Impact of sintering temperature on phase composition,microstructure, and porosification behavior of LTCC substrates

Phase development and changes in crystalline composition of LTCC material during the sintering process were investigated using in-situ X-ray diffraction (XRD) measurements. CeramTape GC was chosen as the chemically simplest model system composed of alumina particles and glass for the investigations. The chemical characterization and microstructural analyses of the tapes sintered with some representative firing profiles were performed by techniques such as (scanning) transmission electron microscope, energy-dispersive X-ray spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and XRD. Moreover, the porosification behavior of LTCC substrates fired at different peak temperatures was studied. These investigations are important for the subsequent wet chemical etching, representing an approach which allows to reduce locally the permittivity of LTCC tapes. Treatment with a KOH solution shows non-selective etching behavior for all substrates. In addition, highly porous silica structures corresponding to Ca and Al depletion from the anorthite phase were observed in all samples after etching treatment.     

Graphical based characterisation and modelling of material gradients in porosified LTCC

The chemical composition of low temperature cofired ceramics (LTCC) allows to locally embed air into sintered substrates by a selective wet chemical etch process. Therefore, LTCC substrate with areas of low permittivity can be created without material combination. The presented graphical method of material component contrast images enables the evaluation of their most important material properties which are their component distribution, their porosification gradient and their residual bearing surface. The graphical method, including focused ion beam and scanning electron microscopy analyses, is applied to different commercially available LTCC types having two porosification states each. Derived mathematical models, which are suitable for finite element method implementation, allow the characterisation of the effective permittivity reduction while keeping a maximum residual surface area for, e.g., metallisation purposes. The shape of the optimum material distribution function features an ‘air pocket’ of small width and a depth being dependent on the application specific operating frequency.     

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.     

Study of Porous Silicon Prepared Using Metal-Induced Etching (MIE): a Comparison with Laser-Induced Etching (LIE)

Porous silicon (p-Si), prepared by two routes (metal induced etching (MIE) and laser induced etching (LIE)) have been studied by comparing the observed surface morphologies using SEM. A uniformly distributed smaller (submicron sized) pores are formed when MIE technique is used because the pore formation is driven by uniformly distributed metal (silver in present case) nanoparticles, deposited prior to the porosification step. Whereas in p-Si, prepared by LIE technique, wider pores with some variation in pore size as compared to MIE technique is observed because a laser having gaussian profile of intensity is used for porosification. Uniformly distribute well-aligned Si nanowires are observed in samples prepared by MIE method as seen using cross-sectional SEM imaging. A single photoluminescence (PL) peak at 1.96 eV corresponding to red emission at room temperature is observed which reveals that the Si nanowires, present in p-Si prepared by MIE, show quantum confinement effect. The single PL peak confirms the presence of uniform sized nanowires in MIE samples. These vertically aligned Si nanowires can be used for field emission application.     

The porosification of fired LTCC substrates by applying a wet chemical etching procedure

In this study, a novel process is presented to generate a defined and homogeneous degree of porosity in fired low temperature co-fired ceramics (LTCC) substrates. For this purpose, a phosphoric-based acid is used which is a standard wet chemical etchant in the MEMS and microelectronic industry for the patterning of aluminium-based conductors and strip lines. Varying the bath temperature between 90 and 130 °C within a time frame of up to 8 h, a maximum penetration depth of 40 μm is achieved. At short etch times up to 5 h, the porosification process is reaction controlled, while at longer exposure times, diffusion-related effects dominate verified by the determination of the corresponding activation energies. In combination with morphological investigations using scanning electron microscopy and micro-X-ray diffraction techniques, it is demonstrated that the anorthite-phase crystallizing during liquid sintering in the vicinity of the Al₂O₃ grains shows a high dissolvability in phosphoric acid and is very important to enable its penetration into the LTCC body. This surface-near process is very attractive for the realization of selected areas on conventional LTCC substrates having modified dielectric properties, especially for high frequency applications.     

Thick-film NTC thermistors and LTCC materials: The dependence of the electrical and microstructural characteristics on the firing temperature

The electrical and microstructural characteristics of 1 kΩ/sq. thick-film NTC thermistors (4993, EMCA Remex) fired either on LTCC (low-temperature cofired ceramic) substrates or buried within LTCC structures were evaluated. The thermistors were fired at different temperatures to study the influence of firing temperature on the electrical characteristics. The results were compared with the characteristics obtained on alumina substrates. The sheet resistivities were higher than the resistivities of thick-film thermistors on alumina substrates. The increase of the sheet resistivities was attributed to the diffusion of the glass phase from the rather glassy LTCC substrates into the NTC thermistors. This was confirmed by EDS analyses. However, the increase in the resistivity was linked to an increase of the beta factors. Therefore, the results show that the evaluated NTC thermistors on LTCC substrates can be used for temperature sensors in MCM-Cs as well as in MEMS LTCC structures. When the thermistors were buried in the LTCC substrates, the LTCC structures delaminated during firing, leading to high sheet resistivities and high noise indices. This delamination is attributed to the different sintering rates of the NTC and LTCC materials.     

Properties of Lead Zirconate Titanate Thick-Film Piezoelectric Actuators on Ceramic Substrates

Lead zirconate titanate (PZT) is a piezoelectric material that can sense or respond to mechanical deformations and can be used in ceramic electro-mechanical systems (C-MEMS). The microstructural, electrical, and piezoelectric characteristics of thick PZT films on low-temperature cofired ceramics (LTCC) and alumina substrates were studied. The PZT composition was prepared with low-melting-point additives in order to decrease the sintering temperature and to be compatible with thick-film technology. The integration of the PZT thick-film materials on ceramic substrates could lead to degradation of the PZT's characteristics due to the interactions between an active PZT layer and a substrate, particularly with glassy LTCC material. To minimize the interactions with LTCC substrates, an intermediate PZT barrier layer was integrated. The value of the piezoelectric coefficient d ₃₃ was found to be up to 120 pC/N on an alumina substrate and approximately 50 on an LTCC substrate. Based on these results, a cantilever-type actuator was designed and fabricated on alumina substrates. Under an applied voltage of 200 V, the maximum tip deflection was about 5 μm.     

Design of LTCC with High Thermal Expansion

New applications of low-temperature co-fired ceramics (LTCC), such as pressure sensors or integrated functional layers, require materials that possess higher coefficients of thermal expansion (CTE). To fabricate LTCC with elevated CTE, two methods of material design are examined: firstly, glass ceramic composites (GCC), which consist of >50 vol% glass in the starting powder, and, secondly, glass-bonded ceramics (GBC), where glass is added as a sintering aid only. The CTE of GBC is mainly determined by the crystalline component. For GCC, the CTE can be well predicted, if CTE and elastic data of each phase in the microstructure are known. A nonlinear characteristic of the CTE versus phase composition was found with increasing E _crystals/ E _glass ratio and absolute CTE difference between the components. The glass composition and glass amount can be used to compensate the fixed properties of a crystalline material in a desired way. However, because the CTE and permittivity of a glass cannot be chosen independently, an optimum glass composition has to be found. For a given LTCC, it is possible to control the devitrification by shifting the glass composition. In this way, the resulting CTE values can be predicted more exactly and tailoring becomes possible. Different LTCC materials, based on the crystalline compounds Ba(La,Nd)₂Ti₄O₁₂, ZrO₂ (Y-TZP), SiO₂ (quartz), and specially developed glasses, possessing an elevated CTE of around 10 × 10⁻⁶ K⁻¹ while showing permittivity ɛ_r between 6 and 63, are introduced.     

LTCC system with new high-ɛr and high-Q material co-fired with conventional low-ɛr base material for wireless communications

The authors have developed a new LTCC material with characteristics of high dielectric constant (ɛ_r), high quality factor (Q) and low temperature coefficient of capacitance (TCC). This material can be co-fired with a conventional base LTCC material and buried resistors with low temperature coefficient of resistance (TCR). The base material which consists of Al₂O₃ filler and glass, has low ɛ_r of 8.7 at 3 GHz. The newly developed LTCC material, which consists of Ba–(Re)–Ti–O filler, Al₂O₃ filler, and glass, has the following characteristics of ɛ_r of 15.1, Q of 900 at 3 GHz, and TCC of −10 ppm/K. The buried resistors consist of RuO₂ and glass. Two different LTCC materials, a resistor material and a silver electrode paste can be co-fired as multi-layer substrates and are regarded as a new LTCC system.Constrained sintering could be applied to this LTCC system and the dimensions of substrates could be controlled with quite high accuracy. This LTCC system is expected to contribute to further miniaturization of RF circuits and the reduction of electrical loss.     

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.     

PZT thick films on LTCC substrates with an interposed alumina barrier layer

PZT thick films (PbZr_0.53Ti_0.47O₃ with the addition of 6% PbO and 2% Pb₅Ge₃O₁₁) with a low sintering temperature were printed and fired on LTCC substrates (951, Du Pont), covered with an alumina barrier layer. The electrical characteristics (remanent polarisation, coercive field, dielectric constant and dielectric loss) of these PZT thick films, together with sets prepared on “unprotected” LTCC substrates and on alumina substrates were compared. Whereas the electrical characteristics of the films on LTCC substrates deteriorated significantly due to interactions between the LTCC substrates and the PZT layers the values obtained for the LTCC/alumina barrier structures were comparable with those on ceramic alumina substrates.     

RF sputter deposition of poly(tetrafluoroethylene) films as masking materials for silicon micromachining

Polymers have been studied extensively because of their wonderful array of properties. Their properties can be tailored by many means and can be made useful in many ways. Polymers can be crosslinked or branched and can provide different properties, such as conduction and passivation. This study dealt with the RF sputter deposition of poly(tetrafluoroethylene) (PTFE) films with the aim of using them as masking materials during the fabrication of various micromachined structures. The films were deposited on silicon substrates at different plasma powers (100, 150, and 200 W) for a constant deposition time (60 min). To test the masking properties, the deposited films were immersed in a 20 wt % aqueous KOH solution at 80°C for 60 min. The films showed lower contact angles and interfacial tension, and this indicated good adhesion of the films to the silicon substrates. Good adhesion is an essential quality of masking materials during micromachining. The structural properties of the as‐deposited and etched films were studied with Fourier transform infrared and X‐ray photoelectron spectroscopy. These indicated that the bonding groups and binding energies of C? F and C? CF matched the reported values well. Furthermore, the presence of C? F and C? CF bonds, even after the etching of silicon substrates in highly alkaline KOH solutions for 60 min, showed that the PTFE films remained unchanged in the etchant and, therefore, could function as good masking materials during the fabrication of micromachined structures. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1183–1192, 2004     

Fabrication of porous silicon by metal-assisted etching using highly ordered gold nanoparticle arrays

ABSTRACT: A simple method for the fabrication of porous silicon (Si) by metal-assisted etching was developed using gold nanoparticles as catalytic sites. Etching masks were prepared by spin-coating of colloidal gold nanoparticles onto Si. Appropriate functionalization of the gold nanoparticle surface prior to the deposition step enabled the formation of quasi-hexagonally ordered arrays by self-assembly which were translated into an array of pores by subsequent etching in a HF solution containing H2O2. The quality of the pattern transfer depended on the chosen preparation conditions for the gold nanoparticle etching mask. The influence of the Si surface properties were investigated by using either hydrophilic or hydrophobic Si substrates resulting from piranha solution or HF treatment, respectively. The polymer coated gold nanoparticles had to be thermally treated in order to provide direct contact at the metal/Si interface which is required for the following metal-assisted etching. Plasma-treatment as well as flame annealing were successfully applied. Best results were obtained for Si substrates which were treated with HF prior to spin-coating and flame annealed in order to remove the polymer matrix. The presented method opens up new resources for the fabrication of porous silicon by metal-assisted etching. Here, the vast variety of metal nanoparticles accessible by well-established wet-chemical synthesis can be employed for the fabrication of the etching masks.     

Thick-film PTC thermistors and LTCC structures: The dependence of the electrical and microstructural characteristics on the firing temperature

The electrical and microstructural characteristics of 1 kΩ/sq thick-film thermistors with high positive temperature coefficients of resistivity, i.e., PTC 5093 (Du Pont) fired either on “green” LTCC (low-temperature co-fired ceramics) substrates or buried within LTCC structures, were evaluated. The thermistors were fired at different temperatures to study the influence of firing temperature on the electrical characteristics. The noise indices of the surface resistors fired at temperatures between 850 °C and 950 °C were very low, around −30 dB. The TCRs of the evaluated PTC thermistors were over 3000 × 10⁻⁶/K. The dependence of the resistivity on the temperature between −25 °C and 125 °C was linear, with the values of R² being better than 0.9999, regardless of the processing conditions. These results show that PTC thermistors co-fired on LTCC substrates can be used for temperature sensors in MCM-Cs as well as in MEMS structures. However, when the thermistors were buried in the LTCC substrates, the LTCC structures delaminated during firing and blisters formed, leading to high sheet resistivities and high noise indices. This delamination is attributed to the different sintering rates of the PTC and LTCC materials as well as to the expansion of the air bubbles captured in the viscous glass of the PTC material.     

Zero Shrinkage of LTCC by Self-Constrained Sintering

Low shrinkage in x and y direction and low tolerances of shrinkage are an indispensable precondition for high-density component configuration. Therefore, zero shrinkage sintering technologies as pressure-assisted sintering and sacrificial tapes have been introduced in the low-temperature co-fired ceramics (LTCC) production by different manufacturers. Disadvantages of these methods are high costs of sintering equipment and an additional process step to remove the sacrificial tapes. In this article, newly developed self-constrained sintering methods are presented. The new technology, HeraLock^®, delivers LTCC modules with a sintering shrinkage in x and y direction of less than 0.2% and with a shrinkage tolerance of ±0.02% without sacrificial layers and external pressure. Each tape is self-constrained by integration of a layer showing no shrinkage in the sintering temperature range of the LTCC. Large area metallization, integration of channels, cavities and passive electronic components are possible without waviness and camber. Self-constrained laminates are an alternative way to produce zero shrinkage LTCC. They consist of tapes sintering at different temperature intervals. Precondition for a successful production of a self-constrained LTCC laminate is the development of well-adapted material and tapes, respectively. This task is very challenging, because sintering range, high-temperature reactivity and thermal expansion coefficient have to be matched and each tape has to fulfill specific functions in the final component, which requires the tailoring of many properties as permittivity, dielectric loss, mechanical strength, and roughness. A self-constrained laminate is introduced in this article. It consists of inner tapes sintering at especially low-temperature range between 650°C and 720°C and outer tapes with an as-fired surface suitable for thin-film processes.     

Ultralow-permittivity glass /Al2O3 composite for LTCC applications

In the field of low temperature co-fired ceramic (LTCC), it remains a challenge to design the performance of LTCC with low permittivity less than 5. Here, a novel glass mixture of K-Al-B-Si-O (KABS) and Zn-B-Si-O (ZBS) is introduced as a sintering aid of alumina to obtain ultralow-permittivity glass/Al₂O₃ composite. Meanwhile, the factors of glass mixture component on microstructure, phase structure and dielectric properties of the composites are considered systematically. The crystal structure measured by X-ray diffraction (XRD) shows that pure crystalline phase of ZnAl₂O₄ spinel can be attained by tailoring the component of the glass mixture. In case of mass ratio of KABS: ZBS equal to 6:4, it favors to efficiently increase the sintering densification of composite, and significantly benefit the low dielectric loss, good mechanical and thermal performances. In detail, the optimal glass/ceramic composites sintered at 850 °C for 2 h exhibit the bulk density of 2.89 g/cm3, ε_r of 4.92 at 14 GHz and Q × f of 6873 GHZ, flexural strength of 202 MPa, thermal expansion coefficient of 5.5 ppm/°C. The above study provides an effective approach for preparing the novel composites as a promising candidate for LTCC applications.     

Aerosol etching by low-pressure impaction

Low-pressure impaction technology has been applied to a new etch process using aerosol, called aerosol jet etching (AJE). Fine droplets (0.1 to 0.3 urn) produced by spray-evaporation-condensation method impinge on a substrate in low-pressure impactor and etches its surface. Investigations were carried out on the control of etchant droplet size, the critical diameter for impaction and the performance of AJE on patterned etching. The patterned etching on SiO₂ film reveals some advantages over conventional wet etching: the economic use of etchant, the reduction of waste disposal and the increase in controllability of etching. To make maximum use of the advantage of AJE, techniques of further decreasing the droplet size and depositing this small droplets on substrates need to be developed.     

Low‐Temperature Co‐Fired Ceramic Substrates for High‐Performance Strain Gauges

Recent advances in the development of high gauge factor thin films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion (9.4 ppm/K) and a low modulus of elasticity (82 GPa) for the application as support material for thin‐film sensors. In the first part, constantan foil strain gauges were fabricated from this material by tape casting, pressure‐assisted sintering, and subsequent lamination of the metal foil on the planar ceramic substrates. The accuracy of the assembled load cells corresponds to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the low‐temperature co‐fired ceramic (LTCC) substrates for fabrication of accurate strain gauges. In the second part, to facilitate the deposition of thin‐film sensor structures to the LTCC substrates, pressure‐assisted sintering step is modified using smooth setters instead of release tapes, which resulted in fabrication of substrates with low average surface roughness of 50 nm. Titanium thin films deposited on these substrates as test coatings exhibited low surface resistances of 850 Ω comparable to thin films on commercial alumina thin‐film substrates with 920 Ω. The presented material design and advances in manufacturing technology are important to promote the development of high‐performance thin‐film strain gauges.     

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.     

Characteristics of thick film resistors embedded in low temperature co-fired ceramic (LTCC) substrates

Commercial thick film resistors were embedded in low temperature co-fired ceramic (LTCC) substrates, and co-fired with substrates at temperatures between 800 and 900 °C. Adding glass frit and amorphous SiO₂ to calcium borosilicate glass ceramic substrates has not only lowered the shrinkage of the substrates, but also improved adhesion and maintained structure integrity of the resistor films. During sintering, the conductive phase particles in the resistor became agglomerated and sedimented, and glass diffused into the LTCC substrate layer. Increasing the dwelling time, the overall resistivity of the co-fired films decreased due to sedimentation of agglomerated conductive particles. The liquid eutectic phases penetrated into the substrates added with either SiO₂ or glass frit that the volume fraction of conductive particles was increased. The resistivity of the embedded resistors was determined by the volume fraction of conductive particles, which was influenced by the conductive particles sedimentation, microstructure of resistor films, and inter-diffusion between the resistors and substrates.