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What is 3D Solid Modeling?
You’ve certainly heard of “3D Solid Modeling”. The term first originated in the verification engineering community and has now become nearly ubiquitous. Mainstream industries like entertainment media, 3D printing and even home improvement design have now adopted the phrase and have made it a symbol of our times. The engineering community has a collective understanding of 3D Solid Modeling which is often different from the mainstream use of the term. 3D Solid Modeling is more than just a virtual representation of an object that looks amazing and can be produced using a 3D printer. For most engineers, 3D Solid Modeling is the gateway to Finite Element Analysis (FEA).
FEA is the numerical analysis of a system that has been broken into a numerous set of small, basic pieces (finite elements) for ease of analysis. Using FEA, an engineer can understand the complexity of a large, complex system by transforming the large system into numerous small pieces (or systems of equations). FEA software can generate relatively simpler equations for each of these smaller pieces, and then solve the complete solution for the larger system by reconstructing the smaller pieces like a puzzle.
The 3D solid model used for FEA must balance the need for geometric fidelity (to the real-world structure) against simulation difficulty and time, since finite element analysis is computationally “expensive”. In order to perform an accurate and yet computationally-reasonable finite element simulation, an engineer needs to carefully “mesh” their 3D solid model – a topic that will be reviewed in a subsequent article.
How to build a 3D Solid Model for FEA… for MEMS Design
Finding a piece of software to generate a solid model is easy. Software that can render 3D virtual representations of objects is readily available. Commercial software vendors offer 3D solid modeling tools designed with the engineering community in mind. These tools allow designers to build 3D geometries from a sketch or construct a complete model from a set of basic building blocks (cubes, spheres, prisms, etc.). These software packages include gloriously rich libraries containing solid modeling features that facilitate the creation of wonderfully complex virtual systems. But are these software packages appropriate for designing a MEMS device?
MEMS devices are not like most real-world structures. MEMS devices display massive aspect ratio differences between the overall size of the structure (on the order of millimeters) and the size of the smallest features (usually on the order of microns). MEMS designs also often include excessively large numbers of geometric parts. These parts are used to model repetitive geometries such as etch release holes or comb rotors and stators making MEMS designs are challenging to recreate for even the most sophisticated 3D solid modeling software engines. Furthermore, MEMS are fabricated like IC chips. MEMS designers must worry about the complexities of lithography resolution, deposition conformality and non-orthogonal sidewall etches – concerns that are unimportant for engineers in other disciplines. So, MEMS designs ideally need specialized 3D modeling and finite element analysis tools.
3D Solid Models from Process and Layout
Assume we have an idea for the next generation MEMS sensor. To efficiently determine the efficacy of our idea, we’ll need to create a 3D solid model to be analyzed using FEA. How should we do this ? An approach that matches the way this sensor will be built leads to a natural solution.
As an example, say we wanted to build a 3D solid model of a commercial MEMS inertial sensor (Figure 1):
Figure 1: Top view of ST Microsystems gyroscope (Courtesy: TechInsights)
MEMS are most frequently conceptualized in layout, using a 2D representation of the 3D structure. An example of a layout file for the MEMS device shown in Figure 1 can be found in Figure 2 (below). The 2D layout file (or layout geometry) is ultimately used to create fabrication masks needed to pattern the thin film deposits during device manufacturing. An extensive amount of time is required to construct this 2D layout file, and we ideally want to leverage this time by using it to generate our 3D solid model. The 2D layout file can generate a 3D model of our MEMS device if we also have a list of the fabrication steps required to “build up” the MEMS device from its initial layout.
Figure 2: Layout representation of ST Gyroscope (reverse engineered using TechInsights teardown)
MEMS are built on wafers, like CMOS chips. No casting, molding, extrusion, or other manufacturing processes common to standard mechanical products are used in MEMS manufacturing. MEMS construction involves thin film surface processes and processes such as bulk micromachining. These processes consist of additive steps used to deposit materials on a substrate (such as a silicon wafer) and subtractive etch steps which pattern the deposits using fabrication masks. Process descriptions resemble recipes, dictating the appropriate deposition thicknesses, photolithography masks, etch depths, and other steps (see Figure 3):
Figure 3: Generic process description
Instead of laboriously regenerating a 2D representation of the MEMS device with standard 3D solid modeling software, a far more efficient solution is to simply use the 2D layout geometry file with a process recipe to directly to create our 3D model. Not only is this solution faster, but it eliminates the possibility for inadvertent translation mistakes, ensuring the highest fidelity match of our 3D model to the actual mask data. The combination of the 2D layout file and the process description yields a 3D virtual representation of the device that will emerge from the fabrication facility (Figure 4).
Figure 4: 3D Solid Model from layout and process description
This 3D Solid Model is the entry point to subsequent finite element analysis and can be used to study the feasibility of our design.
Conclusions
FEA is universally regarded as a necessary component in engineering design, including MEMS design. To conduct efficient and accurate finite element simulation, you need to first create an appropriate 3D solid model. Multiple commercial CAD software systems exist for building 3D models, and these 3D models can be used in subsequent finite element analysis. However, the unique geometry requirements and fabrication techniques employed in both the MEMS and semiconductor industries make adoption of generic solid modeling software difficult, inefficient, and therefore expensive. A 3D solid modeling option that’s aligned with the design and fabrication methodologies used by the MEMS and semiconductor industries is needed. CoventorMP contains software which has been specifically designed for the MEMS and semiconductor industries, and which leverages existing 2D layout files and a description of the manufacturing process to create 3D solid models appropriate for finite element analysis. If you wish to learn more about this topic, please review these additional articles about process-driven design entry (for CoventorWare and MEMS+).
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