Modification of silicon nitride with oligo(ethylene glycol)-terminated organophosphonate monolayers
Introduction
Silicon nitride has found a wide variety of applications in microelectronics and device fabrication [1]. In particular, in nanotechnological and micro-electromechanical systems (MEMS), silicon nitride is often used instead of silicon oxide as a highly selective mask for etching silicon and because of its favorable electrical passivation properties [2,3]. Furthermore, improved device performance compared to other dielectric materials and comparably simple preparation processes increased the interest in silicon nitride for biosensing applications. The material is, for example, widely used in the fabrication of ultrathin membranes for single biomolecule detection applications such as nanopore-based DNA analysis [4] or in the optimization of waveguide technology platforms suitable for label-free optical detection of protein interactions [5]. Despite the increasing implementation of silicon nitride in biosensing device fabrication, there are only few reports in the literature related to reliable strategies to introduce functional target groups on the surface. Generally, thermal or photochemical surface modification procedures with 1-alkenes and 1-alkynes have been applied [6]. For biosensing applications, a common approach is based on hydrogen plasma surface treatment to generate reactive amino functionalities capable to react with bifunctional crosslinkers for subsequent bioreceptor immobilization. [7,8] However, only few examples have been reported in the literature which demonstrate interface optimization on silicon nitride surfaces preventing non-specific protein adsorption or increasing the surface hydrophilicity to preserve bioreceptor functionality [9]. In this context, we report on the fabrication of self-assembled monolayers of phosphonic acids (SAMPs) functional interfaces on silicon nitride. In particular, we have investigated the ability of organophosphonate functional interfaces containing only a few ethylene glycol (EG) units per SAMPs molecule (Fig. 1) to effectively inhibit non-specific surface interactions on silicon nitride. Our work is based on the results obtained on silicon/silicon oxide substrates toward the immobilization of PNA bioreceptors, a synthetic analogue of DNA with unique properties [10]. In this recent contribution we have shown that this class of PEG-organophosphonates can be formed with high reproducibility and homogeneity on silicon oxide and can serve as a reliable anchor platform for bioreceptor immobilization. Organophosphonate chemistry on silicon nitride is more challenging, due to the lack of OH groups on the surface to allow for subsequent surface functionalization. To this end, surface etching is necessary to remove the silicon-rich and nitrogen-depleted native oxide layer resulting in an adequate OH-termination. Applying several surface analysis techniques, including contact angle (CA) measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), we present a detailed study of the morphology, composition, and stability of the functional interface on silicon nitride. In addition, we investigate the protective properties of organophosphonate functional interfaces containing only a few ethylene glycol units per SAMPs molecule against non-specific surface binding by means of fluorescence spectroscopy.
Section snippets
General
Precut (1.0 × 1.0 cm2) chips of undoped silicon covered by 20 nm of thermally grown SiO2 followed by a 50 nm surface layer of silicon nitride (Si3N4) grown by means of low-pressure chemical vapor deposition (LPCVD) were obtained from the Fraunhofer EMFT, Munich. Prior to any surface treatment, all chips were “solvent cleaned” (solv. cleaned) by a sequence of Acetone (Merck, VLSI Selectipur, ≥99.9%) and 2-Propanol (BASF, VLSI Selectipur, ≥99.9%) in an ultrasonication bath (37 kHz) for 10 min,
Results and discussion
Surface characterization by AFM (Fig. 2) confirmed that homogeneous, hole-free monolayers of 2-{2-[2-Hydroxy-ethoxy]-ethoxy}-ethyl phosphonic acid molecules (SAMPs) with a surface roughness (RMS) of 0.55 ± 0.01 nm can be prepared on silicon nitride by carefully tuning the deposition conditions. In particular, etching in an aqueous HF solution is necessary to remove the silicon-rich and nitrogen-depleted native oxide layer and to enhance the number of reactive OH groups. OH coverage depends
Summary and conclusion
SAMPs of 2-{2-[2-Hydroxy-ethoxy]-ethoxy}-ethyl phosphonic acid were used to establish surface functionalization protocols to successfully prepare organophosphonate interfaces on silicon nitride, a material of increasing interest in (bio)sensor device fabrication. A detailed surface analysis including water contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy, indicated that this class of phosphonic acid forms a homogeneous and thermally stable monolayer on
CRediT authorship contribution statement
Johannes D. Bartl: Conceptualization, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. Stefano Gremmo: Investigation, Formal analysis. Martin Stutzmann: Funding acquisition, Resources, Writing - review & editing. Marc Tornow: Funding acquisition, Resources. Anna Cattani-Scholz: Funding acquisition, Project administration, Resources, Validation, Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We acknowledge financial support by the German Research Foundation (CA1076/5-1; TO266/7-1). The authors are grateful to J. Boudaden (Fraunhofer EMFT) for providing Si3N4-coated substrates and to C. Paulus, D. Chryssikos, R. Csiki, F. Casablanca, and G. Pardatscher for experimental support. S. Gremmo is grateful to Prof. Ettore Vittone (University of Turin) for helpful discussions. J. D. Bartl is grateful to A. Hegele for proofreading and fruitful discussions.
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Contributed equally.