James L. Wood

Thermal Stability of W/C Multilayer Films

W(10A)/C(40A), W(15A)/C(15A) and W(40A)/C(10A) periodic multilayer films were prepared by magnetron sputtering and subsequently annealed at 730°C. The resulting change of the layered and crystal structures was studied by X-ray diffraction. The resulting structures largely depended on the thickness ratio of W to C. For example, in the W(15Å)/C(15Å) sample a-W as formed without any peaks of carbon crystals; in W(40Å) C(10Å)only W2C (orthorhombic) peaks appeared.

Measured X-ray Performance of Synthetic Multilayers Compared To Calculated Effects of Layer Imperfection

In connection with the manufacture of OVONYXTM multilayer x-ray optical elements, investigations have been carried out into the effects of various types of layer imperfections on the x-ray optical properties of multilayers. These have included extensive numerical modeling of real multilayers (using a computational scheme based on the complete dynamical theory) including simulations of interface diffusion, deviations from constant d-spacing, and interface roughness. Results are presented for several examples of Hf-Si and W-Si multilayers, including comparison of measurements at Cu-Kato the theoretical model.

New method of manufacturing elastically bent crystals and multilayer mirrors

We present a new high-precision and cost-efficient method of manufacturing aspheric crystals and pseudo-crystals. The concept including calculations, limits of precision, and manufacturing details is discussed. The advantages of this method are illustrated by the application and use of a Johansson bent quartz crystal in an x-ray sequential spectrometer. The resolving power of the spectrometer is compared with the Siemens SRS 300 and the Fisons/ARL 8420 by a comparative measurement of uranium ore. The comparison involves the detection of 0.006 Rb (K(alpha) ) present as background. Options for future investigations and applications of this method are discussed.

Improved thermal stability of sputtered W/C multilayer thin films

Tungsten/carbon (W/C) multilayer thin films prepared by sputtering on unheated Si(100) substrates were encapsulated with various types of layer having low x-ray absorption. Isochronal annealings for 1 hr in the temperature range from 300 to 600 degree(s)C and isothermal annealings at 300 and 400 degree(s)C were carried out under ambient conditions (in air) on coated and uncoated multilayers. The encapsulated layers are: SiNx and SiO2 prepared by plasma enhanced chemical vapor deposition (PECVD) and SiC, Al2O3, C and B4C prepared by sputtering techniques. Previous studies have shown that unprotected W/C multilayers annealed in air exhibit oxidation at relatively low temperatures (approximately 300 degree(s)C). In the present study, we have used Raman scattering (RS), Auger depth profiling and scanning electron microscopy (SEM) to investigate the effects of thermal treatments on the encapsulated W/C multilayers. The results indicate that oxidation of both W and C layers takes place during annealing at temperatures which depend on the type of protective layer. For example, in the isochronal annealing experiments, multilayers coated with C, Al2O3 and B4C suffer oxidation during annealing at 400 degree(s)C, whereas multilayers coated with the other three types of protective films prevent multilayers from oxidation at annealing temperatures as high as 600 degree(s)C. SEM micrographs show that the formation of pinholes through the protective layer occurred during annealing at the temperatures for which oxidation was first detected. Auger profiling shows the loss of compositional modulation in the region reached by oxygen. A WO3 phase is identified by RS in the oxidized region, and the loss of the C layers is most likely due to the formation of carbon oxide vapors.

Influence of electrical isolation on the structure and reflectivity of multilayer coatings deposited on dielectric substrates

Multilayers prepared with electrically isolated or grounded surfaces during deposition are shown to have dramatically different hard-x-ray, soft-x-ray, and neutron reflectivity characteristics. The effect has been observed for (100) silicon wafers, fused silica, and borate glass substrates of different sizes and with different surface roughness and flatness for multilayer structures prepared by rf and dc magnetron sputtering.

Structural changes induced by thermal annealing in W/C multilayers

Tungsten/carbon (W/C) multilayer thin films with a nominal d spacing varying from 2.5 to 14 nm were prepared by magnetron sputtering technique. The thicknesses of the W and C layers were varied from 0.5 to 12 nm. The multilayers were subjected to isochronal anneals in a quartz tube furnace in the range of 300 to 1000 C under high purity Ar flow conditions. X-ray diffraction, Raman scattering and Auger depth profiling were used to characterize the structure of the as-prepared and annealed films. It is found that an overcoat layer of silicon nitride (30-50 nm) prevents the multilayers from oxidation during the 1 hr heat treatment at temperatures as high as 1000 C in Ar flow. In all studied W/C multilayers, the carbon layers are amorphous (up to 12 nm). The tungsten layers are also amorphous when their thicknesses are less than 5 nm. Tungsten layers thicker than 5 nm show crystalline W peaks in addition to the amorphous W feature. Annealing of samples with a silicon nitride protective layer results in several structural changes which depend on annealing temperature, d spacing, the as-deposited W layer structure and the layer thickness ratio of W to C. For W layer thicker than C layer and W layer thickness > 4 nm and/or C layer thickness < 1 nm, the multilayers show the initial crystal formation of microcrystalline W2C occurring at C-W interfaces (that interface in which C was deposited on W) after 600 C anneal, followed by a second crystallization of a-W or a-W and WC at W-C interfaces (W was deposited on C) at the annealing temperature of 900 C. They reveal a relatively small (< 5 %) or essentially no layer expansion. For those multilayers having thin W layers (2 nm) and the same or thicker C layer thicknesses, the initial crystallization takes place at both W-C and C-W interfaces at 900 C or higher. The crystal formed is a-W or a-W and WC. The layer pair period of the multilayers in this group increases monotonically with increasing annealing temperature. Expansion is up to 16 % of the original d spacing and occurs in both W and C layers at approximately equal rates. The expansion in all multilayers is interpreted to be associated mainly with the structural ordering processes in the amorphous W and C layers.