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Continuously tuneable liquid crystal based stripline phase shifter realised in LTCC technology
(2015)
This work presents the design, fabrication and measurements of a low temperature cofired ceramic (LTCC) integrated liquid crystal (LC) phase shifter. The effective permittivity of the phase shifter and therefore its differential phase shift can be tuned continuously by orienting the LC directors with electric fields. The phase-shift demonstrator is designed for Ka-band frequencies around 30 GHz and represents a stripline filled with LC, embedded inside an LTCC multilayer structure, which provides a space-qualified and hermetically-sealed LC cavity within an RF-capable material system. The total length of the device is 37 mm while the phase shifting LC section has a length of 14.6 mm. At 30 GHz it exhibits a differential phase shift of 60° with an insertion loss around 6 dB, resulting in a figure of merit around 10°/dB. An advantage is that the insertion loss is nearly independent of the tuning state of the LC. The response time of the phase shifter, depending on the desired LC orientation, is between 62 ms and 37 s.
In this work, the effect of the pressure-assisted sintering process on buried thick film resistors integrated in LTCC multilayer has been studied. Four commercial resistor pastes with sheet resistivities between 10 kΩ and 10 MΩ/cm were analyzed. First they were characterized by SEM/EDX, XRD and Laser diffraction to determine composition and particle distribution. The pastes consist of isolating particles and of Ruthenium based particles that are supposed to build the conductive phase. The pastes were screen printed on LTCC green tape (DP 951) and buried in four layer laminates. Sintering was done in two ways, pressureless (PLS) and also pressure-assisted (PAS). The pressureless sintered resistors showed electrical resistance values roughly in the range of the nominal sheet resistivity and only relatively small fluctuation within one sample. The PAS samples on the other hand showed significantly higher resistances and larger deviations. The microstructure of the sintered resistors was again investigated by SEM and XRD. It seems that the resistivity is determined by the ratio of the two Ruthenium phases RuO2 and Pb2Ru2O6.5, where RuO2 has the higher conductivity. Buried resistors cannot be trimmed by a laser to adjust the resistance. But we discovered that a refiring step will reduce and normalize the resistivity of the PAS resistors significantly.