Ferroelectric thin and thick films for microsystems

Ferroelectric oxide thin and thick films are interesting materials for microsystem devices because of their wide range of useful properties, particularly the pyroelectric and piezoelectric effects. The ability to grow these films at relatively low temperatures onto a wide range of substrates, including silicon, is especially important. This paper discusses the use of CSD processes to grow high-quality ferroelectric PZT30/70 thin films onto platinised silicon substrates at low temperatures (from 400°C to 575°C), with particular reference to their use in pyroelectric infra-red detector arrays and other MEMS devices. The various factors important to the use of sol-gel processes are discussed, including mechanisms for sol ageing and for perovskite nucleation and growth. The latter is interesting, involving the formation of a transient Pt₃Pb phase that acts as a nucleation layer for the templated growth of the PZT layer. The effects of Mn doping on the resulting materials properties are discussed. It leads to a strong asymmetry in the ferroelectric hysteresis behaviour as well as improved pyroelectric performance. Techniques for increasing the thickness of sol-gel layers, and the reasons for the appearance of nanoporosity, are reviewed. Finally, the use of sol-gel techniques for the fabrication of composite piezoelectric ceramic thick films (10-20 μm thick) at low temperatures (710°C) are discussed.