Local Synthesis and Direct Integration of Carbon Nanotubes into Microsystems for Sensor Applications

Carbon nanotubes (CNTs) have been intensively studied since their discovery more than two decades ago. A lot of research has exploited their extraordinary properties and various applications, meanwhile, revealed the challenges of fabricating the CNT-based device. Major challenges concern the high temperature required for the CNT growth, and the difficulty in handling and maneuvering the CNTs. An innovative approach to overcome these challenges is to locally synthesize and directly assemble CNTs into devices. Following such approach, this thesis developed a fabrication process with a high simplicity, a high controllability, and a CMOS/MEMS compatibility for the local synthesis and direct integration of CNTs into Si microsystems. This thesis covers the total process chain: from synthesis and integration of CNTs, to characterization, and to testing of a proof-of-principle gas sensor. The first key finding of this thesis is a simple and robust method to control the temperature for the growth of CNTs by using only electrical signals. During the growth process, a localized hot region for the growth of CNTs is created by locally heating a Si microelectrode (Joule heating). The induced temperature is monitored through in-situ measurements of the electrical resistance of the Si electrode. The measured resistance provides feedback to control the input power for heating the Si electrode. This pure electrical control enables a simple, automated and parallel process to synthesize locally and integrate CNTs directly into microsystems. The second key finding of this thesis is the diameter dependency for the effect of an applied electric field on the growth orientation of CNTs. A statistical analysis of 1100 CNTs showed that small-diameter CNTs (d 10 nm) were curved and did not align. In the transition regime, CNTs were moderately curved, but the average direction was at small angle with the electric field direction. The third key finding of this thesis is the correlation between local temperature and resulting characteristics of CNTs. A high gradient of temperature along the Si microelectrode due to Joule heating allowed for studying the effect of temperature. At the region where the temperature is highest ( 900oC), the nanostructure of CNTs had the highest degree of order, and the average diameter of CNT was smallest. At regions with lower temperatures, CNTs had a higher degree of defects and disorder, and a lower average diameter. The density of CNTs, however, was highest at the moderate-temperature region ( 850oC). The other contribution of this thesis is preliminary results on the development of CNT-based microsystems towards sensor applications. The preliminary results suggest that: (i) contact resistance at the CNT-Si interface could be reduced by both techniques of local annealing and local deposition of Platinum onto the CNT-Si contacts; (ii) thermal evaporation of metals could be used to functionalize the CNTs in a microsystem where CNTs are suspended and span two microelectrodes

Geometric effects on mixing performance in a novel passive micromixer with trapezoidal-zigzag channels

A novel passive micromixer, called a trapezoidal-zigzag micromixer (TZM), is reported. A TZM is composed of trapezoidal channels in a zigzag and split–recombine arrangement that enables multiple mixing mechanisms, including splitting–recombining, twisting, transversal flows, vortices, and chaotic advection. The effects of geometric parameters of the TZM on mixing performance are systematically investigated by the Taguchi method and numerical simulations in COMSOL Multiphysics. The number of mixing units, the slope angle of the trapezoidal channel, the height of the constriction element, and the width ratio between the middle-trapezoidal channel and the side-trapezoidal channel are the four parameters under study. The mixing performance of the TZM is investigated at three different Reynolds number (Re) values of 0.5, 5, and 50. The results showed that a TZM with six mixing units, a trapezoidal slope angle of 75°, a constricting height of 100 µm, and a width ratio of 0.5 has the highest mixing efficiency. This optimal TZM has a mixing efficiency greater than 85% for Re values from 0.1 to 80. In particular, for Re  ≤  0.9 and Re  ≥  20, the mixing efficiency of the optimal TZM is greater than 90%. The proposed TZM has a higher mixing efficiency and a smaller footprint than previously reported micromixers

Monitoring of Corroded and Loosened Bolts in Steel Structures via Deep Learning and Hough Transforms

In this study, a regional convolutional neural network (RCNN)-based deep learning and Hough line transform (HLT) algorithm are applied to monitor corroded and loosened bolts in steel structures. The monitoring goals are to detect rusted bolts distinguished from non-corroded ones and also to estimate bolt-loosening angles of the identified bolts. The following approaches are performed to achieve the goals. Firstly, a RCNN-based autonomous bolt detection scheme is designed to identify corroded and clean bolts in a captured image. Secondly, a HLT-based image processing algorithm is designed to estimate rotational angles (i.e., bolt-loosening) of cropped bolts. Finally, the accuracy of the proposed framework is experimentally evaluated under various capture distances, perspective distortions, and light intensities. The lab-scale monitoring results indicate that the suggested method accurately acquires rusted bolts for images captured under perspective distortion angles less than 15° and light intensities larger than 63 lux

Excessive functions, Appell polynomials and optimal stopping

The main topic of the thesis is optimal stopping. This is treated in two research articles. In the first article we introduce a new approach to optimal stopping of general strong Markov processes. The approach is based on the representation of excessive functions as expected suprema. We present a variety of examples, in particular, the Novikov-Shiryaev problem for Lévy processes. In the second article on optimal stopping we focus on differentiability of excessive functions of diffusions and apply these results to study the validity of the principle of smooth fit. As an example we discuss optimal stopping of sticky Brownian motion. The third research article offers a survey like discussion on Appell polynomials. The crucial role of Appell polynomials in optimal stopping of Lévy processes was noticed by Novikov and Shiryaev. They described the optimal rule in a large class of problems via these polynomials. We exploit the probabilistic approach to Appell polynomials and show that many classical results are obtained with ease in this framework. In the fourth article we derive a new relationship between the generalized Bernoulli polynomials and the generalized Euler polynomials

Carbon Nanotubes Directly Integrated in CMOS by Local Synthesis - Towards a Wafer-Level Process

Integrating nanomaterials in electronic circuitry and in microsystems is highly desired for fully exploiting the functionality of nanomaterials such as Carbon Nanotubes (CNTs), utilizing the signal processing of micro/nano-electronics such as CMOS. An example device is a CNT-based gas sensor, where the extreme surface-to-volume ratio of CNTs provides ultra-high sensitivity, and direct integration with CMOS enables a device rendering processed, amplified and calibrated signals in a single, low-cost device. The CNTs should be synthesized on-chip directly into electric circuits. One challenge to overcome is the contradiction between temperature requirements for CNT growth (800-1000 °C) and CMOS compatibility (<; 300°C). We can achieve local CNT growth temperatures while keeping the main part of the chip at CMOS compatible temperatures by designing resistive micro-heaters. Synthesized CNTs are directed towards an electrode for desired contact by applying a voltage that gives a guiding electric field. All process parameters are controlled electrically, enabling a wafer-level process compatible with high-volume, low-cost manufacturing. This paper shows results from CNT integration in test vehicles manufactured in MEMS processes (using silicon microheaters), as well as our status towards realizing CNT integration in a purpose-designed CMOS chip (optionally using the chip's metal or polysilicon layers for microheaters)