Experimental Study of In-Plane and Out-of-Plane Adhesions in Microelectromechanical Systems

Two different but similar microdevices were used for studying in-plane (IP) and out-of-plane (OP) adhesions in microelectromechanical systems (MEMS). With these devices, the combined influence of temperature, surface chemistry, contact geometry, and applied load on the adhesion force was investigated. The adhesion force is the maximum force required to separate two contacting surfaces. Distinguished between IP and OP contacts, the adhesion mechanisms for hydrophilic and hydrophobic surfaces were identified. The microdevices, fabricated within the same wafer, may exhibit different IP and OP adhesion mechanisms, dependent upon the wafer surface chemistry. For the hydrophilic surface and IP contact, the dominant adhesion mechanism is hydrogen bonding, while for OP contact, the electrostatic force is dominant. The dominant adhesion mechanism for the hydrophobic surface is the Van-der-Waals force for both IP and OP contacts. Furthermore, the influence of elastic-plastic deformation of interacting asperities on the adhesion force is addressed by experiments at different loads. At constant applied load in a cycling contact-release test, a decreasing and asymptotic behavior of the adhesion force is observed, suggesting strain hardening and reverse plastic deformation of the contacting asperities.