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.     

Influences of the Glass Phase on Densification, Microstructure, and Properties of Low-Temperature Co-Fired Ceramics

Multilayer ceramic devices based on low-temperature co-fired ceramics (LTCC) materials provide a very promising technology. Most LTCC tapes available today contain considerable fractions of glass powders to lower the sintering temperature. However, the glassy phases offer more possibilities to set a proper sintering behavior, on the one hand, and to tailor the desired properties of the final LTCC substrate, on the other. The exploitation of demixing and subsequent crystallizing glass compositions was shown on an example of a low-permittivity (4.4)—low-loss (1.5 × 10⁻³) LTCC with a high quartz content. In another LTCC material, undesired demixing could be restricted and the crystal phase anorthite could be triggered by partial dissolution of alumina in the liquid phase during sintering. To estimate the effect of silver diffusion in the latter material, the surroundings of a pure silver via were studied. A silver-contaminated range of 50 μm was detected. Using model glasses containing silver oxide, a strong influence of dissolved silver on viscosity and crystallization behavior of the liquid phase was demonstrated. The dielectric properties of the sintered substrates were not degraded.     

Influence of Low-Temperature Co-Fired Ceramics Green Tape Characteristics on Shrinkage Behavior

To establish a better understanding of the complex densification and shrinkage processes of low-temperature co-fired ceramics (LTCC) and to improve the dimensional control in the manufacture of LTCC multilayer devices, the influence of glass, composite, and microstructural green tape characteristics on the densification and shrinkage behavior of LTCC materials, with special focus on the development of anisotropy, was investigated. To study the influence of these factors, a commercial LTCC system was analyzed regarding chemical and microstructural composition as well as sintering behavior. The results of the analysis showed that the commercial LTCC system is composed of alumina as a ceramic filler and a CaO–SiO₂–B₂O₃–Al₂O₃ glass. Based on these results, a similar glass was produced. To understand the mechanisms of densification, its wetting behavior and viscosity as a function of temperature were investigated. As developed glass was mixed with an alumina powder and milled down to average grain sizes of 1, 2, and 3 μm, respectively. From these composite powders, slurries were prepared and tape cast. The sintering kinetics including onset temperature, development of viscous flow as well as phase development of both commercial and internally developed LTCC tapes LTCC tapes in relation to their modified composition and green tape structures were analyzed in situ by means of optical dilatometry, thermo-mechanical analysis (TMA), and high-temperature-X-ray diffraction. The viscous behavior of the glass-filler composites was determined by means of cyclic dilatometry in a TMA device.     

Impact of additives and processing on microstructure and dielectric properties of willemite ceramics for LTCC terahertz applications

The paper presents the fabrication procedure, microstructure and dielectric properties of the low temperature cofired ceramics (LTCC) based on Zn₂SiO₄ doped with AlF₃, CaB₄O₇, Li₂TiO₃ and MgTiO₃. The heating microscope studies and differential thermal analysis were used for characterization of the behavior of the green tapes and ceramic samples during heating up to high temperatures. The microstructure and composition were analyzed by scanning electron microscopy, X-ray energy dispersive spectroscopy and XRD method. The dielectric properties were investigated in three frequency regions: 100 Hz–2 MHz, 90–140 GHz and 0.15–3 THz. The developed materials are promising candidates for the LTCC submillimeter wave applications due to a low sintering temperature of 900–980 °C, good compatibility with silver pastes and good dielectric properties – a low dielectric permittivity of 6–6.8, a relatively low dissipation factor of 0.005–0.008 at 1 THz, and a weak temperature dependence of dielectric permittivity.     

Influence of processing and conduction materials on properties of co-fired resistors in LTCC structures

The motivation of this study is the need for fundamental understanding of the effects of processing conditions on the electrical properties of low-temperature co-fired ceramic (LTCC) tapes, those screen printed with commercial thick-film pastes of electronic components for increased reliability. The method of the study is realized by analyzing the physical and chemical effects of mono/multilayer firing and firing temperatures on the temperature coefficient of resistance (TCR) and sheet resistance (SR) values of the positive temperature coefficient (PTC) resistors screen-printed on LTCC tapes. The results are discussed with respect to the information obtained by the scanning electron microscopy (SEM), electro dispersive X-ray analysis (EDXS), X-ray and dilatometry analysis. It is shown that the content of pastes combined with varying processing conditions result in deviation from expected TCR and SR values due to the chemical and/or mechanical reactions.     

Effect of Friction on Inhomogeneous Shrinkage Behavior of Structured LTCC Tapes

Vias, cavities, and other cutouts are significant inhomogeneities in low temperature co-fired ceramics (LTCC) tapes and lead to inhomogeneous shrinkage during sintering, which has a negative effect on the quality of the final multilayer device. The influence of such cutouts on the shrinkage behavior of LTCC tapes was investigated by an exact measurement of the geometry before and after sintering and by in situ observations with an optical dilatometer. The investigations show a strong influence of cutouts on the magnitude of shrinkage inhomogeneities. This effect is more pronounced, if the tapes become thinner, the dimensions of the cutouts become larger, or their position becomes less centric. It is shown that the most important factor on the occurrence of shrinkage inhomogeneities in tapes with cutouts is the static friction of the LTCC material on the setter. Severe warpage is caused by interlocking effects, which occur at bumps of the rough setter surface when the inner edges of the cutouts are pulled over the setter. By using a separating agent between the LTCC tape and the setter, the static friction could be minimized, which eliminates the sintering inhomogeneities.     

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.     

Low-pressure,thermo-compressive lamination

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

Fabrication and fluidic characterization of static micromixers made of low temperature cofired ceramic (LTCC)

Microreaction devices used for chemical synthesis must possess a high resistance against corrosive chemicals. Therefore, microreaction devices were made of glass, steel or ceramics. Photolithographic steps combined with etching processes as well as micropowder blasting or micromilling processes were applied for the formation of appropriate structures. The low temperature cofired ceramics (LTCC) technology combines easy structuring, assembling and packaging techniques with the high chemical resistance of a glass ceramic material. In contrast to the known ceramic technologies, the LTCC technology enables a fast and easy fabrication of microfluidic devices. Here, we present two micromixers made of LTCC and its fluidic characterization. Laser ablation was used for the structuring of green tapes which were layered and cofired to form the micromixers. X-type fluidic barriers were realized inside a squared meandered channel of about 160 mm length. A meandered channel mixer without X-type mixing structures was used as a reference. The pressure drop was measured for aqueous media with various viscosities and the friction factors were calculated. An exponential equation for the friction factor prediction is given. The residence time distribution was determined for both devices by pulse trace experiments and the dispersion model was used to describe the residence time distribution for low Reynolds numbers.     

Synthesis,structural and morphological characteristics,magnetic and optical properties of Co doped ZnO nanoparticles

Zn_1−xCo_xO (x==0.05, 0.10, 0.15) nanoparticles have been synthesized by an alternative wet-chemical synthesis route using the SimAdd technique. The as-obtained powders were investigated by FT-IR spectroscopy, X-ray diffraction and thermal analysis correlated with evolved gas analysis (TG–DTA–FT-IR) in order to determine their chemical nature, crystalline structure and to establish the decomposition sequences. The precipitates are generally amorphous, but low-intensity reflection peaks assigned both to the zinc oxalate dihydrate, and zinc hydroxide can be observed in the recorded patterns, indicating that hydroxy-oxalate precipitates were obtained. The structure, morphology and magnetic properties of the thermally treated samples have been investigated by X-ray diffraction, FT-IR, HRTEM, SAED, UV–vis and EPR. XRD studies reveal a hexagonal wurtzite-type structure for all Zn_1−xCo_xO samples. TEM investigations show particle size between 28 and 37 nm, with spherical and polyhedral shapes and with tendency to form aggregates. The presence of a Co₃O₄ secondary phase was evidenced by XRD, UV–vis and EPR for the Zn_0.85Co_0.15O sample. The ferromagnetic behavior of the samples was revealed. The paper highlights that by varying the cobalt concentration it is possible to modulate the structural, morphological, optical and magnetic properties.     

Adhesion Performance and UV-Curing Behaviors of Interpenetrated Structured Pressure Sensitive Adhesives with 3-MPTS for Si-Wafer Dicing Process

Abstract

As Si-wafers, as used in the electronic industry, become thinner and thinner, it is important to investigate the conditions which are suitable for easily peelable acrylic dicing tapes. In the ‘pick-up’ process, the adhesion strength decreased after UV irradiation as a result of polymer network formation. In this study, interpenetrating polymer network (IPN) structured acrylic pressure sensitive adhesives (PSAs) were investigated with two different types of UV irradiation — a steady UV irradiation and a pulsed UV irradiation of 100 mJ/cm². The PSAs binder contained 2-ethylhexyl acrylate (2-EHA), acrylic acid (AA) and 3-methacryloxypropyl trimethoxysilane (3-MPTS). The hexafunctional monomer, dipentaerythritol hexacrylate (DPHA) and 3-methacryloxypropyl trimethoxysilane (3-MPTS) were used as diluent monomers. The adhesion performance as related to the peel strength and the tack properties on the Si-wafer substrates, was examined with increasing UV dose. The effect of UV-curing on the behavior and viscoelastic properties of the ‘pick-up’ acrylic tapes was investigated using Fourier transform infrared — attenuated total reflectance spectroscopy (FTIR–ATR) and an advanced rheometric expansion system (ARES). It is also necessary to consider the contaminants on the Si-wafer substrates left behind after releasing the dicing tapes, because of possible damage to the Si-wafers and subsequent processes. Field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) analysis revealed little residue on the Si-wafer after removing the tapes and after more than the specific level of UV dose.     

Chemical solution deposition of pure and Gd-doped ceria thin films: Structural,morphological and optical properties

High quality smooth, uniform and crack-free ceria and gadolinium doped ceria (GDC) thin films were prepared on Si and Si/YSZ substrates by chemical solution deposition. The thermal behavior of Gd-Ce-O precursor was investigated by TG-DSC measurements. The phase purity and structure of deposited films were evaluated using X-ray diffraction (XRD) analysis and Raman spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed for the estimation of surface morphological features. Oxidation state of Ce ions in fabricated films was analyzed by X-ray photoelectron spectroscopy (XPS). Optical properties were evaluated by diffuse reflectance UV–vis spectrometry. Thickness of the films can be controlled by applying a certain number of spin coating cycles. A linear relation between the thickness of the films and the number of deposited layers was observed. The single-layer thickness was determined to be approximately 20 nm. The influence of annealing temperature and Gd content on the film structure, morphology and optical properties was studied and discussed. The dependence of an optical band gap as a function of grain size was demonstrated.     

Enhancement of corrosion protection of mild steel by chitosan/ZnO nanoparticle composite membranes

The development of active corrosion protection systems for metallic substrates is an issue of prime importance for many industrial applications. Nanostructured chitosan/ZnO nanoparticle films were coated on mild steel by sol–gel process, dip coating technique. Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corrosion resistance for metal substrates. Further, the sol–gel method is an environmentally friendly technique of surface protection which has traditionally been used for increasing corrosion resistance of metals. Films so formed were characterized by UV–vis absorption spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray fluorescence spectrometry (EDX). Corrosion protection behavior of these coated mild steel substrates in 0.1 N HCl solutions was evaluated by potentiodynamic polarisation studies (Tafel), linear polarisation studies (LPR), electrochemical impedance spectroscopy studies (EIS).     

Nucleation and Growth of Diamond Films on Ni-Cemented Tungsten Carbide: Effects of Substrate Pretreatments

The nucleation and growth of diamond films on Nicemented carbide is investigated. Substrates made of WC with 6 wt% of Ni were submitted to grinding, and then to different pretreatments (scratching, etching, and/or decarburization) before diamond deposition. Diamond synthesis was carried out by hot-filament chemical vapor deposition (HFCVD) using a mixture of CH₄ (1% v/v) and H₂. Depositions were performed for different lengths of time with the substrates at various temperatures. The specimens were analyzed before and after deposition by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffractometry (XRD). Raman spectra showed that the phase purity of the diamond films was not affected by the presence of nickel on the substrate surface. After wet etching pretreatments, the nucleation of diamond was enhanced, mainly at the WC grain boundaries. Continuous films were obtained on scratched and etched substrates. The decarburizing treatment led to the formation of metallic tungsten and of brittle nicke–tungsten carbide phases. These phases reacted in the early stages of diamond film formation with gaseous carbon species with a parallel process which competes with stable diamond nucleus formation. The diamond film formed after long-term deposition on these samples was not continuous.     

Evidence of Electrochemical Resistance on Ternary V-C-N Layers

In this research, vanadium carbo-nitride (VCN) coatings were synthesized via physical vapor deposition with the aim to determinate the electrochemical behavior of the VCN layers on industrial steel substrates. The VCN coatings, deposited at various negative bias voltage were characterized by X-ray diffraction (XRD), exhibiting the crystallography orientations corresponding to a mix of VCN, VC and VN phases while the X-ray photoelectron spectroscopy (XPS) measurements were used to determinate the chemical composition of the metallic carbon-nitride materials. By using electrochemical impedance spectroscopy (EIS) and Tafel curves, it was possible to estimate the electrochemical behavior of the VCN coatings in a sodium chloride (NaCl) solution. Moreover, scanning electron microscopy (SEM) was performed to analyze morphological and chemical surfaces changes on the VCN layer due to the reaction in NaCl/VCN/steel surface interface. The electrochemical behavior of the VCN coatings in relation to the uncoated AISI 8620 steel showed a reduction of 98% in the corrosion rate, indicating that the applied VCN coatings may be a promising material for industrial applications.     

Low-Sintering High-k Materials for an LTCC Application

High-k LTCC tapes with ultralow sintering temperatures were developed from lead-free perovskite powders. Lowering of the sintering temperature from 1250°C down to 900°C has been achieved by means of ultrafine ceramic powders in combination with suitable sintering aids. The tape-casting process has been optimized for ultrafine powders with an enhanced sintering activity. Low-sintering high-k tapes of a thickness down to 40 μm, suitable for LTCC processing, were obtained. The sintering behavior of these high-k tapes has been studied and compared with other LTCC materials. Dielectric properties of the high-k material have been investigated on a multilayer test structure consisting of up to 20 dielectric layers. After metallization with an Ag conductor, the green tapes were stacked and laminated. Sintering of these multilayer stacks at 900°C gives dense ceramic samples. Permittivities up to 2000 have been obtained, together with low dielectric losses. Material compatibility with several Ag/Au-thick-film-paste systems has been tested.     

Cold chemical lamination of ceramic green tapes

The cold chemical lamination (CCL) is a new technique of bonding ceramic green tapes into one 3D structure. Instead of a standard thermo-compression method, new solvent-based lamination is presented. A film of a special chemical agent is put on the green tape surface. The solvent melts the surface. Then the tapes are stacked. The bonding of the green tapes is made at a room temperature. The new method is used for joining green tapes of the low temperature co-fired ceramics (LTCC). A quality of the bonding depends on the solvent type. The cold chemical lamination is examined on two types of the LTCC tapes: DuPont 943 and DuPont 951. Six types of the solvents are analyzed in the paper. The bonding quality and geometry of the test structures are examined. The lamination quality is investigated by the scanning electron microscope.     

Characterization of ultrathin electroactive films synthesized via the self-limiting electropolymerization of o-methoxyaniline

The electropolymerization of o-methoxyaniline under self-limiting deposition conditions yields ultrathin (<20 nm) coatings of an insoluble, low-molecular-weight polymer on planar indium-tin-oxide electrode substrates. The self-limiting nature of the electropolymerization is achieved by using citrate-buffered aqueous electrolytes (pH 4.7) in which the developing polymer that deposits at the electrified interface is neither conductive nor permeable to monomer. Although non-conductive as electrodeposited, the resulting poly(o-methoxyaniline) coating becomes electroactive when transferred to acidic aqueous electrolytes. The morphology and chemical structure of the poly(o-methoxyaniline) coatings are characterized by surface-sensitive methods including atomic force microscopy, specular-reflectance infrared spectroscopy, X-ray photoelectron spectroscopy, and electrochemistry. Fundamental understanding of the structure/property relationships derived from these investigations on planar substrates will ultimately be applied to three-dimensional electrode nanoarchitectures that incorporate such electroactive coatings for enhanced charge-storage functionality.     

Hot Plate Gas Sensors—Are Ceramics Better?

For modern gas sensors, low power consumption is expected. It is well known that with low temperature cofiring technology (LTCC) small compact sensors can be constructed. Compared with standard devices on alumina such sensors consume less power due to their lower thermal conductivity. However, simple replacement of substrate materials is not sufficient. LTCC offers the possibility to structure unfired tapes easily. Therefore, the sensor substrate may have almost any desired shape. In our first investigations, we showed that ceramic hot plates could be successfully constructed in LTCC technology. In contrast to standard configuration of thick-film gas sensors on alumina or even on LTCC, the hot plate principle allows to reduce significantly power consumption. Our tests showed possibilities to further decrease power consumption by laser forming of suspended beams. The obtained results were very promising and induced continuation of these works. This article shows recent results of investigations on hot plate structures. Tapes from different manufacturers have been used for sensor construction. The sensors were made by laser structuring of printed unfired LTCC tapes. Samples were evaluated by measurement and analysis of electrical properties as well as by long-term tests of integrated heaters. Design issues as well as stability issues are discussed in this contribution.     

Influence of Cold Low Pressure Lamination Parameters on the Joining of Low Temperature Co‐Fired Ceramics Green Tapes and Sintered Alumina Substrates

In this study, laminates consisting of sintered alumina substrates and green Low Temperature Co‐fired Ceramics (LTCC) tapes have been produced via Cold Low Pressure Lamination which is based on adhesive tapes for joining of layers at room temperature and pressures <5 MPa. The influences of lamination parameters such as temperature, pressure, and time on the quality of the green and sintered multilayer stack have been determined. If the bottom LTCC layer of an alumina–LTCC–LTCC laminate is metallized by screen printing defects such as crack formation can occur due to stress formation caused by constrained sintering. By adapting the lamination parameters, these stresses can be avoided. Another defect observed is cavities which form along the printed circuit lines. This type of defect is caused by the shrinkage of the circuit line width during firing; by reducing the height of the conductor line during screen printing, the cavity size can be reduced. In addition, different screen‐printed metallization layouts have been tested to determine the influence of line and spaces on the quality of sintered laminates.