Silane Deposition via Gas-Phase Evaporation and High-Resolution

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Silane deposition via gas-phase evaporation and high-resolution surface characterization of the ultra-thin siloxane coatings Walid-Madhat Munief, Florian Heib, Felix Hempel, Xiaoling Lu, Miriam Schwartz, Vivek Pachauri, Rolf Hempelmann, Michael Schmitt, and Sven Ingebrandt Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01044 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Silane deposition via gas-phase evaporation and high-resolution surface characterization of the ultra-thin siloxane coatings Walid-Madhat Muniefa,b, Florian Heibc, Felix Hempela,b, Xiaoling Lua,d, Miriam Schwartzb, Vivek Pachauria,d, Rolf Hempelmannc,e, Michael Schmittf, Sven Ingebrandta,b,d* Author addresses: a Department of Informatics and Microsystem Technology, University of Applied Sciences Kaiserslautern, 66482 Zweibrücken, Germany b RAM Group DE GmbH, Research and Development Center, Zweibrücken, Germany c Department of Physical Chemistry, Saarland University, 66123 Saarbrücken, Germany d Department of Electrical Engineering and Information Technology, Institute of Materials in Electrical Engineering 1, RWTH Aachen University, 52074 Aachen, Germany e Korean Institute of Science and Technology, 66123 Saarbrücken, Germany f Institute for Coatings and Surface Technology, University of Applied Sciences Niederrhein, 47805 Krefeld, Germany

KEYWORDS: Silane, siloxane, gas-phase silanization, organosilicon chemistry, thin-film, wafer-scale deposition, high-precision drop shape analysis (HPDSA), spectroscopic ellipsometry, refractive index, Fourier transform infrared attenuated total reflection absorption spectroscopy.

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Abstract Siloxane coatings for surfaces are essential in many scientific and industrial applications. We describe a straightforward gas-phase evaporation technique in inert atmosphere and introduce a practical and reliable silanization protocol adaptable to different silane types. The primary aim of depositing ultra-thin siloxane films on surfaces is to enable a reproducible and homogenous surface functionalization without agglomeration effects during the layer formation. In order to realize high quality and large area coatings it is fundamental to understand the reaction conditions of the silanes, the process of the siloxane layer formation, and the possible influence of the substrate morphology. We used three typical silane types to exemplify the potential and versatility of our process: Aminopropyltriethoxysilane, Glycidoxypropyltrimethoxysilane, and 1H,1H,2H,2H-perfluorooctyl-trichlorosilane. The ultra-thin siloxane layers, which are generally difficult to characterize, were precisely investigated with high-resolution surface-characterization methods to verify our concept in terms of reproducibility and coating quality. Our results show that this gas-phase evaporation protocol is easily adaptable to all three, widely used silane types also enabling a large area upscale.


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Introduction In the field of surface science, there are different disciplines that have been the subject of intense research for decades.1-5 Self-assembly of molecules onto surfaces is of great interest for science and for industrial applications ever since.6 From the discovery of protective paints for anti-fouling surfaces to the study of single molecule layers with atomic-probe techniques, self-assembly of molecules onto surfaces has come a long way.3, 6-8 Developments in surface engineering accommodate a wide variety of self-assembly systems for unique surfaces and molecule combinations.6, 8 Organosilicon molecules, commonly known as silanes, are one of the most interesting class of molecules for these surface modifications. Thereby the organic side chains of the organosilicon molecules may render the surface properties from superhydrophobic9-10 to super-hydrophilic.11-12 Moreover, they can also act as anchoring molecules with different chemical functionalities.7, 13 Classically, siloxane layers were used for coatings of pH electrodes, for surface modification of microelectromechanical systems (MEMS) and for adjusting the wetting behavior of different surfaces.10, 12, 14 Nowadays, the availability of novel silane compounds with unique chemical and physical properties as well as the advances in micro-/nano-technology have opened up new opportunities for the self-assembly of siloxane coatings.13, 15 The silanization of solid surfaces like silicon and glass wafers has beneficial influence on semiconductor and construction technology. In general, the creation of agglomeration free and ultra-thin siloxane layers, close to a monolayer, is a challenging task. The organosilicon compounds enable the integration of versatile siloxane coatings on silicon-oxide wafers or on devices in lab-on-a-chip (LOC)16-17, micro total analysis systems (µTAS), interdigital transducers (IDT)18, silicon nanowires (SiNW)19, AlGaN/GaN sensors20-21 or even on selfcleaning and heat-reflecting windows.22 The versatile generation of large-area siloxane coatings is of general scientific and industrial interest in order to replace expensive, timeconsuming and complex techniques. A robust approach to generate large-area siloxane layers under controlled reaction conditions for different high-end applications is therefore desired. In the last decades, advances in the formation of siloxane layers were focused on more efficient condensation protocols, ranging from liquid-phase3, 5-6, 13, 20-25 over chemical vapor deposition (CVD)26-33 to gas-phase self-assembly (GPSA).18,

24, 34-35

The wet-chemical

methods require experienced users and well-controlled protocols to create homogeneous layers on silicon oxide surfaces. Batch processing with a high yield and large-scale siloxane deposition is very difficult with liquid phase protocols. The organic liquids have to be 3 ACS Paragon Plus Environment

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exchanged regularly and the protocols produce toxic and sometimes even mutagenic waste in litre scale. Polymeric thin-films can also be generated with plasma enhanced chemical vapor deposition (PECVD) with controllable deposition rates and thicknesses, but expensive equipment is needed. PECVD is a multi-stage deposition process and forms layers in the range from 50 nm to several 100 nm, typically used to create dielectric passivation layers. Monolayer siloxane films are difficult to achieve with this technique. As alternative, to existing methods, we introduce here a novel, straightforward gas-phase silanization on 4-inch silicon-silicon oxide (Si/SiO2) wafers (Figure 1). The three widely-used organosilicons

3-Aminopropyltriethoxysilane

Glycidoxypropyltrimethoxysilane

(3-GPTMS)

and

(3-APTES),

3-

1H,1H,2H,2H-perfluorooctyl-

trichlorosilane (FOTCS) were applied. The main advantage of our method is that the vapor leads to a slow, diffusive transport of the silanes in the gas phase inside of the reactor like illustrated in Figure 1b. The given vapor pressure and gas flow over the substrate surface enables the use of a very low amount of silane solution in µl volumes per process (ideally one molecule would cover an area of around 0.243 nm², which is equivalent to only 7 µmol per 1 m²), which drastically reduces the amount of waste in the process. With our protocol we are able to coat several wafers simultaneously during one synthesis.34 The quality of the resulting siloxane coatings thereby depends on the chemical properties of the constituting silane molecules (APTES, GPTMS and FOTCS) and on the local surface energy of the substrates. Figure 1 (d-f) illustrates the simplified chemical reaction mechanism for the three different silane types. The first step of the condensation reaction is the hydrolysis36 of the organylalkoxy-group in which the inorganic parts of the silane side chains are involved. In the second step under the nucleophilic substitution 1 (SN1)37-38, the respective leaving group (methoxy in GPTMS, ethoxy in APTES and chloride in FOTCS) is released in an elimination reaction. This reaction step transfers the silanol into the siloxane (APTES to Aminopropylsiloxane (APS), GPTMS to Glycidoxypropylsiloxane (GPS) and FOTCS to Fluorooctylsiloxane (FOS)).4 This layer forms covalent bonds to the silicon substrate and the corresponding silane/siloxane thin-film is inter-linked via Si-O-Si bridges.1-2 The formation quality of the silane layers is dependent on the reactive species of the surface, the surface activation, the influence of adsorbed water, which is minimized under vacuum conditions, the molecular structure of the different silanes and their thermodynamic reaction potential of the leaving group.4 This reactive potential is significant to the formation of the siloxane layer and strongly depends on the chain length of the alkoxy-group. The hydrolysis 4 ACS Paragon Plus Environment

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reaction of the inorganic side and the differences of the releasing potential of the alkoxy and the halogen group are the crucial factors concerning the coating quality. The terminal group at the organic chain is the essential factor to engineer the surface towards different applications and chemical functionalizations.1, 37-39

Figure 1. (a) Glovebox set-up used in this study to generate the siloxane layers under controlled gas-phase conditions: (b) Illustration of the vapor flow under vacuum conditions onto a wafer substrate and (c) process upscale with 17 4-inch Si/SiO2 wafers resulting in a macroscopic area of more than 0.5 m². (d)-(f) Simplified reaction mechanisms to illustrate the silane hydrolysis and the condensation reaction on the surface. (d) For APTES some residual ethoxy groups may not be completely hydrolysed and could remain in the resulting siloxane layer. (e) A similar reaction potential can be illustrated for the methoxy group of GPTMS. (f) In the case of FOTCS the favourable thermodynamic properties of the chloride-ions result in a complete hydrolysis and condensation.

In this study, we used different, high-precision surface characterization methods complementing each other to ascertain the physico-chemical properties of the siloxane surfaces. For this purpose atomic force microscopy (AFM), spectroscopic ellipsometry25, 40-41 (SE), Fourier transform infrared external reflection absorption spectroscopy (FTIR-ERAS)425 ACS Paragon Plus Environment

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44

, Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR) and

dynamic contact angle (CA)45-47 measurements were performed to control the individual steps of the process and to characterize the produced siloxane coatings. Additional focus was set on the investigation of the influence of the different reactions and of the leaving group ability from the silanes APTES, GPTMS and FOTCS. The solid vapor-phase synthesis protocol described here is a self-limiting, one-stage deposition concept and enables a mechanically and chemically robust, agglomeration-free, ultra-thin, large area coating with siloxane thicknesses close to monolayer dimensions. Furthermore, the presented technique is scalable, reproducible and cost effective, which makes it useful for many applications, e.g. for biosensors, for the semiconductor technology in general, and for different LOC devices. Experimental All gas-phase evaporation processes were done in a glass desiccator inside a glovebox (GS GLOVEBOX Systemtechnik GmbH – Germany) under inert nitrogen atmosphere (Figures 1a and 1b). In general, all silanes must be handled under inert gas conditions to protect the educts from the reaction with atmospheric gases, which would reduce the silanes’ reactivity.

Wafer preparation: In total 17 4-inch silicon wafers (n-doped, 5-10 mΩ.cm, (100) orientation, MicroChemicals GmbH - Germany) were used as substrates for the silanization experiments. These wafers were cleaned with freshly prepared Caro's acid (H2O2 (30 w %), azeotropic H2SO4 (98 w %) ratio 1:3, T = 120 °C, t = 20 min, Sigma Aldrich Chemie GmbH Germany) to remove all contaminants. Then the native SiO2 was etched in hydrofluoric acid (HF (6%), NH4HF (10%), t = 1 min, Sigma Aldrich Chemie GmbH - Germany), the wafers were washed with DI-water (deionized-water, Milli-Q® Type 1 ultrapure water system, Merck KGaA - Germany) and dried under nitrogen flow. The silicon wafers were then directly transferred to an oxidation furnace (Inotherm Diffusion Oven DS-3900 PC/150 - Germany) and thermally oxidized in O2-atmosphere (T = 1050 °C, t = 3.5 h, O2-flow = 2 sccm). From this batch three Si/SiO2 wafers were taken out and the dry oxide was etched off again with HF-solution (t = 4 min, Sigma Aldrich Chemie GmbH - Germany). Natural oxide was allowed to grow on these wafers in ambient atmosphere. The three wafers with natural oxide (2-4 nm) and five more wafers with dry oxide (141 nm ± 2.1 nm) were used as reference substrates to determine the surface roughness values using AFM characterization. On two of the dry oxide wafers a 200 nm thick copper film was sputtered to enable the FTIR-ERAS


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characterizations as detailed below. The remaining 9 wafers with the 141 nm thick thermal oxide were used for the silane coating experiments.

Surface Activation: To increase the amount of hydroxyl groups as the potential binding partner for the inorganic silane chains, an activation of the oxidized Si/SiO2 wafers was performed with a mild O2 plasma treatment (t = 1 min, 70 W) in a barrel oven (PVA-TePla AG - Germany).

Silanization: The wafers were then directly transferred into a desiccator placed inside of a nitrogen flooded glovebox. The silanes were pipetted into a crystal bowl inside of the desiccator. In dependence of the respective silane educts (APTES, GPTMS and FOTCS Sigma Aldrich Chemie GmbH - Germany) the concentration, volume and temperature parameters were fixed as shown in Table 1. Table 1. Synthesis parameters as we used them for the gas-phase siloxane formation with APTES, GPTMS, and FOTCS, respectively. Silane

Concentration [%]

Density [g/ml]

Boiling point [°C]

Temperature [°C]

Volume [µl]

APTES GPTMS FOTCS

99 99 97

0.946 1.07 1.3

217 120 193

50 30 20

100 100 100

All wafers were positioned at an angle of about 85° with respect to the outlet valve of the desiccator to provide a maximum deposition of the silane onto the substrate (Figure 1b). The pressure inside the desiccator was gradually reduced to 130 mbar so that a continuous vapor flow over the substrates was ensured. After the silanization, all wafers were washed with ethanol (98%, Sigma Aldrich Chemie GmbH - Germany) and DI-water and dried under nitrogen flow to remove physically absorbed impurities. Furthermore, the wafers with APS-, GPS-, and FOS-coatings were transferred into another preheated desiccator to carry out a grafting procedure (T = 90 °C, t = 1 h) to eliminate remaining, uncondensed leaving groups.

Fourier transform infrared spectroscopy: To determine the reaction mechanism and bond formation of the siloxanes, high-resolution FTIR measurements were done in two different configurations. Both techniques cannot be applied directly to the ultra-thin siloxane films on the Si/SiO2 substrates. Therefore, FTIR characterizations were done on alternative substrates as detailed below. Both high-resolution techniques gave valuable insights to the reaction mechanism and thin-film formation processes. The FTIR spectroscopy in ATR configuration using a ZnSe-crystal allowed us to characterize the siloxane particle formation process inside the isotropic liquid material (Figure 2a) and the FTIR spectroscopy in ERA configuration 7 ACS Paragon Plus Environment

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followed the bond formation during the siloxane thin-film deposition process. The latter one, however, needed a 200 nm copper layer as a reflective coating on the substrates (Figure 2b).4243, 48-50

Therefore, these thin-films were formed on CuO surfaces and a comparison to the

siloxane formation on the Si/SiO2 wafers should be discussed with care.

Figure 2. Schematic of (a) the FTIR-ATR configuration generating an evanescent IR wave within the isotropic liquid to identify the characteristic absorption bands of APS (left), GPS (middle) and FOS (right) during sol-gel siloxane particle formation. (b) The geometric configuration for the surface-sensitive FTIR-ERAS characterization: We needed an additional Cu layer for IR light reflection to generate high-resolution spectra. The possible adsorption states of the anisotropic thin-films of APS (left), GPS (middle), and FOS (right) on the Si/SiO2/Cu-surfaces are indicated. Note that the relative orientation of the molecules can also be confirmed by ERAS as discussed below.

Fourier transform infrared attenuated total reflectance spectroscopy: For high-resolution analysis, FTIR-ATR was used as illustrated in Figure 2a. The multi-reflection, horizontal ATR-spectra and the transmission spectra from vapor phase (gas cell, NaCl windows) of the bulk materials were collected with an FTIR-ATR spectrometer (Frontier, Perkin Elmer, Massachusetts - USA). For the sample preparation with the FTIR-ATR spectrometer, the siloxane particles APS, GPS, and FOS were synthesized by a sol-gel protocol, where 1 µl of the respective silane was dissolved in ethanol (98 %, V = 2 ml) as carrier solvent and waterfree phosphoric acid (Sigma Aldrich Chemie GmbH - Germany) as a catalyst. A small droplet of the dispersed siloxane particles was then applied onto the ZnSe-crystal surface and ethanol was evaporated under dry nitrogen flow to minimize drying artefacts during the measurements and to generate constant IR-intensities. Fourier transform infrared external reflection absorption spectroscopy: The FTIR-ERAS measurements as schematically shown in Figure 2b were done with an infrared reflection spectrometer IFS (66v/s Bruker - USA). For these measurements, we needed to sputter a reflective, 200 nm thick copper layer onto one Si/SiO2 test wafer (BAK 591, SenVac GmbH Germany). Then it was cut into 25×25 mm² pieces to fit into the instrument. The metallic Cu layer ensures an appropriate transmission and reflectance of the substrate to characterize the siloxane thin-films in high resolution. After the siloxane coating of these samples following 8 ACS Paragon Plus Environment

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the same protocol as detailed above, they were rinsed with ethanol (99.8 %), dried in nitrogen flow, and immediately transferred to the sample holder of the FTIR-ERA spectrometer. The background spectra were obtained from a bare, 200 nm thick copper layer, where p-polarized IR light at an angle of incidence of 80° was used resulting in surface-sensitive IR-spectra. For the FTIR-ERAS a direct characterization of the siloxane on the Si/SiO2 wafer surface was not possible, because of the high symmetry of the silicon monocrystals, which cannot absorb the IR energy to create lattice vibrations. Atomic Force Microscopy: Surface properties and roughness values were determined via AFM (Dimension Icon AFM Bruker, Germany) in soft tapping mode with a rectangular cantilever (tip radius = 7 nm, f = 300 kHz, k = 26 N/m, Oxford Instruments - Germany). The resulting images were flattened (first order) to correct for the non-linearity of the piezoelectric scanner. For statistical assessment, surface roughness parameters were examined on five different positions for each sample. Spectroscopic Ellipsometry: The thicknesses of the oxide layers and siloxane coatings were measured by spectroscopic ellipsometry (SE, SE-850, Sentech GmbH - Germany). For a high measuring accuracy in relation to the layer height, nine scanning positions on each wafer were investigated. Each position was measured with three different angles of incidence (50°, 60° and 70°) before and after the siloxane coating, on exactly the same positions of the wafers by using a shadow mask. High Precision Drop Shape Analysis: The HPDSA of the siloxane layers was done with an all-purpose contact-angle and drop-shape analysis device (OCA 20 Dataphysics - Germany) equipped with an inclining sample table. The measurements were done with an angular speed of = 0.57°/ (inclining-plate technique), while a video of the drop-shape was recorded at 12.5 frames per second on ten measurement spots repeatedly. A 50 µl drop-volume was maintained on the measuring spots inside a temperature-controlled chamber (30.0±0.2°C).

Results and Discussion We did a very detailed FTIR characterization of the silane reactions to form siloxane in bulk liquid and on surfaces, where the ultra-thin-films are formed. The assignment of the different vibration bands to specific bonds and their vibration modes, as we extracted it from the scientific literature, are displayed in detail in the supplementary material. Peak positions and vibration modes for the silanes involved in our study are shown in Tables S1-S9, whereas the respective spectra are shown in detail in Figures S1-S27. In the main part of the manuscript, 9 ACS Paragon Plus Environment

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we zoom in to the specific regions of the spectra and compare the three silane types and the two different FTIR characterization methods as following. FTIR-ATR: Three siloxane particle types were formed in a sol-gel reaction with H3PO4 as a catalyst.8, 42, 49, 51 The materials were analysed in the isotropic bulk liquid with FITR-ATR and the resulting spectra of APS (Figures 3a and 3b), GPS (Figures 3c and 3d) and FOS (Figures 3e and 3f) are shown. Every spectrum was baseline corrected (marked as normalized transmission [a.u.]). For a further explanation of this procedure please refer to the supplementary information (Description 1). In all cases, the siloxane formation shows two strong absorption bands of almost equal intensities and can be assigned to the Si-O-Si bridge with stretching vibrations ranging from 1120 cm-1 to 1010 cm-1. Also the medium absorption band for the Si-C stretching vibrations of the siloxane at around 800 cm-1 proves the presence of siloxane among the inorganic side chains. The medium absorption bands in the range of 1190-960 cm-1 can be considered as the symmetric Si-O-CH2 vibrations of the residual ethoxy-groups in the APS bulk and the methoxy-groups in the GPS bulk. This demonstrates that the hydrolysis from the reaction of the organylalkoxy-silanes over the monomer formation into the silanol via the SN1 reaction mechanism was not fully completed. During the hydrolysis, product mixtures can be formed on the tetrahedrally coordinated silicon atom, which can be substituted by H2O groups. A closer determination shows that these absorption bands of OH relative to the CH2 vibrational band are more pronounced in the FOS spectrum than in the APS and GPS spectra. This distinction caused by the leaving group potential is thermodynamically and sterically controlled, leading to faster (more complete) hydroxyl formation between 3480 cm-1 to 3250 cm-1 for FOTCS compared to GPTMS and APTES. Detailed kinetic studies without water or bases (APTES contains -NH2) already proved that steric and miscibility aspects influence the kinetics of this reaction.1 The medium absorption band in the wavenumber range from 955 cm-1 to 855 cm-1 can be referred to the OH stretching vibration of the silanol derivate. This remaining silanol interstate product shows that a complete condensation is not given experimentally, as it was theoretically assumed due to miscibility, time and concentration aspects.


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Figure 3. FTIR-ATR spectra of the three siloxane particle types in the isotropic bulk liquids: The characteristic absorption bands can be assigned for APS between 3800-2500 cm-1 and 1200-650 cm-1 (a) and (b), respectively. For GPS (c) and (d) the absorption range is characteristic between 3800-2500 cm-1 and 1240-650 cm-1. In (e) and (f), the vibrational bands for FOS appear between 3800-2500 cm-1 and 1225-650 cm-1.

Furthermore, both APTES and GPTMS have a lower elimination potential than FOTCS (Figures 3e and 3f). The halogen atom at the tetrahedral silicon is more favored due to the stabilized chloride-ion properties. To the best of our knowledge, it is not known if some of the chloride remains attached to the surface or if HCl evaporates completely. The expected reactivity of the three leaving groups can be classified based on these results as following: SiCl- > Si-O-CH3 > Si-O-C2H5. This fundamental difference in the reactivity of the silanes to 11 ACS Paragon Plus Environment

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form the silanol for the condensation reaction on the solid surfaces is of major importance. Quite often, this is an underestimated factor in the typically applied protocols for wet or gasphase silanization. Due to the minimal thickness of the thin-films, the atmospheric humidity is already sufficient for a complete condensation. FTIR-ERAS: The highly sensitive, non-destructive FTIR-ERA spectroscopy allows the investigation of the molecular structures of the thin-films directly on solid surfaces. In contrast to measurements in transmission geometry, the reflection configuration depends on the angle of incidence of the light beam, on the preferential molecular orientation of the layer, and on the polarization of the surfaces.42-43, 50 To generate a sufficient reflection, however, a 200 nm thick copper layer was needed for our experiments. This enabled us to also generate meaningful spectra from the ultra-thin siloxane films. The reflection geometry utilized in our experiments is also dependent, inter alia, on the orientation of the vibration modes relative to the surface. Due to the polarization of the surface, p-polarized light has to be used in a reflection measurement to stimulate vibrations. Henceforth, the absorbance of vibration modes (Figures 4a and 4f) with a vertical component of the dipole moment relative to the surface is strongly enhanced, whereas vibration modes with a horizontal component of the dipole moment relative to the surface vanish. The baseline corrected (marked as normalized transmission) FTIR-ERA spectra of the APS (Figures 4a and 4b), GPS (Figures 4c and 4d), and FOS (Figures 4e and 4f) thin-films illustrate strong absorption bands in the range of 1120-1010 cm-1, which can be identified as the covalent Si-O-Si bridge between the siloxane layer and the solid surface. The additional bands between 2980-2815 cm-1 correspond to the symmetric and asymmetric CH2 vibrations of the carbon chains. When comparing the FTIR-ERA and FTIR-ATR spectra regarding their relative intensities of the vibration modes and respective peak positions, significant differences can be seen. In the case of FOS in both spectra the symmetric C-F stretching vibrations are detected at 1250 cm-1. The adsorption band has a strong intensity in the ATR spectrum, but it appears with a weaker band in the ERA spectrum. The band of the C-F deformation vibration at 800 cm-1 cannot be identified in the ERA spectrum. Furthermore, the intensities of the symmetric and asymmetric CH2 vibrations from 2980-2815 cm-1 clearly increase relative to the intensities of the CF2/3 vibrations and a new, strong band appears at 640 cm-1 for the CH2 rocking and skeletal vibrations. In addition, the broad medium band between 3700-3200 cm-1 indicates the presence of OH-groups, which are most likely caused by adsorbed water from the 12 ACS Paragon Plus Environment

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condensation reaction, which is remaining on the surface and by the presence of silanol groups (medium band at 955 cm-1 Si-O-H stretching vibration).

Figure 4. FTIR-ERA spectra of the siloxane layers on the Cu/SiO2/Si test wafers: The characteristic APS fingerprint appears between (a) 3800-2500 cm-1 and (b) 1200-550 cm-1. The GPS absorption appears in the ranges of (c) 4000-2500 cm-1and (d) 1200-550 cm-1. The absorption bands for FOS are visible from (e) 38002500 cm-1and (f) 1300-550 cm-1. In FTIR-ERAS the copper layer has the additional advantage that also Si-O-Si vibrational bands of the coating can be seen.

A similar situation can be observed in the FTIR-ERA spectra of APS and GPS. In both spectra, a broad band in the range of 780-550 cm-1 is seen, where the typical CH2 rocking and 13 ACS Paragon Plus Environment

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skeletal vibrations are visible. Furthermore, the range of 1500-1250 cm-1 increases in intensity, where the typical CH2 deformation vibrations are appearing. The epoxy function in the GPS spectrum cannot be identified. Instead, a broad medium band between 4000-3000 cm-1 indicates the formation of primary and secondary alcohol functions, which are most likely caused by the hydrolysis of the terminal epoxy group. In the FTIR-ERA spectrum the vibration bands for the primary amine group of the APS are significantly weaker than those in the FTIR-ATR spectra. Furthermore, several bands can be identified, which correspond to CH deformation, wagging and twisting vibrations, which most likely belong to the alkoxy groups. To clarify these relative intensity differences between the ERA and the ATR spectra, the orientations of the functional groups of the siloxane layers relative to the surface must be considered. As schematically shown in Figures 2a and 2b, the absorption of the vibration modes near the surface are strongly enhanced, if the siloxane layers are adsorbed via the oxygen-groups onto the surface and the side chains are orientated vertically to the surface. In this case, the intensities of the vibration modes decrease further from the surface. This could explain why the epoxy function was not detected in the FTIR-ERA spectra. The C-H vibration of the epoxy ring at 3055 cm-1 and the symmetric and asymmetric ring vibrations between 909 cm-1 and 851 cm-1 have a medium intensity in the ATR spectra, which is such weak, that it is not detectable in FTIR-ERAS. The CH2 vibrational band for the FOS layer is even more pronounced compared to the CF2 vibrational bands. The quality and homogeneity of the ultra-thin siloxane films is strongly dependent on the reactivity regarding the hydrolysis reaction to form the silanol for the following condensation reaction. In summary, our ATR and ERAS characterizations clearly demonstrate the formation of Si-O-Si groups in both reactions. Additionally, the ERAS experiments on copper surfaces confirmed that the Si unit was attached to the surface. Hence, also a similar reaction with the silicon oxide of the SiO2/Si wafers is very likely and a crucial factor for a large area and stable thin-film deposition. This reaction pathway should also be accessible with any other layer containing Si(OR)x(O-)y(OH)z units with x + y + z = 4 ∩ x ε {0,1,2,3} ∩ y, z ε {0,1,2,3,4} like fused silica, glass or silicates4. After clarifying the general reaction conditions in our protocol, the nanoscale siloxane morphology and homogeneity of the thin-films was characterized by AFM and SE. Atomic Force Microscopy: With AFM characterizations, we evaluated the average roughness as mean roughness (roughness average Ra) representing the arithmetic average and the root mean square (RMS) roughness (Rq) of the three HF-etched silicon wafers, the five SiO2/Si 14 ACS Paragon Plus Environment

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wafers with the thermal oxide, and the 9 wafers with the APS, GPS and FOS coatings, respectively. Exemplary tapping mode images and all Ra and Rq values of these characterizations are shown in Figure S28 and are evaluated and summarized in Table S10. All the surfaces were statistically evaluated with three different scan sizes of 10×10, 3×3, and 1×1 µm². A summary of the AFM results is given in Table 2. Besides the discussed parameters for the condensation reaction in isotropic bulk liquid via ATR and with ERA configuration on the solid Cu surfaces, another fundamental factor is the surface roughness, which might strongly influence the thin-film formation processes as well. Table 2. Surface roughness values Rq and Ra for the bare wafers with natural oxide, the controls with the grown dry oxide, and for the three different siloxane coatings on the dry oxide with APS, GPS and FOS, respectively. It can be seen that the naturally grown oxide was much rougher than the dry oxide as expected. All three siloxane layers did not increase the roughness of the surfaces significantly. All values are displayed as mean values of 5 scans of all scan areas ± standard deviation. Scan Area

Natural SiO2 Rq // Ra [nm]

Dry SiO2 Rq // Ra [nm]

APS Rq // Ra [nm]

GPS Rq // Ra [nm]

FOS Rq // Ra [nm]

10×10 µm2

0.43 ± 0.03 0.33 ± 0.02

0.10 ± 0.04 0.08 ± 0.03

0.14 ± 0.03 0.11 ± 0.02

0.13 ± 0.03 0.10 ± 0.02

0.11 ± 0.02 0.09 ± 0.01

3×3 µm2

0.37 ± 0.04 0.28 ± 0.02 0.26 ± 0.02 0.20 ± 0.02

0.12 ± 0.04 0.10 ± 0.02 0.14 ± 0.04 0.12 ± 0.03

0.15 ± 0.03 0.12 ± 0.03 0.17 ± 0.03 0.13 ± 0.02

0.15 ± 0.03 0.12 ± 0.02 0.15 ± 0.03 0.12 ± 0.02

0.12 ± 0.02 0.10 ± 0.03 0.12 ± 0.02 0.10 ± 0.01

1×1 µm2

Table 2 shows the average values from the roughness characterizations by AFM. As expected, the silicon wafers with the naturally grown SiO2 layers had the highest roughness. From our results, it can also be seen that the size of the scanning range had only a minor influence and differences lie in between the statistical errors. Only in the case of the natural silicon oxide the scan range had an influence to the extracted values. The starting roughness of the 141 nm thick dry oxide layer was 0.12 ± 0.04 nm or 0.10 ± 0.03 nm for Rq or Ra, respectively. Compared to this the roughness values for the APS coatings slightly increased with 0.15 ± 0.03 nm or 0.12 ± 0.02 nm for Rq or Ra, respectively, whereas all differences lie in the standard deviation (SD) ranges. Eventually, the APS layers were slightly rougher due to the lower reaction potential of the leaving group in this reaction. A small amount of remaining silanol groups may still exist inside these ultra-thin layers, which might lead to a small increase of the surface roughness. In comparison to this, the GPS coatings showed values of 0.14 ± 0.03 nm or 0.11 ± 0.02 nm for Rq or Ra, respectively, very close to the values of the starting SiO2 layers. The FOS coatings had the expected lowest Rq and Ra values with 0.11 ± 0.02 nm or 0.10 ± 0.02 nm, even a bit lower than the starting layer values. In all cases, very homogeneous thin-films 15 ACS Paragon Plus Environment

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without agglomerates or nucleation were observed (see Figure S28 of the supplementary information). Differences in roughness values were very small for all coatings and varied in the range of the SD values for all experiments. As a summary of the AFM characterizations, all three siloxane types formed homogeneous, closed and agglomeration-free thin-films over the 4-inch wafer areas. This is a crucial result to scale up the protocol from micro- to the macroscale areas in a batch processes. Spectroscopic Ellipsometry: We investigated the thicknesses of the siloxane layers with SE. Since the thickness of the starting SiO2 layers varied over the wafers, for a precise determination the SE data had to be modelled as an optical layer stack with refractive indices

n, extinction coefficients k, and respective layer thicknesses. To verify the thermal oxide dlayer as the initial starting surface, a three-layer model consisting of silicon – silicon dioxide – air was used. Afterwards, for the investigation of the siloxane thicknesses the values for the SiO2 layer were kept constant and the model was expanded to a four-layer stack consisting of

silicon – silicon dioxide – siloxane – air. In order to increase the measuring accuracy, every measuring point was evaluated with three angles of incident at exactly the same position on the wafer using a shadow mask. To evaluate the layer thickness of the respective thin-film the refractive indices nj and the absorption indices kj of each sheet were identified and integrated into the Cauchy dispersion relation. For the determination of the unknown siloxane thicknesses, it was necessary to start the fitting from the theoretical height values of the three siloxane systems under the assumption of a monolayer formation. The hypothetical bond lengths were calculated from the tetrahedron formation of the sp3-hybridized silicon atom, the fully stretched carbon chain with sp³ hybridized carbon atoms and the terminal groups R-NH2, R-O-R, and R-F, respectively. These theoretical thicknesses of a stretched chain ~0.946 nm for APS, ~1.249 nm for GPS, and ~1.518 nm for FOS are provided in Table 3. By fitting the optical layer stack model to the experimental data (Ψ and ∆ spectra), the real and imaginary parts of the complex refractive indices nj + ikj were calculated. The refractive indices nj + ikj for the siloxane layers of APS (n = 1.422, k = 0.0001), GPS (n = 1.429, k = 0.0005) and FOS (n = 1.352, k = 0.0025) were obtained. The calculated n and k values are also included in Table 3. The complete SE results and an exemplary Cauchy fit to an exemplary set of multi-angle SE data are provided in Table S11 and Figure S29 of the supplementary material, respectively. The laser spot for SE characterization (length: 6 mm, width: 3 mm) generally recorded a much bigger area compared to the AFM characterizations. All the thicknesses for each siloxane system were statistically determined with 54 measurement points for two wafers each at 50°, 60° and 70° incident angles, respectively. The 16 ACS Paragon Plus Environment

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average thickness of the APS coating was 1.03±0.92 nm very close to its theoretical monolayer value. For the GPS coating (2.42±1.28 nm) and the FOS coating (2.81±1.02 nm) the measured average values were much thicker than the ones of a theoretical monolayer. However, in all cases the SD values were very large indicating that we are at the limit of detection in our SE configuration, especially with the rather thick SiO2 layers underneath the siloxane thin-films, the measurement accuracy is reduced. Table 3: Comparison of the theoretical and the experimental siloxane thicknesses dlayer with the corresponding SiO2 layer underneath. In addition, the SE characterization provided optical constants for all three siloxane thinfilms at 633 nm. Thickness dlayer SiO2 APS

Theoretical [nm] 140 0.964

Experimental [nm] 141.26 ± 1.09 1.03 ± 0.92

Material Stack SiO2 GPS

SiO2 GPS

142 1.294

144.45 ± 1.40 2.42 ± 1.28

SiO2 APS

SiO2 FOS

141 1.518

139.41 ± 0.96 2.81 ± 1.02

SiO2 FOS

n 633

k 633

1.452 1.422 1.452 1.429 1.452 1.352

0 0.0001 0 0.0005 0 0.0025

Our SE results demonstrate how strongly the siloxane coatings are influenced by the topography and the thickness of the silicon oxide surface (compare detailed results provided in the supplementary material Table S11). With our SE analysis we confirmed, that for all three siloxane types APS, GPS, and FOS ultra-thin and homogeneous layers are formed. However, the standard deviations for the layer thickness for SiO2, APS, GPS and FOS were generally large (Table 3 and Table S11 of the supplementary information). We account the reason for this variation to the initial thickness variation of the substrates. Especially the phase difference ∆ in the SE spectrum is closely related to the optical thickness of the thin-film and the high standard deviations of the siloxane thin-films can be explained by non-uniformities of the initial SiO2. Also the direct fits to our layer stack model causes the large SD values. Generally, the accuracy of our SE instrument fluctuated with ± 0.6 nm according to the manufacturer information, which also significantly affects the thickness analysis of the ultrathin siloxanes. As a summary, the characterization of the ultra-thin siloxane coatings was very challenging and at the limit of detection for our SE instrumentation. Contact angle measurements, contact angle hysteresis and motion behaviours: Different exemplary images of sessile drops on APS-, GPS-, and FOS-coated silicon wafers at different inclination angles are illustrated in Figure 5 (an oxidized silicon wafer was used as the reference sample). The dynamic contact angle behaviour in dependence of the inclination angle resulted in a contact angle range for the APS-coated silicon wafer between 45° to 69°, for the GPS-coated 17 ACS Paragon Plus Environment

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silicon wafer between 44° to 59° and for the FOS-coated silicon wafer between 95° to 115°. A more detailed analysis with all data of dynamic uphill and downhill angles in inclining-angle contact angle experiments is provided in the supplementary information (Figure S30 and Tables S12-S19). Our high-precision CA analysis also proves the successful formation of the hydrophilic APS-, GPS- and the hydrophobic FOS-siloxane thin-films with our gas-phase protocol. As illustrated by the exemplary measurements in Figure 5, all three siloxane thinfilm types show a weak pinning of the triple line. This results in small inclination angles φ and minimal changes of the downhill θd(φ) and uphill θu(φ) angle. The stronger decrease of the uphill angle compared to the downhill angle increase is a known aspect of inclining plate CA experiments and is caused by the interdependence between the uphill and the downhill motion. The pinning of the triple line during such wetting experiments is usually caused by surface roughness and/or chemical heterogeneities. The roughness of all investigated material surfaces were in the range between 0.1 nm to 0.2 nm as we confirmed by AFM characterization. Such small roughness values have negligible influence on the contact angle determination in inclining-angle experiments. Also from these characterizations, we can conclude that all three siloxane coatings have a high uniformity and homogeneity.

Figure 5. Example for sessile drops on (a) an APS-coated silicon wafer at an inclination angle of φ = 0° (top), φ = 20° (middle) and an example for an inclining-angle contact angle measurement (bottom). (b) GPS-coated silicon wafer at an inclination angle of φ = 0° (top), φ = 15° (middle) and an example for a contact angle measurement (bottom). (c) FOS-coated silicon wafer at an inclination angle of φ = 0° (top), φ = 14° (middle) and an example for a contact angle measurement (bottom).


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A quantitative characterization of the triple line pinning is shown in Figure 6 on a test surface. Analysis can be done by the determination of the contact angle hysteresis (CAH) ∆θ = θd,e –

θr,e ≥ 0 between the static advancing/downhill (wetting of a new surface) and the static receding/uphill (formerly wetted surface) contact angle.52-54 These experiments were evaluated by the HPDSA method. Details of this analysis and the procedure of the drop shape fitting are described in the supplementary information (Description S2).

Figure 6. Example for the determination of the static downhill θd,e(φ) (a) and static uphill θu,e(φ) angle (b) in dependence on the shift of the triple points ∆XB1o with HPDSA.

As presented in Table 4, the largest CAH with ∆θd = 23.9±4.5° was detected for the uncoated, oxidized silicon wafer. From the siloxane-coated surfaces, the APS coating resulted in the largest CAH with ∆θd = 17.4±2.2°, whereas the CAH values of the GPS and FOS coated silicon were only ∆θd = 8.8±1.6° and ∆θd = 10.4±1.4°, respectively. These results are in accordance to the already discussed FTIR-ATR and FTIR-ERAS experiments, which proved a slightly better homogeneity for the GPS and FOS compared to the APS thin-films. Consequently, the pinning of the triple line due to local surface heterogeneities was larger in the case of APS compared to GPS and FOS. Table 4. Results for the determination of the static downhill θd,e(φ) and static uphill θu,e(φ) angles on the uncoated and siloxane-coated wafers. Siloxane coating

θd,e(φ) [°]

φd [°]

θu,e(φ) [°]

φu [°]

∆θd [°]

Uncoated activated

12.1 ± 2.0

8.3 ± 3.0

5.4 ± 0.7

3.1 ± 1.2

6.6 ± 1.3

Uncoated oxidized

53.5 ±1.0

6.2 ± 1.2

29.6 ± 3.9

29.6 ± 5.3

23.9 ± 4.5

APS

64.7 ± 0.8

6.2 ± 1.3

47.3 ± 1.8

15.6 ± 2.0

17.4 ± 2.2

GPS

55.7 ± 1.2

2.1 ± 1.2

46.9 ± 1.4

9.1 ± 1.2

8.8 ± 1.6

FOS

109.8 ± 3.0

4.4 ± 0.5

99.4 ± 3.5

8.0 ± 1.8

10.4 ± 1.4


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Overall curve shape of the contact angle data by Gompertzian fitting: The contact angles θd,u(φ) relative to the inclination angle φd,u were fitted for every measuring position (10 defined and identical measuring positions for every sample surface). The overall curve shape analysis is one possibility to characterize the dynamic wetting behaviour of a solid-liquid combination55. = ∙ exp exp !"1 The fitting parameters are θshift = θd and θu, amplitude A of the difference between smallest and largest contact angle and k as the slope of the data points of the averaged Gompertzian functions. A detailed description of the Gompertzian fitting procedure can be found in the supplementary information (Description S3).

Figure 7. Overall Gompertzian analysis on (a) an uncoated and oxidized silicon wafer, (b) an APScoated wafer, (c) a GPS-coated wafer, and (d) an FOS-coated wafer. The limits of the fitting range for


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the downhill side φLd and the uphill side φLu as well as the contact angles with lowest standard deviations θd,u are marked with arrows.

The average data of the Gompertzian fittings for the APS-, GPS-, and FOS-coated silicon wafers are presented in Figure 7. The limits of the fitting range for the downhill side Ld and for the uphill side Lu are manually defined at the point when a drop motion from the static (non-moving/slow-moving) to the dynamic range (high velocity) is observed. The Gompertzian function illustrates an idealized behaviour for the contact angles relative to the inclination angle without any pinning of the triple line. That means that even small deviations can be identified by subtracting the fit-function from the measured course of the contact angles. Furthermore, this analysis strategy can describe a huge amount of contact angle data (between 3000 to 16000 images/contact angles per measurement/surface) by only four fitting parameters. Therefore, an average Gompertzian function can be calculated by the fit-functions for every measuring position. This procedure leads to average slopes, which can be used to identify contact angles with smallest standard deviations θd, θu, (Table 4 and 5). The average Gompertzian function describes here the dynamic wetting behaviour of 50 µl droplets of the test liquid on the uncoated and oxidized silicon wafer as well as on the APS-, GPS-, and FOScoated silicon wafers. Table 5. Summary of the averaged data from the Gompertzian fitting for the APS-, GPS-, and FOScoated wafers: + [°-1]

,%&'()

( -° = $/.'

[°]

* [°]

(0. ,0. = $/0. ,0.

[°]

φLd [°]

[°]

uncoated

49.87

8.93

0.113

5.51

51.2

22.80

57.6

APS

60.44

6.05

0.304

2.81

61.0

8.65

65.5

GPS

54.82

5.09

0.238

3.04

55.5

13.33

59.5

FOS

107.41

5.81

0.343

3.18

107.7

9.11

112.5

$%&'()

+ [°-1]

,%&'()

( -° = $/1'

[°]

[°]

φLu [°]

(01 ,01 = $/01 ,01

[°]

* [°]

uncoated

54.87

-49.63

0.042

-21.82

50.5

26.40

32.7

APS

61.66

-29.2

0.095

-12.23

60.5

17.50

45.8

GPS

55.93

-19.18

0.134

-7.01

54.4

13.43

43.4

FOS

108.13

-16.48

0.159

-5.47

106.6

11.33

97.1

Siloxane coating

$%&'()

[°]

[°]

The parameters A and k of the averaged Gompertzian functions are especially interesting. These parameters can be used to characterize the pinning of the droplets on the surface during the dynamic wetting experiments. For example, strong pinning of 21 ACS Paragon Plus Environment

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the triple line results in large A- and small k-values whereas weak pinning results in small A- and large k-values. By using Gompertzian functions, physically meaningful values are only obtainable within the fitting range of 0 ≤ φd,u ≤ φLd,Lu, which starts at minus infinity. In an ideal case (ideal solid surface, atomically flat, rigid, chemically homogeneous) one can expect that for every measuring position onto the sample surface all Gompertzian functions have a common characteristic contact angle/inclination pair θd/φd, and intersection point θu/φu. Due to local inhomogeneities concerning roughness and/or chemical properties, measurements on real solids will result in a variation from the idealized contact angle behaviour. Therefore, the Gompertzian analysis offers the possibility to identify contact angle/inclination angle pairs/ranges with lowest standard deviations. These values characterize the overall dynamic wetting behaviour of a sample surface (Table 6). Table 6. Results of the overall curve shape analysis by Gompertzian fitting to identify the contact angles θd,u with lowest standard deviation on an uncoated and oxidized silicon wafer as well as on the APS-, GPS-, and FOS-coated wafers. Siloxane coating

θd ± σd

φd

θLd ± σLd

φLd

θu ± σu

φu

θLu ± σu

φLu

[°]

[°]

[°]

[°]

[°]

[°]

[°]

[°]

uncoated,

58.0

27.0

57.6

22.8

28.7

31.8

32.6

26.4

oxidized

± 0.5 65.7

± 0.6 10.6

65.6

± 0.6 8.7

52.5

± 0.9 10.6

45.8

17.5

APS ± 0.8

± 1.0

58.9

59.2

± 0.9 12.7

53.1

10.7

GPS ± 0.6

112.5

± 0.3 9.1

13.2

± 0.7

97.6

8.2 ± 2.3

43.6 2.3

± 0.6

112.3 FOS

± 1.2

97.1

11.3

10.3 ± 2.3

± 2.3

± 2.3

The contact angles with the lowest standard deviation θd,u are located within the fitting range and therefore called ‘real’. The downhill angle for APS is located beyond the fitting range and therefore called ‘imaginary’. In this case, it is physically more reasonable to present the angles at the limit of the fitting range. These are also the contact angles, which are influenced less by the motion dynamic of the droplets, which generally results in larger standard deviations.


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Also our very detailed CA analysis shows that all three siloxane coatings result in less pinning indicated by smaller A and larger k values compared to the SiO2 starting surfaces. The GPS and FOS thin-films show a slightly better quality than the APS thinfilms in this analysis method as well. Summary and Conclusion We describe a versatile gas-phase deposition protocol for the formation of ultra-thin siloxane coatings over large areas. For three, commonly used silane types APTES, GPTMS, and FOTCS we optimized the reaction conditions and characterized the siloxane thin-films in detail. The self-limiting reaction of the organofunctional alkoxysilane molecules on surfaces is leading to large area thin-films close to monolayer coverage. With high-resolution FTIR, we gained a deeper understanding of the surface reactions and covalent bond formation. Thereby the reactivity of the silane types strongly depends on the type of their alkoxy-group. In the nanoscale region, the ultra-thin siloxane films revealed smooth and uniform surfaces without any agglomeration or nucleation effects as confirmed by AFM. With spectroscopic ellipsometry, we found that all siloxane thin-films had a thickness close to an idealized monolayer. From these experiments, we extracted the optical constants n and k for each ultra-thin siloxane type, which might be useful to other researchers in the field. We confirmed the large area quality/homogeneity by precise contact angle analysis using temperature-controlled, inclining angle experiments in combination with precise fitting and modelling. In accordance with the FTIR analyses, the coating with APS resulted in slightly more heterogeneous thin-films compared to GPS and FOS. The characterization methods we applied in our study span from nano- over micro- to macro-scale and confirmed homogeneous, ultra-thin siloxane coatings for all three siloxane types. In the field of biosensor research, wet chemical protocols for the formation of siloxane thinfilms are widely used. These procedures also lead to similar coating quality when executed correctly, but with the drawback that scaling up would consume a lot of solvent and would generate a lot of waste. The gas-phase silanization protocol as we described it here could be done with simple tools as well. Contact of the silane material with air could be avoided by using polymeric inflatable bags filled with inert gas in contrast to the glovebox setup as we used it in our study. Activation of oxide surfaces can be done in a plasma cleaner, which is widely accessible in chemical labs. A standard glass desiccator attached to a small-size vacuum pump is sufficient to generate the silane vapor during deposition. It is to be noted that some of the educts from the reaction are aggressive and might damage the pump and its tubing over time. However, once this setup is established, high quality siloxane coatings 23 ACS Paragon Plus Environment

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without the use of any solvents can be realized with minimum consumption of silanes. Most importantly, since no solvent is involved in the process, the protocol is also applicable to fully encapsulated devices with epoxy or polydimethylsiloxane (PDMS) materials that might be incompatible with some of the wet chemical solvents and protocols. Therefore, we are fully convinced that the coating procedure as described in this manuscript is valuable for many researchers in the field, who seek for an adaptable and highly reproducible silanization protocol.

Acknowledgements The authors gratefully acknowledge Prof. Wulff Possart for his support and Prof. Michael Springborg for using the C3MSaar cluster for the HPDSA calculations. W.M.M. acknowledges generous financial support by Euroimmun Medizinische Labordiagnostika AG. X.L. was funded by the EU FP7 ITN project PROSENSE (grant no. 317420, 2012−2016) and F.H. was funded by the Stiftung Rheinland-Pfalz für Innovation, Germany (grant no. 1082).

Author information: * Corresponding author - current address: Department of Electrical Engineering and Information Technology, Institute of Materials in Electrical Engineering 1, RWTH Aachen University, 52074 Aachen, Germany E-Mail: [email protected] Fax: +49 (0)631/3724-5413 Tel.: +49 (0)631/3724-5313 † The high-precision drop shape analysis software is free for scientific use and can be obtained from F. Heib ([email protected]) or M. Schmitt ([email protected]).


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Silane Deposition via Gas-Phase Evaporation and High-Resolution

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