Zhongde Wang

Robust Design of RF-MEMS Cantilever Switches Using Contact Physics Modeling

This paper presents the robust design optimization of an RF-MEMS direct contact cantilever switch for minimum actuation voltage and opening time, and maximum power handling capability. The design variables are the length and thickness of the entire cantilever, the widths of the sections of the cantilever, and the dimple size. The actuation voltage is obtained using a 3-D structural-electrostatic finite-element method (FEM) model, and the opening time is obtained using the same FEM model and the experimental model of adhesion at the contact surfaces developed in our previous work. The model accounts for an unpredictable variance in the contact resistance resulting from the micromachining process for the estimation of the power handling. This is achieved by taking the ratio of the root mean square power of the RF current (ldquosignalrdquo) passing through the switch to the contact temperature (ldquonoiserdquo) resulting from the possible range of the contact resistance. The resulting robust optimization problem is solved using a Strength Pareto Evolutionary Algorithm, to obtain design alternatives exhibiting different tradeoffs among the three objectives. The results show that there exists substantial room for improved designs of RF-MEMS direct-contact switches. It also provides a better understanding of the key factors contributing to the performances of RF-MEMS switches. Most importantly, it provides guidance for further improvements of RF-MEMS switches that exploit complex multiphysics phenomena.

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

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 mum 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)

Comparison of semi-analytical formulations and Gaussian-quadrature rules for quasi-static double-surface potential integrals

This paper presents a comparison of a new semi-analytical expression with Gaussian-quadrature formulas for the quasi-static double-surface potential integrals arising in the boundary integral (BI) models of micron-size objects, such as RF-MEMS switches. The integrals considered are the quasi-static Greens functions for the scalar and vector potentials, with constant or linear basis functions over triangular subdomains. The examples given illustrate that the new semi-analytical formulations can achieve significantly higher solution accuracy and are more efficient when compared to the Gaussian-quadrature formulas.

Analysis of RF-MEMS switches using finite element-boundary integration with moment method

This paper presents a new hybrid methodology for modeling RF-MEMS switches. This method combines the usual finite element-boundary integration (FE-BI) method for the fixed section of the switch, and the method of moments for the movable beam. This approach is intended to address the large 100:1 scale variation within a single computational domain, which also spans a very small fraction of a wavelength.

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

We propose an extended finite element-boundary integral method (EFE-BI) to model the electromagnetic (EM) behavior of RF-MEMS switches over a wide frequency range from UHF to terahertz. Our new method integrates EM with finite element heat transfer analysis to extract heat dissipation on the micrometer-scale switch beam due to the non-uniform radio frequency (RF) current distribution. The developed EFE-BI technique is an extension of the standard finite element-boundary integral (FE-BI) method to allow for accurate characterization of RF-MEMS structures whose entire size is a small fraction of a wavelength (λ/250 or less) and may contain dimensions in the order of λ/50 000 or less. Our model predictions exhibit good agreement with experimental results obtained independent of the current study.

Contact physics modeling and optimization design of RF-MEMS cantilever switches

RF MEMS direct-contact switches exhibit many advantages over the conventional semiconductor switches; however, existing drawbacks such as low power handling, high pull-in voltage and long switch opening time are most critical. This paper presents an optimization design for an RF-MEMS cantilever direct-contact switch to achieve maximum power handling capability, minimum pull-in voltage and switch opening time simultaneously. A 2-step optimization technique is proposed to achieve the optimal design to allow for a power handling capability of 130 mW, a pull-in voltage of 52 V, and a switch opening time 4.4 /spl mu/s presented. The optimization results show that substantial room exists for improving the current designs of RF MEMS direct-contact switches.

Skin effect aggregated heating in RF MEMS suspended structures

This paper presents experimental data together with 2 modeling approaches to demonstrate the increased heating of MEMS suspended structures at radio frequencies due to skin effects. Distinguishable average temperature rises are measured at 2, 13.5, and 18 GHz in a 616 /spl mu/m /spl times/ 20 /spl mu/m /spl times/ 2.7 /spl mu/m suspended coplanar waveguide using 4-wire measurement configuration. Our measurements compare well with: (1) previous electromagnetic simulations and (2) a newly introduced analytical thermal model incorporating only skin effects. Buckling and plastic yielding have been observed during and after measurement. This study provides a simple and quantitative approach for the design of suspended structures such as low loss transmission lines, filters and switches with high power handling capability.

Simultaneous Electrical and Thermal Modeling of a Contact-Type RF MEMS Switch

Improving the power handling capability of direct contact RF MEMS switches requires a knowledge of conditions at the contact. This paper models the temperature rise in a direct contact RF MEMS switch, including the effects of electrical and thermal contact resistance. The maximum temperature in the beam is found to depend strongly on the power dissipation at the contact, with almost no contribution from dissipation due to currents in the rest of the switch. Moreover, the maximum temperature is found to exceed the limit for metal softening for a significant range of values of thermal and electrical contact resistance. Since local contact asperity temperature can be hundreds of degrees higher than the bulk material temperature modeled here, these results underscore the importance of understanding and controlling thermal and electrical contact resistance in the switch.

Effects of dimple geometry on RF MEMS switch heating

This paper presents the effects of the size and location of the contact dimples on the heating of RF MEMS cantilever contact switches. The analyses are based on electromagnetic model with the extended finite element-boundary integral (EFE-BI) method, integrated with thermal model with the finite element (FE) method. Simulation results demonstrate that larger dimples result in lower contact temperature, suggesting the higher power delivery capability of the dimples with a large contact area.

A preconditioner for hybrid matrices arising in RF MEMS switch analysis

Despite the excellent characteristics of RF MEMS switches, they generally suffer from low power-handling capability. This limitation is due to the complex interactions among electromagnetic losses, heat transfer, and mechanical deformations associated with the switches. To understand these failure mechanisms, we proposed a multiphysics model (Jensen, B.D. et al., IEEE Microwave and Wireless Components Letters, vol.13, no.9, p.364-66, 2003). This model is based on an extended finite element-boundary integral (EFE-BI) model that allows efficient modeling of the boundary (MEMS beam for our case) exterior to the volumetric region modeled by the standard FE-BI method. The condition number of the resulting EFE-BI matrix system increases rapidly as the frequency decreases. The matrix condition number required at 2 GHz warrants computational accuracy beyond the capability of normal CPUs. For this reason, our EFE-BI analysis and validation of the code were limited to high frequency cases. We propose a preconditioning approach that lowers the condition number of the system. The proposed approach allows for fast, efficient analysis of RF MEMS switches at practical RF frequencies as low as 500 MHz, which will enable the desired multiphysics modeling.