AFM study of perfluoroalkylsilane and alkylsilane self-assembled monolayers for anti-stiction in MEMS/NEMS

doi:10.1016/j.ultramic.2005.06.034

Ultramicroscopy

Volume 105, Issues 1–4, November 2005, Pages 176-188

https://doi.org/10.1016/j.ultramic.2005.06.034 Get rights and content

Abstract

Recent investigations have revealed the profound influence of stiction/adhesion, friction and wear on the reliability of micro/nanoelectromechanical systems (MEMS/NEMS). Studies have shown that the so-called self-assembled monolayers (SAMs) can be potential anti-stiction lubricants for MEMS/NEMS. Understanding the molecular tribological mechanisms of SAMs is a key to successfully designing and preparing super lubricants for MEMS/NEMS. For this purpose, perfluoroalkylsilane and alkylsilane SAMs have been deposited by a vapor deposition process on silicon with a native oxide layer and silica substrates. The surface roughness, adhesion, friction and wear properties of these coatings have been extensively studied by atomic force microscopy (AFM). Tribological properties on the macroscale have also been investigated for comparison.

Introduction

In micro/nanoelectromechanical systems (MEMS/NEMS), surface forces, such as adhesion and friction, become significant because of the large surface area-to-volume ratio. Recent studies have revealed the profound influence of adhesion, friction and wear on the efficiency, power output, steady-state motion and reliability of MEMS/NEMS devices [1], [2]. In order to improve these properties, various lubrication systems have been investigated including self-assembled monolayers (SAMs). SAMs are ordered molecular assemblies formed by chemical adsorption of an active surfactant on a solid surface. They can be designed by selecting appropriate head and tail groups, and structure of chains [3], [4]. An alkyl carbon backbone with a methyl tail group (CH₃(CH₂)_n–) along with others, such as fluorinated alkanes, have been extensively investigated. In fluorinated alkanes (referred to as perfluoroalkyl SAMs), a part or all of the alkyl hydrogen atoms are replaced by fluorine atoms (CF₃(CF₂)_n(CH₂)_m–). Since fluorinated simple aliphatic compounds have lower surface energies than alkylcarbons, the adhesive force is expected to be reduced [3], [4], [5], which may also result in low friction.

Tribological performances of alkyl SAMs have been investigated using atomic force/friction force microscopy (AFM/FFM) [6]. The effects of load, velocity, humidity and temperature have been studied. The effects of various head and tail groups, substrates and molecule chain lengths have also been investigated. It has been reported that wear resistance depends on the type of SAM, substrate materials and interfacial bond strength [7]. It has also been reported that there is a critical load, above which the wear depth of SAMs increases dramatically. On the other hand, limited data exist on fluorinated SAMs [8]. Contact angles measured using various liquids showed an increase with the molecule chain length for fluorinated SAMs [9]. The structural analysis of the SAMs, conducted using infrared spectroscopy (IR), exhibited a lack of ordering in the methylene segment for the fluorinated alkanephosphonic acid SAMs on aluminum native oxide as compared with the ordered structure for the alkanephosphonic acid SAMs [10]. Fluorinated disulfide SAMs showed comparable friction force as compared with non-fluorinated disulfide SAMs and about 20% larger friction force than alkylthiol SAMs [11]. More conclusive tribological studies comparing fluorinated SAMs and non-fluorinated SAMs are needed.

In this paper, various surface properties of perfluoroalkylsilane SAM deposited on silicon with a native oxide layer and silica substrates, such as contact angle, surface energy, rms roughness, adhesion, friction and wear are evaluated and the results are compared with those acquired from the standard alkylsilane SAMs. Also for certain properties, the data for hexadecane thiol SAMs deposited on gold are compared. Macro scale tests are also performed for comparison.

Section snippets

Chemical structure of SAMs

Fig. 1 shows the chemical structure of the perfluoroalkylsilane (PFTS) and alkylsilanes (ODMS and ODDMS) deposited on hydrated SiO₂ substrates. Details of the structures and acronyms for the SAMs are given in the next section. In the case of Si (1 0 0) substrates, the silanes react with the hydrated native oxide (∼2 nm thick) formed on the Si substrate, therefore, the basic structure of the SAMs deposited onto the two substrates is the same. The thickness of the SAMs was 1.8 nm for PFTS, ∼1.9 nm for

Surface free energy and contact angle measurement

Fig. 4(a) shows a Zisman plot for PFTS/Si, ODMS/Si, ODDMS/Si, PFTS/SiO₂, ODMS/SiO₂ and ODDMS/SiO₂ (SAM/substrate) and a summary of the surface energy values. It was not possible to perform Zisman analysis on Si and SiO₂ substrates because the alkane liquids used for the measurements instantly spread on these surfaces. These surfaces have surface energies even higher than surface tension of the alkane liquids and, in fact, the liquids spread on them. The critical surface tension or surface

Conclusions

The perfluoroalkylsilane SAM (PFTS) and alkylsilane SAM (ODMS and ODDMS) deposited on two types of substrate (Si and SiO₂) were compared in terms of surface energy, water contact angle, rms roughness, adhesion, friction and wear resistance using AFM/FFM (micro/nano scale) and a tribo apparatus (macroscale). Based on this study, we draw the following conclusions:

(1)
Highest water contact angle and lowest surface energy were observed for PFTS/Si compared to ODMS/Si, ODDMS/Si, and Si substrate.

Acknowledgements

Financial support for this work was provided by the National Science Foundation (Contract no. ECS-0301056). The content of this information does not necessarily reflect the position or policy of the Government and no official endorsement should be inferred. The Swiss TopNano21 is acknowledged for partial funding in 5824.4 and 4686.2. We would like to thank J.A. DeRose for proof reading the manuscript and useful discussions.

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AFM study of perfluoroalkylsilane and alkylsilane self-assembled monolayers for anti-stiction in MEMS/NEMS

Abstract

Introduction

Section snippets

Chemical structure of SAMs

Surface free energy and contact angle measurement

Conclusions

Acknowledgements

Ultramicroscopy

Curr. Opin. Colloid Interface Sci.

Mater. Sci. Eng.

Polymer

Tribology Issues and Opportunities in MEMS

Springer Handbook of Nanotechnology

An Introduction to Ultrathin Organic Films: From Langmuir–Blodgett to Self-Assembly

Chem. Rev.

Introduction to Tribology

Self-assembled monolayer for controlling adhesion, friction and wear

J. Phys. Org. Chem.

J. Phys. Chem. B