Development of a Hermetic Barrier Using Vinyl Triethoxysilane (VTEOS) and Sol-Gel Processing

By: Ariel Jackson, Dr. Andrei Jitianu, Professor Lisa C. Klein, Material Matters 2006, 1.3, 11.

Department of Materials Science and Engineering, 
Rutgers University


Humidity barriers are an essential part of microelectronics, micro-electromechanical systems (MEMS) and displays using organic-light emitting diodes (OLEDs). These barriers are required to have low processing temperatures. Hybrid organic-inorganic barrier materials can be applied in a sol-gel process at low temperature, in order to provide hermeticity. The inorganic content is designed to be the barrier, while the hydrophobic, organic content repels water and fills porosity.

Our goal is to develop hermetic barriers for electronics and electrochemical devices. The barrier has to prevent water, water vapor, and gases from permeating the coating and reacting with the device. Many low temperature hybrids have been synthesized using the sol-gel process and methacrylate compounds.1,2 Other hybrids have been discussed in detail.3,4 The concept is that the sol-gel process produces an oxide at low temperatures. Specifically, the process is used to produce silica, either as a network or as an agglomeration of nanosized particles. The hybrid approach incorporates an organic component with the oxide, by either simultaneously or sequentially polymerizing the organic along with the inorganic component.

Typically, the inorganic component is derived from the hydrolysis and condensation polymerization of tetraethyl orthosilicate (TEOS, Aldrich Prod. No. 333859) to form a network of silica, containing bridging oxygens. In this study, we use vinyl triethoxysilane (VTEOS, Aldrich Prod. No. 679275) to generate an oxide network with polymerizable vinyl groups. In this case, the advantage of VTEOS over TEOS is the presence of an inorganic and organic network.

We analyzed the effectiveness of VTEOS in creating a hydrophobic barrier and how ultraviolet (UV) irradiation affected the VTEOS system. The properties of the films were investigated, and contact angle measurements were made to assess hydrophobicity.

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Synthesis Method

The flowchart for synthesizing barrier coatings is shown in Figure 1. First the precursor, vinyl triethoxysilane (VTEOS), was mixed with the solvent ethylene glycol mono-butyl ether (EGMBE). Deionized water and acetic acid (CH3COOH) were added and the solution was stirred for 20 hours. The initiator was benzoyl peroxide (BPO) for free-radical polymerization (0.1 wt %). Once the initiator was added, the solution was stirred for 4 hours before microscope slides and stainless steel coupons were dipped into the solution. A dip coater with a screw drive was used to lower and raise the slide.

The films were allowed to dry over night at room temperature before heating to 70 ºC for 30 minutes. UV irradiation was carried out, followed by a wash in NaOH to remove excess BPO. Finally, the samples were dried and evaluated.

Uniform films were obtained on both glass and stainless steel substrates. Contact angle measurements were acquired using deionized water and calculated using KSV CAM Optical Contact Angle with Pendant Drop Surface Tension Software 3.80. The AFM images were acquired in a contact mode using a Nanoscope IV Scanning probe microscope Veeco™.

Figure 1. Flow chart of the synthesis of hybrid coatings using VTEOS via a sol-gel process.

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Results and Discussion

Two samples are compared. One coating was prepared with four moles of water to one mole of VTEOS and the other one with six moles of water to one mole of VTEOS. Four moles is commonly said to be the stoichiometric water to hydrolyze a tetrafunctional silane, making six moles an excess. This assumes that one mole of water hydrolyzes each ethoxy group, although some condensation polymerization often accompanies further hydrolysis

The results of the contact angle measurement are shown in Figures 2a and 2b. In Figure 2a, the contact angle for the drop of water on the coating prepared with four moles of water is 90.5º. For the sample that used six moles of water, the measured contact angle is 118.4º. This angle is much greater than 90º, and the coating is clearly super-hydrophobic.

Figure 2a. Contact angle u, for coating prepared using four moles H2O: u = 90.5°.
Figure 2b. Contact angle u, for coating prepared using six moles H2O: u = 118.4°.

While samples appeared visually uniform, the AFM scans revealed the surface morphology. The surface roughness for the sample prepared with four moles of water was about 4.71 nm, as shown in Figure 3. The surface roughness adds to the hydrophobicity through the so-called lotus effect.

Figure 3. AFM image of surface of coating prepared with four moles H2O, showing an average roughness of 4.71 nm.

The vinyl groups were found to be on the surface of the coatings, as measured by Attenuated Total Reflectance- Fourier Transform Infrared Spectroscopy(ATR-FTIR spectrum not shown). Further analyses of the spectra are in progress. A schematic of the polymerization of the vinyl groups on the surface is shown in Figure 4. More studies of the surface polymerization are underway.

Figure 4. Schematic of photopolymerization of the vinyl groups of VTEOSderived coatings.

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Hybrid coatings have been prepared by the sol-gel process starting with VTEOS. The surfaces are hydrophobic, with the coating prepared with six moles of water showing a contact angle of 118.4º. The super-hydrophobicity is explained by the roughness of the surface on the nanoscale and by the presence of vinyl groups on the surface. We propose that the simultaneous generation of the two networks, inorganic by a sol-gel process and organic by photopolymerization, is a useful route to hydrophobic, dense, low temperature coatings for hermetic applications.

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  1. A. B. Wojcik, L. C. Klein, J. Sol-Gel Science and Technology, 1995,4, 57.
  2. A.B. Wojcik, L. C. Klein, J. Sol-Gel Science and Technology 1995, 5,77.
  3. A. B. Wojcik, L. C. Klein, Appl. Organometallic Chem. 1997, 11,129.
  4. D. Avnir, L. C. Klein, D. Levy, U. Schubert, A. B. Wojcik, The Chemistry of Organosilicon Compounds Vol. 2, Eds. Z. Rappoport, Y. Apeloig, 1998, Wiley, London, p. 2317.

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