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

¹ Université de Lyon, INSA-Lyon, INL, CNRS UMR 5270, 69621 Villeurbanne, France
² FEMTO-ST, LMARC, CNRS UMR 6174, 25000 Besançon, France
³ AUXITROL SA, Esterline Sensors Group, 18941 Bourges Cedex 9, France

Corresponding author: patrick.delobelle@univ-fcomte.fr

Received: 11 June 2008
Revised: 24 September 2008
Accepted: 2 October 2008
Published online: 14 January 2009

Abstract

The 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 Si₃N₄/SiO₂ 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 Si₃N₄ thin films.