Yogeswaran, Nivasan (2019) Graphene field effect transistor based pressure sensors for tactile sensing applications. PhD thesis, University of Glasgow.
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Abstract
The development of electronic skin emulating human skin's functionality is a growing area of interest due to its prospect in autonomous and interactive robots, prosthesis and wearable health monitoring devices. In an effort to mimic human skin a number of sensors for the detection of various stimuli have been developed including pressure, strain and thermal sensors. Amongst them, a significant effort has been focused on the development of novel pressure sensors due to their potential in the aforementioned applications. A number of strategies have been adopted for the development of pressure sensors and in particular, there has been a growing interest in the development of field effect transistor (FET) based pressure sensors. This is due to the capability to develop large area high spatial resolution active matrix pressure sensor array. In recent times, there has been a growing demand for the development of flexible pressure sensors due to emerging applications such as smart prosthesis, interactive robots, and wearable electronics. The use of conventional material like Si for flexible electronic applications are limited owing to their rigid and brittle nature. This has led to the investigation of various novel materials like organic semiconductors, carbon nanotube, inorganic semiconductor nanowires, and graphene. Amongst them, graphene is an attractive choice owing to its intrinsic material properties such as its electronic and mechanical properties. Further, the complementary metal oxide semiconductor (CMOS) compatibility and ability to grow high-quality graphene over a large area, and its low optical absorption are some of the other attractive features for the development of large area transparent electronic applications. The high mobility of graphene would enable the development of low voltage devices attractive for flexible electronics applications. This thesis presents work on the development of graphene field effect transistor (GFET) based pressure sensors for tactile sensing applications. The developed sensor comprises of two main components: a top-gate GFET and a piezoelectric transducer layer. A commercially available chemical vapour deposition grown monolayer graphene on Cu foil (from Graphenea) was used as the channel material of the transistor. A high-k Al2O3 deposited by atomic layer deposition technique was employed as the top-gate dielectric. In particular, care was taken to ensure a low temperature CMOS compatible process was adopted for the development of GFET. This ensured that the developed fabrication process could be transferred directly for the development of flexible devices. The development of the transfer process, the impact of different polymers (used as supporting layer during the transfer process) on graphene and the optimisation of dielectric deposition process are discussed in the thesis. The piezoelectric transducer layer is another vital component of the developed pressure sensor. In this respect, two piezoelectric materials, lead zirconate titanate (PZT) and aluminium nitride (AlN), have been investigated as the transducer layer. The pressure sensors were characterised with the piezoelectric transducer layer in an extended-gate configuration with GFET. PZT based pressure sensors exhibited a pressure sensitivity of 4.55E-3/kPa for a pressure range between 0 - 94.18 kPa. Though PZT is a better piezoelectric material than AlN, CMOS process incompatibility, non-biocompatibility and high processing temperature often associated with PZT limit its use in the development of flexible electronics especially for wearable applications. Therefore, AlN deposited by low temperature radio frequency magnetron sputtering has been explored as an alternate piezoelectric transducer layer for pressure sensing applications. The use of AlN also evades the need for the high voltage poling process often employed to enhance the piezoelectric property of the material. The AlN deposited via an optimised RF sputtering process reported in the thesis resulted in film with a piezoelectric constant of 5.9 pC/N. Similar to PZT , AlN was also characterised in an extended gate configuration and exhibited a sensitivity of 7.18 E-3 /kPa for a pressure range of 0-9.74 kPa. In an attempt to improve the spatial sensor resolution of sensor and to improve the signal to noise ratio a piezoelectric layer integrated within the top gate dielectric stack was investigated. In this regard,a flexible GFET with a piezoelectric layer integrated with the top-gate dielectric film was developed. The top gate dielectric stack comprise a 15 nm Al2O3(deposited by ALD)/ 90 nm AlN (deposited by RF sputtering). The developed device exhibited typical GFET electrical characteristics. The electron and hole mobility of the developed devices were 1612 cm2/V.s and 1568 cm2/V.s respectively. In addition, the device also displayed a stable electrical response under mechanical bending condition, thereby demonstrating its potential in the development of flexible electronics.
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