Preparation And Dielectric Properties Of Ag-pst Composite Thin Film

With the development of microelectronics and the insatisfaite need for increasingly high miniaturization degree of digital and electronics devices, the electronics devices, such as:RAM, DRAM, in market are becoming smaller and smaller. Though the miniaturization degree of devices increases, yet the demand of the properties increases also, such as storage capacity, response velocity. In the storage and memory devices, the permittivity of dielectric material will determine the miniaturization degree ultimately. In the field of decreasing the miniaturization degree of electronic devices, thin solid films have great advantages over other materials. In view of the two points mentioned above, to meet the increasing small demand of electronic devices, to improve the dielectric constant is of great importance. Besides, according to reference, due to the enhanced inner electric field by the metallic particles introduced into the material, the tenability will be improved also.In recent years, metal/insulator composite will show percolation effect when the volume fraction of metallic material approaches some critical value. The composites show great potential and arouse many researchers' interest. However, for composite thin films, first, thin film is only several hundred nanometers thick. The size of metal particles normally tends to be no smaller than the thickness of the thin film so that large metallic particles are capable of forming conducting path across upper and bottom electrodes. A short circuit current appears in the thin film and the percolation effect disappears. Second, the percolation effect is difficult to appear in the thin film if the size of the conductive particles is too large. According to the percolation theory, the apparent dielectric constant increases by percolation effect only when the conductive particles are blocked by thin barrier of dielectric matrix and thus a large numbers of micro-capacitors form in the direction perpendicular to the surface of the thin film. Hence, the percolation effect disappear in the thin film when the size of metallic particles increase close to the scale of thin film matrix and thus the number of micro-capacitors significantly decrease. It is therefore of great importance to control the particle size to nanometer scale which is much smaller than thin film thickness and to obtain the percolative thin film with high permittivity. Besides, more effective technology and methods should be investigated to control the volume fraction and the size of metal phase, which will be critical to the breakthrough of thin solid film of high dielectric constant and tenability.This dissertation reviewed the development and applications of the microwave tunable ferroelectric thin film materials, introduced the research results and the theory model of the metal particles/ceramic composites. Besides, the various methods of introducing metallic particles on nanometer scale into materials were summarized, such as:Sol-gel technology, ion exchange process, ion implantation, vacuum vapour deposition. The sol-gel technology route was introduced in details. Here, sol-gel technology was adopted to introduce silver nanoparticles into (Pbo.4Sro.6)TiO3 (called PST for convenience hereafter)thin films. By regulating the silver content, heat treatment conditions, etc., silver nanoparticles dispersed polycrystalline PST thin films were obtained. XRD was adopted to study the structure and the phases of the thin films. SEM was used to observe the surface and determine the thickness of the thin film roughly. The dielectric constant and dissipation factor were measured by LCZ meter. The formation of Ag nanoparticles was confirmed by UV-VIS absorption spectra. The size and the relative number of silver nanparticles can also be obtained.The research details and conclusions are listed below:Citric acid and acetylacetone were introduced into sol precursor as complexing agent. A series of Ag-(Pb, Sr)-Ti sols with different Ag addition x of 0-0.8(Ag/Ti molar ratio is called as x for convenience). The thin films were dip-coated and then heat treated in air atmosphere at 600℃. The thin films are proved to be percolative composite material. And the dielectric constant of thin films with x of 0.7 is 6 times as high as pure PST thin film. Yet according to percolation theory, the dissipation factor of thin film will show an non-linear apparent increase also and the experiment data confirm the presumption. However, in the practical electronic devices, the dielectric loss must be very low. In the dissertation, then the methods with which to decrease the dielectric loss of percolative thin films were also discussed. That is, to introduce pure (Pbo.4Sro.6)Ti03 layer of different thickness to the composite thin films to block the path through which the carrier were conducted. The dielectric loss of Ag-PST thin films were decreased while the capacitance of the composite thin films has not deteriorated.Besides, the tunability of thin film with different Ag addition was also investigated. Results showed that the tunability of pure PST thin film was 28%, the tunability for Ag0.3-PST composited thin film was 38% and for Ag0.5-PST is 39.5%. The tunability of thin films with Ag nanoparticles was both higher than that of pure PST thin films.