Bonding technology to extend the SAW filter footprint – An interview with EV Group

For many years, Bulk Acoustic Wave (BAW) filters have been the leading Radio Frequency (RF) filter technology for high frequency bands in mobile communication. Now, the emerging thin-film (TF) Surface Acoustic Wave (SAW) filter technology could disrupt the market. As explained in Yole Développement’s report “5G’s Impact on RF Front-End Module and Connectivity for Cell Phones 2019”, this technology offers some benefits over legacy SAW and Temperature Compensated (TC)-SAW products, while costing less than BAW. TF SAW addresses relatively large bandwidth, while offering a good temperature-compensated performance and an efficient path for integration of multiple bands. Murata already has design wins with its Incredible High Performance (I.H.P.) SAW filter with 2.4 GHz Wi-Fi filter, B25 duplexer, B25-B66 quadplexer. System Plus Consulting has analyzed Murata’s I.H.P. SAW, giving a first cost estimation for this promising technology. It finds a critical role of the engineered substrate enabled by bonding equipment suppliers, among which EV Group (EVG) is a pioneer company. Yole Développement and System Plus Consulting had the chance to explore developments in RF filters with Elisabeth Brandl, Product Marketing Manager at EV Group. Read on to find out what we discussed.

Cédric Malaquin: Could you please describe the position and mission of EVG to our readers?

Elisabeth Brandl: EVG is a supplier for the semiconductor and MEMS industry. We always offer not only equipment, but complete solutions that enable us to adapt our tools to the needs of the market. Our vision to be the first to explore new technologies and serve next generation applications of micro- and nanofabrication technologies enables our customers to successfully market their new product ideas. With different technologies such as bonding and lithography and our spacious clean room, we offer our customers the opportunity to develop processes from scratch and play with new ideas. This framework acts as an innovation incubator, and with the building of another new cleanroom we are able to enhance our capability for process development and pilot line production.

Stéphane Elisabeth: Focusing on the RF acoustic wave filter business, could you describe EVG’s contribution to enabling the growing TC-SAW and upcoming TF-SAW filter markets?

EB: With the introduction of 5G there are new challenges for RF filters. Besides getting tighter requirements for coupling quality, frequency accuracy and higher reflection outside the filter band, the influence of fluctuations in frequency behavior is more severe due to the crowded spectrum,  the frequency dependence on temperature is more critical.

SAW filters have the potential to enable higher frequencies, although a special layer structure is required. Wafer bonding is used in manufacturing to enable SAW filters with a certain multilayer structure that can meet the new requirements for 5G and enable higher frequencies. For this type of application, piezoelectric SAW wafers made of lithium tantalate (LTO) or lithium niobate (LNO) are bonded to receiving wafers. These receiving wafers either already consist of several layers with defined acoustic properties or are a stiff substrate to reduce the coefficient of thermal expansion (CTE) of the piezoelectric LTO/LNO wafer for more stable temperature behavior.

Besides bonding, EVG offers metrology technology to measure film thicknesses or wafer overlay in the bonded stack. Furthermore, lithography technologies, such as coating, development, mask alignment, imprint or newly added maskless exposure lithography complete our portfolio.

CM: Integrated Device Manufacturer (IDM) companies are leading the filter business. However, fabless companies are investing in filter design and working with filter foundries and substrate suppliers to enter the business as well. From which part of this ecosystem do you see more business opportunities for TC-SAW and TF-SAW?

EB: Smartphone manufacturers are among the leading customers for RF front end modules and SAW discretes. Integration strategies are shifting from sourcing the RF modules towards customizing RF modules for the specific product by themselves. For this reason, the supply chain is changing, whereby some discrete manufacturers also manufacture modules, foundries like business models are getting more important, on the same time holding all relevant components for RF modules is becoming increasingly important. Accordingly, direct bonding of LTO and LNO substrates is used for the production of bare SAW die, and here EVG sees opportunities to work with SAW filter manufacturers as well as their suppliers, including substrate manufacturers and crystal growers, as well as other parts of the supply chain. 

Besides the top SAW filter players, we also see major interest in China for TF-SAW, as this technology will play a big role in 5G. With the increasing requirements in SAW filter volume, we see growth in all relevant areas.

SE: The BAW filter market generally is using deposition and trimming techniques. Do you see market traction in going to bonding and layer transfer techniques to serve BAW filter technologies?

EB: For BAW filters, deposition is definitely the key to overall filter performance and shifts requirements away from the substrate. Currently, wafer bonding is used for packaging of BAW filter manufacturing. The main bonding market for piezoelectric engineered substrates is for SAW filters.

CM: How do you see the growth of the bonding market for RF filters?

EB: When we differentiate between bonding for packaging applications and bonding for engineered substrates, we can say that bonding for engineered substrates is a high-growth market driven by the need for tighter specifications for RF filters due to 5G. Bonding for packaging applications is already well established, especially for BAW filters, and it will continue to grow.

CM: What are the main permanent bonding technologies used today? What are the trends?

EB: Direct bonding for engineered substrates and metal bonding for MEMS are already well established in several fields. Hybrid bonding is currently the hottest topic as it is capturing application share not only in backside illuminated image sensors, but also in 3D integration in various forms. Here, development is pushed further because the requirements for hybrid bonding in 3D integration are getting tighter.

SE: Fusion bonding is well known in Image Sensor or MEMS industries. Are there any specific requirements in the RF filter market? What are the technical specifications required at the bonding interface for RF filters?

EB: Indeed, bonding of image sensors, MEMS substrates as well as Silicon-on-Insulator (SOI) wafers has been established in high-volume manufacturing for more than a decade, with EVG having a clear lead of more than 90% market share in some areas. While wafer-to-wafer alignment is very challenging for image sensors, engineered substrates such as SOI and SAW are not alignment-critical. However, the difficulty for TF-SAW comes from its unique material properties. Anisotropic thermal expansion of piezo wafers is the biggest challenge, as fusion bonding interfaces need annealing after bonding for strengthening. Using typical annealing temperature of 300°C always results in wafer breakage. Therefore, modified plasma activation of the substrate prior to bonding facilitates the bond formation at greatly reduced bonding temperatures.

SE: SAW filter plants span wafer sizes from 4-inch to 6-inch. Does EVG’s bonding tool run 4-inch to 6-inch on a single piece of equipment?

EB: EVG’s equipment philosophy is to allow flexibility and innovation for our customers. Therefore, bridge tool capability is always possible and the same tools can be used by our customers to ramp up to larger wafer sizes.

SE: Given the increasing number of filters used in handset for LTE and 5G standards, do you expect a transition to 8-inch? If yes, when?

EB: From an equipment perspective, all our technologies are mature and proven for high volume up to 12” substrate sizes. SAW filters have two major obstacles to increasing their size, namely large wafer availability and the high CTE mismatch to other materials, which currently restricts them to the 6-inch substrate size. The first 8-inch piezoelectric wafers are not expected to arrive any earlier than 2023 in high volume manufacturing. On the other hand, bulk acoustic wave (BAW) devices are deposited on silicon wafers, where size obviously is not a showstopper. Here, 8-inch is the incumbent wafer size for many filter manufacturers.

Interviewee

Elisabeth Brandl, Product Marketing Manager Elisabeth Brandl received her master in technical physics from the Johannes Kepler University Linz, Austria in Semiconductor and Solid State Physics. Since 2014, she has been responsible for Product Marketing Management for temporary bonding and compound semiconductors at EVG.

Interviewers

Cedric Malaquin - Yole Développement

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.

Dr. Stéphane Elisabeth has joined System Plus Consulting’s team in 2016. He has a deep knowledge of Materials characterizations and Electronics systems. He holds an Engineering Degree in Electronics and Numerical Technology, and a PhD in Materials for Microelectronics.


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5G’s Impact on RF Front-End Module and Connectivity for Cell phones 2019
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Murata Incredible High Performance (IHP) SAW Filter

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