Nanoscale Deformation Measurements to Improve Reliability Assessment of Sensors and MEMS

Micro machined micro sensors for gas or flow detection based on physical behaviour of a special layer of a membrane have to fulfil high quality and reliability requirements especially in safety or security applications. Up to now, most of the research studies neglected mechanical issues related to reliability of these structures. In this sense, the study and characterization of the stress distribution on the membranes after fabrication and during their operative life is required. Thin films used in micromachined structures exhibit residual mechanical stress strongly dependent on the layer composition and the deposition process parameters. Often, a deposition of a multilayer is required, and this adds factors like abrupt transitions in thermal, elastic and plastic mismatch across the interfaces that have a direct effect on the resultant stress. Moreover, in operating conditions, a thermal stress originated due to the difference in the thermal expansion coefficient (CTE) of the membrane materials adds to the residual stress of the membrane. The resultant stresses can induce excessive deformation, fracture, delamination and microstructural changes in the material that can lead to the breaking of the structure during the fabrication stage or affect the behaviour of the final device. In the present work, the 2D deformation of a gas and a flow sensor membrane under different thermo-mechanical load states will be analysed by means of the digital image correlation (DIC) techniques based on scanning probe microscopy (SPM) data. With this technique which is introduced as the nanoDAC method (nano Deformation Analysis by Correlation) deformation fields can be determined with nanometer-accuracy. In addition ion milling by focused ion beam (FIB) technique is demonstrated at membrane specimens with residual stresses. Object deformations fields nearby the milling area are measured by fibDAC allowing the evaluation of very local residual stresses. Some principal experiments illustrate the feasibility of the chosen approach