Showing papers by "Samuel Graham published in 2006"

Electrical, Thermal, and Mechanical Characterization of Silicon Microcantilever Heaters

Abstract: Silicon atomic force microscope (AFM) cantilevers having integrated solid-state heaters were originally developed for application to data storage, but have since been applied to metrology, thermophysical property measurements, and nanoscale manufacturing. These applications beyond data storage have strict requirements for mechanical characterization and precise temperature calibration of the cantilever. This paper describes detailed mechanical, electrical, and thermal characterization and calibration of AFM cantilevers having integrated solid-state heaters. Analysis of the cantilever response to electrical excitation in both time and frequency domains aids in resolving heat transfer mechanisms in the cantilever. Raman spectroscopy provides local temperature measurement along the cantilever with resolution near 1 mum and 5degC and also provides local surface stress measurements. Observation of the cantilever mechanical thermal noise spectrum at room temperature and while heated provides insight into cantilever mechanical behavior and compares well with finite-element analysis. The characterization and calibration methodology reported here expands the use of heated AFM cantilevers, particularly the uses for nanomanufacturing and sensing

Room-temperature chemical vapor deposition and mass detection on a heated atomic force microscope cantilever

Abstract: This letter reports the localized room-temperature chemical vapor deposition of carbon nanotubes (CNTs) onto an atomic force microscope cantilever having an integrated heater, using the cantilever self-heating to provide temperatures required for CNT growth. Precise temperature calibration of the cantilever was possible and the CNTs were synthesized at a cantilever heater temperature of 800°C in reactive gases at room temperature. Scanning electron microscopy confirmed the CNTs were vertically aligned and highly localized to only the heater area of the cantilever. The cantilever mechanical resonance decreased from 119.10kHzto118.23kHz upon CNT growth, and then returned to 119.09kHz following cantilever cleaning, indicating a CNT mass of 1.4×10−14kg. This technique for highly local growth and measurement of deposited CNTs creates new opportunities for interfacing nanomaterials with microstructures.

Nanomaterial transfer using hot embossing for flexible electronic devices

Abstract: We demonstrate hot embossing to pattern carbon nanotubes (CNTs) on flexible substrates. Patterns of CNTs grown on both microtextured and flat silicon templates were transferred into polymer substrates, with good replication of both the CNT patterns and surface relief features. The transferred CNTs formed a highly entangled network with electrical resistance of 1kΩ–9MΩ, depending on growth and embossing conditions. The electrical properties showed a strong sensitivity to both light and temperature. This dry transfer process shows promise for high throughput manufacturing of nanomaterial-based flexible electronic devices.

Microwave assisted patterning of vertically aligned carbon nanotubes onto polymer substrates

Abstract: This paper presents a low pressure hot embossing method for transferring patterns of vertically aligned carbon nanotubes into thermoplastic substrates. The procedure utilizes the synthesis of carbon nanotubes in discrete patterns on silicon substrates through the vapor liquid solid growth mechanism. The nanotube pattern and silicon stamp is placed on top of a polycarbonate film and locally heated above the glass transition temperature using microwave processing. The weight of the silicon substrate presses the nanotubes into the polycarbonate, resulting in the complete transfer of vertically aligned patterns. The technique is a rapid processing method, which could be used to integrate aligned nanomaterials with MEMS and flexible electronics that are fabricated on a wide range of thermoplastic polymer materials.

Molding ceramic microstructures on flat and curved surfaces with and without embedded carbon nanotubes

Abstract: This paper explores micromolding fabrication of alumina ceramic microstructures on flat and curved surfaces, the transfer of carbon nanotube (CNT) micropatterns into the ceramic and oxidation inhibition of these CNTs through ceramic encapsulation. Microstructured master mold templates were fabricated from etched silicon, thermally embossed sacrificial polymer and flexible polydimethylsiloxane (PDMS). The polymer templates were themselves made from silicon masters. Thus, once the master is produced, no further access to a microfabrication facility is required. Using the flexible PDMS molds, ceramic structures with mm scale curvature having microstructures on either the inside or the outside of the curved macrostructure were fabricated. It was possible to embed CNTs into the ceramic microstructures. To do this, micropatterned CNTs on silicon were transferred to ceramic via vacuum molding. Multilayered micropatterned CNT–ceramic devices were fabricated, and CNT electrical traces were encapsulated with ceramic to inhibit oxidation. During oxidation trials, encapsulated CNT traces showed an increase in resistance that was 62% less than those that were not encapsulated. The processes described here could allow fabrication of inexpensive 3D ceramic microstructures suitable for high temperature and harsh chemical environments.

Thermal properties and lattice dynamics of polycrystalline MFI zeolite films

Abstract: A study of the thermal properties of the zeolite MFI by a combination of experimental measurements and lattice dynamical modeling is presented. Thermal conductivity data in the range of 150–400 K was obtained through 3ω measurements on polycrystalline zeolite films. While Debye theory is inadequate in predicting the zeolite thermal properties, a detailed calculation of the specific heat using a full set of dispersion relations obtained from atomistic simulations gives excellent agreement with experiments. In addition, the thermal conductivity is successfully reproduced by a phonon relaxation time–based model. The results indicate the possibility of developing a predictive model of the thermal properties of complex zeolite materials.

Flexible microdevices based on carbon nanotubes

Abstract: This work reports the fabrication and testing of flexible carbon nanotube microdevices made using hot embossing material transfer. Both micro-plasma and photodetector devices were made using as-grown unpurified multi-wall carbon nanotubes printed on PMMA substrates. Optical detectors were fabricated by attaching metal wires and monitoring the resistance as a function of light exposure. The electrical resistance of the nanotubes showed a strong sensitivity to light exposure which was also enhanced by heating the devices. While such processes in MWCNTs are not fully understood, the addition of thermal energy is believed to generate additional free charge carriers in the nanotubes. The plasma-generating microdevices consisted of a thin layer of thermoplastic polymer having the CNT electrode on one side and a metal electrode on the reverse side. The devices were electrically tested under atmospheric conditions with 0.01–1 kV ac and at 2.5 kHz, with the plasma igniting near 0.7 kV. The fabrication of these flexible organic devices demonstrates the ability to pattern useful carbon nanotube microdevices in low-cost thermoplastic polymers.

Annealing Effects on Mechanical and Transport Properties of Ni and Ni-Alloy Electrodeposits

Abstract: The effect of annealing at temperatures up to 600 degC on the mechanical properties and the thermal and electrical transport characteristics of nickel and a nickel-manganese electrodeposits are presented. The samples include Ni plated from sulfamate salt with dodecyl sulfate surfactant and from NiSO4 with saccharin additive and a NiMn alloy deposited from a nickel sulfamate bath with added MnCl2. Recrystallization and grain growth, induced by annealing, are shown to strongly affect the mechanical and transport properties. Relatively coarse-grained Ni-sulfamate electrodeposits yielded properties closest to bulk Ni. The incorporation of sulfur (from saccharin additions to the plating electrolyte) or Mn into electrodeposited Ni produces materials with exceptionally fine grain size and with very high yield and ultimate strength. At the same time, the thermal and electrical conductivities are smaller than bulk Ni. Thermal annealing leads to a reduction in strength and an enhancement of the transport properties. The Ni-Mn alloy shows the best temperature stability of the mechanical and transport properties among the tested samples. The observed trends are explained in terms of the influence of microstructure on the mechanical and transport properties

Optical studies of MOCVD-grown GaN-based ferromagnetic semiconductor epilayers and devices

Abstract: Ga1–xMnxN epilayers and p-i-n device structures have been grown by metalorganic chemical vapor deposition. Optical studies were found to investigate the role of Mn concentration and the role of different co-dopants to elucidate the origin of the room temperature ferromagnetism in Ga1–xMnxN epilayers. Increasing Mn concentration was found to significantly affect long-range lattice ordering. This observation was supported by the existence of a disorder-induced Raman mode at 300 cm–1 and local vibrational modes (LVMs) at 669 cm–1 that has been attributed to the formation of self-compensating nitrogen. The E1(TO) phonon frequency was found to linearly increase with Mn composition, which is expressed by (558 + 2.7x) cm–1. The intensity and the linewidth of an absorption band near 1.5 eV was found to increase with Mn concentration, and is assigned to the e to t2 transition in the split 5T2 band of Mn3+. No absorption was detected in Si co-doped Ga1–xMnxN. In this case, a significantly increased EPR signal of Mn2+ ions confirmed the trapping of electrons provided by the shallow Si donor and reduced the available states for ferromagnetic exchange. A change in the measured magnetization is observed under ultraviolet illumination, though the nature of this phenomenon is still not understood. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Manganese-induced long-range lattice disorder and vacancy formation in metal-organic chemical vapor deposition grown and ion-implanted Ga1-xMnxN

Abstract: The structural properties and lattice dynamics of Ga1−xMnxN were studied for Mn concentrations from 0.0% to 1.5%. Ga1−xMnxN layers were fabricated by either Mn incorporation during the metal-organic chemical vapor deposition (MOCVD) growth process or by postgrowth ion implantation into MOCVD-grown GaN epilayers. The crystalline integrity and the absence of major second phase contributions were confirmed by high-resolution x-ray diffraction analysis. Raman spectroscopy showed that increased Mn incorporation in the epilayers significantly affected long-range lattice ordering, revealing a disorder-induced mode at 300cm−1 and a local vibrational mode at 669cm−1. The low intensities of both modes were shown to scale with Mn concentration. These observations support the formation of nitrogen vacancies, even under optimized MOCVD growth conditions. The slight excess of metal components in the growth process compared to undoped GaN growth and the incorporation of Mn deep acceptor levels favors the formation of ni...

Estimating the Effects of Interface Disorder on the Thermal Boundary Resistance Using a Virtual Crystal Approximation

Abstract: An analytical method is presented to estimate the effects of structural disorder on the thermal boundary resistance (TBR) between 2 materials. The current method is an extension of the diffuse mismatch model (DMM) where the interface is modeled as a virtual crystal of finite thickness with properties derived from those of the constituent materials. Using this virtual crystal extension, the predictive capabilities of the diffuse mismatch method are greatly increased with added insight into the sensitivity of materials to interface quality.Copyright © 2006 by ASME

Noncontact surface thermometry for microsystems: LDRD final report.

Abstract: We describe a Laboratory Directed Research and Development (LDRD) effort to develop and apply laser-based thermometry diagnostics for obtaining spatially resolved temperature maps on working microelectromechanical systems (MEMS). The goal of the effort was to cultivate diagnostic approaches that could adequately resolve the extremely fine MEMS device features, required no modifications to MEMS device design, and which did not perturb the delicate operation of these extremely small devices. Two optical diagnostics were used in this study: microscale Raman spectroscopy and microscale thermoreflectance. Both methods use a low-energy, nonperturbing probe laser beam, whose arbitrary wavelength can be selected for a diffraction-limited focus that meets the need for micron-scale spatial resolution. Raman is exploited most frequently, as this technique provides a simple and unambiguous measure of the absolute device temperature for most any MEMS semiconductor or insulator material under steady state operation. Temperatures are obtained from the spectral position and width of readily isolated peaks in the measured Raman spectra with a maximum uncertainty near ±10 K and a spatial resolution of about 1 micron. Application of the Raman technique is demonstrated for V-shaped and flexure-style polycrystalline silicon electrothermal actuators, and for a GaN high-electron-mobility transistor. The potential of the Raman technique for simultaneous measurement of temperature and inplane stress in silicon MEMS is also demonstrated and future Raman-variant diagnostics for ultra spatiotemporal resolution probing are discussed. Microscale thermoreflectance has been developed as a complement for the primary Raman diagnostic. Thermoreflectance exploits the small-but-measurable temperature dependence of surface optical reflectivity for diagnostic purposes. The temperaturedependent reflectance behavior of bulk silicon, SUMMiT-V polycrystalline silicon films and metal surfaces is presented. The results for bulk silicon are applied to silicon-on-insulator (SOI) fabricated actuators, where measured temperatures with a maximum uncertainty near ±9 K, and 0.75-micron inplane spatial resolution, are achieved for the reflectance-based measurements. Reflectance-based temperatures are found to be in good agreement with Raman-measured temperatures from the same device.