AFM Verification of CF_n Surface Treatment Effect and Its Correlation to Stiction Reduction in Microvalves

The purpose of this paper is to show AFM verification of adhesion reduction between valve seat/membrane interfaces by surface coatings from a C₄F₈/Ar plasma in an ICP DRIE. Our check valves utilize a polyimide (PI, Polyimide 5878G, HD Microsystem) membrane on a Si/SiO₂ valve seat. These valves form a seal between a polished Si/SiO₂ substrate and a smooth polymer membrane. PI absorbs moisture up to 3.4% wt per volume, and the SiO₂ surface also has an affinity to water. The smooth PI membrane touches the SiO₂ surface, giving rise to relatively strong van der Waals adhesion. Under humid conditions, hydrogen-bonded stiction can occur at the interface between the PI and SiO₂ during the drying step. The C₄F₈/Ar plasma coating is utilized for the actual device in order to lower the interface adhesion between Si/SiO₂ and PI film. The opening pressures of devices with/without CF_n film are measured. The valves without non-stiction coating did not open with inlet pressures up to 210 KPa. With a non-stiction coating, the valves showed an initial opening pressure of 32.5±11 KPa. AFM pull-off measurements using nano-sized tips and micro sized tips are performed to quantify the effect of the CF_n film-treated surface between solid-solid surface pairs. The original surface pair for the microvalve membrane and seat surface is Si/SiO₂ and PI film. The CF_n film treatment is possible on one or both sides of the surfaces. AFM pull-off testing has been performed to measure the work of adhesion between four possible surface combinations, including SiO₂/PI, CF_n/PI, CF_n/SiO₂, and CF_n/CF_n. The work of adhesion of the surface pairs is obtained using the Johnson-Kendall-Roberts (JKR) theory. Two types of AFM probes were used, a regular nano-sized AFM probe and a one micron particle AFM probe. The work of adhesions obtained for the pairs above are 257.6±37.1, 59.4±29.2, 89.6±18.2, and 41.0±8.2 [mJ/m²] from regular tips, and 159.48±4.0, 41.9±2.0, 65.7±12.2, and 37.4±3.7 [mJ/m²] from the particle tips. The CF_n film treatment reduced the adhesion energy up to 84% for the regular AFM tip results, and up to 76.7% from the particle AFM tip results. The static contact angle of CF_n film with respect to de-ionized water is 116.4 ± 0.9°. The surface coatings from a C₄F₈/Ar plasma in an ICP DRIE can reduce the contact adhesion forces and capillary forces during the fabrication process preventing stiction.