ICMCTF2006 Session H2-1: Thin Films for Next Generation Devices

Wednesday, May 3, 2006 8:30 AM in Room Terrace Pavilion

Wednesday Morning

Start	Invited?	Item
8:30 AM	Invited	H2-1-1 A New Model Describing the Plasma-Assisted MBE Growth of GaN Thin Films and its Alloys with AlN and InN T.D. Moustakas (Boston University) In this paper we propose a new model for the growth of GaN thin films and other III-N compounds and alloys by the plasma-assisted MBE method. In the existing literature it has explicitly assumed that plasma-assisted MBE growth of GaN is a physical vapor phase deposition method, involving the reaction on a heated substrate of Ga-vapor with molecular nitrogen, activated first by a radiofrequency (RF) or microwave plasma. However, contrary to the MBE growth of traditional III-V compounds, which takes place under group-V rich conditions, the growth of atomically smooth GaN takes place under group-III rich conditions of growth. This is because the nitrogen activated species are highly reactive and as a result under N-rich conditions of growth the activated nitrogen species react with gallium on the substrate and limit their lateral mobility, leading to films with rough and faceted surface morphology. We propose that under extreme Ga-rich conditions the GaN growth takes place through the supersaturation of the metallic Ga at the growing surface with nitrogen. According to this model the growth process is a liquid phase epitaxy (LPE) rather than vapor phase epitaxy. Of course the solubility of molecular nitrogen in Ga at the usual growth temperatures of 800 °C is very small but the solubility of atomic nitrogen and other activated nitrogen species is expected to be relatively high. In this model, the GaN growth rate at high temperatures is not limited by the thermal decomposition of GaN but instead by the solubility of the active nitrogen species in Ga at the growth temperature. Since the various nitrogen species (atomic nitrogen, nitrogen radicals etc) are expected to have different solubility in Ga at certain temperatures, the growth rate should depend sensitively on the nature of species produced by the various forms of plasma activation. Experimental evidence will be presented that the growth rate of Ga- polar GaN under extreme Ga-rich conditions does not decrease at growth temperature in excess of 800 °C, a result attributed to higher solubility at higher growth temperatures of active nitrogen in the surface accumulated Ga. In other words, at these high temperatures the decomposition of GaN is overcompensated by the higher solubility of the active nitrogen species in Ga. This is to be contrasted with films grown under nitrogen rich or under Ga / N ratio approximately equal to one. In this case the growth rate was found to decrease abruptly above 750°C for Ga- polar GaN. Such a mechanism can also account for the efficient incorporation of Mg in GaN at high growth temperatures (750-800°C). At such temperatures the vapor pressure of Mg is more than 10 Torr and thus its efficient incorporation in the GaN lattice can only be accounted for by the high solubility of Mg in Ga and its incorporation in the GaN lattice from the liquid phase [1]. [1]. A. Bhattacharyya, W. Li, J. Cabalu, T. D. Moustakas, D. J. Smith and R. L. Hervig, Appl. Phys. Lett., 85, 4956 (2004).
9:10 AM		H2-1-5 Investigation of Structural and Rectifying Junction Properties of Thin ZnO-Poly(Styrene-Acrylic Acid) Nanocomposite Films on p-type (100) Si Substrates H.A. Ali, A.A. Iliadis, L.J. Martinez-Miranda (University of Maryland-College Park); U. Lee (Army Research Laboratory) The nanocomposite-film/p-Si junction properties and structural characteristics of the self-assembled ZnO nanoparticles in polystyrene-acrylic acid diblock copolymer on p-type (100) Si substrates are investigated and reported. The particles were developed in [PS]_m/[PAA]_n with four different block repeat unit ratios, m/n, of 159/63, 139/17,106/17, and 106/4, respectively, to determine the correlation between the particle parameters and the copolymer block lengths (m, n). The ability of the diblock copolymers to microphase separate and form self-assembled spherical nanodomains is used to template the ZnO nanoparticles onto the substrates. AFM images showed particles of spherical morphology with average size and density of about 250nm and 3.5x10⁷/cm², 60nm and 7x10⁸/cm², 60nm and 1x10¹⁰/cm² and 20nm and 1x10¹⁰/cm², were developed with block repeat unit ratios of 159/63, 139/17, 106/17 and 106/4, respectively. We established from AFM evaluations that the nanoparticle average size increased linearly with minority block length n, while the average density decreased exponentially with majority block length m. The x-ray diffraction θ-2θ scan showed the particles to have a wurtzite crystal structure with the (100) diffraction peak being the dominant orientation. Due to smaller dimensions and large separation between the particles, the broad diffraction peaks of ZnO nanoparticles could be detected only when θ was misaligned by 6^o with the substrate to remove the stronger Si peaks from the substrate. Room temperature I-V characteristics of the ZnO nanocomposite film on p-type (100) Si, measured for the first time, exhibited rectifying junction properties with an ideality factor n=1.7, and the transport mechanism in this system was evaluated from the electrical studies and discussed. The support of NSF through grant #ECS0302494 is gratefully acknowledged.
9:30 AM		H2-1-6 The Effect of Annealing Time in La₃Ga₅SiO₁₄ Piezoelectric Films Using Sputtering Deposition F.-W. Wang, Y. Hu, H.-L. Lin (Tatung University, Taiwan) La₃Ga₅SiO₁₄ (LGS) thin films have been grown on (200)-texture MgO buffer layer at 600°C using sputtering technique. The evolution of crystalline phase of the thin films was examined before and after annealing with various annealing time in air by X-ray diffraction and scanning electron microscope. Post-annealing treatment is applied to transfer the as-deposited amorphous films into LGS phase with the annealing temperature higher than 1150°C. In this paper, the crystallization behavior of as-deposited amorphous films is studied as function of annealing time at 1200°C annealing temperature. The morphology and formation mechanism of the crystalline phase of the LGS films are also discussed.
9:50 AM		H2-1-7 Grain Growth in Dielectric-Encapsulated Cu Thin Films T. Sun, B. Yao (University of Central Florida); V. Kumar, K. Barmak (Carnegie Mellon University); K.R. Coffey (University of Central Florida) Grain growth of Cu encapsulated in interconnect structures is an important means to reduce interconnect resistance. This work reports a comparative study of the grain growth of 40 nm-thick Cu films encapsulated with dielectric over- and under-layers. Grain growth in an unencapsulated thin film competes with surface-energy driven dewetting of the film from the substrate. For the latter process, annealing leads to void formation and the eventual formation of a discontinuous film consisting of agglomerated metal islands. We have previously reported that the encapsulation of Cu, Au, Pt, and Ru films with an over-layer of SiO₂ can substantially reduce the tendency of the film to agglomerate. In the current work, we focus on the grain growth of 40 nm-thick Cu films encapsulated by over-and under-layers of Al₂O₃, MgO, SiO₂, and Si₃N₄. The films were all magnetron sputter deposited on SiO₂/Si substrates in an ultra-high vacuum purity environment. Ex-situ annealing was performed at temperatures in the range of 300°C to 1100°C in one atmosphere of a reducing gas. Transmission electron microscopy was used to investigate the grain growth and characterize the microstructure of the as-deposited and annealed films. Films deposited at lower temperatures were found to exhibit more extensive grain growth with annealing. The 40 nm-thick Cu films encapsulated with Al₂O₃ exhibited significant grain growth, with the largest grains over 400 nm in diameter.
10:10 AM		H2-1-8 Electron Transport in Epitaxial Cu(001)/MgO(001) Layers J.M. Purswani (Rensselaer Polytechnic Institute); T. Spila (University of Illinois at Urbana-Champaign); D. Gall (Rensselaer Polytechnic Institute) Cu layers, 6.6 to 100 nm thick, were grown on MgO(001) by ultra-high vacuum magnetron sputter deposition at growth temperatures T_s ranging from 40 to 300 °C. X-ray diffraction θ-2θ scans, ω-rocking curves, and polefigures show that layers grown at T_s = 40 and 100°C are complete Cu(001) single crystals with a rocking curve width of the 002-peak of 2.0° and 1.8°, respectively. In contrast, layers grown at 200 and 300 °C exhibit a strong 001-orientation but contain also grains with 111, 211, and 511 orientations. All these grains have specific preferred polar orientations, that is, they show a local epitaxial relationship with the substrate. The layer thickness was determined by scanning electron microscopy and x-ray reflectivity measurements. The latter also provides values for the surface and buried interface roughness. Layers grown at 40°C are smoothest, with an rms surface and interface roughness of 0.7 and 1.4 nm, respectively, for a 100-nm-thick film. The measured resistivity of 100-nm-thick layers with T_s = 100 °C is 1.76 µΩ-cm. It increases to 2.10, 5.21, and 12.5 µΩ-cm for T_s = 40, 200, and 300°C, respectively, as the lower crystalline quality of these layers increases electron scattering. The resistivity also increases with decreasing layer thickness, due to electron scattering at the free surface and the layer-substrate interface. Fitting the data with a Fuchs-Sondheimer model for surface-scattering indicates almost completely diffuse scattering.
10:30 AM		H2-1-10 Joining Using Self Propagating High Temperature Synthesis of Reactive Multilayered Thin Film Ribbons M. Bai, J.J Moore (Colorado School of Mines); D.P. Adams (Sandia National Laboratory) The production of thin film NiTi alloy from multilayered elemental metal (Ni-Ti) using Self Propagating High Temperature Synthesis (SHS) was studied. SHS is a process in which the exothermic heat released during a propagating combustion reaction is transferred to the adjacent reactants creating a self-sustaining reaction. SHS has three modes of propagation: self-propagation at room temperature, self-propagation at preheated temperature and combustion at the ignition temperature (T_ig). If the reaction is only slightly exothermic it will not generate sufficient thermal energy to be self-propagating at room temperature. Preheating this SHS multi-layer thin film ribbon reaction system will increase the adiabatic temperature (T_ad) thereby providing sufficient energy to create a self-sustaining reaction. The multi-layer ribbon can also be uniformly heated to the ignition temperature (thermal explosion or simultaneous combustion mode) when all the exothermic energy is released instantaneously. For the self-propagating mode the reaction has to be initiated locally at a point or surface by an ignition source. For preheating and simultaneous combustion the whole of the ribbon is heated to the required preheat temperature (between the ambient/room and ignition temperatures), or above the ignition temperature for simultaneous combustion. Previous studies have shown that compact powder assemblies of elemental Ni and Ti requires preheating in order to self propagate; however, multilayered Ni/Ti ribbons were able to react at room temperature in vacuum and in air without preheating. Multilayered ribbons with a total thickness of 5 µm and different bi-layer thicknesses were produced using DC magnetron sputter deposition. This paper discusses the effect of bi-layer thickness, and stoichiometry on the SHS reaction velocity, NiTi structure, and combustion temperature, and discusses the application of these reactive ribbons in material joining processes.
10:50 AM		H2-1-9 Design and Performance of a Microelectromechanical Systems (MES) Vertical-Lift Electrothermal Actuator (VLEA) E.S. Kolesar, T. Htun, W.E. Odom, A.J. Jayachadran (Texas Christian University) The design, finite-element analysis (FEA) modeling, and experimental characterization of a novel microelectromechanical systems (MEMS) device, known as the vertical-lift electrothermal actuator (VLEA), is presented. The MEMS VLEA can function as a novel tool to initially elevate and position various panels and components that are often required to assemble complex 3-D MEMS technologies. The MEMS VLEA device investigated in this research can produce incremental out-of-plane deflections greater than 10 microns. The MEMS VLEA's were designed with the MEMSPro CAD program, and the actuators were fabricated using the Polysilicon Multi-User Microelectromechanical Systems Process (PolyMUMPs) foundry operated by the MEMSCAP Corporation. The finite-element analysis (FEA) modeling was accomplished with the IntelliSuite program. An optical fiber interferometer was used to measure the actuator's out-of-plane deflection. The FEA computations and the experimental results were found to agree extremely well. A relative measure of the reliability of the MEMS VLEA can be appreciated from the fact that it has been operated in its elastic regime for more than 10 million cycles, and no failures have been observed.
11:10 AM		H2-1-11 Multifunctional Thin Film Sensors Based on Amorphous Diamond-Like Carbon for Use in Tribological Applications. H. Lüthje, S. Biehl, R. Bandorf, J.-H. Sick (Fraunhofer Institute for Surface Engineering and Thin Films IST, Germany) Amorphous diamond-like carbon films are well known for their excellent tribological properties. In this paper we will discuss the sensoric properties of multifunctional a-C, a-C:H and a-C:H:Me films with respect to force/load/pressure and temperature measurement. It turned out, that nano structured amorphous carbon films showed an impressive piezorestive effect, which can be used for load measurement on tribological treated surfaces of different machine parts. The advantage over well known sensor techniques are: direct measurement in contact zones, high system stiffness and excellent adaptability to different base materials due to low deposition temperatures. Unlike well known strain gages and piezoresistive sensors, which detect a deformation of the base substrate, this novel sensor can be used in a complete stiff arrangement without any elastic joint. For film preparation rf-plasma CVD processes were used. Substrates have been polished plates of steel 100Cr6 and Si-wafer. Precursor gases have been C₂H₂ and Ar. Besides gas pressure a variation of the substrate power and bias potential was performed. It turned out, that the tribological parameters varied slightly (hardness: 20-30GPa; friction coefficient: 0.10 to 0.15), whereas the electrical parameters showed remarkable differences with respect to the electrical resistivity and piezoresitivity. The paper will also present different applications of amorphous carbon thin film sensors, e.g. smart washers to be used for actual force measurement in screws, ball bearings with local load measurement in the race track of the balls and arrays of piezoresitive pads for measurement of load distribution in mechanically contacts.