Bulge test and AFM point deflection method, two technics for the mechanical characterisation of very low stiffness freestanding films

Abstract

International audienceThe aim of this work is to compare several methods for the determination of very thin films Young's modulus and stress state: the nanoindentation test, the bulge test and the point-deflection method. The tested structures were silicon nitride and silicon nitride/silicon oxide bilayer membranes with different shapes (square or rectangular) and dimensions (from 1 mm to 3 mm). We report new experimental results on submicron thick dielectric membranes with thicknesses down to 100 nm. A Young's modulus of 217 14 GPa have been found for silicon nitride membranes with a residual stress of 411 30 MPa using the bulge test. Using nanoindentation experiments, a Young's modulus higher than 190 GPa has been estimated. The bulge test is still valid for the studied high dimension to thickness ratio membranes and more appropriate to determine the Young's modulus. A mixture law was shown to be possibly applied for SiN/SiO bilayer membranes for the Young's modulus and stress determination. The point deflection method is limited by the very low stiffness of these structures and only the residual stress can be accurately extracted. As the Young's modulus and membrane geometry have no significant influence on the stress determination by means of the point deflection method for the studied membranes (with a high lateral dimension to thickness ratio), more reliable results have been obtained such as 487 40 MPa using an AFM cantilever for load-deflection experiments, for SiN thin films

Polymer Carpets

“… Bulge tests with freestanding polymer carpets yield their Young's modulus. a) The applied pressure as function of the carpet deflection (squares) follows the calculated dependency of thin membranes under tension38 (line) and allows the extraction of the Young's modulus. The inset shows a scheme of the measurement setup.…”

“…Figure 8b shows a clear relation between the Young's modulus and the thickness of the polymer‐carpet layer. It must be pointed out that a polymer carpet is a composite membrane consisting of a crosslinked biphenyl monolayer with a soft polymer‐brush layer and that the measured elastic modulus is a thickness‐weighted average between the two compounds 38. Since the Young's modulus of a single 1‐nm‐thick cABT nanosheet was measured to be 8 GPa, the Young's modulus of the PS component can be calculated (red circles in Figure 8b).…”

Mechanical characterization of carbon nanomembranes from self-assembled monolayers

“…Both methods have a resolution in the range of hundreds of nanometers. Atomic force microscopy (AFM) has also been used for indentation studies on soft [ 14 ] as well as stiff [ 15 ] membranes. In addition, it was recently reported that the curvature of a bulged membrane was determined by AFM, while its deflection was measured with a laser sensor [ 16 ].…”

A microfluidic dosimetry cell to irradiate solutions with poorly penetrating radiations: a step towards online dosimetry for synchrotron beamlines

“…in the hard X-ray domain), one can use thicker membranes to limit the bulging and reach the electronic equilibrium in the medium which ensures a proper dosimetric measurement (Carlsson, 1985). For example, using 3 mm-thick Si 3 N 4 windows will limit the deformation to 1/20th of the one observed here (Martins et al, 2009), i.e. to less than 350 nm, while allowing to work with X-rays down to 10 keV [their secondary electrons have ranges of about 1.5 mm in Si 3 N 4 (Berger et al, 2005)].…”