Darko Belavic

A characterisation of thick film resistors for strain gauge applications

Some commercial thick film resistors with sheet resistivities from 1 kohm/sq. up to 1 Mohm/sq. were evaluated for strain gauge applications. Temperature coefficients of resistivity, noise indices and gauge factors (GFs) were measured. For the same resistor series GFs and noise indices increase with increasing sheet resistivity. However, both GFs and noise indices are different for resistors with the same nominal sheet resistivity but from different resistor series. The results indicated that the microstructure rather than the different chemical composition of the conductive phase in thick film resistors is the primary reason for the different gauge factors.

Characterization of PZT thick films fired on LTCC substrates

Ferroelectric ceramic materials based on solid solutions of Pb(Zr,Ti)O3 (PZT) are used in the electronics industry for sensors and actuators and for electromechanical transducers, to name just a few examples. Thick-film technology, i.e., the deposition of thick-film pastes by screen printing, primarily on alumina substrates, is a relatively simple and convenient method to produce layers with a thickness up to 100 μm. The characteristics of thick-film ferroelectrics are similar to those of bulk materials [1–4]. Low-temperature co-fired ceramics (LTCC) materials, which are sintered at the low temperatures typically used for thick-film processing, i.e., around 850 ◦C, are based either on crystallizable glass [5, 6] or a mixture of glass and ceramics, for example, alumina, silica or cordierite (Mg2Al4Si5O18) [7, 8]. Jones et al. have presented a comparison of the mechanical and chemical characteristics of both green and fired LTCC tapes from different suppliers in [9]. Ceramic multi-chip modules (MCM-C) are multilayer substrates with buried conductor lines. An additional contribution to the smaller size and the higher density of MCM-C is the ability to integrate screenprinted resistors, or sometimes capacitors and inductors. These screen-printed components can be placed either beneath the discrete components on the surface of the multilayer dielectric or buried within the multilayer structure. For an overview of passive integrated components in MCM see, for example [10]. For some applications, for example integrated sensors or micro-actuators, PZT thick-films on LTCC that are sintered at relatively low temperatures (around 850 ◦C) comparable with LTCC’s firing temperatures, would be of interest [11, 12]. The aim of this work was to study the compatibility between LTCC and screenprinted PZT as well as the electrical characteristics of the PZT layer. PZT 53/47 powder (PbZr0.53Ti0.47O3) with an excess 6 mol% of PbO was prepared by mixed-oxide synthesis at 900 ◦C for 1 h from high-purity PbO (litharge) 99.9% (Fluka), ZrO2 99% (Tosoh), and TiO2 99% (Fluka). To this was added 2 wt% of lead germanate, with the composition Pb5Ge3O11 (melting point 738 ◦C) as a sintering aid. Lead germanate (PGO) was also prepared by mixed-oxide synthesis from PbO and GeO2 99% (Ventron) at 700 ◦C. After synthesis, both compositions were ball milled in acetone for 1 h and dried. A thick-film paste was prepared from the PZT (2% PGO) and an organic vehicle (ethyl cellulose, alpha-terpineol and butil carbitol acetate) by mixing on a three roll mill. The green LTCC 951 tape (Du Pont) and alumina ceramics were used for substrates. The thick-film structure was prepared by first printing gold film (Remex 3243) and then the PZT film. The PZT film was printed 6 times with intermediate drying. The gold and PZT layers were cofired at 850 ◦C for 8 h in a closed alumina crucible. The thickness of the PZT films after the thermal treatment was around 50 μm. The green and fired Du Pont LTCC 951 tapes were analyzed by X-ray diffraction (XRD) analysis with a Philips PW 1710 X-ray diffractometer using Cu Kα radiation. X-ray spectra were measured from 2 = 20 ◦ to 2 = 70 ◦ in steps of 0.04 ◦. X-ray spectra are shown in Fig. 1. The unfired material is a mixture of alumina and glass. After firing at 850 ◦C peaks of anorthite ((Na,Ca)(Al,Si)4O8) phase appear. The peaks of alumina and anorthite are denoted by “A” and asterisk, respectively. For the electrical measurements gold electrodes were sputtered onto the PZT films. The values of the remanent polarization and the coercive field were determined from ferroelectric hysteresis curves measured with an Aixact TF Analyzer 2000 at 50 Hz. The real and imaginary parts of the complex dielectric constant were measured with an HP 4284 A Precision LCR Meter at 1 kHz. In Table I the electrical parameters, i.e., remanent polarization Pr, coercive field Ec, dielectric constant e′ and dielectric loss tan δ, of the co-fired LTCC/Au/PZT structure are presented. The electrical characteristics of this structure are compared to the characteristics of a similar structure printed on alumina substrates [13]. Hysteresis loops of the PZT films on the alumina and LTCC substrates are shown in Fig. 2.

Low-frequency noise of thick-film resistors as quality and reliability indicator

Abstract The non-linearity and the noise of thick-film resistors are parameters that can be used to make a prediction of resistor reliability. The noise spectroscopy measurements of thick-film resistors are proposed as a diagnostic tool for the prediction of possible types of failure. The correlation between noise spectral density data and the results of accelerated aging of thick-film resistors at high temperature were made for HS80 and 2000 resistor pastes.

The development of microstructural and electrical characteristics in some thick-film resistors during firing

The development of microstructural and electrical characteristics (sheet resistivities, TCRs, and noise indices) in some thick-film resistors during the firing process has been evaluated. Three 1 kohm/sq. resistor pastes (Du Pont), based on RuO2, ruthenate or a mixture of both conductive phases, were fired at temperatures from 500°C to 950°C. The cell parameters of the RuO2 in the 8031 and the 2031 resistors, the bismuth ruthenate in the 8029 resistors and the lead ruthenate in the 2031 resistors, were calculated from X-ray data. Microstructures were analyzed by SEM and EDS microanalysis. The absolute values of the cold and hot TCRs first decreased to a minimum at 850°C and then increased again with increasing firing temperature. The noise indices of the resistors generally decrease with increasing firing temperature.

Properties of PZT thick films made on LTCC

Purpose – To find properties of screen printed PZT (PbZr0.53Ti0.47O3 with 6 per cent of PbO and 2 per cent of Pb5Ge3O11) thick films layers on LTCC substrate.Design/methodology/approach – The influence of PZT firing time and electrode materials on electrical characteristics and microstructure were examined. A scanning electron microscope (SEM) equipped with an energy‐dispersive X‐ray (EDS) analyser was used for the microstructural and compositional analysis.Findings – Microstructural and compositional analyses have shown the diffusion of SiO2 from LTCC into PZT layers and the diffusion of PbO in the opposite direction. SiO2 presumably forms low permitivity lead based silicates in PZT layer. The new phase deteriorates the piezoelectric properties. The amount of diffused materials was dependent upon the electrode material and increased with increasing firing time. Better properties, i.e. higher remanent polarisation and dielectric constant were achieved for samples with PdAg electrodes and shorter firing ti...

Microstructural, XRD and electrical characterization of some thick film resistors

The microstructural and electrical characteristics (sheet resistivities, TCRs, and noise indices) of some 1 kΩ/sq. and 10 kΩ/sq. thick films were evaluated. The conductive phase was determined by X-ray diffraction (XRD) analysis. The microstructures of fired resistors were investigated by scanning electron microscopy (SEM) and analyzed by energy dispersive spectrometry (EDS). Some resistors were fired for a relatively long time at the highest temperature, i.e., 6 h at 850 °C, to allow the reactions in the material to reach equilibria. Sheet resistivities, temperature coefficients of resistivity, and noise indices of these resistors were compared with “normally” (10 min at 850 °C) fired resistors. After 6 h firing absolute temperature coefficient of resistivity (TCR) values of most resistors increased significantly, while sheet resistivities decreased. Complex impedance analysis showed that in most cases resistors with low noise indices showed nearly ideal resistor response while those with higher noise had a larger imaginary part.

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.

Packaging technologies for pressure‐sensors

Pressure‐sensor miniaturization requires high‐density packaging. This means that designers are constantly faced with all kinds of challenging, and sometimes impossible, requirements. In this paper we will present three examples with specific technologies and aspects of miniaturization and packaging. The first example is a pressure switch, the second a pressure sensor and the third a smart pressure sensor.

Correlation between microstructure and gauge factors of thick film resistors

The change of resistance under applied stress is due partly to. deformation, i.e. changes in the dimensions of the resistor, and partty to the alteration in specific resistivity as a resalt of charrges, in the microstructure of the material [1,2], Tire c ha~. ge of resistance under stress is given by the following equation, where the relative change in specific resistivity is due to the microstructural changes:

An investigation of thick-film resistor, fired at different temperatures, for strain sensors

Some commercial 10 k/spl Omega//sq. thick-film resistors based on RuO/sub 2/, ruthenates or a mixture of RuO/sub 2/ and ruthenates, were evaluated for strain gauge applications. The resistors were fired at different temperatures to determine the influence of firing temperature on the electrical characteristics. The conductive phase in the resistors was determined with X-ray powder-diffraction (XRD) analysis. Microstructures of the thick-film resistors were analysed via SEM and EDS. The temperature coefficients of resistivity, noise indices and gauge factors (GFs) were measured. The results indicate that both the GFs and the noise indices are different for resistors with the same nominal sheet resistivity but which came from different resistor series.