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Using of reactive multilayer systems for room temperature bonding of micro components

: Bräuer, J.; Baum, M.; Wiemer, M.; Gessner, T.

Gessner, T.:
Smart systems integration 2009. CD-ROM : Brussels, Belgium, 10 - 11 March 2009
Berlin: Akademische Verlagsgesellschaft Aka, 2009
ISBN: 978-3-89838-616-6
4 S.
European Conference & Exhibition on Integration Issues of Miniaturized Systems - MEMS, MOEMS, ICs and Electronic Components <3, 2009, Brussels>
Fraunhofer ENAS ()

The setting up of bonded structures, e.g. micromechanical sensors and actuators, using wafer bonding techniques is becoming more important. Today's most used technologies for wafer level packaging are silicon direct bonding and anodic bonding as well as glass frit, eutectic, and adhesive bonding. In addition to further developments of these procedures new technologies with low process temperatures (<350 deg C), or local heat sources have been focused increasingly. Especially, technologies using local and controllable heat sources have been developed recently. With these internal heat sources the required thermal bonding energy acts directly on the join, so that temperature sensitive components would not be damaged. One example for that heat sources is the use of nanostructured reactive multilayers. These systems consist of numerous nanometer thick layers alternating between two elements that react exothermically during intermixing. Once the reaction is initiated by a pulse of energy the reaction can be self-propagating. Self-propagating exothermic reactions or self-propagating high-temperature synthesis (SHS) have been observed in a variety of compacted powders, e.g. Ni/Ti or Ti/Co, and nanostructured multilayer systems, such as Ni/Al. The properties of the powder-based SHS reactions depend strongly on the particle size, shape, processing, and purity as described in different reviews. In comparison to the powder compacts the reactants in multilayered samples are placed in intimate contact and the interface impurities are reduced so that the reaction velocities could be largely increased. Nanoscaled reactive multilayer systems typically consist of two alternating elements. During deposition process the materials undergo partial intermixing which can have dramatic effects on the reaction parameters. To minimize this fact the substrates are usually cooled. The driving force in such a system to self-propagate is the reduction in chemical bond energy. In A-B multilayer systems compounds A(x)B(y) are formed when the constituents, A and B, intermix due to thermally induced atomic diffusion. This local intermixing produces a large amount of heat which is transmitted down to the neighboured constituents and so continues further intermixing. Experimental studies of nanoscaled reactive multilayer systems showed that the enthalpy of formation of A(x)B(y) should be greater than -30 kJ/mole-atom to become self-propagating.