Reduce MEMS design methods

Given that MEMS processes are derived from photolithographic microelectronics processes, it is natural to consider using IC design tools to create masks for MEMS devices. However, there are fundamental differences between IC design and MEMS design, from layout characteristics, verification or simulation types, to the most important construction issues.

　　In MEMS design, the difference in object size in magnitude leads to a common mistake. Two objects may appear connected on the display, but in reality they may be separated by grid points. This error can be costly if not discovered during tapeout. DRCs that can handle curves and polygons can avoid these mistakes.

　　Using IC CAD tools raises some interesting issues when a MEMS design contains a large number of curved objects.

　　Curve objects must be constructed individually and placed on a grid finer than the IC manufacturing process to ensure mechanical "smoothness." In addition, IC layout tools must now handle polygons with thousands of points (which are generated by discrete curve objects).

　　Figure 2: Devices such as electromagnetic actuators require

　　A layout tool that can draw precise curves.

　　Managing this type of data can slow down drawing operations. Handling curves and polygons requires appropriate drawing instructions and precise geometric positioning tools to accurately capture midpoints, radii, and angles. In the IC world, accurately locating a center point or a specific distance to a logic gate may not be as important as in MEMS design. For example, a MEMS designer may need to place a resistor precisely at the center of a curved beam element for mechanical or inertial purposes.

　　Additionally, IC tools may not be able to construct complex curves or objects parametrically, forcing the designer to enter detailed x, y coordinates to draw an approximate version of the same device.

　　Low-cost IC tools usually do not provide algorithm-to-layout generation capabilities, so designers may have to use programs such as Matlab or Excel to create x, y coordinate points and then enter them into the CAD tool. Therefore, these objects are "static" and cannot be edited seamlessly.

　　Unlike the mature IC manufacturing field, MEMS designers must consider the process and the mechanical physics of the device from a very early stage. For example, in MEMS electromagnetic actuators, three-dimensional (3-D) coils are usually difficult to manufacture. In fact, the two-dimensional (2-D) nature of the MEMS manufacturing process often limits the best optimization of the electromechanical design. Therefore, designers have to use novel layering techniques and fixed process trade-offs to achieve the right magnetic field.

　　Closely related to manufacturing are process features and artifacts. Designers can make some corrections and compensations to them directly in CAD tools. MEMS prepares mask data during the layout design stage. In the L-Edit layout editor, all operations on complex polygons (such as arcs, circles and similar graphics) can be completed in a simple step. In some CAD tools, this avoids the time-consuming operation of copying, pasting and scaling each object one by one.

　　MEMS designers should pay special attention to large polygons because when polygonizing curved objects to meet GDSII requirements, the resulting polygons often have a large number of points. Mask companies generally only allow 200 points per polygon. L-Edit can detect these polygons and automatically decompose them into smaller polygons on the GDSII output stream.

　　MEMS processes are often proprietary intellectual property to a company and are unique in terms of material selection, layer order, etc. In fact, there are only a few standard processes. As a result, layout verification is difficult, and startup MEMS companies rarely have the ability to perform automated layout-to-schematic (LVS) checks.

　　However, customized design rule checking (DRC) tools can be used to find simple layout and design errors that crop up in MEMS designs. In the past, the DRC function in low-cost IC tools could only handle 45-degree and right-angle objects. Now, in some better tools, DRC can check the minimum spacing between any polygonal objects across different layers.