ICMCTF2013 Session TS3-1: Energetic Materials and Micro-Structures for Nanomanufacturing

Reactive bonding is a new wafer bonding technique and is becoming an attractive approach for MEMS (Micro-Electro-Mechanical-Systems) encapsulation and packaging due to several advantages: very short bonding time, high bond strength, and internal heating, thus, lower process temperatures compared to traditional bonding techniques, such as glass-frit or anodic bonding [1]. Reactive bonding is based on self-sustaining exothermic reactions in nano scale energetic systems. Such systems typically consist of several alternating layers of two different thin layers. With the application of an initial energy pulse, the system starts to rapidly form intermetallic phases. By choosing material combinations with high negative enthalpy of formation this reaction can be running exothermic and self-propagating [2]. Currently, exothermic and self-propagating reactions are used in Ni/Al foils (NanoFoils©) for different joining applications on the macro scale [3]. Nevertheless, for wafer bonding this method is not similarly applicable due to handling as well as foil patterning limitations [4].

In this study, so called reactive and nano scale multilayer systems (RMS) will be used for room-temperature and hermetic bonding. The bonding approach focuses on the direct deposition and process flow integration of thin Pd/Al-RMS with total thicknesses smaller than 2.5 µm. In addition to that, the used integrated RMS enable the integration into typically used process steps for MEMS device fabrication, such as lithography and waferbonding.

The integrated Pd/Al-RMS were deposited by using alternating DC magnetron sputtering from high purity Al- and Pd-targets. It will be shown that high reaction velocities ranging up to 75 m/s in patterned (minimum lateral dimensions are 20 µm) reactive systems can be achieved. In addition to that, the feasibility for hermetic wafer bonding for up to 150 mm Si-Si as well as glass-Si wafer bonding at room-temperature is presented. Furthermore, it will be shown that high shear strengths (up to 340 MPa) as well as high-temperature stable (up to 400 °C) and reliable (up to 1000 temperature shock cycles at -40°C/+130°C) bond interfaces can be achieved.

[1] P. Ramm et al., in Handbook of Wafer Bonding, Wiley-VCH, Weinheim (2012).

[2] A.B. Mann, et al., Journal of Applied Physics 82 (1997) 1178-1188.

[3] J. Wang, et al., Applied Physics Letters, Vol. 83, (2003), pp. 3987-3989.

[4] B. Boettge et al., Journal of Micromechanics and Microengineering, 20 (2010) pp 064018 1-8.

Self-propagating reactions of two or more elements are a commonly known means to produce ceramic or intermetallic compounds. The nature of self-propagating reactions requires finely dispersed reactants, e.g. in form of powders or PVD-multilayers. During the past decades, substantial research has been conducted on a number of metallic systems. Recently, it has been shown that the Ru-Al system is also capable of self-propagating reactions leading to a direct formation of the intermetallic B2-RuAl phase. This phase shows a favorable combination of properties not commonly encountered in intermetallic compounds. While having a high melting point and good oxidation resistance, RuAl has been reported to exhibit room-temperature ductility.

The self-propagation reactions in these kinds of systems are generally studied regarding ignition criteria, reaction speed and propagation of the combustion front. However, little is experimentally known about the mass transport on atomic scale during the reaction. To analyze the atomic redistribution, we incorporated a thin layer of a third element into Ru-Al multilayer stacks. The elemental distribution is measured after reaction by means of atom probe tomography (APT). This technique combines mass resolution in the ppm-range with near-atomic spatial resolution on a length scale of up to several hundred nanometers and can therefore provide valuable information to support the understanding of transport mechanisms during self-propagating reactions.

Key words: Ti/Al multilayer, TiAl alloy single layer, Heat release, Mechanical properties, Magnetron sputtering

Here we report the use electrophoretic deposition (EPD) as a means to prepare energetic thin films of well-mixed copper (II) oxide/ aluminum (CuO/Al) binary particulate composites. Films were deposited from liquid suspensions of particles onto patterned electrodes. Suspensions were prepared with various particle sizes spanning from nanometer- to micron- sized. The resulting films were examined using electron microscopy and profilometry and their combustion characteristics were analyzed with high-speed videos. The results show that films prepared by EPD display an enhancement in their combustion velocities as the total film thickness increases. Films have been deposited onto patterned electrodes, with very fine feature sizes, which were used for mechanistic investigations of the ignition and combustion. These investigations have lead to a better understanding of the factors that influence the energy release properties of these films. Recent results suggest that films with features large enough to allow gas trapping and pressure unloading, and those with micro-structures which enable more directed transport of hot gases and particles in the desired propagation direction can be used to tailor the reactivity of the composites. These films are also particularly useful for developing thermites for micro-energetic applications.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

LLNL-ABS-597572

Combustion analysis of three different thermites consisting of aluminum (Al) particles with and without surface functionalization, combined with molybdenum trioxide (MoO₃) was performed to study the effect of surface functionalization on flame propagation velocity (FPV). Two types of Al particles had self assembled monolayers (SAMs) of perfluoro tetradecanoic (PFTD) and perfluoro sebacic (PFS) acids around the alumina shell respectively; the other one did not. Flame propagation studies of Al with PFTD/MoO₃ are 86% higher than Al/MoO₃ whereas the thermite comprising of Al with PFS/MoO₃ are almost half of Al/MoO₃. Thermal equilibrium studies were performed using a DSC/TGA to determine activation energy (E_a) of the thermites. Results showed an inverse relationship between FPV and E_a. Fluorine content in the acids and their structural differences contribute to difference in FPV. This study shows that the flame propagation velocity of a thermite is dependent of the nature of surface functionalization of the fuel particles.

Spark ignition and self-propagating reactions in green compacts of continuously low-energy ball-milled aluminum and nickel powders at the NiAl composition were investigated. The microstructure of the as-milled powders showed uniform mechanical mixing and refinement of alternating Al and Ni layers with increasing milling time. XRD analysis of the as-milled powders confirmed nanoscale grain formation and solid-state diffusion of Al into Ni-rich solid solution for milling times beyond 6 h. Interrupted Differential Scanning Calorimetry (DSC) in combination with X-Ray Diffraction (XRD) analysis revealed that the milled powders have an identical phase formation sequence to those of nanoscale magnetron sputtered multilayer foils. Green compacts of powders milled for 11 and 12 h could be ignited using a low-energy spark from a battery, similar to sputtered foils, and they form metallurgically bonded compacts upon completion of the self-propagating reactions. The thermal front velocity measured using a high-speed optical camera was approximately 0.3 m/s. Infrared camera measurements show that the temperatures reach 1911 K, indicating near adiabatic reactions. Post-reaction X-Ray Diffraction (XRD) analysis of green compacts show near identical conversion to the NiAl phase.

This work focused on ball-milled Ni/Al powders and their characterization, but also showed similarities between ball-milled powder pellets and sputtered foils. Ball-milling could have the potential for an economical processing route for generating powders that can be shaped into useful geometries (such as rolled into thin sheets) for thermal manufacturing applications, similar to sputtered nanostructured foils currently used for bonding applications.