Research paper
Compact and flexible meander antenna for Surface Acoustic Wave sensors

https://doi.org/10.1016/j.mee.2020.111322Get rights and content

Highlights

  • Design of wearable and flexible antennas.

  • Fabrication of antennas by means of Multi-material 3D printer.

  • Characterization of the proposed antennas at sub-1GHz frequencies.

Abstract

This paper presents an innovative, flexible and biocompatible Ultra High Frequency meander antenna, operating at about 800 MHz, realized by means of sputtering on a Polyethylene Naphthalate substrate, and by means of a multi-material 3D printer. The fabricated antennas were characterized in terms of scattering parameters, showing a good impedance matching and a bandwidth in the order of tens of megahertz, gain and 3D radiation patterns. A numerical model was also introduced to investigate the limits of the proposed technologies in terms of metal thicknesses. The fabricated antenna could be efficiently integrated with Surface Acoustic Wave resonators to realize compact, wireless, wearable and battery-less sensing platforms.

Introduction

Flexible wearable devices, directly worn on the skin, have gained considerable attention owing to their ease of collecting primary vital signs in real time, both continuously and non-invasively [1]. Power consumption and, in prospect, the creation of self-powered devices is a fundamental issue to be studied [1]: such devices would not require external power to run the sensing activity or generate a read-out signal. In particular, Surface Acoustic Waves (SAW)-based sensors integrated with an antenna, by converting an electrical signal into surface acoustic mechanical vibrations to be coupled with proper sensing mechanisms, generate new totally passive sensing platform for self-powered devices [2].

SAW devices represent one of the most important class of Micro Electro Mechanical Systems (MEMS) thanks to their low cost, low power needs and quick manufacturing. This class of device is usually fabricated on silicon substrate, using crystals of LiNbO3 or LiTaO3 as piezoelectric material, and shows good performances despite the high stiffness of these materials and their bulky dimensions [[3], [4], [5]]. The development of better techniques of deposition of thin film piezoelectric materials brought to a new type of SAW devices based on thin piezoelectric films of aluminium nitride (AlN) or zinc oxide (ZnO) on silicon substrate [6]. Nevertheless, wearable devices require flexible mechanical behaviour, paving the way to the exploitation of flexible material as substrate. Piezoelectric thin film sensors are affirming themselves as the main technology for the development of self-powered wearable technologies, operating across multiple domains as opto- electro-mechanical and biochemical sensing on the human body. The piezoelectric thin film enables the integration of multiple functions onto different substrates such as polymers [1]. Polymers are particularly important for development of flexible and wearable sensing devices, tactile transducers and energy harvesting devices based on piezoelectric thin films [1]. The fundamental challenge is represented by obtaining a thin piezoelectric layer on a flexible substrate with a high effective piezoelectric coefficient (d33) [1]. Flexible polymeric materials and piezoelectric thin films have very different thermal expansion coefficients; this difference can cause the formation of creeps in the thin piezoelectric layer during the high temperature growth. Recently, an aluminium-nitride-(AlN)-based SAW device on a thermoplastic polyethylene naphthalate (PEN) substrate has been fabricated by the low-temperature sputtering deposition of a multilayered AlN-based stack [5]. A large transmission signal amplitude, up to 20 dB, for the Lamb resonance mode below 1 GHz and an electromechanical coupling of the Lamb mode (2.27%–2.91%) were achieved [5]. These are essential characteristics for combining an AlN-based transparent and flexible SAW device with a wireless antenna for wearable electronic communications, paving the way for a new class of battery-less piezoelectric flexible SAW devices for vital parameter (physiological or biochemical) monitoring by mobile communications.

The aim of this work is to realize an antenna to be integrated with a SAW resonator to create a compact, wearable and flexible sensor. In order to work with a SAW resonator, the antenna has to be realized on the same flexible substrate. Since the maximum frequency achievable by the SAW could be limited by the resolution of the photolithography process, it cannot be higher than 800 MHz [5], hence the antenna must be designed to operate at that frequency. Another important requirement regards the small footprint: since the device needs to be compact, so we designed an electrically small antenna (ESA). This kind of antennas are affected by impedance mismatching problems due to their narrow frequency band, hence it also requires an easy scalable geometry to tune the working frequency, if it is necessary. Although some examples of antennas placed on a flexible substrate have been reported in literature [[7], [8], [9]], no one of the existing antennas is well designed to be integrated with an acoustic resonator, since some of them are designed for working at an high frequency to be compatible with SAW resonator technology, and others have a large footprint or a complex geometry to allow an efficient and compact integration and a frequency tuning in case of impedance mismatching. For example, [7] shows a coplanar, flexible antenna placed on a paper substrate of 0.14 mm, with a dimension of about 5 cm and resonant frequency of 2.4 GHz, hence the central frequency is too high to be integrated with a SAW resonator. Furthermore, another configuration is based on a rectangular Planar Inverted-F Antenna (PIFA) of 32x22mm2, placed on a Kapton substrate [8]. The antenna is equally divided in 704 small sub-patches with the grid size of 1 mm2 and the resonant frequency is around 400 MHz. This antenna has some limitations such as complexity and it is not easily scalable. Finally, a compact and flexible flower-shaped coplanar waveguide (CPW)-fed antenna has been proposed in [9]. The chosen substrate is a flexible and biocompatible material, called polyamide with a thickness of 0.025 mm. This antenna resonates at a very high frequency (up 3 GHz) and requires a very complex design which makes very difficult some modification for size reduction. The previous flexible antennas do not meet both requirements of low frequency and small dimensions, necessary for wearable devices integrated with a SAW (~800 MHz).

In this scenario, we report on the design and fabrication of a meander antenna operating at a frequency of about 800 MHz, integrable with the SAW sensor. In particular, we detail the antenna design and describe the fabrication of the proposed antennas by means of both sputtering on a flexible and biocompatible substrate, PEN, and by means of a multi-material 3D printer.

Regarding the technology for the realization of flexible antennas, there are several methods to realize them on flexible substrates such as inkjet printing, screen printing and wax based deposition [[10], [11], [12]]. Inkjet printing generates some driblet of uniform space and size by imposing a periodic perturbation to the ink delivered from a nozzle [10]. Screen printing transfers the ink on the substrate through a stencil covering a fabric mesh [11]. The design of the printed layout is reproduced in positive photolithography using a photosensitive material which is herded after being exposed to a light source. Finally, wax based deposition is a process similar to etching with the advantage of avoiding exposure to ultraviolet light (UV) and the use of aggressive chemical agents. This mask is realized through a wax-based ink and deposited with a solid-ink printer [12].

Section snippets

Design of the meander antenna

The integration of the piezoelectric SAW-based resonator with a transmitting/receiving antenna is sketched in Fig. 1.

The technological challenge is represented by the manufacture of the antenna on flexible substrates having thicknesses of the order of hundreds of microns. Since in a previous paper [5], a flexible Aluminuim Nitride-based Surface Acoustic Waves device was realized on a 125 μm thick PEN substrate, the antenna has to be realized on a substrate with the same thickness.

The proposed

Flexible antenna fabrication and characterization

Two fabrication strategies have been pursued for the meander antenna fabrication on the flexible substrate based on (i) sputtering deposition and wet etching and (ii) multi-material 3D printing, respectively.

Conclusions

In this work, the design, fabrication and characterization of a meander-like antenna on flexible substrates were detailed. Firstly, a FEM-based model was proposed to optimize the scattering parameter and impedance matching of the antenna at 800 MHz taking into account the metal thickness. Then, two different approaches for the fabrication were investigated. In the first, there was an aluminium sputtering deposition followed by wet etching, while in the second the antenna was fabricated by means

Declaration of Competing Interest

None.

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These authors equally contributed to this work.

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