The path to success—layer by layer

"There is only so far you can go with all-metal structures," said the president of Nuvotronics LLC, which recently commercialized a process used to fabricate multilayer, micron-accurate structures from metal, air and polymer dielectrics. "There is only so far you can go with all-metal structures," said the president of Nuvotronics LLC, which recently commercialized a process used to fabricate multilayer, micron-accurate structures from metal, air and polymer dielectrics.

A microcoaxial coupler fabricated using the PolyStrata process. The component contains six layers of metal and dielectric and includes a suspended-center conductor. Photo courtesy of Nuvotronics.

“We currently process over a dozen layers of patterned metal and, typically, three to five layers of dielectric to create complex, 3-D devices that are a millimeter or more thick,” David Sherrer said of his Blackburg, Va.-based company’s PolyStrata fabrication process. Finished parts range from submillimeter to tens of centimeters across and can be monolithically fabricated in batches of hundreds—or millions per year.

The process, he added, “can hold mechanical tolerances of a few microns across many inches.”

The PolyStrata process involves depositing and planarizing metal and dielectric layers in a precision, photo-patterned mold. Each layer can be up to 100μm thick. The thinnest layer is 3μm to 5μm.

Copper is often used for the core metal, due to its excellent electrical, thermal and mechanical properties. The dielectric is a high-temperature, inert photoplastic that is patterned and fused to the metal. A new type of photopolymer is used for the mold.

The final process step involves dissolving the mold material. Completed structures remain attached to the base wafer or substrate, or they can be released from the substrate and function as stand-alone devices.

Unique properties

The PolyStrata process is similar to other rapid-prototyping processes, but it can also be used for volume production on the same platform. “It is extremely scalable,” said Sherrer. “You can use it to make a handful of samples or millions of parts.”

What distinguishes the PolyStrata process is its ability to incorporate permanent polymer dielectrics into the multilayer metal components. “Using dielectrics, we have already been able to monolithically suspend disconnected metal parts on multiple levels, enabling 3-D microelectronics applications,” Sherrer explained.

For example, in order to support freestanding structures, such as the center conductor of a microfabricated air-dielectric coaxial line, a patterned dielectric support can be added. “We have the ability, with any layer, to add a polymer that can be used to suspend metal structures or make mechanical cantilevers—whatever is needed,” said Sherrer.

The PolyStrata process initially was developed to microfabricate microwave and millimeter-wave components and subsystems. An early application was fabricating miniaturized, 3-D coaxial cable networks. Instead of a permanent dielectric filling the space between the center and outer conductors, tiny “tabs,” or “straps,” of the permanent dielectric were used to suspend the center conductor.

The area between the center and outer conductors “is 99.9 percent air, ” said Sherrer. This arrangement minimizes crosstalk—an undesired eff ect that occurs when a transmitted signal loses integrity due to interactions with the non-conductors that usually separate the inner and outer conductor. “We are making complex microwave circuits that can propagate a single TEM (transverse electromagnetic) mode in three dimensions, from 1 to 300 GHz and beyond. These microcoaxial conductors can be side by side with no measurable crosstalk.

“By microfabricating the coax with its correspondingly negligible crosstalk and creating them in multiple layers, we can make microwave circuits and components about as small as they can get,” said Sherrer.

Process history

The PolyStrata process was developed by Rohm and Haas Electronic Materials, where Sherrer was the director of research and product development. The Defense Advanced Research Projects Agency (DARPA), of the U.S. Department of Defense, supplied the grants. “They were looking for a platform technology that could revolutionize RF and microwave components and systems,” said Sherrer.

Two major programs were funded: developing phased arrays for satellite communications and developing hybrid power amplifiers. Both programs were developed in collaboration with BAE Systems Inc., a defense contracting partner, and researchers at the University of Colorado, Boulder.

Nuvotronics LLC was founded in June 2008. Sherrer and other partners purchased the microfabrication business from Rohm and Haas in July. Nuvotronics recently made the technology available to commercial markets.

“It’s an exciting time for us,” said Chris Nichols, a development engineer and senior program manager at Nuvotronics. “We have a fundamentally new way of making miniature devices. We brought over 200 patents with us from Rohm and Haas. We are well positioned to provide our partners with enabling new products and make an impact on the future of micromanufacturing.”

The company is currently focused on the microwave electronics market segment. Besides the two large DARPA programs, Nuvotronics has also begun to make microwave components for various defense subcontractors.

However, noted Sherrer, the “PolyStrata process can be used for all kinds of applications in micromanufacturing.” These include microdevices required for high-density interconnects, microfl uidics, chip microcoolers, highfrequency testing, biomedical devices, valves and fl ow controllers, and automotive and other sensors.

“We are starting multi-user customer runs to make the process more accessible,” Sherrer noted. “Each quarter, we are planning a run for three or four customers to have prototypes made. Instead of assuming we can foresee all the ‘killer applications,’ we think the best approach is making the technology accessible and visible to the diverse industries that may benefit from it and helping them create new value using our process.”

Nuvotronics contact information: Telephone: (540) 552-4610. E-mail: sales@ nuvotronics.com. Web: www.nuvotronics.com.

About the author: Susan Woods is a regular contributor to MICROmanufacturing and Cutting Tool Engineering magazines. E-mail: susanw@jwr.com.