MEMS Pop-Up Gear-driven Mirror
Coventor offers a comprehensive suite of software tools for designing MEMS devices. CoventorMP provides MEMS design automation capabilities, while SEMulator3D is used to build 3D process models of complex MEMS structures.
This example highlights the capabilities of SEMulator3D in designing a MEMS-based Pop-Up Gear-Driven Mirror. The mirror was constructed using the Sandia Ultra-Planar Multi-Level MEMS Technology 5 (SUMMiT VTM) fabrication process. A SEM micrograph of the actual pop-up mirror is shown in Figure 1 (below).
Figure 1: SEM image of pop-up mirror with part of the gear drive assembly. Courtesy of the Sandia National Laboratories.
SUMMiT V fabrication involves a five-layer polycrystalline silicon surface micromachining process using one metal layer (interconnects/ground plane) and four mechanical layers. This mirror device uses all five polysilicon layers.
SEMulator3D can construct highly predictive and accurate 3D process models that reflect the complex interactions between designs and integrated process flows. The 3D process model is built using a series of unit process steps (some requiring masks) to produce a highly accurate “virtual” 3D structure.
The SEMulator3D Process Editor was used to input the fabrication sequence of the gear-driven mirror using a standard library of process steps. Mask design and process information were obtained courtesy of Sandia National Labs. A 3D animation of the fabrication process derived from the process steps is illustrated in Figure 2. SEMulator3D considers the 2D geometry dimensions, materials used and process flow information when automatically building the model and animation sequence.
Figure 2: Animation of the fabrication sequence in SEMulator3D
Pre and post release visualization of on-chip MEMS structures can be accomplished in SEMulator3D by positioning a cross-sectional cut across the device. This capability can identify design errors that would prevent structure release or unintended motion restriction. Cross-section cuts of the gear-driven mirror, pre and post release, can be seen in Figures 3 and 4 (below).
Figure 3: SEMulator3D cross-section view before release
Figure 4: Gear drive assembly after release
In addition to visualizing step-by-step construction of the MEMS device, SEMulator3D provides the MEMS designer with a unique perspective for predicting defects in geometry. It offers the flexibility to run a 3D design rule check (DRC) on the model geometry, rather than using just standard 2D DRC. Layer thicknesses can be automatically extracted at any location in the model. Volume, surface area, and electrical connectivity data can be verified after device construction.
Figure 5: SEMulator3D animation of the pop-up mirror device using motion paths. Motion paths are used to program the motion of mechanical components in order to visualize their movements.
SEMulator3D geometry can also be modeled with surface and volume meshes and exported to industry-standard field-solvers. SEMulator3D uses two sophisticated modeling methods: Voxel Modeling, a fast, robust digital approach, and Surface Evolution, an analog approach capable of modeling a wide range of physical process behavior with great accuracy. SEMulator3D is able to discretize the voxel model with mesh elements, to generate simulation-quality meshes. Both triangle surface and tetrahedral volume meshes can be exported from SEMulator3D to FEA modeling software such as CoventorWare. Figure 6 displays an exported mesh of the gear-drive assembly from SEMulator3D.
Figure 6: Mesh generated on the gear drive assembly
SEMulator3D is able to generate highly accurate models of a MEMS device based upon the actual fabrication process, rather than the idealized geometry customarily used in traditional finite element analysis (FEA). The geometric fidelity of the SEMulator3D model greatly improves FEA simulation accuracy.
References:
http://mems.sandia.gov/gallery/images_mirrors.html
http://www.mems.sandia.gov/tech-info/summit-v.html