High Aspect Ratio 3D Microstructures

We have fabricated flexible electronic devices to test the strain-based change in resistance of a network of single-walled carbon nanotubes (SWCNTs) for use in microscale, high resolution magnetometry. To do this, we first develop a simple, reliable method to obtain catalyst nanoparticles for carbon nanotube growth through indirect, thin-film evaporation. Next we fabricate a two-terminal SWCNT device on a rigid substrate. We then transfer the device, intact, to a flexible substrate for strain testing. Herein, we report progress in growth and measurement techniques.

We used spectroscopic ellipsometry to determine the optical constants of seven ThO₂ thin-film samples, thickness ranging between 28 and 578 nm, for the spectral range of 1.2 to 6.5 eV. The samples were deposited by biased radio-frequency sputtering at DC bias voltages between 0 and - 68 V. The index of refraction, it, does not depend on bias voltage, sputter pressure, deposition rate, or thickness. Specifically, the value of 11 at 3 eV is 1.86 +/- 0.04 for the unbiased samples and 1.86 +/- 0.04 for the biased samples. The average value of n at 3 eV for the thicker samples (d >= 50 nm) was 1.87 +/- 0.05, and 1.85 +/- 0.02 for the thinner samples (d <= 50 nm). (c) 2006 Elsevier B.V. All rights reserved.

A technique for analysis of reflection and transmission data from thin films in the Extreme Ultraviolet (EUV) has been developed. Efforts to deposit films on polyimide membranes to allow for transmission measurements are discussed. Later experiments use thin films of interest deposited directly onto photodiode detectors. This coated photodiode is used for making transmission measurements, and use of a second, uncoated detector allows for the simultaneous collection of reflection data as well. The development of this technique and the analysis of data from thorium thin films and scandium oxide thin films are presented here. EUV measurements were made at the Advanced Light Source (ALS), Beamline 6.3.2, in Berkeley, California.

We have measured the reflectance and transmittance of thorium dioxide thin films from 50-280 eV. We have developed several methods for fitting this data that gives the most reliable values for the complex index of refraction, n = 1 - δ + iβ. These fitting methods included fitting film thickness using interference fringes in highly transmissive areas of the spectrum and fitting reflectance and transmittance data simultaneously. These techniques give more consistent optical constants than solitary unconstrained fitting of reflectance as a function of angle. Using these techniques, we have found approximate optical constants for thorium dioxide in this energy range. We found that the absorption edges of thoria were shifted 4 eV and 2 eV to lower energies from those of thorium. We also found that the peak in δ was shifted by 3 eV to lower energy from that of thorium.

We used spectroscopic ellipsometry to determine the optical constants of seven thin-film ThO2 samples deposited by radio-frequency sputtering, thickness ranging between 24 and 578 nm, for the spectral range of 1.2 to 6.5. We used a hollow-cathode light source and vacuum monochromator to measure constants at 10.2 eV. None of the deposition parameters studied including DC-bias voltages successfully increase the n of (that is, densify) thoria films. The value of n at 3.0 eV is 1.86 ± 0.04. We find compelling evidence to conclude that the direct band gap is at ~5.9 eV, clarifying the results of others, some of whom observed the absorption edge below 4 eV. The edge in the two thickest films is of a narrow feature (FWHM=0.4 eV) with modest absorption (α~6μm-1, k ~0.1). Absorption may go down briefly with increasing energy (from 6.2 to 6.5 eV). But at 10.2 eV absorption is very high and index low as measured by variable-angle reflectometry, α = 47.3 ± 5.5 μm-1 and k = 0.48 ± 0.05, and n =0.87 ± 0.12.