Highly Bendable Piezoelectric Resonators for Flexible Radio‐Frequency Electronics

Abstract

flexible organic or inorganic transistors. However, in many of the emerging applications, such as the Internet of Things and implantable electronic systems, in addition to the aforementioned basic building blocks, functional elements that include wireless RF electronic devices are also essential elements for wireless interconnection and data transmission. RF resonators, such as film bulk acoustic resonators (FBARs), which are traditionally used as the basic building blocks for modern RF filters [12] and oscillators, [13] are a natural candidate for wireless flexible electronics. FBAR-based electronic systems have also been extensively used in the biochemical sensing and actuating domains, [14] e.g., chemical vapors [15] and antibody-antigen reactions for immunosensors. [16] Although FBARs are small in size and light in weight, they have traditionally not been amenable to mechanical bending or stretching because of their rigid and brittle silicon substrate. Therefore, incorporating flexible electronic technology into FBARs is highly demanding and offers much more functionality for next-generation flexible electronic systems. However, since the fabrication process of these FBARs is intrinsically incompatible with the majority of plastic substrates, it is difficult to fabricate flexible FBARs with prominent electrical performance and mechanical flexibility. Previously, a flexible air-gap-type FBAR on a silicon substrate was demonstrated by thinning the silicon wafer to 50 µm. [17] By means of directly fabricating the resonator on polyimide, FBARs integrated on arbitrary substrates (polyimide, glass, and silicon) have also been achieved but with compromised electrical performance. [18] In our previous work, free-standing FBARs on polyethylene terephthalate substrates were demonstrated. [19] However, none of these devices is fully qualified for the rigorous requirements of satisfactory mechanical flexibility and uncompromised electrical performance.In this work, a highly bendable FBAR was introduced to meet these rigorous requirements by placing the FBAR at the mechanical neutral plane using two polyimide thin films. Furthermore, to achieve a high quality factor, the acoustic energy should be efficiently trapped in the material stack by a significant acoustic impedance difference between the FBAR and its surroundings. Therefore, air cavities were created both above

Recent Advances in Flexible RF MEMS

“…Throughout experimental testing, the mechanical and electrical properties of the flexible resonator can still remain stable after 2000 times of bending deformation. Finite element method (FEM) analysis was utilized to evaluate the mechanical properties of the flexible device, and the strain and stress distributions of the flexible FBAR under a bending radius of 0.5 mm are shown in Figure 9 e [ 100 ].…”

Monolithic Heterogeneous Integration of 3D Radio Frequency L−C Elements by Self‐Rolled‐Up Membrane Nanotechnology

“…Miniaturizing device footprint has been a key focus that has led to many novel RF MEMS devices with working frequencies in the range of ≈1-15 GHz . Some of the most significant recent advancements in the field of passive component miniaturization include those facilitated by the use of soft lithography [10] and other methods to enable stretchability, [11,12] frequently employing 2D and biodegradable materials for a diverse array of applications. [13][14][15][16] The swift development of the Internet of Things (IoT) [17][18][19][20] further pushes the advancement of wearable and implantable technologies that are reliant on microsupercapacitors [20][21][22][23][24][25][26][27] and passive L−C network to enable wireless and battery-free sensing.…”

“…[28] Novel L−C filters or resonators structures that have footprints of a few square millimeters have also been constructed using interdigital MEMS, [29] substrate-integrated waveguides, [30] air-bridge structures, [31] and out-of-plane actuation. [32] Reconfigurability, stretchability, and bendability are of great concern to the miniaturization of passive L−C components, [12,33,34] as they enable more efficient use of chip area and versatility for device integration toward a wider set of applications. Given its compatibility with CMOS processing and precise geometric control, rolled-up nanotechnology presents yet another approach to fabricate a plethora of tubular, ring, or helical structures [35][36][37][38][39][40][41][42][43][44] that can be transformed into L−C passive components while continuing the push for miniaturization.…”

“…Miniaturizing device footprint has been a key focus that has led to many novel RF MEMS devices with working frequencies in the range of ≈1–15 GHz . Some of the most significant recent advancements in the field of passive component miniaturization include those facilitated by the use of soft lithography [ 10 ] and other methods to enable stretchability, [ 11,12 ] frequently employing 2D and biodegradable materials for a diverse array of applications. [ 13–16 ]…”

Bending behaviors of flexible acoustic wave devices under non-uniform elasto-plastic deformation

“…Flexible electronics have attracted increasing attentions in recent years owing to their advantages such as portability, deformability and conformability compared to the traditionally rigid and brittle silicon-based counterparts [1][2][3][4] . Over the past decades, numerous studies on high-performance flexible electronics have been investigated, including flexible transistors [5][6] , flexible energy harvesters 7 , flexible resonators 8 , and flexible electronic skins 9 . For realizing a good performance under a large deformation, flexible substrates including polymers 10 , metallic foils 11 or inorganic thin sheets 12 are widely adopted.…”