Papers

AVS2000 Session TF-TuA: Mechanical Properties of Thin Films

Tuesday, October 3, 2000 2:00 PM in Room 203

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		TF-TuA-1 The Nanomechanical Properties of Thin Films J.E. Houston (Sandia National Laboratories.) Material properties and processes can appear remarkably different when viewed at the nanometer level. Mechanically, solids can approach the behavior of perfect single-crystals and inter-particle interactions become dominated by surface energetics and dyna mics. In thin films with nanometer scale grains, dislocation-loop formation can become unfavorable and grain sliding can become the dominant mechanism for plastic deformation. The study of the mechanical properties of these materials is presently enjoying increasing attention due in large part to the rapid development of scanning-probe techniques capable of making measurements on individual grains down to the nanometer level. In this presentation, I will illustrate some of these unique nanoscale effects in various applications of the Interfacial Force Microscope (IFM) to studies of the nanomechanical properties of thin films. The IFM is a scanning force-probe microscopy similar to the Atomic Force Microscope but distinguished by its use of a mechanically stable, zero compliance force sensor. Used in a nanoindenter mode, this sensor offers accurate control of the probe-sample separation and provides a quantitative measure of the film's mechanical behavior. I will illustrate the IFM's unique capabilities w i th examples of near theoretical yield strength for single-crystal surfaces and the effect of surface steps on the strength. In polycrystalline films, I contrast the mechanical behavior as a function of grain size and film thickness and discuss the mechanisms responsible for the interesting observations. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.
2:40 PM		TF-TuA-3 Anomalous Plastic and Elastic Behaviors of Sputter-deposited TiN with 10 or 20 Inserted Thin Al Layers Evaluated by Nanoindentation E. Kusano, N. Kikuchi, K. Tsuda, H. Nanto, A. Kinbara (Kanazawa Institute of Technology, Japan) The hardness enhancement observed for multilayered thin films results from the specialized mechanical properties of the interface region. In this paper, effects of thin Al layers inserted into the TiN matrix thin film on mechanical properties of the coating have been investigated in order to discuss a role of the interface regions made by the Al thin layer insertion. Thin films of TiN with Al thin layers have been deposited by dc magnetron sputtering. The total TiN thickness was kept at 500 nm for all prepared samples. The number of thin Al layers inserted was varied from 2 to 20 for the total Al thickness of 100 to 500 nm. The top layer of the coating was TiN for all sample coatings. Film hardness of prepared samples was estimated by nanoindentation. From a load-unload curve of the nanoindentation, energies consumed to induce plastic deformation and elastic deformation were estimated. A hardness of thin films with 20 Al layers decreased from 11 GPa to 8GPa when the total Al layer thickness increased from 100 to 500 nm. For a constant total Al layer thickness, the film with 20 Al layers yielded higher hardnesses. The film with 20 Al layers with a layer thickness of 5 nm was harder than the monolithic TiN film. While the energy used for plastic deformation during nanoindentation increased with the total Al layer thickness, the energy used for elastic deformation remained constant. Further, the dissipated energy decreased with increasing the number of Al layers inserted. These results obtained by nanoindentation measurement imply that the film becomes more elastic with increasing the number of Al layers inserted. A high microhardness obtained for a film with Al layer thickness of 5nm emphasizes a unique effect of thin Al layers or interface regions on mechanical properties of the coating.
3:00 PM		TF-TuA-4 Tungsten Silicide (WSi2) for the Alternate Gate Metal in Metal-Oxide-Semiconductor (MOS) Devices K. Roh, S. Youn, S. Yang, Y. Roh (Sungkyunkwan University, Korea) Recently, it has been recognized that both achieving low gate resistance and suppressing poly-Si gate depletion are key factors for developing deep submicron MOSFETs. In the present work, tungsten silicide (WSi2) deposited directly on SiO2 is proposed for the alternate gate electrode for deep-submicron MOSFETs. PMOS capacitors were fabricated on 4~7Ω-cm, (100) n-type Si wafers. Thermal oxidation of the Si was carried out at 850°C for 80 s using RTP to grow ~110Å SiO2. WSi2 were then deposited directly on SiO2 in a cold-wall LPCVD system: Deposition temperature and pressure were 350°C and 0.7Torr, respectively. The ratio of SiH4/WF6 flow was changed from 40 to 70. RTP was used for post-deposition annealing at various conditions. Detailed analysis of mechanical properties of WSi2 deposited on SiO2 reveals that a low resistivity can be obtained while satisfying the requirement for the low thermal budget. In addition, HTEM results showed that WSi2-SiO2 interface remains very flat after annealing as-deposited WSi2 films using RTP at 780°C in vacuum. Since F diffusion into SiO2 during the WSi2 deposition and annealing steps has been known to cause the irregular formation of WSi2-SiO2 interface, we attribute the current results to the indirect evidence of negligible F diffusion. In addition, the electrical characteristics of annealed WSi2-SiO2-Si (MOS) capacitors were also improved in view of charge trapping. For example, oxide charging curves monitorted during Fowler-Nordheim tunnel electron injection indicate that the shift of flatband voltage is less for RTP annealed samples as compared to that of as-deposited samples. The phenomenon of gate depletion which has been a serious problem of poly-Si gate is also suppressed in the WSi2 gated MOS capacitors. The C-V data shifted to the positive gate bias after annealing, and we interpret that this positive shift is caused by the workfunction difference that may be caused by the change of Si to W ratio due to the annealing process.
3:20 PM		TF-TuA-5 Structural and Mechanical Properties of TiC/Ti and TiC/B₄C Multilayers Deposited by Pulsed Laser Deposition A.R. Phani, J.E. Krzanowski (University of New Hampshire); J.J. Nainaparampil (Systran, Inc.) Multilayer thin films have been shown to enhance the hardness as well as toughness of hard ceramic coatings, and the results often depend on the type of interlayer used. In the present study, we have investigated multilayers of TiC/Ti (for toughness enhancement) and TiC/B₄C (for hardness enhancement). Films were deposited on 440C steel and silicon substrates by pulsed laser deposition (PLD). Ti, B₄C and TiC targets were ablated in a background gas of 1mTorr Ar at substrate temperatures of 200, 400 and 600C. Samples were deposited having modulation periods ranging from 2.5 to 50 nm. Films were analyzed using nano-indentation hardness, x-ray diffraction, XPS and electron microscopy. Surface morphology and roughness of the samples were measured by scanning electron microscopy and atomic force microscopy, respectively. Tribological studies have also been conducted to evaluate the friction and wear properties of these films. Films with low modulation periods showed lower residual stresses as measured by x-ray diffraction methods. The nano-indentation hardness of compositionally modulated Ti/TiC and TiC/B₄C multilayer films were found to be dependent on composition modulation frequency. TiC/Ti films showed reasonably high hardness values (37GPa) for the lower period samples despite the incorporation of the metal layer, and TiC/B₄C films showed hardness levels up to 42GPa. Film composition depth profiles performed by using XPS showed 1-2 at % oxygen in the deposited films, as well confirming the presence of composition modulations. The mechanisms of hardness enhancement and its relation to tribological properties will also be discussed.
3:40 PM		TF-TuA-6 Structural Studies of AlN Thin Films during Low Temperature RF Sputter Deposition F. Engelmark, G.F. Iriarte, I.V. Katardjiev, M. Ottosson (University of Uppsala, Sweden); P. Muralt (Laboratiore de Ceramique, Switzerland); S. Berg (University of Uppsala, Sweden) AlN is a material used in a wide variety of applications such as electro-acoustic devices, blue diodes, IR-windows, thermal conductors, MIS-structures, IC-packaging, etc. Thin piezoelectric AlN polycrystalline films have been grown on Si and SiO₂ using RF magnetron sputter deposition in an Ar/N₂ gas mixture. The structural properties of the films have been optimized by varying the deposition parameters, such as process pressure, gas mixture, substrate temperature, discharge power, etc. It was found that the best film texture was obtained for a particular set of parameters, namely process pressure of 8 mTorr, substrate temperature 350°C, discharge power 350W and a gas mixture of 25%Ar and 75%N₂. The films as examined by XRD exhibited a columnar structure with a strong (002) texture, and a FWHM rocking curve of 1.6 degrees. A crystallite size of 38 nm was calculated using Sherrers formula. AFM measurements indicated a surface roughness with an rms value of 8 Å. Classical nonapodized transversal SAW filters operating at a frequency of 534 MHz were fabricated to estimate the electro-acoustic properties of the films. The measurements indicated a coupling coefficient of 0.46% and a phase velocity of 4900 m/s. Further, thin epitaxial films were grown on alfa-Al₂O₃(001) under the same deposition conditions. The films exhibited a (001)AlN//(001)alfa-Al₂O₃ plane orientation with a (002) rocking curve FWHM value of about 0.4 degrees, indicating a relatively good alignment of the c-axis or a low dislocation density. The in-plane orientation was [110]AlN//[120]alfa-Al₂O₃ corresponding to a rotation of the AlN film of 30 degrees with respect to the alfa-Al₂O₃(001) surface. Cross-sectional TEM studies indicated a population of both thread and edge dislocations with decreasing concentrations with film thickness.
4:00 PM		TF-TuA-7 Multifunctional Ceramic Films C.R. Aita (University of Wisconsin, Milwaukee) The concept of a film that can perform different functions as required by changing external stimuli has long intrigued scientists and engineers. The nanostructure revolution has made possible new uses for ceramic films that behave as multifunctional "smart" materials. In this paper, we first review new developments in traditional uses for multifunctional films, such as those involving electrochromic and thermochromic materials. Next, we describe new uses for multifunctional ceramic films. The design and development of a biocompatible coating that protects blood-interfacing implants against corrosion and mechanical degradation is used as an example of the challenges that face synthesis and characterization of these multifuctional "smart" materials.
4:40 PM		TF-TuA-9 Quantification of Scratch Resistance and Accelerated Wear for Thin Film Coatings Using the Newly Developed Micro-Tribometer and Testing Procedure C. Gao, N. Gitis (Center for Tribology) Quantification of scratch and accelerated wear resistance of thin films ranging from 1 nm to 1000 nm was achieved by using critical loads, at which the given films were progressively worn through. Progressive worn through of thin carbon films (3 nm to 8 nm) on magnetic disks using a micro-blade under precision motion with linearly increasing loads from 0.2 grams up to 400 grams was clearly observed simultaneously from electrical contact resistance (ECR), friction force and acoustic emission (AE) signals. At the critical load, friction force and AE fluctuated violently and ECR dropped to practically zero. The critical load was found to increase with increasing carbon thickness, as expected. The wear depths at the critical loads were measured post-test using Tencor profiler and optical surface reflection and found to be in excellent agreement with film thickness. A lubricant film as thin as 1 nm on the carbon films enhanced the critical load by a factor of five to ten. The same testing procedure was applied for thicker diamond-like carbon films on silicon substrate ranging from 50 nm up to 1000 nm, but with linearly increasing loading force from 0.2 N up to 40 N. Good correlation was found between critical load and film thickness, and also between critical load and residue film stress also. However, there was no correlation between critical load and nano-hardness. We believe that the nanohardness measurements may not apply for films thinner the sub-micrometer, since the contact stress distributed well into the substrate when nano indentation was made. The successful quantification of scratch resistance and accelerated wear is attributed to precision motion, linearly increasing load mechanism and the contact geometry of the micro-blade. The micro-blade will be described and the mechanism for its effectiveness on surface film evaluations will be discussed, as compared to much less effective counter surfaces, such as a diamond stylus or a stainless steel ball.
5:00 PM		TF-TuA-10 Lubricating Characteristics of Zinc Oxide: Effect of Carbon, Silicon and Boron as Additives J.J. Nainaparampil (Systran Federal/Air Force Research Lab); J.S. Zabinski (Air Force Research Lab) Zinc Oxide, with its wurzite structure, is a unique material that exhibits lubricious qualities and desirable electronic and optical characteristics. There have been numerous studies on the doped zinc oxide thin films profiling it as an optical coating material or as a semiconductor material. But no significant amount of work has been reported yet on this material as a tribological material. Recent work of this lab revealed some of the tribological potentials of this material. Due to its open nature and desirable coordination number, zinc can move around different lattice positions and be substituted with external atoms to become interstitials. Zinc oxide shares the hexagonal structure, which is commonly seen in most of the widely used solid lubricants. The open nature of its crystal structure and similarity of this structure with that of the widely used solid lubricants are quite desirable qualities. To preserve these qualities, and to manipulate the tribological characteristics, additives of ionic radii comparable that of Zn need to be selected. Alumina doped zinc oxide have already been studied and found to have low friction and good wear characteristics. In this work, properties of thin films of zinc oxide, formed with simultaneous addition of Si, C and B will be reported. Si added films showed low friction and long wear life above 350 ÂºC. Deposition direction and oxygen working pressure also had a profound effect on these films. Results of common surface analyzing techniques like XPS, SEM, AFM, and XRD, Raman, Nanoindentation and Friction and Wear will be reported.