There is still room for innovation in FBAR filtering to address 5G and beyond – An interview with Broadcom

5G mobile communications is enabled by the opening of higher frequency bands as well as band refarming. This is leading to a need for new components which must handle more stringent requirements on insertion loss, wider bandwidth and steeper out of band rejection, while always ensuring a smaller footprint at a reduced cost. Even though many technologies have been tried for filtering, the future is through the continuous incremental innovation of current SAW, BAW and FBAR filtering technologies. Broadcom is a market leader in filters for mobile phone RF front ends.

Cédric Malaquin, Technology & Market Analyst at Yole Développement (Yole) discussed with Rich Ruby, Director of FBAR Technology, and William Muller, Principal Technology Strategist, at Broadcom.

They agreed to tell us their view of the future of filtering technologies for 5G applications which has been addressed at the market, technology and intellectual property level in Yole, System Plus Consulting and KnowMade reports, respectively: 5G’s Impact on RF Front End Module and Connectivity for Cell phones 2019 – RF Front-End Modules for Cellphones Patent Monitoring Service – RF Acoustic Wave Filters Patent Landscape Analysis – RF Front-End Module Technical Comparison 2019.

i-Micronews invites you to share the discussion:

Cédric Malaquin (CM): Could you please describe your position and mission at Broadcom to our readers?

William Muller (WM): I use the title Principal Technology Strategist.  My brief is to stay aware of developments in the handset RF Front End (RFFE) space to help direct where my division assigns resources.  To this end I interface with service providers and regulators on matters of direction (wants on their end, capabilities on ours) as well as serving as a delegate to standards bodies including 3GPP.

Rich Ruby (RR): I am the Director of Technology for FBAR.  FBAR (Film Bulk Acoustic Resonator) is Broadcom’s version of Bulk Acoustic Wave (BAW) piezoelectric filtering.  I constantly look at ‘disruptive’ ways to improve our product (size, cost, performance) and also keep an eye out for orthogonal applications of the technology that we develop for the handset markets.

CM: Please introduce Broadcom, its product offerings and the markets that the company targets?

WM: Broadcom is a diverse company, generally focusing on technology for Communications markets, Enterprise Storage, Industrial and Enterprise Software.  The Wireless Semiconductor Division, where Rich and I work, focuses on developing multi-technology modules for the RFFE.  Our customers are leading handset manufacturers.

CM: Could you describe how the previous year (2018) has been for Broadcom, in general?

WM: Broadcom is a publicly traded company.  You can check the filed financial results.  I would say “good”.

CM: In your opinion, what is the impact of 5G in Non-Standalone mode, meaning a dual connectivity of both LTE and 5G?

WM: It appears the majority of early 5G rollouts will be Non-Standalone (NSA).  Largely this is driven by economics, as conversion to a 5G core can take significant investment.  NSA allows the data to be sent over an NR link (NR is the 5G air interface), while network control is done over an LTE link (LTE is the 4G air interface).  This Dual Connectivity (DC, or EN-DC in 3GPP speak) allows the existing 4G network control to function, yet still makes use of the spectral advantages of 5G for the data transmission.  Without this capability, deployment of 5G would be much slower.

NSA by definition means two links have to be maintained to make a call.  Two transmissions simultaneously can lead to significant operational challenges as the signals can interact in ways that de-sense or even block reception.  So NSA comes with many more challenges in linearity, and a stronger need to isolate signals. This increases the utility of high rejection filtering, to minimize interactions, and also of multiplexing, to share antenna ports. Sharing antennas becomes more critical under 5G as 4×4 MIMO (Multiple In Multiple Out) is required by standard for some bands, meaning 4 receive antennas have to be used.

CM: Does Broadcom expect a disruptive innovation in filtering technology to address the continuously higher number of filters in phones without additional space and without a price premium?

WM: By definition disruptive changes are hard to anticipate.  However, I believe continued evolution of present technologies is more likely than any disruptive change.  The installed base of technologies appears to be very capable of supporting the requirements that have so far been identified for 5G.  So far, we have been able to shrink the footprint required per band by about 15-20% per year, compensating for added functionality.  Mature high-volume processes are also typically less costly than radically new processes.  The need for multiple simultaneous connections means tunable filtering is not the answer, due to reduced selectivity inherent in tunable schemes.  I would say the biggest challenges of 5G come not from the filters, where we have answers, but from the antennas, where in many cases we don’t.

RR: I agree tunable filters are not the answer. Due to the existence of carrier aggregated bands, multiple tunable filters would have to be switched in parallel between various ‘approved’ CA bands (ex. CA B1/B3/B7 à CA B25.B66,B30).

To date, tunable filters have not even demonstrated decent performance switching from just one band to another. We have also studied the new guided wave SAW (Surface Acoustic Wave) devices (sometimes called IHP SAWs). Although exciting from the point-of-view that they give new life to the SAW community, given a lack of advantage to bulk acoustics in size or performance we don’t expect this technology to be disruptive.

Periodically A/D and D/A technologies are put forward as a way to replace filters altogether. To achieve the wide dynamic range and the linearity and power handling needed to replace filters (and at a reasonable power budget) will be extremely challenging.  I would say that for the next 5 to 10 years, you will see more and more ‘polishing’ of the piezoelectric filter technologies that do get into phones.

We also looked at novel circulators using FBAR, but do not see an intersection point for this technology and cell phones.  

CM: Does Broadcom expect also to enter the infrastructure market once power levels and requirements of massive MIMO active antennas reduce enough?

WM: It takes a fairly large team to make the kind of product we supply.  So far, we find we do better focusing on integrated solutions for handsets rather than on other applications. It’s hard to compete with the volumes required in the handset space. As to powers in 5G infrastructure reducing:  maybe, but early interest in 128×128 element base stations has hit the reality of cost of network deployment, and nowadays there is much more interest in 32×32 or even 16×16 solutions.  More than a few operators are anticipating a roll out of macro coverage as opposed to the earlier envisioned small cell networks. So the volume of handsets is even more compelling in comparison.

CM: Do you see the industry evolving to 12 inch wafers for FBAR technology? What is the position of Broadcom on that topic?

RR: We constantly evaluate the possibility of moving to 12 inch wafers. As MIMO adds more filters to the phone, the number of filters could eventually justify more than one vendor going to 12 inch. However, having personally been involved with multiple conversions, starting with a 3 inch to 4 inch conversion, I can say that it will not be easy to achieve. For now, we have plenty of capacity to support our customers’ needs.

CM: According to you, will FBAR technology handle the above 3.5 GHz 5G NR bands (N77, N78, N79) which will be the primary bands of 5G?

WM: First, a minor difference of opinion.  In my view it is an oversimplification to claim that these are universally the “primary bands” for 5G.  n41, for example, is likely to be more significant worldwide than either n77 or n79.  In the US many people believe the new millimeter wave bands will be the “primary bands”.  To me “primary” is a reflection of deployment, and my belief is 5G will get deployed on whatever unused spectrum an operator has available.  I do agree, however, that in many parts of the world, including in Europe, this is likely to be on n78.  However, we should not forget that the legacy bands remain important to 5G, especially potentially underutilized ones such as n28, n71, or even n5.

But to address the gist of your question, FBAR can indeed serve these new higher frequency bands.  We can support either n78 or n79 with a single acoustic filter.  We can also support n77 with hybrid structures incorporating L-C structures as well as FBAR resonators.  Such FBAR-based filters can solve coexistence issues with WiFi spectrum, enable asynchronous operation on n78 and n79, and provide a more protected environment for n78 operation.  We believe such solutions will be used in some phones where they are seen as adding significant capability.  We don’t expect such solutions to be universally adopted, at least in early implementations.

RR: I argue that all filters used below 6 GHz will use either piezoelectric technologies or integrated passives devices (IPD).  Piezoelectrics will be used where steep roll-off and complex multiplexing are needed. Where performance is not so important, you will see IPD technology.

But an interesting question is at what frequency does BAW (Bulk Acoustic Wave) filter technology begin to stumble?  We have already demonstrated technologies that can address applications up to 10 GHz. If bands in the 10 to 20 GHz region were to open up, we could support this region as well.  However, at some point adequate filters can be patterned on a low loss substrate or on a die, and also the parasitic losses going in and out of packages become excessive.  So, the frequency is not so much the limiter to BAW devices as are alternate simpler technologies or integration scenarios.

CM: Is there still room to integrate more FBAR filters in already highly densified Power Amplifier modules?

WM: Briefly, yes.  There are coming technology elements that allow for a smaller filter area in the RFFE.  Broadcom has significant effort in that direction.  Also for technology that allows for denser assembly, e.g. double-sided assembly, smaller keep-out regions, denser ball grid arrays, etc.  We have several years of clear roadmap to keep on track with the demands that have been identified to date.

RR: I agree. We are working on reducing the areas of all the components that go into the front end module. Given that filters take up the largest area in today’s modules, we are driven hard to innovate in that direction.

CM: Based on your technical expertise and your vision of the RF Electronics industry, do you foresee a restriction of the number of filtering functions in LTE phone RF front ends?

WM: The restrictions come from cost and size, but as mentioned, for flagship phones at least, we have to date been able to keep pace with industry demands.  From what we see so far, expected future demands are not a concern for us.

RR: I once wrote an Op Ed piece (in 2016) where I predicted ~100 filters per phone. Today, the flagship phones have 60+ filters (driven by diversity and MIMO modules as well as the primary front end module). I strongly believe that ~100 filters/phone will happen. And, I don’t see that number being a big issue.  Should it become clear that much higher numbers of filters are needed, say 300+ filters/phone, then we will see a bifurcation of phones targeting geographical regions (e.g. Asia, Americas, Europe etc.…); supporting the primary region fully for data rates and less so for the other geographical regions.

CM: Once the 5G network becomes standalone, do you expect the filtering function to decrease in the handset?

WM: In a word, “no”.  Legacy bands still need to be supported.  4G will remain active in most networks for an extended horizon, long past the first SA networks.  5G also makes use of Carrier Aggregation (CA).  So I do not foresee a decrease in filtering requirements in any visible future.

RR: Besides the legacy issues, the idea of designing a single phone board that is qualified for a large swath of countries and service providers will always keep the filter count high. The alternative of reducing the number of bands in a phone to support one section of a country or one service provider, starts looking like the early ’Nokia Model’ where at the peak of Nokia’s phone presence, they manufactured and supported over 200 different phone products.

CM: Would you like to add some final words for our readers?

WM: The transition from 3G to LTE brought significant demand for high performance acoustic BAW filtering in smartphones, but only after 1 to 2 years of network densification. FBAR filtering now addresses these needs, and in very high volume.  We see a parallel scenario in 5G.  As filters proliferate in the MIMO-rich NR environment, and as multiplexing needs increase to support CA of new bands as well as DC, we expect our FBAR technology to continue to offer some of the highest performance solutions available.  Some OEMs and operators will even require high performance BAW filtering in their first smartphone implementations.  With expanded 8” capacity, continual effort on solution size reduction, BAW performance improvements and capability to meet requirements up to and beyond 6GHz, Broadcom is well placed to meet the demands of the 5G New Radio era.

RR: Imagine a world without piezoelectric resonators for RF filters.  Phones would be using the earlier ceramic filter technology (ceramic duplexers were 5 X 5 X30 mm³ in volume).  In contrast Broadcom FBAR filter volumes are typically 3000 to 4000 times smaller.  Next, imagine a world where the only piezoelectric filters were SAWs.  SAWs work very well around 1 GHz or lower frequencies, but struggle to meet power, insertion loss, isolation and linearity specs for frequencies at 2 GHz or higher. While SAWs are used at higher frequencies for Rx-only applications, such as in diversity modules, the power limitation is fundamental: SAWs are not good for transmit at higher frequencies.  And Tx is needed to maintain the download link, e.g. for synchronization. However, there is simply not enough bandwidth below 1 GHz to give 100 MHz bandwidth to a user and meet the goal of Gigabit per sec download speeds.  With only SAWs, downloading large data content (movies, streaming, etc.…) would simply not be part of our vocabulary.  FBAR started around 1993 at HP Labs.  It was the first high volume BAW technology for cellular applications; we introduced the first stand-alone FBAR duplexer in 2001.  By 2013 we were in every smartphone sold.  Today, all of the high-performance smartphones use either FBAR or other BAW technology.  In short, you could say FBAR was a key enabler of the modern smartphone.

Interviewees

William Mueller has a BSE from Harvey Mudd College, and an MSEE from UC Berkeley.

William has over 45 years of experience in RF, with emphasis on component level design and RF front end architectures. The last 34 years have been with Avantek-HP-Agilent-Avago-Broadcom (the same work group, different names) where he is presently Principal Technology Strategist for Broadcom Inc.’s Wireless Semiconductor Division.

William is familiar with RF power amplifier and low noise amplifier design, as well as FBAR filter design. He is active in standards bodies (3GPP, MIPI), industry forums (IWPC, GTI, IEEE), and with regulatory (FCC, Ofcom).

William has presented numerous papers at symposia, and holds three patents relating to RF front end components. 

Rich Ruby, PhD (U.C. Berkeley) is Director of Technology, IEEE Fellow.

Rich joined HP Labs in 1984 working on superconductivity, E-beam lithography, X-Ray lithography and packaging. In 1993, he started work on Free Standing Bulk Acoustic Wave Resonator devices (FBAR) and has stayed with that technology since.

He has made many contributions to the acoustic properties, manufacturability and the packaging of FBAR filters and duplexers. Rich commercialized the first FBAR duplexers HPMD7901 and the 7904 back in 2001 to 2003. The first all-silicon, chip-scale packaged FBAR duplexer was introduced in 2004. Today, Avago/Broadcom sells over 2 Billion FBAR filters per year into the mobile market.

Over the years, Rich was awarded the Samuel Silver Award, the Barney Oliver Prize, the Bill Hewlett Award, the C.B. Sawyer Memorial Award, the Institute of American Physics Prize for Industrial Application of Physics and recently Distinguished Alumni of U.C. Berkeley. Rich has over 90 patents in the area of FBAR devices.

Interviewer

As a Technology & Market Analyst, specialized in RF devices & technologies within the Power & Wireless division at Yole Développement (Yole), Cédric Malaquin is involved in the development of technology & market reports as well as the production of custom consulting projects. 
Prior his mission at Yole, Cédric first served Soitec as a process integration engineer during 9 years, then as an electrical characterization engineer during 6 years. He deeply contributed to FDSOI and RFSOI products characterization. He has also authored or co-authored three patents and five international publications in the semiconductor field.
Cédric graduated from Polytech Lille in France with an engineering degree in microelectronics and material sciences.

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