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 from 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 (temperature, relative humidity, contact area, surface energies of contact materials, surface roughness and shape of contact surfaces) - so-called adhesion parameters should be taken into account when investigating the reliability of MEMS devices with respect to adhesion-induced failures. After, the temperature influence on adhesion is experimentally analyzed considering commonly used MEMS materials 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. First, the adhesion between AFM tip and the materials under interest is investigated using the spectroscopy in point of an atomic force microscope. 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 from 0°C to 80°C. 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 (Ra) of investigated polysilicon samples is varied in the range- of ~5-15 nm. The adhesion force between Si₃N₄ and these surfaces increases from 0.94nN to 12.72nN if the roughness decreases from 12nm to 7nm. In order to compare the influence of cantilever type of adhesion testing was done on several samples of the gold using two kinds of cantilevers. As the roughness decreases, the adhesion force increases, respectively. 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. This difference is based on the surface energy of materials. After material testing, flexible MEMS component used in RF switches is investigated in order to determine the adhesion force between him and the substrate. 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, respectively. The adhesion between microcantilever and substrate is obtained based on experimental tests of 11.675μN.