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.     

Cold Chemical Lamination—New Bonding Technique of LTCC Green Tapes

The low-temperature co-fired ceramic (LTCC) technology enables fabrication of sensors, actuators, microfludic devices² and fuel cells. The structures consist of screen-printed components, gas/liquid channels, reactive chambers and mixers. The lamination process determines the quality of such devices. Thermo-compression is the most popular bonding method. The LTCC green tapes are joined together at high temperature (up to 80°C) and high pressure (up to 30 MPa) for 2 to 15 minutes. The method allows good encapsulation of the LTCC structures, but the channels geometry is strongly affected by elevated temperature and pressure. Cold Chemical Lamination (CCL) is a new LTCC green tapes bonding technique, which allows for fabrication of 3D modules. A solvent-based method is used in the CCL lamination instead of the thermo-compression process. A special liquid agent is screen-printed on the green tape in the CCL method. The liquid melts the tape surface. Then the tapes are stacked and compressed at room temperature by a printing roll. The influence of the CCL and the thermo-compression methods on the chamber's geometry quality as well as basic electrical properties of screen-printed resistors (sheet resistance R_φ standard deviation of sheet resistance σ_R, variability coefficient of sheet resistance V_R, and long-term stability) are analyzed and compared in this paper. The bonding quality is examined by a scanning electron microscope (SEM).     

Manufacture of 3D structures by cold low pressure lamination of ceramic green tapes

3D multilayer devices were generated by Laminated Object Manufacturing (LOM), a well-known rapid-prototyping technology. Divergent from this method, commercial ceramic green tapes were used which were laminated by Cold Low Pressure Lamination (CLPL). In contrast to thermo-compression, which works at pressures and elevated temperatures, CLPL allows to join particularly fine, complex structures with cavities or undercuts, because no mass flow occurs. This technique is based on gluing the adjacent tapes by means of an adhesive film at room temperature under a low pressure. After binder burnout and sintering the ceramic laminate has a homogenous and dense microstructure free of interfaces. This modified LOM technique is particularly suitable for the production of Micro Electro Mechanical Systems (MEMS).In the given paper commercial Low Temperature Co-fired Ceramic (LTCC) green tapes were used, which were structured by means of a high-frequency milling plotter and laminated by using CLPL. Various 3D devices of different shape with inlying cavities were manufactured. The quality of the fired and unfired structures of the devices were characterised by different methods and show a high quality surface of the multilayer structures. Process aspects of the CLPL technique are discussed. The results demonstrate the advantages of this method for the fabrication of MEMS.     

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.     

Tape casting of ferroelectric,dielectric, piezoelectric and ferromagnetic materials

Tape casting is a feasible method for preparing ceramic tapes with different electrical and magnetic properties for multilayer ceramic devices. This paper describes the tape casting process for several different electroceramic materials (BST, PZT, NZF and ZSB) utilising similar organic additive and solvent systems. The properties of tapes with different ceramic compositions before and after sintering are investigated, including surface roughness, shrinkage and microstructures. The parameters affecting the casting, shrinkage, lamination, thickness and tensile strength of green tape are also presented. This enables process design for tape which can be used in devices with true integration of dielectric and piezoelectric, ferroelectric and ferromagnetic layers in 3-dimensional multilayer structures.     

Fabrication of embedded microfluidic channels in low temperature co-fired ceramic technology using laser machining and progressive lamination

This paper describes the application of laser micromachining techniques for the fabrication of microfluidic channels in low temperature co-fired ceramic, LTCC, technology. It is shown that embedded cavities can be successfully realised by employing a recently proposed progressive lamination process with no additional fugitive material. Various microfluidic structures have been fabricated and X-ray imaging has been used to assess the quality of the embedded channels after firing. The problem of achieving accurate alignment between LTCC layers is addressed such that deeper channels, spanning more than one layer, can be fabricated using a pre-lamination technique. A number of possible applications for the presented microfluidic structures are discussed and an H-filter particle separator in LTCC is demonstrated.     

Novel cold chemical lamination bonding technique—A simple LTCC thermistor-based flow sensor

The LTCC technique enables fabrication of microfluidic devices. The structures consist of channels, chambers and screen-printed passives. The lamination is a quality-determining process in the manufacture of the fluidic modules. The commonly used bonding method is thermocompression. The tapes are joined together at high pressure (up to 30 MPa) and temperature (up to 80 °C) for 2–15 min. Although these parameters allow good LTCC module encapsulation, the quality of the chamber geometry is strongly affected by high pressure and temperature. The cold chemical lamination (CCL) technique presented in this paper, a solvent-based method, largely avoids these problems. A film of a special solvent is deposited on the green tape, and softens the surface. The tape layers are then stacked and compressed at low pressure, below 100 kPa, at room temperature. The fabrication of a simple LTCC thermistor-based flow sensor is presented here to compare both lamination methods. The test device consists of one buried thermistor screen printed on a bridge hanging in a gas/liquid channel. The basic sensor parameters (measurement range, working temperature, output signal, working pressure and measurement error) are analyzed.     

Processing, Microstructure, and Properties of Laminated Silicon Nitride Stacks

By lamination of silicon nitride tapes, components with complex geometries can be produced. Unstructured tapes can be laminated by common thermal compression. Structured tapes, however, have to be joined by pressureless processes using e.g. pastes as lamination aids because deformation of the structures would occur. These pastes usually contain a binder for maintaining the mechanical contact between the tapes during processing. To prevent the high mass loss of typical organic binders during burnout, pre-ceramic polymers were used in this work. These ceramic precursors convert partly into an inorganic material during heat treatment with a significant reduced mass loss compared with common organic binders. Thus, the porosity in the interlayer of a laminated stack is strongly decreased, which should be favorable for the mechanical and thermal properties. This work discusses the resulting microstructure, strength, and thermal diffusivity data of stacks laminated with pastes containing various precursor contents. These results are compared with those obtained by samples prepared by compression of green tapes. It is found that except for some large pores, the microstructure of the precursor-derived interlayers is qualitatively the same as in the tape material. For stacks made by both lamination methods, strength measurements reveal that the properties parallel and perpendicular to the layers are different. It is shown that the same strength level can be obtained both by using the pressureless route and by the compression method. Unlike the strength, the thermal conductivity does not change with the direction of measurement.     

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.     

Mechanical and lamination properties of alumina green tapes obtained by aqueous tape-casting

Mechanical characterisation and lamination were carried out on alumina green tapes prepared by aqueous tape casting using two acrylic emulsions having different glass transition temperatures (T_g) as binders. The tensile strength and strain were strongly dependent on the binder nature and content. Namely, the mechanical properties of the green tapes reflected those of the binders at room temperature: the green tapes obtained with the higher T_g binder showed a brittle behaviour, whereas those obtained with the lower T_g binder showed an elastoplastic behaviour. The mechanical properties of the green tapes prepared by mixing the two acrylic binders lies in between, giving the possibility of tailoring the flexibility and strength in the range of the values obtained for pure binders. Lamination gave rise to an increase of both green and sintered densities, compared with monolayer specimens, whatever the composition of the binder system. Such improvements significantly depended on lamination pressure, but were insensitive to lamination temperature for the two temperatures tested higher than the T_g of the two binders. ©     

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.     

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.     

Fabrication of membranes and microchannels in low-temperature co-fired ceramic (LTCC) substrate using novel water-based sacrificial carbon pastes

In this work, 3D structuration of LTCC (low-temperature co-fired ceramic) for microfluidics was studied, using two novel sacrificial carbon paste compositions. These pastes are based on graphite with a water-soluble vehicle consisting of polyvinylpyrrolidone binder (PVP) dissolved in propylene glycol (PG), which is not aggressive to green LTCC material. Both examined pastes differ slightly in binder content and added plasticizer, glycerol (G) or trimethylolpropane (TMP). The thermal properties of the sacrificial carbon pastes have been examined using combined thermo-gravimetric analysis (TGA), differential thermal analysis (DTA) and differential thermo-gravimetry (DTG). The sacrificial carbon pastes have been applied to the fabrication of membranes and microchannels in LTCC substrate. A comparison of the obtained features has been made using X-ray tomography and optical profile measurements. Moreover, changes in the composition of the fired LTCC material after co-firing have been studied using X-ray photoelectron spectroscopy (XPS) and surface wettability measurements.     

Effect of Particle Shape on Anisotropic Packing and Shrinkage Behavior of Tape-Cast Glass–Ceramic Composites

In this study, the influence of particle shape anisometry and particle alignment in tape-cast green sheets on the shrinkage behavior of low-temperature co-fired ceramics (LTCCs) was investigated quantitatively. A new method for the characterization of particle shape with the use of a particle image analyzer is presented, and its application to real material systems demonstrated. A commercial LTCC system and three developed composite powders with different average particle sizes were analyzed. After tape casting, particle alignment in the green sheets was analyzed using image analysis of SEM micrographs of cross sections. The investigations showed that the degree of particle alignment correlates significantly with the particle shape and size of the materials. A further increase in particle orientation was seen after the lamination process. Additionally, the powder packing of both single layers and laminates was analyzed by mercury porosity. The anisotropic shrinkage behavior during the sintering process was determined by means of optical dilatometry. The data obtained on the particle morphology, particle orientation in the tapes, and their effects on the shrinkage anisotropy will be discussed.     

Overview on fabrication of three-dimensional structures in multi-layer ceramic substrate

Three-dimensional structures in a multi-layer ceramic substrate are important in realizing ceramic-based meso- and micro-systems. During lamination and/or co-firing, three-dimensional structures, especially those with suspended structures, tend to deform and sag due to the intrinsic nature of the green (un-fired) ceramic material. Fabrication of three-dimensional structures with well-controlled dimensional stability and mechanical integrity remains a challenge. This paper discusses the challenges in fabricating structures in a multi-layer ceramic substrate. An overview is provided of the current state of the art in patterning and lamination techniques for the fabrication of these three-dimensional structures.     

Study of deformation and porosity evolution of low temperature co-fired ceramic for embedded structures fabrication

The deformation behaviors of suspended low temperature co-fired ceramic (LTCC) laminates over a cavity and the evolution of open porosity of LTCC are studied for the fabrication of embedded structures in a multi-layer LTCC platform using carbon material. The effects of the type of LTCC materials (self-constrained and unconstrained LTCC), cavity width, laminate thickness, and lamination conditions on the deformation of the suspended LTCC laminate over a cavity are studied. For suspended three-layers and six-layers LTCC laminates over cavity width ranges from 10 to 25 mm, the self-constrained LTCC laminates were more dimensionally stable (sagged by less than ?120 μm) after sintering as compared to the unconstrained LTCC. The evolution of open porosity and the distribution of open pores in the self-constrained LTCC with changes in sintering temperature and laminate thickness are also studied for process optimization.     

High-density YSZ tapes fabricated via the multi-folding lamination process

A new method of ceramic processing to obtain high green and fully sintered yttria-stabilized zirconia (YSZ) ceramic parts has been studied. The procedure involved slip casting, multi-folding lamination, and sintering. A rheological study revealed correlation between compositional parameters and densities. A particular method of folding and lamination we named multi-folding lamination was proved to be an appropriate route to obtain dense, homogeneous green bodies, reaching density values of ca. 61%. Further studies on the sintered parts were performed in this work, obtaining YSZ sintered tapes suitable for the use in high temperature solid-state devices. This tapes, sintered at 1550 °C, reached values of 98% of theoretical density and average particle sizes within 1.7–12 μm.     

3D-customized thin-walled ceramic mouldings produced by deep drawing of green tapes

Thin-walled 3D-customized ceramic components can be used, e.g. for inert and thermal resistant housings as well as special surface structured kiln furniture. For manufacturing such components, deep drawing of ceramic green tapes has been used. The technology has been almost not applied for processing of ceramic green tapes. A new approach was developed to realize a homogenous ceramic particle packing and hence uniform green density within a deep drawn green tape. After debinding and sintering, defect-free model structures with a dense microstructure (density > 99%) were achieved. Within this study, the deep drawing of ceramic green tapes by using a new approach was investigated. The developed approach brings several beneficial properties together, e.g. a reproducible form deviation and more importantly a homogeneous particle distribution allowing homogeneous and dense structures. With this results, the transformation of non-complex and cheap ceramic green tapes to 3D-customized and near-net-shape thin-walled mouldings becomes possible.     

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.     

Optimization of borosilicate glass/CaTiO3-TiO2 composite via altering prefiring temperature and particle size

Low-temperature co-fired ceramic (LTCC) with middle permittivity is very crucial to the miniaturization of components. Based on our previous study on the glass/CaTiO₃-TiO₂ composite, prefiring temperature and particle size of CaTiO₃-TiO₂ ceramic were optimized in this study to promote the performance of the composites. Comparing with our previous study, after being sintered with 50 wt% glass at 875°C, CaTiO₃-TiO₂ ceramic prefired at 1275°C with particle size of 3.38 μm showed excellent properties of sintering density = 3.33 g/cm³, ε_r = 30.2, tan δ = 0.0005 (7 GHz). In addition, surface roughness of green tapes was also improved after optimization. The material has a good chemical stability and shrinkage matching with silver, making it a very promising candidate material for LTCC applications.