Mechanical characterization of ultrahard and ultrathin films by laser acoustics
The mechanical properties are often the main topic, but at least a necessary precondition in the application of thin films. The indentation methods with monitoring of load and penetrating depth yield local stress-strain curves. Devices are now available with depth resolution down to the manometer range. They are very valuable tools for mechanical characterization for many film systems. But the indentation methods have some intrinsic limitations. Apart from such problems as uncertainties of the tip shape and influence of surface roughness, there are two classes of thin films, which are especially critical: ultra-hard and ultra-thin films. For ultra-hard films (with Vickers hardness HV > 50 GPa) the deformabilities of indenter and film become comparable. For ultra-thin films (with thickness below 100 nm) the plastic zone induced even by very shallow indentations is usually not restricted on the film material but reaches into the underlying substrate. Thus the measured values represent a certain average of the film-substrate composite, where the film properties cannot extracted from in a reliable manner due to the high complexity and non-linearity of the indentation test. The film characterization by Young's modulus determined by surface ultrasonic waves helps to overcome these problems. The laser-acoustic method has proved to be very suitable as reliable, quick and robust technique for testing thin films. The surface acoustic wave method is based on linear-elastic phenomena. Hence, even for deformation fields reaching deep into the substrate, the film properties may be deciphered by strong mathematics. Furthermore, high film hardness is not limiting. (There is only another unavoidable restriction: the attenuation of the substrate material for the high frequency ultrasonic surface waves must be sufficient low.) The measuring method and some representative results are given in chapter 3 and 4, respectively. But at first the basic question must be discussed, if the Young's modulus is suitable for mechanical characterization at all.