Jeremie Bouchaud and Richard Dixon, analysts at iSuppli, refer to Microelectromechanical Systems (MEMS) microphones as “one of the great success stories of MEMS.” By outperforming Electret Condenser Microphone (ECM) technology in size, scalability, and ease of assembly characteristics, it’s no wonder the MEMS microphone market is expanding from about $100 million in 2006 to a projected $300 million in 2013.

Traditional microphone technology has not kept up with market demand and is becoming a stumbling block as demand grows. Previously, microphone suppliers would stack up individual components and assemble microphones one at a time. Most are cylindrical, about 6 mm in diameter by 1-2 mm high, and inexpensive enough to be deployed in a range of applications from phones to toys. The problem is traditional microphones are heat-sensitive, which precludes the use of lead-free solder and the option for surface-mounting into circuits. So, to work around microphones’ intolerability of high temperatures or reflow soldering, most manufacturers of high-volume products resort to an offline task, such as hand assembly or a special insertion machine, at the end of the mainstream assembly line.

Mounted for sound

Knowles has overcome the problems of ECM technology with the SiSonic MEMS microphone, which is batch-produced on silicon wafers and assembled like an integrated circuit except for an air pocket that allows sound waves to vibrate the diaphragm. An outline of the air pocket is shown in Figure 1. The SiSonic is reflowable, so an assembler can place it on a circuit board with a chip insertion machine, just like any other component. Microphones that can fit in with the normal assembly flow help reduce production cycle time, improve quality, and lower overall product cost.

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The MEMS microphone is a bit of a renegade component in that it’s neither a traditional microphone nor a conventional integrated circuit. The difference in the design paradigm is that in MEMS, there are no circuits per se. Knowles doesn’t deal with schematic versus layout in the MEMS group, but instead draws complex polygonal and curved structures.

Electronic Design Automation (EDA) tools for designing these MEMS microphones have shortcomings. Most tools cannot handle complex geometries and are geared toward rectilinear design and layout schema. The Knowles microphone design, for instance, is largely circular and requires a tool that can create and manage toroidal elements (3D circles). And preparing to use a conventional EDA tool requires a significant amount of setup and configuration. While this up-front work might be needed for large circuit designs, where the engineer is working for weeks on a cell, it is hard to justify on smaller circuits.

When designing a cost-efficient MEMS microphone, it is critical to find a tool with a short ramp time and small up-front investment in setup and configuration. The different high-end tools the Knowles design team has used through the years to design microphones have two main drawbacks: an inability to handle complex geometries and too much overhead.

To better meet the challenges of this new paradigm, Knowles chose L-Edit from Tanner EDA for its MEMS design. This solution’s hierarchical architecture provided flexibility to manipulate thousands of repeating elements. The team saved production time creating a variety of parametrically driven shapes. Circles, pie wedges, and tori were instanced slightly differently in every layer because Knowles used them as primitives. Although this was possible in a limited way with a tool like AutoCAD, designers wasted hours going back and forth between mechanical design and EDA tools. The ideal solution was to create and analyze designs in one environment and then send them out for photomask fabrication.

EDA scripting and fab rules

Knowles designs are not especially small – the die size is about 1 mm – but they are intricate. Drawings with 10-12 million objects are common. While MEMS design flow today does not lend itself to direct object generation through standardized libraries, scripting functions make creating and managing thousands of parametric objects extremely easy.

High-end tools have highly specialized scripting languages, but L-Edit users can write scripts using ordinary C/C++ code. The scripting function is flexible and often used to create primitives with a one- or two-page script. For example, in MEMS, Knowles often etches holes through the wafers and cannot have die intersecting the edge of the wafer. This means the dies must be arrayed in a circular pattern. Knowles now starts with an instance of a die and uses L-Edit to make it a rectangular array. The scripts clip the rectangular array to fit within the wafer extents, leaving a few millimeters for an exclusion zone around the edge, thus saving time.

The Knowles R&D team takes advantage of the scripting functions for other tasks as well, such as creating mapping programs for die bonding pick-and-place equipment. The script goes through the database of cells in the layout and automatically generates a wafer map, which is particularly important when working on a matrix design that has several different designs in it. The team can create the maps and assign different letters to different design styles, then tell the die-bonding engineers to pick a particular letter out of the map. The ability to generate the map automatically also helps save time.

Design Rule Checking (DRC) is different in the world of MEMS because there are few set rules. Designers have to create their own rules or work them out on-the-fly with the fab. The cross-sectioning tool in the editor helps with this by allowing Knowles to visualize designs in the third dimension of stacked layers.

Knowles exports to GDSII for handoff to their fabs. Using L-Edit helps the engineers work around two limitations peculiar to many GDS tools. First, the fab can have instances only at 90/180/270/360 degrees, but when Knowles engineers rotate things, they don’t always end up with these particular angles. A script allows them to scan the database for any acute or obtuse angles and flush them out. Each one is then ungrouped, changed to be rectilinear, then later regrouped as it was originally.

Also supported is user-controllable fracturing of polygons, which lets Knowles set the limits for various mask fabrication vendors. Even when dealing with limitations in Knowles’ vendors’ systems, L-Edit has helped overcome problems and interfaced smoothly with the fab.

Summing it all up

Knowles MEMS microphones require creation and manipulation of complex geometry and integration of mask layout data with advanced scripting. High-end EDA tools are not well suited to these types of requirements, and simple mechanical CAD tools can’t perform the requisite tasks.

Knowles uses L-Edit tools to handle complex geometries and manipulate thousands of repeating elements at the heart of the company’s MEMS designs. The results speak for themselves – 1 billion chips and counting.

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Pete Loeppert has been VP of engineering for the Knowles Acoustic Division of Knowles Electronics since 1999. His previous experience includes work at the Gould Electronics corporate research center on a variety of projects including high-speed A/D converters, chemical sensors, fiber-optic systems, and GaAs circuits. Pete received his PhD in Electrical Engineering (specializing in solid-state devices) from Purdue University.

Knowles Electronics 630-250-5930 www.knowles.com

Nicolas Williams is director of product management at Tanner EDA, where he leads product direction from concept to release. His field of specialty is analog EDA and analog, mixed-signal, and RFIC design. Nicolas received his BS and MS degrees in Electrical and Computer Engineering from the University of Tennessee.

Tanner EDA626-471-9700www.tannereda.com