Coating Materials News Vol 10 Issue 2
CMN Takes a Close-up Look at Diverse Reader Interests
In continuing our celebration of CMN’s 10th year of publication, we address in this issue some of the topics most frequently requested by our readers. To illustrate the diversity of interests and applications of our readers, we counted more than 75 separate topics for which information was requested from CERAC through the CMN forum. The most popular topics are, in order of interest:
In this issue, we will cover topics 1 and 2.
Deposition Parameters Recommended for Common Optical Materials
The visible and near-IR spectral ranges are conveniently covered by a set of common materials. Coatings for the UV range below ~400 nm and for the Mid-IR range above ~1000 nm rely on chemically different materials to provide low optical absorption. Metal oxide compounds are the most commonly used materials for the Vis-Near-IR range to produce easily deposited, high performance, and stable thin film layers. Fluoride compounds can be included for the lowest possible refractive index layers when compatible with other materials. Many metal oxides require electron-beam evaporation, while some can be brought to evaporation temperature by resistance heating. Most require reactive conditions to achieve complete oxidation and low optical absorption. This is especially important for high energy laser applications where absorption can lead to catastrophic failure due to heating. Fluorides generally are evaporated by resistance heating, but e-beam can be used successfully at low power.
A large variety of oxide and fluoride compounds have been investigated since the beginning of evaporated coatings in the search for the best materials and combinations, but in the end only a handful are routinely used in production. We concentrate our discussion on this group as being the most useful. Optical coatings are constructed of multi-layers of alternating high- and low-index thin films. For most applications, the higher the ratio of high - to - low index, the better the performance for the lowest layer count. The lowest visible-range index values, near 1.4-1.5, are provided by only two stable and durable materials, silicon dioxide and magnesium fluoride. A larger choice exists of the high index materials, i.e., n > 2. A small number of materials provide intermediate indices ~1.6 -1.8 for use in special designs.
Table 1. Recommended Deposition Parameters for Coating Materials Used in the Visible Spectrum.
The table provides the essential parameters for the evaporation and deposition of these most commonly used optical thin film materials in pure form. More detailed discussion is contained in CMN V5, Issue 4 (1995) and V4, Issue 2 (1994)*. We have discussed in these issues how material properties can be significantly improved in many cases by the addition of a second material as a small weight percentage. The mechanical (density, adhesion) and sometimes optical (low absorption) properties of the oxide compounds can also be improved with additional energy of deposition as provided by ion assisted deposition (IAD), ion plating, activated reactive evaporation using an oxygen plasma, and with other techniques. With the exception of SiO and the fluoride compounds, materials in the table can be sputtered from appropriate targets to produce dense adherent layers, but at lower deposition rates than by e-beam. Titania and MgF2 are discussed in previous issues, for example, CMN V8, Issue 3 (1998). The material list grows slowly because the best candidate compounds in terms of stable film production, ease of deposition, etc. have been tried repeatedly with little improvement. However, progress is made as more experience is gained with improved compositions such as mixtures or with advanced deposition techniques. One example is niobia, Nb2O5, which is proven to grow as very hard, scratch resistant layers by sputter deposition.
Transparent conducting films find application in display screens, touch panel switches, LCD displays, thermal control windows, RF shielding, electrostatic charge prevention, etc. While other compositions have been explored, the transparent conductor material in thin film form commonly applied is tin-doped indium oxide. ITO has a 50 year history in aircraft windows as a defroster, and recently is being applied to automobile windows to control the thermal environment. As a coating for architectural windows, it is used to improve environmental energy efficiency in winter and summer because it permits light to be transmitted but reflects wavelengths near 10 µm, the peak for 300 K blackbody radiation. Reflectance is near 50% at 2 µm and 90% at 10 µm. Therefore it reflects external heat in the summer months and retains internal heating during winter months ("heat mirror"). ITO layers can be patterned by selective etching or lithographic techniques. CMN V4, Issue 4 (1993) discussed ITO deposition parameters, and some articles have been included since then.
There are several techniques for depositing ITO, including plasma enhanced chemical vapor deposition, CVD, spray pyrolysis, etc. CVD techniques require high substrate temperatures, often damaging in themselves. ITO can also be deposited by reactive e-beam evaporation, but the best success for repeatability and consistency in optical and electrical properties is obtained by sputtering. Because sputter processes can be closely controlled, they are repeatable; this is the virtue of the sputter process in general. Sputtering is a low temperature process and therefore is used for coating polymer films in roll-to-roll web coating systems to make films intended for lamination to windows and display panels.
The reason that information on the topic of ITO deposition is so frequently sought is because its electrical and optical properties are critically process dependent, and this is the source for inconsistency from run to run. The successful deposition of a transparent conductor involves more degrees of freedom (parameters) than optical films. Therefore, developing a repeatable deposition process for ITO is much more difficult than for other materials, optical or electrical. To give the reader a sense of the interest in developing transparent conductive coating processes, we note that more than 300 papers have been published on the subject since ~1980. This is indicative of the problems and solutions attempted to perfect ITO deposition. Ideally the goal is to have a highly transparent layer with very low resistivity. Desirable values are >85% visible transmission average and <10 ohms="" q.="" for="" a="" layer=""><1000 å="" thick.="" in="" fact,="" the="" optical="" and="" electrical="" properties="" are="" inversely="" related,="" so="" attempts="" to="" produce="" a="" highly="" transparent="" film="" (visible="" region)="" result="" in="" high="" resistivity,="" and="" vice="" versa.="" so="" a="" compromise="" in="" performance="" must="" be="" made="" for="" the="" intended="">
Another consideration often overlooked is the composition of the substrate. Borosilicate glass substrates produce lower sheet resistances than glasses with a high content of sodium that can diffuse into the film and influence the conductivity.
The deposition parameters for PVD processes that require close control are: oxygen partial pressure, rate of deposition, and substrate temperature. Electron-beam and resistance -heated evaporation require relatively high oxygen pressures, 5 to 10 x 10-5 Torr, a slow rate of 2 Å/s, and a substrate temperature >275 °C. Often post-baking in air is required to reduce optical absorption and to reduce sheet resistance. In the case of sputter deposition (RF or DC magnetron), the operative parameters are: reactive plasma gas composition (oxygen-to-argon ratio), relative flow rates, power density, bias voltage, and plasma current. Some of these deposition parameters are interrelated, reducing the number of variables to a manageable quantity. Reactive sputtering of metal targets can produce higher deposition rates. The target can be an alloy of 9-10% Sn and 91-90% In metals, in which case the rate of oxidation buildup on the target surface becomes another factor to consider since the deposition rate will now decrease with target surface oxidation. Alternatively, and preferably, the target can be a sintered composite of the oxides of these metals. The composition of the target is is 9-10% Sn. Better reproducibility of film stoichiometry has been reported with the oxide targets than with metal targets. The oxide target must have high density, ~90%, for best results. CMN V4, Issue 4 and references therein provide in-depth discussions of the influences and control of the variables. One set of parameters used in RF magnetron sputtering onto 50 ºC substrates included: 4.5 x 10-3 Torr Ar pressure and 200 W power and resulted in a resistivity of 1 x 10-4 ohm-cm (10 ohm /sq.) for a layer 1000 Å thick.
Work has been done with fluorine- and antimony- doped tin oxide, Al- doped ZnO, Cd2SnO4, and others, often with good success in achieving low sheet resistances (<5 ohm/sq.).="" the="" most="" popular="" material,="" however,="" remains="">
*Newsletters beginning with Vol. 6 are available from the CMN archives page of this web site. Contact CERAC for printed copies of referenced newsletters prior to Vol. 6.
If you have a question or a topic you would like us to consider for a future issue of CMN, e-mail your requests to firstname.lastname@example.org or fax them to 414-289-9804.
(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information)
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