Analysis of the adhesion effect in RF-MEMS switches using atomic force microscope

The main scope of this paper is to investigate the adhesion effect on the mechanical response of MEMS components for RF switch application by using an atomic force microscope (AFM). The occurrence and strength of adhesion effects in MEMS devices depend on a variety of environmental, geometrical and operational parameters. The temperature influence on adhesion is experimentally analyzed considering MEMS materials commonly used to fabricate RF switches. The adhesion force is the main factor with influence on stiction. In fact, stiction is the adhesion occurring between the contact surfaces due to surface forces including van der Waals, capillary forces, Casmir forces, and electrostatic forces. The adhesion between AFM tip and the materials under interest is investigated using the spectroscopy in point mode of an AFM. Different coated AFM tip is considered for tests. The AFM spectroscopy-in-point curve gives the direct measurement of tip-sample interaction force as a function of the gap between the tip and sample. The adhesion between tip and sample is characterized by so-called pull-off or pull-out force. The pull-off force is related in current continuum contact mechanics model to the work of adhesion. Using a thermal controller stage the temperature of investigated materials is changed. The mechanical properties variation of MEMS materials as a function of temperature is monitored and the temperature influence on adhesion is analyzed. The influence of the surface roughness on the adhesion forces between AFM tip and samples is determined. Different roughness (R-a) of investigated polysilicon samples is varied in the range of similar to 5 to 15 nm. A high value of adhesion force is determined for the contact between gold AFM tip and gold sample comparatively with the contact between Si3N4 AFM tip and gold material. The contact areas between an AFM tip and substrate is an important parameter for analyzing the adhesion effect. The results from laboratory indicate that the adhesive force increases if the surface roughness decreases because the contact area increases. The adhesion between gold microcantilever and substrate obtained from experimental tests (the AFM spectroscopy-in-point mode) results is obtained. During tests, the AFM tip from Si3N4 coated with gold is applied at the beam free-end and deflects the sample to substrate.