Philippe Houdy

A transmission electron microscopy study of low‐temperature reaction at the Co‐Si interface

An efficient preparation method, which provides wedge‐shaped cross‐section transmission electron microscopy samples, has been developed. It was then used to investigate the structure 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 amorphous silicide layer. When the deposited layer is <3 nm thick, it entirely reacts with the substrate and can form an amorphous silicide as large as 5 nm. Above 4–5 nm thickness, growth of Co crystallites comes in competition with the formation of the amorphous silicide and limits it to 2 nm. The composition of this amorphous silicide is estimated to be Co2Si. In Co/C multilayers, the reactivity between the two materials is negligible, and the coalescence 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...

Grazing x-ray reflection analysis of nanometric scale structures

Grazing x‐ray reflection (GXR) has been performed on thick WN layers and on C/W multilayer mirrors deposited in a diode rf‐sputtering system. In the first case, we show that GXR can provide not only the layer thickness but also precise information on the occurrence of intermediate layers (native oxide superlayer, reactionlike sublayer). The relative thickness of each layer is determined very precisely. The GXR analysis of C/W multilayers can be perfectly interpreted using a physical model which includes a thickness drift during the deposit, an interface layer between W and C layers due to the interdiffusion, and interface roughness. In each case, the GXR simulation models are confirmed by in situ kinetic ellipsometry and ex situ transmission electron microscopy. Moreover, in the W/C multilayer case the soft x‐ray reflectivity calculated at the CKα line assuming the same model agree very well with the experimental value.

Interface effect on tribological properties of titanium–titanium nitride nanolaminated structures

Abstract Nanolaminated titanium–titanium nitride [Ti–TiN] n structures have been deposited on silicon 〈100〉 substrates by RF reactive sputtering in order to study their mechanical properties. Structures with graded Ti/TiN (interface thickness of approx. 2 nm) and with ultra-thin Ti/TiN interfaces (approx. 0.5 nm) have been investigated with multilayer period thicknesses ranging between 20 and 2.5 nm. The deposition followed by in situ kinetic ellipsometry point out the two types of interfaces. Real period thicknesses, measured by X-ray reflectometry, are close to the expected ones. Thin film X-ray diffraction analysis shows the polycrystalline nature of the nanostuctures. In the superlattice, Ti and TiN individual layers are textured [002] and [111], respectively. Tribological tests, carried out using a CSEM pin-on-disc apparatus, point out that the wear resistance of the samples increase when the period thickness is lowered (same total thickness and growth parameters). For small period thickness, the interface quality has a crucial influence on the wear resistance of the samples. Best tribological properties are observed for samples with ultra-thin interfaces and a period thickness of 2.5 nm.

Silicon/silicon oxide and silicon/silicon nitride multilayers for extreme ultraviolet

Si/SiO2 and Si/Si3N4 multilayers have been fabricated using a locally made reactive diode ri-sputtering system. The layer alternation is obtained by modulating a partial pressure of oxygen or nitrogen near the sample using a silicon target with argon as sputtering gas. O2 and N2 partial pressure conditions were optimized to deposit stoichiometric SiO2 and Si3N4 films without significant reaction with the silicon target. In situ kinetic ellipsometry was used to monitor both thick film and multilayer deposition. The different interfaces appear very sharp with a little contamination of the silicon layers especially using oxygen. The multilayers were characterized by grazing x-ray reflection (Cu-K α line), and the reflectivity was measured in the soft x-ray range (120-350 A) by synchrotron radiation. Both Si/SiO2 and Si/Si3N4 multilayers exhibit well-defined Bragg peaks with very narrow bandpasses (two to three times lower than the conventional Mo/Si multilayer), and high absolute reflectivities (up to ≅22% at 130 A). Finally, thermal stability of Si/Si3N4 multilayers was evaluated. We did not find any degradation after annealing up to 800°C, which is extremely high compared to conventional Mo/Si multilayers, which are generally destroyed above 500°C.

Interface analysis of sputtered W-C, Rh-C and Ni-C multilayers for soft X-ray applications

Abstract Diode r.f.-sputtering technique is used to produce W-C, Rh-C and Ni-C ultra-thin layer stacks for application in the soft X-ray domain as optical systems. In situ kinetic ellipsometry is used to calibrate the deposition rates and to analyze precisely the different interface formations. Grazing X-ray reflection at 1.54A is also applied to the same multilayers. The carbon-metal interface is quasi-perfect for tungsten, but a non-negligible interdiffusion occurs for rhodium deposition (≅ 5A) and for the nickel layers (up to 20A). At the metal-carbon interface a smoothing effect of the carbon layer occurs in each case, its magnitude depending on the roughness of the metallic surface (≅ 4Afor tungsten, ≅ 5Afor rhodium and ≅ 7Afor nickel). In the case of the light elements (rhodium and especially nickel), the carbon layer is mixed with the metallic element and becomes pure only after a thickness of some tens of angstro¨ms.

Three materials soft x-ray mirrors: theory and application

A periodic structure alternating three different materials has been investigated and fabricated using a diode RF-sputtering system. Reflectivity was optimized using the wave propagation method. It is concluded that the amorphous character of the rhodium layers is enhanced in the W/Rh/C structures, and good reflectivities have been obtained at the carbon K-alpha line with trilayer structures including a great number of periods (28 percent of reflectivity for 40 periods stack). Some reduction of the boron interdiffusion is observed in the W/Rh/C structures, which leads to better soft X-ray performances at the boron K-alpha line (19 percent of reflectivity for a 40 periods stack).

Radio frequency sputtering of tungsten/tungsten nitride multilayers on GaAs

Thick tungsten nitride films have been deposited on GaAs substrates using a reactive rf‐sputtering system. The optical indexes of the WNx films (determined by in situ kinetic ellipsometry) and their nitrogen contents (determined by Rutherford backscattering), have been determined versus the pressure ratio PN2/Ptot. Using reactive sputtering conditions which provide highly nitrogenated films, W/WNx twenty period multilayers with nanometric layer thicknesses have been deposited. The experiment has also been monitored in situ by kinetic ellipsometry at 1.96 eV and the multilayer has been analyzed ex situ by grazing x‐ray reflection measurements at 1.54 A. The composition of the different layers has been determined precisely by Rutherford backscattering analysis. Both W–WNx and WNx–W interfaces in the multilayer appear very sharp by kinetic ellipsometry. This is confirmed by the occurrence of well defined Bragg peaks on the grazing x‐ray reflectivity curves in spite of the low density contrast between W and W...

Structural characteristics and performances of rf-sputtered Mo/Si and Co/Si multilayers for soft x-ray optics

Precise structural analysis of Mo/Si multilayers deposited by a diode rf-sputtering system has been made using in-situ kinetic ellipsometry, grazing x-ray reflection, x-ray diffraction, Auger profile analysis, Rutherford backscattering, and high-resolution electron microscopy. The main structural imperfections (interface roughness and interdiffusion) have been related to the Mo crystallization and to the molybdenum silicide formation at the interfaces. The comparison to Co/Si multilayers deposited in the same conditions was useful to deduce the influence of the intrinsic properties of these systems on their structural behavior. Silicide layers are formed in real-time during the growth of the samples. They are completely amorphous and their composition is not far from defined compounds (MoSi2 and CoSi2). In Mo/Si multilayers the Mo on Si interface is always thicker than the other interface (approximately equals 15 angstrom compared to 8 angstrom). It is not due to the deposition conditions but to the crystallization of the molybdenum layers which reduces the silicon diffusion at the Si on Mo interface. The higher reactivity of cobalt with silicon produces thicker quasi-symmetric silicide layers (approximately equals 25 angstrom). The thermal behavior of the two systems is also controlled by the interdiffusion and the crystallization of the silicide layers. Absolute soft x-ray reflection was measured on different Mo/Si x-ray mirrors by synchrotron radiation at various wavelengths above the Si L-(alpha) line and related to the structural characteristics. In spite of the occurrence of thin silicide layers at each interface, reflectivities as high as 55% in normal incidence have been obtained at 130 angstrom.

Comparative study of carbon and boron carbide spacing layers inside soft x-ray mirrors

Tungsten, iron, and rhodium materials have been deposited alternatively with carbon and boron carbide by diode RF-sputtering. The multilayer performances have been measured at the carbon or boron K-alpha lines depending on the layer spacing. It is found that tungsten and iron provide multilayers with good optical quality and optimized layer densities. This is related to the amorphous character of the tungsten and iron layers which results in low intrinsic roughness and limited interdiffusion. The experimental reflectivity is a factor of 2/3 lower than the theoretical value for W/C multilayers at 44.7 A. Rhodium layers alternated with carbon are crystallized, which induces significant interface roughness and poor soft X-ray performances. It is concluded that boron-carbide-based multilayers always exhibit lower interface roughness than carbon-based ones.

Ti/TiN multilayers for hard coatings applications: in-situ characterization by real time spectroscopic ellipsometry,

1.Abstract: Single Ti and TiNx thick films and Ti/TiNx multilayers have been deposited by reactive rf- sputtering with in-situ control by spectroscopic ellipsometry. Spectroscopic ellipsometry allows to measure precisely in real time the thickness of the deposited layers after calibration of the optical indices of the materials. For multilayer deposition the interdiffused layers can be followed in-situ and the interdiffusion mechanism can be understood. In this way we have found that the interfaces of this system deposited by rf-sputtering are asymmetric. Different multilayer samples have been realized varying the thickness of each component from 1 to 10nm with the same total thickness of the stack (200nm ). After deposition, the multilayers were characterized by grazing x-ray reflectance (thickness and roughness) and spectroscopic ellipsometry (layer interdiffusion ). Sliding wear tests were made to evaluate the wear lifetime of the samples and to compare the results with those on simple thick films.