Sébastien Gravier

New On-Chip Nanomechanical Testing Laboratory - Applications to Aluminum and Polysilicon Thin Films

The measurement of the mechanical properties of materials with submicrometer dimensions is extremely challenging, from the preparation and manipulation of specimens to the application of small loads and extraction of accurate stresses and strains. A new on-chip nanomechanical testing concept has been developed in order to measure the mechanical properties of submicrometer freestanding thin films allowing various loading configurations and specimen geometries. The basic idea is to use internal stress present in one film to provide the actuation for deforming another film attached to the first film on one side and to the substrate on the other side. The measurement of the displacement resulting from the release of both films gives access to the stress and the strain applied to the test specimen provided the Youngs modulus and mismatch strain of the actuator film are known. Classical microelectromechanical-systems-based microfabrication procedures are used to pattern the test structures and release the films from the substrate. The design procedures, data reduction scheme, and a generic fabrication strategy are described in details and implemented in order to build a suite of test structures with various combinations of dimensions. These structures allow the characterization of different materials and mechanical properties and enable high throughputs of data while avoiding any electrical signal or external actuation. Results obtained on ductile aluminum and brittle polysilicon films demonstrate the potential of the method to determine the Youngs modulus, yield stress or fracture stress, fracture strain, and strain hardening in ductile materials.

Serrated plastic flow behavior and microstructure in a Zr-based bulk metallic glass processed by surface mechanical attrition treatment

The serrated plastic flow, microstructure and residual stress of a Zr55 Cu30 Ni5 Al10 bulk metallic glass (BMG) undergone surface mechanical attrition treatment (SMAT) have been investigated by a combination of compression tests with scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and the incremental hole-drilling strain-gage method. It is found that SMAT leads to various microstructural modifications and residual stress distribution in the surface layers of the Zr-based BMG due to the mechanically-induced nanocrystallization and generation of shear bands. As a result, the BMG alloy exhibits a remarkable work-hardening like behavior and significant increase of plastic strain from less than 1% to 15%, and its plastic deformation dynamics yields a power-law distribution of shear avalanches. Based upon the analysis of the experimental results, it is indicated that this can be connected to the SMAT-induced microstructural modifications and the resulting residual compressive stress in the Zr-based BMG.

A new on chip nanomechanical testing concept applied to ductile and brittle thin film materials

The measurement of the mechanical properties of submicron sized specimens is extremely challenging due to difficulties for manipulating samples, for applying small load and for extracting accurate stresses and strains. A novel, versatile concept of micromachines has been developed to measure the mechanical response, up to large strains and fracture, of films, beams or tubes, with dimensions ranging between 10 to 1000 nm, allowing multiple loading configurations and geometries. This new concept relies on microfabrication techniques destined to the production of microelectronic devices and MEMS. The first idea is to use internal stresses to actuate and impose the load to the tested sample which is directly deposited on a patterned substrate. The second idea is to multiply elementary machines rather than to build on single complex multi-purpose micromachine, see Fig. 1. The stress and strain are directly inferred from a local displacement measurement. Other loading configurations than simple tension have been designed such as shear, biaxial, compression and double cantilever opening.