Some slimline tips to help you with the buckle on your coat

Thin Film Materials
March 4, 2005

Anyone familiar with Subra Suresh's masterly text, Fatigue of Materials (1992), will be eager to read Thin Film Materials, written with L. B. Freund.

The cover picture, taken by Gio Medeiros-Ribeiro at Hewlett-Packard Laboratories, shows a surface of silicon on which germanium has been deposited, imaged with atomic resolution in a scanning tunnelling microscope. Initially the germanium forms a relatively uniform layer, albeit with an increasing concentration of surface defects as the thickness increases. When the film is a small number of atoms thick, the build-up of elastic strain induces the growth to become three dimensional, forming nano-islands in the sea of the wetting layer. The picture shows how different kinds of islands, with a distribution of sizes, can coexist on a surface. It is a splendid way to illustrate the richness of behaviour that can be induced by mechanical effects in thin-film materials.

"The goal of the book is to summarise development in the area of thin film materials that have occurred over the past few decades, with emphasis on the generation of internal stress and its consequences," the preface tells us. The motivation for this is that thin-film technology has made enormous strides in that period, with applications in thermal barrier coatings, tribology, electronic integrated circuits and micro-electro-mechanical systems (Mems) for sensors and actuators.

The start is somewhat unpromising. Perhaps it is necessary to begin with "A classification of thin film configurations", but surely there are more enticing ways to introduce the subject? The overall layout of the sequence of the material is fairly logical and progressive, moving from stress in isotropic films to stress in anisotropic and patterned films; to delamination and fracture; and buckling, bulging and peeling, on to dislocation formation in epitaxial systems and the role of dislocations in strain relaxation; and finally equilibrium and stability of surfaces and the role of stress in mass transport.

The book's ending is duller than the beginning. The final sentence reads:

"The standard unit of measure to characterise interconnect reliability is the FIT, where one FIT equals one failure in 109 device hours." Hardly a line to inspire interest in thin films.

The subject is undoubtedly important and of wide interest. Have the two authors together succeeded in producing a text as definitive as Suresh's earlier book? Well, almost. This book has some of the hallmarks of the master, but there are dips, too. The equations for the stress field at the tip of the crack are wrong; there is a change of Cartesian coordinates from the earlier book, in a way that I find idiosyncratic, and the sxx component is given incorrectly. In places it reads as though each author had a go at writing about a topic, and either it was too much trouble to reconcile the two accounts or perhaps they did not notice the discrepancies.

A fascinating controversy towards the end of the last century surrounded observations in Stranski-Krastinov growth of germanium on silicon. It is from such studies that the cover picture by Medeiros-Ribeiro is taken. On the West Coast of the US, championed by Hewlett-Packard, an equilibrium explanation was given, while on the East Coast, championed by IBM, a kinetic account was favoured. To some extent this reflected a difference in experimental approaches. This subject is treated twice in the book, under the headings "Observations of island formation" and "Formation and interaction of islands". It is the writers' job, not the reader's, to elucidate the relationship between the two sections.

Provided the book is read with critical judgement, Thin Film Materials will prove a valuable resource. It contains a wealth of useful references and good indexes. It is richly illustrated, and there are good exercises after each chapter. For a graduate course in the field, it will be hard to beat.

And if the authors are right, there will be a growing demand for such courses.

Andrew Briggs is professor of nanomaterials, Oxford University.

Thin Film Materials

Author - L. B. Freund and S. Suresh
Publisher - Cambridge University Press
Pages - 750
Price - £45.00
ISBN - 0 521 82281 5

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