ABSTRACT
Properties of thin lms and tiny structures, with typical sizes in the range of a few microns or below, cannot just be guessed from the properties of bulk samples. This is due to two main effects: (1) materials for bulk mechanical testing usually have sizes that are much larger than the microstructural features, like grains or particles, while in thin lms the geometrical and microstructural sizes are usually on the same order of scale. (2) Mechanical behavior is restricted by certain fundamental length scales. Some examples for length scale effects on mechanical properties are shown in Figure 4.1. Elastic properties are recognized by the atomic bonds, with lengths in the range of 1 Å. Plasticity in metals consists of dislocation movements, which are delayed when they try to pass among barriers more closely spaced than about 100 nm. Through fatigue in metals, complex dislocations structures are formed, with typical sizes of a few micrometers. In brittle materials, fracture is started at defects with a critical dimension of several tens of micrometers. As a result, it is predictable that the mechanical properties of a material will basically change as the sample sizes become smaller than these various intrinsic lengths.1-14
Figure 4.1 also shows the usual geometrical sizes of materials used in different technological purposes. In particular, it can be noted that microelectromechanical systems (MEMS) and microelectronics fall precisely in the range where fundamental changes in material properties are expected to happen. It is then essential to measure the mechanical properties at a length scale similar to the feature dimensions used in these technological applications. As a consequence, several specialized testing methods have been developed, mainly in the last 15 years, to study mechanical properties in small sizes. The aim of mechanical testing is to characterize properties such as Young’s modulus, yield, fracture strength, creep, and fatigue resistance. In some cases, the basics of macroscopic testing can be shifted directly to small dimensions, as for instance in microtensile or nanoindentation testing.
