show that the proposed antenna operates from 26.5 to 29.5 GHz for |S11|&lt;−10 dB and AR&lt;3 dB with a peak gain of 6.7 dBic. The antenna element size is only 0.58λ0×0.58λ0×0.056λ0, where λ0 is free-space wavelength at the center frequency of 28 GHz. A sample of the antenna is fabricated and measured to verify the proposed design, which has a good agreement with the simulated ones, indicating that the antenna has potential applications for the fifth generation (5G) mm-Wave n257 (26.5 - 29.5 GHz) frequency band communications and satellite communication systems.