Liquid Crystals

Liquid Crystal Group

The demand on tunable or reconfigurable components at micro- and millimeterwaves increased during the last years. Microwave devices like tunable varactors, filters, or matching networks for mobile communications as well as millimeterwave devices like phase shifters for radar sensing e.g. for automotive adaptive cruise control at 77GHz are required. Liquid crystals (LCs) as tunable microwave dielectrics can satisfy requirements for various applications.

About Liquid Crystals

Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid and those of a solid crystal.

Microwave characterization:

The material behaviour is investigated in cooperation with the Condensed Matter Group at TUD and the company Merck KGaA. The general aim is to find and optimise LC compounds and mixtures with high performance at micro- and millimeterwave frequencies, i.e. with high tunability and low loss. The research done so far resulted in a broad knowledge about which Liquid Crystal materials are suitable for microwave applications.

The extraction of the material parameters is usually done using high resolution methods for material optimization as well as broadband methods which enable the direct design of microwave components based on these materials.

Process 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. To solve this problem a lot can be learned from LC display manufacturing. In fact, with our process technology it is possible to fabricate LC based devices with a 4µm thick LC layer (which is in the range of LC displays) which is constant over an arrea of at least 2cm2. With this technology we can obtain tuning speeds in the range of LC displays which can be considered the limit of possible tuning speed of LC devices.

Another interesting approach to fabricating LC based microwave devices it to use the well established process technology LTCC (Low-Temperature-Cofired-Ceramics) as it is possible to integrate cavities in LTCC which can be filled with LC. It is however not trivial to fabricate these cavities stable and precise which is why this particular technology is being researched in cooperation with Heraeus and the Bundesanstalt fuer Materialforschung in the framework of the BMWi-Projekt „LIQUIDA“.

Simulation

Different simulation and modelling tools have been implemented. They are the basis for the and will be further developed in cooperation with the Institute Theorie Elektromagnetische Felder at TU.

Examples:

Microwave design examples: