A MEMS varactor is a variable capacitance where the capacitance change is mostly realized by changing the electrode spacing. MEMS varactors typically have small tuning ratios (TR) of < 3 because the electrode stroke required for tunability is severely limited. The limitation is explained by the actuator technology that can be used. Normally this is a classical electrostatic or direct Coulomb attraction between the grounded electrode and the signal line (RF-line). Because of such limitations as pull-in, for example, only about one third of the initial gap can be used for capacitance variation. The utilization of the gap is thus inefficient, independent of the initial distance, and leads to a small tuning ratio, which is defined as TR = C/C0 . An additional problem with the pull-in is possible electrostatic sticking of the electrodes.
To overcome the limitations, the grounded varactor electrodes can be adjusted by electrostatic lateral NED actuation. This solves the pull-in problem for MEMS varactors described above because the movement or pull-in of the GND plate is decoupled from the pull-in of the actuator. This allows utilization of almost the entire initial electrode spacing, enabling TR factors of > 10...20. The problem of sticking does not exist here either, because the actuators represent a kind of additional return spring. A special form of electrical voltage can also be used to achieve linearity of tunability. Another advantage of using LNED actuators is that the chip depth and not the chip footprint can be used to increase the electrode area and thus the initial capacitance C0. This leads to cost savings and enables miniaturization of MEMS actuators with larger initial capacitances.
Visit us at booth 1-317/1 and get into touch with our scientists about YOUR specific measurement tasks. The iCampus Cottbus team is looking forward to meeting you!