14934
J. Phys. Chem. C 2008, 112, 14934–14942
Effect of Annealing on Improved Hydrophobicity of Vapor Phase Deposited Self-Assembled Monolayers Arun Kumar Gnanappa,*,† Cian O’Murchu,† Orla Slattery,† Frank Peters,† Tony O’Hara,‡ Bala´zs Aszalo´s-Kiss,§ and Syed A. M. Tofail§ Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland, Memsstar Technology, Starlaw Park, Starlaw Road, LiVingston, United Kingdom, and Materials & Surface Science Institute, UniVersity of Limerick, Limerick, Ireland ReceiVed: May 29, 2008; ReVised Manuscript ReceiVed: July 10, 2008
The hydrolytic stability and the effect of annealing on the improved aging properties of self-assembled monolayer (SAM) hydrophobic coatings have been investigated. Surfaces coated with hydrophobic SAM coatings give higher flow rate and are of wide interest to researchers in the area of microfluidics. Hydrophobic SAM of fluorooctyltrichlorosilane (FOTS) was deposited on silicon by using a vapor phase technique. The aging of the hydrophobic property was examined by using contact angle measurements. It has been found that while such a monolayer suffers from a loss of hydrophobicity with time, annealing can partially recover the hydrophobicity. The mechanism of aging was investigated through Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which revealed very little chemical change in annealed monolayers. Surface roughness and morphological changes due to hydrolytic attack was studied by atomic force microscopy (AFM). A spontaneous formation of silane mounds on the surface of the monolayers was found. These mounds have been irreversibly transformed from initially uniform hydrophobic surface layers. It is highly probable that the compliance of these mounds can reasonably allow hydrophilic microcapillaries to be located around the mounds. Interestingly, the density of these mounds decreases when the monolayer samples are annealed, with no significant chemical change. This novel finding explains the observed recovery of hydrophobicity of the monolayers as primarily a morphological effect that originates from the densification of the monolayers upon annealing. 1. Introduction The application of hydrocarbon or fluorocarbon chlorosilanebased self-assembled monolayers (SAMs) can greatly improve hydrophobic properties of a substrate.1-5 It has been reported that the slip velocity present on a hydrophobic surface results in a significant drag reduction for a microscale channel flow. Among the various available surface coatings and modification techniques, self-assembled monolayers (SAMs) formed from organosilanes are promising candidates as hydrophobic coatings due to their good bonding strength, low surface energy, and good thermal stability. Self-assembled monolayers can be deposited either in a liquid phase process or in a vapor phase process. Liquid phase deposition of SAM coatings has significant disadvantages, such as complicated process control, generation of large amounts of contaminated effluents, insufficient stiction prevention and high production costs. A vapor phase SAM deposition, in contrast, can eliminate some of these limitations6,7 and has been used in the present study for the deposition of SAM from a volatile fluoroalkylsilane FOTS (tridecafluoro-1,1,2,2 tetrahydrooctyltrichlorosilane) CF3(CF2)5(CH2)2SiCl3 precursor. Properties such as bond strength, cross-linking, and density of monolayers mainly depend on the processing method and the chemical composition of the precursor, which in the present * Corresponding author. E-mail:
[email protected]. Phone: +353 21 4904113. Fax: +353 21 4270271. † Tyndall National Institute. ‡ Memsstar Technology. § University of Limerick.
case is silane. Silanes may be distinguished by the number of reactive end groups or by their chain lengths (i.e., number of C-atoms). Trifunctional silanes, e.g., FOTS or perfluorodecyltrichlorosilane (FDTS), usually form densely packed monolayers by horizontal polymerization.8 The desired hydrophobic properties are obtained typically by coating a substrate with SAMs of trifunctional silanes to form by horizontal polymerization densely packed monolayers with low energy.8 These SAM coatings tend to degrade in liquids 9,10 and water,11-16 and are not suitable in microfluidic applications. This is a significant barrier for the use of SAM coatings in MEMS devices which require both chemical and mechanical durability. In particular, microfluidic devices operate in continuous contact with liquids and are often subjected to harsh chemical environments. The hydrophobic protective coatings in such devices must sustain long-term exposure to various liquids including water. While coating deposited under close control of deposition parameters can possess good hydrophobic properties (e.g., a high water contact angle, >110°, and a low surface energy), the hydrolytic stability of the coatings can also play a critical role in defining the long-term durability of such coatings in microfluidic devices and stamps for nanoimprint processes. Here we report the improvement of the quality and durability of SAM deposited from FOTS precursors on a hydrophilic silicon oxide activation layer. While these films have found widespread applications in MEMS technology and elsewhere, their structure and properties are not well understood. The extent of bonding to the surface,17,18 cross-linking,19 and ordering20,21
10.1021/jp804745t CCC: $40.75 2008 American Chemical Society Published on Web 08/29/2008
Hydrophobicity of Vapor Phase Deposited SAMs
J. Phys. Chem. C, Vol. 112, No. 38, 2008 14935
Figure 1. Schematic of the MEMSSTAR SPD module.
Figure 2. Aging and effect of annealing on the vapor phase monolayer.
TABLE 1: Contact Angle under Various Times before and after Annealing sample condition
contact angle
after annealing
fresh 5 days in water 12 days in water
109.4 103.3 68.4
107.4 108.4 94.2
of alkylsilanes are topics of considerable discussion in the literature. Less reported is the stability of the silane-based SAMs in a chemical environment. In the present study, the hydrolytic stability of the vapor phase monolayer film has been investigated with spectroscopic and morphological techniques such as AFM, FTIR, and XPS so as to obtain a detailed understanding of the behavior of the surface of such monolayers in an aqueous environment. 2. Experimental Section 2.1. Film Preparation. The process performed in the MEMSSTAR SPD (surface preparation and deposition) module uses precursor and plasma chemistries for advanced surface engineering and coatings. The schematic of the MEMSSTAR SPD is shown in Figure 1. It is a simple vacuum system in which the vapor deposition chamber is pumped by using a standard dry pump that can achieve a base pressure of 5 mT. The chamber leak-up rate is typically less than 1 mT/min. The precursors are precisely delivered by using a bubbler setup with a nitrogen carrier gas (see Figure 1). Each of the three bubblers
are independently temperature controlled with the associated lines and then the process chamber is heated to an appropriate level. The bubbler arrangement provides precise vapor delivery dependent on the carrier gas flow and the temperature of the bubbler. This method can deliver higher flows of precursor vapor to the process chamber than the stagnant gas approach. The chamber pressure during deposition is controlled and is typically set from base pressure up to 50 T depending on the material and process required. The FOTS-based SAM formation consists of a surface preparation step with O2 plasma followed by the tunable deposition of a monolayer film in the same reaction chamber. The surface preparation by plasma treatment is an important first step of the integrated deposition process. Reactive species generated by the oxygen plasma are used to clean any organic contamination and to ensure uniform hydroxylation of the surface with OH-groups. This increases the silane reaction in the presence of a catalytic agent, which in this case is water. The plasma treatment results in a highly hydrophilic surface as measured by a water contact angle of
