Evolving material structures of small feature sizes

Modern electronic and photonic devices are solid structures of small featur e sizes. During fabrication and use, diffusive processes can relocate matte r, so that the structures evolve over time. A him may break into droplets, and a conducting line may grow cavities. Stress and electric current have l ong been understood as forces that drive the changes. Evidence has accumula ted that, while important, these forces are insufficient to account for div erse experimental phenomena, suggesting forces of other physical origins al so operate. In a structure, collective actions of atoms, electrons, and pho tons contribute to the free energy. When the structure changes its configur ation, the free energy also changes. The free energy change defines a therm odynamic force which, in its turn, drives the configurational change of the structure. This article illustrates the concepts with specific phenomena. Emphasis is placed on physical descriptions of forces of diverse origins, i ncluding elasticity, electrostatics, capillarity, electric current, composi tion gradient, photon dispersion, and electron confinement. The effects of some of these forces are particularly significant in structures of small fe ature sizes, say, between a few to hundreds of nanometers. Insights into th ese forces are increasingly valuable as devices miniaturize. This area of r esearch holds great promises for solid mechanics innovation. (C) 1999 Elsev ier Science Ltd. All rights reserved.