Piezoelectric materials

PZT is currently the dominant piezoelectric material when it comes to achieving high strains and forces at a given voltage (Figure below). It is more than a factor 10 better than AlN or ZnO in this respect. PZT thin film integration onto silicon wafers for MEMS applications started around 1990 and industry interest is growing rapidly. In recent years, thin films of the relaxor material (1-x)[Pb(Mg1/3Nb2/3)O3] - x[PbTiO3] (PMN-PT) has received considerable interest for MEMS actuators due to its very high piezoelectric coefficients.

 

The improvement of piezoelectricity in PZT and related thin films, after Paul Muralt. The numbers beside the dots indicate the film thickness in microns.
The improvement of piezoelectricity in PZT and related thin films, after Paul Muralt. The numbers beside the dots indicate the film thickness in microns.

A lot of progress in improving PZT thin films and their properties has been made during the last 15 years. A good indicator of the progress in quality is the transverse piezoelectric coefficient e31f, relevant to most of the lower frequency (< 10 MHz) applications, and being proportional to stiffness coefficients, indicating as well the film density.

 

Developments under PETMEM: Pulsed laser deposition (PLD) of PMN-PT

 PLD has various advantages, such as high deposition rate, less stringent requirements for the deposition, and the capability of thin films with the correct phase as the target is ablated stoichiometrically. The PLD technology is not limited to piezoelectric thin films, but is an enabler in numerous fields of applications where new (oxide) materials are crucial for the miniaturization and increasing functionality of thin film devices. Not only the stoichiometric control and stabilization of multi-element oxide compounds, but also the ability of integration with existing (CMOS) technology is a key qualifier of PLD above the conventional deposition techniques. In collaboration with the MESA+ institute, SolMateS recently demonstrated the potential of its PLD technology for integration of piezoelectric thin films in CMOS applications. Their PiezoFlare PLD systems are capable of deposition on full 200 mm wafers in a production environment and such a system was recently acquired by Bosch for deposition of piezoelectric PZT. This describes a prototype transistor for reduced power consumption based on Piezoelectric Strain Modulation in FinFETs (Figure below).

 Schematic representation and TEM image of piezoelectric thin film enveloping the FinFET structure.
Schematic representation and TEM image of piezoelectric thin film enveloping the FinFET structure.

For PETMEM, the large piezoelectric coefficient, d33 of PMN-PT makes it the PE material of choice. To quantify, a figure of merit for the optimal PE in a PET can be derived within projection down to a 1D model.

Single phase formation of perovskite PMN-PT systems is difficult but we shall meet the challenge. An electrode structure has to be deposited on the Si wafer. The standard for PZT is Pt, but this leads to the formation of pyrochlore phases in PMN-PT. Thus, lattice matched substrates or template layers like Si/STO/SRO, Si/YSZ/CeO/LNO or must be employed to achieve single phase PMN-PT. A buffer layer of La0.7Sr0.3MnO3 (LSCO) promotes formation of single phase PMN-PT in both an epitaxial stack of Si||YSZ||CeO2||LSCO||PMN-PT and as buffer layer between Pt and PMN-PT giving only oriented growth. PLD has been one of the main the deposition methods of choice in this development.

Single crystal substrates with a suitable lattice constant are several, but SrTiO3, LaAlO3 and MgO with a suitable electronically conductive perovskite such as SrRuO3 or LaNiO3 have been regularly used. Of these STO has the smallest lattice mismatch and was used in combination with miscut Si wafers to grow PMN-PT with very high performance. Complete control of interface between silicon and oxides has been achieved using MBE systems with Sr-based buffers.