Evolving material structures of small feature sizes

Modern electronic and photonic devices are solid structures of small feature sizes. During fabrication and use, di€usive processes can relocate matter, so that the structures evolve over time. A ®lm may break into droplets, and a conducting line may grow cavities. Stress and electric current have long been understood as forces that drive the changes. Evidence has accumulated that, while important, these forces are insucient to account for diverse experimental phenomena, suggesting forces of other physical origins also operate. In a structure, collective actions of atoms, electrons, and photons contribute to the free energy. When the structure changes its con®guration, the free energy also changes. The free energy change de®nes a thermodynamic force which, in its turn, drives the con®gurational change of the structure. This article illustrates the concepts with speci®c phenomena. Emphasis is placed on physical descriptions of forces of diverse origins, including elasticity, electrostatics, capillarity, electric current, composition gradient, photon dispersion, and electron con®nement. The e€ects of some of these forces are particularly signi®cant in structures of small feature sizes, say, between a few to hundreds of nanometers. Insights into these forces are increasingly valuable as devices miniaturize. This area of research holds great promises for solid mechanics innovation