Mechanical Properties and Stress

TF4:

Mechanical Properties and Stress

Oral Presentations

TF4.1.O

REAL-TIME MEASUREMENT OF SrO EPILAYER STRAIN

ON H-TERMINATED Si

H. Asaokaa*, Y. Machidaa, H. Yamamotoa, K. Hojyoua, K. Saikib, A. Komab

aJapan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan

bThe University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

SrO is a well-known buffer layer on Si for SrTiO3 and BaTiO3 which are highly desirable complex oxides for future generation transistor gate dielectric and ferroelectric memory applications. Furthermore, these complex oxides work as good substrates for various functional oxides including high-Tc superconductors. To fabricate integrated devices of semiconductors and such oxide materials, the epitaxial growth of SrO films on Si is very attractive. However, these properties depend strongly on the quality of thin oxide films in which large mechanical stress might be produced during their preparation.

To characterize the film stress as a function of Si surface conditions, we use real-time measurement of substrate curvature during Sr film growth both on bare Si (111) 7x7 and on H-terminated Si (111). The growth of epitaxial Sr films has been achieved at room temperature by MBE.At the beginning of Sr growth on bare Si (111) 7x7, strong compressive stress is generated, indicating an immediate expansion of the surface. The compressive stress in Sr film is a result of strong interfacial strain from lattice mismatch between the film and the substrate.In case of Sr growth on H-terminated Si, however, the maximum total stress in the film is one-third of the film stress on bare Si. Stress-free Sr layers are grown from a thickness of 3 atomic layers corresponding to 1 unite cell. The H-terminated Si whose surfaces have no active dangling bonds are like quasi Van der Waals surfaces. It is known that growth of heteroepitaxial films with weak Van der Waals force leads to a relaxation of lattice matching requirement. Inert Si surfaces also result in the relaxation of interfacial force between films and substrates and thus the lattice mismatch stress is controlled to be minimized.

Real-time measurement also shows change of intrinsic stress during oxidation process after metal film growth on H-terminated Si (111). The epitaxial growth of SrO films has been achieved by oxidizing epitaxial Sr films at room temperature. The sharp interface structure without silicon oxide layer has been also formed on the H-terminated Si. The total stress in the film is affected drastically by introducing the O2 gas. The film stress turns to tensile, indicating a shrinking of the film surface. When Sr films are exposed to the O2 gas, O atoms can penetrate Sr films without changing the position of Sr atoms to form SrO structure, because Sr has a face-centered cubic structure and SrO has a NaCl type structure. The in-plain lattice constant of Sr film changes during the oxidation from 0.430 nm to 0.363 nm, that is, the lattice constant of oxidized film decreases to 94.6 % of the Si lattice. These cause a variable mismatch between the effective lattice parameter of Sr or SrO film and substrate, and lead to the stress change during oxidation process.

*Corresponding author: E-mail:

Full address of the corresponding author:

<Permanent address>

Hidehito ASAOKA

Department of Materials Science, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan

e-mail; , Tel.; +(81)-29-282-5479, Fax; +(81)-29-282-6716

<Temporary address from March 2002 for one year>

Hidehito ASAOKA, c/o Bert Voigtlander, Forschungszentrum Julich / ISG 3, Leo Brandt Str., 52428 Julich,Germany

e-mail; , Tel.: + (49) 2461 61 4116, Fax: + (49) 2461 61 3907

TF4.2.O

Measurements of strain during vapor deposition of thin films and multilayers

A.Rizzo1*, M. Sagace2,M..A. Tagliente1, U. Galietti2, C. Pappalettere2

1ENEA,CR Brindisi, Italy

2DIMEG - Politecnico di Bari, Italy

Zinc selenide (ZnSe), barium fluoride (BaF2) and silver (Ag) single layer and multilayered thin films were deposited by thermal evaporation onto cantilevered substrates Si(100) with native oxide, near room temperature in ultrahigh vacuum. The macroscopic strain in the film during and after deposition was determined from the change in electrical resistance of a strain gauge glued at the substrate back surface. The intrinsic component of the strain was obtained by subtraction of the apparent thermal component, which was obtained by measuring the strain in the heating phase of the deposition process. During the deposition, the strain in the film shows a general trend regardless of the growth parameters: initially it is positive (in terms of stress it means compressive stress) with a linear behavior, then reaches a broad maximum value and increases to become negative (the stress is tensile) towards an asnthotic value after vacuum cooling. After deposition ends, ex-situ x-ray residual stress analysis was also performed by considering the d-sin2 method. The results indicate the films are polycristalline without texture and tensile in-plane stress was measured for all the investigated samples

When a multilayer BaF2/ZnSe/Ag/Si(100) is deposited continuously, the evolution of the features of the strain is different for the several layers but is the same of the corresponding single layers. The BaF2/ZnSe interface strain was determined by measuring the discontinuities in strain associated with the formation of the interface. These discontinuities were measured for both BaF2 on ZnSe and ZnSe on BaF2.

TF4.3.O

Analysis of Deposition Stresses in sputtered Metal Oxides

Robert J. Drese[*], Tom P. L. Pedersen, Matthias Wuttig

I. Physikalisches Institut A, RWTH-Aachen, 52056 Aachen, Germany

Metal oxides have become increasingly important in many fields of application, e.g. as TCO (transparent conducting oxides), hard coatings or coatings for switchable windows. A common way to coat large scale areas with metal oxides is reactive magnetron sputtering. However, thin sputtered films frequently exhibit large stresses originating from the growth of the material. These stresses limit the performance of electronic devices, reduce the adhesion of advanced coatings on glass and may even lead to ablation.

The intrinsic stress has been measured for various metal oxides including ZnOx, ZrOx, NbOx, MoOx, and TaOx. The measurements have been performed using both ex- and in-situ wafer curvature methods. The wafer curvature method utilises the change of curvature in a film-substrate combination upon changing stress in the film. The ex-situ measurement compares the substrate curvature prior to and after deposition, while the in-situ measurement analyses the curvature continuously during the reactive sputter deposition and yields additional information on the growth of the film. The ex-situ apparatus used to measure the stresses uses a single laser beam, which scans the sample, while the in-situ apparatus is based on two stationary laser beams allowing good noise reduction during the measurement.

Our analysis shows that the stresses arising during reactive sputter deposition depend on the oxygen flow, the total pressure during deposition and the deposited material itself. Stresses in these oxides can easily reach the order of GPa. These stresses have e.g. been observed in ZnO, where the maximum state of stress reached 1.4GPa for low total pressure.

Based upon these data, a microscopic model has been developed to elucidate the trends for intrinsic stresses during deposition.

TF4.4.O

NANOMETER SCALE MULTILAYERED COMPOSITE METAL/NITRIDE THIN FILMS : THE TiN/Cu SYSTEM

G. Abadias*, A. Michel, C. Tromas, Y.Y. Tse, C. Jaouen

Laboratoire de Métallurgie Physique, UMR CNRS 6630, Université de Poitiers, SP2MI, Téléport 2, BP 30179, 86962 Futuroscope-Chasseneuil cedex, France

Over the past five years, a considerable effort has been devoted to the elaboration, by various PVD techniques, of nanocomposite coatings for their enhanced mechanical properties [1]. Hard and even superhard materials have been synthesized, either in the form of nitride/nitride or metal/nitride superlattices with bilayer thickness in the 2-10 nm range, or in the form of biphase composite thin films consisting in nanocrystals of one phase (nitride) embedded in a thin matrix of the other phase (nitride or metal).

Recently, it has been shown [2] that combining a soft metallic phase (e.g. Cu) with a hard nitride phase (e.g, ZrN) could improve the toughness of the nanocomposite coating while retaining its high hardness. The elaboration of such composite films in the form of superlattices could be a prized path to gain interesting insights in the microstructural characterization, especially if interfacial effects prove to play a significant role.

This is precisely the aim of the present investigation devoted to a similar metal/nitride system, TiN/Cu, which is also of technological interest for integrated circuits. Multilayered thin films with bilayer periods ranging from 5 to 20 nm and different Cu:TiN ratios have been obtained by alternate deposition of Cu and TiN layers by dual ion beam sputtering. The growth is realized at room temperature on naturally oxidized Si (001) substrates, under a mixed Ar+N2 ion beam with an energy of 25eV. A detailed structural investigation aiming at characterizing the preferential growth orientation, stress/strain state as well as the nature of the interfaces has been performed using XRD in various geometries as well as AFM and HRTEM microscopy techniques. The mechanical properties (hardness and Young modulus) have been obtained from nanoindentation measurements.

We show that pure TiNx thin films deposited on Si under the same growth conditions are stoechiometric (x~0.96), with a texture evolving from (002) to (111) with increasing thickness. The hardness is close to the value of bulk material. XRD experiments reveal that the TiN/Cu superlattices are characterized by i) a well defined periodicity associated with low roughness interfaces, ii) a (002) preferential orientation of both TiN and Cu layers and iii) a non-coherent stacking along the growth direction due to the relatively high disorientation of the Cu grains. HRTEM observations show the presence of some (111) Cu grains as well as a specific contrast located at the interfaces. The evolution of the effective hardness as a function of the bilayer period and the Cu:TiN ratio is also discussed.

[1] For a review see, J. Musil, Surf. Coat. Techn. 125, 322, (2000)

[2] J. Musil, P. Zeman, H. Hruby, P. H. Mayrhofer, Surf. Coat. Techn. 120-121, 179, (1999)

*Corresponding author : Tel : +33 5 49 49 67 48, Fax : +33 5 49 49 66 92

e-mail :

TF4.5.O

Friction and Wear Properties of C-N/MeNx Nanolayer Composites

J.Sobotaa), Z.Bochnicekb, V.Holyb, G.Sorensenc

a) Institute of Scientific Instruments ASCR, Brno, Czech Republic

b) Masaryk University, Brno, Czech Republic

c) Eurconsult, Aarhus, Denmark

Abstract

Nanolayer composite coatings enable creation of stable structure with unique properties, and in particular, combination of properties conventionally considered as excluding one another, such as high hardness and high toughness. Coatings, typically 1-4 micrometers thick, were deposited at a total pressure ranging from 0.9 to 3 Pa at relatively low substrate temperatures not exceeding 200 deg.C. We deposited nanostructured multilayer coatings of the type CN/MeNx, where Me could be Ti, Nb or Zr. Various substrates such as highly polished tungsten carbide, steel and silicon were used. This study concentrated on the tribological properties of C-N/MeNx nanostructured multilayers. The friction coefficient, wear and the film transfer in a ball -on disk tribometer is presented. The samples were measured after the deposition and then thermally treated at a constant temperature in the air. Afterwards the samples were again measured and the temperature treatment at higher temperature was performed until the layer broke down. We investigated also the structure of the coating by means of low-angle x-ray reflection and high angle x-ray diffraction. The potential application of CN/MeNx nanostructured multilayers as hard solid lubricant is discussed.

Keywords: Thermal stability; Friction; X-ray scattering; Carbon; NbN, ZrN, TiN

1 Corresponding author: e-mail: , fax: +4205-4151 4404

Present address: Institute of Scientific Instruments ASCR, Královopolská 147,CZ-612 64 Brno, Czech Republic

[*] Corresponding author: Robert Drese, Tel.: +49-241-8027169, e-mail: