Edmundo da Silva Braga

Structural characterization of transparent semiconducting thin films of SnO2 and In2O3

Abstract The structural characteristics of optically transparent electrically conducting thin films of SnO 2 and of In 2 O 3 deposited onto amorphous substrates and onto monocrystalline silicon substrates were studied by X-ray diffraction. The SnO 2 films, which were prepared by chemical vapor deposition, were polycrystalline with a grain size of 7–15 nm and with a marked (200) preferred orientation. The evaporated In 2 O 3 films had an average grain size of 3–10 nm but showed no preferential orientation.

Fabrication of sharp silicon tips employing anisotropic wet etching and reactive ion etching

Abstract We developed a process to obtain sharper silicon tips by employing anisotropic etching in a KOH solution followed by SF6 plasma etch. The tips were further sharpened using the established thermal oxidation technique to decrease the cone angle and, therefore, obtain smaller curvature radii. We have analyzed the impact of such changes in geometry on a figure of merit associated with the field emission characteristics. An increase in the figure of merit by a factor of three was found in relation to the tips before sharpening.

Recording different geometries of 2D hexagonal photonic crystals by choosing the phase between two-beam interference exposures.

2D hexagonal patterns can be generated by the superimposition of two or three fringe patterns that have been formed by two-wave interference and that have rotations of 60 degrees between them. Superimposing three exposures solves the problem of asymmetry in the cross section of structures, which is caused by double exposure. The resulting structure, however, depends on the phase shift of the third fringe pattern in relation to the previous two. We propose a method for controlling the phase shift, and we demonstrate that three different lattice geometries of hexagonal photonic crystals can be recorded when the phase is chosen.

CF4 plasma etching of materials used in microelectronics manufacturing

Amorphous hydrogenated carbon a-C:H films, deposited on silicon substrates by radio frequency plasma-enhanced chemical vapor deposition (RF PECVD), and AZw 5214 organic photoresist have been etched in a low-pressure and high frequency tetrafluoromethane (CF4) plasma. The etching of Si and SiO2 was also measured in order to determine their selectivities to a-C:H films and AZ 5214 photoresist. The etch rates were measured as a function of RF power in the range of 20‐60 W. Carbona-C:H films were found to be more etch resistant than organic AZ 5214, Si, and SiO2. AZ 5214 demonstrated a relatively high etch rate (300‐700 A ˚ /min). The best etch rate ratios of Si and SiO2 to carbon films were achieved at low RF power. Carbon films can be used as masks for deep pattern transfer to Si and SiO 2 in photolithography. q 2000 Elsevier Science Ltd. All rights reserved.

Optical monitoring of the end point in thin film plasma etching

Abstract An in-process optical technique is described for accurately monitoring the end point in plasma etching processes. A grating pattern is lithographed somewhere in the film to be etched. The grating modulation decreases as the film is etched out and the process may be monitored by measuring the diffraction of a low power He-Ne laser beam aimed at the grating. The etching end point is accurately detected by the disappearance of all diffracted orders. The laser beam does not need to be directed at normal incidence and so any available plasma etching equipment is suitable. The detection is carried out with low cost photovoltaic detectors but simple visual inspection is satisfactory also. Comparative experimental results are presented.

Simulation of the enhancement factor from an individual 3D hemisphere-on-post field emitter by using finite elements method.

This paper presents a 3D computational framework for evaluating electrostatic properties of a single field emitter characterized by the hemisphere-on-post geometry. Numerical simulations employed the finite elements method by using Ansys-Maxwell software. Extensive parametric simulations were focused on the threshold distance from which the emitter field enhancement factor (γ) becomes independent from the anode-substrate gap (G). This investigation allowed demonstrating that the ratio between G and the emitter height (h) is a reliable reference for a broad range of emitter dimensions; furthermore, results permitted establishing G/h ≥ 2.2 as the threshold condition for setting the anode without affecting γ.

Numerical study on performance of pyramidal and conical isotropic etched single emitters

Abstract A full three-dimensional model was implemented in order to investigate the electrical characteristics of conical and pyramidal isotropic etched emitters. The analysis was performed using the finite element method (FEM). The simulations of both emitters were modeled using a combination of tetrahedral and hexahedral elements that are capable of creating a mapped and regular mesh in the vacuum region and an irregular mesh near the surfaces of the emitter. The electric field strengths and electric potentials are computed and can be used to estimate the field enhancement factor as well as the current density using the Fowler–Nordheim (FN) theory. The FEM provides results at nodes located at discrete coordinates in space; therefore, the surface of the emitter can be generated through a function interpolating a set of scattered data points. The emission current is calculated through integration of the current density over the emitter tip surface. The influences of the device geometrical structure on its potential distribution, electric field and emission characteristics are discussed.

Study of the injection molding of a polarizing beam splitter

We describe the replication of a relief grating that behaves like a polarizing beam splitter by injection molding. Measurements of the grating master, nickel shim, and replica, performed by atomic force microscopy, allow establishing a limit for the injection molding technique (currently used in CD fabrication) to aspect ratios of approximately 0.15. Although this limit strongly reduces the diffraction efficiency of the elements as well as their polarizing properties, extinction ratios of approximately 10:1 were measured for the replicas in a large range of wavelengths.

Measurement of optical constants of thin a-C:H films

Abstract Amorphous hydrogenated carbon films (a-C:H) are very interesting materials for optical applications. They are transparent in the near IR part of the spectrum and its refractive index and absorption coefficient may be changed with the deposition parameters. In this paper we measure the optical constants of a-C:H films, deposited by plasma enhanced chemical vapor deposition as a function of the radio frequency power. The measurements were performed by the method of Abeles in λ=633 nm and by the approximation of Lambert–Beer in the transmission measurements using a spectrophotometer. Both methods do not require the use of thick films that are difficult to deposit due to the intrinsic stress of these films. The measurements were compared with that obtained by the fringe peaks in transmission measurements of a thick film. Our results showed that it is possible, in our system, to deposit homogeneous and uniform films with any refractive index chosen between 1.8 and 2.2 at λ=633 nm .

Optical emission end point detecting for monitoring oxygen plasma a-C:H stripping

Abstract The stripping of amorphous hydrogenated carbon films (a-C:H) using an rf oxygen plasma has been monitored using the optical emission from electronically excited CO, H and O species in the visible region of the spectrum. The end point has been detected by monitoring the emission intensity of O reactive species (777.2 nm) and CO emission (483.5 nm) intensity of the etching product.