Skin-Effect Self-Heating in Air-Suspended RF MEMS Transmission-Line Structures

Abstract

Air-suspension of transmission-line structures using microelectromechanical systems (MEMS) technology provides the effective means to suppress substrate losses for radio-frequency (RF) signals. However, heating of these lines augmented by skin effects can be a major concern for RF MEMS reliability. To understand this phenomenon, a thermal energy transport model is developed in a simple analytical form. The model accounts for skin effects that cause Joule heating to be localized near the surface of the RF transmission line. Here, the model is validated through experimental data by measuring the temperature rise in an air-suspended MEMS coplanar waveguide (CPW). For this measurement, a new experimental methodology is also developed allowing direct current (dc) electrical resistance thermometry to be adopted in an RF setup. The modeling and experimental work presented in this paper allow us to provide design rules for preventing thermal and structural failures unique to the RF operation of suspended MEMS transmission-line components. For example, increasing the thickness from 1 to 3 m for a typical transmission line design enhances power handling from 5 to 125 W at 20 GHz, 3.3 to 80 W at 50 GHz, and 2.3 to 56 W at 100 GHz (a 25-fold increase in RF power handling). [1737] Index Terms-Coplanar waveguides (CPWs), failure analysis, microelectromechanical devices, skin effect, transmission lines. I. INTRODUCTION L OW-LOSS and low-dispersion transmission-line structures are critical components in radio-frequency (RF)/microwave circuit design. Air-suspended transmission-line structures, with their suspension above the commonly used complementary metal-oxide-semiconductor (CMOS) or GaAs substrates, can serve as fundamental building blocks to achieve low-loss, low-noise signal transmission for monolithic integration of RF circuits and devices [1]-[5]. They often form microshield transmission lines [6]-[9] as shown in Fig. 1(a)-(c),

Full-wave electromagnetic and thermal modeling for the prediction of heat-dissipation-induced RF-MEMS switch failure

“…An efficient preconditioner was also developed to accurately solve the highly ill conditioned matrix system stemming from the extremely small electrical dimensions of RF MEMS [24]. We remark that the theoretical predictions resulting from the proposed integrated full-wave electromagnetic/thermal model are compared with experimental data [25] and found to be in good agreement.…”

“…The experimental data refer to a device subjected to an RF signal and can be used to validate our model predictions for the selfheating of an RF-MEMS switch in its 'up' state. The details of the design, fabrication and RF/thermal characterization of the testing device can be found in [25]. In brief, we measured the spatially averaged temperature increase in a MEMS RF structure at various frequencies and RF input power using a dc electrical resistance thermometry technique, specially developed for an RF setup.…”

Reconfigurable RF Circuits and RF‐MEMS

“…Already in 2000, Barker and Rebeiz [51] presented a design and optimization of distributed MEMS transmissionline (DMTL) phase shifters for the U-band and W-band with analogue tuning capability of the individual stages, fabricated on quartz substrate. (c) MEMS tunable dielectric-block loaded-line phase shifter Both the TTD network and the DMTL MEMS phase shifter concepts have the disadvantage that the capacitive MEMS switches or tunable capacitor, employed in both concepts, are composed of thin metal bridges that cannot handle large induced current densities at high RF power because of limited heat conductivity to the substrate due to their suspension above the substrate [53][54][55]. Measured phase shift per decibel loss was 70 /dB from 75 to 110 GHz.…”

Continuously variable W-band phase shifters based on MEMS-actuated conductive fingers

“…Topologies presented in the literature are typically based on switched true-time delay-lines (TTD) or periodically capacitiveloaded transmission lines distributed MEMS transmission line (DMTL) [1,2]. Reliability issues and failure mechanisms have been discussed in [5,6]. In [1], Stehle et al demonstrated a 3-bit TTD phase shifter with a maximum phase shift of 3608 and an IL of 5.7 dB at 76.5 GHz (63.28/dB).…”