New Foundry Gold Rush: RF SOI

By Mark LaPedus
About every five years or so, a new and hot market emerges in the specialty foundry business that resembles a frenetic gold rush.

The last big gold rush occurred around 2008, when more than a dozen foundries jumped into the bipolar-CMOS-DMOS (BCD) market to capitalize on the booming power-management sector. Now, the next gold rush is centering on an emerging technology—the radio frequency (RF) silicon-on-insulator (SOI) market.

Today, IBM, STMicroelectronics and TowerJazz offer RF SOI foundry processes for the merchant market. Over time, analysts estimate that a dozen or more foundries could offer RF SOI. Altis Semiconductor and Grace Semiconductor have announced plans to enter the RF SOI fray. Two others, Lapis Semiconductor and Silanna, have put RF SOI on their foundry roadmaps. And sources indicate that GlobalFoundries, MagnaChip and TSMC are developing RF SOI or evaluating the technology.

Foundries are jumping on the RF SOI bandwagon amid a boom for select parts, particularly within the RF front-end for the latest smartphones and tablets. Typically, the RF front-end consists of power amplifiers (PAs), RF switches, tunable capacitors and filters. Generally, the PA and switch are based on gallium arsenide (GaAs), while the tunable capacitors and filters use various technologies.

RF SOI and its variant, silicon-on-sapphire (SOS), recently have made inroads for the RF switch—at the expense of GaAs. Most PAs are still based on GaAs, but the tide is slowly turning. For example, Peregrine Semiconductor is developing an SOS-based PA for a future smartphone at Apple, according to RBC Capital Markets.

Generally, RF chipmakers make GaAs-based devices in their own fabs. Chips based on RF CMOS, RF SOI and SOS generally are outsourced to the foundries. RF SOI is not a difficult technology to develop, but the real issue is that the sector could meet the same fate as BCD. As it turned out, the BCD market was not big enough to support a dozen foundries, prompting a shakeout in the arena.

In all likelihood, there is room for only a handful of RF SOI foundry players. “I would say IBM and TSMC are the only ones that have the economies of scale (in RF SOI),” said Doug Freedman, an analyst at RBC. “IBM is the leader in RF SOI right now, with TSMC trying to play catch-up. There are some other vendors like TowerJazz in the market, as well.”

From a supply/demand perspective, there is already ample RF SOI capacity to meet demand right now. “I have heard that capacity in RF SOI is adequate,” said Christopher Taylor, an analyst with Strategy Analytics. “I would have my doubts about the prospects of serious shortages barring compelling information to the contrary. Also, in light of the fact that RF SOI does not really push into the CMOS, small-node frontier, there is potentially quite a bit of capacity available from older fabs and foundries at the higher nodes.”

Rushing into RF SOI
The stakes are high, especially as RF content continues to increase in the latest mobile devices. In total, the PA market is expected to grow from $1.7 billion in 2008 to $3.8 billion by 2015, according to RBC. The multi-throw RF switch market is projected to grow from $262 million in 2008 to $1.2 billion by 2015, according to RBC. And the tunable capacitor market is expected to reach $500 million by 2016, it said.

“Driving this growth is rising handset and tablet units, which requires a greater amount of PA ICs,” RBC’s Freedman said. “Principally driving (RF switch) growth is rising radio bands. Driving (tunable capacitor) growth is the wider frequency range of bands and the need to reduce antenna size without performance trade-off.”

There is also an increase in design complexity amid a transition from 3G networks to the next-generation, 4G/LTE wireless standard. “LTE and carrier aggregation are thorny problems even in the best of situations,” said Michael Noonen, executive vice present of global sales, marketing, quality and design at GlobalFoundries. “You also want to be as Spartan as possible in the RF front-end design from a battery consumption standpoint.”

GlobalFoundries, which has been expanding its RF process offerings, is “very much interested” in RF SOI, Noonen said. “We have a lot of experience with SOI, but there are also other approaches in RF,” he said.

Indeed, OEMs face a series of complex device and process choices. For years, GaAs has dominated the RF landscape. GaAs has a larger energy gap and is faster than silicon, but it is more expensive to manufacture. RF CMOS, RF SOI, SOS and silicon-germanium (SiGe) are also in the mix. The RF version of SOI combines CMOS with a highly-resistive, thick-film SOI substrate.

RF SOI is an alternative to GaAs, with equivalent insertion loss and noise isolation characteristics. RF SOI also enables OEMs to integrate various chips on the same die. Another technology, SOS, makes use of an insulating sapphire substrate. And SiGe is built with silicon transistors to create RF circuits.
Meanwhile, after years of promises, RF SOI and its variants are finally cracking the RF front-end. OEMs are moving from GaAs pHEMT to RF SOI and SOS for the RF switch, said Paul Boudre, chief operating officer at Soitec, an SOI wafer supplier. “GaAs pHEMT will not disappear, but it will remain for more specific devices,” Boudre said.

Actually, the buzz started when Apple incorporated Peregrine’s SOS-based RF switches in the iPhone 5. Samsung’s Galaxy S4 and other smartphones are also using SOS-based switches, according to RBC. SOS is a proprietary technology that is only offered by Peregrine. Its SOS chips are made on a foundry basis by Lapis, MagnaChip and Silanna.

Rodd Novak, chief marketing officer of Peregrine, said SOS has better insulating properties than RF SOI. SOS also uses sapphire wafers, making it a more expensive than RF SOI. But the overall cost for SOS is declining. This is because sapphire wafers are ramping up in high-volume markets like LEDs, which will impact the cost of SOS, Novak said.

Peregrine recently rolled out a new version of SOS, based on 0.35-micron technology. “Before, we grew an epi (layer) on top of our sapphire process,” Novak said. “Now, we are taking a very clean silicon substrate and bonding that to the sapphire. That process enables better performance.”

Apple to drive SOI?
The fact that Apple and other OEMs have adopted SOS and RF SOI for the RF switch has given the technology some credence. It also has caused a stampede of foundry players looking to enter the RF SOI sweepstakes.

Now, with help from the foundries, RF chipmakers are looking to displace SOS-based switches with traditional and less-expensive RF SOI technology. “RF switches are typically based on GaAs pHEMT, SOS and SOI, with SOI gaining more and more market share away from the other and more expensive technologies,” said Marco Racanelli, senior vice president and general manager at TowerJazz.

In addition to cost, OEMs are also interested in capacity. In one effort to ensure supply, IBM recently signed a second-source foundry deal for its 0.18-micron, RF SOI process with Altis.

Besides the RF switch, the next big market for RF SOI and SOS could be the PA, with Apple emerging as the possible driving force. “We believe that Peregrine is developing a unique integrated PA solution that is targeting the next generation of Apple’s PA product needs,” said RBC’s Freedman. “(This) could add approximately $1.25 in content, assuming (Apple integrates) five to six single PAs in 3G smartphones. We note that in 4G, PA content opportunity rises to approximately $3.00 due to rising single chip PAs per device.”

In another example, Qualcomm recently rolled out the RF360, an RF front-end that includes a PA based on SOI. Today, however, the jury is still out for PAs based on RF SOI and SOS. For the PA, GaAs still has a higher power-efficiency over CMOS.

Still, the handwriting is on the wall for GaAs. “For the PA, SiGe BiCMOS has strong market share in WiFi, while GaAs HBT has strong market share in cellular. RF CMOS is relegated to the very low-end 2G/2.5G cellular space,” TowerJazz’ Racanelli said. “SOI for the PA is only in R&D and may not deliver the best performance by itself. But combined with switches and other functions, (SOI-based PAs) could become relevant as new architectures are adopted. Our view is that SiGe has the best tradeoff in performance. The cost structure is closer to CMOS/SOI. SiGe is likely to gain more ground in the future.”

Also in the RF front-end, there is a tunable capacitor, which tunes the antennae to boost efficiencies. Peregrine is selling SOS-based tunable devices. Paratek and STMicroelectronics are selling components based on barium strontium titanate (BST). And WiSpry is offering a MEMS solution.

“There are two vectors worth exploring here,” GlobalFoundries’ Noonen said. “If you can do something in CMOS, it will be done in CMOS. We will see other ways to approach the problem. Using a tunable capacitor based on MEMs, for instance, you can attack the problem from an entirely different angle.”

Indeed, in the RF front-end, there is no one-size-fits-all technology; OEMs likely will adopt several types of chips and processes. “We will also see more functionality in the RF subsystem,” Noonen said. “The idea is to bring RF into more of a mainstream technology.”