Sculptured Thin Films: Nanoengineered Morphology and Optics

Sculptured Thin Films: Nanoengineered Morphology and Optics
Author(s):    Akhlesh Lakhtakia; Russell Messier
Published:   2005
DOI:             10.1117/3.585322
eISBN: 9780819480415  |  Print ISBN13: 9780819456069  |  Print ISBN10: 0819456063
Description:

Sculptured thin films (STFs) are a class of nanoengineered materials with properties that can be designed and realized in a controllable manner using physical vapor deposition. This text, presented as a course at the SPIE Optical Science and Technology Symposium, couples detailed knowledge of thin-film morphology with the optical response characteristics of STF devices. An accompanying CD contains Mathematica™ programs for use with the presented formalisms. Thus, readers will learn to design and engineer STF materials and devices for future applications, particularly with optical applications. Graduate students in optics and practicing optical engineers will find the text valuable, as well as those interested in emerging nanotechnologies for optical devices.

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A bird's eye view of research on sculptured thin films is presented in this chapter. Its scope is wider than the combined scopes of the remaining chapters, but the discussion is, of course, shallower. We recommend that this chapter be read before any other in this book.

Sculptured thin films (STFs) are nanostructured inorganic materials with anisotropic and unidirectionally varying properties that can be designed and realized in a controllable manner using physical vapor deposition (PVD). This overview chapter traces the development of STFs from their precursors, with emphasis on two seminal events that occurred in 1959 and 1966, as well as on the sculptured columnar morphology of these materials. The defining continuum electromagnetic constitutive relations of STFs are presented, along with a control model to link the nanostructure of STFs to their continuum properties via a local homogenization formalism, for optical purposes. Finally, a tour of accomplished and emerging applications is taken.

1.1 Introduction

In its decadal survey entitled Physics in a New Era conducted during the 1990s, the U.S. National Research Council (NRC) explored research trends and requirements in the materials sciences [1]. A dominant theme that emerged is of nanosciences and nanotechnologies. The nanoscale is Janusian: matter at the 10- to 100-nm length scale exhibits continuum characteristics, but molecules and their clusters of small size can still display their individuality. For that reason, the U.S. National Science Foundation (NSF) has begun to focus on material morphologies and architectures with at least one dimension smaller than 100 nm in its research initiatives.

Among the nanoengineered materials identified by the NRC are STFs [1, p. 123]. These nanostructured inorganic materials with anisotropic and unidirectionally varying properties can be designed and fabricated in a controllable manner [2–5] using PVD, a century-old technique. The ability to virtually instantaneously change the growth direction of their columnar morphology, through simple variations in the direction of the incident vapor flux, leads to a wide spectrum of columnar forms. These forms can be

(i) two-dimensional, ranging from the simple slanted columns and chevrons to the more complex C- and S-shaped morphologies [6]; or

(ii) three-dimensional, including simple helixes and superhelixes [7].

A few examples of STFs are presented in Figs. 1.1 and 1.2, and a representative list of materials deposited as STFs is provided in Table 1.1.

For most optical applications envisioned, the column diameter and the column separation normal to the thickness direction of any STF should be constant.

© 2005 Society of Photo-Optical Instrumentation Engineers

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