New lab-on-chip fracture nanomechanical testing device for extracting properties of thin film materials : silicon nitride

The materials science field had known a lot of progress in the recent years and had gained a lot of interest and more attention are now given to this field in many research laboratories of universities and industries. Thin films have been used in many applications. Integrated circuits (IC) industry is the main field of the use of thin films as well as their development, since the microelectronic industry has been continuously downscaled to keep pace with the Moore’s law for making electronic systems more flexible, faster and less energy consuming. Thus, the thin films are presented as the best candidates with their minuscule dimension and their outstanding electrical properties. In MicroElectroMechanical Systems (MEMS), make use of thin films has a significant impact on their reliability, which can lead to deleterious consequences on the MEMS performance and their proper operation. Thin films can be used as sensors and/or actuators. Moreover, they can be used in packaging solutions and surface coatings to protect, improve or to add some determined properties to bulk materials. The manufacturing process of thin films can be classified into two major categories: Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD) techniques. The deposition technique has a crucial influence on the films structure and composition. Hence, it can be seen as the most important factor that is responsible of the properties of these films. Good use of these materials will definitely require a better understanding of their mechanical behavior. Especially as they show different mechanical properties from bulk materials that can have an impact on the mechanisms that manage fracture behaviour and deformation. Thus, the tests that have been used in the past will not necessarily be useful or/and can be applied to these nanostructures. Throughout the years, many mechanical testing techniques have been developed to extract mechanical properties of thin films, each one of those methods have its own drawbacks and advantages. The testing methods can be divided into two major groups. The first group based on the fabrication of the specimens and the test structure on the same chip, this technique is called on- chip, it has the advantage of avoiding the alignment and the handling of the samples. Unlikely, the first category, this second technique needs to manipulate the samples to put them on the external testing structure. However, in-situ observations can be realized based on this category of testing apparatus. In this master thesis, a new testing apparatus is developed, this test platform based on lab-on-chip technology. From its name, it is obvious that this technique belongs to the on-chip category of testing structures. The lab-on-chip concept, also called internal-stress-driven mechanical loading was developed during the last previous few years, around ten years in UCL by J.-P. Raskin and T. Pardoen. The principle of the lab-on-chip relies on relaxation of the internal stress present in the “actuator” usually made of a thin layer of silicon nitride which, in turn, apply a tensile test to the “specimen”. The new design consists of two actuators, one in the left and the second is in the right side of the specimen beam. The latter incorporates a patterned notch by means of e-beam lithography. After release of the structure ’two actuators +specimen’, the actuators contract pulling on the specimen. In order to initiate a crack propagation starting from the notch tip, the crack will stop after the energy strain rate reaches its critical value. This structure aims to determine the fracture behavior of the samples (here silicon nitride), the design of these devices is inspired from the test used for macroscale materials, which applies a uni-axial tensile to a notched sample with some differences, not just in dimensions also in the type of the forces applied (intern stress of the actuator made of silicon nitride), geometry as well as the fabrication process. The ultimate goal of this new device is to extract the fracture toughness value of any type of materials, at the moment we will focus in this work on the silicon nitride. In the first place, the feasibility and proper operation of the device should be demonstrated. The design and geometry must be optimized. To do so, we first will use finite element analysis, which will help us to determine the best geometry (actuator length, crack length, curvature radius, thickness…) that leads to propagate the crack in the way to facilitate the measurements of the displacement experimentally. Furthermore, the FE simulations are performed for a wide range of crack lengths and other parameters to establish an equation of the stress intensity factor K as a function of the design parameters. This formula can then be used to determine the critical value of the stress intensity factor K_Icand energy release rate G_Ic based on the critical crack length. The latter value will be determined experimentally after the fabrication of these structures at WINFAB , UCL’s cleaning-room.Master de spécialisation en nanotechnologie, Université catholique de Louvain, 201