Aun Shih Teh

Highly sensitive integrated pressure sensor with horizontally oriented carbon nanotube network

This paper presents a functionalized, horizontally oriented carbon nanotube network as a sensing element to enhance the sensitivity of a pressure sensor. The synthesis of horizontally oriented nanotubes from the AuFe catalyst and their deposition onto a mechanically flexible substrate via transfer printing are studied. Nanotube formation on thermally oxidized Si (100) substrates via plasma-enhanced chemical vapor deposition controls the nanotube coverage and orientation on the flexible substrate. These nanotubes can be simply transferred to the flexible substrate without changing their physical structure. When tested under a pressure range of 0 to 50 kPa, the performance of the fabricated pressure sensor reaches as high as approximately 1.68%/kPa, which indicates high sensitivity to a small change of pressure. Such sensitivity may be induced by the slight contact in isolated nanotubes. This nanotube formation, in turn, enhances the modification of the contact and tunneling distance of the nanotubes upon the deformation of the network. Therefore, the horizontally oriented carbon nanotube network has great potential as a sensing element for future transparent sensors.

Effect of Co and Ni nanoparticles formation on carbon nanotubes growth via PECVD

The effect of cobalt (Co) and nickel (Ni) nanoparticle catalysts on the growth of carbon nanotubes (CNTs) were studied, where the CNTs were vertically grown by plasma enhanced chemical vapour deposition (PECVD) method. The growth conditions were fixed at a temperature of 700 °C with a pressure of 1000 mTorr for 40 minutes with various thicknesses of sputtered metal catalysts. Only multi-walled carbon nanotubes are present from the growth as large average diameter of outer tube (∼10–30 nm) were measured for both of the catalysts used. Experimental results show that high density of CNTs was observed especially towards thicker catalysts layers where larger and thicker nanotubes were formed. The nucleation of the catalyst with various thicknesses was also studied as the absorption of the carbon feedstock is dependent on the initial size of the catalyst island. The average diameter of particle size increases from 4 to 10 nm for Co and Ni catalysts. A linear relationship is shown between the nanoparticle size and the diameter of tubes with catalyst thicknesses for both catalysts. The average growth rate of Co catalyst is about 1.5 times higher than Ni catalyst, which indicates that Co catalyst has a better role in growing CNTs with thinner catalyst layer. It is found that Co yields higher growth rate, bigger diameter of nanotube and thicker wall as compared to Ni catalyst. However, variation in Co and Ni catalysts thicknesses did not influence the quality of CNTs grown, as only minor variation in IG/ID ratio from Raman spectra analysis. The study reveals that the catalysts thickness strongly affects not only nanotube diameter and growth rate but also morphology of the nanoparticles formed during the process without influencing the quality of CNTs.

Multiwalled Carbon Nanotube Growth Mechanism on Conductive and Non-Conductive Barriers

We report on the catalytic growth of multiwalled carbon nanotubes by plasma enhanced chemical vapor deposition using Ni and Co catalyst deposited on SiO2, Si3N 4,ITO and TiN Xbarrier layers; layers which are typically used as diffusive barriers of the catalyst material. Results revealed higher growth rates on conductive ITO and TiN Xas compared to non con-ductiveSiO2, and Si3N 4,barriers. Micrograph images reveal the growth mechanism for nanotubes grown on SiO2, Si3N 4 and ITO to be tip growth while base growth was observed for the TiN X barrier layer. Initial conclusion suggests that conductive diffusion barrier surfaces promotes growth rates however it is possible that multiwalled carbon nanotubes grown onSiO2, and Si3N 4,were encumbered as a result of the formation of silicide as shown in the results here.

Thin Film Ag Masking for Deep Glass Micromachining

We report a single thin film and low cost masking material for deep, wet isotropic etching of glass in HF, which has applications in microfluidic devices and systems. With a 100 nm thin silver (Ag) mask, microcavities with an etch depth exceeding 200 μm were achieved and, by further thickening the silver film to 300 nm, etch depths up to 340 μm were observed. The thin film was deposited by evaporation and patterned in a mixture of nitric acid and deionized water at a ratio of 1:3. Silver had good adhesion to glass.

Self-aligned nanostructures by CMOS technology

In semiconductor fabrication, there are various methods that can be employed to form fine structures. Such techniques include a combination of advance lithography and etching, chemical mechanical planarization (CMP), or metal lift-off. However, these techniques may not be the easiest or the most cost effective. When using lithographic methods such as ultraviolet (UV), deep ultraviolet (DUV), extended ultraviolet (EUV), E-Beam [1], and X-ray, there are always resolution and alignment issues such as how small a structure can be produced and how closely and accurately a structure can be aligned to another. Even when lithography issues are resolved, patterning of very fine structures is also a problem. Wet chemical etching is not feasible when trying to produce submicrometer features because of large undercuts due to the isotropic nature of the etch solution. Lift-off with sacrificial resist [2] is a more common solution to produce nanostructures, but the technique does have resist imposed limitations where deposition must take place below 200°C because of resist thermal stability preventing its use with chemical vapour deposition processes. Also, organizing nanostructures into highly ordered array can also prove extremely challenging.