Behaviors of time-dependent and time-independent adhesion forces revealed using an AFM under different humidities and measuremental protocols

Adhesion forces between a tipless cantilever and a silicon wafer were measured on an atomic force microscope (AFM) at two relative humidities (RH). Experimental results show that the behaviors of the adhesion force are largely influenced by the RH and measurement protocols. The capillary contribution to the measured adhesion force is time-dependent at low RH (31±1%) and time-independent at high RH (52±1%) due to different equilibrium times of water bridges. The time-dependent adhesion force increases logarithmically with contact time until saturation is reached. The effects of loading force, approach velocity, retraction velocity and measurement number on the adhesion force can be explained by the logarithmic contact time dependence of the adhesion force. However, the time-independent adhesion force decreases with dwell time, and increases with retraction velocity. No dependence of the adhesion force on loading force and approach velocity is found. The adhesion force by consecutively measurements on the same location shows fluctuations, and the trend is unpredictable.