Bertrand G. Bovard

Rugate filter theory: an overview.

The principal aspects of rugate filter theory are reviewed and expanded to show how the Fouriertransform technique can be used to design rugate filters that fulfill many optical coating functions.

Rugate filter design: the modified Fourier transform technique

The Fourier transform technique developed for the design of variable refractive index coatings such as rugate filters is improved to achieve an accurate correspondence between optical properties and the refractive index profile. An application to the design of narrowband reflectors is presented.

Optical and microstructural properties of hafnium dioxide thin films

Abstract We have applied a variety of analytical tools to educe the composition and morphology of thin films of hafnium dioxide deposited under a variety of conditions. Surface analytical techniques include scanning electron microscopy, transmission electron microscopy, electron diffraction, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. The results from these measurements are correlated with the observed optical properties. Electron diffraction reveals that films deposited below 300 °C are amorphous and spectrophotometry uncovers a small optical inhomogeneity. The refractive index and inhomogeneity are strongly influenced by the oxygen backfill present during film condensation. Ion-assisted deposition (IAD) removes the optical inhomogeneity, increases the refractive index, and eliminates the air-to-vacuum spectral shift observed in films deposited without IAD. In addition, low energy IAD is found to increase the refractive indices of the films without affecting the optical inhomogeneity. This is explained by the preferential sputtering of hydroxide from the growing film surface by the low energy ions.

Derivation of a matrix describing a rugate dielectric thin film

In optical coating calculations, a 2 x 2 matrix formalism is generally used to describe the effect of each layer on the electromagnetic field. In current applications, the layers are considered homogeneous, and, therefore, the matrix used is restricted to this particular case. In this paper, a 2 x 2 matrix describing an inhomogeneous dielectric thin film is derived from Maxwells equations. The inhomogeneity is assumed to vary along the direction perpendicular to the film plane. No restriction is made on the amplitude of its variation. The matrix is illustrated in the case of a rugate filter designed according to Sossis Fourier transform technique. An improved approximation for the Fourier transform technique is then introduced.

Optical constants derivation for an inhomogeneous thin film from in situ transmission measurements

The optical constants of a thin film on its structure. A technique, based on transmission measurements carried out in vacuo, has been developed to derive the profiles of the refractive index and extinction coefficient. The interpretation of the profiles gives information on the layer structure in vacuo. The technique can be used as a means of monitoring the variations of the optical constants with changes in the deposition parameters. This paper presents the technique, which is based on an envelope method, and gives some experimental results.

OPTICAL-CONSTANT CALCULATION OVER AN EXTENDED SPECTRAL REGION : APPLICATION TO TITANIUM DIOXIDE FILM

An iterative algorithm has been developed that takes starting values derived by an envelope method but then minimizes the influence of the envelopes and emphasizes the actual measured data. This combination avoids the difficulties inherent in the accurate drawing of the envelopes and makes it possible to extract the thickness and the optical constants of semiconducting and dielectric films over a wide spectral region, including regions of high absorption.

Ion bombardment-induced retarded moisture adsorption in optical thin films.

Moisture adsorption and desorption in optical filters made of TiO2 and SiO2 can be retarded by bombarding the completed films with 3-keV oxygen ions. This effect is not seen in filters made of ZrO2 and SiO2 We propose that retardation is due primarily to a swelling of the TiO2 surface as material is converted from crystalline to amorphous. In ZrO2, bombardment induces a transition toward more crystalline order, and the effect is not observed. This paper describes our experiments and results and discusses alternate explanations of the retardation effect.

Ion-assisted processing of optical coatings

Abstract The use of ion guns in evaporation chambers has opened a number of material processing possibilities. While ion-assisted deposition (IAD) is certainly the most widely known, other applications have also appeared such as ion processing of substrates and ion beam polishing (IBP). The first part of this presentation is devoted to the study of IAD as a means of growing dense layers which are less affected by moisture than those grown by conventional evaporation. We have applied this technique to a number of materials to study its effects on microstructure, chemical composition, and crystallinity. As a result, several major subsequent applications of IAD are starting to emerge. The first is illustrated by the ability of IAD to tailor the chemical composition of compounds in order to tune their properties. In particular, we describe the synthesis by reactive IAD of aluminum oxynitride (AION), a compound with composition-dependent refractive indices which we have successfully used to fabricate rugate filters. A second application we are at present developing is the control of the microstructure and the intrinsic stress. Finally a third application of IAD is the fabrication of nitrides which can be grown by thermal evaporation but only when nitrogen ions are available at the surface of the growing film. Other uses of ion beams are ion beam processing of subtrates and IBP. We present some results regarding the increase in roughness observed in ion bombarded substrates such as germanium, ZnSe, and ZnS. Then, we describe IBP as a means of smoothing surfaces. IBP has been developed to polish rough diamond films produced by microwave-assisted chemical vapor deposition. By achieving r.m.s. roughness values below 5 nm we have opened a range of optical applications for diamond-coated silicon.

Development of an automated scanning monochromator for monitoring thin films.

A scanning monochromator system for the monitoring of thin-film deposition in a box coater is described. The system employs data from both a quartz crystal oscillator and a wideband transmission spectrometer. The spectrometer uses a holographic grating as its dispersive element and a CCD array to collect the data. All data are sent to a microcomputer where the information is displayed, stored, and analyzed. Several applications, including measurement of optical constants of inhomogeneous films and characterization of moisture adsorption, are discussed.

Repeatability of the composition of titanium oxide films produced by evaporation of Ti 2 O 3

Congruent vaporization is a process that yields constant vapor species. Ti(2)O(3) was continuously electron-beam evaporated to produce titanium oxide thin films. Rutherford backscattering spectrometry was employed to study the evolution of the composition of these films. It seems that congruent vaporization can be established in a coating plant. TiO(2) films produced by conventional reactive deposition tend to contain mixtures of titanium oxides. Increasing the transmission of TiO(2) films becomes an issue of increasing the TiO(2) component in the films by adequate reactive evaporation.