Mechanisms of Stress and Structure Evolution During Processing of Polycrystalline Thin Films

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

TECHNICAL SUMMARY Polycrystalline films are used in a wide range of micro- and nano-scale devices and systems in which their stress state and grain structure profoundly affect their performance, properties and reliability. Stress and structure evolution during film formation and subsequent processing are known to be strongly coupled. In-situ measurements of stress evolution have revealed a complex range of phenomenology, including evolution between tensile and compressive stress states, and apparently reversible stress changes during growth interruptions. Both ex-situ and in-situ characterization of grain structures and surface topography have also revealed complex processes that include grain growth and texture evolution during and after island coalescence, network formation during island coalescence, retention of deep trenches at grain boundaries, and changes of surface topography during and after film island coalescence. This phenomenology is not accounted for in current understandings of the linkage between stress and structure evolution. In this program, stress evolution will be studied during deposition of a range of materials under a range of conditions. Use of different materials and deposition temperatures will allow observations of behavior that are intermediate to those that have been the focus of prior studies. Use of materials with higher melting temperatures will also allow quenching of surface and grain structures for ex-situ characterization. Surface topography will be monitored during deposition, and during interruptions of depositions, using light scattering techniques. The angle of incidence of the atomic deposition flux will also be varied in order to controllably promote different levels of surface roughness. The size and spacing of the initial islands from which films are formed will also be varied using templated dewetting techniques. Through these studies, understandings will be developed that will allow application-specific engineering of structures and properties of polycrystalline thin films and the micro/nanostructures patterned from them. NON-TECHNICAL SUMMARY The electrical devices and integrated circuits that power our cell phones, computers, and the Internet are created using very thin layers of metals and semiconductors that are formed on flat surfaces of materials like silicon. Extremely small patterns are then made in these films to create millions of devices that are connected to make circuits of devices. Sometimes the devices have parts that physically move as they work. Layers, or films, of different materials are usually made by essentially spraying atoms onto a flat surface. Once the atoms arrive on the surface, they move around to form small crystals and these crystals grow until they run into each other to form a continuous film. The way these crystals form and grow can vary tremendously, depending on how the atoms are sprayed and on which atoms are used. These variations strongly affect the properties of a film, including how easily electrons can move through them or how easily the moving parts can be moved. This makes it very difficult to make complex integrated circuits and limits what can be made. This research program focuses on understanding why these variations occur and on how they can be controlled, so that new devices and circuits can be made. To do this, the researchers will change the way the atoms are sprayed and measure how the properties and structure of films change, while they are being formed.