Overcoat Layer Research Articles

Dielectric Tensor Characterization for Overcoated Amorphous TbFe Alloys

AbstractAmorphous thin films of TbFe. sputter-deposited on quartz substrates with compositions supporting polar Kerr magneto-optical behavior, and overcoated with a transparent protective layer, are studied to determine their dielectric tensor elements. Refleetivity measurements over a range of angles of incidence are made in an apparatus that corrects for the overcoat layer and exploits magnetization reversal to enhance the measurement of r⊥, the complex reflectivity associated with the magneto-optical signal. Conventional Fresnel reflectivities are also found. The theoretical model for reflectivities from a thick film magneto-optical medium at oblique incidenci is developed and used within a Levenberg-Marquardt nonlinear estimation algorithm to estimate sequentially the dielectric tensor elements, considering their widely varying magnitudes and sensitivities to errors in reflectivity data. The interpretation of the dielectric elements in terms of magneto-optical performance and structural perpendicular anisotropy is discussed.

Scandium oxide coatings for high-power UV laser applications

Scandium oxide has proved to be a damage-resistant high-index material in laser coatings for use at 248 nm. The results of damage threshold measurements on laser reflectors and antireflection coatings of various designs, material combinations, and deposition temperatures are presented. The most significant effects are observed for overcoat layers on high reflectors and undercoat layers on antireflection coatings. Thresholds >6 J/cm(2) for 20 nsec pulses were observed in both cases.

Increases in magnetic hyperfine field at the surface of ultra-thin epitaxial Fe films (invited)

Previous investigations of ultra-thin epitaxial Fe (110) films grown on Ag (111) have indicated that there is a measurable increase at 4.2 K in the magnetic hyperfine field compared with bulk value in iron atoms in the first three surface layers of the films. As these films were produced in ultra-vacuum and then removed for study by transmission Mössbauer spectroscopy it was necessary to provide a protective overcoat to prevent oxidation of the surface. Because of this it was not possible to rule out interaction of the Fe surface with the overcoat layers as a cause of the increased surface hyperfine field. A series of new films have been made of isotopically pure 56Fe. A thin probe layer of 57Fe ranging in thickness from 4–10 Å was then deposited on the film surface and a variety of different overcoat layers subsequently chosen to protect these films. Overcoating materials included Ag, Au, Cu, Ge, MnF2, and NaCl. While there were differences in the hyperfine fields and isomer shifts of the 57Fe probe layer associated with different overcoat materials, the overall results suggest that the increased hyperfine field is related to an increased localization of the 3d electrons for Fe atoms at or near the surface. An observation of a decrease in 4s electron density indicated by the isomer shift data is consistent with this view and is in qualitative agreement with the theoretical work of Wang and Freeman.

The effect of overcoats on the ablative writing characteristics of tellurium films

The change in the ablative writing sensitivity of Te films due to SiO2 and polymethyl methacrylate overcoats of various thicknesses were measured. These experimental data were compared to results of thermal calculations. We found that thermal losses of the absorbed laser energy to the overcoat layer is usually not negligible even for very thin (≲500 Å thick) overcoat layers. In addition to thermal effects which degrade the writing sensitivity of Te films, we found that rigid overcoats combined with rigid substrates can constrain the ablative writing process of Te and hence cause further degradation in the writing sensitivity and in the read-during-write signal.

Surface aging mechanisms of AC plasma display panels

Excellent life performance is being achieved for ac plasma display panels through understanding of the causes of voltage changes, surface overcoat material research, and adjustment of the manufacturing process to eliminate contaminants. It has been found that surface aging phenomena are dominant in many practical cases. Because of prefereutial operation patterns (i.e., alpha numerics), a differential aging pattern may evolve. The differential pattern is the result of voltage changes which are different for heavily operated and lightly or nonoperated cells. It is shown that the nature and extent of surface changes are dependent on overcoat material selection and the presence (or absence) of contaminants introduced during manufacture. With the use of thin overcoat layers in contact with the gas, it is necessary to be aware of the possibility of surface film removal by sputtering. The effects of significant variables: gas pressure and composition surface material, and operating voltage are discussed.