John F. Flintoff

Fabrication and characterization of diamond moth eye antireflective surfaces on Ge

An antireflective coating on Ge which combines a polycrystalline diamond film with a surface relief (moth eye) structure has been designed and successfully fabricated. This progressive gradation in the effective refractive index between air and the substrate has reduced Fresnel reflection losses to below 1%. This provides a means of overcoming the high refractive index and surface roughness considerations that often limit optical application of polycrystalline diamond thin films.

Microstructure and orientation effects in diamond thin films

The microstructure and orientation of diamond thin films grown by plasma assisted chemical vapor deposition have been studied as functions of growth temperature, substrate identity, and substrate pre‐treatment. Results indicate that for growth temperatures below 650 °C, competition between film growth and etching can lead to preferential (110) oriented films on a variety of substrate materials. This orientation can be globally sustained during growth by the occurrence of (111) planar defects.

MICROSTRUCTURAL CONTROL OF DIAMOND THIN FILMS BY MICROLITHOGRAPHIC PATTERNING

Microlithographic patterning has been used to elucidate the mechanisms controlling diamond film nucleation and grain growth. The approach is capable of establishing a degree of control over diamond nucleation on the substrate, which can be used to improve film uniformity and enhance fine grained microstructure. The observed microstructures in the patterned films are consistent with an intrinsic growth mechanism based upon defect‐initiated renucleation.

Adhesion improvement in diamond films by microlithographic patterning

The use of microlithographic surface patterning has been investigated as a means of modifying the nucleation and adhesion of diamond films on non-compatible substrates. Significant improvements in film adhesion were achieved using this technique, to the point that interfacial integrity was maintained even at stress levels which induced subsurface fracture in the supporting substrate.

Direct observation of the defect structure of polycrystalline diamond by scanning electron microscopy

A method has been demonstrated to directly observe the surface crystallography and defect structures in diamond films by scanning electron microscopy. Individual diamond crystals in the polycrystalline films are polished to a rms smoothness of less than 2 nm using iron metal at temperatures in excess of 725 °C. In the absence of topography, the detailed microstructure of the films can be characterized by secondary electron imaging in a scanning electron microscope by charge‐induced electron contrast which shows strong beam voltage dependence. It is hypothesized that defects and grain boundaries form a connected pathway in the film which has greater conductivity than the generally insulating diamond and creates the charge‐induced contrast.

The formation of surface layers and reaction products in the leaching of defense borosilicate nuclear waste glass

The development of surface alteration layers and solid products in the reaction of borosilicate nuclear waste glass with aqueous solutions has been studied as a function of time, surface morphology, water flow rates, and solution composition. A physical description of the growth of the surface layers has been developed, and several solid reaction products have been identified. The results of the study support a saturation based description of long-term radionuclide release from glass waste forms and demonstrate the complex dependence of short-term leaching experiments upon both surface and solution variables.

Residual stress measurements on polycrystalline diamond

The magnitude of the residual stresses in thick samples of polycrystalline diamond were measured by the sine squared, angular resolved x-ray diffraction (XRD) technique and by detailed analysis of electron channeling patterns from individual grains in polished diamond films using a scanning electron microscope. The XRD measurements were made on samples produced by both plasma torch and microwave plasma low pressure growth techniques with a range of microstructures. Results show that residual levels of stress +/- 0.3 GPa can be generated inside the thinner films on substrates by thermal expansion mismatches, while average residual stress in free standing 0.5 to 2 mm thick diamond plates is negligible. Within the individual grains of the thicker films, localized stress variations on the order of +/- 0.5 to 0.8 GPa can be distinguished.

Polishing of polycrystalline diamond films

Optically smooth surfaces can be produced on initially rough polycrystalline diamond film through the combined use of reactive ion etching and high temperature lapping on Fe metai Protective thin film barriers are first applied to the diamond surface to restrict the reactiv oxygen or hydrogen ion etching process to regions of greatest roughness. When the overaJ surface roughness has been reduced sufficiently by etching mechanical lapping of the surfac on an Fe plate at temperatures of 730C-900C in the presence of hydrogen can be used t produce surface roughnesses of less than 10 nm as measured by profilimetry. The tw techniques are complementary for flat surfaces while the reactive etching process alone can b used with shaped substrates to produce a surface finish suitable for LWIR optical applications. 1.

Preferential Dissolution Phenomena of Nuclear Waste Materials

Preferential dissolution of polyphase nuclear waste materials in short term leach tests can exaggerate radionuclide release rates when extrapolated to the lifetime of the waste form. Possible preferential leach phenomena are associated with the presence of cracks, intergranular phases and readily soluble phases. The rate of dissolution and the microstructural connectivity of the most soluble phase determine the period over which perferential dissolution is observable. The connectivity of phases is amenable to control during processing by altering the starting green density of the precursor powders.

Microstructure of diamond films as a function of deposition conditions

The microstructure of polycrystalline diamond films grown by microwave plasma assisted chemical vapor deposition (PACVD) have been observed as a function of growth temperature substrate identity and surface condition. Our highest microwave PACVD growth rates have been achieved in (1 10) axis normal oriented polycrystalline diamond films. Results indicate thai at growth temperatures below 650C kinetically dominated processes induce the formation of a preferential (1 10) axis normal orientation in diamond films with micron scale microstructure1 This orientation can be sustained on silicon boron nitride and silicon nitride substrates to filrr thicknesses in excess of 60 microns through the occurrence of (I I I) twin defects. Such fiIm have the high density and generally uniform microstructure required for optical applications. 1.