RF Driving Next-Gen Processes

By Dave Lammers
GLOBALFOUNDRIES is expanding its RF capabilities in two important ways: moving RF SOI manufacturing to larger wafers and a new technology platform at its East Fishkill 300mm fab. Secondly, RF IP development plays a key role in the 22FDX® platform.</em>
Successful semiconductor companies are facing an interesting challenge: they must be able to create solutions for the fast-growing automotive and Internet of Things markets by combining three technologies that, historically, often have been separated: processors and other digital cores; memories, and RF.

Subramani Kengeri, vice president of the CMOS Platforms business unit at GF, said “going forward, the SoCs for every emerging market will have a radio. With the 22FDX platform we have a cost-effective solution, with RF and analog on the same technology as digital.”

RF and Digital Convergence

Fully depleted SOI has advantages for on-chip RF. The planar transistors in the 22FDX technology have less variability than finFETs on a bulk silicon substrate, where the ability to control both the height and width of the fin is challenging.
“FD-SOI technology provides better transistor-matching characteristics. Because 22FDX is planar, with much lower variability, that helps in building cleaner RF and analog alongside high-performance digital,” Kengeri said.
To accelerate the 22FDX rollout, GF contracted with INVECAS, a Santa Clara-based IP vendor, for 22nm libraries and higher-level IP offerings that are exclusive to GF silicon. Other eco-system partners are engaged in the development of silicon-proven WiFi and Bluetooth cores as a priority.
“Over 45 customers are in various stages of engagement and lead customers have taped out test chips. All the top five EDA providers have announced support for 22FDX. We are on track to qualifying the technology later this year,” Kengeri said, with high-volume production to quickly follow.

RF SOI Moving to 300mm

GF also is working on developing its next-generation RF SOI process, which continues its leadership in RF front-end silicon technologies. The foundry recently reached an important milestone in shipping its 20 billionth RF SOI chip. As the performance requirements for RF SOI technologies become more challenging and the demand continues to increase as a function of smartphone radio complexity, GF is working to address the next wave of innovation in mobile RF fronts. Enabling manufacturing on 300mm wafers is an important component of that strategy.
Peter Rabbeni, senior director of the RF business unit, said “we have already proven that 300mm can provide a number of additional benefits besides capacity enhancement. The availability of new materials and smaller lithography are some of the capabilities of 300mm manufacturing that benefit device performance.”
One of the key differentiators of RF SOI is that circuits are built on an engineered substrate — much different than the SOI substrate used for digital applications such as lowpower microcontrollers — which has characteristics that are better suited for high-performance RF. The substrate characteristics support the high isolation and low harmonic response needed in RF front end circuits, preventing radio interference and preserving signal fidelity, Rabbeni explained.
GF worked closely together with substrate suppliers to develop an RF SOI technology which meets the stringent harmonic and linearity requirements that today’s RF front end switches and tuners need.
LTE communications and carrier aggregation require next-generation RF SOI with improved insertion loss and linearity. Carrier aggregation, for example, introduces data rate expansion methods which binds two or more carriers to a single data stream. This introduces certain complexities in the RF path that need to be accounted for to insure any nonlinear products that are generated by this operation are minimized.
“Another key trend we are observing is the integration of more digital content. There is significant adoption of the MIPI interface for RF front end control, for example, and this is now becoming a larger percentage of the die,” he said.
Beyond the needs of the LTE standard, Rabbeni said the next-generation RF SOI process will set the foundation for the 5G cellular standard. Though the final 5G standard has yet to be ratified, customers are already developing 5G demonstration systems to intersect the 2018 and 2020 Olympics. Millimeter wave frequency operation seems best suited to deliver on the promises of 5G, including low latency, spectral efficiency, and high cell edge data rates. “If this is the direction the industry takes, more integration will be required than today’s RF SOI technologies can achieve,” Rabbeni said.
Customers may integrate the beam formers, power amplifiers, phase-shifters, LNA’s (low noise amplifiers), and even some portions of the transceiver into a single chip under high-speed digital control. “For certain, going forward there is an expectation our customers will want significantly more integration with RF SOI. We are leveraging much of the learning that has been achieved on our 45SOI technology to help make this leap forward,” he said.
In the end, it all comes down to the enablement which helps designers get to market quickly with their product. “We spend significant effort and take a lot of pride in providing very accurate models and high quality process design kits (PDKs), so that customers can be confident that what they simulate is exactly how the silicon performs when it comes out of the fab. We have decades of manufacturing experience in RF silicon technologies. There are not many large foundries that can make that claim.”
These growing market opportunities have led to “a big focus on the transformation of our manufacturing capacity. We have made some very focused decisions on capacity additions for RF SOI and silicon germanium to make sure we can meet the anticipated demand. Expansion to meet the coming demand fromChina is a big focus for us,” Rabbeni said.
As China’s cellphone users move to 4G- and LTE-capable handsets, and as the 5G standard begins to take hold, the demand for RF SOI and SiGe-based chips could expand quickly, just as it did several years ago.

Watching the 300mm Move

Joanne Itow, managing director at Semico Research (Phoenix), said she is watching closely to see how the transition to 300mm RF SOI wafers works out at GF and other foundry vendors.
When IBM’s Burlington operation developed a silicon-based path to RF front end ICs, “the switch to RF SOI and away from GaAs was pretty quick, as the benefits were obvious. The foundries that have the ability to move to 300mm wafers have a leg up. Just having that option is a real plus,” Itow said.
Itow said she is watching to see how the SOI wafer suppliers, primarily Soitec (Grenoble, France), respond with a reliable supply of 300mm RF SOI wafers, and how the foundries and customers take advantage of the larger wafer sizes.
“We are looking at the next move to bringing products on to 300mm capacity. What we are being told by the foundries sounds good, and it sounds as if GF is in the right place, getting ready for the right markets. Now we will have to wait and see if it will work out or not,” she said.