| The Ferroelectric materials for their excellent dielectric, piezoelectric, ferroelectric, pyroelectric, electro-opticial, phonon-optic and nonlinear polarization properties have been widely applied in many fields especially in microwave tunable devices, such as phase shifter in phased array antenna, resonantor and filter etc. As far as microwave tunable dielectric materials, Barium strontium titanate(BST) and its doped series have been investigated intensively, which have about 50% tunability. Their dielectric loss is about 10-2, and figure of merit (FOM) is less than 300. Recently, Cross etc. found that lead strontium titanate (PST) has bigger tunability and lower loss, besides this, PST is a perovskite ferroelectric material which has good solid solution. Because of the expedient modulation of curie temperature Tc to room temperature, the larger dielectric-temperature coefficients, PST materials are favorable to be used in microwave tunable devices. Compared with BST materials, especially in thin films, PST has smaller ferroelectric critical size, lower crystallization temperature, and compatible fabrication with Si micro-electronics, so it can meet the need of the high quality si-based integrate circuit (IC). MoreOver, it is important to promote the development of the miniaturization and integration for the modern devices. At present there are a few research on the PST materials, especially on the thin films, and the properties of the thin film is far away to the expectation. In this thesis, the sol-gel preparation and characteristics of the PST thin films were investigated in detail, and got the high quality PST thin films.This thesis reviewed the development of the microwave tunable ferroelectric thin film materials, summarized the application of sol-gel on the materials, and introduced the principle and application of the microwave tunability in brief.The PST thin films in which the perovskite structure primarily formed were fabricated successfully on ITO/glass substrates by the sol-gel method. It is found that both the formation of the perovskite and contents of the crystalline phase in this PST thin film are dependent on the preparation processing and activity of an atom movedfor nucleation and crystallization. By rapid thermal processing (RTP), high active atoms are excited while decomposing the gel precursor film, and consequently, much more contents of crystalline phase are obtained even at relatively lower temperature. 14% more contents of crystalline phase appear in PST thin film at 600 C by RTP than at the same temperature for a long and constant calcination time. The lowest temperature of formation of the proveskite phase in sol-gel derivede PST thin film is approximately 500 C by RTP, which is 50 C lower than that by relatively long and constant calcinations.It can be found that the crystallization temperature would change with the Pb/Sr ratio in PST. The crystallization temperature of the PST film with Pb/Sr ratio of 80/20 is at least 50?C lower than that of the PST film with Pb/Sr ratio of 20/80. At the same time, the contents of crystalline phase wound increase with the contents of Pb in the thin films. When the contents of the Pb is larger than that of the Sr, the crystalline phase structures is changed from the cubic phase structure to the tetragonal stucture. In another words, the Tc temperature will increase with the Pb contents, when the Pb contends is equal to the Sr contends, the Tc temperatue is at about room temperature. The structure can be controlled by controlling the Pb contents in a certain temperature. The higher dielectric-temperature coefficient can hence be kept near the applied temperature, improving the dielectric tunability of the materials.The PST thin films which have high tunability and FOM were fabricated successfully. The tunability of Pb0.4Sr0.6TiO3 thin film is about 83%~85% under the 100 kHz, and keep constant with the change of frequency. The dielectric loss decrease with the increase of the frequency, and has the trend to decrease with the incr...
|
PDF Full Text Request