Voltage Contrast In Scanning Electron Microscope Research Articles

Characterizing the Electrical Properties of Anisotropic, 3D-Printed Conductive Sheets for Sensor Applications

This paper introduces characterization techniques to investigate electrical properties of 3D-printed conductors. It presents the combination of a physical model to describe frequency dependent electrical properties of 3D-printed conductors; the use of infrared thermography in combination with Joule heating to characterize electrical anisotropy in 3D-printed sheets; and the use of the voltage contrast scanning electron microscopy method (VCSEM) to determine potential distributions in 3D-printed sheets. By means of lock-in thermography, infrared (IR) measurements are improved and amplitude modulation enables lock-in thermography at excitation frequencies above the thermal cut-off frequency. Measurements on sensor samples show the potential of the methods for characterizing sheet-like, conductive structures. The characterization methods allow improvement of 3D-printed sensor designs and exploit electrical properties of 3D-printed conductors.

Multiple contacts investigation of single silicon nanowires with the active voltage contrast scanning electron microscopy technique

A method for analysing the formation of electrical contacts to single silicon nanowires (Si NWs) by exploiting scanning electron microscopy (SEM) images, using active secondary electrons voltage contrast, is presented. Our approach clearly demonstrates the advantages of the proposed technique in analysing multiple contacts to a single nanowire simultaneously, in comparison to the conventional voltage contrast technique, where only two contact structures can be analysed, mainly for studies of the material dopant’s profile. The SEM is equipped with an in-lens detector, which collects the secondary electrons generated during electron beam exposure of the sample. Biasing the contacts with different voltages has been used to analyse the metal to Si NW contacts. The secondary electrons are sensitive to the potential distribution and the contrast of the SEM image changes depending on the number of secondary electrons detected. The Si NWs also vary their contrast together with the electrodes if they are properly electrically contacted. The basic tools and fixtures required for such measurements, and the corresponding image processing algorithms are described.

Dispersion and its relation to carbon nanotube concentration in polyimide nanocomposites

Characterization of Carbon Nanotube (CNT) dispersion in a polyimide matrix and its effect on nanocomposite mechanical properties is presented in this paper. CNT bundle aspect ratio, measured by voltage-contrast scanning electron microscopy, is determined to be the quantitative measurement of dispersion and is found to decrease with increase in nanotube concentration. The reduction of CNT bundle aspect ratio with concentration is shown to explain the less effective reinforcement observed in composites as CNT concentration is increased beyond the electrical percolation concentration. It is shown that increase in CNT concentration beyond percolation concentration does not yield proportional improvement in elastic modulus because CNT aspect ratio systematically decreases as concentration increases. A modified Cox micromechanical model that accounts for the actual nanotube bundle aspect ratio as a function of concentration, nanotube waviness and orientation is shown to predict the observed nanocomposite elastic modulus dependence upon concentration.

Imaging conduction pathways in carbon nanotube network transistors by voltage-contrastscanning electron microscopy

The performance of field-effect transistors based on single-walled carbon nanotube(SWCNT) networks depends on the electrical percolation of semiconducting and metallicnanotube pathways within the network. We present voltage-contrast scanningelectron microscopy (VC-SEM) as a new tool for imaging percolation in a SWCNTnetwork with nano-scale resolution. Under external bias, the secondary-electroncontrast of SWCNTs depends on their conductivity, and therefore it is possibleto image the preferred conduction pathways within a network by following thecontrast evolution under bias in a scanning electron microscope. The experimentalVC-SEM results are correlated to percolation models of SWCNT-bundle networks.

A Scalable, CMOS‐Compatible Assembly of Ambipolar Semiconducting Single‐Walled Carbon Nanotube Devices

Semiconducting single-walled carbon nanotubes are integrated into high-density arrays using dielectrophoresis, which is a CMOS-compatible, bottom-up assembly technique. The devices are statistically analyzed by voltage-contrast scanning electron microscopy and electron transport measurements. Annealing and the choice of parylene substrate are shown to improve device performance.

Toward Single-Chirality Carbon Nanotube Device Arrays

The large-scale integration of devices consisting of individual single-walled carbon nanotubes (SWCNT), all of the same chirality, is a critical step toward their electronic, optoelectronic, and electromechanical application. Here, the authors realize two related goals, the first of which is the fabrication of high-density, single-chirality SWCNT device arrays by dielectrophoretic assembly from monodisperse SWCNT solution obtained by polymer-mediated sorting. Such arrays are ideal for correlating measurements using various techniques across multiple identical devices, which is the second goal. The arrays are characterized by voltage-contrast scanning electron microscopy, electron transport, photoluminescence (PL), and Raman spectroscopy and show identical signatures as expected for single-chirality SWCNTs. In the assembled nanotubes, a large D peak in Raman spectra, a large dark-exciton peak in PL spectra as well as lowered conductance and slow switching in electron transport are all shown to be correlated to each other. By comparison to control samples, we conclude that these are the result of scattering from electronic and not structural defects resulting from the polymer wrapping, similar to what has been predicted for DNA wrapping.

Imaging defects and junctions in single-walled carbon nanotubes by voltage-contrast scanning electron microscopy

Voltage-contrast scanning electron microscopy is demonstrated as a new technique to locate and characterize defects in single-walled carbon nanotubes. This method images the surface potential along and surrounding a nanotube in device configuration and it is used here to study the following: (a) structural point-defects formed during nanotube growth, (b) nano-scale gap formed by high-current electrical breakdown, (c) electronic defect such as electron-irradiation induced metal-insulator transition, and (d) charge injection into the substrate which causes hysteresis in nanotube devices. The in situ characterization of defect healing under high bias is also shown. The origin of voltage-contrast, the influence of the above defects on the contrast profiles and optimum imaging conditions are discussed.

Imaging electronic structure of carbon nanotubes by voltage-contrast scanning electron microscopy

We introduce voltage-contrast scanning electron microscopy (VC-SEM) for visual characterization of the electronic properties of single-walled carbon nanotubes. VC-SEM involves tuning the electronic band structure and imaging the potential profi le along the length of the nanotube. The resultant secondary electron contrast allows to distinguish between metallic and semiconducting carbon nanotubes and to follow the switching of semiconducting nanotube devices, as confi rmed by in situ electrical transport measurements. We demonstrate that high-density arrays of individual nanotube devices can be rapidly and simultaneously characterized. A leakage current model in combination with fi nite element simulations of the device electrostatics is presented in order to explain the observed contrast evolution of the nanotube and surface electrodes. This work serves to fill a void in electronic characterization of molecular device architectures.

Exploiting voltage contrast scanning electron microscopy to investigate conductive polymer composite resettable fuse devices

Voltage contrast scanning electron microscopy has been exploited to investigate the operating stability of conductive polymer composite resettable fuse devices. It has been shown that the technique enables the active region of the device to be imaged and, hence, its position and shape to be compared after a succession of operating cycles and as a consequence of the position of the attached surface electrodes. This leads to a useful diagnostic tool for the investigation of the configuration of such devices and the morphology of the conductive polymer composite material of which they are made.

Latchup Paths in Bipolar Integrated Circuits

A combination of experimental techniques was used to find latchup paths in bipolar circuits. Voltage contrast scanning electron microscopy was applied as an internal probe to analyze latchup paths. Data obtained on six device types show a variety of path combinations and interactions responsible for latchup.

Characterization of Emitter-Collector Shorts by Anodization, Voltage Contrast Sem, and Tem

AbstractChemical anodization and voltage contrast scanning electron microscopy (VC-SEM) have been used to identify electrically faulty structures in a bipolar test array. Direct comparison of these techniques was achieved by examining the same emitters with each method. VC-SEM is shown to be a useful technique for delineating E-C shorts because of its nondestructive and purely electrical nature. Further investigations by transmission electron microscopy revealed dislocations in many short-circuited emitters and occasionally in unshorted devices. This confirmed prior observations that crystallographic defects in silicon devices may sometimes be, but are not always, electrically active.

Lateral dopant transport during laser recrystallization of polysilicon

Significant lateral dopant transport occurs during seeded laser recrystallization of polysilicon. Both spreading-resistance measurements and voltage-contrast scanning electron microscopy show that phosphorus, arsenic, and boron are transported several tens of microns in the direction of the beam travel during recrystallization, while much less transport is observed in the other directions. The dopant motion orthogonal to the beam scan is consistent with liquid-phase diffusion.