Carbon nanotube field effect transistors as electromechanical transducers

Abstract

It is difficult to imagine a world without sensors. We are surrounded by sensors Sensors almost constantly. Consumer electronics, cars, trains, medical diagnostic applications or automation techniques are just a few examples of devices that are packed with sensors in order to increase functionality. The success of sensors has been Miniaturization made possible by continuous system miniaturization over the last few decades. The driving factors behind miniaturization are cost reduction and increase of device functionality at similar or smaller size. The main enablers of miniaturization are novel materials and new process technologies. This work reports on further miniaturization of membrane based pressure sensors which is made possible by the use of nanoscaled single walled carbon nanotube (SWNT) piezoresistive transducer elements. Design, fabrication and characterization of pressure sensors with diameters down to 40 μm are presented. The ultra small sensors made of circular SiO2/Al2O3 thin film membranes show Ultra Small Sensors linear pressure-resistance transfer characteristics. The strain gauge element is a long channel (L = 1.5 μm) and large diameter (d = 2.1 nm) SWNT transistor with a side gate as the third terminal. The measured sensor performance are maximum output sensitivity, So = 0.15± 0.015∆R/R/bar and a resolution of 15 mbar, which are achieved by the high piezoresistance of the SWNT strain gauge with gauge factors (GFs) up to GF = 450± 54, at the same time the power consumption of ∼100 nW is exceptionally low for piezoresistive sensors. The measured pressure range is between pmin = 0 mbar and pmax = 200 mbar. pmax corresponds to a maximum applied strain, ǫ at the location of the strain gauge of ǫmax = 4.6 · 10−5. These final sensor results could only be achieved through a series of incremental development steps of process technologies for carbon nanotube based electronics. First, a stable fabrication process for SWNT field effect transistors (CNFETs) Process Development has been developed. The fabrication process is based on CVD growth of SWNTs, contacting the SWNT channel by source and drain metal electrodes and encapsulation by a dielectric layer to protect them from the ambient. Second, CNFETs Carbon Nanotube Transistors are thoroughly characterized by electrical measurements. Encapsulated CNFETs show n-type unior ambipolar Ids-Vg gate characteristics. Their low frequency 1/ f noise amplitudes are between 10−6 and 10−3, depending on the SWNT type and measurement location of the Ids-Vgcharacteristics. Long term stability of encapsulated CNFETs is demonstrated over a period of 260 days. Moreover, CNFETs made of large diameter SWNTs contacted by Pd metal show reproducible