Recently, copper thin films have many application fields in micro-electro-mechanical systems (MEMS) such as RF MEMS switch, inductor coils, etc. Many electrical and thermal properties of copper thin films for MEMS applications have been well known, but the characterization techniques on mechanical properties of copper thin films have not been standardized yet. But in order to design and fabricate the MEMS device for working actuating functions, it is very important to know the mechanical properties of structural thin films, especially elastic modulus, Poisson’s ratio, yield strength, tensile strength, fatigue and residual stress. Among these properties, MEMS devices are usually operating in elastic range, the elastic modulus and yield strength are more crucial properties. In order to evaluate these mechanical properties, many micro-mechanical test techniques have been developed in last decades. Recently, nanoindentation test and micro-tensile test are the most widely accepted test technique for reliability, however the analysis technique needs to be improved. In this study, micro-mechanical properties of copper thin films were characterized by the nanoindentation, micro-cantilever beam bending and micro-tensile test techniques and their reliabilities were discussed.
The fabrication process of specimens, available for micro-mechanical tests, is as following. The MEMS fabrication process which are composed of electroplating, lithography and silicon bulk micromachining technology was used to make the copper cantilever microbeams and guided dog-bone specimen with film thickness ranged from 3㎛ to 12㎛.
The load-displacement curves of copper films were obtained by nanoindentation test using CSM methods. From these curves, elastic modulus and hardness were calculated using Snedden stiffness equation and Oliver-Pharr methods as a function of film thickness. As a result, elastic modulus and hardness of copper films are increased with decreasing the thickness of films. By microcantilever beam bending test, the load-beam deflection curves of copper thin films were deformed. The beams were mechanically deflected by a Nanoindenter, and the elastic modulus and the yield strength of copper thin films were analyzed using the pole figure test by XRD techniques. Measured elastic modulus and yield strength are decreased with increasing the thickness of copper films. The measured mechanical properties of copper films are compared to those obtained by indenting the films supported by their substrates. Load-displacement curve were also obtained by micro-tensile test and the ultimate tensile strength and elastic modulus of copper films as a function of film thickness were calculated from this curve. As film thickness of copper films were increased, ultimate tensile strength was reduced but elongation was increased. Elastic modulus of copper thin films has been calculated from cyclic loading and unloading process, similar to fatigue test.
In order to analyze the variation of elastic modulus of copper films as a function of thickness, pole-figure test using XRD techniques was performed. The copper films have preferred crystallographic orientation, so called texture. Therefore we could convert the texture of copper films with Euler angle to ODF(Orientation distribution function) and using the Voigt model elastic modulus of copper films as a function of film thickness could be predicted. By applying these methods, the theoretical estimation of elastic modulus shows a good agreement with the measured one of the copper films by nanoindentation techniques.
