Ferromagnetic nanostructures by laser manipulation

Lithography based on laser focusing of a beam of neutral iron atoms shows great promise for creating nanomagnetic structures. Laser focusing is a relatively new area, where successful experiments have been performed with, e.g., chromium atoms. Iron is perhaps one of the most difficult elements for this kind of experiment. Nevertheless, it is the ferromagnetic element most suitable for laser focusing. The production of well isolated sub–10 nm structures of iron requires two improvements to conventional laser focusing: chromatic aberration is reduced by using a supersonic beam of Fe atoms, whereas spherical aberration is reduced by implementing an additional mechanical grating placed just upstream of the standing light wave. In this thesis a description of the nanostructure project, preliminary results, and future plans are presented. The road to the fabrication of Fe nanostructures is full of challenges, and some of them have already been met. One of the most pronounced results is the development of a supersonic Fe beam source. Since these kind of beam sources are commercially not available, considerable effort has been put into the development of such a tool. The resulting source, as shown in Chapter 3, has demonstrated excellent performance. An Fe beam intensity in the range of 1015 to 1016 atoms/s/sr can be produced, which gives an ultimate deposition rate of the Fe structures in the range of 10 nm/h. Furthermore, the axial beam temperature is exceptionally low for atomic metal beams. The beam speed ratio, defined as the ratio of the mean axial velocity to the velocity spread, has been measured to be 11 for the Fe beam. As a result, chromatic aberration of the standing light wave (nanolenses) that focus the atoms to nanostructures, which up till now has been one of the limiting factors for the size of the structures in similar experiments, will not play a significant role anymore. Another result is the development of a UV laser system for laser manipulation of the atomic Fe beam, as described in Chapter 4. The requirements are a relatively high UV laser power of 500 mW at 372 nm, and a frequency stabilizing within 3 ?? 10-9 and locked to the Fe resonance frequency. These requirements have been met. Using cavity-enhanced second harmonic generation of a laser beam of 2 W at 744 nm a power of more than 500 mW has been obtained at 372 nm. Polarization spectroscopy applied to a hollow Fe cathode discharge has been used to obtain an error signal with a width of 40 MHz. Using this signal to lock the laser, a frequency stabilizing of better than 1 MHz results on the fundamental frequency, or 2 MHz on the second harmonic beam at 372 nm. In similar experiments performed so far the standing light wave is used as a thick lens. However, calculations presented in Chapter 5 show that using the standing wave in the thin lens regime, the focused structures are smaller by a factor of 3 at similar light field parameters. Channeling of atoms in a standing wave to produce small structures has also been investigated, since channeling is much more tolerant for errors in substrate placement and laser instability. However, this process requires a relatively high laser power of up to 1000 mW, which is not available. The resulting structure widths are then in the range of 10 to 30 nm, albeit with excellent contrast. Different models have been used for the calculations, and their validity has been checked. As a result, a thorough understanding of the process of laser focusing has been achieved. The next step that has to be taken experimentally, is laser cooling of the Fe beam. A well collimated beam of less than 200 ??rad angular divergence is necessary in order to obtain sub- 10 nm structures. Since the Fe beam is already "collimated axially" by using the supersonic Fe beam source, the transverse collimation is the main limiting factor to the focused Fe structure width. Numerical simulations presented in Chapter 6 have shown that a collimation of 100 ??rad can be achieved using an acceptable number of scattered photons. We are now at the verge of having the first laser cooled atomic Fe beam. Experimental results on laser cooling are to be expected soon. It is clear that several challenges are still present on the road to the fabrication of Fe nanostructures. However, the most important steps have already been taken, i.e., the development of a supersonic Fe beam source and UV laser system

Influence of reactor wall conditions on etch processes in inductively coupled fluorocarbon plasmas

The influence of reactor wall conditions on the characteristics of high density fluorocarbon plasma etch processes has been studied. Results obtained during the etching of oxide, nitride, and silicon in an inductively coupled plasma source fed with various feedgases, such as CHF3, C3F6, and C3F6/H2, indicate that the reactor wall temperature is an important parameter in the etch process. Adequate temperature control can increase oxide etch selectivity over nitride and silicon. The loss of fluorocarbon species from the plasma to the walls is reduced as the wall temperature increased. The fluorocarbon deposition on a cooled substrate surface increases concomitantly, resulting in a more efficient suppression of silicon and nitride etch rates, whereas oxide etch rates remain nearly constant

On the fluid mechanics of bilabial plosives

In this paper we present a review of some fluid mechanical phenomena involved in bilabial plosive sound production. As a basis for further discussion, firstly an in vivo experimental set-up is described. The order of magnitude of some important geometrical and fluid dynamical quantities is presented. Different theoretical flow models are then discussed and evaluated using in vitro measurements on a replica of the lips and using numerical simulations

Influence of reactor wall conditions on etch processes in inductively coupled fluorocarbon plasmas

The influence of reactor wall conditions on the characteristics of high density fluorocarbon plasma etch processes has been studied. Results obtained during the etching of oxide, nitride, and silicon in an inductively coupled plasma source fed with various feedgases, such as CHF3, C3F6, and C3F6/H2, indicate that the reactor wall temperature is an important parameter in the etch process. Adequate temperature control can increase oxide etch selectivity over nitride and silicon. The loss of fluorocarbon species from the plasma to the walls is reduced as the wall temperature increased. The fluorocarbon deposition on a cooled substrate surface increases concomitantly, resulting in a more efficient suppression of silicon and nitride etch rates, whereas oxide etch rates remain nearly constant

Supersonic Fe beam source for chromatic aberration-free laser focusing of atoms

A monochromatic Fe beam is generated by heated supersonic expansion of argon seeded with Fe vapor. At a nozzle temperature of 1930 K and 800 torr argon inlet pressure the Fe beam has an axial velocity spread of 8% and intensity of 3x10^15 s^-1 sr^-1, corresponding to a deposition rate of 10 nm/h at 150 mm from the nozzle. The two-chamber alumina crucibles are chemically stable for liquid Fe. With 400 mm^3 Fe we have operated for more than 200 hours without reloading. The power consumption at 1930 K is 750 W. Temperature stability at constant power (without feedback) is better than 30 K. The source is intended for deposition of nanostructures by laser focusing of the Fe beam. The small axial velocity spread virtually eliminates the increase in focal spot size due to chromatic aberration

Laser frequency stabilization using an Fe-Ar hollow cathode discharge cell

Polarization spectroscopy of an Fe-Ar hollow cathode discharge cell was used to lock a frequency-doubled Ti:sapphire laser to the 372-nm5D45F5 transition of 56Fe. The discharge cell produced a density of 1018 m-3 ground-state 56Fe atoms at a temperature of 650 K, this density being comparable to a conventional oven at 1500 K. Saturated absorption spectroscopy and two schemes of polarization spectroscopy were compared with respect to signal-to-background ratio and the effect of velocity-changing collisions. The laser was locked within 0.2 MHz for hours by feedback of the dispersive polarization spectroscopy signal

High-rate silicon nitride deposition for photovoltaics : from fundamentals to industrial application

The development of a novel plasma technique for high rate (> 1 nm/s) silicon nitride deposition for multifunctional antireflection coatings on crystalline silicon solar cells is described. The research has involved the analysis of the structural and optical properties of the silicon nitride films as obtained under different operating conditions using the N2-SiH4 and NH3-SiH4 reactant mixture. Furthermore, the fundamental plasma processes and the film growth mechanism have been studied in terms of plasma chemistry, plasma species, and their contribution to film growth. The feasibility of the application of the high-rate deposited silicon nitride as a bulk passivating antireflection coating has been proven by lab-scale experiments on multicrystalline silicon solar cells and the successful transfer of the technique into an industrial inline deposition system for high-volume production of solar cells is reported

A novel commercial plasma source for ultrahigh-rate deposition of silicon nitride for solar cells

Recently, a new inline tool for high rate deposition of silicon nitride for (multi) crystalline silicon solar cells, the "DEPx", was introduced. This tool uses the so called expanding thermal plasma (ETP) technique operated on an Ar-NH3 gas mixture to dissociate SiH4. We will discuss some of the important properties of this remote and high density plasma in terms of ion densities, radical densities (N, NH, NH2) and the NH3 consumption degree