Young's modulus and residual stress state of freestanding thin membranes are characterized in this work by means of wafer level experimental techniques. RF MEMS Switches manufactured by Raytheon Systems Co. are investigated using a new method that combines a Membrane Deflection Experiment (MDE) and numerical simulations. It is found that the thin aluminum alloy membranes used in the RF MEMS devices have a Young's modulus of 70±10 GPa in the plane of the membrane, and a residual tensile stress of 4±1 MPa. The accuracy of the identified parameters is confirmed by sensitivity studies to geometric aspects of the specimens and loads. It is found that changes in initial residual stress affect the load-deflection curves at small values of deflection. By contrast, variations in Young's modulus result in changes in load-deflection curvature at large displacements. These features are very important to decouple both effects in the process of identification of the parameters.