Fabrication and characterization of piezoelectric thin films on si wafers and fibers

摘要

Ferroelectric films are very promising materials in many areas of modern technologies such as microactuators, microtransducers, sensors and phase modulators. The application area of ferroelectric films is growing due to the need for miniaturization and integration of electronic components. Ferroelectric films have several advantages over bulk material, such as smaller size, less weight, easier integration to integrated circuit technology, lower operating voltage, higher speed, and the ability to fabricate micro-level structures. Lead Zirconate Titanate (PZT) film is an appropriate candidate for the above mentioned applications due to it's excellent piezoelectric properties. The principal aim of this study is the optimization of PZT film deposition process parameters on Si wafers using a single metallic PZT target. The optimized process parameters for wafers are transferred to the PZT coating on fiber substrates, showing good flexibility and high tensile strength. The objective is to investigate the piezo properties of the PZT coated fibers for sensor application. PZT thin films have been deposited by pulsed DC sputtering process on Ti/Pt/Ti coated Si wafers. The influence of various processing parameters (substrate temperature, oxygen partial pressure, annealing conditions) on the properties of PZT films have been studied. It has been found that 490°C is the optimum substrate temperature to obtain crystalline and homogeneous films with excellent adhesion. The deposited films have been processed by conventional annealing (CA) and rapid thermal annealing (RTA) to obtain pure pervoskite phase. Different crystallographic orientations have been observed for different annealing procedures (110 for CA and 111 for RTA) for films deposited at low O2 partial pressure. In order to investigate the depth profile, a novel GD-OES method has been developed. The obtained results revealed deficiency of oxygen within the pervoskite structure. Using higher oxygen partial pressure within the plasma and adequate post annealing methods (CA and RTA) results in pure PZT pervoskite structure. Hardness and Young's modulus of these oxygen rich samples are almost three times higher than the oxygen deficient films, indicating a improved crystallization in oxygen rich samples. CA and RTA processed films repeatedly show different crystallographic orientations (100 for CA and 110 for RTA). Annealed samples show dense and multi-crystalline structure with grain sizes in the range of 100 to 300 nm. Depth analysis revealed constant elemental concentrations close to the target value. It has been found that CA processed samples show higher polarization and remanence than RTA processed samples because of larger grain sizes and different polarization axis. CA and RTA processed samples have 100 and 110 polarization axis. Grains can be easily polarized in 100 direction compare to the hard 110 direction, results in higher polarizations. The optimized process parameters have been used for PZT deposition on Gold/glass fiber, steel fiber and copper fiber. Some modification of parameters were essential due to following reasons: different substrate geometry, different sticking of Ti atoms on the fiber surface, oxidation of fiber surfaces and temperature distribution through the fibers. RTA processing at 650°C has been found the best optimum temperature to obtain pervoskite structure. All the coated fibers revealed (110) preferred orientation. A dense and crystalline structure has been observed on Gold/glass fibers and on steel fibers. Grain sizes are in the range of 100 to 400 nm range. Some micro cracks have been observed on steel and Cu fibers, due to large difference in thermal expansion coefficients between PZT and substrates. Atomic concentration of Pb, Zr, Ti and O atoms have been found in stoichiometric ratio and close to our target value. PZT coated steel fibers have been investigated as force sensors. Sensor sensitivity and piezoelectric coefficient, d31 has been found to be 1.95 V/N and 870 pm/V, which is comparable to the literature value (500 pm/V on wafers from multi target).