Post-CMOS wafer level growth of carbon nanotubes for low-cost microsensors--a proof of concept

Here we demonstrate a novel technique to grow carbon nanotubes (CNTs) on addressable localized areas, at wafer level, on a fully processed CMOS substrate. The CNTs were grown using tungsten micro-heaters (local growth technique) at elevated temperature on wafer scale by connecting adjacent micro-heaters through metal tracks in the scribe lane. The electrical and optical characterization show that the CNTs are identical and reproducible. We believe this wafer level integration of CNTs with CMOS circuitry enables the low-cost mass production of CNT sensors, such as chemical sensors.     

Transmission electron microscope imaging of single-walled carbon nanotube interactions and mechanics on nitride grids

A method for analysing systems of isolated single-walled carbon nanotubes is of paramount importance if their structural characteristics are to be fully understood and utilized. Here we offer a simple technique for analysing such systems, with unprecedented contrast, using transmission electron microscope imaging of carbon nanotubes suspended over large holes in a silicon nitride grid. The nanotubes are grown directly on the viewing grids, using the chemical vapour deposition process, thus avoiding the use of chemicals or aggressive treatments. This method is simultaneously non-invasive, reusable, allows the analysis of multiple structures based on carbon nanotubes and is quickly implemented.     

Scalable fabrication of carbon nanotube/polymer nanocomposite membranes for high flux gas transport

We present a simple, fast, and practical route to vertically align carbon nanotubes on a porous support using a combination of self-assembly and filtration methods. The advantage of this approach is that it can be easily scaled up to large surface areas, allowing the fabrication of membranes for practical gas separation applications. The gas transport properties of thus constructed nanotube/polymer nanocomposite membranes are analogous to those of carbon nanotube membranes grown by chemical vapor deposition. This paper shows the first data for transport of gas mixtures through carbon nanotube membranes. The permeation of gas mixtures through the membranes exhibits different properties than those observed using single-gas experiments, confirming that non-Knudsen transport occurs.     

Carbon Nanotubes Grown by RF Heating and Their Morphological and Structural Properties

Multiwall and single-wall carbon nanotubes were synthesized on Fe-Co/CaCO₃ and a Fe-Co/MgO catalyst system, respectively, by using two different catalytic chemical vapor deposition methods, external furnace (EF) heating and radio frequency (RF) excitation. The carbon nanotubes synthesized with radio frequency excitation have a smaller outer diameter, fewer layers (smaller outer/inner diameter ratio), and better crystalline properties than the nanotubes grown with external furnace heating. The radio frequency process was found to be responsible for a faster growth rate of the carbon nanotubes over longer periods of time due to a higher localized heating. These findings can be explained by the skin currents induced in the metallic catalytic clusters, which keep the catalysts active for longer periods of time and diminish the amount of noncrystalline carbon formed in the synthesis process.     

Transmission electron microscopy study of individual carbon nanotube breakdown caused by Joule heating in air

We present repeated structural and electrical measurements on individual multiwalled carbon nanotubes, alternating between electrical measurements under ambient conditions and transmission electron microscopy (TEM). The multiwalled carbon nanotubes made by chemical vapor deposition were manipulated onto cantilever electrodes extending from a specially designed microfabricated chip. Repeated TEM investigations were then made of the progressive destruction of the nanotube structure induced by Joule heating in air. The electrical measurements indicate that the studied nanotubes behave as diffusive conductors with remarkably predictable electrical properties despite extensive structural damage.     

On the role of activation mode in the plasma- and hot filaments-enhanced catalytic chemical vapour deposition of vertically aligned carbon nanotubes

Catalytic chemical vapor deposition (CCVD) with different activation modes (thermal; hot filaments-enhanced; direct current plasma-enhanced and both hot filament and direct current plasma-enhanced) are achieved in order to grow vertically aligned carbon nanotubes (VA CNTs). By widely varying the power of the different activation sources of the gas (plasma, hot filaments, substrate heating) while keeping identical the substrate temperature (973 K) and the catalyst preparation, the results point out the important role of ions in the nucleation of carbon nanotubes (CNTs), as well as the etching behaviour of highly activated radicals such as H˙ in the selective growth of vertically aligned films of CNTs. Moreover, it is demonstrated that, within the deposition conditions (temperature, pressure, flow rate) used in this study, oriented carbon nanotubes can be grown only when both ions, mainly generated by the gas discharge plasma, and highly reactive radicals, mainly formed by the hot filaments, are produced in the gas phase. We propose that highly energetic ions are needed to nucleate the carbon nanotubes by increasing the carbon concentration gradient whereas the highly reactive radicals allow the selective growth of vertically aligned CNTs by preventing carbon deposition on the whole surface through chemical etching of edge carbons in graphene sheets.     

CMOS monolithic metal-oxide sensor system comprising a microhotplate and associated circuitry

A gas sensor system fabricated in industrial CMOS technology is presented, which includes, for the first time, a microhotplate and the necessary driving and control circuitry on a single chip. Post-complementary-metal-oxide-semiconductor (CMOS) fabrication steps, such as micromachining of the membrane structure, the deposition of noble metal on the electrodes, and the processing of the sensitive metal-oxide layer, have been developed to be fully compatible with the industrial CMOS process. Temperatures up to 350/spl deg/C were reached on the hotplates using a low-voltage power supply (5 V). A symmetric hotplate design with a temperature homogeneity of better than 2% in the heated area was realized. The integrated temperature controller regulates the membrane temperature with a resolution of /spl plusmn/0.3/spl deg/C in the tracking mode. The temperature increase on the bulk chip owing to heat transfer through the membrane is less than 2% of the respective membrane operation temperature (6/spl deg/C at 350/spl deg/C membrane temperature). The gas sensing performance of the sensor was assessed by test measurements with carbon monoxide (CO). The gas tests evidenced a limit of detection of less than 5 ppm CO.     

Controlling electron-beam-induced carbon deposition on carbon nanotubes by Joule heating

Electron-beam-induced deposition (EBID) of carbon on the surface of carbon nanotubes was well controlled by passing different electrical currents through the nanotubes. The control is due to Joule heating and the temperature of the carbon nanotubes was estimated. The deposition rate was found to increase and then decrease with the temperature and was maximized at about 310?K and approached zero at about 400?K. The method can be used to control the deposition rate of EBID in nanowelding and nanofabrication and to eliminate amorphous carbon contamination in in situ study of nanostructures.     

Nanoscale memory cell based on a nanoelectromechanical switched capacitor

The demand for increased information storage densities has pushed silicon technology to its limits and led to a focus on research on novel materials and device structures, such as magnetoresistive random access memory and carbon nanotube field-effect transistors, for ultra-large-scale integrated memory. Electromechanical devices are suitable for memory applications because of their excellent 'ON-OFF' ratios and fast switching characteristics, but they involve larger cells and more complex fabrication processes than silicon-based arrangements. Nanoelectromechanical devices based on carbon nanotubes have been reported previously, but it is still not possible to control the number and spatial location of nanotubes over large areas with the precision needed for the production of integrated circuits. Here we report a novel nanoelectromechanical switched capacitor structure based on vertically aligned multiwalled carbon nanotubes in which the mechanical movement of a nanotube relative to a carbon nanotube based capacitor defines 'ON' and 'OFF' states. The carbon nanotubes are grown with controlled dimensions at pre-defined locations on a silicon substrate in a process that could be made compatible with existing silicon technology, and the vertical orientation allows for a significant decrease in cell area over conventional devices. We have written data to the structure and it should be possible to read data with standard dynamic random access memory sensing circuitry. Simulations suggest that the use of high-k dielectrics in the capacitors will increase the capacitance to the levels needed for dynamic random access memory applications.     

Secondary nanotube growth on aligned carbon nanofibre arrays for superior field emission

We report substantial improvement of the field emission properties from aligned carbon nanotubes grown on aligned carbon nanofibres by a two-stage plasma enhanced chemical vapour deposition (PECVD) process. The threshold field decreased from 15.0 to 3.6 V/microm after the secondary growth. The field enhancement factor increased from 240 to 1480. This technique allows for superior emission of electrons for carbon nanotube/nanofibre arrays grown directly on highly doped silicon for direct integration in large area displays.     

辉光放电和压强对准直碳纳米管生长的影响

利用负偏压增强热丝化学气相沉积系统在沉积有过渡层Ta和催化剂层NiFe的Si村底上制备出准直碳纳米管，并用扫描电子显微镜研完了它们的生长和结构，结果表明辉光放电和压强对其生长和结构有极大的影响。若无辉光放电产生，碳纳米管是弯曲的，有辉光放电时，碳纳米管是准直的。当压强较大时，准直碳纳米管较容易生长，并且随着压强的减小，其平均直径减小和平均长度增大。但压强为5Pa时，准直碳纳米管却不能够生长。最后，分析和讨论了辉光放电和压强对准直碳纳米管生长和结构的影响。     

Room temperature growth of single-wall coiled carbon nanotubes and Y-branches

Straight carbon nanotubes, carbon nanotube “knees,” Y-branches of carbon nanotubes and coiled carbon nanotubes were grown on a graphite substrate held at room temperature by the decomposition of fullerene under moderate heating (450 °C) in the presence of 200-nm Ni particles. The grown structures were investigated without any further manipulation by STM. The growth and the chemical stability of the carbon nanostructures containing nonhexagonal rings are discussed.     

Micromachined silicon transmission electron microscopy grids for direct characterization of as-grown nanotubes

Transmission electron microscopy (TEM) is a key technique in the structural characterization of carbon nanotubes. For device applications, carbon nanotubes are typically grown by chemical vapour deposition (CVD) on silicon substrates. However, TEM requires very thin samples, which are electron transparent. Therefore, for TEM analysis, CVD grown nanotubes are typically deposited on commercial TEM grids by post-processing. However, this procedure can damage the nanotubes, and it does not work reliably if the nanotube density is too low. The ability to do TEM directly on as-grown nanotubes on the silicon substrate would solve these problems. For this purpose, we have fabricated micromachined silicon TEM grids with narrow open slits on them. Since the nanotubes grown on these substrates are suspended freely over the open slits, the micromachined substrates form a natural TEM grid for direct imaging of CVD grown nanotubes. Furthermore, the background noise is significantly reduced during micro-Raman spectroscopy, resulting in a better signal-to-noise ratio. As a result, these micromachined Si substrates provide a low cost, mass producible, efficient, and reliable platform for direct TEM, SEM, AFM, and Raman characterization of as-grown nanotubes. These grids can be used for characterizing a wide range of other nanomaterials, including peapods, nanowires, and nanofibres.     

真空度对碳纳米管生长过程的影响

利用等离子体增强热丝化学气相沉积系统在沉积有过渡层Ta和催化剂层NiFe的Si衬底上制备出准直碳纳米管，并用扫描电子显微镜研究了它们的生长和结构，结果表明真空度对其生长和结构有较大的影响。当真空度为4000Pa和2000Pa时，准直碳纳米管较容易生长，并且真空度为2000Pa时生长的碳纳米管平均长度大于真空度为4000Pa时碳纳米管的平均长度。但真空度为667Pa时碳纳米管生长困难。根据热力学和辉光放电理论，分析了真空度对准直碳纳米管生长和结构的影响。     

High frequency piezoelectric MEMS ultrasound transducers

High-frequency ultrasound array transducers using piezoelectric thin films on larger structures are being developed for high-resolution imaging systems. The increase in resolution is achieved by a simultaneous increase in operating frequency (30 MHz to about 1 GHz) and close coupling of the electronic circuitry. Two different processing methods were explored to fabricate array transducers. In one implementation, a xylophone bar transducer was prototyped, using thin film PbZr(0.52)Ti(0.48)O(3) (PZT) as the active piezoelectric layer. In the other, the piezoelectric transducer was prepared by mist deposition of PZT films over electroplated Ni posts. Because the PZT films are excited through the film thickness, the drive voltages of these transducers are low, and close coupling of the electronic circuitry is possible. A complementary metal-oxidesemiconductor (CMOS) transceiver chip for a 16-element array was fabricated in 0.35-microm process technology. The ultrasound front-end chip contains beam-forming electronics, receiver circuitry, and analog-to-digital converters with 3-Kbyte on-chip buffer memory.     

Raman study on single-walled carbon nanotubes with different laser excitation energies

The industrial use of carbon nanotubes is increasing day by day; therefore, it is very important to identify the nature of carbon nanotubes in a bundle. In this study, we have used the Raman spectroscopic analysis on vertically aligned single-walled carbon nanotubes (SWCNTs) grown by the chemical vapour deposition (CVD) technique. The grown sample is excited with two laser excitation wavelengths, 633 nm from He-Ne laser and 514·5 nm from Ar⁺ laser. Raman spectrum in the backscattering geometry provides the characteristic spectra of SWCNTs with its radial breathing mode (RBM), defect-induced disorder mode (D band), and high-energy modes (G and M bands). The Raman signal positions of the spectra in RBM, G and M bands confirm the grown sample to be of semiconducting type in nature.     

Impact behavior and fractographic study of carbon nanotubes grafted carbon fiber-reinforced epoxy matrix multi-scale hybrid composites

Carbon nanotubes are the most promising reinforcement for high performance composites. Multiwall carbon nanotubes were directly grown onto the carbon fiber surface by catalytic thermal chemical vapor deposition technique. Multi-scale hybrid composites were fabricated using the carbon nanotubes grown fibers with epoxy matrix. Morphology of the grown carbon nanotubes was investigated using field emission scanning electron microscopy and transmission electron microscopy. The fabricated composites were subjected to impact tests which showed 48.7% and 42.2% higher energy absorption in Charpy and Izod impact tests respectively. Fractographic analysis of the impact tested specimens revealed the presence of carbon nanotubes both at the fiber surface and within the matrix which explained the reason for improved energy absorption capability of these composites. Carbon nanotubes presence at various cracks formed during loading provided a direct evidence of micro crack bridging. Thus the enhanced fracture strength of these composites is attributed to stronger fiber–matrix interfacial bonding and simultaneous matrix strengthening due to the grown carbon nanotubes.     

A Novel Saw Device in CMOS: Design, Modeling, and Fabrication

The design, finite element modeling, fabrication, and characterization of a novel surface acoustic wave (SAW) delay line for bio/chemical and telecommunication applications in CMOS technology are introduced. A full modeling was carried out. The devices are designed in a standard semiconductor foundry 1.5-mum two-metal two-poly process. A unique maskless postprocessing sequence is designed and completed. The three postprocessing steps are fully compatible with any standard integrated circuit technology such as CMOS. This allows any signal control/processing circuitry to be easily integrated on the same chip. ZnO is used as the piezoelectric material for SAW generation. A thorough characterization and patterning optimization of the sputtered ZnO was carried out. The major novelties that are introduced in the SAW delay line features are the embedded heater elements for temperature control, compensation, and acoustic absorbers that are designed to eliminate edge reflections and minimize triple transit interference that is amplified by edge reflections. Both of these attributes are designed by using CMOS materials without disturbing SAW performance     

Direct spinning of carbon nanotube fibres from liquid feedstock

Carbon nanotubes are regarded by many as being the next generation in high performance materials due to their unique properties. The Cambridge Process was developed to utilize these unique properties by directly spinning carbon nanotube fibres drawn from an aerogel sock. The sock is formed from carbon nanotubes grown via a catalytic chemical vapour deposition (CVD) process. Due to the nature of CVD, the process is readily scalable. Kilometres of fibre can be made at a rate of 20 m/min. Altering process parameters (catalyst concentration, feedstock injection rate, furnace temperature, and gas flow rate) allows the production of nanotubes of a desired morphology. The fibres have been characterized with scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman Spectroscopy in order to confirm nanotube composition and orientation. The fibre possesses mechanical and electrical properties that rival or exceed those of present-day materials. Mechanical properties can be enhanced by increasing the degree of orientation of the nanotubes with the long axis of the fibre and by overall densification. These effects can be accomplished through drawing the fibre and solvent treatment.     

Dielectrophoretic integration of nanodevices with CMOS VLSI circuitry

We present a novel platform for the development and deployment of nanosensors in integrated systems. The nanosensor technology is based on cylindrical structures grown using porous membranes as templates. These nanostructures are manipulated using dielectrophoretic forces, thus allowing their individual assembly and characterization. This assembly also enables the development of "mixed-mode" integrated circuits that include readout, signal processing, and communications circuitry, as well as the requisite layout for the post-integrated-circuit assembly of the nanostructures. We report the results of assembly experiments performed on complementary metal-oxide-semiconductor (CMOS) circuitry designed using Freescale semiconductor's HiP6W low-voltage 0.18-/spl mu/m Si/SiGe BiCMOS process.