Low Temperature Co-fired Ceramic Substrate Research Articles

Electro-thermal simulation and fabrication of LTCC hotplate with lead-free interconnects

PurposeThe purpose of this paper is to explore a new possibility of providing high-temperature stable lead-free interconnections for low-temperature co-fired ceramics (LTCC) hotplate. For gas-sensing application, a temperature range of 200°C-400°C is usually required by the sensing film to detect different gases which imply the requirement of thermally stable interconnects. To observe the effect of parameters influencing power of the device, electro-thermal simulation of LTCC hotplate is also presented. Simulated LTCC hotplate is fabricated using the LTCC technology.Design/methodology/approachThe proposed task is to fabricate LTCC hotplate with interconnects through vertical access. Dedicated via-holes generated on the LTCC hotplate are used to provide the interconnections. These interconnections are based on adherence and bonding mechanism between LTCC and thick film. COMSOL software is used for finite element method (FEM) simulation of the LTCC hotplate structure.FindingsThermal reliability of these interconnections is tested by continuous operation of hotplate at 350°C for 175 h and cycling durability test performed at 500°C. Additionally, vibration test is also carried out for the hotplate with no damage observed in the interconnections. An optimized firing profile to reproduce these interconnections along with the experimental flowchart is presented.Research limitations/implicationsResearch activity includes design and fabrication of LTCC hotplate with metal to thick-film based interconnections through vertical access. Research work on interconnections based on adherence of LTCC and thick film is limited.Practical implicationsA new way of providing lead-free and reliable interconnections will be useful for gas sensor fabricated on LTCC substrate. The FEM results are useful for optimizing the design for developing low-power LTCC hotplate.Originality/valueAdherence and bonding mechanism between LTCC and thick film can be used to provide interconnections for LTCC devices. Methodology for providing such interconnections is discussed.

Selective metallization of silicon and ceramic substrates

PurposeThe purpose of this paper is to present the possibility of technology of chemical metallization for the production of electrodes and resistors based on Ni–P alloy on silicon (Si), alundum (Al2O3) and low temperature cofired ceramic (LTCC) substrates. The developed technology provides low cost in any form.Design/methodology/approachDuring the study monocrystalline Si plates and Al2O3 and LTCC substrates were used. On the surface of the substrates, the electrodes (resistors) by the electroless metallization were made. Subsequently, the electrical parameters of obtained structures were measured. Afterwards, trial soldering was made to demonstrate that the layer is fully soldered.FindingsOptimal parameters of the metallization bath were specified. As a result of the research conducted, it has been stated that the most appropriate way leading to the production of soldered metal layers with good adhesion to the portion of selectively activated Si plate and Al2O3 and LTCC substrates comprises the following technology: masking, selective activation, nickel-plating of activated plate. Such obtained metal layers have a great variety of application; in particular they can be used for the preparation of electric contacts in Si solar cells, production of electrodes and resistors and production of electrodes in thermoelectric structures.Originality/valueThe paper presents a new, unpublished method of manufacturing electrodes (resistors) on Si plate and Al2O3 and LTCC substrates.

Tailored and deep porosification of LTCC substrates with phosphoric acid

Low temperature co-fired ceramics (LTCC) as an advanced technology for robust assembly of electronic components, has attracted significant attention in a wide application range such as in wireless communication or automotive radar systems. However, accurate designs of micromachined devices operated at high frequencies require substrates with regions of tailored permittivities. Introduction of controlled porosity into the substrate via wet-chemical etching procedure, is a promising approach for permittivity reduction which can be applied to commercially available LTCC without necessitating to alter their composition or sintering process. In the present study, by selective dissolution of celsian phase a very deep porosification (highest reported so far) could be realized while preserving the surface quality. Also, by a careful selection of the etching parameters, the depth of porosification and hence the permittivity reduction can be delicately tailored. Laser ablation inductively coupled plasma mass spectrometry was used for the investigation of chemical compositions of substrates.

An LC Wireless Microfluidic Sensor Based on Low Temperature Co-Fired Ceramic (LTCC) Technology.

This work reports a novel wireless microfluidic biosensor based on low temperature co-fired ceramic (LTCC) technology. The wireless biosensor consists of a planar spiral inductor and parallel plate capacitor (LC) resonant antenna, which integrates with microchannel bends in the LTCC substrate. The wireless response of the biosensor was associated to the changes of its resonant frequency due to the alteration in the permittivity of the liquid flow in the microchannel. The wireless sensing performance to different organic liquids with permittivity from 3 to 78.5 was presented. The measured results are in good agreement with the theoretical calculation. The wireless detection for the concentration of glucose in water solution was investigated, and an excellent linear response and repeatability were obtained. This kind of LC wireless microfluidic sensor is very promising in establishing wireless lab-on-a-chip for biomedical and chemical applications.

Vertical LTCC Integrated Rectangular Waveguide and Transitions for Millimeter-Wave Applications

A new vertical guiding structure in low-temperature cofired ceramics (LTCC) substrates is proposed. This vertical LTCC integrated waveguide (V-LIW) allows transmitting signals and realizing components in the body of the LTCC substrate perpendicular to its surface. This vertical integration leads to considerable footprint reduction while opening the door to higher frequency coverage as well as easy implementation of $E$ -plane structures. Discontinuities in V-LIW and transitions between the proposed V-LIW and conventional horizontal guiding structures such as coplanar waveguides, standard rectangular waveguides, and substrate integrated waveguides are also introduced. Applications to the design of V-band and W-band filters, couplers, and power dividers are presented. Several prototypes are fabricated with measurement results showing a good agreement with simulation data.

Comprehensive (S)TEM characterization of polycrystalline GaN/AlN layers grown on LTCC substrates

Three GaN layers grown on polished AlN buffer films previously sputtered on Low-Temperature Co-fired Ceramic substrates have been studied by X-Ray Diffraction, Atomic Force Microscopy and Electron Microscopy related techniques. This allowed to assess the quality of the whole fabricated III-N/LTCC heterostructures considering factors such as superficial roughness, crystallinity, structural features (e.g.: lattice defects) and chemical homogeneity. All the AlN and GaN films were chemically uniform. However, for different regions and growth conditions, properties like the GaN average grain size or roughness varied significantly, and polycrystalline or partially single-crystalline GaN areas, with an elevated density of lattice defects, were identified. The particular surface structure of the LTCC substrate was regarded as key for the formation of these features, as it notably affected the AlN structure. Additional characteristics, like a different degree of c-axis orientation of the GaN grains or formation of small cubic GaN domains depending on the fabrication conditions, were also detected.

A SiC BJT-Based Negative Resistance Oscillator for High-Temperature Applications

This brief presents a 59.5 MHz negative resistance oscillator for high-temperature operation. The oscillator employs an in-house 4H-silicon carbide BJT, integrated with the required circuit passives on a low-temperature co-fired ceramic substrate. Measurements show that the oscillator operates from room temperature up to 400 °C. The oscillator delivers an output power of 11.2 dBm into a 50 $\Omega $ load at 25 °C, which decreases to 8.4 dBm at 400 °C. The oscillation frequency varies by 3.3% in the entire temperature range. The oscillator is biased with a collector current of 35 mA from a 12-V supply and has a maximum dc power consumption of 431 mW.

A Module-Level Spring-Interconnected Stack Power Module

This paper presents the design, fabrication, and characterization of a module-level spring-interconnected stack power module for power semiconductor devices. The benefit of multilayer design capability offered by a low-temperature cofired ceramic (LTCC) substrate is used for this module-level power module structure. A half-bridge configuration is demonstrated using two standalone wire-bondless power modules with parallel power semiconductor devices stacked on top of each other. An interconnection scheme consisting of conductive copper clamps and a spring-loaded LTCC interposer is adopted to form a module-level 3-D package structure. The low loop and stray parasitic inductances for the 3-D stack are achieved using an antiparallel current path configuration along with the wire-bondless approach and a low-profile spring interposer. For demonstration, silicon insulated-gate bipolar transistors and silicon carbide (SiC) Schottky barrier diodes are used to fabricate the hybrid 3-D stack. Static and transient electrical characteristics of the 3-D stack power module are evaluated through simulation and experimental measurements and compared with a wire-bonded half-bridge power module to investigate the performance improvement in switching characteristics of the 3-D module. High-voltage isolation test, leakage current test, and thermal cycling are performed to validate the spring-interconnected module-level 3-D stack structure.

Feasibility of an automotive radar antenna at 77 GHz on LTCC substrate

The feasibility of microstrip planar antenna arrays printed on a low-temperature co-fired ceramic (LTCC) substrate for an automotive long-range radar application at the frequency of 77 GHz is investigated. The antenna configuration consists of three receiving (RX) and two transmitting (TX) microstrip patch antennas. The full three-dimensional electromagnetic simulations show, despite the high permittivity and losses of the LTCC substrate, that the achievable impedance matching is wider than 1 GHz and the combination of TX and RX radiation patterns satisfies typical requirements in the azimuthal and elevation planes. Problematic, however, is the reduced antenna gain that can be achieved for a given radiation pattern shape due to the relatively high-dielectric losses.

On the porosification of LTCC substrates with sodium hydroxide

Outstanding material properties of low temperature co-fired ceramics (LTCC) make them the technology of choice when targeting the realization of robust, highly integrated substrates or packaging solutions for micromachined devices. However, the relatively high permittivity limits their utilization in high-frequency applications, as in addition defined areas with reduced permittivity in one single LTCC layer would be most beneficial. The wet-chemical porosification method under alkaline conditions is a novel and most advantageous approach which can be applied for permittivity reduction through locally embedding air into the LTCC surface in its as-fired state with low impact on the surface characteristics. The resulting porosity of the LTCC substrates after the etching process is strongly affected by the LTCC tape composition as well as the etch parameters. Since the chemical composition of the tape is desired to be unaltered, optimizing the etching parameters, namely etchant concentration, etching time, and bath temperature is a reasonable procedure which allows tailoring the porosification process for the LTCC tapes in their as-fired state.In this work the etching behavior and surface morphology of the LTCC substrates have been studied, both qualitatively and quantitatively, and detailed gravimetric and roughness measurements, as well as porosification depth investigations, were carried out. The conducted analyses suggest a dominating reaction-controlled mechanism for the etching process. In addition to generating a tailored porosity in the surface-near region, overall thickness reduction of the LTCC through the overall dissolution of the LTCC surface under a defined etching condition was also possible. The experimental results were finally fitted into a nonlinear polynomial model for providing an experimental basis in order to identify the most crucial parameters to achieve a tailored porosity.

Design, fabrication and characterization of laser patterned LTCC micro hotplate with stable interconnects for gas sensor platform

The present paper presents design, fabrication and characterization of low temperature co-fired ceramics (LTCC)-based micro hotplates. Laser irradiation is used to pattern fired conductive paste over LTCC substrate to generate fine meander shaped heater patterns. These micro hotplates consumes low power (~ 500 mW for 300 °C) compared to standard alumina technology due to low thermal conductivity (3–4 W/(m-K) of LTCC substrate. This technology is inexpensive for small series compared production as compared to the silicon technology. The structuring ability of LTCC green tapes is utilized to provide dedicated through via-holes in micro hotplate design which lowers the thermal mass and helps in providing special fused electrical interconnects. These connections are found thermally stable and results are very promising. Thermal properties such as temperature distribution and power consumption have been investigated using FEM (Finite Element Method) simulations on COMSOL software.

76–81 GHz LTCC antenna for an automotive miniature radar frontend

AbstractFor a fully 360° detection around a vehicle, novel automotive radar system concepts consist of up to eight radar sensors. The existing sensor-mounting areas, such as front grill or bumper corners would no longer be sufficient. Therefore, additional mounting positions such as B-pillars and side skirts have to be considered, where the radar can observe the side area of the vehicle. However, these new mounting positions usually offer significantly less space, than the established mounting areas. The solution is, to build separate miniature radar frontends that can be placed all over the vehicle and are connected to one central signal processing and power supply unit. Investigations for a miniature radar frontend have been done, based on RF360 low loss non-shrinkage low-temperature cofired ceramic (LTCC) substrate. For the automotive radar band (76–81 GHz), an array antenna has been simulated, manufactured, and the radiation pattern has been measured. A first sensor with a miniature radar frontend based on an LTCC multilayer has been designed and manufactured.

Copper Plated through-Holes for 3D Electro-Thermal Systems

Three-dimensional (3D) packaging/integration of thermal devices, multi-chips, passives, sensors, electro-optics etc. is a key technical approach to optimize power density and create 3D electro-thermal systems that are optimally designed to route both thermal and electrical power. 3D electro-thermal systems are operated at high power density and temperature. These systems have to overcome the issues of high thermal-mechanical stress and poor reliability. Interconnects with plated-through holes (PTH) fabricated by using NANO3D’s eLOCOSTM process materials for thermal and mechanical management will enable the 3D electro-thermal system to withstand high thermal stress and high temperature. Low-cost electrochemical metallization was used because it drastically reduces the cost of 3D power systems by replacing costly, high-overhead metallization processes. Electroless Ni and Cu PTH. 10 mil and 20 mil PTH were fabricated in Low Temperature and High Temperature Co-fired Ceramic substrates (LTCC and HTCC, respectively). The ceramic surfaces were pre-cleaned and etched in organic solvent and alkaline solutions before an activation step. Surface activations for electroless plating were performed by using one of two methods: tin-based sensitization or silanization followed by palladium activation. A conformal barrier layer was applied to ceramic substrates via electroless plating. Nickel barrier and copper seed layers were electrolessly plated on activated LTCC substrates. Both nickel and copper readily plate inside the vias as well as on exposed surfaces. Cu PTH. Copper electroplating was performed in a suppression-based plating solution on LTCC/HTCC blanket and patterned via substrates after Ni/Cu barrier/seed layer deposition. Pulse-reverse plating in the plating solution containing two suppressors/surfactants was used to completely fill the holes and obtained a smooth copper surface (Fig. 1). The reverse-current step removes copper from the surface. Reverse-current step combined with the agitation of the plating solution are important for hole fill because it removes peaks, bulges and ridges preferentially. This eliminates buildup along the hole (via) edges that might cause the tops and bottoms to fill before the center, leaving large crevices within the via. Blanket plated substrates were analyzed in various ways post electroplating, including optical microscopy, profilometer, and adhesion testing. For Instron testing, many samples broke at the epoxy site rather than the plated copper layer. Adhesion between the copper and substrate was over 2.6 MPa. Testing Results. Thermal shock testing of NANO3D’s PTH substrates showed promising results for the adhesion of the Cu to the ceramic substrates. Only one of the samples showed obvious delamination after 500 cycles of thermal shock (Fig. 2). Of the five samples of commercial DBC used, only two alumina samples survived the 500 cycles. Both the aluminum nitride DBC showed signs of cracking in the ceramic after 200 cycles (Fig. 2). One alumina sample started showing signs of delamination around 200 cycles as well. SAM images showed the delamination in the failed samples and showed minimal delamination in a few others that was not visible under the microscope. Figure 1

Stability of miniaturized non-trimmed thick- and thin-film resistors

This paper is focused on reliability tests of non-trimmed miniaturized thin-film and thick-film resistors. Thick-film resistors are screen printed by polymer paste on LTCC (Low Temperature Co-fired Ceramic) substrate by two different approaches. Nonstandard precise screen printing process provide tolerance of resistivity less than 5% and thus further trimming is not necessary. OhmegaPly material with Nickel Phosphorous (NiP) metal alloy is used for thin-film resistors fabricated by subtractive process. Miniaturized resistors have dimensions 0.5 × 0.5 mm, and thus 1 square, with thickness 1 μm for thin-film and 20 μm for thick-film resistors. Stability of miniaturized resistors were tested by humidity test, thermal shocks, long-term thermal ageing, direct current stress, current pulses and simulation of soldering process using VPS (Vapour Phase Soldering). Resistivity of resistors is measured by four wire method before and after each set of test and relative change of resistivity is plotted in graphs. Influence of every test on each type of resistor is analysed.

Crystallization kinetics, sintering, microstructure, and properties of low temperature co-fired magnesium aluminum silicate glass-ceramic

We investigated the effects of sintering temperature and time on the dielectric and mechanical properties, sintering, microstructure, crystallization, and thermal expansion coefficient of Magnesium aluminum silicate glass-ceramic for the applications of low temperature co-fired ceramics substrates. The glass-ceramic has been prepared by melt quenching route. The crystallization kinetics of glass-ceramic was investigated using non-isothermal methods by differential thermal analysis. The activation energy for crystallization E was found to be 201.92 kJ/mol. Mg0.6Al1.2Si1.8O6 was the first crystalline phase forming in the glass-ceramic. With the increase of sintering temperature, the phases of ZrO2 and indialite precipitate successively. The coefficient of thermal expansion of glass-ceramic increased with the increase of the sintering temperature and time. With the increase of sintering temperature or holding time, the density, bending strength and dielectric constant of the material all increased first and then decreased. The samples sintered at 900 °C for 2 h showed the best properties: high bulk density (2.695 g/cm3), low dielectric constant (6.21) and low dielectric loss (1.2 ∗ 10−3), good bending strength (163 MPa) and low coefficient of thermal expansion (6.35 ∗ 10−6 K−1). The results showed that the prepared glass-ceramic can be considered as a potential candidate for the application of low temperature co-fired ceramic substrates.

Stable interconnections for LTCC micro-heater using isothermal solidification technique

PurposeThe purpose of this work is to explore the forms of intermetallic phase compounds (IMPCs) in Pt/In/Au and Pt/In/Ag joints by using isothermal solidification. This lead-free technique leads to formation of IMPCs having high-temperature stable joints for platinum-based micro-heater gas sensor fabricated on low temperature co-fired ceramic (LTCC) substrate.Design/methodology/approachProposed task is to make an interconnection for Pt micro-heater electrode pad to the silver and gold thick-films printed on LTCC substrate. Both Pt/In/Au and Pt/In/Ag configured joints with different interactive areas prepared at 190 and 220°C to study temperature and contact surface area effects on ultimate tensile strength of the joints, for a 20 s reaction time, at 0.2 MPa applied pressure. Those delaminated joint interfaces studied under SEM, EDAX and XRD.FindingsIMPCs identified through material analysis using diffraction analysis of XRD data are InPt3, AgIn2, AgPt, AgPt3, Au9In4 and other stoichiometric compounds. The interactive surface area between thick-films and temperature increment shows improvement in the formations of IMPCs and mechanical stability of joints. These IMPCs-based joints have improved the mechanical stability to the joints to sustain even at high operating temperatures. Elemental mapping of the weak joint contact interface shows unwanted oxide formations also reported. Physical inter-locking followed by the diffusion phenomenon on the silver substrate strengthen the interconnection has been noticed.Research limitations/implicationsInert gas environment creation inside the chamber to isolate the lead-free joint placed between heating stamp pads to avoid oxide formations at the interface while cooling which adds up to the cost of manufacturing. Most of the oxides at a joint-interface increase minute to moderate resistance with respect to the level of oxides took place. These oxides contributed heat certainly damage the micro-heater based gas sensors while functioning.Practical implicationsThese isothermal solidification-based lead-free solder joints formation replace the existing lead-based packaging techniques. These lead-free interconnections on ceramic or LTCC substrate are reliable and durable, especially those designed to work for heavy-duty engines, even at severe environment conditions.Originality/valuePlatinum micro-heater-based gas sensors handles over a wide-range of temperatures about 300 to 500°C. The specific temperature level of different oxide films (SnO2) on the micro-heater is capable of detecting various specific gases. This feature of platinum based gas sensor demands durable and mechanically stable joints for continuous monitoring.

Porosification behaviour of LTCC substrates with potassium hydroxide

The demand to meet advanced substrate requirements in terms of electrical, mechanical, thermal, and dielectric properties has led to an increasing interest in low temperature co-fired ceramics (LTCC). However, LTCC materials suffer from high permittivity. We recently showed that the wet-chemical porosification under acidic conditions allows the reduction of the permittivity of LTCCs in the as-fired state. In the present study, potassium hydroxide solution was employed as an alternative etchant which features a suitable bearing plane for further metallization lines. Various characterization techniques, including scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, and electron energy loss spectroscopy were used for investigation of the morphology and chemical composition of the substrates. Three-dimensional information of the surface topography was acquired by means of MeX® Alicona software and the obtained roughness parameters confirmed the advantage of the proposed approach over acid treatment when targeting an enhanced surface quality.

基于等刚度原理的LTCC基板密集孔区域研究

由于低温共烧陶瓷(LTCC：low temperature co-fired ceramic)基板上大片的密集孔区域的存在，对LTCC基板进行有限元仿真分析时出现模型网格数量极大，且网格质量较差等问题，导致仿真分析结果误差较大，计算时间长。本文针对上述问题，采用等刚度原理对密集孔区域进行等效，得到LTCC基板密集孔区域的简化模型及等效弹性模量，并对等效前后的模型进行有限元分析，验证等效的正确性。密集孔区域的等效研究达到了简化模型、提高计算精度和减少计算时间的目的，为以后具有相似特征的LTCC基板的有限元分析奠定基础。 Due to the presence of a large area of dense holes in the LTCC (low temperature co-fired ceramic) substrate, the number of LTCC substrate model meshes is very large and the mesh quality of the LTCC substrate is very poor during finite element simulation analysis, which leads to the error of the large simulation analysis, and the long calculation time. In this paper, in order to solve the above problems, the equivalent stiffness principle is used to obtain the equivalent area of dense holes. The simplified model and the equivalent elastic modulus of dense holes in the LTCC substrate are obtained. The models before and after the equivalent are validated by the finite element analysis to make sure the simplified model is right. The equivalent study of the dense hole area achieves the purpose of simplifying the model, improving the calculation precision and reducing the calculation time, and lays the foundation for the finite element analysis of the LTCC substrate with similar characteristics in the future.

Compact Lumped-Element LTCC Bandpass Filter for Low-Loss VHF-Band Applications

A compact multilayer lumped-element bandpass filter which employs a novel third-order schematic for the low-loss wide passband along with high selectivity is proposed in this letter. The filter characteristics are analyzed mathematically and graphically when the practical implementations of the filter get discussed in detail afterward. A prototype filter of extremely small size 0.004λ g × 0.004λ g × 0.0003λ g is realized in a ten-layer low-temperature co-fired ceramic substrate. Experimental results demonstrate that the proposed filter achieves a wide fractional bandwidth of 62% with remarkably low insertion loss, which shows great potentials for miniaturized high-resolution very-high-frequency communications applications.

Embedded Passive Circuit Elements in Low Temperature Co-Fired Ceramic Substrates for Aerospace Electronics

Abstract The packaging technology plays a critical role in the development of environmentally robust microelectronic systems. There is great demand for microelectronic systems that can withstand harsh environments and high temperatures in numerous applications including down-hole drilling, geothermal drilling, aerospace, heavy industrial, and automotive industries. In these applications, both sensors and interface electronics could be subjected to temperatures of 200°C and beyond. Standard plastic or epoxy based printed circuit boards are not reliable at these temperatures for long term operation. Packaging techniques such as Low Temperature Co-fired Ceramic (LTCC) offer a versatile and reliable alternative to form electronic substrates. LTCC is a multi-layer packaging system that also provides the ability to embedded passive components, such as resistors and capacitors, and thermal management structures within the substrate. This work evaluates the performance of LTCC embedded passive elements from 25°C to 225°C. A variety of passive component designs are evaluated using theoretical calculations, simulations, and measurements of fabricated test structures.