Phase Change Materials for Actuation in MEMS

Phase change materials (Sb and Te alloys) are used for optical data storage in commercial phase change memories, such as rewritable compact discs (CD±RW) and rewritable digital video disks (DVD±RW, DVD-RAM). Recently, they have also shown high potential for the development of phase change random access memories (PC-RAMs or PRAMs), which might replace flash memories in the future. In this project, thin films of phase change materials are systematically analyzed with regard to their transformation behavior under laser-induced amorphization and crystallization. The goal of this project is to gain a better understanding of the relationship among the laser parameters, the material-specific transformation kinetics, and the involved volume changes (and associated mechanical stresses) over a wide range of alloy compositions.

The approach to pursuing this goal is to use microfabricated SiN cantilevers as substrates for thin film deposition: The SiN cantilevers are manufactured by chemical vapor deposition of low-stress SiN on Si wafers, patterning the SiN film using optical lithography and revealing the cantilevers using dry etching and wet etching. Thin films of phase change materials are subsequently sputter-deposited on these SiN cantilevers and are locally switched by laser heating from the amorphous to the crystalline phase (and vice versa). The associated stresses induce a cantilever bending, which is measured by optical microscopy and non-contact interferometry as a function of laser annealing parameters, laser quench rate and alloy composition. Additionally, amorphous films are hot-stage crystallized, which allows the study of the kinetics associated with the crystallization process as well as the force associated with the cantilever bending.

The results of this project will help to increase the number of write-erase cycles and the data transfer rate in phase change memories and may lead to other applications of phase change materials in MEMS actuation.