A ‘nano-battering ram’ for measuring surface forces: obtaining force–distance curves and sidewall stiction data with a MEMS device

Abstract

MEMS (micro-electromechanical systems) devices often suffer from stiction induced reliability problems. Although a theoretical framework to describe stiction in MEMS is under development in the scientific community, experimental data are still scarce. We have developed a MEMS device that can be used to electronically measure sidewall surface forces in situ, i.e. on chip. To be able to use it, we have developed a sensitive electronic readout system that detects comb drive capacitance variations with 10 aF resolution. In addition, we use a model to correct the measured capacitance changes for the ground-plane-induced levitation effect of the comb drive. Our first results for the interaction between oxidized polycrystalline silicon sidewall surfaces show that at a relative humidity (RH) of 45% and a temperature of 27 °C the stiction force amounts to 6.7 ± 1.6 kPa, which we assume to be mainly caused by capillary condensation. This stiction force was found to be independent of both the normal force with which the surfaces were pressed against each other prior to the stiction measurement and the age of the contact, showing that the contact is fully elastic, and capillary condensation is instantaneous on the time scale of the measurement (milliseconds to seconds). We also use the device to measure complete force–distance curves, so that these results can be compared with AFM (atomic force microscope) measurements.