Page 56 - MEMS Mechanical Sensors

3.2 Simulation and Design Tools 45

Figure 3.4 Layout of a micromachined accelerometer.

Once the layout and fabrication process flow have been generated, the Solid
Model tool is used to build a three-dimensional model using the two-dimensional
layout geometry from the mask files and the deposit/etch and thickness information
from the fabrication process file.
The next step performs the finite element model creation and meshing of the
device. The meshing tool creates a three-dimensional mesh based on the model
created by the Solid Model tool and the process file created by the process editor.
The user can choose between various mesh elements such as tetrahedral, bricks, and
hexahedral according to the device geometry. The result is stored in a file containing
all model input and output parameters such as the geometry and material proper-
ties, conductor and dielectric types, and mesh information used by the various simu-
lation solvers.
Analyzer is the core of CoventorWare, consisting of various solvers such as
MemCap, MemMech, CoSolveEM, SimMan (Simulation Manager), and some spe-
cialized solvers, which are briefly described next. All use finite element and bound-
ary element techniques for solving the differential equations for each physical
domain.
MemCap is the electrostatic solver that computes a charge matrix based on
voltage conditions or a voltage matrix based on charge conditions for the MEMS
design under investigation. Secondary effects such as fringe capacitances and the
influence of a lossy media on the electric behavior can be simulated as well.
MemMech is the mechanical solver that analyses structural, displacement,
modal, harmonic, stress, and contact steady-state thermomechanical properties.
Figure 3.5 shows the modal analysis of the accelerometer shown in Figure 3.3,
which computes and visualizes the natural frequencies of the mechanical structure.
CoSolveEM is a coupled electromechanical solver that combines the electro-
static and mechanical solution. It also can perform pull-in voltage and hysteresis
analysis efficiently in one sweep.