While the behaviour of large amounts of liquid is dictated by gravity, surface forces become dominant at small scales. They have for example the remarkable ability to make droplets stick to their substrates (even if they are inclined), which is a practical issue in many cases (windshields, window panes, greenhouses, or microfluidic devices). Here we describe how this problem can be overcome with super-hydrophobic materials. These materials are often developed thanks to micro-textures, which decorate a solid surface, and we describe the way such textures modify the wettability of that solid. We conclude by showing the unusual dynamics of drops in a super-hydrophobic situation.

https://iopscience.iop.org/article/10.1088/0034-4885/68/11/R01/pdf

Non-sticking drops

Starting from the background of nanofluidics in other disciplines, this paper describes the present state of research in this field and discusses possible directions of development. Emphasis is put on the very diverse background of nanofluidics in biology, chemistry, physics and engineering and the valuable knowledge available in these disciplines. First, the forces that play a role on the nanoscale are discussed and then a summary is given of some different theoretical treatments. Subsequently, an overview is given of the different phenomena occurring on the nanoscale and their present applications. Finally, some possible future applications are discussed.

Nanofluidics: what is it and what can we expect from it?

We have investigated the potential of self-assembled monolayer (SAM) coatings for the purpose of adhesion reduction in microelectromechanical systems (MEMS). Two types of SAM coatings, derived from the precursor molecules octadecyltrichlorosilane [CH/sub 3/(CH/sub 2/)/sub 17/SiCl/sub 3/, OTS] and 1H,1H,2H,2H-perfluorodecyltrichlorosilane [CF/sub 3/(CF/sub 2/)/sub 7/(CH/sub 2/)/sub 2/SiCl/sub 3/, FDTS], were applied to polycrystalline silicon microstructures in a liquid-based process. Due to the hydrophobicity of these coatings, the water capillary forces responsible for the phenomenon known as release-related stiction are eliminated, and SAM-coated cantilever beams 2 /spl mu/m thick, 2 /spl mu/m above the substrate, and up to 2 mm in length emerge dry and free standing when removed from the final water rinse. The effects of SAM coating on adhesion encountered during device operation, termed in-use stiction, were characterized using arrays of cantilever beams of varying lengths. Structures made with a polycrystalline silicon of 3-nm rms roughness gave apparent works of adhesion of 30 and 8 /spl mu/J/m/sup 2/ for the OTS and FDTS SAM coatings, respectively, in comparison to 56 mJ/m/sup 2/ for standard oxide-coated structures. These results demonstrate that OTS coating reduces adhesion by more than three orders of magnitude over the conventional process and that the fluorinated SAM can lessen it further by four times. With regard to thermal stability, both SAM coatings can withstand heat treatment for 5 min at 450/spl deg/C in an N/sub 2/ ambient. In air, the OTS film begins to degrade at 150/spl deg/C while the fluorinated coating remains intact up to 400/spl deg/C. Therefore, both types of SAM coatings are compatible with several MEMS packaging techniques, with the FDTS monolayers exhibiting superior stiction and thermal stability properties to those derived from OTS. Furthermore, the FDTS formation does not require any chlorinated solvents such as carbon tetrachloride, which has been banned from industrial use, making the latter coating an industrially viable antistiction treatment.

Alkyltrichlorosilane-based self-assembled monolayer films for stiction reduction in silicon micromachines

Solid–solid adhesion occurs at contacting asperities in two contacting solids. A thin liquid film with a small contact angle, present at the interface, can result in the so-called liquid-mediated adhesion. This may result in high adhesion during normal pull and high static friction during sliding, both commonly referred to as “stiction.” The problem of high stiction is especially important in an interface involving two very smooth surfaces under lightly loaded conditions. This article provides a critical and comprehensive review of mechanisms of adhesion and stiction, various measurement techniques, and methods used to reduce stiction in magnetic storage devices and micro/nanoelectromechanical systems.

Adhesion and stiction: Mechanisms, measurement techniques, and methods for reduction

Pull-in instability as an inherently nonlinear and crucial effect continues to become increasingly important for the design of electrostatic MEMS and NEMS devices and ever more interesting scientifically. This review reports not only the overview of the pull-in phenomenon in electrostatically actuated MEMS and NEMS devices, but also the physical principles that have enabled fundamental insights into the pull-in instability as well as pull-in induced failures. Pull-in governing equations and conditions to characterize and predict the static, dynamic and resonant pull-in behaviors are summarized. Specifically, we have described and discussed on various state-of-the-art approaches for extending the travel range, controlling the pull-in instability and further enhancing the performance of MEMS and NEMS devices with electrostatic actuation and sensing. A number of recent activities and achievements methods for control of torsional electrostatic micromirrors are introduced. The on-going development in pull-in applications that are being used to develop a fundamental understanding of pull-in instability from negative to positive influences is included and highlighted. Future research trends and challenges are further outlined.

Electrostatic pull-in instability in MEMS/NEMS: A review

Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces.

/pdf/stiction-in-surface-micromachining-s5wfxdssmg.pdf

Tonny Sonnenberg

Papers

Stiction in surface micromachining