Alkoxyl monolayers as anti-stiction coatings in Si-based MEMS devices

Abstract

Reducing surface energy is key to the success of many microelectromechanical systems (MEMS). In this report we present a strategy for the ef cient assembly of alkoxyl monolayers onto a silicon surface to control surface energy. This is achieved by an all-liquid process in which the hydrogen terminated silicon surface resulting from aqueous HF etching is coated with a close-packed alkoxyl monolayer. The adhesion to silicon surface is reduced by a factor of 40 by the monolayer coating and friction coef cient of the coated surface is only 10 ¡2 . These coatings are successfully implemented in a model MEMS structure: cantilever beam array (CBA). Release-stictionis eliminated for polycrystalline silicon beams with a thickness of only 2 ¹m but with lengths up to 2 mm. Electrostatic actuation of coated beams in a controlled environment shows that the monolayer coating prevents in-use stiction at relative humidity as high as 90%.

Improved Substrate Protection and Self-Healing of Boundary Lubrication Film Consisting of Polydimethylsiloxane with Cationic Side Groups

“…The most well-known example is self-assembled monolayers (SAMs) [7][8][9][10][11][12][13][14]. However, the bound layer alone cannot provide reliable lubrication.…”

1,2-Epoxyalkane: Another Precursor for Fabricating Alkoxy Self-Assembled Monolayers on Hydrogen-Terminated Si(111)

“…The SiO 2 components did not exist after the grafting time increased to ≥45 min, as indicated by the unapparent peak at 103.5 eV. Therefore, exposure to UV light for ≥45 min produced EC 16 -SAM, which protected the H−Si substrate from oxidation.…”

“…Surface functionalization of semiconductors, especially silicon (Si), has recently gained interest because of their usage in different applications such as electronics and biosensing. − Covalent assembly of organic molecules on silicon dioxide (SiO 2 )-free Si substrate is considered to be a promising and challenging functionalization process by which the organic precursors are bonded to the Si substrate through covalent bonds, resulting in self-assembled monolayers (SAMs). − Such covalently-bonded SAMs have considerable chemical and mechanical stability. ,, The absence of the insulating SiO 2 layers allows for electron movement to the substrate through the monolayers, which promotes the use of these SAMs in electronic applications. ,, In addition, these SAMs are highly dense and can therefore be used as protecting layers and to reduce the friction. ,− Furthermore, assembly of the organic precursors occurs on the step-terrace structure of hydrogen-terminated silicon (H–Si), and any conversions occurring at the well-defined morphological structures of SAMs can be easily distinguished. ,− Thus, these SAMs can be used in microelectromechanical systems, molecular charge storage, electronics, biosensing, and lithography. ,,− …”

“…Recent studies found that SAMs formed on the SiO 2 -free Si substrate may have Si–C–C, Si–O–C, Si–S–C, Si–Se–C, and Si–Te–C linkages. ,,− The Si–C–C linkages between the monolayers and the Si substrate can be obtained via the hydrosilylation of alk-1-ene and alky-1-ne or through grafting of Grignard reagents on H–Si substrates. − Alkoxy-SAMs have been reported to be produced from the grafting of alkan-1-ol and alkan-1-al on H–Si substrates, and these SAMs have Si–O–C linkages. ,,− Moreover, alkan-1-thiol and dialkyl disulfide are used for forming the Si–S–C-bonded SAMs. ,,, Dialkyldiselenide and diphenyl ditelluride have been used for forming SAMs on H–Si, and these molecules were bonded to the substrate through Si–Se–C and Si–Te–C linkages, respectively. , The formation of these SAMs on H–Si substrates can be thermally, photochemically, or catalytically induced. − ,,− …”

Silicon Based Nanocoatings on Metal Alloys and Their Role in Surface Engineering

“…These special features of organosilanes offer unique opportunities for planned assembly of extremely robust supramolecular systems, capable of unusual structural and dynamic characteristics [138]. Alkyltrichlorosilanes are very important technologically because of the reactivity of the tri-silyl chemical moiety with the hydroxyl groups on semiconductor silicon and insulator silica surfaces, for example in microelectromechanical (MEMS) systems [149,150]. Typical of SAM precursors, alkyltrichlorosilanes have three parts: the headgroup (-SiCl 3 ) which provides chemisorption to the surface, the tail group (-CH 3 ) which provides a surface with the desired chemical and physical properties and the alkyl chain (CH 2 ) n that links the two and makes a significant contribution towards the mechanical stability of the SAM.…”