Large tuning ratio high aspect ratio variable capacitors using leveraged bending

Variable capacitors are a key component in Radio Frequency Micro Electro-Mechanical Systems (RF MEMS). They comprise fixed and flexible electrodes. Deformation, or actuation, of the flexible electrode changes the capacitance of the capacitor. This way, electrical properties of high frequency circuits can be modified. Traditionally, variable capacitors are based on a planar layout architecture, while a newer, vertical-wall, quasi three-dimensional approach theoretically enables increased device performance. Such devices depend on high aspect ratios, i.e. relatively high micro structures with very thin walls and gaps. A few vertical-wall variable capacitors made of nickel or gold have been fabricated to date, using deep X-ray lithography and subsequent electroplating (Achenbach et al. 2006; Klymyshyn et al. 2007, 2010) as the fabrication approach. They feature, amongst others, excellent quality factors of Q ≤ 95 at 5.6 GHz with 50 Ω reactance, but suffer from a very limited tuning range of the capacitance value (tuning ratio of, e.g., 1.38:1). The devices presented here exploit the same architecture and materials selection, resulting in similar, excellent Q-factors, but feature a different electrode layout approach, referred to as leveraged-bending. This layout is based on pulling a flexible electrode sideways, towards a fixed electrode, increasing the capacitance when actuating the variable capacitor. The leveraged bending approach theoretically enables infinitely high tuning ratios for components with perfect structure accuracy. To date, a significantly increased tuning ratio of 1.9:1 has been demonstrated. Limiting factors are an electrically non-ideal layout geometry chosen as a compromise to increase the fabrication yield, and structure deviations of ~1.6 μm from CAD layout to the electroplated component. Electrostatic actuation requires voltages between 0 and 72 V for capacitance values on the order of C = 0.3 pF at device dimensions of about 1.5 mm overall length, 5–10 μm gap and wall widths, and 100 μm metal height. Device performance measured with a vector network analyzer is in 97 % agreement with simulation results based on two-dimensional electrostatic-structural coupling (ANSYS Multiphysics) and three-dimensional electromagnetic field simulations (Ansoft HFSS). These simulations also indicate that an optimized gap geometry will allow to reduce the actuation voltage required by up to 40 %.