Examination of Polymer Brush Formation on SI Wafers by GATR-FTIR

Polymer brush systems are made up of ordered assemblies of polymeric chains that are terminally absorbed or grafted onto an interface and surface at one or more tethering points.

The last few years has seen an increased interest in the formation of these surface immobilized thin films because of their significant use in a wide range of applications, such as controlled gene/drug delivery, thermo-responsive adhesives, nanopatterning, tailoring of surface properties, microelectronics, and biomedical devices.

In order to form polymer brushes from flat silica surfaces, multiple reactions must occur at the surface to efficiently functionalize the silicon dioxide layer with active sites and for the resulting polymerization from these active sites.

Compared to other FTIR methods, grazing angle attenuated total reflectance (GATR)-Fourier transform infrared (FTIR) spectroscopy is particularly beneficial as it enables direct collection of spectra from silica wafer substrates without causing any damage to the surface functionality or requirement for a suitable substrate, such as a silicon ATR crystal (Figure 1).

This article analyzes the utilization of GATR-FTIR to differentiate surface modifications of silica wafers with polymer brushes and thin films. For better sensitivity, this technique needs optical contact between the ATR crystal and the sample.

Figure 1. GATR Ge-ATR accessory

Experiment

All substrates were first cleaned in a 30:70 v/v hydrogen peroxide and concentrated sulfuric acid solution in order to remove any surface contaminates from the Czochraclaki-grown <111> Si wafers. Following this, an 11-carbon tertiary bromo-isobutyrate initiator was then deposited in an anhydrous toluene solution onto the cleaned Si wafers as shown in Figure 2a.

Figure 2. Reaction sequence for formation of a diblock copolymer brush on flat si wafers.

Methylene chloride, methanol, and toluene were used sequentially to clean the wafers. GATR- FTIR spectra were acquired at a resolution of 8 cm^-1, with 256 sample scans and 64 background scans.

Samples with earlier deposited bromo-initiator were subjected to a modified atom transfer addition (ATA) reaction in order to transform the terminal bromine to a dithioester endgroup (Figure 2b). Spectra were collected and wafers were cleaned.

Finally, homopolymer and diblock copolymer brushes of poly(styrene) (PSty) and PSty-bpoly(methyl acrylate) PMA were developed from the dithioester moieties (Figure 2c and 2d, respectively). After cleaning the samples, GATR-FTIR spectra were gathered.

Results and Discussion

Figure 3a shows the GATR-FTIR spectrum of the immobilized bromoinitiator, displaying peaks at around 2930 and 2850 cm^-1, which are respectively allocated to the C-H stretching and CH₂ stretching vibrations, as well as peaks at around 1740 cm^-1, which is allocated to the carbonyl stretching vibration of the ester group.

Figure 3. GATR-FTIR of surface immobilized (a) 11-C tertiary initiator and (b) dithioester surface structure.

Following the deposition and characterization of bromo silane initiator, a customized ATA reaction was performed to transform the terminal bromine to a dithioester end group (Figure 2b). The sample’s GATR-FTIR spectrum following reaction with a dithioester containing compound (Figure 3b) highlights a few perceptible variations to that of the immobilized bromosilane initiator spectrum.

This is attributed to the comparatively weak intensity of aromatic C-C stretches and C-H stretches, particularly when only a single aromatic ring exists for each immobilized molecule, and to the fact that the C=S stretching vibration takes place in the finger print region. This spectral region is subject to huge variation in the GATR-FTIR of silicon wafers because of the intense absorbance of the lattice bands and native silicon dioxide.

A PSty homopolymer brush was synthesized in order to establish the efficiency of the immobilized dithioester surface towards surface initiated polymerizations (Figure 2c). The GATR-FTIR spectra for the PSty homopolymer brush (Figure 4) established the presence of PSty due to the anticipated appearance of aromatic C=C aromatic doublets at 1420 - 1480 cm^-1 and C-H stretching around 3100 cm^-1.

A PSty-b-PMA diblock copolymer brush was developed using a homopolymer brush of PSty (Figure 2d). The GATR-FTIR spectra validated the inclusion of MA in the development of the PSty-b-PMA diblock copolymer brush (Figure 5) because of the appearance of a carbonyl stretch at 1720 cm^-1 and an increase in the CH2 stretch at around 2920 cm^-1.

Figure 4. GATR-FTIR of PSty homopolymer brush.

Figure 5. GATR-FTIR of PSty-b-PMA diblock copolymer brush.

Together with GATR-FTIR spectroscopy, every single Si wafer system was characterized by x-ray photoelectron spectroscopy, goniometry, and ellipsometry. As some amount of force is needed to establish a good contact between the treated wafer and the ATR crystal, ellipsometry was used to measure the thicknesses, before and after GATR-FTIR collection.

Since these samples have an apparent change in thickness, a sample wafer at each alteration step was mainly kept to collect spectra and not to measure the thicknesss.

Conclusion

The GATR Ge-FTIR accessory has been shown to be a useful tool in spectral description of films, whose characteristic stretches are noticeable in the 1300 - 3300 cm^{- 1} spectral range. Conversely, other peaks existing in the fingerprint region are masked by the native silicon oxide stretches of the Si wafer.

This issue can be prevented by using floatzone Si wafers. The GATRFTIR technique is nondestructive, but the required high contact force does affect the thickness of these polymer brush structures.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

For more information on this source, please visit Harrick Scientific Products, Inc.