3D MEMS Design Entry
MEMS designs are inherently process dependent and three-dimensional, so MEMS+ includes an intuitive, 3D user interface that makes design entry easy and fast.
The user first enters materials and a process description in separate tabs and then moves to design entry in the Innovator tab, assembling the device by selecting components from the MEMS Component Library and setting the parameters for each component.
While MEMS and IC design share aspects related to manufacturing, they differ in the impact manufacturing has on their design flows. IC components are standardized within a fabrication process while MEMS components are not. For instance, a transistor (an IC component) is created out of specific layers deposited on the silicon substrate during the fabrication process, and these layers cannot be changed by the IC designer; but a mechanical suspension beam 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 often 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 may be changed as development progresses. The flexibility to change the description of the fabrication process is required for MEMS design, but is missing from conventional IC design environments.
The MEMS+ environment addresses the specific needs of MEMS designers by providing two integrated editors for specifying all process-specific data: the material editor and the process editor:
Intuitive Design Assembly
Users assemble a design in Innovator by inserting parameterized components from the MEMS Component Library. This is somewhat analogous to creating a layout by inserting pre-defined parametric layout cells. The main difference is that it is done in 3-D, which is more natural for MEMS designers who are used to working in 3D CAD environments. Unlike 3D CAD environments, there is a sophisticated behavioral model associated with each MEMS component. The component models are automatically connected by a wizard.
The relationship between the 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 design entry environment allows the model to have process-related parameters whose specification is not fixed, but rather is tied by reference to the manufacturing process.
Users can define a hierarchy of symbolic variable names and use those names in place of literal values, or combine them in algebraic expressions, to define any material property, any process parameter (e.g. layer thickness), or any component parameter (e.g. width of a suspension). A combination of variables and algebraic equations allows for properties to be mutually dependent on other properties, environmental variables (e.g. temperature and pressure) or even abstract variables such as the equipment settings for a given fabrication process. This makes MEMS+ models entirely parametric. Users can also control which parameters and electrical and mechanical “pins” are exposed to users of the models in the supported simulation environments.
One Design, Multiple Simulation Options
Design entry does not depend on the target simulation environment; every design is compatible with all supported simulation environments, including the built-in simulator, MATLAB, Simulink and the Cadence Virtuoso environment.
- 3-D design entry is tied to the manufacturing process
- Built-in, parametric material database and process description
- The material data, process file and Innovator schematics are stored in separate text files using the standard XML format
- Graphical variable definition and exposure
- Easy setup of mechanical boundary conditions and electrical ports
- Interactive, graphical design entry, and
- Python scripting interface allows scripted design entry
- GDSII or 3D ACIS SAT files can be imported into Innovator for comparison with a MEMS+ model
- Standard output filters include GDSII layout and ACIS SAT 3D solid models
Example: Entering an accelerometer design
This example shows how library components are used to create a complex accelerometer design. The designer first adds a “Serpentine” component that will be re-used several times to form the suspension for the accelerometer’s proof mass: