3-D MEMS Design Entry
3-D MEMS Modeling
MEMS devices are inherently three-dimensional, which is why it is essential to have an intuitive 3-D user interface that guides the user through the 3-D design process. The MEMS+ Innovator plug-in makes the MEMS design process clear and understandable.
Innovator includes a standard MEMS component library where each component is represented by a 3-D solid model and a corresponding behavioral model. The user builds the MEMS design by connecting the components. The MEMS component library can be thought of as the equivalent of the BSIM library in the IC design world –although it is proprietary rather than open-source. The library represents basic building blocks, such as rigid and flexible plates, suspensions, electrodes, contact and electrostatic comb drives.
While MEMS and IC design share aspects related to manufacturing, they differ in the impact manufacturing has on their design flows. In particular, the micro-fabrication processes for IC devices are standardized. IC components are fixed within a fabrication process, while MEMS components are not. For instance, a transistor (an IC component) is created out of specific layers implanted and deposited on the silicon substrate during the fabrication process, and these layers cannot be changed by the IC designer; but a mechanical beam component that is part of a MEMS design can be placed on any of several “mechanical” layers, and that layer is a design choice.
In addition, it is sometimes necessary to tailor the fabrication process to a particular MEMS device in order to achieve the design goals for the device. Thus, the fabrication process is an important “free parameter” in MEMS designs that often needs to be refined as the development progresses. The flexibility to change the description of the fabrication process is missing from conventional IC design environments. In addition, behavioral models of electrical IC components cannot be parameterized in terms of the process parameters. In MEMS design, the parameters of the process description can be varied as part of the design; thus, the models must be parameterized with respect to the process parameters.
The MEMS+ environment addresses the specific needs of MEMS designers by providing two builtd-in editors that are used to specify all relevant fab-specific data, the material editor and the process editor:
All material and process step properties (such as Modulus of elasticity, layer thicknesses, side-wall angles etc.) can be defined as absolute values, variables or algebraic equations. A combination of variables and algebraic equations allows for properties to be mutually dependent on other properties, environmental variables (e.g. temperature and humidity) or even entirely abstract variables such as the equipment settings of a given fabrication process.
The MEMS+ design environment gives the provider of the material and process database full control over which variables are to be exposed in the Innovator plug-in. The connection between the MEMS+ library components in Innovator and the fabrication data is established by a common layer property that allows the user to assign a given structure to one of more layer names:
The separation of the materials and process data from the MEMS design and simulation environment allows the model to have process-related parameters whose specification is not fixed, but rather is tied by reference to the manufacturing process data.
Description of Innovator
MEMS+ Innovator is a tool to graphically enter designs using parameterized generators similar to standard layout tool. The main difference is that it is done in 3-D.
There are two main features that make MEMS+ Innovator unique:
- The MEMS designer constructs the MEMS design in a 3-D view (Innovator). This direct creation in a 3-D view is more natural for MEMS designers, who are used to working in a 3-D CAD environment. The resulting 3-D view differs from a typical feature-based 3-D CAD modeling tool in that there is an underlying behavioral model associated with each MEMS component. (Watch this video for a short tutorial)
- The high order finite elements of the complete MEMS design that are shared in the standard EDA environment can be fully parameterized with respect to manufacturing- and design-dependent variables.
- 3-D solid model creation is tied to MEMS manufacturing process
- Built-in, flexible and expandable Material Database and Process Editor
- The material data, process file and Innovator schematics are stored separate files using the standard XML format
- Graphical variable definition and exposure
- Easy setup of mechanical boundary conditions and electrical ports
- Support for script or GUI-driven design entry
- GDSII or 3-D SAT files can be imported into the Innovator canvas for comparison with a MEMS+ model
- Standard output filters for GDSII and 3-D SAT
Customized parameterization of the MEMS models
Customized parameterization means just that: the MEMS designer or MEMS design kit controls which variables, parameters and electrical and mechanical “pins” are exposed to the user of the models or layout cells that are created from a given Innovator schematic.
Simulating the MEMS models
Because MEMS+ is built on a comprehensive library of MEMS building blocks with underlying high order finite elements, MEMS+ Innovator schematics can be automatically converted into simulation models for Simulink or the Cadence design environment.
Creating a MEMS design in MEMS+ Innovator
In MEMS+ Innovator the model library is used to construct the 3-D MEMS design. One of the library components is the “Serpentine”. The figure below illustrates the use of this building block.
Another powerful component of the library is the “RigidPlate”. Here the designer continued to add multi-segment plates to the design with parameterized geometry and etch-holes:
The final design is fully parameterized according to the designer’s specifications and consists of serpentine suspensions, rigid plates, and electrostatic comb structures: