To better understand the proximity effect of electron-beam lithography and its limitation on aspect ratios in a thick resist film (>1.0μm), the simulation of isolated trench patterning has been performed using the Monte Carlo simulation software SELID™. Various key process parameters, including beam energy, exposure dose, substrate, acid diffusion length, and development time have been analyzed by means of the point-spread function, secondary electron distribution, energy distributions, and developed resist profiles. Exposure dose and development time are optimized to achieve vertical sidewalls, and the acid diffusion length has been adjusted to slightly change the top and bottom dimensions of the patterned trench. The simulation results show that increasing the beam energy significantly reduces the forward proximity effect and thereby increases the trench aspect ratio. In contrast, the substrate generally has only a minor effect in terms of the resist profile because its backscattering effect covers a long range up to 10.0μm from incident point (50keV). The achievable maximum aspect ratio for a given resist thickness was found to scale approximately linearly with beam energy. The aspect ratio in a 1.0μm thick resist film increased from 3:1 to 20:1 as the beam energy was increased from 10to100keV, and from 5:1 to 10:1 for a 4.0μm resist film as the beam energy was increased from 50to100keV. Finally, experimental data using a 50keV Leica e-beam writing tool is presented, and they agree with the results from our simulations.

The carbon nanotube-metal nanoparticle complex has attracted a lot of research interest because of their potential applications in catalysis and gas sensing. Here we introduce a novel electrochemical method to realize in-situ decoration of single-walled carbon nanotube field effect transistors (CNT-FET) with metal nanoparticles using a sacrificial electrode. In this process, metal atoms are first ionized into an electrolyte solution by applying a potential difference between the sacrificial electrode and the grounded source/drain electrodes connecting the nanotube of the CNT-FET. The positive metal ions migrate under the influence of the electric field, and deposit on the grounded nanotube as metal nanoparticles. This method provides for better control over the quantity and size of the deposited nanoparticles compared to other decoration methods. We demonstrate successful deposition of Au and Ag nanoparticles on carbon nanotube field effect devices, with the quantity and size of the nanoparticles varying as a function of the applied potential. We show that the metal nanoparticle size can vary from 10 nm to over 300 nm, and the spatial distribution can change from very scarce decoration to a near continuous coating. Such metal nanoparticles have potential applications in chemical sensors, as they interact with gas molecules and generate an electrical signal in the nanotube, which can be detected. They can also be explored as biological anchoring sites for bio-functionalization of the nanotube, which is critical to developing highly sensitive and selective bio-sensors.

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