The modification of surfaces by the deposition of a robust overlayer provides an excellent handle with which to tune the properties of a bulk substrate to those of interest. Such control over the surface properties becomes increasingly important with the continuing efforts at down-sizing the active components in optoelectronic devices, and the corresponding increase in the surface area/volume ratio. Relevant properties to tune include the degree to which a surface is wetted by water or oil. Analogously, for biosensing applications there is an increasing interest in so-called “romantic surfaces”: surfaces that repel all biological entities, apart from one, to which it binds strongly. Such systems require both long lasting and highly specific tuning of the surface properties. This Review presents one approach to obtain robust surface modifications of the surface of oxides, namely the covalent attachment of monolayers.

Covalent surface modification of oxide surfaces.

Nanoscale patterning of materials is widely used in a variety of device applications. Area selective atomic layer deposition (ALD) has shown promise for deposition of patterned structures with subnanometer thickness control. However, the current process is limited in its ability to achieve good selectivity for thicker films formed at higher number of ALD cycles. In this report, we demonstrate a strategy for achieving selective film deposition via a self-correcting process on patterned Cu/SiO2 substrates. We employ the intrinsically selective adsorption of octadecylphosphonic acid self-assembled monolayers on Cu over SiO2 surfaces to selectively create a resist layer only on Cu. ALD is then performed on the patterns to deposit a dielectric film. A mild etchant is subsequently used to selectively remove any residual dielectric film deposited on the Cu surface while leaving the dielectric film on SiO2 unaffected. The selectivity achieved after this treatment, measured by compositional analysis, is found to b...

Self-Correcting Process for High Quality Patterning by Atomic Layer Deposition.

Superlubricity is defined as a sliding regime in which friction, or the resistance to sliding of two relatively moving surfaces, almost vanishes. From a practical point of view, the development and use of new materials that can enable superlubricity (coefficient of friction, COF < 0.01) in moving mechanical systems will have huge positive impact on energy-saving and emission reduction. In this work, the use of a new two-dimensional material, black phosphorus (BP) as a high-performance water-based lubricant additive that can significantly reduce friction and achieve superlubricity has been explored. A lowest COF value of 0.0006 ever measured by an application-orientated ball-on-plate tribometer has been found. Robust superlubricity in the aqueous solution with ultrafine BP nanosheets modified by NaOH (BP-OH) has been observed for a wide range of additive concentrations, contact pressures, and sliding velocities owing to the very low shear resistance of the water layer retained by BP-OH nanosheets. This finding has the potential of opening up a new approach to dramatically reduce or even eliminate friction by using BP nanomaterials as lubricant additives.

Superlubricity of Black Phosphorus as Lubricant Additive.

Both area selective atomic layer deposition (ALD) and area selective molecular layer deposition (MLD) are demonstrated on Cu/SiO2 patterns using octadecylphos- phonic acid (ODPA) self-assembled monolayers as a resist layer. X-ray photoelectron spectroscopy and Auger electron spectroscopy confirm that during a metal oxide ALD process, no growth occurs on ODPA-protected Cu, whereas the metal oxide grows on SiO2 regions of the substrate, for up to 36 nm of metal oxide. The results also show that ODPA blocks the Cu surface from MLD, preventing polyurea deposition for up to 6 nm of film thickness.

A New Resist for Area Selective Atomic and Molecular Layer Deposition on Metal−Dielectric Patterns

Wet chemical surface functionalization of oxide-free silicon

Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the bonding geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity an...

Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces.

Deposition of thin films and grafting of organic molecules on semiconductor surfaces, particularly oxide surfaces, are widely studied as means of passivation and functionalization for a variety of applications. However, organic functionalization of silicon oxide is challenging, as the currently used molecules (silanes and phosphonates) do not form layers that are stable in aqueous environments and present challenges during the grafting process. For instance, the chemical grafting of phosphonates requires high temperature (140 °C) to perform. Modification of SiO2 surfaces with metal oxides is an attractive alternative since strong bonds can be established between metal oxides and relevant molecules (silanes, phosphonates). While such modification is possible using vapor-phase methods, such as atomic layer deposition and physical vapor-phase deposition, wet chemical processing is inexpensive and technologically very attractive. We describe here a simple wet chemical method to deposit an ultrathin layer of m...

Controlled, Low-Coverage Metal Oxide Activation of Silicon for Organic Functionalization: Unraveling the Phosphonate Bond

Several reports on the chemical termination of silicon nitride films after HF etching, an important process in the microelectronics industry, are inconsistent claiming N-Hx, Si-H, or fluorine termination An investigation combining infrared and x-ray photoelectron spectroscopies with atomic force and scanning electron microscopy imaging reveals that under some processing conditions, salt microcrystals are formed and stabilized on the surface, resulting from products of Si3N4 etching Rinsing in deionized water immediately after HF etching for at least 30 s avoids such deposition and yields a smooth surface without evidence of Si-H termination Instead, fluorine and oxygen are found to terminate a sizeable fraction of the surface in the form of Si-F and possibly Si-OH bonds The relatively unique fluorine termination is remarkably stable in both air and water and could lead to further chemical functionalization pathways

Morphology and chemical termination of HF-etched Si3N4 surfaces

Here the microscopic mechanism that leads to the surprising formation of a nanopattern upon methanol reacting with a H-terminated Si(111) surface [Michalak et al., Nat. Mater. 2010, 9, 266–271] is reinvestigated from both theory and experiment. First-principles calculations determine the fully OCH3-terminated Si(111) surface as the thermodynamic ground state in the presence of methanol, seemingly in contrast to the experimentally found nanopattern. At 65 °C the presence of H2 initiates desorption of the methoxy groups and thereby leads to a dynamic equilibrium of the reaction of methanol with the H-terminated Si(111) surface, which is a 1/3 OCH3-terminated and 2/3 H-terminated Si(111) surface, with all OCH3 groups standing as NNNs, corresponding to the nanopattern observed earlier. Investigating fluorine group termination of Si(111) as a test system the present study demonstrates that the combination of theoretical and experimental techniques used here is in fact sufficiently sensitive to resolve complex ...

Nanopatterning on H-Terminated Si(111) Explained as Dynamic Equilibrium of the Chemical Reaction with Methanol

Semiconductor surface cleaning, passivation and functionalization are critical steps for applications such as microelectronics, MEMS, sensors, and solar energy harvesting. Progress has relied in part on the development of surface chemical treatments. However, the ability to characterize surfaces at all steps of processing has also been essential and will be emphasized here. This work focuses on the chemical modification of oxide-free, hydrogen-passivated silicon, including wet chemical treatments, atomic layer deposition, and physical deposition. Characterization methods include in-situ infrared absorption spectroscopy, low energy ion scattering, x-ray photoelectron spectroscopy and mass spectrometry, and is complemented by ex-situ spectroscopic ellipsometry and atomic force microscopy.

Tatiana Peixoto

Papers

Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces.

Controlled, Low-Coverage Metal Oxide Activation of Silicon for Organic Functionalization: Unraveling the Phosphonate Bond

Morphology and chemical termination of HF-etched Si3N4 surfaces

Nanopatterning on H-Terminated Si(111) Explained as Dynamic Equilibrium of the Chemical Reaction with Methanol

Characterization of Semiconductor Surfaces during Surface Conditioning and Functionalization