Ainissa G. Ramirez

Crystallization and phase transformations in amorphous NiTi thin films for microelectromechanical systems

Amorphous sputtered nickel–titanium thin films were deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. Their associated microstructure was monitored directly and simultaneously with in situ transmission electron microscopy. These electron-transparent membranes constrained the NiTi films and rendered it possible for observation of the complete transformation cycle, which includes: the crystallization of the amorphous phase to austenite phase (cubic B2 structure) with heating; and the conversion of austenite (B2) to martensite (monoclinic B19′ structure) with cooling. Electron micrographs show the nucleation and growth of grains occurs at a temperature of 470°C and at a rate that indicates a polymorphic transformation. The onset of martensitic transformation occurs between 25 and 35°C. Calorimetric measurements are consistent with the observed crystallization.

Bonding nature of rare-earth-containing lead-free solders

The ability of rare-earth-containing lead-free solders to wet and bond to silica was investigated. Small additions of Lu (0.5–2 wt. %) added to eutectic Sn–Ag or Au–Sn solder render it directly solderable to a silicon oxide surface. The bonding is attributed to the migration of the rare-earth element to the solder–silica interface for chemical reaction and the creation of an interfacial layer that contains a rare-earth oxide. It was found that additions of rare-earth materials did not significantly modify the solidification microstructure or the melting point. Such oxide-bondable solders can be useful for assembly of various optical communication devices.

Grain size estimations from the direct measurement of nucleation and growth

Microstructures that emerge during the crystallization of amorphous materials depend on nucleation and growth kinetics. The ability to predict these final microstructures, particularly the average grain size, would allow better control of material properties. Well-established crystallization theories have proposed mathematical models to describe these microstructures. What remains missing, however, is an independent experimental verification of the microstructures these models predict. Here, we report in situ transmission-electron-microscopy experimental methods that assess independently the nucleation and growth rates of crystallizing grains. A consequence of having a separate, experimentally-determined description of nucleation and growth is the ability to predict the average grain size over a broad range of temperatures. The results from these experimental methods verify the theoretical models that were posed several decades ago.

Nanoindentation of Ni–Ti Thin Films

Ni–Ti thin films of various compositions were sputtered-deposited on silicon substrates. Their mechanical properties (hardness and Youngs modulus) were then determined using a nanoindenter equipped with a Berkovich tip. This paper examines the effects of composition on the mechanical properties (hardness and Youngs modulus) of the sputter deposited Ni–Ti thin films. This is of particular interest since the actuation properties of these shape memory alloy films are compositionally sensitive. The surface-induced deformation is revealed via Atomic Force Microscopy (AFM) images of the indented surfaces. Which show evidence of material pile-up that increases with increasing load. The measured Youngs moduli are also shown to provide qualitative measures of the extent of stress-induced phase transformation in small volumes of Ni–Ti films.

Fabrication and Field Emission Properties of Carbon Nanotube Cathodes

A variety of carbon nanotube films have been fabricated and tested as cold cathodes. A spray deposition technique was developed for processing as-grown bulk nanotubes, both single-walled and multi-walled, into films of randomly oriented nanotubes. Films of randomly oriented multi-walled nanotubes were grown using thermal chemical vapor deposition, and arrays of well-aligned multi-walled nanotubes have been fabricated using a microwave plasma enhanced chemical vapor deposition technique. The emission current-voltage (I-V) characteristics of these nanotube cathodes have been measured. Both multi-walled (random and aligned) and single-walled carbon nanotubes exhibit low turn-on fields (∼ 2 V/μm to generate 1 nA) and threshold fields ( 2 ). Significantly, these cathodes were capable of operation at very large current densities (> 1A/cm 2 ), making them candidates for application in a variety of vacuum microelectronic devices.

Crystallization of amorphous carbon thin films in the presence of magnetic media

Amorphous carbon thin films, which are often used as protective coatings for magnetic hard disks, were deposited in a carbon/cobalt alloy/carbon trilayers (20/10/20 nm) and subjected to thermal annealing and cooling. The associated microstructural changes were analyzed by in situ transmission electron microscopy (TEM). TEM micrographs show that the amorphous carbon in contact with the magnetic media increases in graphitic content at annealing temperatures near 400 °C. It fully crystallizes between 500 and 600 °C. The microstructural changes at these temperatures suggest that the metals of the magnetic layers mediate graphitization, similar to the behavior of other eutectic metal metalloid systems (e.g., Al–Si, Ag–Ge). Calorimetric and magnetic measurements are consistent with a graphitization mechanism that includes a diffusional process. This article presents the experiments and proposes a graphitization mechanism.

Universal solders for direct and powerful bonding on semiconductors, diamond, and optical materials

The surfaces of electronic and optical materials such as nitrides, carbides, oxides, sulfides, fluorides, selenides, diamond, silicon, and GaAs are known to be very difficult to bond with low melting point solders (<300 °C). We have achieved a direct and powerful bonding on these surfaces by using low temperature solders doped with rare-earth elements. The rare earth is stored in micron-scale, finely-dispersed intermetallic islands (Sn3Lu or Au4Lu), and when released, causes chemical reactions at the interface producing strong bonds. These solders directly bond to semiconductor surfaces and provide ohmic contacts. They can be useful for providing direct electrical contacts and interconnects in a variety of electronic assemblies, dimensionally stable and reliable bonding in optical fiber, laser, or thermal management assemblies.

Structural relaxation and crystallization of NiTi thin film metallic glasses

This letter demonstrates the effects of structural relaxation on the crystallization and phase transformation behavior of NiTi thin films. Heat treatments below the glass transformation temperature produce films with a greater hardness than as-deposited films. The reduction in free volume occurring during film relaxation plays a role. Using scanning electron microscopy, structural relaxation was found to decrease the overall crystallization time and increase the nucleation rate, thus modifying the resulting microstructures. Structural relaxation had little effect on the phase transformation temperatures of fully crystallized films, but slightly increased the resulting actuation force during transformation.

Experimental determination of kinetic parameters for crystallizing amorphous NiTi thin films

The crystallization of amorphous NiTi thin films was studied using in situ transmission electron microscopy (TEM) methods. Samples were subjected to heating conditions within the microscope and the microstructural development was monitored and recorded. The nucleation rate and the growth rate were determined experimentally by noting the number of new grains per frame and their change in size. These parameters were compared to the conventional method of kinetic analysis using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. In it, the amount transformed is related to fitting parameters that describe the overall crystallization rate. The individual kinetic rates found directly with the TEM methods have considerable agreement with the overall rate determined by the conventional JMAK analysis. This quantitative analysis provides the groundwork for the control of microstructures and properties in NiTi shape memory alloy thin films.

Magnetically driven three-dimensional manipulation and inductive heating of magnetic-dispersion containing metal alloys

Fundamental to the development of three-dimensional microelectronic fabrication is a material that enables vertical geometries. Here we show low-melting-point metal alloys containing iron dispersions that can be remotely manipulated by magnetic fields to create vertical geometries and thus enable novel three-dimensional assemblies. These iron dispersions enhance the mechanical properties needed for strong, reliable interconnects without significantly altering the electrical properties of the alloys. Additionally, these iron dispersions act as susceptors for magnetic induction heating, allowing the rapid melting of these novel alloys at temperatures lower than those usually reported for conventional metal alloys. By localizing high temperatures and by reducing temperature excursions, the materials and methods described have potential in a variety of device fabrication applications.