Tunable Microwave Devices

Tunable Microwave Devices

The demand for tunable or reconfigurable components at micro- and millimeter wave frequencies increased during the last years. Tunable microwave devices for reconfigurable systems such as tunable varactors, filters, or matching networks are becoming more and more important. Applications can be found in modern communication systems of the 5th generation (5G) and in radar sensing as for automotive applications. Implementing reconfigurability into radio frequency circuits will be the enabling technology required for the final realization of software-defined-systems.

The Microwave Engineering Group at IMP of TU Darmstadt started almost 20 years ago with research in the field of tunable microwave devices. The focus of the research was and still is the exploration of functional materials, which can adapt their electromagnetic properties. Beside these materials, also semiconductor-based and micro-electro-mechanical systems (MEMS) are used for tunable devices.

The most interesting materials for application in high-frequency circuits are ferroelectric materials such as Barium Strontium Titanate (BST) or Liquid Crystals (LC). The latter are well known from their optical application in liquid crystal displays for many years. Both materials can change their effective permittivity for an RF signal depending on the state of polarization. The difference between the these two classes of materials can be found in the used polarization mechanism for tuning. For LC, the orientation polarization of molecules is used, while BST-based components are utilizing the ionic polarization by a displacement of the titanium ion in the crystal lattice.

Liquid Crystal Technology

Together with our cooperation partners we established the liquid crystal technology for microwave applications. Since many years, IMP is working in close cooperation with the Performance Materials Division of Merck on the synthesis and implementation of liquid crystals for micro and millimeter wave frequencies. Targeted frequency range is above 15 GHz up to several THz.

An overview on recent development can be found in H. Maune et al. „Microwave Liquid Crystal Technology“, Crystals 2018, 8, 355 .

Microwave Liquid Crystal Technology from Darmstadt is a synonym for high performance tunable microwave components, not only in academia, but also in industry. While our papers on LC technology have won many prizes at international conferences, a team of alumni of IMP founded a spin-off to commercialize this technology for smart antenna systems. The first antennas for satellite communication have been announced recently. You will find more information on the spin-off Alcan Systems at http://www.alcansystems.com.

The name “liquid crystal” is a combination of the main properties of this material, as it exhibits a phase of matter that has properties between those of a conventional liquid and those of a solid crystal. LC used for microwave applications can be described as uniaxial anisotropic material, featuring different effective permittivities, depending on the orientation to an applied RF field. The effective permittivity of the LC material is depending on the orientation of the applied RF field. The orientation of the LC molecules can be controlled by electric or magnetic fields and by mechanical anchoring at surfaces. Tunable components are usually based on transmission lines with tunable propagation constant. The simplest geometry is based on a microstrip line.

Research at IMP

The research done at IMP can be grouped into following categories:

Microwave characterization

The material properties must be investigated in order to realize optimal components. The aim is to find and optimize LC with high performance at micro- and millimeter wave frequencies, i.e. with high tunability and low loss. The extraction of the material parameters is usually done using high resolution methods based on the cavity perturbation method. At IMP, different resonator setups are available from several GHz up to 110 GHz. For broad-band material characterization transmission line based methods have been implemented.

Processing technology

The main challenge which must be tackled when fabricating LC based microwave devices, is to create thin and stable cavities which can be filled with LC. The LC layer must be thin the ensure fast tuning speeds. Many different processing technologies have been investigated in the last years. The most important ones are: Planar LCD-like structures with spacer materials, hollow waveguide structures with integrated biasing systems, LTCC (Low-Temperature-Cofired-Ceramics) cavities, and tunable dielectric waveguides.

Simulation

Different simulation and modelling tools have been implemented which can simulate the director dynamics of the LC together with the electromagnetic properties in a multi-physics simulation.

Team Microwave Liquid Crsytal Technology

Name Arbeitsgebiet(e) E-Mail
Ersin Polat M. Sc.Tunable LC mmWave-Filters, Characterization of LC
Henning Tesmer M.Sc.Tunable Dielectric Waveguides and Components
Dongwei Wang M.Sc.Tunable Devices with Slow-Wave Effects

Ferroelectrics (Barium Strontium Titanate, BST)

The work on ferroelectric materials for tunable microwave components has been started many years ago in cooperation with the Universität Karlsruhe and the Forschungszentrum Karlsruhe. Since many years, the focus of this collaboration is research on BST thick films. Together with our colleagues from material science at TU Darmstadt (ATFT) and industrial partners, we also investigate different materials for thin-film and bulk ceramic ferroelectric devices. The targeted frequency range is below 15 GHz.

Components implementing ferroelectric materials use the electric-field dependent material’s polarization to realize a tunable capacitance. As for semiconductor devices, the capacitance of a varactor can be changed by an electrostatic field applied to the electrodes. The most prominent material in this class is Barium Strontium Titanate. Different technologies can be applied for the processing of BST-based tunable components. Thin-film components are produced by evaporation methods such as sputtering or pulsed laser deposition (PLD). These films usually have a thickness of less than 500nm and show a single or polycrystalline behavior. Thick-film components are resulting from various printing technologies such as screen printing or ink-jet printing. The poly-crystalline films have a thickness above 1 µm after sintering. This sintering step requires a temperature of about 1250°C. By implementing composite materials, e.g. by adding ZnO-B2O3, this temperature can be reduced to around 800°C, making BST composite materials compatible to low temperature co-fired ceramics (LTCC). Poly-crystalline cylindrical bulk ceramic pellets are obtained from uniaxial compressed green bodys after sintering at 1250°C. The end faces of the pellet are metallized in a screen-printing process, forming a tunable parallel plate capacitor with a thickness of 1mm and a diameter of 8-16mm. Applications for these pellets are tunable high-power matching networks for industrial plasma depositioning or etching processes.

Research at IMP

The research done at IMP can be grouped into following categories:

Microwave characterization

The material properties must be investigated in order to realize optimal components. A very important research task for ferroelectrics, is the material optimization, which is always combined with the material characterization. BST components are fabricated on wafers of different materials and sizes. Not only established technologies such as on-wafer probing or dielectric probe kits are used to obtain the materials properties from measured scattering parameters, but custom characterization fixtures are developed and verified.

Processing technology

As already mentioned the processing of the BST can be realized in different technologies, ranging from semiconductor-like pulsed laser deposition (PLD) to “simple” screen-printing technologies. Also, LTCC and ink-jet printing of ferroelectric devices has been shown by IMP.

Simulation

Different simulation and modelling tools have been implemented, which are able to simulate micro- and macroscopic effects of non-linear ferroelectric materials.

Team Ferroelectrics

Name Arbeitsgebiet(e) E-Mail
Prannoy Agrawal M.Sc.Modelling of BST Composites
Dominik Walk M. Sc.Varactors Based on Oxides

Publications