Optical Thin Films by CVD: Structure/Property Relationships

Monday, April 30, 2001 1:30 PM in Room Sunset

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1:30 PM C5-1-1 Gas-Phase Chemistry in the On-Line Deposition of Coatings on Float Glass
M.D. Allendorf (Sandia National Laboratories)
Chemical reactions in the gas phase can be the rate-limiting process in the on-line deposition of films on float glass. In particular, deposition of tin oxide from organotin compounds such as tetramethyltin and diethyltindichloride is controlled by radical-chain reactions occurring in the gas phase above the substrate. Such reactions determine the species that interact with the glass and affect key properties that determine the optical properties of the glass. Of primary concern is the deposition rate and its uniformity. Relatively high activation barriers for tin oxide CVD chemistries (25 - 30 kcal/mol) make the deposition rate highly sensitive to substrate temperature. Temperature nonuniformities can thus lead to thickness variations that are observable. In addition, byproducts of precursor decomposition, such as hydrogen chloride, can react with components of the glass to cause defects. One such example is the formation of sodium chloride crystals by reaction of chlorine with sodium oxide in float glass. The resulting cubic crystals disappear during subsequent processing steps, leading to pinhole defects in the coating. In this presentation, the factors controlling gas-phase chemistry during the chemical vapor deposition of coatings will be described. These processes will then discussed in the context to the formation of known morphological defects, such as those described above. Theoretical and experimental techniques used to explore this chemistry will be addressed, including quantum-chemistry techniques for predicting thermodynamic data of gas-phase species, transition-state theory for modeling chemical reaction rates, and high-temperature experimental methods used to measure the kinetics of key gas-phase reactions.
2:10 PM C5-1-3 Characterization of Nanostructured Hydrogenated Silicon Nitride Thin Films Produced by Plasma Modulated Chemical Vapour Deposition
E. Bertran, A. Pinyol (Universitat de Barcelona, Spain)
Nanostructured hydrogenated silicon nitride (ns-SiN:H) thin films have been produced by plasma modulated chemical vapour deposition in presence of nanoparticles. This technique consisted of a burst modulation of the RF-power applied to the cathode. The plasma used is obtained from a gas mixture of SiH@sub 4@ and N@sub 2@ or NH@sub 3@. Samples have been deposited on glass for optical measurements and on metallic electrodes in order to carry out the electrical and electro-chemical characterization. We have studied how the deposition parameters (pressure, gas mixture, modulated plasma frequency and duty cycle) affect the properties of the obtained films. The structure of the samples has been characterized using surface analysis and spectroscopy techniques (TEM, XPS, Raman and FTIR). The behaviour of the diffusion of H@super +@ and Li@super +@ ions in the ns-SiN:H, and the effect of the nanometric structure of the sample on electrical transport and dielectrical frequency responses have also been discussed. This work focuses on the potential applications of these films to solid state electrochromism.
2:30 PM C5-1-4 Optical Properties of Thin Films Produced by High Density Plasma Enhanced Chemical Vapor Deposition @footnote 1@
J.B.O. Caughman, D.B. Beach, G.E. Jellison, Jr. (Oak Ridge National Laboratory)
Silicon nitride and tantalum oxide films are being studied for use as waveguides for optical interconnect applications. The films have been deposited using a plasma enhanced CVD process using a high density inductively coupled plasma source. The inductively coupled source operates at 13.56 MHz and couples power to the plasma via a flat spiral coil. A high density plasma (i.e. nitrogen/hydrogen or oxygen) is produced in the ionization region of the source, and a precursor containing the material to be deposited (silane or tantalum metalorganic) is injected downstream. The plasma produces atomic species that interact with the precursor in the gas phase as well as at the surface of the growing film. The deposition rate varies as the plasma coupling makes the transition from predominantly capacitive coupling to inductive coupling, which is related to the amount of the atomic species being produced. Gas phase composition is being determined by in-situ mass spectroscopy and optical emission spectroscopy, and the optical properties of the films are being determined by using spectroscopic ellipsometry. The stoichiometry of the films has been determined by either Rutherford Backscattering or Auger electron analysis. The optical properties of the films can vary with processing conditions, with typical films having a bandgap of 4.0 - 4.4 eV and a refractive index of 1.95 - 2.19. The relationship between atomic species production and film properties will be presented.@FootnoteText@ @footnote 1@ Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725..
3:10 PM C5-1-6 Innovative and Cost-effective Synthesis of Indium Doped Tin Oxide Films Using ESAVD Method
K.L. Choy, R. Chandrasekhar (Imperial College of Science, Technology and Medicine, United Kingdom)
This paper presents the significant progress in the synthesis of optical tin oxide-based thin films using a novel and cost-effective Electrostatic Spray Assisted Vapour Deposition (ESAVD). ESAVD is a variant of CVD process which involves spraying atomised precursor droplets across an electric field where the precursor droplets undergo decomposition and chemical reaction near the vicinity of the heated substrate and deposit a stable solid film. The process can be performed in an open atmosphere without the use of sophisticated reactor and vacuum system. This simple and cost-effective technique can potentially be incorporated for on-line large scale processing application. The ESAVD is used for the synthesis of undoped and indium doped tin oxide films. The structure, stoichiometry, crystallinity, texture and film thickness can be controlled by optimising the deposition conditions. The microstructure, optical transparency and conductivity of the deposited films are characterised and compared with those prepared using sputtering and CVD techniques. .
3:30 PM C5-1-7 The Influence of Film Composition on the Optical and Thermal Properties of Solar Control Coatings
D. Russo, J. Brotzman, C. McKown, C. Roger, J. Stricker (ATOFINA Chemicals, Inc.)
ATOFINA has developed a new type of solar control glazing product, Certincoat@super ®@ SunE@super TM@, that combines the properties of high solar absorption in the near infrared region of the spectrum and low emissivity with neutral reflected color, low haze, and high visible light transmission in a simple two-layer coating based on doped tin oxide. When the dopant is antimony, the tin oxide layer (TOSb) absorbs solar energy; when the dopant is fluorine, the tin oxide (TOF) reflects mid-range infrared radiation. SunE@super TM@ coatings can be deposited under typical atmospheric pressure chemical vapor deposition (APCVD) conditions used by existing manufacturers of low emissivity products utilizing readily available organotin precursors such as monobutyltin trichloride or dimethyltin dichloride. By the use of optical models and lab experimentation, we will show that the solar heat gain, visible transmission and reflected film color depend on the antimony concentration and the layer thicknesses. Glazings with a neutral reflected color can be produced over a wide range of visible transmission (43-71%) and solar heat gain coefficient (0.47-0.60, center of glass) without the incorporation of a traditional iridescence suppressing layer. Film haze due to surface roughness can be reduced significantly by the inclusion or exclusion of certain additives, such as trifluoroacetic acid, during the deposition process which modify the film morphology. Windows in both single and double pane configurations can be fashioned with SunE@super TM@ that meet or exceed the Energy Star window labeling specifications of the U. S. Department of Energy for Southern climate zones.
3:50 PM C5-1-8 Haze Reduction in Solar Control Coated Glass
J. Szanyi, S. Walck, J. Sopko (PPG Industries, Inc., Glass Technology Center)
Residential windows with low solar heat gain coefficients (SHGC) are desirable in warm climates in order to decrease the cost of air conditioning. A double layer CVD SnO@sub 2@-based film stack on clear float glass provides both low SHGC and low emissivity. However, in order to achieve the desired solar performance and aesthetic attributes of the product, the total film thickness must be greater than 5000Ã. Due to the epitaxial growth of the top F:SnO@sub 2@ layer on the bottom Sb:SnO@sub 2@ film, the resultant "thick" film contains large, columnar crystals. This film structure ultimately leads to high surface rugosity, and consequently to unacceptably high overall haze. In order to prevent the epitaxial growth of large F:SnO@sub 2@ crystals on top of the Sb:SnO@sub 2@ layer, and therefore significantly reduce the haze, an amorphous layer was inserted between the two crystalline SnO@sub 2@ films. The presence of this amorphous layer on top of the crystalline Sb:SnO@sub 2@ film forces the top F:SnOS@sub 2@ film to nucleate on the amorphous layer, and grow smaller crystals than it would if the epitaxial growth was allowed. In this paper, the results of the XRD, SEM and TEM investigations on CVD prepared double layer structures will be presented. We will discuss how the structural properties determine the performance, particularly the haze, of the deposited thin film stack.
4:10 PM C5-1-9 Optical Coatings Deposited from an Ultrasonically Generated Aerosol
M. Langlet (Domaine Universitaire, France)
Two deposition techniques, based on the ultrasonic pulverisation of a metalorganic liquid solution, have been developed at INP Grenoble (France) and tailored for the preparation of optical oxide coatings. The ultrasonic pulverisation leads to the formation of an aerosol that is then deposited at the surface of a substrate. Both techniques are low cost deposition processes (no vacuum technology) fully compatible with in-line technologies for high throughput rate production. The first technique (Pyrosol process) is based on the vapour phase pyrolysis of the aerosol at the surface of a heated substrate. In that sense, it can be considered as a true CVD technique. The main advantage of Pyrosol process is that the metalorganic precursors are transported in the form of a thermally stable cold aersosol, which allows to prevent drawbacks due to the warm vapour transport in classical CVD (premature pyrolysis or condensation). The second technique (Aerosol-gel process) is based on the sol-gel transformation of a liquid film deposited at room temperature from the aerosol. The so-formed inorganic film can then be heat-treated at a temperature that depends on the targeted application. Because no vapour phase deposition is involved, Aerosol-gel process cannot be strictly considered as a true CVD technique. Nevertheless, it is a gaseous transport assisted deposition technique that shows many similarities with CVD. The main interest of this technique is that it takes fully advantage of the sol-gel chemistry (low temperature preparation, large range of film compositions, preparation of original materials non accessible through other techniques). In this paper, we describe the preparation of thin films using both techniques for various application fields in optics (magneto-optical recording, ophthalmic optics, integrated active optics).