Projects / Programmes
Tunable ferroelectric thin film capacitors for agile microwave antennas
| Code |
Science |
Field |
Subfield |
| 2.09.01 |
Engineering sciences and technologies |
Electronic components and technologies |
Materials for electronic components |
| Code |
Science |
Field |
| T001 |
Technological sciences |
Electronics and Electrical technology |
| Code |
Science |
Field |
| 2.02 |
Engineering and Technology |
Electrical engineering, Electronic engineering, Information engineering |
ferroelectric varactor, frequency-agile antenna, chemical solution deposition
Researchers (12)
Organisations (3)
Abstract
At present, the fast development of the communication technologies requires increased data rates and at the same time less-expensive, miniaturized and more efficient devices. Antennas are an important part of these technologies. However, broadband and multiband antennas require more space for fast data transmission. This is strongly disadvantageous, because the device housings are getting smaller, as well as the space for their installation. This trend is especially obvious in small handheld communication devices and in space applications, where micro satellites (total volume below ( 1000 cm3) are employed in a large number of missions.
The solution to this problem may be the use of frequency-agile antennas, which cover a wide frequency range and at the same time occupy less space. There are only a few reports in the literature on development of frequency-tunable dipole antennas and the solutions in use today exhibit poor functional properties, either in terms of a low frequency agility, or the resonant frequency can be changed only at discrete frequencies.
Our proposal is to develop a frequency-reconfigurable planar antenna, which will enable low-loss variable capacitance properties. The technical objective is to develop a novel, highly-integrated and highly efficient antenna design based on ferroelectric varactors technology. Compared to competitive semiconductor technology, ferroelectrics have a fast response exhibit low-leakage currents resulting in lower power dissipation and they represent a cost-effective solution. In addition, their functional response is irradiation-resistant, which is strongly beneficial for space applications.
The proposed project is divided into four work packages. In the first we will define the structure and dimensions of the antenna. In this frame we will select the substrate, active ferroelectric, and electrode material, define the frequency of operation and numerically simulate the antenna operation.
We will employ chemical solution deposition derived (Ba,Sr)TiO3 (BST) thin films as the active part of the varactors. With the aim of the optimization of their functional properties, i.e., electric-field-dependent permittivity (tunability) and dielectric losses (dissipation factor), we will focus on the control of the synthesis of the precursor solutions. These will be deposited on alumina substrates by the spin-coating method and further heated in rapid thermal annealing furnaces. The films will be characterized by means of microstructure, compositions and broadband dielectric properties. The relations between the processing parameters and microstructure, as well as between the microstructure and functional properties will be established. In addition, the leakage current of the films will be studied.
In the work package 3 we will focus on patterning and deposition of the interdigital capacitor structures. The geometry will be defined by the simulations of the antenna operation. The patterning will be performed by the lift-off photolithography. The varactor structures will be characterized to evaluate their performance, i.e., the capacitance agility. An effort will be made to design the capacitor in a manner, which will diminish the dissipation of the GHz-electromagnetic wave in the metal.
In the final, fourth work package we will finalize the antenna and characterize its performance, such as impedance, return loss, radiation pattern, gain and efficiency, by indoor and outdoor measurements. As a result we will produce the prototype microstrip antenna.
The proposed project is also a great opportunity to merge the state-of-the-art knowledge of all participators, ranging from the materials science to microwave device engineering.
Significance for science
Development of wireless and mobile communication technologies requires fast and efficient data transfer at smaller size and lower cost. Small frequency agile planar antennas, which work in a wide bandwidth are very attractive for applications. Our goal was to develop a highly integrated and efficient design of a dipolar antenna, based on the technology of electrically tunable ferroelectric varactors. The antenna with the lateral dimensions of ~20 mm x 20 mm and thickness of about 250 µm, operating in the X band, has the input of the microwave radiation at the back of the alumina substrate. Interdigital varactors of ~2 um-thick copper are positioned on the top. To achieve the desired frequency and polarization agility we added the structures of filter and phase shifters, which are also based on ferroelectric thin films. The simulated operating frequencies were between 7.4 and 8.4 GHz with required polarization agility. Polished polycrystalline aluminium oxide was selected as a substrate, while BaxSr1-xTiO3 solid solution (BST) with x = 0.5, 0.4, 0.3, was chosen as the active ferroelectric film. The BST films, with the thicknesses of ~250 nm, were prepared by chemical solution deposition, deposited by spin coating and rapid thermally annealed. Based on the microstructural analysis we decided that 900 °C is the optimal crystallization temperature. Integranular cracks appeared at thicknesses above 250-300 nm, which we ascribed to thermal stresses. Dielectric properties of the BST films were measured in the kHz- and GHz-frequency range. The observed drop of the permittivity in the thicker films was correlated with the appearance of the cracks. By rigorous analysis we concluded that the non-monotonous permittivity-thickness dependence is a result of the complex interrelationship between grain size, stresses and their relaxation. We also determined the crucial importance of the microstructure for the stability of the BST films against cosmic radiation. Using BST (x = 0.3) films on alumina substrate the frequency and polarization agile antenna system was designed and fabricated. The antenna system, consisting of the slot antenna, filter and phase shifter, allows a shift of the resonant frequency and polarization in the X-band.
Significance for the country
Research of wireless and mobile communication technologies is important for Slovenia and also for our participation in European Space Agency projects. Members of the project team have been active in the Centre of Excellence SPACE.SI, and started to collaborate in research of voltage tunable ferroelectric thin films as active elements of frequency and polarization agile microwave antennas. The project team consisted of researchers from materials for electronic components and telecommunications. The goal of the project was to realize a highly integrated and efficiant antenna design which would base on tunable ferroelectric capacitors. By numerical modelling the structure and dimensions of the antenna were determined. Ferroelectric (BaxSr1-x)TiO3 (BST) thin films were prepared by chemical solution deposition and rapid thermal annealing. The required capacitance, its voltage tunability and dielectric losses of planar BST capacitors on alumina substrates were obtained by the design of chemical composition, processing conditions, microstructure and film thickness. Interdigital capacitors with 5 micrometre gaps between fingers were patterned by UV lift-off photolithography and sputtering of copper electrodes. The antenna prototype for X-band was fabricated and characterized. Two young researchers finished their PhDs in 2016 with topics related to the project, one from ‘Jožef Stefan’ Institute and one from Faculty of Electrical Engineering, University of Ljubljana. Their results contributed a lot to the success of the project. It is also important to note that the findings obtained in the frame of the project are included in the course Electrodynamics at the Faculty of Electrical Engineering, University of Ljubljana, [COBISS.SI-ID 11306836]. The results of the project, namely application of ferroelectrics in microwave devices were presented at the professional seminar Radio communications (Faculty of Electrical Engineering, University of Ljubljana, 2013, 2014). This central Slovenian event gathers about 100 experts on telecommunications and it enables transfer of expertise between academia and industry.
Most important scientific results
Annual report
2013,
2014,
2015,
final report
Most important socioeconomically and culturally relevant results
Annual report
2013,
2014,
2015,
final report
