Low‐Temperature Co‐Fired Ceramics System for Light Absorbance Measurement

The article describes technology of the low‐temperature co‐fired ceramics (LTCC) structure that enables light absorbance measurements of liquid sample. The manufactured ceramic structure contains buried microfluidic channels. The structure consists of two co‐fired glass windows that separate the light source and detector from the test solution. A construction of an electronic measurement system is described as well. The signal from three light‐emitting diodes (LED)s — red, green, and blue — can be used in the absorbance measurements. The light intensity is measured by the TCS 3414CS (TAOS, Plano, TX) color detector. Optical properties of the fabricated microfluidic LTCC system is investigated with several concentrations of potassium permanganate (KMnO₄) in water solution. The system can be applied in microbiology for constant monitoring of bacterial growth.     

Design and Fabrication of Transparent and Gas‐Tight Optical Windows in Low‐Temperature Co‐Fired Ceramics

Low‐temperature co‐fired ceramics (LTCC) enable the fabrication of microfluidic elements such as channels and embedded cavities in electrical devices. Hence, LTCC facilitate the realization of complex and integrated microfluidic devices. Examples can be applied in many areas like reaction chambers for synthesis of chemical compounds. However, for many applications it is necessary to have an optically transparent interface to the surroundings. The integration of optical windows in LTCC opens up a wide field of new and innovative applications such as the observation of chemiluminescent reactions. These chemical reactions emit electromagnetic radiation and thus offer a method for noninvasive detection. Thin glasses (≤500 μm) were bonded by thermocompression onto a LTCC substrate. As the bonding agent, a glass frit paste was used. Borosilicate glasses, fused silica as well as silicon were successfully bonded onto LTCC. To join materials with a large coefficient of thermal expansion mismatch (i.e., fused silica and LTCC), it is necessary to limit the heat input to the bond interface. Therefore, a heating structure was integrated into the LTCC substrate beneath the bond interface. This bonding process provides a gas‐tight optical port with a high bond strength.     

Fabrication of microfluidic chips using controlled dissolution of 3D printed scaffolds

Microfluidic chips are commonly fabricated using soft lithography, which often requires a clean room and micropatterning equipment. Recently, microfluidic chips are increasingly fabricated using 3D printing, but this technology is still limited in smallest channel printability, transparency, supports residue, and biocompatibility. In this work, a simple, fast, and inexpensive step is introduced to fabricate polydimethylsiloxane (PDMS) microfluidic chips using enhanced internal scaffold removal (eISR). It is found that final channel dimension decreases by 0.22 ± 0.02 μm/revolution with a 7% error using eISR. Surface topology is inspected after dissolution using scanning electron microscopy. A T-junction device, bifurcation channels, and curved channels are fabricated to demonstrate the usability of eISR in multiple applications. Compared to previous methods, eISR provides acrylonitrile–butadiene–styrene dissolution before PDMS casting to achieve thinner and smoother channels produced using a commercial 3D printer.     

Integration of transparent glass window with LTCC technology for μTAS application

The LTCC substrate makes it possible to build various microsystems which integrate not only passive components such as resistors, capacitors and inductors but also 3D structures such as cavities and channels. Nevertheless non-transparency is a main limitation of the LTCC-based microfluidic systems. The goal of this paper is to present technology which allows an optical transparent element to integrate with LTCC co-firing process. A micrototal analysis system (μTAS), which is based on the LTCC–glass technology, enables optical measurements. The study shows that integration of sodium glass material is feasible not only with zero-shrinkage LTCC (HL 2000, HL 800) but also with a standard one (DP 951). A FEA (finite element analysis) is used to calculate stress inside the LTCC–glass structure. A series of LTCC–glass windows with different sizes and shapes is investigated to observe size limitation of the integration method. The example ceramic–glass structures (chambers, mixer) with glass windows are made in order to present the possibilities of this new technology.     

Electrospun nanofibers for microfluidic analytical systems

Nanofibers embedded with the functional polymers exhibited a charged surface. These fibers were incorporated into microfluidic channels to provide high surface area and functional surfaces within a microfluidic system. The positively or negatively charged nanofibers were fabricated by blending of hydrophilic PVA with functional polymers with available amine groups or carboxyl groups, respectively. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to confirm the presence and location of blend components in the spun fibers. Thermally stimulated current (TSC) measurements confirmed that surface-charged nanofibers showed positive or negative currents according to the functional polymers embedded with the PVA polymer. The surface-charged nanofibers were incorporated into microfluidic channels. Poly(vinyl alcohol) (PVA) blend nanofibers formulated to create variations in fiber surface chemistry were electrospun to form patterns around gold microelectrodes on a poly(methyl methacrylate) (PMMA) chip surface. These nanofiber patterns were integrated into polymer-based microfluidic channels to create a functionalized microfluidic system with potential applications in bioanalysis. Spinning conditions and microelectrodes were optimized to enable an alignment of the nanofibers across the microfluidic channel. Importantly, nanofibers within the assembled microfluidic channels were robust and did not break or wash out of the channel under extreme fluid flow conditions at linear velocities up to 13.6 mm/s.     

LTCC Microfluidic System

A simple and inexpensive low-temperature cofired ceramic (LTCC) microfluidic device with integrated optical fibers is designed, manufactured, and tested with positive results. Fluidic channels, mixer, detector, optical fiber, light source, light detector, heater, and temperature sensor are integrated in one LTCC module. The optical system in the LTCC microsystem permits measurements of light transmittance and fluorescence. The design, technology, and results of the module's evaluation are presented.     

Sintering behavior and biocompatibility of a low temperature co‐fired ceramic for microfluidic biosensors

A low temperature co‐fired ceramic (LTCC) has been formulated and evaluated for in‐vitro microfluidic sensors and cell culture applications. Using a 75/25 vol% glass to alumina ratio, high density was achieved for sintering temperatures <900°C. No toxicity was observed in the leachate medium obtained by soaking LTCC in cell medium for 5 days. The human umbilical vein endothelial cells (HUVECs) also attached on the fibronectin‐coated LTCC after 14 hours and proliferated after 74 hours. On the basis of these results, the current LTCC formulation is a viable candidate for the continued development of LTCC‐based microfluidic biosensors.     

Poly(dimethylsiloxane) as a material for fabricating microfluidic devices

This Account summarizes techniques for fabrication and applications in biomedicine of microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The methods and applications described focus on the exploitation of the physical and chemical properties of PDMS in the fabrication or actuation of the devices. Fabrication of channels in PDMS is simple, and it can be used to incorporate other materials and structures through encapsulation or sealing (both reversible and irreversible).     

Carbon burnout and densification of self-constrained LTCC for fabrication of embedded structures in a multi-layer platform

Carbon burnout and densification of self-constrained low temperature co-fired ceramic (LTCC) are investigated using thermal analysis techniques. Slow heating rates and holding at a temperature higher than initial crystallization temperature of the glass component show evidence of retarding the densification of the self-constrained LTCC. Based on these results, it is proposed that the fabrication of embedded structures in a multi-layer self-constrained LTCC platform could be achieved by controlling carbon burnout with a multi-step co-firing profile, which can ensure complete carbon burnout without affecting the densification of LTCC structures. Using this approach, fabrication of an embedded cavity with dimensions of 10 mm × 10 mm × 0.50 mm in a self-constrained LTCC platform is demonstrated.     

Vacuum-Assisted Microfluidic Lithography of Ceramic Microstructures

The method to fabricate complex shaped micro-patterned ceramic structures has been developed. Vacuum-assisted infiltrating the suspensions to the micro channels generated by the contact of polydimethylsiloxane mold to the substrate enables simple micro patterning of ceramics with complex structures in a relatively large area in short time. The use of well-dispersed ethanol-based suspensions of solid loading ∼20 vol% plays an important role in a successful pattern formation without defects. The current process, called microfluidic lithography, is applicable to the entire range of ceramic materials which can be processed to colloidal suspension with relatively low viscosity. It is demonstrated that the interdigitated ceramic structures with 50 μm in the width composed of Al₂O₃ and NiO on a Si substrate were fabricated in an area of 5 mm × 5 mm.     

Fabrication of embedded fine line inductors by constrained sintering and formulation of photoimageable conductive paste

Abstract

For miniaturisation and precision of electronic device, the new technologies such as photoimageable thick film process were combined with conventional thick film process, and constrained sintering with near zero shrinkage in the x and y direction has been proposed. In this research, photoimageable conductive paste for forming embedded components via constrained sintering by low temperature cofired ceramic (LTCC) technology was formulated. Afterwards, by optimising paste formulation, formation process of fine line and sintering method, miniaturised LTCC components especially embedded fine line inductors were fabricated and their properties such as line resolution, surface morphology and yield were investigated. As a result, embedded fine line inductors formed by constrained sintering with fine line resolution of 20 μm and yield over 90% were acquired.     

Monomorph piezoelectric wideband energy harvester integrated into LTCC

In this paper the first fully LTCC embedded piezoelectric vibration harvester is demonstrated and characterized. Using ordinary LTCC processes a 39 mm × 39 mm × 3 mm package containing 25 mm co-fired PZT discs was made. Three laser cut beams of different lengths provided a 5.4% frequency bandwidth for 3 dB attenuation and a power of 32 μW at 1 g acceleration delivered into a 33.9 kΩ resistive load. The packaged structure was compared to a bare monomorph reference sample and showed less crosstalk, better frequency control and more power generated. The experiments showed that integration of LTCC and PZT bulk materials by co-firing is a very feasible way to realise energy harvesters for wireless technologies, sensors and autonomous System-On-a-Package.     

Low temperature co‐fired ceramics based modular fluidic system for efficient optical measurements

In this paper some issues on development and functionality of the modular low temperature co‐fired ceramics (LTCC)‐based microfluidic system are discussed. Design, fabrication, and performance of an exemplary LTCC microfluidic system with separated electronic and microfluidic parts are presented. This microsystem utilizes absorbance and fluorescence measurement for determination of various chemical compounds in liquid test samples. According to performed measurements the analytical signal of the presented LTCC microfluidic system is characterized by good signal‐to‐noise ratio and repeatability.     

Strength reliability of 3D low temperature co-fired multilayer ceramics under biaxial loading

Low temperature co-fired ceramics (LTCCs) are multilayered ceramic based components, which can be used as high precision electronic devices in highly loaded environments. In many applications, LTCC end components are exposed to mechanical stresses, which may yield different types of failure coming from different locations, thus decreasing the mechanical reliability of the device. The aim of this work is to assess the mechanical strength of LTCC parts and investigate the influence of the metal internal structure (supporting the maximum load) on the local fracture response. Strength of different positions (e.g. near vias, metal-pads, ceramic layers) has been measured under biaxial loading and compared with a reference bulk LTCC. The strength results were interpreted in the framework of Weibull theory. Fractographic analyses revealed a significant effect of the first metallisation layer below the tensile surface on the strength reliability of the structure, which should be considered to optimise LTCC designs.     

Novel Microsystem Applications with New Techniques in Low-Temperature Co-Fired Ceramics

Low-temperature co-fired ceramic (LTCC) enables development and testing of critical elements on microsystem boards as well as nonmicroelectronic meso-scale applications. We describe silicon-based microelectromechanical systems packaging and LTCC meso-scale applications. Microfluidic interposers permit rapid testing of varied silicon designs. The application of LTCC to micro-high-performance liquid chromatography (μ-HPLC) demonstrates performance advantages at very high pressures. At intermediate pressures, a ceramic thermal cell lyser has lysed bacteria spores without damaging the proteins. The stability and sensitivity of LTCC/chemiresistor smart channels are comparable to the performance of silicon-based chemiresistors. A variant of the use of sacrificial volume materials has created channels, suspended thick films, cavities, and techniques for pressure and flow sensing. We report on inductors, diaphragms, cantilevers, antennae, switch structures, and thermal sensors suspended in air. The development of "functional-as-released" moving parts has resulted in wheels, impellers, tethered plates, and related new LTCC mechanical roles for actuation and sensing. High-temperature metal-to-LTCC joining has been developed with metal thin films for the strong, hermetic interfaces necessary for pins, leads, and tubes.     

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.     

Fabrication of an Inductively Coupled Plasma Antenna in Low Temperature Co‐Fired Ceramic

A miniature electrostatic thruster is being developed in Low Temperature Co‐fired Ceramic (LTCC) at Boise State University. The thruster is composed of an antenna to create the plasma, a cylinder to contain the plasma, and grids to extract the plasma beam at high velocity. In this work, the development of the inductively coupled plasma (ICP) antenna in LTCC will be presented. This antenna is fabricated using DuPont 951 LTCC tape. A Direct Write dispenser is used to apply silver paste for the spiral ICP antenna. Using LTCC allows for the antenna to be embedded in the device under a thin sheet of LTCC dielectric, which protects the antenna from ion back bombardment during operation. This thin sheet is the seventh layer of the total device, with the ICP antenna one layer below the top. The design of the antenna is based on the research done by J. Hopwood. This article discusses the fabrication and performance of the ICP antennas in LTCC. These ICP antennas are operated at pressures from 10 mTorr to 1 Torr with radio frequencies (RF) of 500 MHz to 1 GHz to inductively couple with low‐pressure argon to produce plasma. The performance of the antennas will be verified with data showing the start and stop power of the plasma at various pressures and an electric field map of the RF field above the antenna.     

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.     

Fabrication and Applications of Microfluidic Devices: A Review

Microfluidics is a relatively newly emerged field based on the combined principles of physics, chemistry, biology, fluid dynamics, microelectronics, and material science. Various materials can be processed into miniaturized chips containing channels and chambers in the microscale range. A diverse repertoire of methods can be chosen to manufacture such platforms of desired size, shape, and geometry. Whether they are used alone or in combination with other devices, microfluidic chips can be employed in nanoparticle preparation, drug encapsulation, delivery, and targeting, cell analysis, diagnosis, and cell culture. This paper presents microfluidic technology in terms of the available platform materials and fabrication techniques, also focusing on the biomedical applications of these remarkable devices.     

A passive microfluidic valve fabricated from a hydrogel filled with carbon nanotubes

A new hydrogel composite was prepared by incorporating carbon nanotubes into agarose, and its mechanical, thermal, and structural characteristics were analyzed. A microfluidic unit element, a passive microfluidic valve, was fabricated from the hydrogel composite and embedded into a microfluidic device. The composite was found to exhibit satisfactory mechanical properties and a linear swelling feature for application as a passive valve in the microfluidic channel. The device operated successfully with different liquids.