Other examples
In addition to passive optical material like Si3N4, and according to applications, it turns out to be very interesting to use active materials such as electro-optical or magneto-optical materials for optical modulation, pyroelectric materials such as IR sensors or fluorescent materials to make the sources. Most of these interesting active materials are complex oxides, often made up of perovskite structures which, normally, in a massive solid state, have to reach high temperatures to obtain the phase purity or the desired micro-structures. In order to make their integration possible in the structures of silicon micro-electronics or on micro-mechanical devices, these materials are set up in the form of thin film, either by vapor phase deposition techniques (for instance by pulverization, PLD, MOCVD, MBE) either by chemical methods (for instance the sol-gel process). In general, the degree of crystallinity, orientation, micro-structuration, stoichiometry and intrinsic stress depend on the deposition conditions, the substrate nature and temperature. In most cases, the film made show a reduced activity compared with the corresponding massive material. For instance, an electro-optical modulator get by laying a BaTiO3 film on a MgO single-crystal substrate shows an effective electro-optical coefficient of 50 pm/V, less than an order of magnitude weaker than the measured value for the single-crystal. We notice a similar phenomenon for piezoelectric properties. These phenomena are said to be due to stress effect imposed by the substrate, by the defaults and at the interfaces level. Stamp layers, between silicon and active oxides, have an essential role to avoid interdiffusion and oxidation but they also act as “germ” layers to help the phases nucleation and appropriate crystallographic orientations. There is an excellent journal about many of these problems in an article written by P. Muralt in 2000 (Journal of Micromechanics and Microengineering, Vol 10, p 136).