Research Programs in the NIST Materials Reliability Division:
Metrology for Nanoscale Properties
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Conductive AFM using Nanotubes involves the development of probes for high-frequency electronic circuitry as well as for measuring electrical properties of biological specimens. We are mounting carbon nanotubes on the end of conductive atomic force microscope (AFM) tips and scanning them across a sample. This provides an electrical conduction map of the surface of the sample with nanometer resolution. Very thin films are prevalent in today’s new optical and electrical circuitry. Films of only a few nanometers are routinely deposited on structures for devices such as narrow bandwidth filters used in the telecommunications industry for separating the individual data signals from the main carrier signal. Since nanometer-thick films are only a few atomic layers the completeness of the film is crucial in the operation of the device. Visualizing thin film coverage has always been an issue. Two widely used techniques are optical and scanned probe based. Optical techniques in general average over several square micrometers, and thus have difficulty resolving defects that are nanometers in dimension. Scanned probe techniques can resolve the defects if there is enough vertical contrast around the edge of the defect. However for very thin coatings it is difficult to distinguish the film from the substrate. To explore this thin film coverage a nanometer-thick dielectric film is deposited on a conductive surface. By scanning a nanotube tip across the surface, flaws can be distinguished in the dielectric film as changes in the electrical conductivity. The figure to the right shows an SEM micrograph of an AFM tip modified with carbon nanotubes. Contact: P. Rice, (303)497-7601, paulrice<at>boulder.nist.gov
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Nanoscale Mechanical Properties focuses on development of in-situ measurement techniques for investigating modulus and stiffness on the nanoscale. Ever-decreasing length scales in many industries present a serious challenge for materials characterization. Tools must be developed to accommodate submicrometer dimensions. Specifically, the need to determine nanoscale mechanical properties exists in applications from microelectronics to biotechnology. Knowledge of mechanical properties such as elastic modulus and interfacial quality (defects, adhesion, strain) is critical to the successful development of new film materials and device assemblies. Likewise, such information could help assess the integrity or reliability of biocompatible coatings, tissue scaffolding, and so forth. To meet these needs, we are developing nondestructive tools that exploit the spatial resolution of atomic force microscopy (AFM). Our approach is called atomic force acoustic microscopy (AFAM) and involves vibrating the cantilever at ultrasonic frequencies to excite mechanical resonances. The resonant frequencies shift as the tip comes in contact with a sample. By measuring the resonant frequencies under both free-space and surface-coupled conditions, quantitative information about the sample’s elastic properties can be extracted. The small tip diameter (~10-100 nm) means that we can obtain in-situ elastic-property information with nanoscale spatial resolution. Furthermore, AFM’s scanning ability enables 2D imaging of mechanical properties. The figure to the right shows a comparison of modulus measurement methods for AFAM in comparison to two other techniques sometimes used by researchers. Contact: D. Hurley, (303)497-3081, hurley<at>boulder.nist.gov
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Brillouin Light Scattering addresses characterization of the interactions between magnetic modes and thermal phonons, which are important for magnetic-storage devices, spin-valve sensors, and other thin film magnetic devices. The Brillouin light scattering process involves the inelastic scattering of incident photons with elastic waves (phonons) or spin waves (magnons) in a material. This scattering can involve either the generation of waves (Stokes process) or annihilation of waves (anti-Stokes process) in the material. Fabry-Perot interferometric techniques for measuring the shifts in photon frequencies arising from Brillouin scattering have evolved rapidly over the past couple of decades, such that they now provide powerful and practical methods of characterizing the dynamic properties of bulk and thin-film materials. In our division, these techniques are being pursued because of their capability to characterize both elastic and spin waves at gigahertz frequencies in thin-film material with surface areas typically on the order of 50 mm in diameter. The research is currently focused on the interactions of magnons and phonons in ferromagnetic thin films. This subject is of great importance with respect to maximizing the speed of magnetic-storage devices, spin-valve sensors, and other thin-film magnetic devices. The coupling of directly excited spin waves to other waves in the material determines the time to achieve equilibration of the magnetization during a switching event. Brillouin light scattering may provide a particularly powerful tool for probing these interactions, because the detection generally can be switched between magnons and phonons simply by rotating a polarizing filter in the path of the scattered light. The plan of research includes measurements of changes in the populations of magnons and phonons induced by ferromagnetic resonant excitation of permalloy (Ni81Fe19) thin films having varying amounts of rare-earth doping, which affects the damping constants of magnetic switching. Also, measurements will seek to determine whether rare-earth doping affects the peak line widths, which are proportional to the damping constants of the individual acoustic and spin waves. The figure to the right shows typical magnon and phonon spectra from Ni81Fe19 (1.2% Tb). Contact: W. Johnson, (303)497-5805, wjohnson<at>boulder.nist.gov |
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Last modified on June 20, 2005
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