Ronald S. Goeke

Atomic-layer deposition of wear-resistant coatings for microelectromechanical devices

Friction and wear are major concerns in the performance and reliability of microelectromechanical systems (MEMS) devices employing sliding contacts. While many tribological coating materials are available, most traditional surface coating processes are unable to apply conformal coatings to the high aspect ratio (height/width) structures typical of MEMS devices. We demonstrate that thin, conformal, wear-resistant coatings can be applied to Si surface-micromachined structures by atomic-layer deposition (ALD). For this demonstration, we apply 10-nm-thick films of Al2O3 using a binary reaction sequence with precursors of trimethyl aluminum and water. Deposition is carried out in a viscous flow reactor at 1 Torr and 168 °C, with N2 as a carrier gas. Cross-section transmission electron microscopy analysis shows that films are uniform to within 5% on MEMS device structures with aspect ratio ranging from 0 to >100. Films are stoichiometric Al2O3, with no evidence of contamination from other species, and are amorp...

Atomic layer deposition of tungsten disulphide solid lubricant thin films

The synthesis and characterization of crystalline tungsten disulphide (WS 2 ) solid lubricant thin films grown by atomic layer deposition (ALD) using WF 6 and H 2 S gas precursors was studied. A new catalytic route was established to promote nucleation and growth of WS 2 films on silicon surfaces with native oxide. Scanning electron microscopy with energy dispersive spectroscopy and Raman spectroscopy were used to determine the film morphology, composition, and crystallinity. The films exhibited solid lubricating behavior with a steady-state friction coefficient of 0.04 in a dry nitrogen environment.

Trapping of Mobile Pt Species by PdO Nanoparticles under Oxidizing Conditions

Pt is an active catalyst for diesel exhaust catalysis but is known to sinter and form large particles under oxidizing conditions. Pd is added to improve the performance of the Pt catalysts. To investigate the role of Pd, we introduced metallic Pt nanoparticles via physical vapor deposition to a sample containing PdO nanoparticles. When the catalyst was aged in air, the Pt particles disappeared, and the Pt was captured by the PdO, forming bimetallic Pt-Pd nanoparticles. The formation of metallic Pt-Pd alloys under oxidizing conditions is indeed remarkable but is consistent with bulk thermodynamics. The results show that mobile Pt species are effectively trapped by PdO, representing a novel mechanism by which Ostwald ripening is slowed down. The results have implications for the development of sinter-resistant catalysts and help explain the improved performance and durability of Pt-Pd in automotive exhaust catalytic converters.

Ion beam modification of topological insulator bismuth selenide

We demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.

Electrical resistivity of Au-ZnO nanocomposite films

The electrical resistivity of electron beam codeposited gold and zinc oxide (Au-ZnO) films was investigated over the full composition range. The electrical resistivity was shown to increase monotonically with increasing ZnO content, with three characteristic regimes of behavior associated primarily with (1) grain boundary electron scattering due to grain refinement at ZnO volume fractions below 0.3, (2) percolation theory for ZnO volume fractions at and above the percolation threshold (fc = 0.85), and (3) a transition region between these where it was proposed that resistivity was influenced by the formation of Au-Zn complexes due to an oxygen deficiency in the deposited ZnO. The electrical resistivity of the composite films remained below 100 μΩ cm for ZnO volume fractions below 0.5. A model combining the general effective media equation and Mayadas-Shatzkes grain boundary electron scattering model was shown to generally describe the composition dependence of electrical resistivity for the investigated o...

On the thermal stability of physical vapor deposited oxide-hardened nanocrystalline gold thin films

We describe a correlation between electrical resistivity and grain size for PVD synthesized polycrystalline oxide-hardened metal-matrix thin films in oxide-dilute (<5 vol. % oxide phase) compositions. The correlation is based on the Mayadas-Shatzkes (M-S) electron scattering model, predictive of grain size evolution as a function of composition in the oxide-dilute regime for 2 μm thick Au-ZnO films. We describe a technique to investigate grain boundary (GB) mobility and the thermal stability of GBs based on in situ electrical resistivity measurements during annealing experiments, interpreted using a combination of the M-S model and the Michels et al. model describing solute drag stabilized grain growth kinetics. Using this technique, activation energy and pre-exponential Arrhenius parameter values of Ea = 21.6 kJ/mol and Ao = 2.3 × 10−17 m2/s for Au-1 vol. % ZnO and Ea = 12.7 kJ/mol and Ao = 3.1 × 10−18 m2/s for Au-2 vol. % ZnO were determined. In the oxide-dilute regime, the grain size reduction of the A...

Nanoparticle Size Effects on the Electrochemical Dissolution Rate of Pt

Proton exchange membrane fuel cells are being extensively studied as power sources because of their technological advantages such as high energy efficiency and environmental friendliness. The most effective catalyst in these systems consists of nanoparticles of Pt or Pt-based alloys on carbon supports. Understanding the role of the nanoparticle size and structure on the catalytic activity and degradation is needed to optimize the fuel cell performance and reduce the noble metal loading. One of the more significant causes of fuel cell performance degradation is the cathode catalyst deactivation. There are four mechanisms considered relevant to the loss of electrochemically active surface area of Pt in the fuel cell electrodes that contribute to cathode catalyst degradation, including: catalyst particle sintering such as Ostwald ripening, migration and coalescence, carbon corrosion and catalyst dissolution. The dissolution of the Pt nanoparticles is fundamental to a few of these mechanisms. Most approaches to study this role utilize membrane electrode assemblies (MEAs), which results in a complex system where it is difficult to deconvolute the effects of the metal nanoparticles. Our research addresses this need by taking a fundamental approach to study the electrocatalyst using a model support. We have recently shown this type of 2D model catalyst as a very useful tool for studying the sintering of Pd particles at high temperatures. We now apply this model system to study Pt nanoparticle electrochemical activity degradation. Electrochemical processes at the nanometer scale may dissolve Pt at potentials more negative than predicted based on the Nernst equation alone. An electrochemical equivalent of the Gibbs-Thomson equation can provide the effect of interfacial energy on the solubility in terms of particle radius. This curvature effect on EPt can be calculated by adding this effect to the Nernst equation.

Micro-concentrator module for Microsystems-Enabled Photovoltaics: Optical performance characterization, modelling and analysis

Prototype micro-concentrating optical modules developed for Microsystems-Enabled Photovoltaics (MEPV) were characterized, modelled, and analyzed. Laboratory and field tests were performed to evaluate the optical performance of the modules. An optical simulation model was constructed based on measured sub-component and material properties, showing a good agreement with on-sun experimental results. Performance of a fully-packaged module integrating 100X micro-scale solar cells (-250μm in diameter) was characterized and projected based on experimental and simulation results. Characterization results and analyses indicate that the micro-concentrator module developed provide desirable performance while maintaining a compact physical profile. The development of a next generation micro-concentrator module is also described.

Design of wearable binoculars with on-demand zoom

Sandia has developed an optical design for wearable binoculars utilizing freeform surfaces and switchable mirrors. The goals of the effort included a design lightweight enough to be worn by the user while providing a useful field of view and magnification as well as non-mechanical switching between normal and zoomed vision. Sandia’s approach is a four mirror, off-axis system taking advantage of the weight savings and chromatic performance of a reflective system. The system incorporates an electrochromic mirror on the final surface before the eye allowing the user to switch between viewing modes. Results from a prototype of a monocular version with 6.6x magnification will be presented. The individual mirrors, including three off-axis aspheres and one true freeform, were fabricated using a diamond-turning based process. A slow-slide servo process was used for the freeform element. Surface roughness and form measurement of the freeform mirror will be presented as well as the expected impact on performance. The alignment and assembly procedure will be reviewed as well as the measured optical performance of the prototype. In parallel to the optical design work, development of an electrochromic mirror has provided a working device with faster switching than current state of the art. Switchable absorbers have been demonstrated with switching times less than 0.5 seconds. The deposition process and characterization of these devices will be presented. Finally, details of an updated optical design with additional freeform surfaces will be presented as well as plans for integrating the electrochromic mirror into the system.

Gas Permeation Measurements on Low Temperature Cofired Ceramics

Commercial low temperature cofired ceramic (LTCC) technology is established in microelectronics and microsystems packaging, multichip and radio frequency (RF) modules, and sensors. The ability to combine structural considerations with embedded traces and components using laminated glass-ceramic tapes has created solutions to unconventional packaging requirements of micro-electro-mechanical systems (MEMS) devices. Many MEMS devices such as resonators are very sensitive to pressure and require packaging in a vacuum environment. Attaining and maintaining desirable pressure levels in sealed vacuum packages requires knowledge of the permeation characteristics of the vacuum envelope and the sealing materials. An experimental system to measure the time dependent gas permeation through LTCC at temperatures from room temperature to 500°C has been developed. This system utilizes a membrane technique in which a gas is allowed to permeate through a test sample, held at a constant temperature, into a high vacuum chamb...