We have briefly analyzed the market structure of acoustic filters before, and this article will explain the technical development trend of acoustic filters.
1. TC-SAW
For surface acoustic wave devices, it is very sensitive to temperature. At higher temperatures, the hardness of the substrate material tends to decrease, and thus the speed of the acoustic waves. The impact of this limitation is getting worse as the guard band gets narrower and the specified operating temperature range for consumer devices is wider (typically -20°C to 85°C).
An alternative is to use a temperature compensated (TC-SAW) filter, which is an additional coating on the structure of the IDT that increases stiffness as temperature increases. The temperature coefficient of frequency (TCF) of temperature uncompensated SAW devices is typically around -45ppm/°C, while TC-SAW filters drop to -15 to -25ppm/°C. However, since the temperature compensation process requires double the mask layer, the TC-SAW filter is more complex and relatively expensive to manufacture.
At present, the TC-SAW technology is becoming more and more mature, and foreign manufacturers have basically launched corresponding products, which have achieved many applications in the RF front-end of mobile phones, while the domestic technology still needs to be explored.
2. High frequency SAW
Ordinary SAW is basically below 2 GHz, and Murata has developed IHPSAW (Incredible High Performance-SAW) that overcomes the weakness of conventional surface acoustic waves. Murata intends to take SAW technology to the extreme (below 4GHz), and the current mass production frequency can reach 3.5GHz.
Figure basic structure of IHPSAW
IHPSAW can achieve the same or higher characteristics than BAW, and has 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 value of its Q value characteristic exceeds 3000, which is a significant improvement compared to the previous SAW with a Qmax of about 1000.
(2) Low TCF: It achieves good temperature characteristics by simultaneously controlling the coefficient of linear expansion and the speed of sound. The TCF conversion of SAW in the past is very large (about -40ppm/°C), but IHPSAW can improve it to less than ±8ppm/°C.
(3) High heat dissipation: IDT will generate heat after inputting a high-power signal to the RF filter, and inputting a higher power may destroy the electrode due to the heating of the IDT, resulting in failure. The IHPSAW can efficiently dissipate the heat generated by the electrodes from the substrate side, and can reduce the temperature rise during energization to less than half of the conventional SAW. The two effects of low TCF and high heat dissipation make it work stably even at high temperatures.
3. New Bulk Acoustic Wave Filter
BAW filters currently on the market are basically based on polycrystalline thin film technology. The Bulk ONEâ„¢ BAW technology invented by start-up Akoustis Technologies, Inc. uses a single crystal AlN-on-SiC resonator, which is said to improve performance by 30%.
Figure Monocrystalline silicon BAW technology for high frequency applications
Akoustis Technologies, Inc. (formerly known as Danlax, Corp.) was incorporated on April 10, 2013 under the laws of the State of Nevada, and is headquartered in Huntsville, North Carolina. On April 15, 2015, the company changed its name to Akoustis Technologies. In March 2017, it landed on Nasdaq.
Akoustis has announced three commercial filter products: the first is a commercial 5.2 GHz BAW RF filter for tri-band WiFi router applications; the second is a 3.8 GHz BAW RF filter for radar applications; the third The AKF-1652 is a 5.2 GHz BAW RF filter for future 4G LTE and 5G mobile devices
4. Package miniaturization
The miniaturization of the filter package mainly refers to the use of wafer-level packaging technology.
Qorvo's CuFlig interconnect technology uses copper pillar bumps instead of wire bonds. Wafer-level packaging filters eliminate ceramic packaging, enabling smaller size and thinner devices.
Fig. Comparative advantages of CuFlip technology over wire bonding
RF360 company DSSP (Die-Sized SAW Packaging) and TFAP technology (Thin-Film Acoustic Packaging, thin-film acoustic packaging technology), realize the miniaturization of products, and can provide 2in1, even 4in1 filter mode Group.
New standard package sizes for different product categories: duplexer 1.8mm*1.4mm, 2in1 filter: 1.5mm*1.1mm, single filter: 1.1mm*0.9mm.
Figure RF360 Miniaturization of SAW Filters, Duplexers and Multiplexers
Figure DSSP Package Diagram
Figure BAW filter using TFAP technology
5. Integrated modularization of RF front-end
International manufacturers have been committed to the integration and modularization of RF front-end, such as the Qualcomm RF360 solution; Murata's modules that integrate filters, RF switches, and matching circuits; Qorvo RF Fusion solutions, etc.
Qualcomm's POP3D design uses advanced 3D packaging technology that integrates a single-chip multimode power amplifier and antenna switch (AS) in a single package, and integrates the filter and duplexer into a single substrate, which is then placed on the base assembly On top of that, integrated into a single "3D" chipset combination, thereby reducing overall complexity and eliminating the wire bonds common in today's RF front-end modules.
Figure Qualcomm RF POP 3D design CMOS front-end
The Qorvo RFFusion solution consists of three modular solutions to achieve full coverage of high, mid and low frequency spectrum regions. Each module integrates a power amplifier (PA), switches and filters.
Figure Qorvo's multi-module 2017 RF Fusion solution
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