Interference Effects In X-Ray Specular Reflectivity from Thin Films
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BSTRACT A strong resonance effect in the specular reflectivity of a C layer deposited on a Ni surface has been measured. The e.m. field intensity distribution has been monitored by measuring the fluorescence intensity of an ultra-thin Ti layer placed in the C film. The reflected intensity and the fluorescence yield of Ti and Ar (this last trapped in the film because of the deposition by sputtering) have been measured in different samples with a different location of the Ti layer. The results show that with this method a nondestructive, accurate of 10-2 determination of the Ti position can be achieved with a relative accuracy INTRODUCTION It is well known that x-rays impinging on solid surfaces under a certain critical angle, which depends on the incoming energy and on the nature on the reflecting surface, are totally reflected[1]. Due to the interference between the incoming and the reflected beams, a standing wave is formed above the surface with a periodicity D which depends on the incident
angle cc and on the wavelength X: D = V/2 sin(o). If a thin film is present on the reflecting surface, an interesting phenomenon can take place, provided certain conditions are met: let's call the thin film medium 1, and the reflecting surface medium 2. The air (or the vacuum) will be medium 0. (See Fig. 1). If medium 1 is lighter than medium 2, than for a certain photon energy it will be possible to find an incident angle which is larger than the critical angle ot. for medium 1 but smaller than ot. for medium 2. In these conditions the incoming beam will penetrate easily through medium 1 where a refraction effect will take place, but it will be totally reflected from medium 2, with the consequent formation of a standing wave field in medium 1. If the thickness of medium 1 is properly chosen, we can find the standing wave nodes on the medium 0-medium 1 and on the medium 1-medium 2 interfaces. If these conditions are met, a constructive interference effect will take place, with a considerable enhancement of the e.m. field intensity in medium 1 [3,4]. Previous measurements [4] dealt with Langmuir-
Blodgett films on a gold mirror surface studied with a powerful synchrotron radiation source. In this paper we demonstrated the potential applications of this method in the study of sputtered
inorganic films with a conventional X-ray source.
381 Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
Medium 0 (Air)
Medium 2 (NI)
Fig. 1 Sketch of the incident, refracted and reflecied beams at the air-C and C-Ni interfaces. When the resonance takes place, the incident beam is in phase with the beam traveling in the C film which reflected at the C-air interface. The reflectivity of a N-layers system can be calculated taking into account all reflection and transmission processes of a monochromatic incident plane wave at the N+1 interfaces of the system. For a single interface between two media the reflection coefficients can be evaluated through the Fresnel equations. If the system consists of two interfaces (a thin film
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