Piezo-tunneling strain sensors integrated on plastic by combining vacuum thin film coatings and 3D printing technologies
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TECHNICAL PAPER
Piezo-tunneling strain sensors integrated on plastic by combining vacuum thin film coatings and 3D printing technologies Re´mi Rafael1 • Etienne Puyoo1 • Christophe Malhaire1 Received: 20 March 2020 / Accepted: 30 March 2020 Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract There is a growing demand for the integration of sensor functions on flexible substrates for wearable electronics, robotics or medical monitoring. For this, it is necessary to develop strain gauges both sensitive and integrable at low cost with a low thermal budget. The gauge factor of metal/insulator/metal piezo-tunneling strain sensors is first measured as a function of applied current and polarity, for different electrode materials (Al, Pt or Pd) and insulator (Al2O3) thicknesses. A maximum gauge factor of 90 is obtained with an Al/Al2O3 (10 nm)/Al junction and top electrode injection. Results are discussed based on the Fowler–Nordheim model and it is shown that the electron effective mass in Al2O3 most likely plays a major role in the observed mechano-sensitivity. Next, the feasibility of a low-pressure sensor demonstrator based on a 3Dprinting process on a polymer substrate is shown with a sensitivity of 0.19 bar-1 in the 0–450 mbar range.
1 Introduction Piezoresistive sensors are of major interest for the development of both wearable and conformable electronics industry sectors (Amjadi et al. 2016). Flexible strain sensors integrating piezoresistive elements can indeed be used to develop tactile sensing functions (Park et al. 2016), human motion and medical monitoring (Tolvanen et al. 2018; Puyoo et al. 2018; Kang et al. 2014; Choi et al. 2017), and multirecognition functions (Su et al. 2016). They can also be used in the field of robotics, aerospace engineering and transportation to develop lightweight plastic Micro-Electro-Mechanical Systems (MEMS) that can be conformably integrated on non-planar surfaces (Xiao et al. 2008). Among the different approaches which have been proposed in literature these past few years, piezo-tunneling strain sensors give rise to gauge factors in the range of 10 to 300 which compete with that of monocrystalline silicon (standard for piezoresisitive MEMS) (Herrmann et al. 2007; Schlicke et al. 2016; Jiang et al. 2015; Puyoo et al. 2017; Zhu & McNamara 2015; Rafael et al. 2017). The principle of piezo-tunneling strain & Etienne Puyoo [email protected] 1
INL UMR 5270, CNRS, INSA de Lyon, Institut Des Nanotechnologies de Lyon, Universite´ de Lyon, 69621 Villeurbanne, France
sensor is based on the fact that the electrical resistance of a tunnel junction exponentially varies with its parameters (junction length, electron effective mass, barrier height). For example, when submitted to mechanical stress, the variation of the tunnel junction length implies an exponential variation of the tunnel resistance which gives rise to these high gauge factors. This concept has been first exploited with metallic nanoparticle assemblie
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