RF Switch and Variable Capacitor Design and Simulation

In the past decade, RF MEMS switches have found low-volume niche applications in the aerospace, defense, telecoms infrastructure and RF ATE markets. Now there is increasing interest in RF MEMS for the high-volume mobile device market, as an innovative solution to solving the RF design challenges that currently limit the functionality and performance of today’s radio technologies. For example, RF MEMS devices offer a means to implement low-attenuation antenna tuning for different base bands and tunable power amplifiers. RF-MEMS switches and relays offer wide bandwidth, low insertion loss, excellent isolation and outstanding linearity.

RF MEMS can be manufactured using standard semiconductor processing equipment, so offering the possibility of a high-volume, low-cost integration via System on Chip, MEMS on CMOS or Integrated Passive Devices (IPD) technologies. Yole Development predicts that the RF MEMS Switch / Variable Capacitor Market for mobile devices will grow to over $100M in 2015, as shown in Figure 1.

Figure 1 2012 – 2018 RF MEMS Switch / Variable Capacitor Market (Yole Development)

Figure 1 2012 – 2018 RF MEMS Switch / Variable Capacitor Market (Yole Development)

Design Challenges

RF MEMS switches and variable capacitor designs typically make use of the so-called “pull-in” instability to achieve low-power actuation and latching. The high degree of non-linearity combined with mechanical contact and manufacturing effects such as prestress and stress gradients make it especially challenging to achieve high-yield, high-reliability designs. Consequently, these devices have been brought to market only after years of costly design-fabricate-and-test cycles. Fast, accurate simulations of these devices in advance of fabrication can eliminate some of the time-consuming design spins.

On one hand, the complex physics and transient behavior in these devices cannot be accurately modeled with analytic formulae. On the other hand, it is very difficult to simulate the opening and closing transients with conventional volume-based finite element tools because of the dramatic change in the air gap between the open and closed states of the device, which requires sophisticated mesh morphing or re-meshing at each iteration of the solver. Static simulations of the switch can take many hours of computing time, while transient simulations are nearly infeasible. This severely limits the value of conventional finite element tools for design exploration and optimization. MEMS+ and CoventorWare address these challenges, and together provide a comprehensive platform for designing RF switches and varactors.

Figure 2 Coventor’s Novel Hybrid-Simulation MEMS RF Switch and Varactor Design Platform

Figure 2 Coventor’s Novel Hybrid-Simulation MEMS RF Switch and Varactor Design Platform

A New Hybrid Design Methodology

Rapid Design Exploration with High-Order Finite Elements

Coventor’s hybrid design methodology uses proprietary MEMS-specific, high-order finite elements as a central core to the design flow. Using MEMS+, designers can easily construct a 3D device from a handful of high-order elements, as shown in the RF Switch model in Figure 3. After model construction, the full range of coupled complex multi-physics can be simulated in MEMS+ itself, MATLAB, Simulink or Cadence. Results are achieved in a time that is by orders of magnitude faster than conventional finite element simulations.

Figure 3 IHP Nanotech RF Switch quarter model in MEMS+ showing mechanical elements. The model also contains electrostatic and fluid elements, for clarity these are not illustrated [1].

Figure 3 IHP Nanotech RF Switch quarter model in MEMS+ showing mechanical elements. The model also contains electrostatic and fluid elements, for clarity these are not illustrated [1].

The accuracy and speed of MEMS+ simulations make it feasible to perform the large number of simulations needed to fully explore the design space, optimize the design, and investigate sensitivity to process variations. For example, the pull-in, release and meta-stable states of a switch or varactor are very sensitive to process variations and are critical to achieving desired performance and yield. Understanding this sensitivity by varying many parameters in a conventional traditional finite element approach could take weeks of simulation. With the MEMS+, these simulations can be completed within a matter of hours, with each point simulation taking only a few minutes.

RF switches also exhibit complex transient behavior, such as contact bouncing and contact stiction. These phenomena, together with contact forces and resistance, must be accurately modeled for devices fabricated with multi-layer thin films and design details such as stand-off dimples. MEMS+ models are fully capable of addressing these challenges.

Verification And Detailed Analysis with FEM/BEM Field Solvers

The MEMS+ design flow does not preclude the use of conventional finite element analysis. MEMS+ can export designs in widely used 2D and 3D formats, for further analysis in other tools. CoventorWare Designer, for example, can directly import MEMS+ models and generate a mesh for CoventorWare Analyzer. Here, specific simulations can be undertaken to study package deformation effects, stress concentrations and verify various aspects of the MEMS+ models.

Figure 4 CoventorWare Analyzer for Low Order Element Design and Verification

Figure 4: CoventorWare Analyzer for Detailed Design and Verification

MEMS + IC Circuit and System Simulation

RF switches and varactors are often combined in arrays and must be integrated with control electronics. Co-simulation of the MEMS devices with the electronics is required to determine the overall performance and assure that the final product meets design specs. Unlike conventional finite element models, MEMS+ models can be easily included in Simulink flow diagrams and Cadence schematics. And unlike hand-crafted behavioral models, MEMS+ models accurately capture the complex physics of switches and varactors that may have a significant impact on the electronics design.

Figure 5 Voltage Controlled Oscillator implemented in Cadence using MEMS+ Varactor model based on a Two-Parallel-Plate Tunable Capacitor Configuration [2]

Figure 5 Voltage Controlled Oscillator implemented in Cadence using MEMS+ Varactor model based on a Two-Parallel-Plate Tunable Capacitor Configuration [2]

A Complete Platform for RF Switch and Variable Capacitor Design

MEMS+ is a unique environment for quickly developing RF MEMS products, including ohmic switches and varactors, and their associated control circuits. MEMS+ simulation models are parametric and accurately capture the complex physics of the RF MEMS switches and varactors while being sufficiently computationally efficient to allow simulation of the MEMS and IC together, with reasonable CPU time. The parameters in MEMS+ models may include manufacturing variables such as material properties and pre-stress, as well as geometric properties of the design. The sophistication and accuracy of these models enable optimization of the MEMS and IC design, both for performance and yield. CoventorWare’s field solvers complement MEMS+ models, and can be used to investigate design details and verify the accuracy of MEMS+ models.

References

  • A. Mehdaoui, S. Rouvillois, G. Schröpfer, G. Lorenz, M. Kaynak, M. Wietstruck. “Residual Stress and Switching Transient Studies for BiCMOS Embedded RF MEMS Switch Using Advanced Electro-Mechanical Models.” 14th International Symposium on RF MEMS and RF Microsystems (MEMSWAVE 2013), Germany, July 2013.
  • Jun Zou, Chang Liu, Jose E. Schutt-Aine, “Development of a wide-tuning range two-parallel-plate tunable capacitor for integrated wireless communication systems”, Int. J. RF Microwave Computed Aided Eng., Vol.11, No. 5, pp. 322-329, Sept. 2001.

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