Low-temperature Co-fired Ceramic Technique Research Articles

Preparation of Sol-Gel-Derived CaO-B2O3-SiO2 Glass/Al2O3 Composites with High Flexural Strength and Low Dielectric Constant for LTCC Application.

Low-temperature co-fired ceramic (LTCC) substrate materials are widely applied in electronic components due to their excellent microwave dielectric properties. However, the absence of LTCC materials with a lower dielectric constant and higher mechanical strength restricts the creation of integrated and minified electronic devices. In this work, sol-gel-derived CaO-B2O3-SiO2 (CBS) glass/Al2O3 composites with high flexural strength and low dielectric constant were successfully prepared using the LTCC technique. Among the composites sintered at different temperatures, the composites sintered at 870 °C for 2 hours possess a dielectric constant of 6.3 (10 GHz), a dielectric loss of 0.2%, a flexural strength of 245 MPa, and a CTE of 5.3 × 10-6 K-1, demonstrating its great potential for applications in the electronic package field. By analyzing the CBS glass' physical characteristics, it was found that the sol-gel-derived glass has an extremely low dielectric constant of 3.6 and does not crystallize or react with Al2O3 at the sintering temperature, which is conducive to improving the flexural strength and reducing the dielectric constant of CBS glass/Al2O3 composites.

MXene/SnO2 heterojunction based chemical gas sensors

Two dimensional (2D) MXene decorated by the SnO2 nanoparticles has been successfully synthesized by the hydrothermal method. The formed MXene/SnO2 heterojunction based chemirestistive-type sensors exhibit an excellent sensitivity and selectivity to different concentrations of NH3 from 0.5 to 100 ppm at room temperature. The enhanced sensing properties can be attributed to two major factors. On one hand, 2D MXene provides a matrix which has a unique, selective adsorption ability to ammonia and good conductivity that makes the room-temperature sensing of the heterojunction based sensor feasible. On the other hand, the difference in the Fermi level of the 2D MXene and SnO2 drives the charge transfer at the interface of the heterojunctions enriching the electrons on the surface region of SnO2 and consequently enhancing the sensitivity. Furthermore, a wireless sensor made from an inductor-capacitor (LC) antenna and the as-synthesized MXene/SnO2 heterojunctions fabricated by the low-temperature-cofired ceramic technique (LTCC) was preliminary investigated. The wireless sensor shows a even faster response/recovery speeds (<30 s) and more stable response to different concentrations of ammonia when the response was expressed in the mode of resonant frequency indicating a promising practical application.

Microwave Characteristics of Thin-Film Passivation on Ceramic Substrate by Using DC Reactive Magnetron Sputtering

This paper presents a novel method to change the dielectric constant and enhance the average power-handling capability (APHC) to a thin-film coplanar waveguide (CPW) line through dc reactive magnetron sputtering. This paper discusses the effects of material properties, microwave characteristics, and APHC on the thin-film CPW line. An aluminum nitride (AlN) thin film can be deposited on the surface of a ceramic substrate to provide a substrate with a smooth surface. Using the proposed thin-film passivation on the ceramic substrate can effectively improve the thermal conductivity and enhance the APHC of a thin-film CPW line. In a physical experiment, a thin-film CPW line with a 3- $\mu \text{m}$ -thick AlN thin film was deposited on a 1000- $\mu \text{m}$ -thick ceramic substrate. Microwave losses (including conductor loss $\alpha _{c}(f)$ and dielectric loss $\alpha _{d}(f)$ ) dielectric constant and APHC were extracted from S-parameters that were measured up to 10 GHz. The method can be applied in ceramic fabrication processes, particularly in low-temperature cofired ceramic techniques.

Synthesis and Implementation of LTCC Bandpass Filter With Harmonic Suppression

In this paper, we utilize the coupling matrices to design a compact low-temperature cofired ceramic (LTCC) bandpass filter with wide stopband and high selectivity. One coupling matrix at the fundamental frequency is synthesized to obtain required passband responses. The other one at the third harmonic is manipulated to realize wide stopband. Two transmission zeros are appointed at both sides of the passband to enhance the skirt selectivity. They are generated by multiple cross-coupling paths, which are easy to realize in the multilayer LTCC structure. To reduce the circuit size, two coupled quarter-wavelength resonators are folded vertically and horizontally on different layers. An experimental filter is designed for verification. High skirt selectivity and wide stopband are observed in the measurement. Due to the LTCC techniques, the proposed circuit has a compact size of $2~\text {mm}\times 1.7~\text {mm}\times 2$ mm or $0.042\lambda _{g}\times 0.035\lambda _{g}\times 0.042\lambda _{g}$ . It is suitable for system-in-package applications.

Microwave and Millimeter-Wave LTCC Filters Using Discriminating Coupling for Mode Suppression

In this paper, a new discriminating coupling mechanism, which is realized by choosing suitable coupling region between the feeding lines and resonators, is proposed to suppress the unwanted resonance modes without affecting the desirable operating modes. Utilizing such a coupling mechanism, two low-temperature cofired ceramic (LTCC) bandpass filters (BPFs) are designed operating at microwave and millimeter-wave bands. One BPF realizes a wide stopband, since the third and fifth harmonics are suppressed by the discriminating coupling. The other one is an oversized 60-GHz BPF with the fundamental mode suppressed by the discriminating coupling. Due to the multilayer characteristics of the LTCC techniques, the circuit layout can be controlled flexibly to realize desirable discriminating coupling. The two filters are theoretically analyzed and implemented. Good agreement between the simulation and the measurement validates the proposed method.

Tunable SIW Structures: Antennas, VCOs, and Filters

Due to their high quality factor (hundreds), high power handling, and good isolation, SIW structures are excellent candidates for a variety of microwave devices, including filters, antennas, VCOs, and isolators. However, their use has been hindered due to their narrow BW and high sensitivity to the fabrication process. Tuning the SIW-based microwave structures has the advantages of covering more bands, the possibility of post-fabrication fine-tuning, and less crosstalk sensitivity. However, the conventional tuning methods applicable to regular microwave structures are not effective for SIWs. Finding a way to tune these structures over a wide tuning range while maintaining their high Q seems to be very appealing and, at the same time, very challenging. To address all these points in more detail, the main focus of this article was on frequency-tunable/ reconfigurable SIW structures. The reported tuning methods applied to SIW structures so far are studied, and their pros and cons are discussed in detail. Also, separate libraries for SIW-based tunable filters, antennas, and VCOs are provided.

Analysis and Design of a Novel Low-Loss Hollow Substrate Integrated Waveguide

In this paper, a novel hollow substrate integrated waveguide (HSIW) is presented for realizing low-loss millimeter-wave (mm-wave) transmission lines embedded in multi-chip modules. A new analysis method for the HSIW is proposed by treating it as a combination of a two-dielectric loaded rectangular waveguide (RWG) and standard substrate integrated waveguide, where an effective dielectric constant, e e , is introduced. An HSIW prototype in the Ka-band is fabricated using a progressive-lamination low-temperature co-fired ceramic technique. The measured results agree well with theoretical calculations and simulations. An average of 0.009-dB/mm loss is achieved in Ka-band, which is comparable to an air-filled RWG. This shows that the technique has great potential for further development to realize highly integrated mm-wave modules.

Design, Fabrication and Characterization of Fused Silica-Based Composites with an LTCC-Derived FSS

Laminated composites with a frequency selective surfaces (FSS) or more complex metamaterials are potential radome materials due to their unique characteristics of electromagnetic wave transmission. For making high-temperature resistant radomes, metamaterials or laminated composites with an FSS should be based on ceramic substrates. However, the processing methods for ceramic metamaterials are very limited and the conventionally used LTCC technique suffers from the shortcoming of large sintering shrinkage rates, which unfortunately impede the production of ceramic-based metamaterials. In this paper, a novel method of a low temperature co-fired ceramic (LTCC) technique combined with a technique of ceramic joining via green tapes was developed to fabricate the fused silica ceramic laminates sandwiched with the FSS. It was found that the newly developed composites with the FSS unit cells of the Ag-Pd strips exhibited near zero shrinkage of the unit cells, showed predictable transmission efficiencies of electromagnetic microwaves, and were able to overcome the poor transmission efficiencies below 11 GHz of the pure fused silica ceramic plates with an identical thickness.

Compact tunable bandpass filter with wide tuning range and enhanced stopband characteristics

A compact tunable bandpass filter (BPF) with wide tuning range and enhanced stopband characteristics based on low-temperature co-fired ceramic technique is proposed. The wide tuning range is realised by properly introducing an axial coupling structure. Three transmission zeros are obtained near the passband for enhanced frequency selectivity by introducing mixed electric/magnetic (E/M) coupling between the source/load and the taped inductor. Back-to-back varactor diodes are adopted to improve linearity characteristics. A tunable BPF with a 120% tuning range from 370 to 850 MHz has been designed and fabricated to verify the proposed design concept. The measured results are in good agreement with the electromagnetic simulation. The circuit occupies only 4.0 × 3.0 × 1.6 mm3.

Fabrication of 3-D Metamaterials Using LTCC Techniques for High-Frequency Applications

Metamaterials are artificially engineered metallo-dielectric microstructures that display strong resonance behavior although their electrical size is . Individually they behave like LC oscillators and collectively they give rise to effective permittivity and permeability that are highly dispersive in the resonance region and may even become negative. In this paper, metamaterials with 3-D interconnects are designed for low-temperature co-fired ceramic (LTCC) fabrication. The fabricated materials are characterized experimentally using a free-space measurement system in the 33-110 GHz range. Dupont 951 is chosen as the substrate with silver ink for metallization. Three-layer and five-layer samples were fabricated. The fabricated samples exhibit electric resonance, magnetic resonance, or both, depending on the orientation and geometry of the metallic microstructure. The materials are passive and may be modeled using series and/or parallel LC circuits. LTCC metamaterials are proposed for packaging applications in microwave integrated circuits that may require embedded passive inductors, capacitors, resistor elements, and circuits that are functional at required frequencies and are inactive at other frequencies.

Design aspects of microwave components with LTCC technique

Abstract Low temperature co-fired ceramic (LTCC) technology, widely used in the automotive industry, is now being employed in microwave applications. Several commercial materials with low dielectric losses at microwave frequencies and adequate thermomechanical properties have been introduced. Computer-aided design of three-dimensional circuits has also become available. These advances together with high-quality manufacturing technology have placed LTCCs at the forefront in the development of miniature microwave devices. The paper outlines LTCC technology placing emphasis on those essentials of the materials and processing technologies about which the microwave circuit designer needs to be aware. The discussion is illustrated by examples.The crucial issue of component reliability is also addressed. Although the integration of passive components into the structure improves reliability, the joints between the LTCC module and PCB remain as significant ‘weak link’. Therefore, thermomechanical and structural design is a key to reliable LTCC assemblies.Finally, some future trends the LTCC technology for microwave applications are outlined.

Characterization of Embedded Spiral Inductors in Co-Fired Ceramic Substrates

Embedded planarized inductors can be fabricated using the low-temperature co-fired ceramic technique. Larger inductance is obtained when using a spiral pattern, because the mutual inductance between adjacent conductors has a positive sign. In addition, because of their simple geometry and easy fabrication, spiral-type inductors are widely used. For embedded five- and ten-turn spiral coils, the inductances of about 0.3 and 1.2 µH were obtained, respectively. The value of inductance was almost unchanged in the frequency range between 500 kHz and 40 MHz. The quality factor reaches a peak with increasing frequency and then falls. The peak values (about 10 MHz) of the five- and ten-turn spiral coils are about 20 and 32, respectively.