Reviewing Advances in Coating Materials and Processes Coatings

Volume 15 / Issue 4 December, 2005

Reviewing Advances in Coating Coatings on Materials and Processes Polymer Optics

From the premier issue of Coating Professional meetings, symposiums, Glass and metal surfaces present few Materials News 15 years ago, we have and publications report many of the ad- coating problems, but the unique chemi- attempted to review and summarize for vances, but it is safe to speculate that cal and physical properties of polymers our readers current advances in our proprietary and trade secret develop- present a number of coating chal- technology. The growing application ments in private industry might equal lenges. Polymer optics are continually of optical coatings and the higher de- or exceed the volume of publicly dis- replacing glass optics in a number of mands placed on them by tributed information. Coating applications: critical (medical, scientific, state-of-the-art developments have en- practitioners, while motivated to stay military) as well as commercial. The list couraged pace-keeping development of current with the state of their art, are of polymeric materials used in the opti- coating materials and their deposition busy people and must often be satis- cal industry includes: polycarbonate technologies. Emerging industries pro- fied to accept summaries of the (PC), polymethyl methacrylate ducing display screens, digital image advances in progress. One goal of (PMMA), polyethylene-terephthalate projection systems, energy conserva- CMN is to acquaint our readers with (PET), acrylic (AC), polyether sulphone tion in automobile and architectural new developments. (PES) and polyetherimide. The oph- windows, and others have placed chal- thalmic industry is a large supplier of lenging demands on the thin-film Progress in Coating AR-coated lenses [3], the computer and coating industry. We can add to that entertainment industries consume large list highly specialized precision filters Materials Science volumes of metallic coatings on poly- for telecom, commonplace AR coatings A major accomplishment in materials carbonate for DVD and CD recording, on eyeglasses, anti-counterfeiting coat- engineering is the demonstration that and architectural thermal-control coatings on currency, and colored pre-melted forms of many oxide com- ings are made of coated plastic PET decorative finishes on household ap- pounds provide processing films laminated into the window. Rea- pliances. Medical and military advantages over tablets and piece- sons for the increasing use of polymer requirements continue to push the re- forms, without sacrificing optical AC and PES optics include the ease of quirements envelope. performance. The elimination of the molding high volumes of precision need to process multiple melts and vent- lenses, lower coat and weight, and sim- ing to build a solid, non-spitting, plicity of mounting. Display panels are non-outgassing evaporation source often made using PES, PET or PC coated saves valuable chamber time and as- with a sputter-deposited AR design that sures better repeatability of coating includes a transparent conductive layer CERAC offers optical coating properties [1]. of ITO. The ITO layer can act as an RFI materials in a variety of forms shield or touch contact circuit. The re- suitable for evaporation or Mixtures of oxide and fluoride com- fractive indices of polymers used in sputtering, including our pounds continue to provide optical components range from ~1.4 to time-saving pre-melted evaporants. advantages over “pure” materials, and 1.64 (polyetherimide), and surfaces re- in some cases, produce improvements quire anti-reflection coatings in many Contact us at [email protected] or approximating high-energy deposition devices. Polymer components are also call 414-289-9800 for more techniques. Specifically, densely used as substrates for mirrors and col- information! packed, amorphous growth from mixed ored parts, the latter requiring composites produces film layers with multi-layers of dielectric coating.

continued on page 2 continued on page 3 3 appear and grow into sparsely populated columns with resultant large void volume between them. Limited mobility and line-of-sight adatom arrival contribute to the growth of the self-shadowing co- lumnar open microstructure that pure oxide compounds often ex- hibit. The relationship between energy and growth microstructure was illustrated in Figure 1 of CMN V13, Issue 3 (Sept. 2004). The presence of foreign species admixed with the host oxide interferes with crystalline growth and encourages denser microstructure. Ion as- sisted deposition (IAD) is a process enhancement that produces dense, hard, adherent films, and has been incorporated in commercial coating production for at least 10 years. Energies in IAD ap- Advances in Coating Materials from page 1 proach hundreds of eV and a high ratio high environmental stability and im- from the use of specific mixed materials of energetic ion-to-species can be proved optical mechanical and chemical deposited by e-beam; sputter targets achieved with high current densities. durability. It has been established that are available for some materials (only The transfer of kinetic energy through- stability of crystalline phase of many transparent materials are included). out the growing structure results in the refractory oxides can be achieved over Only high- and medium-index materials dissipation of singular growth struc- a high temperature range by adding a are listed because the choice for the low tures, thus encouraging dense, random few wt-% of another oxide compound index is mostly limited to silica, which microstructure. Varieties of IAD equip- such as yttria to form a solid solution. in pure form or doped with alumina ment are available to accomplish the While co-evaporation of two oxide grows with a dense glass form. More same basic function: to provide added compounds from separate evaporation discussion of the materials listed can energy for increasing layer packing den- sources is useful in establishing the be found in the reference column. sity through increased surface energy optimum mixing ratio, and can be used and momentum transfer. in production, pre-mixed material prepa- Progress in Deposition rations are more convenient. Films Sputter deposition is a high-energy deposited from the mixture possess Techniques process that also results in hard, dense greater hardness, higher homogeneous E-beam and sputter deposition are the film layers. Many variations in sputter refractive indices, smoother surfaces, basic production processes used in the techniques have been developed; the and lower stress than the pure host ma- industry. The refractory oxide com- primary differences in technique are terial as a result of the uniform and pounds and their mixtures require the adapted to sputtering metals vs dielec- stable crystalline state and higher pack- high thermal energy of an electron beam tric compounds. The process pressure ing density [2]. Optimum optical to reach their evaporation temperatures, in sputtering is higher than for E-beam properties such as highest refractive often 2000° C. Alternatively, sputter evaporation, as a result the growth is indices with low absorption and scat- processes produce sufficiently high en- not line-of-sight, and a more random ter and high resistance to water ergy to move material from the target to structure grows that exhibits relatively absorption were demonstrated with condense on the substrate. Some of the high density. By varying sputter pa-

CeO2 - SiO2 composite films [2]. material mixtures mentioned are avail- rameters, large or small crystals can be able in sputter-target form. grown, and stress can be controlled. Mixtures designed for e-beam evapo- Energies are generally lower than can ration for tribological or E-beam evaporated species have low be achieved by IAD. Sputtering is eas- high-temperature exposure coating are energies (<10 eV) upon impingement at ily adapted to high volume, large area, proving useful in optical applications. the substrate surface, therefore the ki- automated production. We have dis-

Many of these are based on ZrO2 or netic energy available for high mobility cussed IAD and sputter deposition Y2O3, materials that are inherently hard on the surface is marginal. Free mobil- extensively in previous issues of CMN. in pure form, as the host material. Table ity insures dense surface coverage in I lists some applications that benefit two dimensions before isolated islands 2 Coatings on Polymer Optics from page 1 Several properties peculiar to polymer plasma-surface interactions that clean References surfaces require different consider- and create chemical bond energy to ations and techniques to achieve stable, which an oxide or metal can strongly 1. CMN V14, Issue 2 June 2004. adherent optical coatings [4]. The high adhere (surface activation). The 2. Won Hoe Koo, Soon Moon Leong, water absorption (up to 0.45%), low tem- plasma can be created in an oxygen or Sang Hun Choi, Hong Koo Balk, perature limit (<125° C), high thermal nitrogen atmosphere, as required for Se Jong Lee, and Sung Man Lee, J. expansion compared with the coating the polymeric materials involved. Vac. Sci. Technol. A 22(5), 2048 materials (~2X), non-reactive surfaces, More recently, plasma-enhanced CVD (2004). and low hardness are the more impor- has been applied to polymer coating 3. CMN: V7, Issue 4 (1997), V8, Issue tant issues that must be accommodated. because the process permits control 3 (1998), and V10, Issue 4 (2000). When we attempt to apply typical coat- of the plasma-surface interactions re- 4. Ludvik Martinu and Jolanta E. ing techniques to plastics, failures sulting in stronger bonds, and because Klemberg-Sapieha, “Optical Coat- appear in the forms of: high stress and can it be used to generate graded-in- ings on Plastics”, in Optical water content leading to poor coating dex films and thus rugate filters [4] Interference Coatings, N. Kaiser adhesion and short-term physical sta- employed on visors as a countermea- and H. K Pulker (Eds), Springer bility, surface figure deformation from sure to laser blinding. Energetic N or 2003. excessive process temperatures, and O ion plasmas and UV exposure modify process instabilities from out gassing the polymer surface and create a of water and additive / plasticizer com- denser and harder interphase layer <0.5 ponents. mm thick. Present in this interlayer are reactive cross-linked and functional CERAC Coating Materials News The coating deposition process and the groups that can chemically bond to is a quarterly publication of materials used must be engineered to oxide and metal depositions. The CERAC, inc. accommodate the diffusion of water and PECVD process is more efficient at A subsidiary of Williams Advanced Materials Inc. the thermal properties associated with producing these active groups than P.O.Box 1178 polymers. Either can result in adhesion classical PVD because of the dense Milwaukee, WI 53201-1178 loss: the first by weakening the bond plasma and higher energies involved. Phone: 414-289-9800 between polymer and coating inter- FAX: 414-289-9805 faces; the latter by the presence of stress The most popular coating materials for web: www.cerac.com e-mail: [email protected] due to thermal coefficient mismatch. polymers such as PC are TiO2 (index = Chemical hardcoats are applied to the 2.2) and SiO2 (n = 1.45). This couple Editor: surfaces of eyeglass lenses and data adheres well and can satisfy most de- David Sanchez disks, and their characteristics also be- signs for AR and filter coatings. Sr. Materials & Applications Scientist come part of the coating process. Often Ophthalmic coaters also use mixed Advanced Technologies Group CERAC, inc. this treatment reduces the chemical and high- and low-index materials based on physical mismatch presented by the bare titania and silica that were developed Principal Contributor: polymer surface. Sputter deposition is for lower stress and low temperature Samuel Pellicori used to deposit low-e window coatings deposition. As new applications for Pellicori Optical Consulting for thermal control. Such coatings con- polymeric materials appear, new and P.O. Box 60723 Santa Barbara, CA 93160 sist of thin silver between dielectrics and improved coating material combina- Phone/FAX: 805-682-1922 often ITO to reflect the IR. Large area tions will be developed for them. It is e-mail: [email protected] roll-to-roll coaters produce millions of clear that the solution for stable and sq. meters annually. E-beam systems durable coatings on plastics requires For a free subscription to CMN, please produce the majority of ophthalmic coat- close interaction between the chemis- E-mail your name and address to [email protected] or send us a fax ings; when augmented with IAD, try of the polymer and the entire at 414-289-9805. substrate temperatures can be kept low. coating process. Both techniques employ energetic ions Guest articles or topic suggestions are and are low temperature processes. welcome. Questions and comments can be e-mailed to [email protected] Proper surface preparation is necessary or faxed to 414-289-9805. to obtain adherent and stable coatings. An electronic version of this publication Some techniques were discussed ear- can be accessed from the Technical lier in CMN [3]. For many polymers, a Publications page of the CERAC web ~20 Å deposit of Cr provides a good site at www.cerac.com. From there, link to the CMN Archives to view back nucleation and adhesion precursor. The issues. Cr metal converts to CrOx which actively binds to the following TiO2 layer. More ©Copyright 2006, CERAC, inc. vigorous pre-treatments consist of 3 PRSRT STD U. S. POSTAGE PAID MILWAUKEE, WI P.O.Box 1178 PERMIT NO. Milwaukee, WI 53201-1178 2418 USA

CERAC will be exhibiting at the following upcoming trade shows: - Photonics West Jan. 24-26, 2006 San Jose, CA Booth# 2016 - Society of Vacuum Coaters (SVC) April 24-25, 2006 Washington, DC