Xuhui Sun

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.

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.

Circuit Modeling of High-Frequency Electrical Conduction in Carbon Nanofibers

We show that the simplest possible circuit model of high-frequency electrical conduction in carbon nanofibers from 0.1 to 50 GHz is a frequency-independent resistor in parallel with a frequency-independent capacitor. The resistance is experimentally determined and represents the total dc resistance of the nanofiber and its contacts with the electrodes. The capacitance is obtained as a free parameter and has not been previously observed. The experimental method utilizes a ground-signal-ground test structure whose two-port scattering parameters (S-parameters) can be described to within plusmn0.5 dB and plusmn2deg using a simple lumped-element circuit model. The nanostructure is placed in the signal path of the test structure, and its equivalent circuit is deduced by determining what additional elements must be added to the test structure circuit model to reproduce the resulting changes in the S-parameters. This methodology is applicable to nanowires and nanotubes.

Temperature Modeling for Carbon Nanofiber Breakdown

Carbon nanofibers (CNF) are proposed for electrical interconnect applications because of their relatively large current capacity and ability to form well-aligned one-dimensional structures. It is experimentally determined that nanofibers that are suspended between two electrodes breakdown at or near the nanofiber center. Based on published property values a simple model is used to calculate the temperature and quantify the effect of heat generation at the CNF/electrode interface on the nanofiber temperature. The model has the capability to separately account for the substrate temperature and the temperature at the CNF/electrode junction. It is determined that the CNF reaches a temperature at which carbon oxidation is likely to occur.Copyright

Frequency-Independent

We demonstrate that a frequency-independent parallel RC circuit is the simplest model that accurately describes high-frequency electrical conduction in 1-D nanostructures. The resistance is determined from dc measurement, and the capacitance is extracted directly from the measured S-parameters for a ground-signal-ground test structure, without using any fitting parameter. The methodology is applied to carbon nanofibers, and the RC model yields results that are within ±0.5 dB and ±5° of the measured S-parameters up to 50 GHz. The model is further justified by examining the relationship between S- and Y-parameters of the test network.

RC

We study electrical conduction in carbon nanofibers from 0.1 to 30 GHz, by measuring the S-parameters of a ground-signal-ground test structure in which a nanofiber forms part of the signal path. If the nanofiber is modeled as a resistor, the S-parameters are reproduced well by a simple, but realistic, lumped RC circuit model. This implies that, as at low frequencies, nanofibers behave as resistors all the way up to microwave frequencies.

Circuit Model for One-Dimensional Carbon Nanostructures

We describe a test structure optimized for studying high-frequency electrical transport in 1-D nanoscale systems. The test structure exhibits lower transmission than previously reported structures, enabling capacitances less than 1 fF to be detected in the frequency response of the nanoscale system. The scattering parameters (S-parameters) of the test structure are describable to within ±0.5dB and ±2° from 0.1 to 50 GHz using a simple lumped-element RC circuit model whose elements are all measured experimentally.

Measurements and Circuit Model of Carbon Nanofibers at Microwave Frequencies

Carbon nanofibers (CNF) are studied in a horizontal configuration as a model for on-chip interconnects. The electrical performance is determined by both CNF resistivity and contact resistance with electrodes. Reliability and current capacity are determined by Joule heating in the system and the thermal accompanying transport. We show that current capacity can be modeled by accounting for the nature of the contacts with the substrate and with the electrodes.

Test Structure to Extract Circuit Models of Nanostructures Operating at High Frequencies

Growth behavior of vertically aligned carbon nanotubes (CNTs) grown on different underlayer metals is investigated. The average diameter, diameter distribution, density, and growth rate of CNTs exhibit strong correlation with the choice of metal and catalyst. These observations are analyzed in terms of the interactions between the catalyst and the underlayer metal.

Current capacity and thermal transport in carbon nanofiber interconnects

This paper describes a current-sensing technique to extract the resistances of carbon nanostructures in via interconnects. Test structures designed and fabricated for via applications contain carbon nanofiber (CNF)-metal composites embedded in silicon dioxide (SiO2). Electrical characterization of single CNFs is performed using an atomic force microscope (AFM). This technique yields a metal-CNF contact resistance of 6.4 k¿ and a lowest CNF resistivity of 1.89e-4 ¿-cm.