Change In Contact Resistance Research Articles

Electron microscopy characterization of Au/Ni contacts to p-type InP

The change in contact resistance produced by annealing Au/Ni metallizations top-type InP, has been correlated to the microstructural changes by means of electron microscopy techniques. The contacts were initially non-ohmic, but decreased in resistance by a factor of 24 upon annealing to temperatures of up to 400° C. The metallization was quite unstable,i.e. the Ni reacted readily with InP, even during deposition. Annealing was characterized by the formation of a series of Ni-P phases, as well as inward diffusion of Au and outward diffusion of In, which resulted in the formation of Au-In compounds. These reactions aided Ni doping of the InP, leading to the lower contact resistance.

Transmission/scanning transmission electron microscopy investigation of Au/Cr contacts to p-type InP

The change in contact resistance produced by the annealing of Au/Cr metallizations to p-type InP has been correlated with the microstructural changes examined by electron microscopy. The contacts were initially non-ohmic. Their resistance decreased, but remained non-ohmic, even after annealing to temperatures up to 450°C. Transmission and scanning transmission electron microscopy of cross-section samples revealed the formation, at elevated temperatures, of CrP and AuIn intermetallic layers at the contact/semiconductor interface. Chromium appeared to initiate compound formation through the dissociation of InP, although gold likely assisted in driving the reactions. CrP formed first, at temperatures below 350°C, as a polycrystalline, fine-grained layer between the Cr and InP. The formation of islands of AuIn followed at about 400°C and these grew into a continuous layer upon further annealing.

On the resistivity anomalies above 500 K in some high-Tcceramic superconductors

The observation of an anomalous rise in resistance above 500 K in a Ti-doped YBa2Cu3O7 superconducting system is reported. Detailed investigations on the changes in contact resistance under various annealing conditions reveal that this anomaly definitely originates from the sample. The authors observed an increase in resistance with time in a CO2 atmosphere at temperatures above 300 K and also an increase in BaCO3 present from infrared absorption measurements on the samples after the experiments. These results suggest that the resistance anomaly reported here is possibly related to the reaction of chemisorbed of CO2 in the sample at elevated temperatures. The possibility of a structural transition accompanied by oxygen loss, however, cannot be ruled out.

New instrument for measuring contact resistance developed for studying electrical contact phenomena

A new experimental apparatus for measuring the electrical contact resistance is described. This apparatus is essentially a conventional balance, modified for the examination of electrical contact resistance changes of silver and some other metal specimens due to increases and decreases in load and current. The results obtained show that plastic deformation takes place at the contact spots in response to an initial increase in load and elastic deformation occurs on a subsequent decrease in load. When a pair of Nichrome rods, which have a much higher resistivity than the silver specimen, are brought into contact with each other, the areas of the contact spots appear to be larger than those calculated. A fritting phenomenon, first studied in detail by Holm, occurred on the contact surfaces of the rods when they were subjected to heat and oxidation in the course of the experiment.

Barrier effect of selective chemical vapor deposited tungsten films

Thin selective chemical vapor deposited tungsten films are evaluated as a barrier metal for reaction between Al–1%Si electrode and Si substrate. Change in contact resistance and junction leakage current after annealing at 500 °C in N2 are measured. It is found that an increase of contact resistance at a very small contact hole of 2.25 μm2 is very slight. On the other hand, contact resistance in Al–1%Si electrode increases to one or two magnitudes with annealing of 210 min. Junction leakage current in a tungsten electrode is almost the same as that in Al–1%Si electrode. And it is also found that a thin tungsten layer protects the diffusion of aluminum into the Si substrate. From these results, it is concluded that thin selective chemical vapor deposited tungsten films have a good barrier effect for the reaction between Al–1%Si electrode and Si substrate.

Thermal enhancement of the electrical conductivities of alkali metal-doped polyacetylene complexes

Alkali metal-doped polyacetylene exhibits higher thermal stability than either the parent polymer or acceptor-doped complexes. For the Na, K, Rb and Cs complexes in which a channel structure is indicated, this stability appears to be related to the packing arrangement, which interferes with interchain reactions. In contrast with the case for acceptor doping, the conductivities of the alkali metal-doped cis-polyacetylenes are lower than for correspondingly doped trans-polyacetylenes. However, thermal annealing of the K-doped or Rb-doped polyacetylene (obtained using the naphthalide in THF) results in dramatic enhancement of conductivity for the doped cis-polymer, but not for the doped trans-polymer. After about a six-fold increase in room-temperature conductivity, the conductivity of K-doped polyacetylene is stable for many hours at 200 °C. Cis-polyacetylene films doped in the same way with other alkali metals do not show this enhancement. Significant changes in unit cell parameters, ESR linewidths, electrical anisotropy and contact resistance also occur during the annealing of K-doped cis-polyacetylene. Results presented suggest that thermal annealing of the K-doped polyacetylene eliminates residual conformational defects in partially chemically-isomerized polymer chains, thereby increasing crystalline perfection, increasing the effective conjugation length and increasing the hybridization of alkali metal and carbon orbitals. These changes probably result in the observed conductivity enhancement, the dramatic increase in ESR linewidth (6 – 12 G to 40 – 60 G) and the decrease in unit cell parameter.

Mo/TiW contact for VLSI applications

A molybdenum and titanium-tungsten double-layer contact metallization was developed for VLSI circuit applications. A thin TiW contact layer produced good ohmic contact and thermal stability With n+silicon and p+silicon up to 650°C without spiking. Thick Mo over the TiW layer has high conductivity, good step coverage, low electromigration, and smooth surface morphology. Both metal layers are sequentially sputtered in one pumpdown. The pattern formation process, including the etching Of approximately 1-µm-size contacts and tightly pitched 1.5-µm-wide metal lines is described. The contact resistance was evaluated as a function of processing conditions and high-temperature annealing. The contact resistance values for n+silicon contacts were always less then 35 Ω . µm2and that for the p+silicon contact resistance was within a range of 35 to 85 Ω . µm2after annealing at 400°C. Annealing experiments showed that occasional larger values of p+contact rsistance could be reduced by annealing at 625°C. There was no junction leakage degradation for annealing temperatures below 600°C. The mechanism, and cause, of the contact resistance and leakage current change with sintering temperature was studied by means of X-ray, SIMS, TEM, and SEM.

Survey of Contact Fretting in Electrical Connectors

Fretting in mated electronic connectors can lead to unacceptable increases of contact resistance. Although this phenomenon has been known for many years, it was not until interest developed in replacing gold by certain low-cost contact materials, such as tin-lead solder plate, that the incidence of fretting problems rose dramatically. Over the past 10 years there has been considerable research on the origins of this contact resistance instability, on methods for its measurement, and on finding ways to avoid the problem. This paper is a critical review of fretting based on published work by the author and by other investigators. Included are a discussion of mechanisms of transfer, wear, oxidation, and frictional polymerization which apply to the fretting of contacts; the effect of operational parameters such as cycle rate and force on contact resistance change; a survey of materials whose fretting behavior has been determined; and an analysis of the different roles which lubricants play in controlling fretting according to the contact materials that are involved. Connector design, the particular conditions of field use, expected connector lifetimes, and their reliability requirements determine whether contact materials will be satisfactory. Hardware tests which tend to cause fretting, such as shock and vibration, should be employed before new contact materials and designs are incorporated in connectors.

Effect of Fretting on the Contact Resistance of Palladium Electroplate Having a Gold Flash, Cobalt-Gold Electroplate, and DG R156

Contact resistance stability is a function of the contact materials, design, and conditions of connector use. The purpose of the present work was to identify certain potentially unstable materials systems when fretting occurs at the contact interface. In previous studies of fretting, it was found that variable resistance can be caused by wearout of the finish to base substrate or underplate, or to the formation of frictional polymers when platinum group contact metals are used. An evaluation of the fretting of cobalt-gold electroplate, palladium electroplate having a pure gold flash, and DG R156 was made with these materials mated to themselves or to each other. A bench test apparatus was employed with wire contacts joined in crossed rod configuration. It was found that mated to themselves and for the conditions of this test, unlubricated cobalt-gold plate performed well, although DG R156 was nearly as good and gold flashed palladium was poor. Unlubricated cobalt-gold plate mated to DG R156 was good, followed by cobalt-gold plate joined to gold flashed palladium, which was marginal. Again, for the conditions of this test, gold flashed palladium versus DG R156 was poor. Lubrication with a thin film of a polyphenylether attenuated the contact resistance changes in all cases by lowering the rate of wear of the material or, with palladium-containing systems, by causing the frictional polymers that formed to be nonadherent. The tendency of a connector to degrade by fretting depends on its design. If material systems are to be conducted which perform poorly in the bench test, it is recommended that connector tests be conducted which tend to cause micromotion at the contact interface. Procedures involving shock, vibration, and cycling temperature may be employed for this purpose.

Effect of HCl and Cl<inf>2</inf>on Pd Inlay Coupons and Pd Connector Contacts

Field exposure studies indicate that a resistive palladium chloride film may grow on Pd coupons when the ambient contains both high relative humidity (RH) and chlorine. This behavior raises a concern about the use of palladium in separable connectors. Most connectors are shielded systems, and the contacts are subject to wipe on mating. Therefore contact resistance probe measurements on coupon surfaces may not be representative of the behavior of connector contacts. A study on a moderately shielded connector system was initiated to compare the effects of chlorine and HCl vapors in high relative humidities on connectors with the effects on coupon surfaces. A number of material combinations including Pd against Pd, Pd against gold, and 60Pd40Ag against gold were tested in the study. It is shown that the high contact resistance values measured on the coupons are not representative of the contact resistance changes detected on connector contacts subjected to an accelerated aging treatment in the mated condition. The changes in contact resistance are much lower on connector contacts than on coupons. Palladium and 60Pd40Ag against gold are much superior in performance to Pd against Pd when exposed to Cl <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> environments. Exposure of mated connector contacts to HCl at high relative humidity results in little degradation of contact resistance compared to chlorine exposure.

Field Studies of Contact Materials: Contact Resistance Behavior of Some Base and Noble Metals

The contact resistance of various metals was determined after exposure of from three months to four years in telephone central offices and in nonair-conditioned locations. The logarithm of median contact resistance was stable for some materials, but with most it increased on aging according to linear, parabolic, or logarithmic time relationships. The kinetics of contact resistance change depended on the intrinsic chemical reactivity of the material, the air pollutants which were present in the exposure site, and especially on the relative humidity. Contact resistance-force behaviors were affected also by the topographic and hardness characteristics of the metals. The overall ranking in central offices on the basis of stability of contact resistance from best to worst was 1) gold; 2)high gold-silver alloys, palladium, high palladium-silver alloys, 60Sn40Pb, and tin; 3) copper, high copper alloys, and nickel; and 4) silver. Where relative humidity was only slightly less well-contro11ed, as in a nonair-conditioned room in an urban central office building, except for gold the contact resistances of all materials were from one and two orders of magnitude greater after 40 months of exposure, and often displayed different kinetic behaviors. Palladium was exposed in a total of 18 sites and its contact resistance was found to be highly variable, even exceeding 1000 \Omega in less than 1 year when relative humidity was high and reactive chlorine-containing air pollutants were present. Pore corrosion of gold electrodeposits and creep of tarnish films from adjacent base metals did not occur in central offices except when the substrate was silver and the level of sulfiding pollutants was high.

Effect of Fretting on the Contact Resistance of Palladium

The fretting of palladium contacts was studied at conditions relevant to their use in separable electronic connectors. Wrought, clad, and electroplated palladium mated to wrought palladium develop high contact resistance due to the formation of friction polymer. This can occur in ordinary room air which contains traces of organic air pollutants. Contact resistance changes diminish as force is increased. Static contact resistance measurements were made in small steps along wear tracks during fretting tests. It was found that the contact resistance increases were greatest at or near the ends due to the accumulation of polymer, and that the numbers of fretting cycles required to produce high contact resistance diminish sharply as track length is increased from 10 to 160 #m. Thin coatings of a polyphenyl ether contact lubricant significantly improve the stability of contact resistance, and contacts flooded with this fluid showed little degradation. This is attributable to its ability to disperse the polymer which is formed. Likewise, palladium contacts immersed in toluene, a polymer precursor commonly used in laboratory studies of friction polymerization, display stable contact resistance. These findings are compared to those of previous studies of the contact resistance of palladium during fretting, and it is shown that differences in interpretation of results are due to variations in the apparatus which were used.

In Situ AES Characterization of Rotating Electrical Contacts

The electrical contact resistance and surface chemical composition of a rotating copper slip ring in contact with two wire brushes were investigated in situ under ultrahigh vacuum (UHV) conditions as a function of the number of revolutions of the slip ring. The initial surface of the copper slip ring was examined by Auger electron spectroscopy (AES) techniques and found to be almost completely covered by surface impurities largely consisting of carbon. As the slip ring was rotated in contact with the brushes, the impurity concentration declined sharply to less than ten at% after several hundred revolutions. A corresponding decrease in electrical contact resistance and a sharp increase in friction and wear were also observed. Regardless of polarity, the brush with the higher contact pressure had the lower contact resistance. The in situ experiments were terminated when the motor turning the slip ring could no longer overcome the adhesive forces between the brushes and slip ring. Subsequent scanning electron microscopy observations of the brush and slip ring surfaces gave supplementary information on the mechanisms of friction, wear and surface cleaning during rotation. For a given experiment the change in contact resistance during rotation divided by the change in impurity concentration on the surface of the slip ring is approximately the same for the positive and negative interfaces. This parallelism of contact resistance and surface impurity concentration suggests that the former is caused to a significant extent by the latter. Since the surface impurities are mostly carbon atoms, it is concluded that carbon impurities are largely responsible for the observed contact resistance.

Wear of zirconium alloys due to fretting and periodic impacting

Impact fretting of zirconium alloys was studied in demineralized water at temperatures up to 97 °C. Results showing the effects of motion parameters, test duration, material combination and temperature (23–97 °C) are presented. The correlation between the electrical contact resistance and wear is good for zirconium alloy combinations. Continuous monitoring of the change in electrical contact resistance helped to reveal the wear process during tests.

Contact Resistance Measurements for Observation of Fluid Film Effects in Journal Bearings

Wear studies of material combinations, sliding or rolling in contact, frequently produce very inconsistent results. Thin film effects are often cited as the cause of these disparities. One means of detecting the presence of these films is by the use of electrical contact measurements to determine the degree of metal-to-metal contact. This paper describes an experimental study of the wear behavior of grease-lubricated, cast bronze sleeve bearings running against steel, shafts at high loads, to 24.8 megapascals and a low sliding velocity of 1.6 m/min. The effects of the casting method, shaft speed and shaft finish were investigated. Bearing torques, temperatures, contact resistances, and wear rates were monitored, during the tests. Neither the torque nor the temperature measurements could be related consistently to the wear data. On the other hand, the electrical contact resistance measurements correlated very well. Changes in wear rates were accompanied by corresponding changes in contact resistance. A reg...

The effect of silicon on the adhesion of noble metals

The effect of silicon on the adhesion of noble metals has been studied using a twist-compression bonding test and an ultrahigh vacuum test. The specimen surfaces were analysed using ion microprobe mass analysis and electron spectroscopy for chemical analysis. It was found that silicon, whether introduced on the surface as an impurity or present as an alloy component, was effective in preventing adhesion in air because of the formation of silicon oxide. The oxidation rate of silicon and metal-silicon alloy systems is qualitatively discussed considering changes in adhesion coefficient and contact resistance. Twist-compression bonding tests of noble-metal alloys containing an active additive such as silicon were found to be affected by the surrounding atmosphere.

Effect of the electric arc and the ambient air on the contact resistance of silver, tungsten, and silver-tungsten contacts

Contact pairs of Ag, W, and Ag-W were operated 6000 times in a 20-A 110-V ac circuit. The effect of contact-resistance change was observed by measuring the temperature rise (TR) at the fixed contact while passing the steady-state 20 A. The TR was measured before the switching had begun and after each 1000 operations. A large number of contact pairs were used in order to ensure a good statistical interpretation of the data. The chemical composition of the contact surfaces was measured using a Debye-Scherer x-ray analysis. The distribution of contact resistance for Ag contacts remained the same throughout the experiment. For W contacts, the distribution was initially higher than that for Ag: the result of conductivity and hardness differences. During the switching experiments, the formation of tungsten oxides on the contact surface resulted in high contact-resistance values. For Ag-W, the distribution was initially closer to that for Ag contacts: the result of a thin surface layer of Ag on the Ag-W. During the switching, the Ag-W contact-resistance distribution gradually increased in dispersion and had higher values than those measured for W contacts. The x-ray data showed that not only were tungsten oxides formed, but also there was a high probability that some of the free Ag on the contact surface formed an insulating silver tungstate.

Design and test of a sliding plug-in conductor connector for compressed gas-insulated cables

This paper describes the theoretical analysis, development testing, and final design and successful testing of a sliding plug-in conductor connector for compressed gas-insulated cables using SF 6 insulation. Particular attention was paid to the change of contact resistance and contact temperature with time, contact welding, and contact wearing.

Stress Relaxation Processes in Wire Wrapped Joints

A standard system of determining stress relaxation behavior in wrapped joints has been developed. The system uses extended length wrapped terminals with i00 turns of wire and monitors the elastic stress reduction in terms of the relaxation of axial twist in the terminal by means of an optical magnification technique. The effect of various joint parameters on stress relaxation behavior of secondary wrapped joints has been evaluated and correlated with Contact resistance changes observed in similar joints after quaiification approval testing. The effects of various wrapping wire Parameters and of terminal cross section On the stress relaxation behavior of primary Wrapped joints has also been investigated.

Compatibility of Gold Deposits for Connector Contacts

A forty way, two part, printed circuit board connector has been the test vehicle for the evaluation of wear of a number of gold deposits in similar and dissimilar combinations. The proprietary electrolytes, from four suppliers, consisted of four acid cyanide types (cobalt or nickel hardened) two noncyanide (sulphite) and two neutral cyanide organically hardened golds. Assessment of wear was made by scanning electron microscope examination of wear tracks and by changes in contact resistance when the connectors were subsequently exposed to a corrosive environment. The acid Cyanide deposits exhibited least wear, the sulphite types wore excessively by cold pressure welding to produce platelets of debris, and the neutral cyanide golds were of mixed extent of wear. A hypothesis involving a polymer lubrication mechanism is proposed to explain the Iow wear of the acid golds. Scanning electron micrographs support this pro- posal.