Cement-based controlled electrical resistivity materials

Low-cost controlled electrical resistivity materials based on Portland cement and exhibiting low values of the relative dielectric constant have been attained. The materials are cement paste containing short electrically-conducting fibers. With steel fibers (0.1 vol.%), the resistivity and relative dielectric constant (10 kHz) are 8 × 10⁴ cm and 20, respectively. With carbon fibers (1.0 vol.%) and silica fume, these quantities are 8×10²Ω-cm and 49, respectively.     

Electrically Nonconductive Thermal Pastes with Carbon as the Thermally Conductive Component

Electrically nonconductive thermal pastes have been attained using carbon (carbon black or graphite) as the conductive component and ceramic (fumed alumina or exfoliated clay) as the nonconductive component. For graphite particles (5 μm), both clay and alumina are effective in breaking up the electrical connectivity, resulting in pastes with electrical resistivity up to 10¹³Ω·cm and thermal contact conductance (between copper surfaces of roughness 15 μm) up to 9 × 10⁴ W/m²·°C. For carbon black (30 nm), clay is more effective than alumina, providing a paste with resistivity 10¹¹ Ω·cm and thermal contact conductance 7 × 10⁴ W/m²·°C. Carbon black increases the thermal stability, whereas either graphite or alumina decreases the thermal stability. The antioxidation effect of carbon black is further increased by the presence of clay up to 1.5 vol.%. The addition of clay (up to 0.6 vol.%) or alumina (up to 2.5 vol.%) to graphite paste enhances the thermal stability.     

Improving the electrical and mechanical behavior of electrically conductive paint by partial replacement of silver by carbon black

Partial replacement of silver particles by carbon black (low cost) in electrically conductive paint was found to decrease the electrical resistivity and increase the scratch resistance of the resulting thick film, which is for use in electrical interconnections. An effective carbon black content is 0.055 of the total filler volume. By using a total solid volume fraction of 0.1969 and a silane-propanol (1:1 by weight) solution as the vehicle, a paint that gives a thick film with resistivity 2 × 10⁻³ Ω·cm has been attained.     

Silver Particle Carbon-Matrix Composites as Thick Films for Electrical Applications

Silver particle (3 μm) carbon-matrix composites in the form of thick films (around 100 μm thick) on alumina, as prepared from pastes comprising silver and mesophase pitch particles (14 μm), have been attained. The films on alumina were fired at 650°C in nitrogen to convert pitch to carbon. The volume electrical resistivity attained ranged from 10⁻⁵ Ω cm to 10⁴ Ω cm, depending on the silver volume fraction. The percolation threshold was 12 vol% silver.     

Combined Use of Magnetic and Electrically Conductive Fillers in a Polymer Matrix for Electromagnetic Interference Shielding

The combined use of a highly magnetic filler (mumetal) in low proportion and a highly conductive filler in high proportion in a polymer matrix provides a composite material that is more effective for electromagnetic interference shielding than the use of a highly magnetic filler alone or the use of a highly conductive filler alone. Mumetal is effective (due to absorption) when it is in a composite material of DC electrical resistivity below 10 Ω cm, as provided by conductive fillers, which contribute to shielding by reflection and allow paths for eddy current.     

Electrically Conductive Metal Polymer Nanocomposites for Electronics Applications

An electrically conductive nanocomposite composed of thermoplastic elastomer and nanosized silver particles was developed. Nanosized silver particles were produced by the liquid flame spraying method. Nanocomposites were produced employing a batch mixing process in the melt state. The percolation curve and the minimum resistivity as a function of silver content were defined. A plasticized styrene block-copolymer was used as the matrix polymer. The results showed that the agglomeration of the silver particles has a major influence on the percolation threshold and the resistivity of the compound. With slightly agglomerated silver particles a percolation threshold with a silver content of 13–16 vol.% was achieved. The corresponding resistivity was 2.0 × 10⁻¹ Ω cm. With heavily agglomerated particles the resistivity is high (2.9 × 10³ Ω cm), even with a silver content of 20 vol.%. With a low primary silver particle size (under 100 nm), the resistivity of the compound was high (5.6 × 10⁵ Ω cm).     

Thermally stable Ge/Cu/Ti ohmic contacts to n-type GaN

The performance of a novel Ge/Cu/Ti metallization scheme on n-type GaN has been investigated for obtaining thermally and electrically stable low-resistance ohmic contacts. Isochronal (2 min.) anneals in the 600–740°C temperature range and isothermal (690°C) anneals for 2–10 min. duration were performed in inert atmosphere. For the 690°C isothermal schedule, ohmic behavior was observed after annealing for 3 min. or longer with a lowest contact resistivity of 9.1 × 10⁻⁵ Ωcm² after the 10 min. anneal for a net donor doping concentration of 9.2 × 10¹⁷ cm^−Ω3. Mean roughness (R_a) for anneals at 690°C was almost constant at around 5 nm, up to an annealing duration of 10 min., which indicates a good thermal stability of the contact scheme.     

High conductivity copper-boron alloys obtained by low temperature annealing

The electrical behavior during annealing of copper films with a nominal concentration of 2 at.% boron has been investigated. The evolution of the resistivity of the film was monitored using an in situ technique, in which the film was rampannealed at constant ramp rates. At temperature of 150–200°C, the resistivity of the Cu(B) undergoes a first drop. This is followed by one or two such drops in resistivity, so that after completion of a ramp-anneal from 50°C to 750°C, the room temperature resistivity decreases from the initial value of 13 μΩ cm to 2.1 μΩcm, close to that of bulk copper. Isothermal annealing of the film also leads to substantial decreases in resistivity, from 13 μΩcm to 3 μΩ cm after annealing at 350°C for 8 h and to 2.5 μΩ cm at 400°C for 4 h. These results show that a dramatic reduction in resistivity of Cu(B) alloys takes place at temperatures below 400°C, suggesting possible applications for silicon device interconnections.     

Carbon-black thixotropic thermal pastes for improving thermal contacts

This paper addresses thermal interface materials for thermal conduction of excess heat for microelectronic applications. Carbon black (30 nm) thixotropic paste based on polyol ethers is comparable to carbon black fluidic paste based on polyethylene glycol (PEG) in its effectiveness as a thermal paste, and in its dependence on pressure history. Prior pressure (up to 0.69 MPa) application is helpful. The optimum carbon black content is 2.4 vol.% for the thixotropic paste. The thermal contact conductance across copper surfaces is 30 × 10⁴ and 11 × 10⁴ W/m²-°C for surface roughness of 0.05 μm and 15 μm, respectively. The volume electrical resistivity is 3 × 10³ Ω-cm. Boron nitride (BN) (5–11 μm) and graphite (5 μm) thixotropic pastes are less effective than carbon black thixotropic paste by up to 70% and 25%, respectively, in thermal contact conductance, due to low conformability.     

Pressure electrical contact improved by carbon black paste

Pressure and pressureless electrical contacts were evaluated by measuring the contact electrical resistivity between copper mating surfaces. Pressure electrical contacts with a contact resistivity of 2×10⁻⁵ Ω·cm² have been attained using a carbon black paste of a thickness of less than 25 μm as the interface material. In contrast, a pressureless contact with silver paint as the interface material exhibits a higher resistivity of 3×10⁻⁵ Ω·cm² or above. A pressureless contact with colloidal graphite as the interface material exhibits the same high contact resistivity (1×10⁻⁴ Ω·cm²) as a pressure contact without any interface material. On the other hand, pressureless contacts involving solder and silver epoxy exhibit lower contact resistivity than carbon black pressure contacts.     

Comparative study of electrically conductive thick films with and without glass

An air-fireable, glass-free, electrically conductive thick-film material (96.6% Ag, 1.38% Cu, 0.28% Al, 0.35% Ti, and 1.39% Sn by weight) and a conventional glass-containing, electrically conductive thick-film materials (96.6% Ag and 3.4% glass frit by weight), both on alumina substrates, were studied by electrical, mechanical, thermal, and microscopic methods. The volume electrical resistivity of the glass-free thick film (2.5×10⁻⁶ Ω·cm, 30-μm thick) is lower than that of the glass-containing thick film (3.9×10⁻⁶ Ω·cm, 19-μm thick), with each film processed at its optimum firing temperature. The optimum firing temperature is 930°C and 850°C for glass-free and glass-containing thick films, respectively, as indicated by the criteria of low resistivity and high scratch resistance. The glass-free thick film has a higher scratch resistance than the glass-containing thick film, both fired at their respective optimum temperatures, suggesting that the former has higher bond strength to the alumina substrate. The formation process of the glass-free and glass-containing thick films is similar. The process involves solid-state diffusion of silver, which results in a silver network and grain boundaries. However, the sintering of silver particulates in the glass-containing thick film is enhanced by the viscous flow of glass.     

Electrical transport properties of single GaN and InN nanowires

The transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN). The as-grown GaN nanowires were insulating and exhibited n-type conductivity (n ≈ 2×10¹⁷ cm⁻³, mobility of 30 cm²/V s) after annealing at 700°C. A simple fabrication process for GaN nanowire field-effect transistors on Si substrates was employed to measure the temperature dependence of resistance. The transport was dominated by tunneling in these annealed nanowires. InN nanowires showed resistivity on the order of 4×10⁻⁴ Ω cm and the specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10⁻⁷ Ω cm². For In N nanowires with diameters <100 nm, the total resistance did not increase linearly with length/square of radius ratio but decreased exponentially, presumably due to more pronounced surface effect. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN nanowires.     

Oxidation Prevention and Electrical Property Enhancement of Copper-Filled Isotropically Conductive Adhesives

This paper describes the development and characterization of isotropically conductive adhesives (ICAs) incorporating copper (Cu) powders as electrically conductive fillers, along with a silane coupling agent for oxidation protection of copper powders, for environmentally friendly, low cost and high thermal reliability applications in microelectronics packaging. The effect of silane coupling agent materials and concentration on the electrical conductivity, thermal stability and reliability of Cu-filled ICAs was investigated for potential alternatives of conventional silver-filled ICAs. The surface characteristics of silane thin films on copper surfaces, such as their hydrophobicity and thermal stability, were also evaluated to compare the performance of antioxidant behaviors of different silane coupling agents for Cu-filled ICAs. The low contact resistance and high thermal stability of the contact resistance of Cu-filled ICAs were achieved by addition of an optimized silane coupling agent. Greater thermal stability and improved reliability of Cu-filled ICAs under high temperature and humidity conditions were achieved with a silane coupling agent of high molecular weight and hydrophobicity. The bulk resistivity of ∼10⁻⁴ Ωcm of Cu-filled ICAs was achieved with bimodal filler loading.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

Microstructural and electrical investigation of Ni/Au ohmic contact on p-type GaN

Electrical properties of Ni/Au ohmic contacts on p-type GaN were interpreted with the change of microstructure observed under transmission electron microscopy. The contact resistivity was decreased from 1.3×10⁻² to 6.1×10⁻⁴ Ωcm² after annealing at 600°C. The reduction is due to the dissolution of Ga atoms into Au−Ni solid solution formed during annealing, via the generation of Ga vacancies. Thus, net concentration of holes increased below the contact, resulting in the reduction of contact resistivity. At 800°C, N atoms decomposed; reacted with Ni, and forming cubic Ni₄N. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to the increase of contact resistivity to 3.8×10⁻² Ωcm².     

Z-Axis anisotropic electrical conductor films in adhesive and standalone forms for electrical interconnection

Z-axis electrical conductor films in adhesive and standalone forms were made from nickel particles (one per conduction path) and a polymer matrix (polyvinylidene fluoride for the standalone film and epoxy for the adhesive film). The standalone film was found to exhibit resilience, conducting path pitch 125 μm and z-axis resistance 2 Ω. 4 Ω, 17, 48 Ω, and 160 Ω per connection at z-axis pressures of 1.39 MPa, 0.037 MPa, 0.0046 MPa, and 0 MPa respectively; each connection consisted of one nickel particle, which protruded from both sides of the film; the film exhibited no stress relaxation due to the absence of polymer deformation under z-axis pressure. The adhesive film exhibited pitch 64 μm and z-axis resistance 52 Ω per connection at zero pressure.     

Effect of Migration and Condensation of Pre-existing Voids on Increase in Bump Resistance of Flip Chips on Flexible Substrates during Electromigration

In Pb-free solder joints formed by reflowing a bump of solder paste, voids are formed within the solder due to the residue of flux in the reflow process. These voids migrate toward the cathode contact during electromigration under current stressing. Accompanying the electromigration, resistance jumps of a few 100 mΩ were observed. It was postulated that a jump occurs when a void touches the cathode contact. This study investigated the effect of the void migration and condensation on the change in bump resistance using three-dimensional (3D) simulations and finite element analysis. It was found that there was negligible change in bump resistance during void migration towards the high-current-density region before touching the cathode contact opening. When a small void condensed on the contact opening and depleted 18.4% of the area, the bump resistance increased only 0.4 mΩ. Even when a large void depleted 81.6% of the opening, the increase in bump resistance was 3.3 mΩ. These values are approximately two orders of magnitude smaller than those reported in the literature for the change in resistance due to void migration in flip chips on flexible substrates. We conclude that the reported change in resistance was most likely that of the Al or Cu interconnection in the flip-chip samples.     

Effect of heating on the electrical resistivity of conductive adhesive and soldered joints

Thermal cycling from room temperature to 60°C was found to cause the contact resistivity of a silver-epoxy conductive adhesive joint to decrease irreversibly, due to an irreversible decrease of the thickness of the joint. This effect was much smaller for a soldered joint cycled to 40°C. An extended period of current on-off cycling caused a slight irreversible increase in the contact resistivity of the adhesive and soldered joints, but thermal cycling using a heater did not. Within each thermal cycle, the contact resistivity increased reversibly with increasing temperature, due to the increase in volume resistivity of the solder or adhesive. Temperature variation caused fractional changes in contact resistivity up to 48% and 6% for adhesive and soldered joints, respectively.     

Thermal Stability of Nitride-Based Diffusion Barriers for Ohmic Contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The use of TaN, TiN, and ZrN diffusion barriers for Ti/Al-based contacts on n-GaN (n ∼ 3 × 10¹⁷ cm⁻³) is reported. The annealing temperature (600–1,000°C) dependence of the Ohmic contact characteristics using a Ti/Al/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, deposited by sputtering was investigated by contact resistance measurements and Auger electron spectroscopy (AES). The as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ≥600°C. A minimum specific contact resistivity of ∼6 × 10⁻⁵ Ω-cm⁻² was obtained after annealing over a broad range of temperatures (600–900°C for 60 s), comparable to that achieved using a conventional Ti/Al/Pt/Au scheme on the same samples. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts at 350°C was examined; each contact structure showed an increase in contact resistance by a factor of three to four over 24 days at 350°C in air. AES profiling showed that the aging had little effect on the contact structure of the nitride stacks.     

Analytical equations for predicting the thermal properties of isotropic conductive adhesives

This paper investigates a set of theoretical equations for analyzing the thermal properties of isotropic conductive adhesives (ICAs) containing several types of Cu filler particles. The thermal conductivity of ICAs containing randomly dispersed filler particles can be simulated well by Bruggeman’s equation for spherical particles and by Kanari’s equation for flake particles. The effect on the thermal conductivity of any residual voids can be taken into account in the analysis by the additional application of Bruggeman’s or Kanari’s equations with the appropriate shape factor. The linear thermal expansion coefficient of the ICAs was analyzed using Schapery’s scheme. The thermal expansion coefficients of ICAs with 40–50 vol.% of filler particles range between Schapery’s upper and lower limits. As the particle size of the filler decreases, the thermal expansion coefficient starts to approach the lower limit at a lowr volume fraction of the filler particles. The transition behavior of the thermal expansion coefficient is related to the characteristics of the network structure formed by percolating the filler particles.