Contents

Structured Coatings and Nano-Engineered Materials 

TiO₂ as a Reactive Coating

Volume 12 Issue 4
December, 2002

Two different applications and procedures for coatings and materials are discussed in this issue. Demands placed on the coating industry for optical and mechanical (wear-resistant) and also coatings for the semi-conductor industry sometimes require performance that cannot be satisfied by existing coating materials. In this case, coatings must be engineered from known or common materials. Solid materials grown in thin film form fall into two general classifications, depending on application: passive as for the majority of optical and mechanical coatings, and active, as for electro-optical applications. The latter include transparent conductive coatings and photo-active coatings. The brief discussions presented here will introduce the reader to these alternate thin film technologies.

Structured Coatings and Nano-Engineered Materials

For those special cases where a material or a deposition process does not produce all the required properties, materials and their structures can be manufactured to satisfy the need. The concept and practice of tailoring the structure of a thin film to modify its properties is not new. Chemical etching or ion exchange at the surface of glass is a well known technique for obtaining a refractive index low enough to provide some degree of surface reflection reduction. Perfect AR of glass of index 1.52 requires a coating of index 1.23, lower than any solid thin film material. However, chemical etching to introduce a surface layer on glass that is permeated with microscopic air-filled cavities can approach this effective low index. Similarly, leaching heavy ions from the upper surface layer of soda lime glass can also reduce the effective index. While these surface-modification techniques are simpler and less expensive methods for anti-reflecting glass than vacuum deposition, the resultant surfaces have low mechanical and cleaning durability.

Most fluoride compounds and oxide compounds, when grown under conditions of low adatom energy and thus low surface mobility, form a columnar microstructure. Under typical deposition conditions, the substrate is rotated in a two-axis planetary motion to average the variety of vapor-stream incidence angles, thus causing the columns to grow perpendicular to the substrate. Another older concept recently reintroduced for controlling the structure of thin solid films is that of altering the deposition (incidence) angle during film growth, whereby the columns grow at an oblique angle to the substrate and the microstructure of the film layer grows to be inhomogeneous and anisotropic. Shadowing among the growing columns produces an open, porous structure that is generally undesirable because of its low cohesive strength and tendency to adsorb water and other vapors on the column walls, leading to instability of optical properties. The growth conditions that favor a columnar microstructure can be exploited to create "sculptured" films for special purposes. For example, with slow rotation about specific rotation axes, helical structures can result. Without rotation, a tilted linear structure of isolated columns results. Dichroism is observed in oriented anisotropic optical films to make polarizers. Some applications of these anisotropic structures are as film-type two-dimensional polarizers or retarders, surfaces for orienting liquid crystal materials, as chemical agent detectors, micro-channel biosensors, pressure transducers, microelectronic devices. [1, 2] Oriented anisotropic magnetic and piezoelectric films can also be produced.

The ‘microstructure’ of a deposited thin solid film is characterized by its orientation and crystal phase. Columnar layers are composed of micro-crystals predominately randomly assembled and stacked in a vertical column whose width generally tapers from wide at the substrate to narrow at the top due to shadowing by neighbor columns. The micro-structure can be either growth or nucleation controlled, and influenced by substrate temperature and roughness, and adatom energy. In nucleation-controlled growth selection, the micro-structure of the grown layer is determined by that condensing micro-structure that possesses the highest nucleation rate. This rate is influenced by substrate surface state and energy. Specifically, surface contamination can increase the energy barrier to nucleation; surface defect density is directly related to nucleation site density; and surface mobility is related to arrival energy. In growth-controlled microstructure selection, all thermodynamically possible crystalline states have equal nucleation probability, but those that have a higher growth rate will propagate and predominate in spite of a mixed initial interlayer of competing phases. This type of microstructure often exhibits inhomogeneity in its optical properties (n & k) with thickness. Other undesirable features of crystalline micro-structure films is their higher stress and lower packing density and high internal surface (grain boundary) area leading to environmental and mechanical instability.

Some oxide compounds are known to exhibit inhomogeneous optical properties because of the particular micro-structural growth selection as described above. Notable among these are ZrO₂, TiO₂, ITO, CeO₂, and other refractory oxides. One method of reducing or eliminating the problem is to introduce ‘impurities’ that serve to destroy single or large-size crystal growth and thus produce a structure with no long-range order. Such a structure is essentially amorphous, and exhibits a more compact, dense micro-structure. Often glass-forming materials such as silica, MgO or yttria are added. Past issues of CMN have described such mixed or composite materials, which are available either as solid solution preparations or produced by co-evaporation / co-sputtering.

A different technique for producing growth-controlled micro-structures is the process of depositing nanolaminates. In such a structure, many alternating nanometer-thickness layers of two materials are deposited under conditions that control the growth nature of each layer. Individual layer thicknesses range from ~2 nm to ~20 nm. This engineered thin film structure is useful for creating nucleation, transition, or buffer layers between materials of otherwise incompatible chemical, physical or mechanical properties (TCE, or lattice mismatch being two examples). Nanolaminate architectures composed of rf-excited reactive sputter-deposited TiO₂ and ZrO₂ layers were investigated in recent research [3]. The thermodynamic evolution of ZrO₂ polymorphs crystallizing from a melt follows the trend: cubic (2360° –2680° C) to tetragonal (~1075°- 2360° C) to monoclinic (<1075° C). The question explored was how the microstructure of ZrO₂ evolves as a function of the relative ZrO₂ / TiO₂ layer thicknesses and the total number of layers. When the thicknesses were < 4nm / 1.5 nm respectively, no crystal order was present for a bilayer count up to 68. Only with ZrO₂ thickness increased to ~11 nm and 25 bilayers was cubic ZrO₂ detected. The single c-ZrO₂ crystallite spanned several bilayers. Increasing the TiO2 layer thickness causes other crystal morphologies to be suppressed. Thus, it is possible to engineer and control the properties of the nanolaminate.

Thin film biosensors are used for the rapid immunoassay detection of infectious diseases and viruses in the physician’s office or hospital. The principle of operation is the appearance of a color change when a specific biologic agent is present. The detector is a substrate coated with an interference layer that reflects a particular color that then shifts when an antigen is reacted with a specific enzyme. The reaction causes the precipitation of a layer whose optical thickness optically interferes with the thin film coating to cause the color shift. Figure 1 shows a simplified diagram illustrating the composition of a thin-film bioassay sensor [4]. The silicon substrate is coated with an anti-reflecting layer of silicon nitride or Diamond Like Carbon (DLC) tuned in combination with the biological attachment polymer and antibody capture layers to anti-reflect a portion of the visible spectrum. As the sample and reagent react, an optically transparent layer is formed that causes selective reflection of a color indicative of the thickness and thus the concentration of the biological agent. DLC is useful because of its efficient biological capture ability. Other metals and their oxides are used as required for different target assays. The detection limits of optical biosensors range from ng/ml to pg/ml.

TiO₂ as a Reactive Coating

References

 *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 marketing@cerac.com or fax them to 414-289-9804. We also encourage contributions from other writers. Contact the CERAC marketing department via e-mail for more details on submitting an article.

(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information)

Editor:
Russ DeLong
CERAC, inc.
P.O. Box 1178 | Milwaukee, WI 53201
Phone: 414-289-9800 | FAX: 414-289-9805
e-mail: marketing@cerac.com

Principal Contributor:
Samuel Pellicori
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922
e-mail: pellopt@silcom.com

Phone:  414-289-9800 /  FAX: 414-289-9805  /   info@cerac.com