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

[+] Author Affiliations
Jeahyeong Han, Daniel Joe, Rich I. Masel, Mark A. Shannon

University of Illinois - Urbana-Champaign, Urbana, IL

Paper No. DETC2008-49842, pp. 543-545; 3 pages
doi:10.1115/DETC2008-49842
From:
  • ASME 2008 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference
  • Volume 4: 20th International Conference on Design Theory and Methodology; Second International Conference on Micro- and Nanosystems
  • Brooklyn, New York, USA, August 3–6, 2008
  • Conference Sponsors: Design Engineering Division and Computers in Engineering Division
  • ISBN: 978-0-7918-4328-4 | eISBN: 0-7918-3831-5
  • Copyright © 2008 by ASME

abstract

The purpose of this paper is to show AFM verification of adhesion reduction between valve seat/membrane interfaces by surface coatings from a C4 F8 /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 C4 F8 /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 CFn 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 CFn 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 CFn 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, CFn /PI, CFn /SiO2 , and CFn /CFn . 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 CFn 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 CFn film with respect to de-ionized water is 116.4 ± 0.9°. The surface coatings from a C4 F8 /Ar plasma in an ICP DRIE can reduce the contact adhesion forces and capillary forces during the fabrication process preventing stiction.

Copyright © 2008 by ASME

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