RuO2-based Thick Film Research Articles

Microstructural Analysis and Transport Properties of RuO2-Based Thick Film Resistors

Microstructural Analysis and Transport Properties of RuO₂-Based Thick Film Resistors

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.

Noise resolution of RuO2-based resistance thermometers

Low-frequency noise was measured for RuO2-based thick film resistors at liquid helium temperatures down to 0.36 K. The 1∕f-type spectrum and squared voltage dependence of power spectral density observed at low voltages attribute the noise as coming from equilibrium resistance fluctuations. Measurements carried out at different temperatures show that the magnitude of noise intensity (index) increases significantly as temperature goes down. Due to this fact, the resolution of RuO2 thermometers increases above the instrument resolution. The quantity which describes a sensor resolution is defined and calculated for RuO2 thick film sensor. Some remarks on measurement strategy and sensor optimization are supplied.

RuO2-based Low Temperature Sensors with ?Tuned? Resistivity Dependencies

The paper presents low temperature properties of RuO2-based thick film resistors (temperature sensors) prepared from DuPont paste under various technology conditions. It is shown that the temperature dependence of their resistance R(T), in the case when all sensors are prepared from the same starting paste, can be strongly influenced by the manufacturing process (above all by the firing temperature and firing time). The paper also presents the results on the low temperature magnetic field dependence of RuO2-based sensors.

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.

Correlation between electric force microscopy and scanning electron microscopy for the characterization of percolative conduction in electronic devices

Abstract The correlation between electric force inicroscopy (EFM) and scanning electron microscopy (SEM) techniques provided interpretation of the contrast features of electric force images and gave an insight into the two-dimensional (2D) electrical transport properties of RuO2-based thick film resistors (TFRs). From the comparison between EFM, tapping-mode atomic force microscopy, SEM (secondary electrons, specimen current and X-ray energy-dispersive spectroscopy) of TFRs on the same specimen area, it turned out that the conduction mechanism was related to the grain size and to how RuO2 crystals were distributed in the glassy matrix. Dark areas in EFM images corresponded to conductive-insulating interfaces. In SEM the same areas were the interfaces between the RuO2 crystals and the insulating matrix. The 2D percolative path of the electrical current near the surface was observed in connection with the net of RuO2 grains not homogeneously dispersed in the insulating matrix.

Conduction Mechanism in RuO2-Based Thick Films

The paper presents resistivity characteristics of RuO2-based thick resistive films (temperature and magnetic field dependences), and analyzes them from the point of view of the possible conduction mechanism at low temperatures. It is shown that simple models based on tunnelling of charge carriers between conductive RuO2 grains and on variable range hopping of carriers between localized impurity states in the glass matrix via thermal activation do not provide a satisfactory explanation for the electrical conductivity of the investigated thick film resistors. We suggest a new mechanism based on tunnelling of electrons through graded barriers between conductive particles which might explain the observed behaviour.

Magnetoresistance of RuO2-Based thick film resistors

In a series of samples whose composition was systematically changed, we have studied the magnetoresistance (ΔR/R = [R(B)-R]/R) of RuO 2 -based thick film resistors (TFR's) in magnetic fields (B) up to 20 Tesla and in a temperature range (1.2 K < T < 40 K) in which their resistance exhibits insulating behavior at the boundary between the strong and the weak localization. At the higher temperatures, ΔR/R exhibits a positive bump that does not depend on the RuO 2 concentration but it changes with the concentration of magnetic Mn impurities diluted in the glassy matrix. For T ≤ 20 K magnetoresistance is entirely negative and it has a quadratic magnetic field dependence at low field. We use the weak localization theory to relate these features of the high temperature magnetoresistance to the composition of TFR's. At low temperature (T < 4.2 K) the negative magnetoresistance shows some peculiarities. The quadratic term shrinks within a vanishing magnetic field range and the magnetoresistance linearly increases in a wide range of B. At the strongest fields and the lowest temperature ΔR/R shows a tendency to saturation. Although these results do not enable to discriminate among different mechanisms, we note that the low temperature (T < 4.2 K) behavior is essentially consistent with recent theory of Nguen et al.

Low Temperature Electronic Transport in RuO2-Based Cermet Resistors

We have studied the temperature (T) dependence of resistance (R) of RuO2-based thick films down to 1.2 K. Samples were prepared from inks containing conductive RuO2 powders (⩽ 10% wt.), high lead-silicate glass particles and Mn (⩽ 1.4% wt.). We found that the resistance fits the exponential law R = R 0 exp(T 0/T)x with x = 1/4 and the most resistive samples show a cross-over to the x = 1/2 regime as the temperature decreases. Both the fitting parameters R 0 and T 0 scale down as the RuO2 fraction increases and they are affected in a similar way by a change of the Mn content. The presence of the two regimes is similar to that observed in n-type GaAs [Phys. Rev. B 39, 8059 (1989)] and n-type CdSe [Phys. Rev. Lett. 64, 2687 (1990)] whilst it disagrees with the behaviour predicted for grain to grain hopping [Phys. Rev. B 27, 2583 (1983)] and with that expected for resonant tunneling between metallic particles [J. Appl. Phys. 48, 5152 (1977)]. We conclude that in our systems the driving charge transport mechanism is electron hopping within the glassy matrix. Since in our case the hopping length is of the same order of the localization length, the puzzling questions arising from our experiments are whether and how the variable range hopping model can be extended beyond its conventional limits.

Low temperature electronic transport in RuO2-based cermet resistors

We have studied the temperature (T) dependence of resistance (R) of RuO2-based thick films down to 1.2 K. Samples were prepared from inks containing conductive RuO2 powders (⩽ 10% wt.), high lead-silicate glass particles and Mn (⩽ 1.4% wt.). We found that the resistance fits the exponential law R = R0 exp(T0/T)x with x = 1/4 and the most resistive samples show a cross-over to the x = 1/2 regime as the temperature decreases. Both the fitting parameters R0 and T0 scale down as the RuO2 fraction increases and they are affected in a similar way by a change of the Mn content. The presence of the two regimes is similar to that observed in n-type GaAs [Phys. Rev. B 39, 8059 (1989)] and n-type CdSe [Phys. Rev. Lett. 64, 2687 (1990)] whilst it disagrees with the behaviour predicted for grain to grain hopping [Phys. Rev. B 27, 2583 (1983)] and with that expected for resonant tunneling between metallic particles [J. Appl. Phys. 48, 5152 (1977)]. We conclude that in our systems the driving charge transport mechanism is electron hopping within the glassy matrix. Since in our case the hopping length is of the same order of the localization length, the puzzling questions arising from our experiments are whether and how the variable range hopping model can be extended beyond its conventional limits.

Electronic transport in RuO2-based thick film resistors at low temperatures

The electrical conductivity of various RuO2-based thick film paste resistors was investigated in the temperature range between 50 mK and 20K. It is shown that models based on variable range hopping and simple models based on tunnelling of charge carriers between conductive grains do not provide a satisfactory explanation for the temperature dependence of the electrical conductivity in these materials at very low temperatures. We suggest a new mechanism based on tunnelling of electrons through graded barriers between conductive particles.

Resistance adjustment in RuO2-based thick film strain-gauges by laser irradiation

Resistance adjustment in RuO2-based thick film strain-gauges by laser irradiation

Influence of the preparing conditions on the physicochemical characteristics of glasses for thick film hybrid microelectronics

Seven batches of a high-lead glass were used for the preparation of RuO2-based thick film resistors. Investigation of their electrical properties showed a lack of reproducibility of results whose origin was related to changes of the physicochemical properties of the glassy matrix. A systematic investigation of the glass batches, both in form of frit powders and screen printed and fired layers, was carried out with several spectroscopies to detect changes in composition and structure. The spectroscopic methods included x-ray Energy Dispersive Fluorescence (EDS), Scanning Electron Microscopy (SEM), Atomic Absorption (AA), diffuse optical reflection of the powders and specular reflection of the layers, optical transmission, and other complementary methods. The dissolution of Al, due to interaction between the glasses and the alumina substrate, as well as the diffusivity and solubility of Ag due to interaction with the Ag-bearing terminations were measured. The results demonstrated that, apart from small compositional differences, the various batches were characterized by differences in residual stresses, redox reactions, and “microstructure.” The latter was responsible for very notable differences in the optical properties of the glasses, which in turn are closely related with the difference in atomic solubility and diffusivity. Optical spectroscopies have been found to be a suitable means for testing reproducible preparation methods of glass frits for thick-film hybrid microelectronics.

Size effect on the electrical conduction and noise of RuO2-based thick film resistors

Temperature dependence of sheet resistance for a generic RuO2-based resistor with a composition of 20wt.% RuO2-80wt.% glass (63wt.% PbO-25wt.% B2O3-12wt.% SiO2) is evaluated. A combined tunnel/parallel conduction model is employed to describe the resistance behavior with respect to the temperature variation. The geometry of the resistive film, such as the aspect ratio and thickness, cast a significant effect on the electrical characteristic of the thick film assembly. It is observed that shorter resistive films exhibit smaller resistivity as compared to that of the longer film. Thinner resistive films have smaller resistivity as compared to the thicker ones. In addition,1/f noise is the dominating contribution in the thick film resistor. The presence of1/f noise can be qualitatively explained with the aid of the tunneling mechanism.

Temperature dependence of the electrical conduction in RuO2-based thick film resistors

In this paper, the temperature dependence of resistance of two generic RuO2-based resistors is investigated. The resistor compositions studied are 80 wt.% glass (63 wt.% PbO - 25 wt.% B2O3 - 12 wt.% SiO2, designated as G1) - 20 wt.% RuO2 and 80 wt.% glass (55.5 wt.% PbO - 22 wt.% B2O3 - 10.5 wt.% SiO2 - 12 wt.% Al2O3, designated as G2) - 20 wt.% RuO2. The sheet resistance of resistor 80 wt.% G1 - 20 wt.% RuO2 fired at 850° C decreases as the temperature is increased from 100 K to ∼400 K, remains a minimum value at temperatures 400 K ∼690 K, and then increases as the temperature is further raised. A negative temperature coefficient of resistance (TCR) of ∼-480 ppm/°C is obtained from 100 K to 500 K. TheTCR becomes less negative when temperature increases. Three models for conduction mechanism of thick film resistors are employed to explain the experimental results. A modified model, consisting of both tunneling and parallel conduction approaches, is proposed to elucidate the change in slope in the resistance-inverse temperature curve as well as the temperature dependence of the resistance. In addition, an equivalent circuit model is proposed to describe the electrical behavior of the thick film resistors.

Surface-modified RuO2-based thick film resistors using Nd:YAG laser

An RuO2-based thick film resistor (TFR) is a cermet-type resistor which consists of RuO2 particles and glass. Paste containing organic vehicles is printed onto an insulating substrate, and subsequent firing at about 900 °C makes the cermet-type resistors. TFRs are widely utilized as electrical resistive materials in electric devices such as hybrid ICs (integrated circuits) [see, for example, R. W. Vest, Ceram. Bull. 65, 631 (1986)]. The features of an advanced laser application process to control the electrical resistivity of the TFRs were proposed. This new process is an application of surface-modification using laser beams. In this paper, a mechanism of the surface-modification of the TFR is considered using results of morphology observation, x-ray diffraction (XRD), and x-ray photoemission spectroscopy (XPS). The following points should be noticed. (1) In the surface-modifying process using normal mode pulse Nd:YAG laser beams, glass in the TFR is modified and supersaturated with ruthenium impurities because of rapid heating and rapid cooling rates in the irradiation process; the specific resistivity of the modified glass is extremely decreased, estimated at 10−3 Ω cm at most. (2) No new crystalline phase in the ruthenium-oxygen system is detected after the surface modification by results of x-ray diffraction (XRD) analysis. (3) The valence state of the ruthenium ions in the modified glass is more positively charged than that in crystalline RuO2 , while the valence states of lead and oxygen ions in the modified glass are more negatively charged than in as-fired glass before surface modification according to the results obtained by x-ray photoemission spectroscopy (XPS) analysis. It is suggested that if ruthenium ions introduce electrons into the modified glass and if these electrons behave as itinerant electron carriers in the modified glass, a decrease in the resistivity of TFR after the surface modification is quite reasonable for the results of XPS.

Experimental study of electrical conduction in RuO2-based thick resistive films

Electrical conduction and noise of thick resistive films prepared from RuO2 and lead borosilicate glass were studied. Two series of pastes were prepared differing by the specific surface area of RuO2 powder (77 m2/g and 24 m2/g). It was concluded that two regions on resistanceR versus volume fractionv of conducting component characteristics may be specified. For small values ofv resistance obeys percolative power law revealing universal value of critical exponentt∼-2.0. Continuous-inverted Swiss cheese model was proposed to describe this region. When attempting to use power law forR versusv curve for largev special care should be taken as values ofv are far from critical volume fraction at variance with basic assumption of percolation theory as well as obtained values of critical exponentst=5.63±0.56 and κ=6.64±0.82 are higher than any ones predicted by theory.

The Relationship Between the Electrical Properties of Thick Film Resistors and the Thermal Expansion Coefficient of the Substrates

The temperature characteristics of RuO2-based thick film resistors on various substratcs having different thermal expansion coefficient have been investigated.It became clear that, if the thermal expansion coefficient of the substrate is larger than that of the thick film resistor, a compression is being exerted by the substrate on the as-fired resistor at low temperature. As temperature rises, the resistance value increases, and the TCR becomes positive.On the contrary, if the thermal expansion coefficient of the resistor is larger than that of the substrate, the as-fired resistor is being stretched by the substrate at low temperature. As temperature rises, the resistancc value decreases, and the TCR becomes negative.

Electrical conduction and strain sensitivity in RuO2 thick film resistors

We have measured the resistance and strain characteristics of RuO2-based thick film resistors and find that they differ from Ru-based pyrochlore (A2Ru2O7) resistors. The dependence of resistance on RuO2 volume concentration (x), for different (average) particle sizes, obeys a critical conduction expression of the form R=R0/(x−xc)t, with t≊3. The strain dependence of resistance or strain gauge factor (γ) approximately follows the phenomenological expression γ=γ0+Atx/(x−xc), with γ0 between 2 and 3. Pyrochlore resistors obey the same critical conduction behavior, but we had previously found that t≊2. Further, γ is larger for pyrochlore resistors for comparable resistivity and particle size. We account for the differences by postulating that in RuO2 resistors sintered conduction pathways dominate at low resistance, and both sintered interfaces and tunnel junctions contribute to the high resistance behavior. This is in contrast to pyrochlore resistors for which we believe that tunnel junctions always dominate electrical conduction.