Coventor's MEMS Design Blog

MEMSCAP PolyMUMPs MEMS Variable Capacitor Design

Variable Capacitor

The PolyMUMPS varactor is a gap-tuning structure, see image below and schematic below. The design consists of a three parallel-plates with the central one suspended by T-shaped arms between two fixed electrodes. One electrode lies on the silicon substrate coated with nitride (poly 0), the second rigidly suspended above the mobile plate (poly 2). Thus the device consist of two variable capacitors in series. One is used for RF applications (e.g. a VCO) and the other is considered as a parasitic. (The one with the smallest capacitance which means the bottom capacitor in this case).

mems variable capacitor 3d model 

3D Solid model in CoventorWare DESIGNER

mems variable capacitor schematic

Hierarchical schematic in CoventorWare ARCHITECT3D of the Variable Capacitor.
For process related questions a much more detailed 3D model and cross sections are often needed.

Cross section of MEMS variable capacitor

Process accurate 3D model in SEMulator3D

Voltage controlled oscillator

 

A voltage controlled oscillator (VCO) can be constructed from the variable capacitor. The schematic is shown below and is based on a cross-coupled PMOS architecture with 2 LC tank resonators.

MEMS VCO schematic 

VCO with 2 variable MEMS capacitors

Tutorial Overview

The above material and more is covered in a short course and is scheduled for 2 short days and contains 3 exercises. There is an additional excerise, excercise 4 which can be used to extend the course duration
Exercise 1: Model design in Architect
  • Material Property Database review
  • Process view (Design kit)
  • Schematic creation including hierarchical design of the tether arms
  • Scene3D simulation result visualization
  • Layout export
Exercise 2: Varactor simulation in Architect
  • Mechanical Resonance Frequency analysis (including a vary loop)
  • DC transfer
  • Transient analysis
  • Adding an electronic circuit to create a Voltage Controlled Oscillator (VCO)
  • Initiate oscillation (holding specific nodes)
  • AC analysis to see frequency change with bottom electrode voltage* *
Exercise 3: FEA in Analyser : Considering the top electrode as mobile (fixed on 2 opposite sides only).
  • Process modification
  • 3D solid generation
  • Meshing with partition of arms
  • Initial position of middle and top plates in MemMech due to residual stresses (including tiedlink automatically created)
  • Capacitance calculation in MemElectro on the deformed shape obtained in MemMech
Exercise 4 additional FEA in Analyzer:
  • Memmech mechanical+modal 6 frequencies
  • MemElectro as above
  • CoSolveEM with detect pull-in on the bottom electrode

Additional resources

  1. Dec and Suyama, 1998, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 46, NO. 12, DECEMBER 1998, Micromachined Electro-Mechanically Tunable Capacitors and Their Applications to RF IC's
  2. PolyMUMPs Design Handbook release 11.0
  3. Dec and Suyama, 2000, "Microwave MEMS-based Voltage-Controlled Oscillators", IEEE Transactions on Microwave Theory and Techniques, vol 48, no 11
  4. To find CoventorWare manuals please go to: Start >All Programs >Coventor >Coventorware2008 >documentation or browse to ...\Coventor\CoventorWare2008\docs

 

This entry was posted in mems simulation, variable capacitor, mems oscillator, mems design