Table of contents

The popularity of thick-film sensors has grown significantly over the past decade. The technology allows the production of low-cost, miniaturized and rugged sensors for use in a wide variety of applications. For any readers not familiar with the subject, it is worth noting that thick-film refers to the enabling technology employed and not simply to the thickness of the films themselves. Thick-film sensors are fabricated using screen printing techniques, not too dissimilar to those used in the production of T-shirts, mugs and pencils.

Thick-film technology for the production of hybrid microelectronic circuits became popular around forty years ago. It was overshadowed as a result of the impact of silicon technology in the 1960s. With the widespread use of surface mounted devices in the 1980s, thick-film techniques once again found favour, largely as a means of depositing solder pastes onto circuit boards. Its use as an enabling technology for modern sensor development is comparatively new and many of the key research papers were written within the past fifteen years.

This special issue brings together a collection of the latest research papers in the field of thick-film sensors. There are contributions from research groups in eight different countries around the world, indicating the spread in popularity of the subject. We wish to offer our thanks to all the authors who have contributed articles, to those referees involved in the peer review procedure and also to the Editorial Board and staff of Measurement Science and Technology for bringing the subject of thick-film sensors to a wider audience.

Neil White and John Turner University of Southampton, UK

A review of thick-film sensors is presented. The evolution of the technology to a successful enabling mechanism for solid-state sensors is described. Many examples of applications in the major signal domains (mechanical, radiant, thermal, magnetic and chemical) are cited. The important characteristics of the technology relating to hybrid circuits, support structures and primary sensing elements are illustrated. The future of thick-film sensors is discussed in the light of the rapid development of new materials for sensors that have emerged over recent years.

This paper deals with ferromagnetic thick-film materials and with sensors relying on the resistance change due to an external magnetic field, usually in the range from to . The research for the development of thick-film magnetoresistors (TF-MRs) is briefly summarized from Ni-based air-fireable pastes to -based films fireable in atmosphere. The different materials are compared, regarding their magnetoresistive performances (mainly in terms of magnetoresistive anisotropy and temperature sensitivity). Some applications are described, covering the area of position sensing (displacement, proximity switch, angle) and the measurement of dynamic quantities (angular rotation) either for digital or analogue control systems.

The harmonic generation effect in superconducting Bi(Pb) - Sr - Ca - Cu - O screen printed thick film is reported. When an ac field of frequency 10 kHz is applied to the film, odd harmonics corresponding to frequencies 30, 50 and 70 kHz are observed. The addition of a dc field , induces the generation of even harmonics at frequencies 20, 40, 60, 80 kHz. The amplitudes of these harmonics are studied as a function of and . In the low-field region the amplitude of the even harmonics has been found to vary linearly with with the maximum amplitude change found for the second harmonic. The development of a high- thick film magnetometer based on the measurement of the amplitude of the second harmonic is described. The sensitivity of this magnetometer has been found to depend on and on the frequency of the ac field. The lowest field noise was found to be T.

The production of thick-film (TF) PLZT layers for pyroelectric sensors prepared from perovskite-type (Pb,La)(Zr,Ti) powders is described. The powder manufacture, paste preparation and layer production processes are detailed and flow charts of the powder and layer production and diagrams of the layer structure are presented.

The relative permittivity and dielectric loss of PLZT layers were measured as functions of temperature. The temperature dependence of the pyroelectric coefficient was measured and the influence of the poling electric field on the pyroelectric coefficient was investigated. The room-temperature pyroelectric coefficients of the PLZT layers are of the order of and are comparable with commonly used materials, but thick-film technology offers the advantage of a mature and simplified production route. The experimental results made it possible to define parameters for the production process necessary to achieve improved layers for pyroelectric sensors.

Densification and adherence of thick films are usually achieved by the addition of a glass frit in the screen printing ink. However, the glass may have a detrimental effect on the electrical properties of the active component as a ferroelectric. The addition of a new step - the application of a uniaxial or isostatic pressure on the dry film - to the standard processing of thick films, enables one to obtain a good densification of the films, even without the use of glass.

The efficiency of this new densification process of PZT-based films or `supported type microceramics' is demonstrated. Pyroelectric properties of the screen printed layers are compared to those of ceramics of the same composition.

Resonant piezo-layers (RPLs) manufactured in thick-film technology are proposed as highly sensitive gravimetric sensors. RPLs rely on a PZT-based screen printable paste which is used for the fabrication of planar elements on alumina substrates behaving as thickness-mode resonators. When operated at resonance, these elements are sensitive to a mass load applied on their free moving face, working as resonant microbalances with a frequency output, reaching a mass sensitivity of 114 kHz . This paper describes the paste preparation, background theory, frequency-tracking oscillator and the characterization of the sensors and anticipates their application in sensing arrays for chemical measurements.

A new design for a silicon-based micropump is described. Passive cantilever valves are produced by boron etch stop and fusion bonding. Tests of these valves show good performance, as no flow could be detected in the reverse direction. Initial experiments on a thick-film screen printed piezoelectric membrane actuator were undertaken. A study of suitable inks for electrodes on different insulation layers on silicon yielded silicon dioxide and cermet gold ink as the most satisfactory combination. Deflection measurements of a mm PZT (lead zirconate titanate) - bimorph membrane gave movement at an applied voltage of 100 V.

A quasi-static simulation package of the flow through a micropump is also presented. The valve action is simulated using ANSYS coupled with FLOW3D. The piezoelectric membrane deflection is simulated with ANSYS. A differential equation for the combined actuation of membrane and valves is solved numerically with Maple. Pump rates of up to and a maximum backpressure of up to 70 kPa for a driving voltage of 40 V have been modelled using bulk values for PZT-5H. A pump rate of up to and a maximum backpressure of up to 35 kPa at 100 V driving voltage are predicted using thick-film parameters extracted from the measurements.

In this paper a novel strain gauge system on stainless steel and 96% alumina substrates is described incorporating a Wheatstone bridge network, thick-film piezoresistive strain gauges, instrumentation, and a novel thermal and resistor mismatch tolerance correction software. The resistors in the Wheatstone bridge are fabricated with a novel mixture of ruthenium, bismuth and indium oxides and three different thick-film layouts are used. Mechanical measurements are performed using the cantilever beam set-up. The linearity, hysteresis, repeatability, reproducibility, creep, stability and temperature effects are measured for the novel thick-film pastes. The linearity and temperature effect on the output of the different bridge layouts are also examined. The printing of resistors using thick-film technology can result in resistor tolerances of %. Also, thick-film piezoresistors are intrinsically cross sensitive to temperature. These two properties cause an offset voltage to appear at the output of the bridge under no-load/load conditions and a subsequent error in the calculated applied strain. In order to compensate for the offset error voltage, novel correction software has been developed and implemented using a microcontroller. The error in the predicted strain was measured before and after application of the correction software under different bridge layouts and temperature conditions. The error was reduced from a maximum of 45% to approximately 1%. The effectiveness of the correction software in reducing the error suggests that this technique can be used without the need for trimming or bridge balancing.

Various methodologies are applied to the measurement of pressure. Among them, the capacitive pressure sensor is one of the most useful devices due to its high performance-to-price ratio, reliability and low power consumption. However, the early versions of capacitive pressure transducers had a general reputation of being highly nonlinear. This paper discusses a design approach to overcome this problem by choosing a new electrode shaping and coupling the sensor with an electronic circuit based on a relaxation oscillator. Moreover, the output signal in the form of a frequency makes the distance transmission and the interfacing with digital systems easier. The theory of operation is presented with a description of the sensor's implementation in thick-film technology on ceramic substrates and with experimental results on its characterization.

This paper discusses thick-film resistive temperature sensors investigated at the Technical University of Wroclaw. The technology and electrical properties of resistance temperature detectors, thermistors, low-temperature thermometers and heating elements are presented. The R(T) curve of chosen air-fireable Ni - P based films agrees with Ni wire. The initially aged thermistors from the system can operate in the range from room temperature up to 673 K. The commercial thick-film resistors modified by the negative TCR drivers (e.g. powder) are fully suitable for low-temperature measurements in the range from 20 to 100 K. The integrated gas sensors need heaters because temperature influences their sensitivity, selectivity and response time. The thick-film compatible system based on commercially available resistive and conductive inks allows continuous long-term electrical heating of the sensor up to 673 K. The admissible operating temperature is much higher for the heaters made from conductive inks; for example fritless platinum heaters are satisfactory up to 1073 K.

Thick-film CdS/CdTe photovoltaic cells have been prepared on borosilicate glass substrates by sequential screen printing and sintering steps. The residual amount of acts as a flux in a sintered CdS film and the sintering temperature of the CdTe film has a strong influence on the spectral response of the cell. The cells fabricated with the sintered CdS film having a larger amount of residual Cl ion and the cells sintered at a higher temperature show a poor response in the short wavelength. This is due to a formation of solid solution at the CdS/CdTe interface during the CdTe sintering. The screen printed CdS/CdTe cells also exhibit a higher spectral sensitivity in the longer wavelength region compared with typical CdS/CdTe cells. This may be due to the narrowing of the band gap resulting from layer.

The cells prepared under suitable conditions show a spectral sensitivity nearly constant in the range of 530 to 850 nm. cell has shown an increase in the spectral response at 500 nm wavelength region. These cells may be used as a visible-light-radiation sensor with a constant sensitivity in a wider wavelength region.

The results of an investigation of thick-film humidity sensors based on ceramic materials are discussed in the paper. Thick-film technology is very promising for the production of low cost sensors based on ceramic materials. The technology gives the possibility for reproducible production of sensors with a defined microstructure, determined porosity and proper structure of grains and grain boundaries. Ceramic sensors have advantages over polymer sensors due to their better thermal stability and resistance to chemicals. Properties of thick-film planar humidity sensors based on - (ZCT) ceramics are presented. The influence of and Si additives, as well as firing temperature, on the sensor characteristics are discussed. Impedance spectroscopy measurements determined the correlation between the technological parameters and the electrical properties of humidity sensors. The role of each part of the sensitive material in the electrical conduction process is determined on the basis of measurements and calculated equivalent circuits. The proposed model describes the frequency characteristics at various relative humidities with very good fit to the experimental data. A new approach to the modelling of the impedance frequency dependence by means of an equivalent circuit yields very promising results for sensors.

This paper shows the long route between gas-sensitive tin oxide material and a reliable prototype for selective methane sensing. A good reproducibility of fabrication is a prerequisite to any development of a cheap sensing device. Hybrid thick-film technology appears well adapted for this purpose, on the condition that the rheological properties of the pastes are well controlled.

Sensitivity was shown to be modified by the thickness of the tin oxide layer, the addition of palladium, the addition of a glass binder, the nature of the electrode metal and the presence of a coating.

Methane selectivity was achieved by adapting a concept proposed previously, based on the differential catalytic activity of palladium and platinum versus the oxidation of methane at and by combining two sensing principles on the same alumina substrate.

As for long-term stability, much information could be gained by annealing at temperatures higher than that of operation. Metal interdiffusion in the sensor connections (Kirkendall effect) and between the catalysts and the other metallic parts of the sensor was demonstrated to be a major cause of instability. Moreover, the firing and annealing conditions used in the fabrication process of our sensors, including that of the Pt heating resistor, were also shown to influence the reliability.

Radiofrequency-SQUID (superconducting quantum interference device) sensors have been fabricated using natural grain boundary weak-links in high- Br - Sr - Ca - Cu - O screen printed thick films. The films have predominantly high- 2223 phase and are c-axis oriented with large size grains ( - ) and of 104 K. These sensors showed good voltage - flux (V - ) characteristics at 77 K. The rf-SQUIDs have been operated in the flux-locked-loop mode at 77 K and showed a flux noise density in the white noise region. The potential of these SQUIDs in non-destructive detection of defects in steel plates has been demonstrated.