Experimental study of out-of-plane adhesion force evolution (and regression) for MEMS accelerometers

The work investigates in-operation adhesion evolution in out-of-plane (OOP) test devices fabricated in an industrial surface micromachining process. The aim is to mimic the behavior of OOP MEMS accelerometers in terms of package environment (pressure of ∼ 100 mbar), characteristic frequency and quality factor, so that the contact dynamics during impact events is representative of such devices. The experiments include: (i) statistical measurements of pristine adhesion forces; (ii) application of dynamic impacts at known contact velocity (∼ 1 cm/s) and adhesion evolution monitoring as a function of the elapsed collisions; (iii) adhesion force monitoring within 40 days after the former test ends. With a sub-nN measurement resolution, the results show an adhesion force increase during cycles up to a few hundred nN, followed by a regression (likely due to a slow relaxation mechanism) when the device is left still.