Digital Light Processing (DLP) Mirror
Coventor offers a comprehensive suite of software tools for designing integrated MEMS and IC devices. SEMulator3D is a Coventor software platform that is used to build 3D process models of complex MEMS structures and CMOS circuits, and to visualize the electrical connectivity of the modeled device.
One of the best known examples of a monolithically integrated CMOS circuit and MEMS device is the Texas Instruments Digital MicroMirror Device. The TI Digital MicroMirror is comprised of a large number of microscale MEMS-based digital light switches. Each switch controls one pixel of light. The switches are rotated using electrostatic attraction, and are controlled by an underlying SRAM cell. A virtual 3D representation of a single pixel, constructed using SEMulator3D, is shown in Figure 1.
Figure 1: Isometric and cross-section view of single pixel model created with SEMulator3D
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. In Figures 2 and 3, the single pixel micromirror model built in SEMulator3D is compared to an actual SEM photo of a similar device. The actual SEM photomicrographs (courtesy of Texas Instruments) can be seen on the left, with the equivalent SEMulator3D model on the right.
Figure 2: Ion Mill and SEMulator3D model cross section
Figure 3: SEM of DMD with mirror removed and SEMulator3D model
Each digital micromirror is addressed (or controlled) by an SRAM memory cell. SEMulator3D can accurately model both the underlying CMOS memory circuit as well as the MEMS device integrated above it (see Figure 4, below).
Figure 4: Example of 6T SRAM CMOS model before the micromirror is built. (Top left) Exploded view of DLP mirror and memory cell. (Bottom left) 6T SRAM. (Top right) 3D model of circuitry. (Bottom right) Top view of MEMS DLP 3D model.
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 5: SEMulator3D voxel model and mesh model
Advanced options in SEMulator3D allow for meshes to be refined in areas where a denser mesh is required.
Figure 6: Meshed model with a much finer mesh on the hinge element, wherein the maximum stresses are expected to develop.
SEMulator3D meshes can be used with the FEA tool of choice. Surface meshes are exported in .ans, .dxf, .stl and .obj formats with volume meshes exported in .unv and .ans formats. Figure 7 displays a .unv mesh imported into the CoventorWare Preprocessor.
Figure 7: Discrete model from SEMulator3D shown in the CoventorWare Preprocessor. The model has been rendered to reflect separate conductor regions, supporting additional finite element analysis in the CoSolve module of CoventorWare.
Once the discrete model has been imported, surfaces, parts, conductors, and other features can be identified and named. Boundary conditions can then be assigned as part of an electrostatic-mechanics simulation.
Figure 8: Face selection shows that model edges are well defined and correctly identified in the CoventorWare Preprocessor.
A CoventorWare CoSolveEM voltage trajectory analysis can predict the mechanical response of the micromirror under electrostatic actuation, including the calculated deflection until contact is achieved (Figure 9) and the angle of rotation as a function of applied voltage (Figure 10).
Figure 9: Calculated mirror displacement under electrostatic actuation
Figure 10: Projected Angle of Rotation as a function of applied voltage
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. Mechanical analysis, like predicting regions of micromirror stress concentrations, can be accurately and quickly completed using realistic MEMS device geometry generated within SEMulator3D.