Optical Thin Film Technology: Mechanical Properties

1 3 Volume 17 / Issue 3 September, 2007continued on page 2 Optical Thin Film Technology: Mechanical PropertiesIntroductionFunctional Optical coatings are used inapplications such as reflection reduc-tion (Anti-Reflection), color separation(image projectors, cameras), spectralbandpass filtering (medical, environ-mental diagnosis and monitoring), highreflection (mirror), and other specializedapplications. These coatings are com-posed of thin layers of alternating high-and low-index materials, and their spec-tral operating ranges are determined bythe Optical Properties , refractive indexand absorption, of their composite lay-ers. The physical and chemical naturesof the thin-film layers as they are con-densed and grow into solid films aredetermined by the deposition technology for film depositionneeds to consider the Mechanical andenvironmental conditions under whichthe micrometer-thin coatings must sur-vive and function.

2 Common examplesof coatings that require high-durabil-ity are eye glasses, automobilewindscreens, aircraft canopies, andother windows deployed in harsh en-vironments that include high-velocityrain and sand erosion. The durabilityof coatings to various abrasive andchemical agents is determined to somedegree by the chemical composition ofthe layers, but primarily by their cohe-sive and adhesive strengths and hard-ness, which are functions of the depo-sition process. Durability to mechani-cal forces is influenced by the natureof the microstructural growth of the lay-ers during their condensation on thesubstrate surface. That microstructure,in turn, determines the magnitude andsign of the residual stress built into thethin-film multi-layer / substrate V14, Issue 3 (Sept 2004) pre-sented a structure zone model of filmgrowth as affected by deposition CompensationTechniques for Oxide-Compound CoatingsIn this section, we expand on previousdiscussions and introduce techniquesfor achieving a balance or neutraliza-tion between tensive and compressivestresses.

3 Background discussionsabout stress in coatings can be foundin previous CMN issues: V15, Issue 2(June 2005), which discusses the originsof intrinsic stress in film layers, and V16,Issue 2 (June 2006) which is concernedwith development of stress during thegrowth review, with tensive stresses theforces lie in the plane of the film / sub-strate, and left unchecked can causethin substrates to bend from planar toconcave shape or upon release of thetensive stress, cause the film to con-tract into mud flat cracks. Fluoride-compounds exhibit tensive stress, whileSilicon dioxide (SiO2) layers exhibit com-pressive stress with characteristic buck-ling due to expansive forces parallel tothe substrate.

4 In contrast, high-indexoxide-compound films, especially sput-tered refractory compounds, generallyexhibit tensive stress Properties . Thinsubstrates obtain a concave in the latter category includeZrO2, TiO2, HfO2, etc. But Nb2O5, Ta2O5and Al2O3 can be in either stress statedepending on whether they are depos-ited by reactive evaporation or by dioxide is paired with one of theabove high-index materials in opticalcoatings that function from UV wave-lengths to >2 mm in the IR. The intentis to compensate the opposing forcesand achieve a low- or zero-stress coat-ing. Intrinsic stress forces accumulatewith thickness, while extrinsic stressesincrease with thermal expansion differ-ences between the substrate and thecoating.

5 Intrinsic stresses are gener-ated by the deposition process and thegrowth microstructure; these param-eters are intimately related as illus-trated in the structure zone model re-ferred to above. High deposition en-ergy produces a densely packedsmall-scale microstructure that is alsohighly stressed. While films that growin the environment of high-energy bom-bardment such as IAD and ion platingare physically hard and exhibit no wa-ter-induced Optical changes, such filmscan exhibit adhesion failure, microcrack-caused scatter, brittleness, orsubstrate bending due to more subtle effect of coatingstress is the generation of stress-in-2duced birefringence. This is a problemfor phase-sensitive Optical compo-nents involved in interferometry, pola-rimetry, ultra-fast laser optics, and verynarrow bandwidth filters such as usedin telecom.

6 The effects of coating-re-lated stress can be detected by view-ing between crossed polarizers. Col-ored fringes will be seen if the optic ismechanically distorted or the coatingitself causes phase has evolved to minimizestress and obtain and preserve the de-sirable Mechanical and Optical proper-ties in multi-layer oxide-compound coat-ings. Some approaches are:1. Selection of the appropriate deposi-tion process (reactive e-beam, IAD,plasma assisted, ion plating, sputter-ing, etc),2. Modification of process parameters(IAD energy, rate, deposition atmo-sphere and pressure),3. Design of alternative material com-positions (admixtures, pre-process-ing),4. Introducing stress compensation bydesign between layers and substrate,and5.

7 Post-deposition are several ways toward ap-proach 3. One is using designed ma-terials based on the admixture of for-eign materials with a host, a techniquethat we have discussed extensively inthese issues, see for example CMN V17,Issue 1, March 2007 and older discus-sions of CERAC IRXTM. In anothermethod, co-evaporation or co-sputter-ing of high- and low-index materials isemployed to compose a gradient be-tween the two limits [1]. Alternatinglayers deposited lack a discrete inter-face and thus interface stress is alsograded. An index gradient is also es-tablished. We have discussed tech-niques 1-3 previously; in this issue weemphasize the 4th and 5th a typical multi-layer design consist-ing of quarter-wave Optical thicknesses,the low-index layers (SiO2) are physi-cally thicker than the TiO2 high-indexlayers.

8 The stack will have net compres-sive stress if it is sputter deposited. Theability to substitute Nb2O5 for the high-index layer material can provide an im-mediate solution since sputtered niobiais tensive. Alternatively, annealing theSiO2 / TiO2 coating combination canincrease the tensive stress and movethe net stress toward a lower or possi-bly neutral value. Post-deposition in airat temperatures of 300-400 C for 2-4hours is a typical annealing actual time and temperature valuesto be used depend on the substratematerial s thermal expansion coefficient( glass vs. fused silica) and on thespecific high-index material. Compen-sation can be achieved in some casesfrom the excess extrinsic tensive stresscreated in a coating that is depositedon a fused silica substrate when it iscooled to room temperature due to thelow TCE of that substrate.

9 Other ef-fects of high-temperature post-deposi-tion annealing are the removal of waterfrom film pores and crystallization oflayers, both events cause physical vol-ume shrinkage and cause higher tensivestress. The growth of large micro-crys-talline grain sizes common for the high-index materials can be thwarted in sput-ter deposition by interposing very thinlayers of silica within thick high-n lay-ers. In addition to increasing the pack-ing density of the layer, Optical scatteris reduced [2]. These benefits are alsoachieved by the use of mixed materialevaporated by E-beam or sputtered [1].Crystallization changes generally re-quire post-deposition temperatures>500 C, and TCE differences can causeadhesion failure before the benefits ofcrystal size changes can be CMNs have discussed the high-temperature transitions between ana-tase and rutile phases for titania, andphase changes in refractory materialssuch as zirconia that lead to inhomoge-neous growth and refractive indexvariation with GrowthMechanicsFilm growth microstructure is deter-mined by the energetics of the arrivingadatoms and the substrate surface con-dition onto which they condense toform a solid film.

10 When the kinetic en-ergy of the adatoms is low, they tend toinitiate growth at randomly distributed,low-energy, point defect sites ratherthan grow with uniform area coverageas they would when surface mobility ishigh. Point defects can be residual pol-ishing imperfection or micro-particulateejection from the start-up of evapora-tion or sputtering. To understand therole of the surface energy, we refer againto the structure zone model. Island3 CERAC Coating Materials Newsis a quarterly publication ofCERAC, subsidiary of Williams AdvancedMaterials 1178 Milwaukee, WI 53201-1178 Phone: 414-289-9800 FAX: 414-289-9805web: : SanchezSr. Materials & Applications ScientistCERAC, Contributor:Samuel PellicoriPellicori Optical Box 60723 Santa Barbara, CA 93160 Phone/FAX: 805-682-1922e-mail: a free subscription to CMN, pleaseE-mail your name and address or sendus a fax at 414-289-9805.