A. Safi

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.

MEMS-based microstructures for nanomechanical characterization of thin films

The measurement of mechanical properties of thin films is a major issue for the design of reliable microelectronic devices, microsensors or thin coatings. New simple microstructures actuated through the release of internally stressed long beams made of high temperature, low pressure chemical vapour deposition silicon nitride have been developed to test under uniaxial tension submicron thin film material specimens. The relative displacement between a fixed and a moving cursor is used to determine the strain applied to the specimen. The stress is inferred based on the mismatch strain and Youngs modulus of the silicon nitride actuator beam. By multiplying the tensile test microstructures with different lengths, the full stress-strain curve characterizing the thin material sample is generated from which the elastic stiffness, yield strength, ductility and fracture stress can be extracted. The potential of the method is demonstrated through applications on both brittle and ductile thin films. The Youngs modulus of 238 GPa for a 373 nm thick silicon nitride film is extracted and size effects are observed for the yield strength of pure aluminium with a value of 220 and 550 MPa, respectively, for 373 and 205 nm thick films. An original variant of the procedure based on this new test microstructure for measuring Youngs modulus is also presented.

Ductility of thin metallic films

Depending on the loading conditions, geometry and material characteristics, the ductility of thin metallic films is controlled either by the resistance to plastic localization or by the resistance to internal damage. New on-chip tensile tests performed on submicron aluminium films show significant strain hardening capacity leading to relatively good resistance to necking, while damage occurs through void nucleation at grain boundaries followed by their growth and coalescence. These results are discussed in the light of several other studies presented in the recent literature in order to unravel the origins of the frequently reported poor ductility of thin metallic films, and the various means existing to improve it.

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.

TEM study of the NiTi shape memory thin film

Recently, NiTi thin films have received a growing interest owing to the large stress induced transformation strain and the high work density involved in that transformation