Global Trends in Acoustic Filter Technology Development

Jan 02,2025
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1 TC-SAW

For surface acoustic wave devices, they are highly sensitive to temperature. At higher temperatures, the hardness of the substrate material tends to decrease, resulting in a decrease in sound wave velocity. Due to the increasingly narrow protection frequency band and the large specified operating temperature range of consumer devices (usually -20 ℃ to 85 ℃), the impact of this limitation is becoming more and more severe.

An alternative method is to use a temperature compensated (TC-SAW) filter, which is a coating on the structure of the IDT that enhances its stiffness when the temperature rises. The frequency temperature coefficient (TCF) of temperature uncompensated SAW devices is typically around -45ppm/℃, while TC-SAW filters drop to -15 to -25ppm/℃. However, due to the need for doubled mask layers in temperature compensation processes, TC-SAW filters are more complex and have relatively higher manufacturing costs.

At present, TC-SAW technology is becoming increasingly mature, and foreign manufacturers have basically launched corresponding products, which have been widely used in the RF front-end of mobile phones. However, the domestic process still needs to be explored.

2、 High frequency SAW

Ordinary SAW is basically below 2GHz, and Murata has developed I. to overcome the weaknesses of previous surface acoustic waves H.P.SAW（Incredible High Performance-SAW）。 Murata Yi has taken SAW technology to the extreme (below 4GHz), and the currently mass-produced frequency can reach 3.5GHz.

Figure I H. The basic structure of P.SAW

I. H.P.SAW can achieve the same or higher characteristics as BAW, and combines the advantages of BAW's temperature characteristics and high heat dissipation, as follows:

(1) High Q-value: The trial production results of the resonator in the 1.9GHz frequency band show that the peak Q-value characteristic exceeds 3000, which is significantly improved compared to SAW with a Qmax of around 1000 in the past.

(2) Low TCF: It achieves good temperature characteristics by simultaneously controlling the coefficient of linear expansion and sound velocity. In the past, the TCF conversion of SAW was very large (about -40ppm/℃), but I.H.P.SAW can improve it to below ± 8ppm/℃.

(3) High heat dissipation: When a high-power signal is input to the RF filter, the IDT generates heat. If a higher power is input, the IDT may cause damage to the electrodes due to heat generation, leading to malfunctions. I. H.P.SAW can efficiently dissipate the heat generated by the electrode from one side of the substrate, reducing the temperature rise during power on to less than half that of traditional SAW. The two effects of low TCF and high heat dissipation enable it to work stably even at high temperatures.

3、 New type of bulk acoustic wave filter

At present, the bulk acoustic wave filters on the market are basically based on polycrystalline thin film technology. And the Bulk ONE invented by startup Akustis Technologies, Inc? BAW technology uses single crystal AlN on SiC resonators, which are claimed to improve performance by 30%.

Single crystal silicon BAW technology for high-frequency applications

Akoustis Technology Company (formerly known as Danlax, Corp.) was registered and established under Nevada state law on April 10, 2013, with its headquarters located in Huntsville, North Carolina. On April 15, 2015, the company was renamed Akoustis Technology Company. In March 2017, it was listed on NASDAQ.

Akoustis has announced the launch of three commercial filter products: the first is a commercial 5.2 GHz BAW RF filter for tri band WiFi router applications; The second one is a 3.8 GHz BAW RF filter designed for radar applications; The third AKF-1652 is a 5.2 GHz BAW RF filter designed for future 4G LTE and 5G mobile devices

4、 Packaging miniaturization

The miniaturization of filter packaging mainly refers to the use of wafer level packaging technology.

Qorvo's CuFlig interconnect technology uses copper pillar bumps instead of wire bonding. Wafer level packaged filters eliminate ceramic packaging, allowing for smaller sizes and thinner equipment.

Comparative advantages of CuFlip technology over wire bonding

RF360's DSSP (Die Sized SAW Packaging) and TFAP technology (Thin Film Acoustic Packaging) have achieved product miniaturization and can provide 2-in-1 or even 4-in1 filter modules.

New standard package sizes for different product categories: Duplex 1.8mm1.4mm, 2-in-1 filter 1.5mm1.1mm, single filter 1.1mm * 0.9mm.

Miniaturization of RF360 acoustic meter filters, duplexers, and multiplexers

DSSP packaging diagram

BAW filter using TFAP technology in the figure

5、 RF front-end integration and modularization

International giants have been committed to the integration and modularization of RF front-end, such as Qualcomm RF360 solution; Murata integrates modules such as filters, RF switches, and matching circuits; Qorvo RF Fusion solution, etc.

Qualcomm POP3D design utilizes advanced 3D packaging technology, integrating a single-chip multi-mode power amplifier and antenna switch (AS) into a single package, and integrating filters and duplexers into a single substrate. The substrate is then placed on top of the base components to form a single "3D" chipset combination, thereby reducing overall complexity and eliminating the common wire bonding in RF front-end modules today.

Qualcomm RF POP 3D Design CMOS Front End

The Qorvo RFFusion solution includes three modular solutions that achieve full coverage of high, medium, and low frequency spectrum areas. Each module integrates power amplifiers (PA), switches, and filters.