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7416. A transmission electron microscopy study of low-temperature reaction at the Co-Si interface
Ciassmed abstracts 7414 7423
both a resistively heated oven and a thermocouple has been integrated within the sample holder. O Haase et al, Rev Scient lnslrum, 61, 1990, 1480 1483. 54 7414. Low energy electron microscopy of surface processes After a brief discussion of the basic features of low energy electron microscopy, the application of this technique to the study of processes on clean surfaces (Si( 111 )-(7 x 7) ( l × 1) phase transition, Si(100) growth and sublimation) and in surface layers (Au and Cu on Si(II1) and Mo(110)) is illustrated by video recordings. E Bauer et al, Vacuum, 41, 1990, 5 10. 54 7415. A scanning tunneling microscopy study of the surface morphology of supported gold particles Scanning tunneling microscopy (STM) was used to examine the surface structure of supported gold particles. STM images of annealed gold particles showed that particles larger than 150/~ have a polyhedral shape and a relatively smooth surface morphology. However, the surfaces of the particles smaller than 60 ~ remained highly corrugated even after long annealing times. We believe that the difference in morphology of large and small particles m a y have some important implications for supported metals catalysis. P A Thomas et al, J l/ac Sci Tcchnol, A8, 1990, 3653 3656. 54 7416. A transmission electron microscopy study of low-temperature reaction at the Co-Si interface An efficient preparation method, which provides wedge-shaped crosssection transmission electron microscopy samples, has been developcd. It was then used to investigate the slructure of as-deposited cobalt multilayers on silicon substrates by rf plasma sputtering. It was found that an extended reaction takes place between Co and Si probably during the deposition. The cobalt atoms react with the silicon substrate to form an a m o r p h o u s silieide layer. When the deposited layer is < 3 nm thick, it entirely reacts with the substrate and can form an a m o r p h o u s silieidc as large as 5 nm. Above 4 5 nm thickness, growth of Co crystallites comes in competition with the formation of the a m o r p h o u s silicide and limits it to 2 nm. The composition of this a m o r p h o u s silicide is estimated to be Co2Si. In Co/C multilayers, the reactivity between the two materials is negligible, and the coalescencc thickness of cobalt is 2 3 nm. At 2 nm, the cobalt layers are noncontinuous and very rough, whereas at 3 nm the critical thickness for crystalline nuclei coalescence has already been reached. The cobalt layers are then polycrystalline and havc a reasonable roughncss. P Ruterana et al, J Appl Phys, 68, 1990, 1033 1037. 54 7417. A versatile scanning tunnelling microscope for use in air A scanning tunnelling microscope for use in air is described. Dcspitc the apparent simplicity of the stage it is capable of high performancc and this is largely due to its inherent stability resulting from miniaturisation and the use of stressed cantilever beams. It also employs a novel threeaxis piezo scanner assembly which provides a fast response. Because of its stability, the microscope can be used in areas suffering from high levels of acoustic noise and vibration without additional isolation. W S Steer et al, Meas Sci Technol, 1, 1990, 881 886. 54 7418. Observation of gold thin film growth with reflection electron microscopy We have studied the morphology of gold thin films, grown on a m o r p h o u s subslrates which were held at fixed temperatures, with reflection electron microscopy (REM). The grazing incidence geometry of this technique permitted us to observe nucleated growth, anisolropic growth, and colu m n a r growth with high resolution. The observed growth is compared with the kinetics predicted by diffusion constants. In addition, we have observed whisker growth under conditions of high effective supersaturation that can be distinguished from previously reported whisker growth. The features observed with R E M are virtually invisible when examined by high resolution scanning electron microscopy. T Jack et al, Thin Solid Filmx, 187, 1990, 133 140. 54 7419. Performance limits for the scanning tunneling microscope Thc electromechanical performance limits of a tube scanner type, scanning tunneling microscope are analyzed with respect to the three figures 596
of merit: noise level, scan speed, and scan range. A simple tradeoll" between the tube resonant frequency and scan range is defined. For a critically damped scanner tube, a good balance between scan speed and scan range is achieved with a preamplifier bandwidth equal to the resonant frequency of the scanner tube, in which case the closed loop control system bandwidth ( 9 0 phase margin) is 2/~ times the resonant frequency of the tube, for a proportional-integral controller. This control loop bandwidth is shown to be compatible with a noise performance that is limited by shot noise in the tunnel junction. T Tiedje and A Brown, J Appl Phys, 68, 1990, 649 654. 54 7420. Spatial coherence of anisotropic and astigmatic sources in interference electron microscopy and holography The lateral coherence properties of two illumination modes in electron interferometry and holography are investigated within the framework of the mutual coherence function. It is shown that the results obtained can be considered particular realizations of the general, anisotropic, Gaussian Schnell model, which plays an important role in the classical coherence theory in optics. Another property of this model is described : the ratio between the coherence and illumination areas is constant along every section of the bcam. This invariant parametcr is shown to be related to another geometric optical invariant of the beam, i.e. its ctendue. Numerical calculations showing the relation between illumination and coherence areas in the image, Fresnel, and far-field (Fraunhofer) regions are presented and discussed with regard to the experimental implications. F F Medina and G Pozzi, J Opt Soc Am, 7, 1990, 1027 1033. 54 7421. Application of a semi-empirical sputtering model to secondary electron emission A semi-empirical sputtering model is demonstrated to provide a good description of electron-induced secondary electron emission. A simple formula for the yield of secondary electrons as a traction of primary energy, incident angle, and material parameters is obtained. In comparison with the most commonly employed semi-empirical model, the present model provides a superior fit to the universal energy dependence curve, a better description of energy and mass effects at off-normal incidence, and additional insights into factors affecting the magnitude of the yield. The bulk and surface contributions to the yield magnitude are separately deduced from experimental data in the lfierature. Trends in these contributions arc assessed for metals and insulators. S A Schwarz, .I Appl Phys, 68, 1990, 2382 2391.
55. X-RAY A N D P H O T O E L E C T R O N S P E C T R O S C O P Y 55 7422. Spin-resolved core and valence electron photoemission from nonepitaxially grown Ph layers on Pt(I 11) The core and valence levels of Pb adsorbates on P t ( l l l ) have been studied by means of spin, angle and energy-resolved photoemission with circularly polarized synchrotron radiation of BESSY. For an eight layer Stranski Krastanov grown Pb adsorbate (which is thus non-epitaxial) the d core level peaks show a spin orbit splitting of 2.7 eV and no shift with photon energy. Spin-resolved photoemission I\~r normal light incidence and normal electron emission from the 5d~,: level reveals a m , splitting in addition to the spin orbit splitting and in addition to the core level shifts known. The valence levels were studied with photon energies of about 12 eV. Two peaks corresponding to the 6p~ ~ stud 6p/.2-derived levels could be detected for the highly symmetrical set-up of normal light incidence and normal electron emission. Off normal electron emission from the 6p, 2-dcrived level yields the opposite sign for the spin polarization component parallel to the photon spin when compared with the normal electron emission results. This is in agreement with earlier photoionization studies on free Pb atoms. B Vogt et al, Vacuum, 41, 1990, 1118 1120. 55 7423. X-ray reflectivity study of SiO2 on Si X-ray reflectivity is a powerful probe of thin film morphology. It can be used to nondestructively determine the interface and surface roughness, thickness, and density of the layers. This paper presents the application of the technique to the case of SiO2 on Si. Experimental reflectivity results with a dynamic range of IW have been obtained on a number ot" oxide layers prepared using processing conditions. The reflcctivity oscillations caused by oxide layers could be easily observed over this entire range,
both a resistively heated oven and a thermocouple has been integrated within the sample holder. O Haase et al, Rev Scient lnslrum, 61, 1990, 1480 1483. 54 7414. Low energy electron microscopy of surface processes After a brief discussion of the basic features of low energy electron microscopy, the application of this technique to the study of processes on clean surfaces (Si( 111 )-(7 x 7) ( l × 1) phase transition, Si(100) growth and sublimation) and in surface layers (Au and Cu on Si(II1) and Mo(110)) is illustrated by video recordings. E Bauer et al, Vacuum, 41, 1990, 5 10. 54 7415. A scanning tunneling microscopy study of the surface morphology of supported gold particles Scanning tunneling microscopy (STM) was used to examine the surface structure of supported gold particles. STM images of annealed gold particles showed that particles larger than 150/~ have a polyhedral shape and a relatively smooth surface morphology. However, the surfaces of the particles smaller than 60 ~ remained highly corrugated even after long annealing times. We believe that the difference in morphology of large and small particles m a y have some important implications for supported metals catalysis. P A Thomas et al, J l/ac Sci Tcchnol, A8, 1990, 3653 3656. 54 7416. A transmission electron microscopy study of low-temperature reaction at the Co-Si interface An efficient preparation method, which provides wedge-shaped crosssection transmission electron microscopy samples, has been developcd. It was then used to investigate the slructure of as-deposited cobalt multilayers on silicon substrates by rf plasma sputtering. It was found that an extended reaction takes place between Co and Si probably during the deposition. The cobalt atoms react with the silicon substrate to form an a m o r p h o u s silieide layer. When the deposited layer is < 3 nm thick, it entirely reacts with the substrate and can form an a m o r p h o u s silieidc as large as 5 nm. Above 4 5 nm thickness, growth of Co crystallites comes in competition with the formation of the a m o r p h o u s silicide and limits it to 2 nm. The composition of this a m o r p h o u s silicide is estimated to be Co2Si. In Co/C multilayers, the reactivity between the two materials is negligible, and the coalescencc thickness of cobalt is 2 3 nm. At 2 nm, the cobalt layers are noncontinuous and very rough, whereas at 3 nm the critical thickness for crystalline nuclei coalescence has already been reached. The cobalt layers are then polycrystalline and havc a reasonable roughncss. P Ruterana et al, J Appl Phys, 68, 1990, 1033 1037. 54 7417. A versatile scanning tunnelling microscope for use in air A scanning tunnelling microscope for use in air is described. Dcspitc the apparent simplicity of the stage it is capable of high performancc and this is largely due to its inherent stability resulting from miniaturisation and the use of stressed cantilever beams. It also employs a novel threeaxis piezo scanner assembly which provides a fast response. Because of its stability, the microscope can be used in areas suffering from high levels of acoustic noise and vibration without additional isolation. W S Steer et al, Meas Sci Technol, 1, 1990, 881 886. 54 7418. Observation of gold thin film growth with reflection electron microscopy We have studied the morphology of gold thin films, grown on a m o r p h o u s subslrates which were held at fixed temperatures, with reflection electron microscopy (REM). The grazing incidence geometry of this technique permitted us to observe nucleated growth, anisolropic growth, and colu m n a r growth with high resolution. The observed growth is compared with the kinetics predicted by diffusion constants. In addition, we have observed whisker growth under conditions of high effective supersaturation that can be distinguished from previously reported whisker growth. The features observed with R E M are virtually invisible when examined by high resolution scanning electron microscopy. T Jack et al, Thin Solid Filmx, 187, 1990, 133 140. 54 7419. Performance limits for the scanning tunneling microscope Thc electromechanical performance limits of a tube scanner type, scanning tunneling microscope are analyzed with respect to the three figures 596
of merit: noise level, scan speed, and scan range. A simple tradeoll" between the tube resonant frequency and scan range is defined. For a critically damped scanner tube, a good balance between scan speed and scan range is achieved with a preamplifier bandwidth equal to the resonant frequency of the scanner tube, in which case the closed loop control system bandwidth ( 9 0 phase margin) is 2/~ times the resonant frequency of the tube, for a proportional-integral controller. This control loop bandwidth is shown to be compatible with a noise performance that is limited by shot noise in the tunnel junction. T Tiedje and A Brown, J Appl Phys, 68, 1990, 649 654. 54 7420. Spatial coherence of anisotropic and astigmatic sources in interference electron microscopy and holography The lateral coherence properties of two illumination modes in electron interferometry and holography are investigated within the framework of the mutual coherence function. It is shown that the results obtained can be considered particular realizations of the general, anisotropic, Gaussian Schnell model, which plays an important role in the classical coherence theory in optics. Another property of this model is described : the ratio between the coherence and illumination areas is constant along every section of the bcam. This invariant parametcr is shown to be related to another geometric optical invariant of the beam, i.e. its ctendue. Numerical calculations showing the relation between illumination and coherence areas in the image, Fresnel, and far-field (Fraunhofer) regions are presented and discussed with regard to the experimental implications. F F Medina and G Pozzi, J Opt Soc Am, 7, 1990, 1027 1033. 54 7421. Application of a semi-empirical sputtering model to secondary electron emission A semi-empirical sputtering model is demonstrated to provide a good description of electron-induced secondary electron emission. A simple formula for the yield of secondary electrons as a traction of primary energy, incident angle, and material parameters is obtained. In comparison with the most commonly employed semi-empirical model, the present model provides a superior fit to the universal energy dependence curve, a better description of energy and mass effects at off-normal incidence, and additional insights into factors affecting the magnitude of the yield. The bulk and surface contributions to the yield magnitude are separately deduced from experimental data in the lfierature. Trends in these contributions arc assessed for metals and insulators. S A Schwarz, .I Appl Phys, 68, 1990, 2382 2391.
55. X-RAY A N D P H O T O E L E C T R O N S P E C T R O S C O P Y 55 7422. Spin-resolved core and valence electron photoemission from nonepitaxially grown Ph layers on Pt(I 11) The core and valence levels of Pb adsorbates on P t ( l l l ) have been studied by means of spin, angle and energy-resolved photoemission with circularly polarized synchrotron radiation of BESSY. For an eight layer Stranski Krastanov grown Pb adsorbate (which is thus non-epitaxial) the d core level peaks show a spin orbit splitting of 2.7 eV and no shift with photon energy. Spin-resolved photoemission I\~r normal light incidence and normal electron emission from the 5d~,: level reveals a m , splitting in addition to the spin orbit splitting and in addition to the core level shifts known. The valence levels were studied with photon energies of about 12 eV. Two peaks corresponding to the 6p~ ~ stud 6p/.2-derived levels could be detected for the highly symmetrical set-up of normal light incidence and normal electron emission. Off normal electron emission from the 6p, 2-dcrived level yields the opposite sign for the spin polarization component parallel to the photon spin when compared with the normal electron emission results. This is in agreement with earlier photoionization studies on free Pb atoms. B Vogt et al, Vacuum, 41, 1990, 1118 1120. 55 7423. X-ray reflectivity study of SiO2 on Si X-ray reflectivity is a powerful probe of thin film morphology. It can be used to nondestructively determine the interface and surface roughness, thickness, and density of the layers. This paper presents the application of the technique to the case of SiO2 on Si. Experimental reflectivity results with a dynamic range of IW have been obtained on a number ot" oxide layers prepared using processing conditions. The reflcctivity oscillations caused by oxide layers could be easily observed over this entire range,