Verilog-A Large-Signal RF MEMS Model Library

DOCUMENTATION DOWNLOAD SCREENSHOTS SUPPORT

This SourceForge project hosts an open-source Verilog-A [1,2] library of large-signal RF MEMS capacitor, resonator and switch models [3,4]. The electrostatically-actuated RF MEMS capacitor and switch models are based on a nonlinear damped mass spring system and a nonlinear capacitive transducer, whereas the electrostatically-actuated RF MEMS resonator model is based on a nonlinear damped mass-spring system and two nonlinear capacitive transducers. The reasons for nonlinear behavior are:

• The contact force consists of the attractive van der Waals force and the repulsive nuclear contact force, which are nonlinear with beam displacement.
• The damping coefficient which takes into account the anchor loss and the squeeze-film damping at atmospheric pressure, is nonlinear with beam displacement. Squeeze-film damping involves gas dynamics.
• The electrostatic force is nonlinear time-varying with beam displacement and drive voltage. Electrostatic force strengthening allows the bias voltage of RF MEMS switches and switched capacitors to be reduced with beam displacement. Electrostatic force strengthening in vibrating RF MEMS resonators leads to noise aliasing.
• The spring force is nonlinear with beam displacement, although it is often linearized (Hooke's law of elasticity). Taking into account the cubic spring force coefficient, k₃ [N/m³], is necessary to model spring-hardening and spring-softening of Duffing resonators.

The Verilog-A models can be used with SPICE solvers for DC, small-signal (AC, noise, S-parameters (SP)) and large-signal (harmonic balance (HB), periodic steady-state (PSS), quasi-periodic steady-state (QPSS), transient) simulation of analog/RF circuits based on RF MEMS components. Some remarks on simulation:

• Large-signal simulation (1 dB compression point, third order intercept) of high-Q resonator based analog/RF circuits: While transient analysis is feasible, it requires a large number of time steps to reach the steady-state regime. It is therefore recommended to use the HB method, which directly converges to the steady-state solution, with a limited number of harmonics.
• Noise simulation: Supply-voltage noise should be generated by the test bench.
• Power consumption simulation: At rest, electrostatically-actuated RF MEMS capacitors and switches exhibit no static (DC) power consumption. While switching, electrostatically-actuated RF MEMS capacitors and switches do exhibit dynamic (AC) power consumption (10-100 nJ per switching cycle). The dynamic power consumption is due to current transients, which (dis)charge the capacitive transducer. It is therefore recommended to perform a transient simulation.
• Switching time simulation of switch based analog/RF circuits: RF MEMS switches are slow compared to III-V compound semiconductor and silicon semiconductor switches. It is therefore recommended to perform an envelope simulation.

The Verilog-A models allow for specification of beam dimensions and material properties (Poisson ratio, Young's modulus, etc...). Examples of schematics based on gallium nitride (GaN), polysilicon, silicon carbide (SiC) fixed-fixed beam RF MEMS resonators and oscillators, as well as golden capacitive fixed-fixed beam and ohmic cantilever RF MEMS switches, are available upon request. A user guide is being written.

Coming soon

• Inclusion of non-electromechanical multiphysics effects, such as acoustoelectric, electrothermal, piezoelectric, pyroelectric and thermomechanical effects. I will probably start with implementation of electrothermal and thermomechanical effects, because it allows analysis of heat handling, which is of importance in the heterogeneous integration with GaN high electron mobility transistor (HEMT) technology. It also allows analysis of infrared (IR) detection properties.
• Verilog-A models of comb-drive, disc, free-free beam and ring RF MEMS resonators
• Verilog-AMS models of analog mixed-signal circuits based on RF MEMS components, such as a digital phase locked loop (DPLL) based on a phase frequency detector (PFD) with charge pump (CP) output, and Δ/Σ fractional-N frequency synthesizers

Examples

Requirements

A SPICE solver supporting Verilog-A/Verilog-AMS, such as Agilent ADS, Cadence SpectreRF, or Synopsys HSPICE, is required. Some remarks on installing the Verilog-A models:

• 1-DOF Models:
  • Agilent ADS (see [5])
    • Copy the files with ael extension into the networks subdirectory of the ADS project.
    • If necessary, make a veriloga subdirectory in the ADS project.
    • Copy the files with va extension into the veriloga subdirectory of the ADS project.
    • Close and reopen the ADS project.
    • Create a new design.
    • Insert a random component and swap it with OHMIC_CANTILEVER_RF_MEMS_SWITCH or RF_MEMS_CAPACITOR.
  • Cadence (see [6])
    • CIW: File → New → Cellview...
    • Create New File: Cell Name: OHMIC_CANTILEVER_RF_MEMS_SWITCH or RF_MEMS_CAPACITOR, View Name: veriloga, Tool: VerilogA-Editor. Click "OK".
    • The editor will appear. Copy/paste the Verilog-A code into the editor. Save it and exit the editor.
    • A dialog box will appear and ask you if you want to create a new symbol. Click "Yes".
    • The Symbol Generation Options window will appear. Make appropriate changes and click "OK".
    • Create a new schematic and insert an instance of the OHMIC_CANTILEVER_RF_MEMS_SWITCH component or the RF_MEMS_CAPACITOR component.
  • I hope the Verilog-A models will be incorporated in the next release.
• Multiple DOF Models (by Dr. J. Iannacci):
  • Cadence (see [6])
    • The RF MEMS compact model library is treated by Cadence as a standard design kit. This means it has to be copied in a proper location of the file system which we suppose to be /users/OPUS.
    • If we now name the two libraries as MEMS_MODELER_CONTAINER_VerilogA and MEMS_MODELER_CONTAINER_SpectreHDL, the components within each of them will be seen as subdirectories of the two following paths: /users/OPUS/MEMS_MODELER_CONTAINER_VerilogA and /users/OPUS/MEMS_MODELER_CONTAINER_SpectreHDL.
    • The previous absolute paths must then be added to the Library Path Editor within Cadence in order to make the two libraries visible and usable within Cadence together with the other pre-existing libraries and design kits.
    • The new configuration must be saved in the cds.lib file.
    • The RF MEMS compact models deal with mixed domain magnitudes, i.e. electrical and mechanical. Proper definitions for the electrical and mechanical magnitudes are declared within the libraries. Moreover, for each of them two parameters (abstol and blowup) used by the Spectre simulator to define convergence and non-convergence cases are defined for all the magnitudes.
    • In the VerilogA library the implementation definitions are included in the file /users/OPUS/MEMS_MODELER_CONTAINER_VerilogA/discipline/discipline.h and no special settings are required to properly link such a file.
    • In the SpectreHDL the implementation information are defined within the file /users/OPUS/MEMS_MODELER_CONTAINER_SpectreHDL/quantity.spectre. This file must be linked to make it visible during simulations. To do this a line containing the information on where the file is located must be added to the cds.lib file (absolute path: /users/OPUS/ cds.lib) before the lines referring to the actual libraries and design kits. In this case the line has to be as follows: DEFINE MEMS_MODELER_CONTAINER_SpectreHDL /users/OPUS/MEMS_MODELER_CONTAINER_SpectreHDL.

Links

[1] Verilog-AMS Language Reference Manual, Version 2.4, Accellera Organization, Inc.

[2] Hidden State in SpectreRF, Version 1c, May 27, 2003, Ken Kundert, Designer's Guide Consulting, Inc.

[3] Modeling RF MEMS Devices, Koen Van Caekenberghe, IEEE Microwave Magazine, Volume 13, Issue 1, January - February 2012 Page(s): 83 - 110.

[4] Mixed-Domain Fast Simulation of RF and Microwave MEMS-based Complex Networks within Standard IC Development Frameworks, Jacopo Iannacci, Fondazione Bruno Kessler - FBK, MemSRaD Research Unit, Italy

[5] Using Verilog-A in Advanced Design System

[6] Cadence Mixed-Signal Tutorial