Leszek J. Golonka

New design of an SnO2 gas sensor on low temperature cofiring ceramics

Abstract A new design for thick film gas sensors was investigated. The sensors were made by the low temperature cofiring ceramics (LTCC) technique with a platinum heater buried inside the multilayer structure. SnO2 or SnO2 with Pd as catalyst thick films were used as gas sensitive materials. The properties of the gas sensors were measured with methane and carbon monoxide. The results of our study show that LTCC can be successfully applied in sensor technology as well as confirming the correctness of the sensor design.

Properties of laser cut LTCC heaters

This article describes fabrication and properties of buried microheaters made inside low temperature co-fired ceramics (LTCC) structures. Laser cutting is used for meander pattern generation in dried Pt, PtAu or PdAg conductive pads. The electrical characterisation of microheaters is based on measurement and analysis of R(T) dependence in the range from 20∞C to 850∞C, measurement and analysis of thermal dynamic properties, long-term high-temperature passive or active ageing and behaviour of the heater in a pulse operation mode. The presented results are very promising for application of LTCC microheaters in various microsystem devices. ” 2000 Elsevier Science Ltd. All rights reserved.

Laser treatment of LTCC for 3D structures and elements fabrication

This paper presents possibility of laser application for fabrication of 3D elements and structures. The Aurel NAVS‐30 Laser Trimming and Cutting System with special software was used. It was applied successfully for fabrication of vias (minimum diameter – 50 μm) in fired and unfired LTCC ceramics and channels with width between 100 μm and 5 mm. The achievements and problems are presented and discussed. The influence of lamination process on quality of vias and channels as well as the problems connected with interaction of laser beam with ceramic tapes are shown. Three‐dimensional resistors and microfluidic system were successfully designed and fabricated based on our investigations. Chosen electrical and thermal parameters of constructed devices are shown, too.

Thick-film humidity sensors

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.

Thick-film resistive temperature sensors

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.

New application of LTCC technology

The paper presents general information on Low Temperature Cofired Ceramics technology and properties of fired materials. Special techniques developed for making LTCC microsystems are shortly described and an overview of sensors, actuators and microsystems is given.

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.

Anomalous behaviour of new thick film gas sensitive composition

Abstract Anomalous properties of gas sensitive film based on SnO2 are presented. The gas sensitive film was prepared by adding CeO2 and rhodium. The gas sensor was made on alumina substrate by the screen printing technique. The resistance characteristics of these gas sensors were investigated. It was found that resistance of the sensors increased in the presence of reducing gases. This behaviour was in contrast to typical properties of SnO2 sensors. Our sensors, based on the new composition, exhibited very short response time in CO and CH4.

A PDMS/LTCC bonding technique for microfluidic application

A novel bonding method of glass-covered low-temperature co-fired ceramics (LTCC) to transparent polydimethylsiloxane (PDMS) polymer is reported in this paper. The irreversible bonding between both materials was achieved by exposing their surfaces to an oxygen plasma. The influence of different plasma treatment process parameters (system power, time of surfaces activation) and glass/ceramics firing temperatures (Tmax = 700–875 °C, co-fired, post-fired) on the bonding process was investigated. Scanning electron microscopy (SEM) was used to study the glass surface quality after firing at various temperatures. Contact angle measurements, x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) and atomic force microscopy (AFM) were used to investigate properties of the PDMS and glass-covered LTCC surfaces before and after oxygen plasma treatment.

Embedded passive components for MCM

MCMs often have a large number of passive components connected to a small number of active devices. Integration of passive components into the MCM substrate improves electrical properties and reliability, and also reduces the cost, size and weight of electronic systems. The embedded components are mostly used in MCM-D (thin film) and MCM-C (thick film) modules. The use of embedded elements for MCM-L is at the early stage of development. The basic information on embedded passives as well as the activity of Dresden University of Technology and Wroclaw University of Technology in the area of LTCC buried passives is presented.