Progress on Key Technologies of Cavity-structured Thin Film Bulk Acoustic Wave Filter

doi:10.15541/jim20240355

Abstract

Abstract:

With upgrading of communication technology and driving of 5G communication applications, the explosive number of filters required by various smart devices promoted prosperity of the filter market. However, the demanded performance also becomes increasingly stringent, including broad bandwidth, high frequency, high power capacity, miniaturization, integration, and low cost, in both academia and industry. To meet these strict requirements, the thin film bulk acoustic resonator (FBAR) filters have emerged as one of the most promising types of filters with commercial success. But currently they are still facing difficulties such as insufficient performance, complex fabrication process, relatively higher cost, and technological constraints. This paper reviews the relevant issues and key technologies in FBAR filters in three aspects: theoretical research on devices and structural optimization, preparation and optimization of high-performance piezoelectric materials, and development of novel processes and technological integration. The purpose of this paper is to delineate the trajectory of technological advancements and iterations in FBAR filters for scholars in the research field, with the expectation of providing several considerations for future research directions and pathways.

Key words: FBAR filter, theoretical research, AlN piezoelectric film, technology integration, review

CLC Number:

TN713

TAO Guilong, ZHI Guowei, LUO Tianyou, OUYANG Peidong, YI Xinyan, LI Guoqiang. Progress on Key Technologies of Cavity-structured Thin Film Bulk Acoustic Wave Filter[J]. Journal of Inorganic Materials, 2025, 40(2): 128-144.

Figures/Tables 19

Table 1 Technical characteristics of SAW filter and BAW filter

Filter type	SAW filter	BAW filter
Characteristic	High stability, low insertion loss (2-4 dB)	High stability, low insertion loss (0.8-1.5 dB), high power tolerance
Applicable frequency range	10 MHz-3 GHz	1.5-6 GHz, the maximum up to over 10 GHz
Advantage	Smaller than the traditional ceramic filter, flexible, mature technology, high reliability	Suitable for high frequency, insensitive to temperature changes, miniaturized vertical propagation in acoustic wave, decreased size according to frequency increase
Limitation	Poor thermal stability, decreased Q-value when operating frequency exceeds 1.5 GHz	High manufacturing cost, complex manufacturing process

Fig. 1 Operating principle of the L-type FBAR filter (a) Electrical characteristics of the idealized FBAR; (b) Fundamental unit of the L-type structure; (c) Transmission response

Fig. 2 FBAR and the equivalent circuit model of piezoelectric thin film (a) Schematic structure of FBAR; (b) Mason equivalent circuit model of the piezoelectric thin film

Fig. 3 Nonlinear distortion of the duplexer[23] (a) Circuit diagram; (b) Signal distribution

Fig. 4 BAW resonator based on AlN thin film[25] (a) Nonlinear circuit model; (b) Second harmonic response; (c) Third harmonic response

Fig. 5 Resonator with edge Bragg reflection layer structure[31]

Fig. 6 (a) Cross-section and (b) top view of FBAR[35]

Fig. 7 Discretized model and electrode feed lines of BAW resonator[36] (a) Schematic representation of discretized resonator model; (b) Two resonators in series and connected by broad lead; (c) Single resonator connected by narrow lead

Fig. 8 3.55 GHz FBAR filter based on single-crystal and polycrystalline AlN[54] (a) Measured power sweep at right band edge of the filters; (b) Measured insertion loss of the filters versus input power

Fig. 9 Single crystal AlN-FBAR filter[55] (a) Actual layout; (b) Simulated and measured transmission response curves

Fig. 10 Measured values of transmission response of FBAR filter[64]

Fig. 11 Experimental results of FBAR filter fabricated with Al0.8Sc0.2N film[65] (a) Transmission response of FBAR filter; (b) Return loss of the FBAR filter

Fig. 12 Effect of Sc-dopant concentration on crystal quality and surface morphology of Al1−xScxN thin films[69] (a-c) TEM dark-field images and electron diffraction patterns for 10%, 31%, and 42% Sc content; (d-f) Corresponding SEM plane views

Fig. 13 Performance comparison of FBAR obtained by two-step (Sample 1) and single PVD (Sample 2) processes[86] (a) Frequency impedance characteristic curves; (b) Smith chart

Fig. 14 Schematic diagram for SABAR process[87] (a) Combination of PLD and MOCVD AlN film; (b) Bottom electrode and bonding layer sputtered on top of AlN layer; (c) Resonator structure

Fig. 15 N77-band hybrid filter based on FBAR and IPD technology[88] (a) Design schematic; (b) Simulated transmission response curve

Fig. 16 N41-band hybrid filter based on FBAR and IPD technology[89]

Fig. 17 Convolutional hybrid network for the filter parameter prediction[94]

Fig. 18 LTE 4G/5G radio frequency front end module structure[98]

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