Two master thesis presentations were held on November 9th 2016 at Silex in Järfälla during the largest Stockholm November snowstorm in 100 years.
Mikael Granberg presented his thesis "Investigation of Methods to Improve PZT Sol-Gel Deposition Process for Energy Harvesting Applications"
The purpose of this work was to investigate ways to modify Silex sol-gel deposition of PZT (PbZrxTi(1-x)O3, Lead Zirconate Titanate) to improve its properties for energy harvesting applications. A number of methods to improve the figure of merit for energy harvesting (FOM= e312/ε), cause self-polarization, increase lifetime, reduce cost, increase throughput or simplify processing were tested.
In order to create a barrier preventing lead diffusion into the substrate, a method to oxidize the bottom electrode’s Ti adhesion layer into TiO2 by RTA (Rapid Thermal Anneal) was tested. Oxidation was successfully achieved and was found to aid in self-polarization, thereby increasing the FOM for films without post-processing polarization. An extended lifetime is expected, but has not yet been confirmed by testing. A seed layer of a different material was tested and compared to a PZT-based seed layer. The new seed layer was found to give highly (100) crystalline PZT with improved self-polarized e31,f and FOM. The new seed layer was also found to be less sensitive to processing variations. Oxygen control during crystallization of the PZT was used in an attempt to generate PZT layers with oxygen vacancies. These hypoxic layers were intended to polarize the film, but were found to reduce the FOM and lead to partial delamination of the film due to stress.
A different type of PZT sol-gel was tested as an alternative to the PZT sol-gel in use at Silex. The tested solution was found to result in PZT films with similar properties to those generated by the original type, but the tested type allowed for single layer thicknesses nearly three times thicker than the original type, thereby increasing the throughput and reducing manufacturing costs.
Jesper Scott-Robbert presented his thesis "Fabrication and Characterization of doped thin film PZT"
Piezoelectric MEMS is used to fabricate a wide variation of sensing and actuating devises. The most common piezoelectric material for MEMS is PZT which has been intensively investigated. In order to improve the performance of PZT and create materials optimized for specific applications, altered versions of PZT are being investigated. One way to alter the behavior of PZT is to introduce dopants.
In this work, doped and non-doped PZT films have been fabricated using the sol-gel deposition process and the e31,f value of these films has been measured. Two types of dopants have been used to see if these dopants could boost the e31,f value making the film more suitable for energy harvesting applications. Furthermore, processes alteration has been performed to increase the quality and throughput of the PZT film fabricated at Silex Microsystems. The quality of the film could be seen by inspecting the level of non-uniform areas in regards to color and clarity of the film. The quality was improved and the color and clarity uniformity across the wafer was visibly improved. The throughput of the PZT deposition process was increased by ~33% by finding an alternative process requiring fewer crystallization steps. One type of dopant gives an e31 increase of ~12% compared to the highest e31 value previously obtained at Silex Microsystems using non-doped PZT.
Authors: Thorbjörn Ebefors, Mikael Granberg, Jesper Scott-Robbert, Johanna Anteroinen