Lecture on "Fabrication of carbon nanotube field effect transistors based on individual SWCNTs for NO2 sensors"

El divendres 21 de juny, de 12:00 a 13:00, a l'aula seminari B4-212, als segon pis de l'edifici B4 del Campus Nord, Miroslav Haluska del ETH de Zurich farà una xerrada que porta per títol Fabrication of carbon nanotube field effect transistors based on individual SWCNTs for NO₂ sensors.

Abstract

Field effect transistors (FETs) utilizing individual single‐walled carbon nanotubes (SWCNTs) as active channels are promising for sensor applications, for example as SWCNT‐based chemical sensors with low power consumption for NO2 detection [1, 2, 3]. For a successful and reliable integration of  SWCNTs into future devices, some obstacles still remain to be solved. One of the biggest issues is related to the electrical characteristics of carbon nanotube FETs (CNFETs), which exhibit variations that cannot be explained solely by the different intrinsic properties of the incorporated SWCNTs. The individual fabrication steps can also affect the CNFET characteristics by altering the nanotube properties and/or interfaces between the nanotube, metal and substrate, respectively. In this presentation, we focus on the identification of fabrication steps affecting the properties of SWCNTs and CNFETs. Changes in the SWCNT properties and the final CNFET characteristics were monitored by Raman spectroscopy and electrical measurements, respectively.

SWCNTs were synthesized on Si/SiO2 substrates catalyzed by ferritin‐based Fe nanoparticles by lowpressure chemical vapor deposition (LPCVD) at 850 °C in CH4/H2 [4]. As‐grown SWCNTs were electrically contacted by using either standard photo or electron‐beam lithography. Each CNFET consists of Cr/Au source and drain electrodes connected by a SWCNT. The back gate electrode is formed by the highly p‐doped Si substrate. The chips on which CNFET devices have been fabricated were completely passivated by an Al2O3 film formed by atomic layer deposition after contact metal deposition and lift‐off. Finally, sensor windows were defined by etching the passivation film in the middle part of CNFET channels, while the metal‐nanotube contact areas remained passivated [2]. The presence of carbonaceous clusters was detected by Raman spectroscopy after lithographic processes had been applied. We attribute the origin of these carbonaceous impurities to the photoresist residues remaining on the chip even after cleaning with N‐methyl‐2‐pyrrolidone (NMP) at 80 °C [5]. The clusters were often attached to SWCNTs. These impurities can increase the metalnanotube contact resistance, broaden the Isd‐Vg hysteresis, and dope the nanotubes , all in an uncontrolled manner.

By monitoring the CNFET gas sensor fabrication flow, we have identified some problematic fabrication steps and suggested preventing the direct exposure of the nanotubes to the image resist by separation layers [6, 7] or by resist‐free fabrication flows [8]. The CNFETs fabricated using separation layers exhibit n‐type transistor characteristics with on‐state resistances from 25 to 46 kΩ. CNFET characteristics of devices with suspended nanotube channels will be shown to demonstrate the influence of the substrate on the CNFET performance.

References:
[1] J. Kong et al., Science 287 (2000) 622.
[2] M. Mattmann et al., Appl. Phys. Lett. 94 (2009) 18350294 (2009) 183502.
[3] T. Helbling et al., Nanotechnology 20 (2009) 434010.
[4] L. Durrer et al., Nanotechnology 20 (2009) 355601.
[5] L. Durrer, PhD thesis, ETH No. 18947, “Controlled Single‐Walled Carbon Nanotube growth forsensing applications” (2010).
[6] S.‐W. Lee et al.: 36th Int. conf. on Micro and Nano Engineering (MNE), Genoa, Italy, 2010.
[7] W. Liu et al., Int. Con. on Solid‐State Sensors, Actuators and Microsystems, Barcelona, 2013.
[8] M. Muoth et al., IEEE MEMS 2013, Taipei, Taiwan, 2013, pp. 496‐499.