The introduction of shape memory alloys (SMAs) into novel microsystems products is hampered by various constraints imposed by process technologies of SMA film or foil fabrication, thermo-mechanical treatment and, in particular, monolithic integration. Often, the costly piece by piece processing and pick-and-place assembly remain as the only solution. Many technology combinations promising novel functionality in various applications cannot be realized today. In order to overcome these problems, the technologies of transfer bonding on a full wafer scale and selective transfer bonding are investigated.

SMA materials to be transferred comprise either sputtered thin films or free-standing foils produced, e.g., by cold rolling. In either case, the thermo-mechanical treatment is performed in free-standing form to avoid any kind of temperature incompatibilities, such as interdiffusion or thermal stress. The SMA material is then fixed on a substrate to perform the micromachining. The resulting array of SMA microdevices is then bonded to a transfer substrate and released from the temporary substrate. In the case of wafer transfer bonding, the complete composite of transfer substrate and micromachined SMA layer is transferred and bonded to the final target wafer. Alternatively, individual SMA microdevices are transferred by selectively removing the bonding layer. Both technologies are compatible to target wafers of temperature-sensitive polymers, since critical steps are performed separately. The temporary and transfer substrate may be identical.

In the present study, the transfer technologies are tested for cold-rolled NiTi foils of 20µm thickness, temporary substrates of alumina, transfer substrates of glass and target substrates of a polymer. Different adhesive coatings are used as bonding layers, which provide sufficient bonding strength during processing, but act as a sacrificial layer later on. The batch fabrication process is demonstrated for the example of microvalves.

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