Advanced CAD System for Electromagnetic MEMS
Interactive Analysis (Academia)

Contractor: Stanford University
Agreement Number: F30602-96-2-0308
Project Number: HJ1500-3221-0599
DARPA Order Number: N-7-0504/AO E117/12
Contract Period: September 13, 1996 - September 12, 1999
Sponsor: Department of the Air Force
Technical POC: Robert W. Dutton

1. Research Status

TASK 1. SOFTWARE DEVELOPMENT (Prof. Robert Dutton)

1. Develop and demonstrate fully-integrated, FEM-based solver capabilities to model structural and process dependent behaviors of MEMS devices. This will include layered materials, stress dependences and other process induced/created factors.
2. Develop application-specific lumped models and parameter extraction schemes for steering design optimization and level of details in physical models.

a) enhanced gridding capabilities compatible with VLSI TCAD.
b) object-oriented design of layered access to model definition that allows fast prototyping of new and realistic physical and constitutive models.
c) enhanced element technology for discretization of multi-physics system of equations.
d) fully-integrated MEMS fabrication and device simulation with detailed material and stress analyses monitored by the test MEMS structure discussed below.
e) development of lumped models for specific MEMS applications of interest to DARPA.

3) PROGRESS:

a) 3D tetrahedra mesh for the microswitch has been generated from Stanford's TCAD gridder (CAMINO) based on the oct-tree algorithm. The geometry has been parametrized to investigate performance tradeoffs. Presently the geometry is described by boundary representation, and ongoing discussion has been started on geometry representation for composite CAD (tiger team).
b) Two dial-an-operator PDE solvers initially designed for semiconductor process simulation (ALAMODE from Stanford and PROPHET from Lucent) had been connected to CAMINO. Efforts on prototyped capabilities for the electrostatic and isotropic-elastic models for MEMS devices have started.
c) A unified grid representation for the multi-physics systems is considered to avoid manual grid generation and error (or even instability) from interpolation. Several techniques for less stiff mechanical systems on tetrahedral elements have been investigated.
d) The residual stress from fabrication modeled by conventional TCAD (Stanford's SUPREM for oxidation and SPEEDIE for etching and deposition) can be transferred to the PDE solvers in (b) by the SWR (Semiconductor Wafer Representation) field server. Schemes for tracking boundary movement and stress evolution simultaneously are under investigation.
e) A two-lump compact model for the MEMS microswitch has been constructed that can account for the effects of inertia, viscoelasticity, the actuating circuit and residual stress. Parameter extraction from electrical measurements is described. Device characteristics (both static and dynamic) and geometrical scaling are successfully modeled. Design tradeoffs between actuating voltage and switching time are clearly identified.

TASK 2. CHARACTERIZATION OF MEMS MATERIAL MODELS (Prof. John Bravman)

Develop extraction schemes of materials properties using in-situ test MEMS structures, in combination with the CAD tools developed above, to support predictive modeling. The scope of material characterization includes the use of multilayer and graded structures reliability issues such as fracture and fatigue. This closed-loop process of test structures, simulation and extraction/validation of models is the key link between groups in CAD/Material Science/MEMS developers.

2) APPROACH:

a) mechanical characterization of MEMS materials with dependencies on processing and microstructure.
b) reliability study including fracture and fatigue.
c) feedback to the constitutive models in the FEM-based solver in Task 1.

3) PROGRESS:

a) reliability subtask regarding fatigue has been studied and determined that interaction of the constitutive pieces required results of other task (this subtask now on hold).
b) test structure work:

• grain structure impact on constitutive formulation identified as one key area of productive overlap
• joint effort on progress related to the new test structure described below in Task 3.
• a new method for extraction of Poisson's ratio for thin films has been developed and is being tested.

TASK 3. MEMS DEVICE MODELING AND DESIGN (Prof. Greg Kovacs)

Design and implementation of MEMS structures for in-situ materials parameter extraction mentioned above, which will be applicable to MEMS devices of interest to DARPA. The experimental measurements made with these structures will facilitate the critical evaluation of models, parameters and overall simulation accuracy for MEMS devices. The geometry and process flow design will support not only the application side (such as micromachined RF switches) but also the underlying need to quantify and understand MEMS device limits resulting from materials and process dependences.

2) APPROACH:

a) design and implementation of in-situ test structures.
b) design and implementation of example prototype MEMS device.
c) measurement, parameter extraction, modeling and testing.

3) PROGRESS:

a) TI interaction (MAFET) provides:

• test structures to validate the compact model for the RF switch described in Task 1.
• procurement of electron gun (equipment) to physically alter metal film structure.

• new structure conceived for calibration of physical (and CAD) parameters
• improved understanding of the effects of thin film materials parameters on device characteristics (including understanding of constitutive formulation) under investigation (jointly with Bravman's group).

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