Patrick Wilhite

Length dependence of current-induced breakdown in carbon nanofiber interconnects

Current-induced breakdown is investigated for carbon nanofibers (CNF) for potential interconnect applications. The measured maximum current density in the suspended CNF is inversely proportional to the nanofiber length and is independent of diameter. This relationship can be described with a heat transport model that takes into account Joule heating and heat diffusion along the CNF, assuming that breakdown occurs when and where the temperature reaches a threshold or critical value.

Contact resistance in carbon nanostructure via interconnects

We present an in-depth electrical characterization of contact resistance in carbon nanostructure via interconnects. Test structures designed and fabricated for via applications contain vertically aligned arrays of carbon nanofibers (CNFs) grown on a thin titanium film on silicon substrate and embedded in silicon dioxide. Current-voltage measurements are performed on single CNFs using atomic force microscope current-sensing technique. By analyzing the dependence of measured resistance on CNF diameter, we extract the CNF resistivity and the metal-CNF contact resistance.

Application of Carbon Nanotubes to Thermal Interface Materials

Improvements in thermal interface materials (TIMs) can enhance heat transfer in electronics packages and reduce high temperatures. TIMs are generally composed of highly conductive particle fillers and a matrix that allows for good surface wetting and compliance of the material during application. Two types of TIMs are tested based on the addition of carbon nanotubes (CNTs): one mixed with a commercial TIM product and the other only CNTs and silicone oil. The materials are tested using an in-house apparatus that allows for the simultaneous measurement of temperature, pressure, heat flux, and TIM thickness. Results show that addition of large quantities of CNTs degrades the performance of the commercial TIM, while the CNT-silicone oil mixtures showed improved performance at high pressures. Thickness and pressure measurements indicate that the CNT-thermal grease mixtures are more compliant, with a small increase in bulk thermal conductivity over the range of tested pressures. [DOI: 10.1115/1.4003864]

Tunneling between carbon nanofiber and gold electrodes

In a carbon nanofiber (CNF)-metal system such as a bridge between two gold electrodes, passing high current (current stressing) reduces the total resistance of the system (CNF resistance RCNF plus contact resistance Rc) by orders of magnitude. The role of current stressing is modeled as a reduction in the interfacial tunneling gap with transport characteristics attributed to tunneling between Au and CNF. The model predicts a reduction in Rc and gradual disappearance of the nonlinearity in the current-voltage (I-V) characteristics as Rc decreases. These results are consistent with measured I-V behavior.

Metal?nanocarbon contacts

To realize nanocarbons in general and carbon nanotube (CNT) in particular as on-chip interconnect materials, the contact resistance stemming from the metal?CNT interface must be well understood and minimized. Understanding the complex mechanisms at the interface can lead to effective contact resistance reduction. In this study, we compile existing published results and understanding for two metal?CNT contact geometries, sidewall or side contact and end contact, and address key performance characteristics which lead to low contact resistance. Side contacts typically result in contact resistances >1 k?, whereas end contacts, such as that for as-grown vertically aligned CNTs on a metal underlayer, can be substantially lower. The lower contact resistance for the latter is due largely to strong bonding between edge carbon atoms with atoms on the metal surface, while carrier transport across a side-contacted interface via tunneling is generally associated with high contact resistance. Analyses of high-resolution images of interface nanostructures for various metal?CNT structures, along with their measured electrical characteristics, provide the necessary knowledge for continuous improvements of techniques to reduce contact resistance. Such contact engineering approach is described for both side and end-contacted structures.

The effect of catalysts and underlayer metals on the properties of PECVD-grown carbon nanostructures

The growth behaviors and contact resistances of vertically aligned carbon nanotubes (CNTs) and carbon nanofibers (CNFs) grown on different underlayer metals are investigated. The average diameter, diameter distribution, density, growth rate and contact resistance exhibit strong correlation with the choice of catalyst/underlayer combination. These observations are analyzed in terms of interactions between the catalyst and the underlayer metal. The CNT via test structure has been designed and fabricated to make current-voltage measurements on single CNTs using a nanomanipulator under scanning electron microscopy (SEM) imaging. By analyzing the dependence of measured resistance on CNT diameter, the CNT-metal contact resistance can be extracted. The contact resistances between as-grown CNTs and different underlayer metals are determined. Relationships between contact resistances and various combinations of catalysts and underlayer metals are investigated.

Resistance Determination for Sub-100-nm Carbon Nanotube Vias

We report resistance results from carbon nanotube (CNT) vias of widths from 150 to 60 nm for potential application in integrated circuits technology. Selective CNT growth inside the vias with an areal density of 2×1011/cm2 is achieved with a statistical average resistance of 1.7 kΩ with standard deviation between 420 Ω and 7.1 kΩ, and lowest resistance of 150 Ω for 60 nm vias, the lowest reported value for sub-100 nm-CNT vias. Statistical analysis yields a best-case projected value of 295 Ω for a 30 nm via, within one order of magnitude of its copper and tungsten counterparts.

Extraction of contact resistance in carbon nanofiber via interconnects with varying lengths

A method to extract the contact resistance and bulk resistivity of vertically grown carbon nanofibers (CNFs) or similar one-dimensional nanostructures is described. Using a silicon-compatible process to fabricate a terrace test structure needed for the CNF length variation, the contact resistance is extracted by measuring in situ the resistances of individual CNFs with different lengths and within a small range of diameters using a nanoprober inside a scanning electron microscope. Accurate determination of contact resistances for various combinations of catalysts and underlayer metals can lead to eventual optimization of materials’ growth and device fabrication processes for CNF via interconnects.

Change in carbon nanofiber resistance from ambient to vacuum

The electrical properties of carbon nanofibers (CNFs) can be affected by adsorbed gas species. In this study, we compare the resistance values of CNF devices in a horizontal configuration in air and under vacuum. CNFs in air are observed to possess lower current capacities compared to those in vacuum. Further, Joule heating due to current stressing can result in desorption of gas molecules responsible for carrier trapping, leading to lower resistances and higher breakdown currents in vacuum, where most adsorbed gaseous species are evacuated before any significant re-adsorption can occur. A model is proposed to describe these observations, and is used to estimate the number of adsorbed molecules on a CNF device.

Electron-beam and ion-beam-induced deposited tungsten contacts for carbon nanofiber interconnects

Ion-beam-induced deposition (IBID) and electron-beam-induced deposition (EBID) with tungsten (W) are evaluated for engineering electrical contacts with carbon nanofibers (CNFs). While a different tungsten-containing precursor gas is utilized for each technique, the resulting tungsten deposits result in significant contact resistance reduction. The performance of CNF devices with W contacts is examined and conduction across these contacts is analyzed. IBID-W, while yielding lower contact resistance than EBID-W, can be problematic in the presence of on-chip semiconducting devices, whereas EBID-W provides substantial contact resistance reduction that can be further improved by current stressing. Significant differences between IBID-W and EBID-W are observed at the electrode contact interfaces using high-resolution transmission electron microscopy. These differences are consistent with the observed electrical behaviors of their respective test devices.