SAW/BAW New Market Entrants Offer New Approaches
Editor’s Note: With the increasing number of cellular bands for 4G/LTE, the mobile RF front-end’s critical component has shifted from the power amplifier to the filter. Surface acoustic wave (SAW), and, more recently, bulk acoustic wave (BAW) filter technology has been addressing the challenges in the mobile RF front-end that currently uses 40+ filters (and growing). This market growth has attracted new market entrants so Microwave Journal compiled information from three such companies—Akoustis, OnScale and Resonant—offering new solutions for the SAW/BAW market.
XBAW RF Filter Blazing Into Higher Frequency Spectrum
BAW RF filters are high performance semiconductor components primarily used in mobile smart phones. They address the stringent size requirement for high levels of integration and provide superior performance compared to SAW and ceramic filters, therefore improving the battery life and reducing the number of dropped calls to end users. These high performance components offer low insertion loss and high selectivity required to meet the demanding coexistence requirement for difficult FDD and high frequency TDD 4G/LTE and emerging 5G bands. Current multimode, multiband mid- to high-tier smartphones utilize > 50 filters and experts foresee that including 5G bands will push that number to > 70 filters.
Solidly mounted resonator (SMR) and film bulk acoustic resonator (FBAR) are the two dominant BAW resonator technologies currently utilized in BAW RF filters due to their high Q-factor, high operating frequency and good power handling. The BAW RF filter market is currently served by a duopoly that has historically supplied > 95 percent of the market, where both company’s core material technology is based on a sputtered poly-crystalline piezo-electric aluminum nitride (AlN) deposited by physical vapor deposition (PVD) techniques.
Single Crystal RF BAW Filter Technology
Akoustis Technologies is an emerging new entrant in the projected $5.8 billion BAW filter market dominated by mobile RF filters.1 Leveraging a patented BAW resonator process (called XBAW) combined with an integrated design and manufacturing (IDM) business model, Akoustis is blazing new territory and focused on becoming the first commercial supplier of BAW RF filters for applications above 3 GHz.
Akoustis has introduced a new approach of utilizing high purity, single crystal piezo-electric AlN material in BAW RF filters (see Figure 1). Epitaxially grown, metal-organic chemical vapor phase deposition (MOCVD) single crystal AlN has inherently higher crystal quality compared to PVD poly-crystal AlN. This improved crystal quality has shown improvements in acoustic velocity and piezo-electric mechanical coupling coefficients. In addition, the thermal conductivity of single crystal AlN is 2× higher than poly-crystal AlN that degrades as film thickness decreases, which may result in a constraint on power capability for traditional FBAR resonators, especially at higher frequencies. In all BAW technologies shown in Figure 1, the resonance frequency is determined by the thickness of the material stack and the effective propagation velocity of the acoustic wave. A higher propagation velocity in the AlN piezo-material results in higher operating frequencies for the same thickness. These three factors; improved acoustic velocity, improved piezo-electric coefficients and improved thermal conductivity enable XBAW RF filters constructed from single crystal, epitaxially grown MOCVD-AlN piezo-electric materials to offer better performance (power handling, insertion loss, bandwidth and skirt steepness) than PVD-AlN based BAW RF filters, especially for high frequency and high-power applications (see Figure 2).
In June 2017, Akoustis completed the strategic acquisition of a MEMS fab located in Canandaigua, N.Y. With this acquisition and subsequent consolidation of all its manufacturing processes, Akoustis now has an internal, ISO-9001 certified 122,000 sq. ft. commercial wafer-manufacturing capability which includes class 100/1000 cleanroom facility, tooled for 150 mm diameter wafers and an operations team to conduct research, development and production of its XBAW RF Filters. In addition, Akoustis is in the process of transitioning DoD Trusted Foundry accreditation for MEMs wafer processing, packaging and assembly, enabling Akoustis to be a supplier for DoD programs requiring specialized filters and Trusted Foundry certification.
RF BAW Filter Markets
Akoustis is the only pure play BAW RF filter company targeting the mobile high band 4G/LTE and emerging 5G applications. This market is by far the largest and made up of filter competitors engaging mobile phone OEMs and ODMs, RF front-end (RFFE) module manufacturers (some with captive BAW filter technology) and transceiver manufacturers. The push to higher frequency and the wide bandwidth requirement necessary to support the enhanced Mobile Broadband (eMBB) feature of 5G will tax existing SAW and poly-crystal BAW filter technology. Single crystal RF BAW technology will enable the development of higher performance, wider bandwidth BAW RF filters for 5G n41, n77, n78 and n79 bands (or sub bands) operating in 2.6 to 5 GHz spectrum with bandwidths that range from 200 to 900 MHz.
Beyond mobile, there are two additional markets that will be well served with access to single crystal RF BAW technology. Advanced Wi-Fi CPE architectures including 802.11ac multi-user MIMO (MU-MIMO) are experiencing faster uptake, driving the demand for smaller components as the complexity within Wi-Fi infrastructure devices is increasing. This trend is expected to continue, especially as 802.11ax is finalized and implemented in next generation tri-band routers that operate at 2.4, 5.2 and 5.6 GHz, simultaneously. Ultra-small passband 5.2 GHz BAW RF filters provide low 1.2 dB typical insertion loss over 160 MHz covering U-NII-1 and U-NII-2A bands with typical 52 dB attenuation across 345 MHz to meet the stringent rejection requirements enabling coexistence with U-NII-2C and U-NII-3 bands (see Figure 3). Incumbent Dielectric Resonator (DR) filters are 23× larger and require shield cans to mitigate interference issues degrading isolation performance.
The infrastructure market is looking at full dimension-MIMO or Massive MIMO architectures which use large antenna array each with its own transceiver configuration to offer much higher spectral efficiency. These new basestation systems will probably be the primary solution for emerging 5G and an alternative to traditional macro-cell BTS for 4.5G and 4.9G LTE networks. FD-MIMO architectures support both FDD and TDD bands and offer 1 to 4 W average powers in 32T32R to 64T64R configurations operating in the 2 to 5 GHz spectrum. These large array systems will need an alternative filter technology to enable size/weight reduction and high volume, surface mount assembly. Traditionally macro-cell style cavity filters are large in size and typically require manual assembly so are not ideal for FD-MIMO systems. Poly-crystal BAW RF filters are used in pico- and micro-cell BTS but may fall short on power handling above 1 W. High-power single crystal technology offers a potential paradigm shift to the major BTS OEMs developing 5G FD-MIMO systems. Akoustis has demonstrated BAW RF filter die mounted on standard laminate capable of handling > 10 W average power at 2.6 GHz (see Figure 4). This power handling provides plenty of power margin headroom for the design of RF BAW filters that offer smaller form factor, surface mount assembly at semiconductor price levels.
Akoustis Technologies is a new entrant to the multi-billion RF filter market and blazing its own path through material science innovation in single crystal piezo-electric enabling high performance BAW RF filters in the 3 to 6 GHz spectrum for emerging 5G mobile, Wi-Fi and infrastructure applications. Beyond these largest markets, Akoustis is eyeing additional markets such as automotive C-V2X (or DSRC) and military IF/RF filters for L-, S-, C- and X-Band phase radar and communication systems.
Enabling Design of Next-Generation RF Filters for 5G
While 4G LTE and LTE-Advanced technologies are still being deployed worldwide, the next generation in wireless communication promises a paradigm shift in throughput, latency and scalability. By 2025, the emerging wireless 5G market is expected to reach a total value of $250 billion.2 SAW filters and BAW filters are already used in 4G devices and will compete for the emerging 5G market. Adoption of 5G will see a significant increase in the number of filters required in a handset, with 4G models already employing 40+ filters. This puts the onus on 5G manufacturers to rapidly innovate new filter designs to capture a share of the growing market. Such innovations tend to offer a “winner take all” prospect such as the FBAR filter ushering in an entirely new product that captured a large percentage of the 4G/LTE market segment.
To help drive this new level of innovation, OnScale has developed a cloud-enabled simulation platform optimized for Multiphysics analysis of piezoelectric devices such as SAWs and BAWs. This approach is being used to reduce cost, risk and time to market for these products.
Optimization of SAW/BAW filters is a challenging task due to the complexity and size of the devices. OnScale is well-suited for these kinds of problems, taking a highly efficient Finite Element Analysis (FEA) approach and seamlessly deploying this on the cloud. Figure 5 shows a SAW filter with 100 interdigitated pairs and 20 grating fingers modeled in full 3D. The zoomed-in portion shows the simulated surface velocity at a given time-step. The entire model can be run in a matter of hours which is a feat even the most powerful legacy solvers on the market are typically incapable of doing.
Optimization of this design is achieved using the cloud, where hundreds of these models can be simulated simultaneously to allow exploration of a design space defined by the variation of specific parameters. One of these iterations reveals a sweet spot where Q is maximized and spurs are minimized in the impedance of the device. Figure 6 shows the chosen design’s impedance versus frequency in this example.
FBAR Design Example
FBAR filters, unlike their surface and bulk silicon counterparts, use piezoelectric thin films over cavities with resonant frequencies between 100 MHz and 10 GHz. A range of different shapes and sizes can be used depending on the performance requirements, with early designs using square shapes and more advanced designs using pentagons. Figure 7a shows a layout of a pentagonal FBAR resonator that has been imported into OnScale from a GDSII file. An image of the instantaneous surface velocity from the simulation is shown in Figure 7b.
A major challenge for designers is ensuring that filters do not support strong lateral resonances that corrupt passband performance. Resonators with non-parallel sides, such as those shown in Figure 8, support weaker lateral resonances than ones with parallel sides. However, optimizing these shapes empirically is expensive and time consuming. Ideally, an engineer would use full 3D simulation for this optimization process, but this is considered impractical due to the extremely large computational requirements and time demanded by legacy FEM tools. The cloud method solves this problem, delivering rapid insights into these complex electro-mechanical systems and opens entirely new solution spaces for engineers to explore.
To demonstrate this capability, a 3D model of a pentagonal FBAR was constructed and a generic algorithm was used to optimize the design of the filter to minimize lateral resonances. Genetic algorithms mimic the process of natural selection to guide successive populations of candidate designs towards a global optimum. Each population of designs was simulated in parallel on the cloud, as shown in Figure 9.
The model was run for 52 generations and a total of 3,640 designs were investigated. It ran for a total of 68 hours and utilized 8.67 GB of memory. The simulation tool was connected to MATLAB’s Global Optimization Toolbox, which allowed various parameters to be tracked during the run including the current best design. The optimal designs were found to have edges angled relative to the substrate edges to avoid strong reflections, whereas the worst design had three edges close to parallel with the substrate causing increased lateral mode activity.
The results can be seen in Figure 10, where the best pentagonal design shows a significant reduction in ripple when compared to the square device, which was the starting point for the exercise. It is important to note that each of the 3,640 designs were simulated in full 3D, a study that would take a legacy solver nearly a year to complete on the same computing resources. The results provided are indicative of the type of design improvements that can be achieved with cloud based CAE using OnScale’s solvers.
Despite the lack of standards, 5G is promising faster data rates for mobile phones and will be an enabler for autonomous vehicles and the IoT. The move from 4G to 5G represents orders of magnitude higher data rates at frequency bands beyond 3 GHz. However, legacy CAE tools are incapable of performing complete 3D design studies, which are a critical step in optimizing the design and improving the time to market for these highly complex structures. OnScale’s cloud solvers open the possibility of doing this analysis in parallel, reducing prototyping costs and speeding time to market.
Infinite Synthesized Networks Deliver RF Filter Design Tools for 5G
Santa Barbara, Calif.
Where 4G/LTE was a single specification for high speed mobile device services, 5G is a family of technologies designed to serve different use cases ranging from ultra-broadband fixed wireless to low data-rate IoT services. The transition to this new network technology will result in dramatic increases in filter and RFFE complexity.
5G devices will exist in a mobile device environment that includes more complexity, more components (particularly filters), more performance demands, smaller size and lower cost components, plus dual connectivity between cellular and Wi-Fi networks. More bandwidth will be needed, which will require higher frequency components, more carrier aggregation (CA), more complex MIMO antennas, new and adaptable waveforms and improved interference mitigation.
5G RFFE designs for all wireless-enabled products will be driven by cost, power efficiency and available space within the mobile device. The requirements for 5G filters will include complex multiplexing, increasing integration, more filters and the capability to handle much higher frequencies than are currently in use.
Resonant Infinite Synthesized Networks
To address these needs, Resonant has developed a comprehensive filter Electronic Design Automation (EDA) platform called Infinite Synthesized Networks (ISN). Resonant’s ISN platform brings together the following elements:
- Modern filter theory.
- Finite element modeling, both electro-magnetic and acoustic.
- Novel optimization algorithms.
- Ecosystem of foundry and packaging/back-end partners.
ISN was initially focused on designing acoustic wave filters, which are a key design block for the RFFE. ISN is specifically intended to solve many of the 5G challenges that will face design engineers: speed, flexibility and tools that drive down system cost. As of August 2018, more than 10 companies have committed to produce more than 60 devices using ISN.
Figure 11 is a schematic that shows the design-to-mask flow through the ISN process. Testing has proven that ISN’s models are highly accurate and reflect physical details of the filter structures, matching the measured performance of manufactured filters, not only in loss and isolation but also in power handling and linearity. Thus, ISN is a capable platform for quickly, efficiently and cost-effectively scaling filter design to meet emerging 5G demand.
Traditional acoustic wave filter design uses a ladder structure and empirical models (linked to a particular fab manufacturer). This typically results in an iterative approach to filter development that involves multiple foundry runs and can take months or more. The ISN platform enables filter design teams to create novel filter structures that outperform traditional filter designs, in a smaller footprint and using lower-cost technologies. Figure 12 shows how closely ISN-modeled performance tracks the actual data measured on a Band 3 duplexer.
ISN’s grounding in fundamental materials physics, while optimizing for high-volume design screening, enables designs that are unconstrained by traditional acoustic wave filter design techniques. Consequently, a designer using ISN can create multiplexers, wide passbands and high-power performance, and predict manufacturing yields as well, before a design is committed to mass production.
Thousands of designs can be developed simultaneously and screened to maximize the ultimate performance of the device. Leveraging the expertise of filter design engineers for an increasing number of more complex designs can be achieved using ISN.
Implications for the 5G RF Front-End
ISN can be used to develop RF filters for 4G/LTE and other wireless networks, but it is especially impactful for 5G designs that need the high performance, small size and complex passband design benefits of the design tool.
The current state-of-the-art for a 4G/LTE mobile smartphone RFFE separates the frequency spectrum into low-band (698 to 960 MHz), mid-band (1710 to 2200 MHz) and high-band (2400 to 3800 MHz) frequencies, which isolates the RF components, minimizes cross-talk and optimizes the entire power amplifier-filter-switch path (see Figure 13). Although integration of components is logical, the increasing complexity of 5G limits the number of manufacturers that have the expertise to execute on such a complex RF sub-system.
5G RFFEs for all wireless-enabled products will be driven by cost, power efficiency and available space within the unit. So they will need to be small, highly efficient and able to be manufactured in large quantities to meet fast-growing global demand. To commercialize affordable custom parts for IoT devices in particular, RFFEs will need to be designed with a minimum number of components and manufacturing volumes will have to increase dramatically from current levels to reduce per-unit cost. In the current environment, most IoT devices are being built with low-cost parts originally developed for high-volume mobile phone production.
As we move toward 5G, the complexity of the RFFE continues to increase. For instance, in addition to the main antenna path modules, diversity antennas provide both link robustness and increased downlink data rates. Designers are increasingly using receive diversity modules to process the diversity path, comprised of receive (Rx) filters and switches (and increasingly incorporating LNAs). Wireless carriers demanding higher 5G data rates will drive more carrier aggregation, creating more potential interference. Consequently, the onus on RFFE designers moving forward will be to reduce complexity, reduce cost, while at the same time improving performance.
5G Filter Requirements
The growth in the number of filters, and the ever more demanding performance requirements, make RF filtering the critical pain point of the RFFE. The basic requirements for a 5G filter includes complex multiplexing driven by CA and increasing integration to maintain high performance of the RFFE. Maximizing PA efficiency on the uplink, and receiver sensitivity on the downlink, will require optimization of the entire RF chain. As complexity increases, it will be crucial to understand the RF chain and any interactions between elements.
Isolation, loss and power handling requirements continue to create new performance challenges. Filters in the RF chain are a major contributor to loss, which is critical for total Tx efficiency (and ultimately for the current draw for the PA and battery life), and the total noise figure in the Rx path (and ultimately for the SNR and the data rate). Figure 14 shows the estimated losses from each component in the Tx path.
LTE, which is optimized for high speed data, demanded significantly higher power than 3G protocols such as CDMA. And as such, the requirements for isolation and minimizing leakage into the Rx path, and vice versa, grew. This will only be further exacerbated by high-power user equipment (HPUE), which uses more Tx power for improved cell edge coverage. In addition, power durability of progressively smaller filters becomes a major concern.
For 5G, frequencies greater than 6 GHz will require different filter technology than the current acoustic wave filters used in mobile devices. Significant advances will be needed to reduce size and cost. The 5G RFFE for mobile broadband will be extremely complex and that the goal for filter design will be to both simplify the design process and the RFFE itself.
Innovations that enable 5G RFFEs will need to include a low-loss triplexer (to minimize the number of antennas), multi-mode, multi-band PAs and multi-band filters (to reduce the number of filters and switches), all of which will need to be optimized as a complete system to reduce matching components.
With RF complexity expected to grow significantly in 5G devices, the time is right for a filter design tool that can design better, more complex components in a time and capital efficient way. ISN delivers on this need with highly accurate, highly integrated and highly manufacturable filters with complex features.n
- “RF Front-Ends for Mobile Devices 2018,” Mobile Experts Inc., 2018.
- “5G Wireless Market Worth $250 Billion by 2025: $6 Billion Spend Forecast on R&D for 2015–2020,” PR Newswire, March 2016.
- R. Ruby, “A Decade of FBAR Success and What Is Needed for Another Successful Decade,” 2011 Symposium on Piezoelectricity, Acoustic Waves and Device Applications (SPAWDA), 2011, pp. 365–369.