Critical balance: Going fabless while designing MEMS for package and test
By Jay Lyman
Manufacturers and other MEMS players have come to realize the importance of incorporating packaging into the design and manufacturing process. However, they are still facing the difficult task of balancing a design for manufacturability approach with outsourcing of fabrication, an increasing necessity given the requirements and cost of high-volume facilities.
Companies navigating this confluence of design forethought and outsourcing must know what manufacturing variations they and their MEMS products can live with, and come up with a strategy that will enable the high volumes and high yields necessary to find a manufacturing partner, according to Farrokh Ayazi, associate professor in the School of Electrical and Computer Engineering at the Georgia Institute of Technology.
“If you don’t maintain a high volume, the manufacturing houses that maintain tight specifications are not going to take you seriously,” Ayazi said. “They won’t even talk to you.”
Before getting to volume, organizations focused on MEMS solutions are waking up to the fact that there must be a compelling, workable application for their products, given that the days of investing in technology for its own sake have passed.
At the same time, organizations must make sure they are designing for manufacturability from the start, including packaging, which is no small task, according to Concurrent Design and Drafting Principal Tom Ortman, whose Texas-based firm provides engineering design and build services.
“When you move that part of the process up front, you’ve essentially created more variables and more complexity,” he said. “This doesn’t make it easier doing these things in parallel rather than sequentially. It is clearly a more difficult process to be able to account for variables and added levels of complexity.”
Add the outsourcing of MEMS fabrication to the mix, and it becomes an even more difficult proposition. However, this is not to say it can’t be done, say Ortman and other experts, who stress that the selection of the right manufacturer becomes even more critical.
“You would be best served by having a good understanding of a manufacturer’s capabilities, and be able to design a product within or to those capabilities, optimizing manufacturer capabilities so when you get to manufacturing, they’re not seeing it for the first time and you have designed things that would work well within those capabilities, not outside their abilities or dimensional size. The trick in that is kind of a subtle one,” Ortman added. “You want to choose the best vendor to design a product, yet you don’t have a product to design yet. You don’t have anything to show him at the beginning.”
In addition to capabilities, MEMS companies looking for a foundry partner must also consider other factors, including qualifications, availability, and cost, Ortman said. He said the model of combining design for manufacturability with outsourcing of MEMS fabrication can work, provided the fabless company has two things: the ability to dedicate skills in smaller, niche application areas; and a business team capable of supporting the effort with money, supplies and the right foundry partner.
“Those two things really are the two pieces of the puzzle that work extremely well at a macro level,” Ortman said, indicating a large number of fabless MEMS and semiconductor companies are leveraging the foundries in order to “do what they do best.”
Know their limits, and yours
Ayazi, who researches micro and nano electromechanical resonators at the Microsystems Packaging Research Center, stressed the sensitivity to process variation in MEMS fabrication. He highlighted industry awareness of the need for design that includes packaging, but said there is less awareness of the need for a wider design for manufacturability approach.
“The solution to that is to design your product to be process tolerant,” he said, indicating that the manufacturers are able to specify their parameters, and expect the same of their fabless partners.
“It will all come down to yield and cost,” Ayazi said. “If the variations drop the yield, then you don’t have a deal with a manufacturer.”
In addition, he stressed the need for both a robust design that can be manufactured and the use of critical materials that are well characterized and reproducible. He also said fabless MEMS companies should expect that some common questions from the manufacturers - including variation, yield and cost - will all be considered hand-in-hand with packaging.
“Those are the types of questions you’re going to get as soon as you start talking to them,” he said.
‘S’ is for systems
Dave Monk, an operations manager with Freescale Semiconductor, says it’s important to remember that the “S” in MEMS stands for systems. MEMS products must be designed as systems from the start. For Freescale, this has meant pulling different organizations and teams together to collaborate, thereby necessitating fewer processes.
“You really do a design for an entire system,” he said.
Monk also said time has taught him that materials changes need to be taken seriously, and the proper investment needs to be put forth. Once an organization focuses on a particular MEMS process and application and has factored in packaging and manufacturing, then it is ready to take advantage of the manufacturing power of a foundry, which enables more entrepreneurial spirit, according to Monk.
“It’s a little bit of a self-fulfilling prophecy,” he said. “To get to that level of maturity, you can go out to the foundries.”
Monk also referred to a maturation of the MEMS industry, which while still 15 to 20 years behind the fabless CMOS model, is rapidly advancing. Going back to the beginning of the trend toward outsourcing of fabrication in CMOS, Monk said there was not the same level of design maturity that exists today in MEMS.
“I see the foundry mentality growing up,” he said.
What is more similar to the old CMOS outsourcing model is the need for fabless ventures to be able to provide some basic business metrics, including cost and yield, for their foundry partners, Monk said.
“MEMS is getting to where those cost metrics are more important,” he said.
Take measured steps
Given the novelty of MEMS technology compared to semiconductors, it can be difficult to take the fabless route right away, according to Brad Nelson, vice president of operations at TeraVicta Technologies, which makes MEMS switches for electronics, telecom and other applications.
“In the case of MEMS, the technology is so immature relative to building integrated circuits,” Nelson said. Rather than initially look outside for foundries for manufacturing and packaging, TeraVicta incorporated design into its own small fab facility to get started before seeking a foundry partner.
“From day one, we’ve concentrated on having all the fundamental expertise, as well as a small fab to develop the technology,” he said. “For high volume, we’ve hired a high-volume manufacturer who will transfer our technology.”
Nelson explained TeraVicta’s manufacturing partner, Hong Kong-based China Resources Semiconductor, was willing to take TeraVicta’s technology and run with it - something TeraVicta or other small MEMS companies cannot do on their own. “They were willing to invest in our technology and transfer our technology,” he said.
Support your outsourcing
Nelson stressed that until organizations can whittle MEMS manufacturing down to a handful of processes, which is not currently the case, the fabless model will continue to pose unique challenges. However, once an organization is ready to take their MEMS to higher volume, the usual rules of outsourcing apply, Nelson said.
“You have to work very closely with them, which means spending significant resources to make them and yourself succeed, instead of (the) over-the-wall design and mask, make me X units per month (type of approach),” he said.
For TeraVicta, that has meant substantial travel for both the company’s and its foundry’s engineers.
“In the case of new technology, it’s also about helping them understand,” Nelson said. “You have to build that infrastructure while you’re having to maintain a significant infrastructure of your own,” he added, indicating this task is often underestimated by fabless semiconductor companies.
Nelson said he does not see the challenge getting any easier soon given the many, various MEMS flavors and many processes within those. And while even large companies that outsource significant chunks of their volume sometimes struggle to maintain mindshare and fab capacity, companies can leverage design and outsourcing together with the right approach.
“We haven’t achieved a perfect balance, but we’re going into it with our eyes open,” Nelson said.
China Resources Semiconductor Co. Ltd.
Concurrent Design and Drafting
Georgia Tech College of Engineering Microsystems Packaging Research Center