Srecko Macek

Capacitive pressure sensors realized with LTCC technology

This work is focused on pressure sensors designed as a ceramic capsule consisting of a circular edge-clamped deformable diaphragm, which is bonded to the rigid ring, and the ring is fixed on the base substrate. These three elements form the cavity of the pressure sensor. The capacitive pressure sensor is based on changes of the capacitance values between two electrodes. One thick-film electrode is deposited on the diaphragm and the other on the rigid substrate. The distance between electrodes and the area of electrodes define the initial capacitance of the capacitive pressure sensor, and together with the geometry and flexibility of the diaphragm define the sensitivity of the sensor. The diaphragm with the diameter of 9 mm made with low-temperature cofired ceramic (LTCC) has a thickness of 200 mum and the distance between electrodes is about 70 mum. The initial capacitance is around 10 pF. The capacitive ceramic pressure sensor is the part of the electronic conditioning circuit with the frequency output. The typical output frequency is about 10 kHz and sensitivities are between 2.5 and 3.5 Hz/kPa.

PZT thick films for pressure sensors: Characterisation of materials and devices

Different piezoelectric materials can be used in micro-electro-mechanical systems (MEMS) for transducers i.e. actuators and sensors of mechanical quantities. Lead zirconate titanates (PZTs) are the most common ceramic materials used as piezoelectric transducers. The ceramic MEMS are made by LTCC (low temperature cofired ceramic) and thick-film technology. They have been extensively used due to their superior piezoelectric properties. The use of thick-film PZT materials on LTCC substrate is not a trivial task due to the interaction between the printed PZT layers and the LTCC substrates during firing. The microstructural, electrical and piezoelectric characteristics of the thick PZT films on relatively inert alumina substrates and on LTCC tapes were studied. To minimise the possible interactions between glassy LTCC substrates and active PZT films for some samples the intermediate barrier layers were used. To characterise the thick PZT films on ceramic substrate the dielectric permittivities (epsiv), dielectric losses (tgdelta), and piezoelectric coefficients (d33) were measured. The thick-film piezoelectric resonant pressure sensor on LTCC structure was designed and samples were fabricated (Figure 2). The thick-film PZT actuator on the ceramic diaphragm induces the vibration of the diaphragm at its resonant frequency. The applied pressure bends the diaphragm and generates additional stress, which shifts the resonant frequency. This is used as the output signal of the piezoelectric resonant pressure sensor. The resonant frequencies are shifted around 26 kHz. The calculated sensitivities are 2.5 Hz/kPa.

Design study for a capacitive ceramic pressure sensor

Purpose – The purpose of this paper is to consider a capacitive pressure sensor fabricated using low‐temperature cofired ceramic (LTCC) materials and technology as a candidate for an energy‐autonomous sensor application. Designing the 3D capacitive sensor structure, with the cofired thick‐film electrodes inside the narrow air gap in the LTCC substrate, was a challenging task, particularly due to the presence of the parasitic elements influencing the sensors characteristics.Design/methodology/approach – In this work, different design variants for the thick‐film electrodes of the capacitive sensing structure were studied and compared. The test sensors were designed for the pressure range 0‐10 kPa and manufactured with readout electronics based on a capacitance‐to‐digital conversion.Findings – The typical sensitivity obtained was 4 fF/kPa, and the temperature coefficient of the sensitivity was 0.03%/°C. The design variant with the guard‐ring electrode showed the best rms resolution of 50 Pa. One drawback of...

Experimental and numerical analyses of thick‐film piezoceramic structures for miniaturised sensors and actuators

Purpose – The successful use of piezoceramic thick films in sensors and actuators requires a thorough understanding of their electrical and electromechanical characteristics. Since these characteristics depend not only on the materials composition but also on its compatibility with various substrates and a number of processing parameters, accurate measurements of the materials parameters are essential. Here, the aim of this paper is to present a procedure for characterising lead‐zirconate‐titanate (PZT) thick films on pre‐fired low‐temperature co‐fired ceramic (LTCC) substrates performed in order to determine the material parameters for numerical modelling.Design/methodology/approach – Owing to the lack of standard procedures for measuring the elastic and piezoelectric properties of the films, the compliance parameters were evaluated from the results of nano‐indentation tests, and a substrate‐flexure method was used to evaluate the transverse piezoelectric coefficients.Findings – The validation of the m...

Design of a Capacitive LTCC-based Pressure Sensor

A capacitive pressure sensor was realized by using low-temperature co-fired ceramic (LTCC) materials and technology. The paper will present the design considerations of a sensing element, and compares the experimental data to the theoretical design, with the aim to improve the sensors characteristics. Special points of attention are the linearity, the temperature behaviour, the pressure media and the behaviour of the reference pressure medium.

Thick Film Interconnections for Sensor Applications

Signal processing for some sensor applications demands circuits with specific characteristics, such as miniaturization, high functional density, high number of input ports, and high signal speed. For some sensors operating in industrial environments protection from electromagnetic interference (EMI) is also required. Thick film technology is convenient way to realize such circuits, but some special technological processes must be used. In this paper some of these will be presented through some examples of thick film multilayer hybrids. First is the multipoint pressure measuring system for medical applications. It is designed for measuring 720 pressure points in a test mattress for hospital patients. The second example is thick film multilayer interconnections for sensor array readout with high function density, high signal speed, and low interlayer capacitances. The third example is of thick film hybrid circuits for proximity switches.