CMOS MEMS technology upgrades next-generation communications and electronics products.

By Kaigham (Ken) J. Gabriel, Ph.D.

From the advent of the telephone and the world's first radio broadcast to the development of the electronic hearing aid and the cellular phone, technology has transformed the way people use sound at work, home and play.

Innovation continues to fuel the rapid development of increasingly sophisticated communication and electronics products. However, the performance limitations of the conventional microphones and speakers that currently capture and reproduce sound impose unnecessary constraints on the capabilities of mobile phones, hearing aids and many other products.

Acoustic system-on-chips* eliminate these constraints by combining the functionality of multiple microphones or speakers with microelectronics and software onto a single, standard complementary metal-oxide semiconductor (CMOS) chip. As a result, these chips create significant performance, design, manufacturing and cost advantages over conventional acoustic components.

The Technology

The chips are based on microelectromechanical systems (MEMS) technology, a commercially proven approach to fabrication that integrates microelectronics with microstructures and electromechanical systems. This method is improved by building MEMS structures directly out of CMOS semiconductor materials.

The acoustic CMOS MEMS technology enables fabrication of an integrated membrane or membrane arrays that perform the functions necessary to capture or reproduce sound. Successful integrated CMOS MEMS manufacturing has been demonstrated in six different processes, ranging from a three-metal 0.6 µm process to a six-metal 0.18 µm process.

The chips on a wafer come from a standard CMOS fabrication facility covered with a layer of protective glass. Regions meant for mechanical structures are patterned in one of the metal layers, usually the top-most layer. The oxide is etched anisotropically down to the silicon substrate, with the metal layers acting as a mask to define the mesh structure. The underlying silicon substrate is etched with an anisotropic deep etch followed by an isotropic etch to release the metal dielectric layers from the silicon substrate underneath. At this point, the membrane mesh structure releases from the underlying silicon and the desired cavity forms. In the final step, the released CMOS MEMS structure is coated with a polymer in a chemical vapor deposition process. The polymer conforms to all sides of the beams until all gaps are sealed, creating an airtight membrane suspended over the gap. Metal layers inside the beams allow the membrane to be treated as an electrode for either capacitive sensing or electrostatic actuation (Figure 1).

Acoustic CMOS MEMS is a technology platform that enables arrays of transducers and integrated signal processing with improved sound quality and lower installed cost. This platform offers numerous capabilities that disrupt conventional microphone and speaker technologies as well as other current discrete components necessary to capture, process and reproduce sound. Its capabilities include integrating multiple microphones or speakers onto a single chip, reusable or flexible designs, analog, digital and mixed signal chips, and programmable chips for advanced functions (Figure 2).

Figure 1. The CMOS MEMS structure is coated with a polymer, which conforms to all sides of the beams until all the gaps are sealed, in a chemical vapor deposition process. Metal layers inside the beams allow the membrane to be treated as an electrode for either capacitive sensing or electrostatic actuation.Click here to enlarge image

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Cost and Performance Advantages

As a result of this robust CMOS MEMS platform, the chips create cost and performance advantages across 10 different markets that currently consume billions of microphones, speakers and other discrete acoustic components annually. These advantages include:

Component Reduction — The chips combine signal processing circuitry, software and transducers onto a single, standard CMOS semiconductor chip, reducing discrete components and increasing valuable device board space.

Improved Design — The chips can be designed and configured rapidly and cost effectively to meet specific requirements of different device and system architectures. And, with chips that integrate analog-to-digital or digital-to-analog signal converters, product designers have flexibility due to reduced electromagnetic interference.

Acoustic Solutions — The chips offer acoustic solutions such as directionality and noise suppression that improve quality and consumer experience with mobile phones, hearing aids and other products.

Surface Mountable — Unlike conventional microphones or speakers, which must be hand-assembled into products, the chips can be integrated into products using existing surface mount technology processes.

Figure 2. Acoustic CMOS MEMS capabilities include integrating multiple microphones or speakers onto a single chip, reusable or flexible designs, analog, digital and mixed signal chips, and programmable chips for advanced functions.Click here to enlarge image

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Conclusion

Acoustic CMOS MEMS technology offers a paradigm shift for next-generation communications and electronics products. This technology could change the landscape of device development and manufacturing, open and expand markets and business opportunities, and create an improved product.

*Akustica's acoustic system-on-chips, microphone chips and speaker chips.

KAIGHAM (KEN) J. GABRIEL, Ph.D., chairman and chief technology officer, may be contacted at Akustica Inc., River Park Commons, 2403 Sidney St., Suite 270, Pittsburgh, PA 15203; (412) 390-1730; E-mail: [email protected].