C. D. Wang

Enhancement of the reflectivity of soft-x-ray Co/C multilayers at grazing incidence by thermal treatment

We report a significant increase in the reflectivity of a soft x-ray Co/C multilayer at grazing incidence after low-temperature annealing. By monitoring the enhancement of the first-order modulation peak on annealing, an effective interdiffusion coefficient as low as has been measured, which is nearly equal to the true macroscopic interdiffusion coefficient as the modulation wavelength of the Co/C multilayer studied is well above the critical wavelength. The negative true macroscopic interdiffusion coefficient indicates that there is a tendency to phase separation in the Co - C system, which can be interpreted as the positive enthalpy of mixing calculated on the basis of Miedemas macroscopic atom model. The decrease in reflectivity above some annealing temperature is also discussed and explained as the competition between interfacial sharpness and roughness. This is the first report of observations of enhanced reflectivity in the Co/C multilayered system.

TEMPERATURE DEPENDENCE OF MICROSTRUCTURE AND MAGNETIC PROPERTIES OF CO/TI MULTILAYER THIN FILMS

The microstructural and magnetic properties of amorphous Co/Ti multilayer films and their variation with temperature are investigated by transmission electron microscopy (TEM) analysis and thermomagnetic measurements. Thermomagnetic curves showed two peaks at about 400 and 520 °C. The evolution of the structure monitored in the hot stage of the TEM was found to be consistent with the magnetic changes. The first peak of the saturation magnetization Ms at 400 °C was associated with the transformation from amorphous ferromagnetism to paramagnetism due to the amorphous Co existing in the film. Ms began to increase corresponding to the crystallization point of the ferromagnetic Co phase, which decreased with increasing amounts of Co in the film. Ms reached its maximum at 520 °C and then decreased because the phase transition occurred at a temperature greater than 520 °C and approached completion at 650 °C. The amorphous phase and crystalline phase formation and phase transition during annealing were observed i...

Interdiffusion in CoN/CN soft X-ray multilayer mirrors

Abstract The interdiffusion in the amorphous CoN/CN soft X-ray multilayers is investigated quantitatively by monitoring the enhancement of the first order modulation peak on annealing in the temperature range of 473–523 K. The effective interdiffusion coefficient measured is as low as 10 −25 m 2 s −1 . The comparison with the results of Co/C system [H.L. Bai et al., Thin Solid Films, 286 (1996) 176–183] indicates that the following three features are noticeable: (1) the interdiffusion critical wavelengths were calculated as 2.00–2.04 nm at temperatures ranging from 473 to 523 K, which are equal to those of Co/C multilayers within the experimental error, indicating that the interdiffusion behaviours in the CoN/CN multilayers are still decided by the thermodynamic properties of the Co C system, (2) the effective interdiffusivities and macroscopic diffusion coefficients are lower, and (3) the activation energy for diffusion is higher. The results imply that it is possible to improve the thermal stability of Co/C multilayers by doping with N. The mechanisms resulting in the features are clarified in detail.

Structural stability of heat-treated Co/C soft X-ray multilayers fabricated by dual-facing-target sputtering

Thermal stability of Co/C multilayers prepared by a dual-facing-target sputtering system was studied. A picture of the thermally induced changes in the microstructure was obtained using complementary measurement techniques including low-angle and high-angle X-ray diffraction, transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. It was found that the period expansion, reflectivity change and compound formation, that were observed after annealing are caused by structural changes both in the sublayers and at the interfaces. Below 400°C, the period expansion is mainly caused by the graphitization of the amorphous carbon layers, and a significant increase in the reflectivity at grazing incidence was observed. By 500°C, the crystallization and agglomeration of Co layers induce an enormous period expansion and a serious decrease in reflectivity. A small amount of carbide is found to form at this temperature. Our results imply that new multilayer structures, e.g., compound multilayers will have to be developed for use at high temperatures or under high X-ray incident flux.

Period expansion of Co/C and CoN/CN soft x-ray multilayers after annealing

Period expansion of Co/C and CoN/CN soft x-ray multilayers has been investigated by x-ray diffraction and Raman spectroscopy. Below the anneal temperature of 400 °C, the period expansion (< 12%) of Co/C multilayers is mainly caused by the graphitization of the amorphous carbon layers. By 500 °C, the crystallization and agglomeration of Co layers induce an enormous period expansion (∼40%). The period expansion of CoN/CN multilayers is only 4% at 400 °C, which is much smaller than that of Co/C multilayers. The interface patterns of the CoN/CN multilayers still exist even if they were annealed at 700 °C. The Raman spectroscopy analyses indicate that the formation of the sp3 bonding can be suppressed effectively by doping N atoms, and thus the period expansion is decreased considerably at annealing temperatures below 600 °C. The significant suppression of grain growth above 600 °C is believed to be attributed to the coexistence of hcp and fcc Co structures induced by interstitial N atoms, which cause the high...

THERMAL EVOLUTION OF CARBON IN ANNEALED CO/C SOFT X-RAY MULTILAYERS

The structures of the carbon sublayers in the annealed Co/C soft x‐ray multilayers fabricated using a dual‐facing‐target sputtering system have been characterized by x‐ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy (RS). The results suggest that the structural variations in the carbon layers can be roughly divided into three stages, i.e. ordering, crystalline and grain growth stages. In the ordering stage with annealing temperatures below 400 °C, the upward shift of D and G lines in Raman spectra indicates that the amorphous carbon layers are changing from ones with bond‐angle disorder and fourfold‐bonding only to ones containing threefold‐bonding. In the crystalline stage, the amorphous carbon layers in the as‐deposited multilayers crystallize to graphite crystallites in the annealing temperature range of 500–600 °C. The rapid increase in the intensity ratio of D line to G line and dramatic decrease in linewidth further confirm this substantial structural change. In...

Interdiffusion in CoC soft X-ray multilayer mirrors

Abstract The interdiffusion behaviours in the amorphous Co C multilayers prepared by dual-facing-target sputtering techniques are investigated quantitatively by monitoring the enhancement of the first-order modulation peak on annealing in the temperature rang of ∼473–523 K. Effective interdiffusion coefficients as low as 10−25 m2 s−1 were measured. The negative time macroscopic interdiffusion coefficients were obtained by measuring the effective interdiffusion coefficient as a function of modulation wavelength, indicating that there is a tendency to phase separation in the CoC system. This result is also predicted by the positive enthalpy of mixing, calculated based on Miedemas macroscopic atom model. The temperature dependence of the true macroscopic interdiffusion coefficients can be described as D = −1.66 x 10−10 exp[−(128±10) KJ mol−1/RT] m2s−1. The existence of a critical wavelength below which the effective interdiffusion coefficient changes its sign is proposed, and it was calculated to be ∼19.8–20.5 A at temperatures ranging from 473 K to 523 K. The spread values of the critical wavelength are believed to be attributed to the composition dependence of the gradient energy coefficient k and the second derivative of the Helmholtz energy f0n.

TEM and Raman scattering investigation of carbon in annealed Co/C soft X-ray multilayers

The structures of the carbon sublayers in the annealed Co/C soft X-ray multilayers fabricated by using a dual-facing-target sputtering system have been characterized by transmission electron microscopy (TEM) and Raman spectroscopy (RS). The results suggest that the structural variations in the carbon layers can be roughly divided into three stages, i.e. ordering, crystalline and grain growth stages. At the ordering stage with annealing temperatures below 400°C, the amorphous carbon layers change from ones of bond-angle disorder and fourfold-bonding only to ones of threefold-bonding. At the crystalline stage, the amorphous carbon layers in the as-deposited multilayers crystallize to graphite crystallites in the annealing temperature range of 500–600°C. At the grain growth stage, the specimens are annealed at temperatures higher than 700°C. A growth in the graphite crystallite dimensions is observed, which is consistent with the TEM results.

Aging effect of Co/C soft x-ray multilayer mirrors

The effect of aging on Co/C multilayer x-ray mirrors is presented. One significant result is the enhancement of the reflectivity at grazing incidence with time for the Co/C multilayers with C-on-top stored in air or argon and Co-on-top stored in argon. This can be interpreted in terms of a Co–C phase-separation at Co–C interfaces due to the positive enthalpy of Co–C mixing. Results also show that oxidation of the surface of Co-on-top Co/C multilayers plays an important role in the decrease of reflectivity. The oxidation can be prevented by storing the multilayers in an oxygen-free atmosphere or by depositing amorphous carbon as the top layer. The reflectivity of tarnished multilayers can be restored by removing the oxides by wet chemical methods.

Improvement of the thermal stability of Co/C soft x-ray multilayers through doping with nitrogen

The thermal stability of Co/C soft x-ray multilayers is improved by 100 - 200 centigrade degrees through doping with N. The low-angle x-ray diffraction of CoN/CN soft x-ray multilayers indicates that their period expansion is only 4% at , and the interface pattern of the multilayers still exists even if they are annealed at . High-angle x-ray diffraction and transmission electron microscopy analyses reveal that this crystalline process is significantly retarded by doping with N atoms, leading to a smaller grain size at higher annealing temperatures. Raman spectroscopy analyses of the multilayers give evidence that the formation of the bonding is suppressed effectively by doping with N atoms, and thus the period expansion is decreased considerably. The strong covalent bonding between N atoms and the ionic bonding between Co and N atoms can slow down the structural relaxation. The significant suppression of grain growth is believed to be attributable to the coexistence of hcp and fcc Co structures at annealing temperatures higher than .