What are the different forms of wafer-level packaging for BAW?

Source：新声半导体 Author：新声半导体 Time：2024-05-27

Packaging technology is crucial in the semiconductor industry, as it not only protects the chips but also provides electrical connections and thermal pathways. Categorized by different packaging forms and application fields, semiconductor packaging technologies mainly include:

Figure 1. Packaging process evolution

Distinct from conventional chip packaging methods, Wafer Level Packaging (WLP) technology involves encapsulating and testing the entire wafer before cutting the packaged wafer into individual chips. Among all packaging forms, WLP is the only method that does not increase the area on the wafer base. This approach enhances production efficiency, reduces costs, and improves performance. Additionally, WLP can achieve a packaged filter thickness between 0.2-0.3mm, significantly lower than that of CSP. With the widespread adoption of slim smartphones, especially with the large-scale promotion of folding screens, the reduction of RF device thickness has become a hard requirement, making the importance of WLP increasingly evident.

Over the past 30 years, with the continuous emergence of various MEMS products, advanced MEMS packaging technologies have been developed in abundance. MEMS packaging is based on IC packaging and has evolved to include various WLP structures and processes. There are five key technologies in WLP, which are:

1. hrough Silicon Via（TSV）

By drilling holes through silicon wafers and filling them with conductive materials, vertical electrical connection channels are formed, enabling direct interconnectivity between chips or different levels within a chip's structure.

2. Redistribution Layer（RDL）

By redistributing and rerouting interconnect lines on the surface of a chip or package, it provides more flexible and efficient electrical connections, supporting a variety of I/O configurations and the demands of high-density interconnectivity.

3. Bumping

By forming one or more tiny metal bump structures on the I/O pads of a chip, it achieves an efficient and reliable electrical interconnection between the chip and the packaging substrate or other chips.

4. Wafer Bonding

By closely joining two or more highly polished wafers together to form a single composite structure, this bonding technique aims to create a stable, high-strength bond suitable for device structures where access to the underlying wafer is not required again.

5. Plating

Utilizing the principles of electrolysis to deposit a layer of metal or alloy coating on the surface of a conductive object. In WLP technology, plating is involved in the metal filling of TSVs, the formation of bumps, and the fabrication of RDLs.

BAW filters are radio frequency MEMS devices with a three-dimensional mechanical structure (vibrating resonator units), which require a stable external environment to operate effectively. This characteristic means that the packaging of BAW filters differs significantly from traditional IC packaging and involves a more complex process. While IC packaging is planar and two-dimensional, wafer-level packaging processes such as WLCSP and FO-WLP are designed to integrate more I/O pins; BAW filter packaging, on the other hand, is three-dimensional. Beyond the pins, it is crucial to provide a sealed cavity that allows the resonator to vibrate freely in a stable environment. Therefore, BAW filters require a "3D WLP" approach.

Currently, the mature WLP technology routes for BAW filters include:

Avago：Shaped Si Cap + Au-Au Bonding (Microcap)

Qorvo：Polymer Cap + Organic Bonding

Skyworks：Copper Filled TSV + Cu-Sn Bonding

Newsonic：Raw Si Cap + Organic Bonding（SiRoof®)

Avago：Shaped Si Cap + Au-Au Bonding

Avago's FBAR technology——Microcap achieved WLP around 2003, and laid out core technology patents, the essence of which is the shaped silicon cap and gold bonding (US8232845B2). At that time, the wafer-level packaging technology for MEMS was not yet perfected, and traditional SAW filters were still using the old ceramic housing and gold wire bonding packaging methods. Avago opted for the relatively mature Metal Diffusion packaging, using gold as the material. Avago's Microcap technical core includes TSVs constructed on the Cap Wafer, silicon bumps and rings that form the cavity, and Au Gaskets structures for bonding and electrical connection. By the rational combination of TSVs and silicon bumps, effective electrical interconnection is achieved while ensuring sealing.

Figure 2. Avago's wafer-level package patent

Avago's unique WLP technology, Microcap, has been instrumental in the large-scale commercialization of FBAR over the subsequent 20 years. This all-silicon approach has greatly facilitated back-end packaging, ensuring that Avago can achieve higher density integration in module solutions. A 2019 research report from Yole shows that Avago integrated 19 FBAR filters in a high-frequency transmit module (measuring 7.22×6.23mm with a thickness of 0.76mm), with the area of each individual filter being only half of 1mm².

Figure 3. Avago: Shaped Si Cap + Au-Au Bonding

Qorvo：Polymer Cap + Organic Bonding

Qorvo's Polymer Roof WLP solution came a few years after Avago's and employs a double-layer dry film packaging: an Polymer Cap and Organic Bonding. Unlike the traditional suspended structure, the core vibration structure of the SMR is not entirely suspended but only vibrates freely on one side. By This design the process flow is greatly simplifies, reduces manufacturing complexity, and allows the resonator to withstand more processing steps during the WLP packaging phase, thereby improving yield and reliability.

Qorvo's adoption of a double-layer dry film scheme in WLP technology is one of its distinctive features. The first layer of dry film primarily serves to construct the sidewalls of the cavity, ensuring the stability of the internal structure; the second layer of dry film acts like a roof, capping the top to form a complete packaging structure, providing protection for the internal devices. Electrical interconnects utilize an advanced copper pillar technology, which forms vertical interconnects directly on the wafer, significantly enhancing integration.

It is worth mentioning that the dry film material used in Qorvo's WLP technology is a high-performance photosensitive resin, which is costly and has a limited number of suppliers, posing challenges to the supply chain. The supply of this material is entirely sourced from Japanese manufacturers.

Figure 4. Qorvo: Polymer Cap + Organic Bonding

Skyworks：Copper Filled TSV + Cu-Sn Bonding

Skyworks introduced its FBAR product series around 2017, characterized by solid copper-filled TSVs (Copper Filled Through Silicon Vias) and copper-tin bonding (Cu-Sn Bonding). Unlike traditional gold material bonding techniques, Skyworks pioneered the use of copper-tin alloy as the bonding material, a choice that not only optimized costs but also enhanced the thermal stability and reliability of the bonding process.

It is particularly worth mentioning that Skyworks has adopted an innovative strategy in the design of the bonding ring, choosing to construct a sealed bonding ring with thick copper material. This copper ring is not only significantly thick but also serves as the sidewall of the cavity structure, simplifying the manufacturing process.

Figure 5. Skyworks：Copper Filled TSV + Cu-Sn Bonding

In terms of internal interconnect implementation, Skyworks' FBAR products utilize solid copper pillars as TSVs, which not only enhance the current carrying capacity but also significantly improve thermal conductivity, benefiting the device's heat dissipation. At the same time, the bumps used for electrical connections are also made entirely of copper, forming a unified metal system with the TSVs. The extensive use of thick copper materials greatly reduces material costs, providing Skyworks' FBAR products with higher cost-effectiveness and allowing them to stand out in the competitive market. This innovation has solidified Skyworks' leading position in the RF component market, offering its customers more efficient and reliable solutions for product design in the 5G era.

Newsonic：Raw Si Cap + Organic Bonding

Newsonic launched its original D-BAW product series with the SiRoof packaging solution in 2021. The product features a raw silicon cap (Raw Si Cap) combined with organic dry film bonding (Organic Bonding), and Newsonic has laid out core technology patents in the United States and China, US20220103147A1 and CN113556098B, respectively. In the SiRoof design, the layout of TSVs differs from conventional WLP practices by placing the electrically interconnected TSVs on one side of the device, not on the cap side, This design means that the current path does not need to cross the bonding interface, effectively avoiding potential reliability issues caused by residual stresses in the bonding interface, the reliability level is consistent with Avago's, significantly enhancing the product's stability and durability. Compared to the Microcap, which involves multiple photomasks and complex processing steps, Newsonic's SiRoof wafer requires only one photomask, akin to a roof (Roof) covering the walls formed by the organic bonding layer, overall shortening the manufacturing cycle and reducing costs.

Figure 6. Newsonic’s Double-sided Processed technology: Raw Si Cap + Organic Bonding

BAW WLP's cross-industry grafting bears fruit

How did SiRoof born?

As is well known, the desire for imagery is a thriving need for all of humanity, and people's lives have been completely transformed by mobile phone cameras. In the 1990s, mobile phone photography technology began to gradually develop and rapidly spread, followed by a sharp increase in the number and performance demands of smartphone camera sensors.

Figure 7.Netizens jokingly refer to modern camera modules as "mahjong tiles."

Also in the 1990s, thanks to the low power consumption, high integration, and low cost advantages of CMOS technology, CMOS image sensors began to gradually replace charge-coupled devices (CCDs) as the core of mobile phone camera imaging technology. Today, approximately 7 billion CIS are produced globally each year, with seven-tenths used in smartphones. Around the year 2000, the demand for higher resolution, lower power consumption, and smaller size in CIS led to the rapid evolution of CIS WLP technology.

In the early 2010s, the Israeli company Shellcase developed wafer-level through-silicon via (TSV) packaging technology for CIS sensors. This technology enabled efficient vertical interconnects between chips, significantly increasing data transfer speeds and power efficiency. It has been widely applied in three-dimensional stacking (3D Stacking) packaging structures, where multiple layers of circuits and devices can be stacked together, markedly enhancing functional integration and performance while reducing the packaging volume.

Figure 8. Modern TSV-based packaging of CIS WLP [6]

Against the backdrop of the rapid development of China's semiconductor industry in the early 2010s, CIS WLP packaging technology gradually achieved domestication in China and has since established mature production lines. To date, the core process of TSV has become very advanced and mature.

Interestingly, BAW WLP also has similar needs to reduce electrical interconnect parasitics, ensure reliability and sealing, and minimize size. Newsonic sought to collaborate with a leading manufacturer of CIS WLP to attempt direct grafting of CIS WLP onto DBAW wafers. To their surprise, the process was fully compatible and achieved rapid success, resulting in the SiRoof approach. Subsequently, the two parties collaborated to overcome process challenges such as narrow bezel wafer bonding and complex dielectric etching, further enhancing reliability margins in mass production.

Figure 9. Actual photo of the TSV and neatly arranged Bumps in Newsonic's improved DBAW SiRoof.

The Horizontal Development of BAW WLP Technology and Intellectual Property

nspired by the WLP technology in the CIS industry, it has subsequently integrated advanced packaging technology points such as SiP. Newsonic extends the technology and IP family of BAW WLP horizontally, including the patent layout of eight categories such as package structure and process method, as shown in Figure 10.

Figure 10. Patent layout of Newsonic's core key technology WLP

Newsonic’s Next generation BAW WLP: Glass-Cap
Wafer-level packaging technology (WLP) for BAW filters is still iterating with endless possibilities, and in the direction of further improving production efficiency, reducing costs without losing performance, Newsonic is developing new packaging technologies based on cheaper materials and manufactured glass wafer cap WLP (Figure 11).

Figure 11. Newsonic’s Next generation: Glass-Cap BAW WLP

In conclusion

Avago, Skyworks, and Qorvo have been innovating and developing their own BAW WLP technologies independently, with Avago and Skyworks primarily based on "metal bonding" and Qorvo based on "double-layer dry film." Newsonic, on the other hand, has developed advanced and stable SiRoof technology by standing on the shoulders of CIS giants, fully integrating the advanced features of TSV in the CIS industry, the low-cost and huge capacity, and combining it with the newly developed narrow frame organic bonding and technology, flat untreated SiRoof material, to achieve its own BAW WLP.

In "The Three-Body Problem," there is a classic description of the theory of technological explosion: "...the history of Earth life spans over a billion years, while modern technology has developed in just three hundred years. From the perspective of the cosmic time scale, this is not development at all, it's an explosion! The possibility of technological leaps is like the explosive powder hidden within every civilization. If internal or external factors ignite it, it will explode in an instant..."

The development of smartphones globally over the past two decades has not only changed people's lives, but also given rise to a Renaissance-like technological explosion in the "ecosystem of smartphones" industry, with the RF communication industry and the CIS industry being two of its members. In this brilliant era, we are not only shining on our own, but also borrowing the light of others.

Reference
[1] United States Patent US9219464 - Bulk Acoustic Wave (BAW) Resonator Structure Having an Electrode with a Cantilevered Portion and a Piezoelectric Layer with Multiple Dopants: https://patentimages.storage.googleapis.com/a4/25/02/1e0ba32593084f/US9219464.pdf
[2] United States Patent US8232845B2 - Packaged Device with Acoustic Resonator and Electronic Circuitry and Method of Making the Same: https://patentimages.storage.googleapis.com/8f/92/b0/ed6ee6c47a65d3/US8232845.pdf
[3] United States Patent US8143082B2 - Wafer Bonding of Micro-electro Mechanical Systems to Active Circuitry: https://patentimages.storage.googleapis.com/9d/59/fa/6486ad02d1a6c3/US8143082.pdf
[4] United States Patent US20220103147A1 - Lithium Niobate or Lithium Tantalate FBAR Structure and Fabricating Method Thereof
[5] People's Republic of China State Intellectual Property Office Invention Patent CN113556098B - Bulk Acoustic Wave Resonator Packaging Structure
[6] Ma Shuying, Wang Jiao, Liu Yi, et al. A Brief Analysis of CMOS Image Sensor Wafer-Level Packaging Technology [J]. Electronics & Packaging, 2021, 21(10): 100108

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