The industry research firm Yole Développement says that more than 65 percent of substrates used in fabricating switches for handsets are SOI-based. This is a high-growth part of the market, putting up double-digit increases.
Like a standard SOI wafer, an RF SOI substrate has an active (“top”) layer on which CMOS transistors are built, with an isolating (“BOx”) layer under it. The main difference is that the bottom “base” layer under the layer of isolation is a high-resistivity material. This reduces noise and interference, which helps the finished die reach its target performance in terms of signal integrity, handling RF power and integration density. SOI-based devices can reach a figure of merit for on-series resistance and off-equivalent capacitance (Ron.Coff ) below 200 fs (femtoseconds) with potential for further reduction. This directly relates to improved device performance and smaller die size.
At Semicon West SOI wafer manufacturer Soitec announced that its SOI technologies are now mainstream for manufacturing switches and antenna-tuners, key RF components for cell phones and tablets.
There are also new challenges as the industry moves from from 3G to 4G/LTE and further LTE Advanced using carrier aggregation. With SOI, designers can beat the demanding linearity requirements such as intermodulation distortion (IMD), going far beyond -110 dBm, thereby helping avoid interference between networks, says Soitec.
“RF SOI technologies enable the device integration, cost effectiveness and high performance needed for high-volume 3G and LTE applications,” explains Bernard Aspar, vice president, Communication & Power Business Unit at Soitec. “RF, with over 100 percent revenue growth last year, remains a strategic market in which we have been continuously investing for more than a decade.”
Aspar says that as the leading supplier of engineered wafers, Soitec is looking to catch the next growth wave in the RF market.
Based on recent demonstrations, Soitec sees power amplifiers as likely be the next RF components based on SOI. The technology enables highly tunable amplifiers to address multi-region requirements on a single platform. The RF SOI substrates also offer a path towards further integration, such as more mixed-signal and digital content.
Soitec explains that its approach is to offer a wide choice of engineered substrates, so that RF device manufacturers can choose the solution that aligns best with their market strategies – from low-cost GSM handsets to multi-band, multi-mode LTE smartphones and tablets.
In a recent press release, the SOI wafer leader Soitec said that chips built on its SOI wafers were found in over half of the smartphones and tablets in the market worldwide. 50%? That’s a lot! How do they figure that? The answer: RF.
With all the talk right now about FD-SOI for application processors, the importance of the RF chips might seem to get a little lost. Don’t expect that to last. If app chips are the heart of the smartphone, RF is really the soul, the raison d’etre, if you will.
Soitec’s wafer shipments for RF apps have increased by 400 percent in the last two years. In their current fiscal year (which ends this month), the company says it will have shipped over 200,000 engineered wafers to customers making chips for mobile comm. Those wafers translate into about 2.5 billion ICs for RF front-end module apps, which covers half of the 600 million smart phones and 100 million tablets expected to be produced this year.
Soitec, of course, does several flavors of SOI (including bonded silicon-on-sapphire aka BSOS, and high-resistivity (HR) SOI, which Soitec markets as their Wave SOI™ product line) as well as epitaxial GaAs wafers. It all adds up.
In terms of chips and substrates, the RF side of the mobile world is much more complicated than the app side. Different functions have different needs, and those needs have traditionally been best met by disparate starting substrates. Devices can have eight of more chips and modules, and the chips in any given set can have different starting substrates, depending on the critical parameters.
The advent of LTE – “long-term evolution” aka 4G – will have a phenomenal impact on the RF components market, with analysts looking for RF components to almost double in value over the next five years. Look for an alphabet soup of new chip modules designed to handle the enormous complexity of evermore frequency bands.
Front-end modules (FEMs), which handle the back-and-forth of signals between the transceiver and the antenna, already contain multiple parts, including switches, power amplifiers, antenna tuning, power management and filters. With FEM real-estate reduction tracking at 15%/year and market growth continuing to increase at 15%/year for at least another five years, the quest is on for better, cheaper FEM solutions. Some are targeting SoCs, some will be multi-chip modules.
A couple years ago, Soitec put together a really useful white paper on substrate technologies for RF. You can see, for example, that in choosing a substrate for switches, linear resistivity is the key parameter. This is something that can be addressed by several substrates, including GaAs, SoS and HR-SOI: the deciding factors are the trade-offs between performance and cost.
There are huge opportunities in RF for the greater SOI & engineered substrates communities, so in coming issues of ASN, we’ll be covering more about the apps. If you’re working on one yourself and would like us to cover it, or submit an article, let us know.
Next up look for an article by Professor Jean-Pierre Raskin of UCL. His group is did pioneering work on HR-SOI in the late ’90’s. Now, in collaboration with Soitec, he’s been working on a new generation of HR-SOI with enhanced signal integrity (eSI – referred to in the early papers as “trap-rich”), which holds great promise for further FEM integration.