MEMS RF Switch and Variable Capacitor Design and Simulation

In the past decade, RF MEMS switches have been used in low-volume applications within the aerospace, defense, telecommunications and RF automated test equipment markets. There is now an increasing interest in using RF MEMS switches within the high-volume mobile device market, to overcome RF design challenges and improve the functionality and performance of today’s radio technologies. RF MEMS devices can enable low-attenuation antenna tuning for different base bands and tunable power amplifiers. RF-MEMS switches and relays also provide wide bandwidth, low insertion loss, excellent isolation and outstanding linearity.

RF MEMS can be manufactured using standard semiconductor processing equipment, and offer the possibility of high-volume, low-cost integration with surrounding circuitry via System on Chip (SoC), MEMS on CMOS or Integrated Passive Devices (IPD) technologies.

Design Challenges

Electrostatically actuated RF MEMS switches and variable capacitors use “pull-in” instability to achieve low-power actuation and latching. The high degree of non-linearity, coupled with mechanical contact and manufacturing effects such as thin-film stress gradients, make it especially challenging to create a high-yielding and reliable design. Consequently, many of these devices have only been brought to market after years of costly silicon learning cycles. Fast, accurate simulation of these devices, completed prior to fabrication, can eliminate many of these time-consuming design cycles.

Modeling the behavior of these non-linear RF MEMS components is not simple.  The complex physics and transient behavior of these devices cannot be accurately modeled with analytic formulae. Unfortunately, it is also very difficult to simulate the opening and closing transients of these devices using conventional volume-based finite element tools, due to the dramatic change in the air gap between the open and closed states of the device.   Any finite element analysis (FEA) simulation requires sophisticated mesh morphing or re-meshing at each iteration of the solver. Static simulations of the switch in an FEA tool can take many hours of computing time, while transient simulations are nearly infeasible. This severely restricts 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.

Rapid Design Exploration and Optimization with MEMS+

Using MEMS+, designers can easily construct a 3D device model from a handful of high-order elements (such as Bernouilli beams), as shown below. After model construction, the full range of coupled, complex multi-physics can be simulated in MEMS+ itself, or in conjunction with the MATLAB^®, Simulink^® or Cadence^® design environments. Results are achieved orders of magnitude faster than using conventional finite element simulations.

Due to its speed and accuracy, MEMS+ allows a designer to perform a large number of simulations in a reasonable timeframe and fully explore the design space.   This deep analysis can be used to optimize the design and investigate sensitivity to process variations. For example, the pull-in, release and meta-stable states of a MEMS RF switch (or varactor) are very sensitive to process variations.   Understanding the effect of these variations is critical to achieving desired performance and yield. To understand this sensitivity, a designer needs to vary many parameters during finite element modeling.   This approach could take weeks of simulation time using conventional finite element tools. With MEMS+, these simulations can be completed within a matter of hours, with each individual simulation requiring only a few minutes to execute.

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 and include support for designs with multi-layer thin films and stand-off dimples. MEMS+ models are fully capable of addressing these challenges.

Verification And Detailed Analysis with CoventorWare

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 can directly import MEMS+ models, and generate a mesh for the CoventorWare Analyzer software. Specific simulations can then be undertaken to verify quasi-static results such as pull-in and lift-off voltage, or investigate additional design results such as stress concentrations.

MEMS+IC Circuit and System Simulation

RF switches and varactors are often combined in arrays and must be integrated with their surrounding control electronics. Co-simulation of the MEMS devices with the electronics is required to determine overall performance and to ensure that the final product meets design specifications. Unlike conventional finite element models, MEMS+ models can easily be included in Simulink flow diagrams and Cadence circuit design schematics. Unlike hand-crafted models, MEMS+ models accurately capture the complex physics of switches and varactors.  Understanding this physics is critical, since it has a significant impact on the final electronics design.

A Complete Platform for RF Switch and Varactor Design

MEMS+ is a unique environment for quickly developing RF MEMS products, including ohmic switches,  varactors and their associated control circuitry. MEMS+ simulation models are parametric, and accurately capture the complex physics of the RF MEMS devices in a computationally efficient manner.  This computational efficiency enables fast simulation of the MEMS design within its control circuit. MEMS+ models can be explored parametrically by varying manufacturing specifications such as material properties and thin-film stress gradients, as well as geometric properties of the design. The sophistication and accuracy of the MEMS+ models enable co-optimization of the RF MEMS and IC design for optimal performance and yield. CoventorWare’s field solvers complement the MEMS+ models, and can be used to verify design details along with the MEMS+ model results.

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.