MEMS Hitting The Curved Ball

Many of today’s design tools ignore the specialised requirements of MEMS design. Paul Double takes a look at what’s needed.

Microelectromechanical-system (MEMS) devices are moving into many fields of industry, providing the basis for novel applications of semiconductor technology in fields as diverse as medicine and high-resolution mobile-phone displays. However, designing MEMS devices is not necessarily a straightforward process for these specialists working with the technology. MEMS is an IC-design discipline that currently has little in the way of support from the large EDA tools vendors, which mainly focus on deep-submicron CMOS IC design. At first sight, MEMS design looks as though it could easily be satisfied using traditional tools and techniques.

The geometries used in MEMS design are typically at 1µm or larger. These geometries can indeed be handled easily by today's design and layout tools, which are dealing with geometries an order of magnitude smaller. However, many tools ignore the specialised requirements of MEMS design. These requirements range from the shapes that need to be described through to workflow considerations.

The most apparent difference between conventional CMOS design and MEMS is the use of irregular and rounded shapes in the latter. CMOS designs are almost always performed using rectangular shapes. Only polysilicon and metal layers sometimes show any variation from Manhattan structures, and then only to achieve 45° angles for denser routing.

Because MEMS devices must be able to provide mechanical, optical, or fluid-control functions, a much wider variety of shapes is needed. The shapes will often be based on arcs and curves as well as multi-sided polygons that use arbitrary angles. An understanding of height in the tool is also important, since MEMS features can be much thicker than those made using conventional CMOS processes—on the order of 10µm thick rather than 1µm or less.

The need to support geometries that are not seen in CMOS design is a major factor when choosing a MEMS layout tool. The tool needs to support curves and arbitrary angles for the sides of polygons. Most IC tools do not support the precision drawing of curves, such as those needed for micro-turbines. The end result is that, using CMOS-only tools, the designer has to synthesise curved shapes and unusual angles using short line segments, which are laborious and time-consuming to create. These line segments may further not work well with the fracturing settings used to create the final masks.

Mask-creation tools, being designed for CMOS device production, tend to write the mask using short straight line segments, although some tools can write 45° lines without converting them into horizontal and vertical line segments. The finer the fracturing, the more time it will take to write the mask, and the more expensive the mask will be to create. If the shapes for a MEMS device created using a CMOS-oriented tool are already defined using short line segments, then the fracturing strategy for the mask is effectively set, as it is too time-consuming to tune.

With a tool that understands Bezier curves and arbitrary angles natively, it is possible to tune fracturing to the best balance of cost and performance. It is only when the mask file is created that a decision is made on how short each line segment must be, which will affect the final accuracy of the device. As a result, the tool needs to be able to support the process that will render a curve into a form suitable for fracturing.

The shapes used in MEMS design are typically much bigger than those used in CMOS design. A transistor made for a 1µm process may cover an area of just 20 x 20µm with its minimum dimension being the 1µm-long gate element. However, a MEMS part, such as a cantilever, may measure 1000µm from end to end, but it still needs features to be defined along its length that are less than 5µm across. The problem for the layout engineer is that to construct such an element using a conventional tool, they will have to zoom in and out many times to define those tiny features accurately on such a large element.

A better and more natural way for MEMS designers to work is to have the smaller features snap to corners, midpoints, or other points based on relative distances without zooming in. Tools designed with the MEMS designer in mind will have commands to make this possible.

Defining arbitrary shapes using just drawing functions can be tedious, so tools oriented to the MEMS designer add functions that allow logical operations on combinations of simple shapes. These make it easy to construct complex shapes and define features, such as release holes for a free-standing plate or a flow channel that needs a metal border of a certain width around it. Using a logical operation, it is possible to "grow" a line to define a border to a specified width. This is much quicker than having to draw and align the border by hand.

Further, any change to the flow channel can be accommodated in the border by maintaining the logical link.

Another way to incorporate complex shapes is to take them from a mechanical CAD tool such as Autocad. This is simple as long as a DXF import filter is in place and the tool is designed to accommodate the differences between mechanical and electronic design tools—something that not many CMOSfocused layout tools can achieve. "Zero width" lines are often used in mechanical CAD files to define shapes, but mask-writing tools do not recognise these lines. For successful maskmaking, all features must be in the form of enclosed polygons. If the shape is a compound object, such as a square with a hole in the centre, it must be drawn in a specific manner—with only one enclosing boundary—so that the mask manufacturer knows which areas are "clear" or "dark." For MEMS designers, this rule can become cumbersome, for example, when designing a free-standing plate with thousands of release holes. Conversion utilities that understand these issues can greatly simplify the import of mask-compatible shapes.

Export to DXF is an important feature for many MEMS designers, since special packaging and encapsulation needs to be made specifically for each design. By exporting the design in DXF form, errors in alignment can be avoided. Also, it's a much easier process to deal with when there are late design changes.

Easy import functions and the ability to construct complex shapes using logical functions plays into another aspect of MEMS design: an efficient workflow. In contrast to CMOS design, where the focus of design is on extensive simulation before sending a design to the mask house, the early stage of evolution of MEMS technology means that physical prototyping is much more commonly employed. Although finite-element simulation can be used to help guide MEMS design, many teams use simulation only for the first steps. Tuning of the design is often performed with manufacturing samples.

IC production techniques allow the fabrication of many different small IC designs using one reticle. MEMS engineers take advantage of this to produce variations of a design. These variations can be used to help debug a design or show which structures work best for a given process. Creating the variations is potentially a time-consuming process. However, the use of macros coupled with an easier import process or logical shapecreation functions can do much to streamline the job for the engineer. For example, if the purpose is to determine the best number of supports for a cantilever or the minimum effective bend radius for a fluid channel, macros can be created to generate the necessary basic shapes and their variations.

A further productivity improvement considered for tools designed to handle MEMS projects is support for custom design-rule checks. The specialised nature of most MEMS designs means that customer design rules often have to be used on top of those supplied by the foundry. A layout tool optimised for the MEMS environment should make it easy to produce those rules and check the design against them before it proceeds to the mask-making stage.

IC-layout tool suppliers are bridging the gap between the needs of MEMS designers and the traditional needs of IC designers. The suppliers who have made the most progress have worked hard to understand the needs of the MEMS industry, and have implemented features that greatly improve their workflow. Before purchasing a tool, MEMS designers must check to see if it includes functions such as DXF input and output, arc and arbitrary-angle drawing tools with support for logical functions, as well as design-rule checks. Otherwise, they may not get the full benefit of layout automation for their specialised needs.

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