Size Effect in Cleavage Cracking in Polycrystalline Thin Films

The reliability of polycrystalline thin films is essential to assuring safe performance of micro/nano-electromechanical systems. Usually, they are of through-thickness grain structures and are brittle at working temperatures, and therefore their fracture properties are dominated by the resistances offered by grain boundaries to cleavage cracking [1,2]. As a cleavage crack front propagates across a high-angle grain boundary, it would first penetrate across a number of break-through points [3], and the persistent grain boundary areas would then be separated through shear fracture or ligament bending [4,5]. It is, therefore, envisioned that as the film thickness is smaller than the characteristic distance between the break-through points, which is often in the range of 0.5-5 microns, the crack front transmission can be significantly confined by the film surfaces, leading to an either beneficial or detrimental size effect. That is, the fracture toughness of the polycrystalline thin film is not a material constant; rather, it highly depends on the film thickness. Since this important phenomenon has not received the necessary attention, we propose to carry out a systematic study on fracture resistances of bicrystal silicon films. The film thickness will range from 1 to 1000 microns, and the crystallographic orientations across the grain boundaries will be controlled precisely so that the size effect and the geometrical factors can be analyzed separately. The study will starts with thick films. Once the crack front transmission process is relatively well understood, it will be extended to thin films. This project will shed light on crack-boundary interactions in confining microenvironments, which has both great scientific interest and immense technological importance to the development of fine-structured devices