STM/SEM method for testing bending strength of MEMS beams

The observed fracture strength of materials when used at NEMS and MEMS scales are generally higher than the bulk properties, particularly as the size decreases to nanometers. Such high strength offers the possibility of designing devices that sustain very high stresses, for example tensioned oscillators, or energy storage components. To study the fracture of micron scale cantilevers, we propose a new test method that makes use of an ultra high vacuum STM/SEM combination and digital image processing. The experiment consists of using a slightly blunted STM tip to load a cantilever beam. While deflecting the cantilever, a sequence of digital images of the deformed beam is acquired with the SEM, from the first load up to the point of fracture. The elastic modulus of the beam is determined before the fracture test in a separate experiment by use of resonant frequency measurement. The sequence of SEM images is processed to determine the fracture strength by first creating a (mathematical) model cantilever of the same size and cross section as the tested cantilever. Using large deflection beam theory, the deflected shapes of this imaginary cantilever are obtained for increasing loads. Synthetic images of the deformed model cantilever are formed by mapping onto the computer screen using projection angles and magnifications determined from a separate calibration test. The final step is to compare the synthetic images to the digital ones captured by the SEM. The applied load on the model beams is iterated to obtain the best fit with the processed SEM images. Once the load that causes the beam to fracture is known, the fracture strength can be easily determined. To demonstrate the procedures we present initial results for untreated silicon cantilevers as well as for samples heat treated in vacuum by applying resistance heating prior to testing.