How SOI wafers for RF predict LTE-A/5G device performance


Soitec has developed an innovative metrology and metric for ensuring that devices built on our latest SOI wafers for RF will meet the draconian demands of LTE-Advanced (LTE-A) and 5G network standards.

For smartphones and tablets to handle LTE-A and 5G, they need RF devices with much higher linearity than those running over the current 2G, 3G, 4G and LTE network generations. These next generation network standards require mobile devices to support more bands, higher frequency bands, and emission and reception on adjacent bands with downlink and uplink carrier aggregation. (Carrier aggregation refers to the simultaneous reception of multiple frequency bands to improve data throughput.)

Soitec recently announced eSI90, our newest generation of trap-rich, high-resistivity SOI wafers for LTE-A and 5G. eSI90 extends our existing line of eSITM (enhanced Signal Integrity) wafers, the first generation of which are currently being used by leading manufactures to produce more than a billion RF devices every quarter.

This article gives an overview of how Soitec developed a new metric using innovative metrology on its wafers in order to predict the RF performance of final devices manufactured on eSI substrates. (Readers wanting greater detail can also consult our complete white paper on the subject, which is freely available to download here.)

Wafer specs evolve to meet new standards

To address the different communication standards and functions used in front-end modules, Soitec, the leader in SOI technology, has developed two flavors of RF-SOI products – high-resistivity (HR)-SOI and Enhanced Signal Integrity TM (eSI) SOI – both of which are compatible with standard CMOS processes. While standard HR-SOI wafers (which we introduced over a decade ago) are capable of meeting 2G or 3G requirements, eSI SOI can achieve much higher linearity and isolation specifications, allowing designers to address some of the most stringent LTE requirements. (We detailed how advanced RF design challenges are solved by eSI wafers in a 2013 ASN article – you can still read it here.) This paves the way for integrating more functions on a device with better RF performance at competitive cost.

eSI_SoitecUCLwafer
Soitec’s enhanced Signal Integrity™ (eSI) wafers integrate a trap-rich layer under the insulating BoX in a high-resistivity (HR) SOI wafer (Image courtesy of Soitec)

 

eSI wafers leverage the addition of a “trap-rich” layer to high-resistivity (HR) SOI wafers, an approach that was developed by UCL and Soitec (that project was covered in an ASN piece explaining the technical details at the time – you can read it here).

 

Change at all levels

The IIP3 linearity requirements for 3G are +65dBm. For LTE, they increased to +72dBm, and for LTE-A, they are over +90dBm. For RF designers, this has added substantially to the complexity of RF Front-End Modules (FEMs), and entails multiple changes for each of the main functions: switches, power amplifiers, power management and antenna tuners.

RFSOI_FEM_3G
Example of Front-End Module Block Diagram for 3G
RFSOI_FEM_LTE
Example of Front-End Module Block Diagram for LTE

 

These latest front-end modules need to support more bands, higher frequency bands from 700 MHz to 3.5 GHz, larger bands from 20 MHz to100 MHz and carrier aggregation downlink and uplink, sometimes on adjacent bands. This means:

  • A proliferation of switches on top of the antenna switch including diversity, power-mode and antenna-swapping switches
  • Advanced, tunable power-amplifier architectures to achieve compact and cost-effective multi-mode, multi-band transmission in a single broadband power amplifier
  • Advanced power management: with an envelope-tracking system approach, the efficiency of broadband power amplifiers will be close to or as good as that of single-band power amplifiers
  • Advanced wide-band antenna: with an antenna-tuner system performing either impedance matching and/or aperture tuning, an antenna can efficiently cover bands with frequencies from 700 MHz to 3.5 GHz with optimum efficiency and a smaller footprint

To meet the required performance, many changes are happening at all levels, from systems, architectures, design, manufacturing processes, devices – right down to where it all starts: the substrates. The substrates on which RF devices are manufactured have a significant impact on the level of performance that the final chips will be capable of achieving.

Characterizing eSI wafers

To quantify the performance designers can expect from an eSI SOI substrate, Soitec has now developed an innovative characterization method based on spreading resistance profiling (SRP), which can predict the 2nd harmonic distortion (HD2) performance of a coplanar waveguide. This solution is used today throughout the Soitec eSI product line to ensure the substrates will enable the expected RF performance in the finished devices.

We predict the RF harmonic distortion performance of the substrate immediately after the eSI SOI substrates are fabricated and before any devices are manufactured on them. This prediction is provided through a metric we call the harmonic quality factor (HQF).

HQF correlates with the second harmonic distortion generated from a 900-MHz signal applied to a coplanar waveguide (CPW) deposited on the substrate.

The CPWs are implemented on sample test wafers by depositing aluminum metal lines on the buried oxide of eSI SOI wafers after the Smart Cut process has been completed and the top silicium removed.

Then a 900-MHZ fundamental tone is applied on one end of the CPW line and the HD2 signal is measured at the other, providing a value of the HD2 generated by the substrate. Then, using the same wafers, a Spreading Resistance Profiling (SRP) technique measures the resistivity of the material at different depths under the buried oxide.

Next, we use a proprietary algorithm to compute the series of measures. The algorithm, tuned to match various HD2 values, takes into account the resistivity of the substrates weighted by the depth of the measure, and gives us the HQF.

Soitec has implemented this metrology on its production eSI SOI wafers and is sampling products to carry the HQF measurement.

To address different market requirements, we set our HQFmax specification at -80 dBm for eSI-G1 (first-generation eSI product) and at -90 dBm for our eSI90 (second-generation eSI product).

Soitec_RFSOI_eSI90_HQF
HQF specifications for Soitec’s 1st and 2nd generation eSI products (eSI-G1i and eSI90, respectively) correlated with linearity requirements.

Conclusion

HQF metrology, conducted at the substrate level, provides a reliable measure of the finished devices’ RF performance. It is now being used by Soitec to report the expected RF linearity performance of ICs manufactured with RF-SOI substrates.

As a solution addressing the current and next generation of RF standards, eSI SOI wafers are enabling this market by meeting some of the most difficult LTE and LTE Advanced linearity requirements. Soitec is able to provide its customers with the eSI SOI substrates that meet their desired level of RF performance.

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