Parallel Assembly of Carbon Nanotubes for NEMS Applications (Parallelle assemblage van koolstof nanobuizen voor NEMS applicaties)
Parallel Assembly of Carbon Nanotubes for NEMS Applications
Pathangi Sriraman, Hari; S0182926
The main theme of this Ph.D. thesis is about the use of single-walled carbonnanotubes (SWCNTs) as mechanical components for (nano) -electromechanical system(NEMS) applications. The objective of this work is to develop an industry-relevanttechnique to fabricate devices comprising of horizontally aligned individual SWCNTssuspended over the substrate (to ensure their mechanical functionality) at multiple andspecific on-chip sites in parallel, preferably in a CMOS-compatible process. The majormotivation for the choice of SWCNTs as mechanical resonators stems from their verysmall physical dimensions. SWCNTs have an extremely small diameter (usually about0.5-4 nm) and have a low spring constant, making them very flexible. Also, they havea very low linear mass density. Moreover, the enhanced mechanical robustness ofSWCNTs compared to Si and their superior transduction properties compared to Sibased materials make them a promising candidate for NEMS applications. Hence,SWCNTs are of great interest in the technology roadmap for scaling down thedimensions of mechanical devices, as they are bottom-up chemical structures and canbe relatively easily synthesized.In this thesis, we first show the successful design and fabrication of templatestructures on which parallel growth of CNTs by chemical vapor deposition (CVD) orCNT assembly by dielectrophoresis (DEP) at multiple on-chip positions could becarried out in parallel. The capacitive coupling design in the samples was takenadvantage of to assemble CNTs by DEP on to multiple electrode pairs in parallel.Electrochemical deposition (ECD) was successfully used to deposit the catalystnanoparticles required for CNT growth. It offered excellent control in depositingcatalyst nanoparticles of the required diameter and density. CNTs were grown in alocalized manner with a good control on the growth density. However, CNTs could notbe grown with the desired directionality on the templates. Therefore, the experimentsof directional CVD growth were discontinued and the process of dielectrophoresis wasused to fabricate horizontally suspended CNTs bridged between the electrode pairs.Preliminary DEP experiments with MWCNTs resulted in successful alignment ofCNTs and assembly at specific on-chip positions with high spatial selectivity. A strongdependence on the electrode angle towards the density of the deposited CNTs isobserved. Best results are obtained on the electrodes with 45O tip angle, which hadabout 40% of them bridged with single CNTs.Further, the DEP experiments were optimized with SWCNT dispersions. Theaggregation factor of SWCNTs in liquid media has been successfully quantified usingoptical absorption spectroscopy. A modified approach to estimate the SWCNTaggregation factor in dispersions was proposed and verified. The dielectrophoresisprocess was further optimized with SWCNTs, especially the effect of the frequency ofthe electric field and the concentration of the SWCNT dispersion. With the optimalDEP conditions, 40% of the electrode pairs that were bridged by nanotubes had anindividual SWCNT assembly across a total on-chip area of 1x0.1 cm2. The study ofDEP results over such large area scales is critical for predicting the yield of wafer-scaleprocesses.DC electrical measurements could be performed with the as-assembled nanotubesafter DEP. The gate dependence measurements of the drain current of the suspendedSWCNTs revealed the presence of both metallic and semiconducting nanotubesdeposited by DEP. Thermal annealing of SWCNTs in the presence of forming gas wasfound to decrease the total resistance of the nanotube device. Post -DEP Pt ECD on theelectrodes of the nanotube devices creates a Schottky barrier diode with truly rectifyingproperties.Electromechanical measurements demonstrate a significant change in thenormalized resistance of the suspended SWCNTs as a function of the actuationfrequency applied at the substrate. We observe either an increasing (upward) or adecreasing (downward) peak for different devices. The frequency at which theresistance maximum (or the minimum) is measured was found to be in reasonableagreement with the calculated mechanical resonance frequency of the suspendedSWCNT device assuming a spring model with low tension. The measured resonancefrequency of the devices ranged between 26 MHz and 35 MHz. The piezoresistanceeffect of SWCNTs could cause this significant change in the normalized resistance attheir mechanical resonance frequency. We have also established that the chirality ofthe SWCNT determines whether the SWCNT resistance increases, decreases orremains unchanged during mechanical resonance.Given the scalability of the DEP process, arrays of such devices assembled inparallel could be of interest, for instance as gas or chemical sensors. The resultsobtained in this thesis represent a successful groundwork laid towards amanufacturable technique to fabricate arrays of SWCNT resonators over a largesample area. In addition to the development and optimization of the fabricationtechnique, the electromechanical measurements of the resonators clearly establish themechanical functionality of the resonators fabricated using CMOS-compatiblematerials and methods and hence their relevance in NEMS applications. Furthermore, aclear insight into the fundamental understanding about the behavior of SWCNToscillators during mechanical resonance was obtained from our measurement data.