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Addressing MEMS Challenges (Part I)

Addressing MEMS Challenges (Part I)

Overcoming design challenges for mircoeclectromechanical systems (MEMS) in an IC-focused environment. By Nicolas Williams, product marketing manager and Qi Jing, technical marketing engineer, Mentor Graphics

Microelectro-mechanical systems (MEMS) have been growing rapidly ever since it became possible to fabricate MEMS devices using modified semiconductor device fabrication technologies. Layout tools that are widely used in IC design naturally become the tool of choice for MEMS layout design.

Although IC layout tools are quite mature and feature-rich for IC applications, many of them lack the capabilities to efficiently handle the challenges MEMS layout imposes. That is why unique MEMS-oriented features are needed in IC tools to address the specific requirements of MEMS layout design and to make the design process fast, easy and accurate.

A big difference between MEMS layout and IC layout is the use of unique, irregular shapes. Unlike conventional CMOS IC design, where layout shapes are usually Manhattan style (such as rectangles and rectilinear polygons) or polygons with 45-degree edges for routing, MEMS design utilises a much broader variety of geometries, due to its wide applications in the mechanical, optical, magnetic, fluidic and biological fields.

The support and ease of use for implementation of irregular shapes, including curves and all-angle polygons, becomes a critical criterion differentiating MEMS-oriented CAD tools from conventional IC-oriented tools. Most layout and verification tools are focused and optimised on IC designs and are not suitable for irregular shapes needed for MEMS designs. MEMS designers need layout and verification tools that can handle challenges that curved or all-angle objects presents. Also, designers need tips and tricks on how to handle false errors that result from rules that are optimised for orthogonal geometry.

Overcoming Limitations Of Mechanical CAD Tool

Unlike traditional mechanical CAD tools, where “zero-width” lines are common, MEMS layout requires all geometry to be represented as “closed” or “filled” polygons. This is needed to define the light and dark regions of the mask. One limitation with some mechanical CAD tools is that they cannot represent a filled polygon easily and any drawing done in these tools usually results in the polygon being represented as several “zero-width” line segments.

When importing DXF files (a common format used to transfer geometry from mechanical CAD tools), MEMS can search for segments having endpoints within the user-specified tolerance and try to reconstruct closed polygons as seen in Figure 1.

If a closed polygon is found, the individual segments will be replaced by a single polygon. The endpoints of the line segments do not have to match exactly. MEMS allows the user to specify the largest gap between segment endpoints when joining them into a polygon. When sending the MEMS design to the foundry for fabrication, the user typically will export the design in GDSII format. Since the GDSII format does not support curves, a conversion is needed for circles, pie wedges, curved-sided polygons and tori to all-angle polygons that approximate the curve when GDSII mask data is exported.

MEMS automatically performs this conversion, and additionally issues warnings if the design contains a wire or polygon with more than 200 vertices, since GDSII has a limit of 8,192 vertices, and 200 vertices is a traditional best practice limit. The user can then specify a maximum number of vertices that each polygon should have and if it exceeds this maximum, the polygon will be automatically fractured into smaller polygons with fewer vertices by Tanner L-Edit MEMS.

All-angle polygons can also be converted back to curved polygons. Sometimes, a GDSII file, where curves are not preserved, needs to be read back in for design revision; or curves need to be recovered from the result of an advanced editing operation such as Boolean operations, making it easier to edit.

To achieve good curve recovery, MEMS searches all-angle polygons for arcs with eight or more vertices and replaces the multiple adjacent segments with curved edges, provided that those vertices lie on an arc with no more than one manufacturing unit radius error (Figure 2). Such conversion capabilities make it much more convenient and accurate for users to re-edit curved objects.

Curve Conversion To All-Angle Edges

Curved polygons need to be converted to allangle polygons when doing some advanced editing operations, when running design rule checking (DRC) and when exporting to GDSII. The all-angle approximation must represent the actual curve as accurately as possible.

In some CAD tools, curves are converted based on a specific number of vertices, which does not guarantee the precision between curves of different sizes. MEMS converts curves based on the manufacturing grid, which adjusts the number of vertices to use during conversion based on the size of the curves to have maximum precision.

To show the difference between the approach of Tanner L-Edit MEMS and other CAD tools, three circles with a 5-μm, 50-μm, and 250- μm radius were converted in Figure 3 to all angle polygons using a fixed number of vertices which is common in other CAD tools.

They were also converted using the MEMS approach. Notice that for small curves such as the 5-um radius circle, both approaches do a good job approximating the curve compared to the original curve and have about the same error.

For larger curves, however, the error rate increases for the fixed number of vertices method to be as much as 0.3 μm for the 250 μm circle. Since MEMS uses the manufacturing grid to calculate the number of vertices, the error is on average, less than the manufacturing grid of 0.01 μm.

Even though edges are smoothed when fabricated, this error can affect how the resulting MEMS structure performs if the error is too high. Also, this conversion error can cause problems when doing Boolean operations on curved geometry and can cause many false DRC errors.

To be continued… Addressing MES Challenges (Part II)

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