Jingxun Juzhen's new FBAR RF filter helps the 5G market

In recent years, the scale of the RF front-end market has continued to expand. According to the Yole Development report, the filter market will grow by about 3 times from 2017 to 2023. Not long ago, Jingxun Juzhen Technology Co., Ltd. officially released its independently developed B41 full-band FBAR filter in Zhuhai. It can reduce the package size while supporting high power, and has excellent performance such as large bandwidth, high out-of-band suppression and low insertion loss.

RF filters are widely used in wireless communications to select and process signals within a specific frequency bandwidth. The core of the filter is a set of resonators, whose quality is given by the quality factor Q and the K2eff of the piezoelectric film. To maximize the performance of the filter, the resonator uses an independent bulk acoustic film to form a bulk acoustic wave filter (FBAR). Since the film is only supported around the edge where it is used, there is an air cavity between the bottom electrode and the carrier wafer, which affects the further improvement of the FBAR filter performance.

In this case, Jingxun Juzhen Company filed an invention patent entitled "Method for manufacturing single crystal piezoelectric RF resonators and filters with improved cavities" on June 21, 2018 (application number: 201810640995.6), and the applicant was Zhuhai Jingxun Juzhen Technology Co., Ltd.

This patent proposes an FBAR filter device, including a resonator array, each resonator including a single crystal piezoelectric film sandwiched between a first and a second metal electrode, wherein the first electrode is supported by a supporting film on an air cavity, and the air cavity is embedded in a silicon dioxide layer on a silicon cover and has a silicon through hole passing through the silicon cover and entering the air cavity.

Figure 1 Flowchart of cavity-defined resonators and filters

Figure 1 shows a flow chart of the design of piezoelectric RF resonators and filters with improved cavity definition. The first part A is manufactured first, providing a support film on a silicon dioxide block on a silicon handle, with grooves passing through the silicon dioxide layer. The second part B consists of a piezoelectric film on a removable carrier substrate, and then a metal electrode is sandwiched between the piezoelectric film and the support film. Then in step D, the carrier substrate is removed, the piezoelectric layer and the electrode are trimmed, and a passivation layer, an adhesion layer and an upper electrode are applied. A rigid frame is manufactured around each filter array, and a cover array with a bonding layer is manufactured using the frame and the solder terminals, and the filter array is placed. Finally, the silicon cover etched holes through the silicon cover to the silicon dioxide are thinned, as well as part of the ceramic film and the plugged holes.

Figure 2 Flow chart of making metal layer support film

The support film in Figure 1 consists of a silicon dioxide layer deposited onto a silicon cover, and is made as shown in Figure 2. Etching techniques are used to penetrate the film, the silicon dioxide, and an etch-resistant coating such as silicon nitride is deposited on the support film 31 and deposited into the groove pattern, creating alignment marks on the cover and lining the grooves with the etch-resistant coating, which is then filled with the filter. Next, the filler layer is removed, the lower support film is polished, and the adhesion layer, electrode layer, and bonding layer are deposited. Since the filter array includes a support film and a single crystal film that is difficult to manufacture with precise dimensions, as well as a passivation layer and electrodes, it is best to manufacture a sealing ring on the cover and attach the cover with the sealing ring and the contact stack to the rest of the structure.

The above is the FBAR filter improved by Jingxun Juzhen. Its air cavity is embedded in the silicon dioxide layer of the silicon handle, which can greatly improve the performance of the filter. With the development of 5G, this filter will surely be widely used in the RF front-end market to give full play to its performance advantages.