Papers

AVS1997 Session TF2-WeA: In Situ Characterization

Wednesday, October 22, 1997 2:00 PM in Room B3/4

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		TF2-WeA-1 Application of In-Situ Mass Spectroscopy of Recoil Ions for MBE Growth of GaN Thin Films E. Kim, I.E. Berishev, A. Bensaoula (University of Houston); A. Shultz, K. Waters (Ionwerks) Low energy ion scattering mass spectroscopy of recoiled ions (MSRI) is a novel and promising surface characterization technique which allows real time monitoring of the film chemical composition at pressures up to 1 mTorr. The correlation between surface composition with optical and electronic properties of the resulting layers is important but has not been established so far. MSRI is particularly suited for studies of group III nitrides growth because of its high sensitivity to surface H, N and C. In this paper we will present our MSRI and RHEED results from GaN and InGaN films grown by MBE with either ECR plasma or ammonia for nitrogen sources, and by CBE using tri-ethyl gallium and ammonia. We will demonstrate that the surface composition as determined by MSRI in real time can be directly correlated with the RHEED reconstruction patterns but more importantly with ex-situ photoluminescence properties of the final layer. We also present results showing that indium incorporation is greatly reduced in the presence of ammonia and depends strongly on III/V ratio and substrate temperature. In the case of CBE growth, buffer layer growth parameters (ECR and TEGA) are critical to the final layer crystal quality. Also the growth rate, 2D to 3D transitions, and surface composition (carbon and hydrogen signals) are found to be very sensitive to the growth temperature and TeGa flux. Finally, MSRI spectra will be shown for each step of the growth and for all the above mentioned growth technique. We will thus demonstrate that optimization of the growth conditions can be done in real time through the combination of MSRI and RHEED.
2:20 PM		TF2-WeA-2 In Situ Real-Time Studies of AlN and GaN Growth by Low-Energy Electron Microscopy (LEEM) A. Pavlovska, E.G. Bauer, V.M. Torres, J.L. Edwards, R.B. Doak, I.S.T. Tsong (Arizona State University); V. Ramachandran, R.M. Feenstra (Carnegie Mellon University) We have conducted in situ growth experiments of AlN and GaN on Si(111) and 6H-SiC(0001) substrates in the low-energy electron microscope (LEEM). The Al and Ga flux species were supplied by evaporative sources while the nitrogen was supplied by either a RF plasma source or by a helium supersonic beam seeded with ammonia. In the case of the Si(111) substrates, a SiN-(8x8) layer was first formed by nitridation of the Si surface, after which Al was deposited to displace the Si to form an AlN buffer layer for GaN deposition. The LEED pattern showed that the AlN layer grew epitaxially in parallel orientation with the Si substrate. For 6H-SiC(0001) substrates, the surfaces were prepared by high-temperature hydrogen etching. After such treatment, the SiC surface exhibited clear (1x1) and (√3x√3) LEED patterns, and the LEEM images showed well-defined steps and terraces. GaN depositions at 650°C made with the N-atom plasma source in the LEEM showed clear nucleation at the steps and growth of three-dimensional faceted crystals. ¹Work supported by the Office of Naval Research (N00014-95-1-0122), and NSF-MRSEC (DMR-9632635).
2:40 PM		TF2-WeA-3 Determination of Slowly Varying Refractive Index Profiles from In Situ Spectrophotometric Measurements D. Poitras, L. Martinu (École Polytechnique, Canada) Optical reflectance monitoring is often used for in situ determination of the thickness and refractive index of homogeneous coatings. However, to deal with inhomogeneous, graded refractive index layers, one has to divide the refractive index profile n(z) in a large number of sublayers, a process which increases the number of parameters involved and which complicates the calculation. We developed analytical methods which relate the variation of the reflectance with time, R(t), to n(z) in the cases of thick (≥ 500 nm) and thin (≤ 100 nm) graded layers. In the first case, a reflectance envelope method based on a WKBJ matrix solution is found and applied to graded silicon nitride (SiN_1.3) and silicon oxynitride (SiO_xN_y) layers deposited in a low pressure plasma from SiH₄/NH₃ and SiH₄/NH₃/N₂O mixtures. Variation of the refractive index in these films is related to changes in composition (SiO_xN_y) and in packing density (SiN_1.3)¹. In the case of thin graded layers, a MacLaurin series is obtained from the reflectance formula and it is used to elucidate the occurrence of graded layers at the early stage of SiN_1.3 films deposition on polymeric substrates such as polycarbonate (PC)². Limitations of the methods are discussed in the context of low pressure plasma deposition of optical, protective and barrier coatings. ¹D. Poitras, P. Leroux, J. E. Klemberg-Sapieha and L. Martinu, Opt. Eng. 35, 2693 (1996). ²J. E. Klemberg-Sapieha, D. Poitras, L. Martinu, N. L. S. Yamasaki and C. W. Lantman, J. Vac. Sci. Technol. A, 15, (1997) May/June.
3:00 PM		TF2-WeA-4 In Situ Sputter Deposition Plasma Charaterization for Tailoring Ceramic Film Growth C.R. Aita (University of Wisconsin, Milwaukee) Reactive sputter deposition is widely used to grow ceramic films at low temperature, produce metastable ceramic phases, and fabricate artificial multicomponent structures. Understanding the growth environment is essential for the deposition process to be controllable and reproducible. Here, we review optical emission spectroscopy (OES) as a non-invasive in situ technique for characterizing plasma volume chemistry during ceramic film growth. This technique is applied to selected binary oxide and nitride systems, including the oxides of Zr, Y, V, Nb, and Ti, and the nitrides of Al and B. By comparing the results from model binary systems, we show how chemical information obtained by OES was used to develop "rules" for obtaining general growth characteristics (e.g., dense vs. rare polymorph, crystalline vs. amorphous structure). We then apply these rules to growth of complex ceramic systems, including nanolaminates and nanocomposites. Supported by ARO Grant Nos. DAAH04-93-G-0238, DAAH04-95-1-0242, AFOSR Grant No. F49620-95-1-0467, and the Johnson Controls Foundation.
3:40 PM		TF2-WeA-6 Characterization by Real Time Spectroscopic Ellipsometry of Carbon and SiC Thin Films Grown by Pulsed Laser Deposition G. Soto, E.C. Samano, R. Machorro (Instituto de Fisica-UNAM, Mexico) The optical, mechanical and chemical properies of carbon and SiC thin films are so unique that they have become very attractive in many applications. For instance, diamond-like carbon films (DLC) have been used as hard coatings due to its hardness to wear resistance and shear stress, SiC is a prototype for manufacturing semiconductors devices with a remarkable performance under high temperature, high power and high frequency. The properties of these films depend on the deposition technique and growth control during processing. Pulsed Laser Deposition (PLD) satisfies the requirements of selectivity and performance for layer-by-layer growth control. Although PLD affords high deposition control, the properties of a film-to be-grown are still difficult to predict. The in situ monitoring and control of the deposition process is a way to handle the properties of thin films. Kinetic ellipsometry arises as the right choice because it is a non-damaging and non-intrusive technique. The experiment is based on the photoevaporation of pyrolitic graphite and SiC targets using a KrF excimer laser in a UHV system. The laser power density was varied from 2 to 5 W/cm². The ablated species are deposited on clean Si (111) and quartz substrates. The films growth is monitored by kinetic ellipsometry at fixed photon energy of 2.5 eV. The different stages of the deposition process are momentarily interrupted to analyze the film properties in the 1.5 to 5 eV photon energy range. In particular, the films crystallinity strongly depend on the experimental parameters. The volumetric composition and microstructural identification of the films studied by ellipsometry are compared to XPS and AES spectra. The films crystallinity is corroborated by ex situ X-ray diffraction.
4:00 PM		TF2-WeA-7 Metalorganic Chemical Vapor Deposition of Tin Oxide in the Environmental Scanning Electron Microscope Chamber M.J. Nystrom, R.E. Cavicchi, S.A. Wight, F. DiMeo (National Institute of Standards & Technology) Metal oxide thin film conductance sensors are an emerging technology for the measurement of gas concentrations in mixed atmospheres. The gas sensing properties of these materials are dictated by the microstructure of the polycrystalline films. While metalorganic chemical vapor deposition (MOCVD) is the process of choice for obtaining these thin layers, the typical reactor geometry is not amenable to morphological characterization during deposition. In this study, tin oxide thin films were deposited by low pressure metalorganic chemical vapor deposition in the chamber of an environmental scanning electron microscope (ESEM). This provided for the in situ monitoring of thin film nucleation, coalescence, and growth by both secondary electron and backscatter electron imaging techniques. The tetramethyltin precursor gas and the oxygen reactant gas were introduced directly into the microscope chamber, and the films were grown from this static mixture. Deposition occurred on micromachined silicon microhotplate devices which incorporated a heating element, temperature sensor, and electrical contacts. Growth of the films as monitored by in situ electrical conductance measurements was correlated to grain growth imaged by the electron microscope. While the electron beam was not observed to significantly affect film structure, transient changes in conductivity in the growing film induced by rastering of the microscope beam were observed. To prevent carbon contamination of the tin oxide films deposited in the microscope, rigorous cleaning and pumping of the chamber were necessary.
4:20 PM		TF2-WeA-8 Quantitative Analysis of Real Time In Situ Infrared Spectra for the Study of Surface Reactions: Theory and Experiments. A. von Keudell, J.R. Abelson (University of Illinois, Urbana) In situ real time infrared spectroscopy is widely used as a diagnostic for the study of surface reactions during plasma processing of materials. In order to enhance the signal of a Fourier transform infrared (FTIR) spectrometer, films are usually deposited on metal substrates and analyzed with a straight forward optical modeling of the measured spectra. The use of multiple internal reflection (MIR) provides even larger signals, but the quantitative analysis of these spectra is rather complex and can be misleading. The substrate configuration of a ‘optical cavity’ consisting of an oxidized wafer with an Al backside coating, however, combines a straight forward optical modeling of the results with a high signal enhancement and an ease of use for in situ real time infrared spectroscopy. This has already been proofed by the analysis of surface reactions during growth of a-Si:H by reactive magnetron sputtering. However, in order to achieve submonolayer sensitivity with this method, the analysis of the resulting spectra has to take into account the partial incoherence of the IR light in the silicon wafer and the influence of the Fourier transformation on the spectral signature. In this paper the theory for this analysis of the IR spectra and, as an example, experimental data on the HD exchange reaction on microcrystalline silicon with submonolayer resolution will be presented.
4:40 PM		TF2-WeA-9 Temperature Induced Phase Transitions in Polystyrene Films and Colloid Particles Studied with a Quartz Crystal Microbalance K. Revesz, M. Rodahl, B. Kasemo (Chalmers University of Technology, Sweden) We have developed a method to measure the resonant frequency (f) and the energy dissipation factor (D) of a freely oscillating quartz crystal microbalance (QCM). The method consists of suddenly switching off the QCM driver circuit after which the decay of the QCM oscillation is recorded. By numerical fitting of an exponentially damped sinusoidal to the recorded decay curve, both f and D are obtained simultaneously. A change in the mass coupled to the QCM (shear) oscillation induces a proportional shift in f. The coupled mass of a film coating the QCM decreases the more fluidlike and less viscous it is. D is a measure of how much energy is lost in this coupled mass which depends on the film viscoelasticity, density, and thickness. f has a large and non-linear temperature (T) coefficient concealing any temperature induced shifts in f caused by changes in film properties. However, D varies only slightly during a T sweep for a naked crystal making it suitable for studies of temperature induced changes in thin films. We have applied the QCM method to measure temperature induced phase transitions in thin (<200 nm) polystyrene (PS) films, spin coated onto a 10 MHz, AT-cut QCM, as a function of T. When the PS film is heated, D initially decreases (indicating an apparent stiffening of the film) but then starts to increase at ~100 C indicating a viscosity change in the PS film. We have also studied 6 µm PS spheres adsorbed to the QCM electrode surface as a function of T. These PS spheres dampen the QCM much more than PS films per unit mass. The spheres also exhibit a more complex D vs T trace with a large hysteresis. Mechanisms for the measured damping and hysteresis will be discussed. We will also discuss the possibilities and limits of the QCM technique for studies of solid to liquid phase transitions in thin films and colloid particles.