Microwave MEMS devices designed for process robustness and operational reliability

Abstract

, antti ra ¤isa ¤nen 2 and joachim oberhammer 1 This paper presents an overview on novel microwave micro-electromechanical systems (MEMS) device concepts developed in our research group during the last 5 years, which are specifically designed for addressing some fundamental problems for reliable device operation and robustness to process parameter variation. In contrast to conventional solutions, the presented device concepts are targeted at eliminating their respective failure modes rather than reducing or controlling them. Novel concepts of MEMS phase shifters, tunable microwave surfaces, reconfigurable leaky-wave antennas, multi-stable switches, and tunable capacitors are presented, featuring the following innovative design elements: dielectric-less actuators to overcome dielectric charging; reversing active/passive functions in MEMS switch actuators to improve recovery from contact stiction; symmetrical anti-parallel metallization for full stress-control and temperature compensation of composite dielectric/metal layers for free-standing structures; monocrystalline silicon as structural material for superior mechanical performance; and eliminating thin metallic bridges for high-power handling. This paper summarizes the design, fabrication, and measurement of devices featuring these concepts, enhanced by new characterization data, and discusses them in the context of the conventional MEMS device design.Keywords: RF MEMS, Reliability, MEMS design, Phase shifter, Tuneable capacitor, MEMS switch [12]. In general, RF MEMS devices are characterized by near-ideal signal handling performance in terms of insertion loss, isolation, linearity, large tuning range, and by keeping these performance parameters over a very large bandwidth [4,13]. I . I N T R O D U C T I O NMicrowave MEMS are RF MEMS devices that operate with signal frequencies above 30 GHz. With applications moving to higher frequencies, the performance advantages of MEMS devices over their competitors are getting larger. Also, at frequencies where the signal wavelengths are getting closer to the device dimensions, it is possible to miniaturize a complete RF system on a chip, and different ways of interaction between the microwave signals and the micromechanics lead to new possibilities of RF MEMS devices [47].Operational reliability, i.e. the ability of devices to work as specified over the whole lifetime, and robustness to process parameter variations, are key issues in RF MEMS design [14,48].Conventional RF MEMS designs are still characterized by some fundamental problems. Thin metallic bridges, for instance, found in many RF MEMS devices, such as switches and phase shifters, are susceptible to temperature-accelerated creep and fatigue, limiting the device life-time [14]. In contrast, monocrystalline silicon is a very robust MEMS structural material that is at the same time also suitable as RF dielectric material if high-resistivity silicon (HRS) is used [15,49].

Multi-Position RF MEMS Tunable Capacitors Using Laterally Moving Sidewalls of 3-D Micromachined Transmission Lines

“…Neither the stoppers nor other actuator elements showed any signs of wear when inspected in the SEM after the lifetime measurements. The life-time characterization agrees very well with other silicon-core, allmetal RF MEMS devices developed by the authors [38].…”

Millimeter-Wave Near-Field Probe Designed for High-Resolution Skin Cancer Diagnosis

“…5. A number of probe prototypes with different tip sizes were fabricated simultaneously from a 4-in 600-m-thick high-resistivity silicon substrate (HRSS) wafer ( 4000 cm, , of 6 10 measured at 100 GHz [40]). A 3-m-high thermal oxide layer, which is patterned by a standard photolithography process and silicon dioxide dry etching, is used as a masking layer.…”

Optimization of Micromachined Millimeter-Wave Planar Silicon Lens Antennas With Concentric and Shifted Matching Regions

“…The Eigenmode Solver implemented in [24] was used for this simulation. Since the relative permeability of silicon is very close to 1 in the W-band [15], we have n = √ eff where n is the calculated refractive index. Based on the simulated dispersion relation (i.e., the relation between the wave number k and the angular frequency ω), the permittivity was extracted according to eff = (ck/ω) 2 where c is the speed of light in vacuum.…”

“…Planar lenses have many similarities with non-planar lenses, where there are many good references, see e.g., [8][9][10][11][12][13]. High-resistivity silicon is an excellent dielectric material for millimeter-wave planar lenses, due to its very small losses and accurate micromaching fabrication processes [14,15]. However, homogeneous lenses of high permittivity materials, such as silicon, suffer from reflections at the airdielectric interface caused by the large difference in impedance compared to free-space (see e.g., [9]).…”