High Resolution Electron Microscopy Study of As-Prepared and Annealed Tungsten-Carbon Nultilayers
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HIGH RESOLUTION ELECTRON MICROSCOPY STUDY OF AS-PREPARED AND ANNEALED TUNGSTEN-CARBON NULTILAYERS 2
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Tai D. NGUYEN', Ronald GRONSKY AND Jeffrey B. KORTRIGHT 1 Center for X-ray Optics, Accelerator and Fusion Research Division, 2 National Center for Electron Microscopy, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720
ABSTRACT A series of sputtered tungsten-carbon multilayer structures with periods ranging from 2 to 12 nm in the as-prepared state and after annealing at 500°C for 4 hours has been studied with high resolution transmission electron microscopy. The evolution with annealing of the microstructure of these multilayers depends on their period. As-prepared structures appear predominantly amorphous from TEM imaging and diffraction. Annealing results in crystallization of the W-rich layers into WC in the larger period samples, and less complete or no crystallization in the smaller period samples. X-ray scattering reveals that annealing expands the period in a systematic way. The layers remain remarkably well-defined after annealing under these conditions.
INTRODUCTION Periodic multilayer structures with periods ranging from roughly I to 10 run are becoming widely used as Bragg diffracting elements for x-rays ranging in wavelengths from about 0.1 to 20 nm. Multilayers having tungsten and carbon as the primary constituents of the individual alternating layers were some of the first to demonstrate utility in x-ray optical applications [1,2]. Applications which would span a long time of service or in which the multilayers may be subject to thermal annealing or harsh radiation environments has prompted study of the structure and stability of these ultrathin structures. Various techniques have been used to characterize the tungsten-carbon multilayer system and related systems showing optical contrast in the x-ray region such as W-Re/C, W/Si and Mo/Si. These include x-ray techniques measuring the low-angle multilayer interference peaks from the composition modulation, and the scattering [3,4] and EXAFS [5] from the interatomic structure within the layers. Transmission electron microscopy techniques have provided information about the quality of the layering and the nature of the interatomic arrangements within the layers [6-10]. The general structural picture of the tungsten-carbon multilayer system that is emerging from previous studies is that as-prepared samples with small enough periods are predominantly amorphous. There is evidence of intermixing of W and C in as-prepared, amorphous multilayers [3]. Annealing at moderate temperatures leads to an expansion of the d-spacing while the layered structure remains at least somewhat intact [11,3-7]. Associated with this annealing are atomic rearrangements towards those like in the tungsten carbides, which may or may not result in crystallization of the structures [35,10]. It has been proposed [7] that the expansion of these structures on annealing results from the agglomeration of the tungst
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