Papers

ICMCTF2006 Session E3-2: Modelling Mechanical Properties of Thin Films and Coatings

Wednesday, May 3, 2006 1:30 PM in Room California

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		E3-2-1 The Relationship Between Strain Generation and Relaxation, Composition and Electrical Performance in Strained Si/SiGe MOS Technology S.H. Olsen, S.J. Bull, P. Dobrosz, A.G. O'Neill (University of Newcastle Upon Tyne, United Kingdom) Strain can enhance the performance of many semiconductor devices. Tensile strain in silicon improves electronic transport properties compared with bulk silicon. Using strained Si as the channel material of MOSFET devices enables faster switching speeds at state-of-the-art dimensions and lower power technologies at modest geometries. This paper presents the key trade-offs encountered in optimising strained Si devices. The 4.2% lattice mismatch between Si and Ge leads to tensile strained Si when epitaxially grown on a relaxed SiGe virtual substrate. The degree of strain (and therefore mobility enhancement) can be adjusted by the virtual substrate Ge content. However if the strained layer is grown above a critical thickness relaxation occurs with the introduction of defects. Since critical thickness is inversely proportional to strain, virtual substrate composition and channel thickness are vital material parameters for realising high performance in strained Si technology. Strained layers are also prone to relaxation if they are subjected to high thermal budgets such as those used in device processing. In this work the effects of virtual substrate composition, channel thickness and high temperature processing on MOSFET operation are presented. Techniques used to evaluate epitaxial layer composition and strain are critically examined. Channel strain is studied using Raman spectroscopy, defect etching and transmission electron microscopy and is related to electrical measurements. Threading dislocation density is found to correlate with leakage currents while the peak position of Raman spectra, commonly used to assess strain, is shown to be less sensitive at identifying strain relaxation. Defects arising from partial relaxation are found to have a greater impact on device leakage than on carrier mobility. Challenges associated with analysing strain in layers grown on thin virtual substrates aimed at maximising performance advantages are also discussed.
2:10 PM		E3-2-3 The Extended Hertzian Theory for Normal and Lateral Loads Applied to the Determination of Intrinsic Stresses via Indentation Experiments - an in Principle Feasibility Study N. Schwarzer (Saxonian Institute of Surface Mechanics, Germany) Since the presentation of the complete elastic field of an extended Hertzian approach by the author ([1] treating the problem of mechanical contact of two bodies with shape functions of symmetry of revolution of the type z(r)=c₀ r² + c₂ r⁴ + c₄ r⁶ + c₆ r⁸) quite a few publications have appeared showing some useful applications of this new theory in analysing normal indentation data. They are mainly dedicated to the problem of parameter identification using the so called "concept of the effectively shaped indenter". In this paper these new analysing techniques are applied to the determination of intrinsic stresses directly via indentation experiments. Therefore not only normal but also lateral loading must be taken into account [2]. The new techniques are demonstrated on hypothetic examples and - wherever possible - compared with real experimental results. [1] N. Schwarzer: "Elastic Surface Deformation due to Indenters with Arbitrary symmetry of revolution", J. Phys. D: Appl. Phys., 37 (2004) 2761-2772 [2] FunnyIndenter: software for the evaluation of the elastic field of arbitrary combinations of normal and tangential loads of symmetry of revolution, free test version available at: http://www.esae.de/products.
2:30 PM		E3-2-4 Determining Viscoelastic Properties from Instrumented Indentation Y.-T. Cheng, W.Y. Ni (General Motors Research and Development Center); C.-M. Cheng (Institute of Mechanics, China) Instrumented indentation is playing an important role in characterization of small-scale mechanical behavior of "soft" matters, such as polymers, composites, and biomaterials that exhibit viscoelastic behavior. Modeling of indentation into viscoelastic solids thus forms the basis for analyzing indentation experiments in these materials. In this presentation, we will discuss our recent results on modeling instrumented indentation in linear viscoelastic solids. We first examine the relationships between initial unloading slope, contact depth, and viscoelastic properties for various loading conditions, including load- or displacement-control. We then discuss several commonly used methods, such as the "hold-at-peak-load" and "hold-at-the-maximum-depth" techniques, as well as the constant indentation strain-rate method. Finally, we will discuss methods of obtaining storage and loss modulus using dynamic indentation using either spherical or pyramidal indenters. These studies should help improve instrumented indentation techniques for measuring quasi-static and dynamic properties of viscoelastic solids.
2:50 PM		E3-2-5 Nanocontact Morphology Model Considering Surface Roughness During Nanoindentation J.-Y. Kim, J.-J. Lee, D. Kwon (Seoul National University, Korea) Nanoindentation is extensively used to evaluate mechanical properties of materials at small-volume. However, it still has problems to measure at nanometer scale because of intrinsic error factors, such as surface roughness, imperfection of indenter tip geometry, environment effect. The surface of micro-materials that is not processed by intentional surface treatment is usually rough. Surface roughness is main error factor when it is large relatively to indentation depth. New analysis method of nanoindentation results was developed by modeling the nanocontact morphology during nanoindentation on rough surface. When nanoindentation is performed on rough surface, initial contact is happened at the peak and reference plane of surface should be mean height of surface profile. From these two concepts, the relation to determine contact depth was derived, where pile-up/sink-in effect was considered also. If surface roughness is not considered for determination of contact depth, contact depth is overestimated, consequently, hardness and Young's modulus decreases as surface roughness increases at the same indentation depth. We performed mechanical and electro polishing to control the surface roughness for pure tungsten (pile-up happen) and nickel (sink-in happen). The variation in hardness and Young's modulus with average surface roughness was measured and the developed model was verified.
3:10 PM		E3-2-6 On the Relationship Between Plastic Zone Radius and Residual Depth During Nanoindentation J. Chen, S.J. Bull (University of Newcastle Upon Tyne, United Kingdom) The relationship between plastic zone size and indentation depth has been studied by a number of workers and an expression relating the indentation radius to the plastic zone radius has been given by Lawn et al (1980) based on microindentation testing. In this study, the relationship between the plastic zone radius and residual indentation depth was examined using finite element analysis for conical indentation in elastic-perfectly plastic bulk materials. The simulation shows that the Lawn method overestimates the plastic zone for different materials with a wide range of Young's modulus over hardness ratio and for indenters with different geometry. A new expression relating the plastic zone radius and residual depth is proposed by fitting the simulated data. The energy-based model developed at Newcastle University to predict the hardness and Young's modulus of coated glass has been modified using the new relationship between plastic zone radius and residual depth. It is found that the prediction of hardness and Young's modulus of the coated glass shows better agreement with the experimental results after modification.
3:30 PM		E3-2-7 Numerical Analysis on the Contact Stresses Developed During the Indentation of Coated Systems with Substrates with Orthotropic Properties N.K. Fukumasu, R.M. Souza (University of Sao Paulo, Brazil) Many works in the literature have analyzed the contact stresses developed during the indentation of coated systems. In general, the damage observed in systems with soft substrates is characterized by circular cracks that propagate at the edge of the indentations. In systems with soft substrates, recent works have also shown that the amount of substrate indentation pile-up presents direct relationship with a peak in radial stresses at film surface and, consequently, with the amount of indentation circular cracks. In this work, a series of finite element analyses was conducted to simulate the indentation of systems with an elastic film and an elastic-plastic substrate. Two values of film thickness were selected and, in each simulation, substrates were divided into two layers with different plastic properties. The layer on the bottom was considered isotropic and the layer attached to the film presented plastic properties along the loading direction (z) that were different from the plastic properties along the radial and tangential directions. Different ratios between axial and radial/tangential properties were considered. Results indicated that the amount of substrate pile-up and, consequently, the peak in radial stresses at film surface, could be significantly reduced in coated systems with substrates with orthotropic plastic properties.
3:50 PM		E3-2-9 Elastic Properties of Polycrystalline Gold Thin Films: Simulation and X-Ray Diffraction Experiments D. Faurie, P.O. Renault, E. Le Bourhis, Ph. Goudeau, O. Castelnau, R. Brenner (Universitat de Poitiers, France) Polycrystalline thin solid films exhibit physical, chemical and mechanical properties that can differ from their bulk counterparts. Thin films elaborated by physical vapor deposition can exhibit a microstructure with high volume proportion of surface and interface atoms, high defect density, columnar structure and preferential orientation (texture). This particular microstructure may induce noticeable modifications of the elastic properties. Therefore, we have developed an in situ tensile tester in a four-circle goniometer on synchrotron beam line (DW22, LURE facility, France). Preliminary works allowed us to study elastic behavior of strongly textured gold films deposited onto polyimide substrate using Crystallite Group Method. Nevertheless, due to the strong elastic anisotropy of gold and to the columnar structure inherent of thin films, suitable grain interaction model has to be used in order to better estimate or bound the elastic constants of thin films. In that frame, the use of homogenization methods is very helpful. Simulations using self-consistent model has been carried out and compared to experiments on textured gold films. We discuss the effect of grain shape (morphological) texture on the elastic behavior of thin films.
4:10 PM		E3-2-10 Determination of Young's Modulus and Yield Stress of Porous Low-k Materials by Nanoindentation M. Herrmann (Chemnitz University of Technology, Germany); N. Schwarzer (Saxonian Institute of Surface Mechanics, Germany); F. Richter (Chemnitz University of Technology, Germany) For successful integration of porous low-k dielectrics into damascene technology, the full compatibility to the mechanical load arising during the chemical mechanical polishing (CMP) process is required. To achieve this, mechanical properties of the porous material have to be known in order to understand the material behaviour as well as the failure mechanisms under loading conditions. Frequently, Young's modulus is used to describe the mechanical behaviour of porous materials, and standard techniques like the Oliver&Pharr method, continuous stiffness measurement, surface elastic wave measurement or BRILLOUIN scattering are applied to determine it. However, since those standard methods were developed for homogeneous materials it is not surprising that different methods usually deliver different values for one and the same porous sample. The aim of this work is to extend the applicability of indentation methods for the determination of Young's modulus and yield strength to the investigation of porous thin film materials. For the determination of Young's modulus we have utilized the Oliver&Pharr method together with a special extrapolation technique to correct for the substrate influence. To determine the yield strength of the porous films one of us (N.S.) has proposed an extension of Pharr's concept of the effectively shaped indenter, the so-called extrapolation method [1]. Our investigations were done on mesoporous SiO₂ xerogel as well as MSQ-based films on silicon substrates with porosities of 30 up to 57 vol%. The film thickness was in the range of 600 nm. We found Young's moduli values between 1 and 4 GPa, depending on material (xerogel or MSQ) and porosity. The yield strength was in the order of 75-150 MPa and decreased with increasing porosity. [1] René Puschmann, Norbert Schwarzer, Frank Richter, Swantje Frühauf, Stefan E. Schulz: "A usable concept for the indentation of thin porous films", Z. Metallkunde 96 (11) (2005).
4:30 PM		E3-2-11 Improvements on Industrial Adhesion Evaluation for PVD and PACVD Coatings O. Massler, R. Mertens, E. Kuhnt, M. Ante (Balzers AG, Liechtenstein); A. Stutzer, D. Grosskreutz (Balzers Verschleisschutz GmbH, Liechtenstein) The evaluation of the adhesion of PVD and PACVD coatings on industrial scale is performed with the well known Rockwell test. The deformation around a Rockwell hardness indent is used to determine, when a coating starts to crack and delaminate. This investigation of the influence parameters of the system shows, that the reproducibility of this type of measurement is difficult to achieve. Finite element simulation has been used to identify the most important influence parameters of the method. The results have been compared to results on real surfaces by measuring deformation and evaluating the adhesion result. As a result of the study, an approach is proposed to improve the reproducibility of the adhesion evaluation and gain better control of this important characteristic.