Microstructure development and electrical properties of RuO2-based lead-free thick film resistors

Lead-free thick film resistive compositions suitable for hybrid microelectronics were prepared. The compositions were made with RuO₂ as the conducting phase and bismuthate glasses. This blend of bismuthate glasses constitutes a suitable choice for avoiding negative effects such as devitrification, bleeding out of the glass on alumina substrates, anomalous distribution of conductive grains in the glassy matrix and phase separation observed in other systems. The morphology, microstructure and electrical properties have been studied. X-ray diffraction (XRD), electron scanning microscopy (SEM) and energy dispersion spectroscopy (EDS) show that a defect pyrochlore phase of bismuth titanate formed at about 700 °C in all the compositions studied. Transmission electron microscopy (TEM) analysis of the original RuO₂ powder shows that a single grain is made of many smaller grains of different crystalline orientations. The sheet resistance spans two decades by changing the RuO₂ fraction from about 14–52 wt%. The resistors exhibit good reproducibility and their temperature coefficient of resistance is in the range of ± 300 ppm/°C.     

The effects of processing conditions on the resistivity and microstructure of ruthenate-based thick film resistors

Elevated temperature processing parameters affect the microstructure and electrical behaviour of thick film resistors on alumina substrates. Blended resistors (DuPont QS87 series) with a nominal sheet resistivity of 56 kΩ/□ and temperature coefficient (TCR) less than ±100 p.p.m K-1 were fired in a laboratory process that simulated production ramp rates and atmosphere. Resistances were measured in situ during firing in a three-factor, replicated experiment with two levels and centrepoints for peak temperature, firing time and probe current. Room temperature resistance values after firing show a strong correlation to temperature and time, which both increase resistance and flatten the R(T) curve around room temperature. In situ resistance during firing shows a weaker correlation, inverse with temperature because the thermally activated glass conduction has a greater share of the composite conduction at firing temperature. X-ray diffration (XRD) shows lead ruthenate, alumina, and zirconium silicate present in the resistors. The ruthenate lattice parameters increase with increasing firing temperature and time. Qualitative particle coarsening is observed with increasing firing temperature and time by transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDX) shows lead ruthenate, CuBi ruthenate and zirconium silicate crystallites dispersed in a lead silicate glass matrix, without much particle chaining. Resistance changes are attributed to increased separation of ruthenate particles by coarsening. This revised version was published online in August 2006 with corrections to the Cover Date.     

Environment friendly perovskite ruthenate based thick film resistors

It is well known that the dielectric matrix of air-fireable thick film resistors (TFRs) presently used in hybrid microelectronics and passive components invariably consists of a high-lead silicate glass. However, the current trend in the electronic industry is to restrict and eliminate the hazardous elements viz. lead, cadmium etc. from electronic components. An attempt to develop suitable RuO₂-based or pyrochlore ruthenate based Pb-Cd free TFRs has been only partially successful till now. We report here the preliminary results of a study aimed to investigate the feasibility of CaRuO₃ perovskite-based lead-free TFRs. Our results showed that sheet resistances higher than 1 kΩ/sq. can be easily achieved in a controlled way, with hot and cold temperature coefficients of resistance (TCR) in the range of 325-580 ppm/°C and 180-500 ppm/°C, respectively. Similarly, the compositions also exhibit negligible piezoresistive effects with gauge factor, GF < 1. Additionally, the resistors do not exhibit negative structural features, like bleeding or devitrification of glass, observed in previous attempts to develop reliable lead-free TFRs.     

Conductive powder preparation and electrical properties of RuO2 thick film resistors

RuO₂ powders were prepared under different conditions to vary their properties. A study was made of the powder properties and electrical characteristics of thick film resistors (TFRs) containing an RuO₂ powder as a conducting component. Both crystalline size and specific surface area varied with calcination temperature but the change in the washing solution had different effects upon the two relationships. Variations in the total pore volume produce differences in the specific surface area at the same crystalline size. The resistivity of TFRs decreases and the temperature coefficient of resistance (TCR) becomes more positive with decreasing crystalline size. TFRs having conductive powder of higher specific surface area at the same crystalline size show lower resistivity and higher TCR values. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of glass modifiers on the electrical properties of thallium oxide thick film resistors

Glass frits used in preparing thallium oxide thick film resistors are modified by adding 10% (wt.) modifiers into basic lead borosilicate glass (PbO:B₂O₃:SiO₂=50:30:10). A wide range of sheet resistivity is observed for modified resistors. The temperature coefficients of resistance (TCR) of ZnO-modified and ZrO₂-modified resistors are smaller than ?200 ppm/°C, while larger than ?8000 ppm/°C for MnO₂-modified resistors. The sheet resistivity (?_s) has a hyperbolic relationship with the contents of thallium oxide. Current noise indices show a linear dependency on log ?_s. Decreasing the contents of thallium oxide will increase the current noise, also the sheet resistivity will be increased. The relationship between current noise indices and the sheet resistivity of the resistors, as well as the contents of thallium oxide, can be explained by a model based on individual noise generator approximation. The effects of glass modifiers on the sheet resistivity is discussed in view of the ionic potential, $\text{Z}\text{R}{}_{\mathrm{a}}$, of the modifiers. Where Z is the valence and R_a is the ionic radius of the metallic ions of the modifiers. The relationship between log ?s and ionic potentials of the modifiers, for 40% (wt.) Tl₂O₃ resistors, can be obtained as a straight line.     

Influence of the contacts and firing process on the properties of thick film resistors on alumina and dielectrics

This paper describes the behaviour of thick film ruthenium resistors when printed and fired onto standard 96% alumina substrates and onto alumina substrates covered with a dielectric film during long-term thermal ageing. It was observed that the behaviour of the resistors on alumina and dielectric depends upon the firing process of the resistors and the kind of terminations. An analysis of the effects of thermal ageing was made as well as some SEM observations of diffusion processes between the resistor and its terminations.     

Microstructural, XRD and electrical characterization of some thick film resistors

The microstructural and electrical characteristics (sheet resistivities, TCRs, and noise indices) of some 1 k/sq. and 10 k/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.     

Electrical properties of thick film resistors from noise measurements

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Electrical conduction in thick film paste resistors

A formalism presented recently to describe the conductivity in particulate systems by effective medium theory is shown to be applicable to all families of thick film resistors. The thick film resistor is considered as being a mixture of relatively large glass particles and smaller conductive particles which are surrounded by a variably thin glass layer. The large variation in the resistance of the contact between particles that is expected to occur, combined with the particle size difference, is shown to result in the type of resistivity dependence on composition that is observed experimentally. The temperature dependence of the conductivity is predicted within the same formalism using data on extracted conductive particles and expected forms of the conductivity of the thin glass layers. The resulting behaviour is shown to be at least qualitatively correct.     

Lead-free thick film resistors: an explorative investigation

Pastes prepared with seven lead-free glass particles were screen-printed and fired on 96%-Al₂O₃ substrates. The fired films were studied in terms of sintering, devitrification, crack propagation and bleeding. The most promising glass compositions were selected for the preparation of RuO₂-based thick film resistors (TFRs), on pre-fired PtAu- or PdAg-based terminations; electrical properties of the resistors (sheet resistance vs. RuO₂ volume fraction, temperature coefficient of resistance and size effects) were investigated. The study evidenced a myriad of complex phenomena, including devitrification, relevant bleeding of the glass on alumina substrates, anomalous distribution of conductive grains in the glassy matrix, phase separations. Contrarily to what happens in traditional systems, it is difficult to get a wide resistance range simply by changing the fraction of the conductive phase. These results emphasize the criticality in the choice of the glass composition for the preparation of lead-free thick film resistors.     

Chemical constitution and conduction mechanisms in thick film resistors

This paper re-examines the data produced in a study of the behaviour of thick film resistors on dielectrics. It uses the published data to investigate the physicochemical nature of the conduction in six combinations of resistors on dielectrics and alumina. The results appear to confirm earlier observations that the presence of barium oxide in thick film resistor glass matrices has a major impact on the conduction mechanism. It is suggested that the cause is the totally different nature of two ions of similar size, possibly giving rise to localized regions of barium-lead compounds.     

TaAlN thin film resistors with improved electrical properties

When TaAl films are sputtered in the presence of nitrogen they exhibit a high sheet resistivity of the order of 2000–4000 μΩ cm. To understand their performance as resistive elements, the properties of TaAlN films covering a wide range of aluminium and nitrogen compositions were investigated. The increase in sheet resistivity and the changes in the structure of the film due to increases in the aluminium and nitrogen concentrations were studied. The variation in the temperature coefficient of resistance and the change in resistance for different aging temperatures for films with sheet resistances of 50–100 Ω/? and 600 Ω/? were also studied. The formation voltage limits in anodizing the TaAlN films are presented. The results are compared with data reported by other investigators. We suggest a suitable range of sputtering conditions with reference to the aluminium area in the TaAl target and the nitrogen partial pressure to fabricate thin film resistors of optimum performance for use in hybrid circuits.     

用激光微细熔覆法直写电阻浆料制备厚膜电阻

介绍了一种激光微细熔覆法直写电阻浆料制备厚膜电阻的新技术,并利用该技术在陶瓷基板上制备出厚膜电阻。文章中展示了部分实验结果．并分析了其附着机理。由于激光微细熔覆直写电阻技术具有无需掩模、图形由计算机控制、分辨率高、柔性化制造程度高等优点,显示谊技术在电子工业中将具有广阔的应用前景。     

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.     

The effect of hydrogen on the conductivity of Ag-Pd thick film resistors

The resistance of silver-palladium thick film resistors decreases and their surface conduction type changes upon heating in a flow of hydrogen at temperatures within 50–100°C or hydrogenation in an aqueous acid electrolyte at room temperature. These effects are due to the reduction of PdO (present in the Ag-Pd film) to Pd by hydrogen entering into the resistor material. In the electrolyte, the resistance of samples starts decreasing at the moment of the current being switched on.     

Structural stability and high-temperature electrical properties of cation-ordered/disordered perovskite LaBaCoO

A-site cation-disordered/ordered perovskite oxide LaBaCoO was synthesized by sol-gel method. Disordered LaBaCoO has higher conductivities than the ordered counterpart due to higher concentration of charge carriers, and is more stable at temperatures up to 850 °C and in argon, air and oxygen. Different conduction behavior in air/oxygen and argon suggests a p-type to n-type conduction transition caused by changes in chemical defects and charge compensation mode. Two conduction transitions associated with thermal-driven reduction of Co⁴⁺ to Co³⁺ and Co³⁺ to Co²⁺ were observed in air and oxygen. High conductivities and long-term structural stabilities suggested that LaBaCoO is a promising material as solid oxide fuel cell cathode or oxygen permeation membrane.     

Preparation of ultra-fine copper powder and its lead-free conductive thick film

In this paper, non-agglomerated monodispersed ultra-fine copper metallic powders have been synthesized with chemical reduction method. Fine lead-free glass powders were also prepared by solid synthesis process. Thick film paste prepared by above-mentioned copper metallic powders and lead-free glass powders was applied as conductive paste of MLCC. Mixture of glass and zinc oxide give the thick film a high adhesion strength which is attributed to the rough interface from interfacial reaction between glass and chip, and a good densification. Diffusion of metal between copper thick film and nickel thick film is clear. Ni-Cu solid solution appears under high temperature firing.