Kyung-Hwang Lee

Origin of N 1s spectrum in amorphous carbon nitride obtained by X-ray photoelectron spectroscopy

This paper focuses on determining a reasonable peak assignment for the N 1s spectrum of amorphous carbon nitride (a-CN) measured by X-ray photoelectron spectroscopy (XPS). Unfortunately, several peaks of a-CN have not been identified, that is, there has yet to be any shared understanding of their chemical bonding states. We investigated this by monitoring changes in spectra before and after vacuum ultraviolet (VUV) irradiation, thus obtaining information about the changes of chemical bonds. The observed changes were discussed based on the chemical shifts of components of a-CN determined from ab-initio molecular orbital (MO) calculations. We have proposed the following peak assignment for N 1s. The peaks located at approximately (1) 397.6, (2) 398.2, (3) 399.0, (4) 400.0, (5) 400.9 and (6) 401.9 eV were assigned to the chemical bonding states of (1) β-C3N4, (2) pyridine, (3) formonitril, (4) methylmethyleneamine, (5) pyrido[2,1,6-de]quinolizine, and (6) a nitroso group. This assignment did not agree with oft-quoted ones. However, only this peak assignment provides a reasonable interpretation for the changes we observed in a-CN introduced by VUV irradiation, and allows us to understand the chemical bonding states of a-CN film.

Self-Assembled Monolayers Directly Attached to Silicon Substrates Formed from 1-Hexadecene by Thermal, Ultraviolet, and Visible Light Activation Methods

Using 1-hexadecene as a precursor, self-assembled monolayers (SAMs) were fabricated on hydrogen-terminated silicon (111) [H-Si(111)] surfaces without forming an interfacial oxide layer on the basis of thermal (180 °C, 2 h), ultraviolet (UV; 500 mW cm-2, 10 h), and visible-light activation (330 mW cm-2, 16 h) processes. As characterized by water contact angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and ellipsometry, the hexadecyl SAM fabricated by the visible-light process had a highly-ordered molecular arrangement and a closely-packed methyl-terminated surface similar to the SAMs prepared by the thermal and UV activation processes. The photo-irradiation wavelength dependence of the visible-light activation process was further studied at irradiation wavelengths of 400, 550, and 700 nm. The SAM formation reaction was certainly promoted at all the wavelengths, even at 700 nm. However, oxidation of the Si surface became apparent due to the slow rate in SAM growth, and thus the monolayer coverage of SAM at 700 nm became smaller. The reaction rate became faster with the decreasing wavelength for activation, probably due to the increase in the light adsorption coefficient of Si. The excitation of Si, namely, the generation of hole/electron pairs at the Si substrate, is assumed to be the rate-controlling step of the visible-light activation process.

Surface Chemical Conversion of Organosilane Self-Assembled Monolayers with Active Oxygen Species Generated by Vacuum Ultraviolet Irradiation of Atmospheric Oxygen Molecules

The chemical conversion of the top surface of n-octadecyltrimethoxy silane self-assembled monolayers (ODS-SAMs) on oxide-covered Si substrates using active oxygen species generated from atmospheric oxygen molecules irradiated with vacuum ultraviolet (VUV) light at 172 nm in wavelength has been studied on the basis of water contact angle measurements, ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy. An ODS-SAM whose water contact angle was 104° on average was prepared using chemical vapor deposition with substrate and vapor temperatures of 150 °C. The VUV treatment of an ODS-SAM sample was carried out by placing the sample in air and then irradiating the sample surface with a Xe-excimer lamp. The distance between the lamp and the sample was regulated so that the VUV light emitted from the lamp was almost entirely absorbed by atmospheric oxygen molecules to generate active oxygen species, such as ozone and atomic oxygen before reaching the sample surface. Hence, the surface chemical conversion of the ODS-SAM was primarily promoted through chemical reactions with the active oxygen species. Photochemical changes in the ODS-SAM were found to be the generation of polar functional groups, such as –COOH, –CHO, and –OH, on the surface and the subsequent etching of the monolayer. Irradiation parameters, such as irradiation time, were optimized to achieve a better functionalization of the SAM top surface while minimizing the etching depth of the ODS-SAM. The ability to graft another SAM onto the modified ODS-SAM bearing polar functional groups was demonstrated by the formation of alkylsilane bilayers.

Photochemical Oxidation of Chloromethylphenylsiloxane Self-assembled Monolayer Amplified with Atmospheric Oxygen and Its Application to Micropatterning

An excimer lamp irradiating vacuum ultraviolet (VUV) light 172 nm in wavelength has been applied to the photochemical conversion and micropatterning of a p-chloromethylphenylsilyl self-assembled monolayer (CMPhS-SAM) in the presence of atmospheric oxygen. The terminal functional groups of the CMPhS-SAM, the –CH2Cl, were photochemically converted to polar functional groups, mostly –COOH. This chemical conversion was accelerated with atomic oxygen species photochemically generated through VUV excitation of atmospheric oxygen molecules. By irradiating a CMPhS-SAM sample with VUV thorough a photomask, a microchemical pattern consisting of hydrophobic and hydrophilic regions was formed on the SAM surface. Furthermore, by selectively depositing n-octadecyltrimethoxysilane (ODS) molecules on the hydrophilic region, a binary microstructure consisting of CMPhS and ODS was successfully fabricated with an optimum spatial resolution of 1 µm.

Characteristics and high water-repellency of a-C:H films deposited by r.f. PECVD

Abstract Hydrogenated amorphous carbon (a-C:H) films have been prepared by r.f. plasma-enhanced chemical vapor deposition at low bias voltage from 0 to −40 V with CH 4 and H 2 as raw materials. The a-C:H films deposited with no bias applied showed characteristics of polymeric films with a large fluorescence level while the a-C:H films deposited at a bias voltage of r.f. 150 W showed characteristics from diamond-like carbon to graphitic nature with a significantly reduced fluorescence level. High water-repellency of over 140° in a contact angle obtained at a-C:H/trimetylmethoxysilane/Si structures. This high water-repellency is due to CH groups on surfaces and the roughness.

Correlation between wear-resistance and chemical structure of CNx films synthesized by shielded arc ion plating

Abstract Amorphous carbon nitride (a-CN x ) thin films have been synthesized by shielded arc ion plating under various deposition conditions. Nanohardness and wear resistance of the films were evaluated with a nanoindenter interfaced with an atomic force microscope. The chemical structures of the synthesized a-CN x thin films were studied by Fourier transformed infrared, X-ray photoelectron and Raman spectroscopies. The nitrogen concentration of the films decreased with the increase in negative substrate bias (− V b ). All of the films prepared with applying substrate bias voltage showed wear resistive properties. The proper − V b and incorporation of nitrogen content in the film lead to cross-linking between graphite-like layers which improve nanohardness and wear resistance of a-CN x .

Organic monolayers covalently bonded to Si as ultra thin photoresist films in vacuum ultra-violet lithography

In this paper, we report on the application of our VUV patterning method to a SAM covalently attached to Si substrates as an alchoxy (Si-O-R) form. Although, we have, so far, focused mainly on the VUV-patterning of organosilane SAMs prepared on oxide-covered Si substrates, the organosilane SAMs are not satisfactory durable as a masking material in some types of chemical etching processes. For example, the underlying oxide layer is readily damaged with hydrofluoric (HF) acid and the SAMs are exfoliated in a short time. SAMs covalently bonded to Si through Si-C or Si-O-C bonds are promising as alternatives.

Nano- and micro-tribological characteristics of amorphous carbon films prepared by shielded arc ion plating

Abstract Amorphous carbon (a-C) thin films have been prepared by shielded arc ion plating (SAIP) at an argon gas pressure of 1 Pa by applying a substrate bias voltage between 0 and ∼−500 V. The chemical structure of the a-C thin films was studied by visible Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The nanohardness and wear resistance of the films were measured with a nanoindenter interfaced with an atomic force microscope (AFM). The friction coefficient of the films was determined with a SUJ2 bearing ball using a ball-on-disc tribo-tester. The maximum nanohardness of 42 GPa was obtained when the film was prepared at a negative substrate bias (Vb) of −100 V. This film also showed the highest wear resistance and lowest friction coefficient, although the bearing ball was highly worn down. The nanohardness and wear resistance decreased between −200 and −500 V. The Raman spectroscopy and XPS results showed that the sp3 fraction increased by applying suitable Vb of −100 V.

Scanning capacitance microscopy for alkylsilane-monolayer-covered si substrate patterned by scanning probe lithography

The surface and interfaces of Si substrates covered with an alkylsilane self-assembled monolayer (SAM) were locally modified using the probe tip of an atomic force microscope (AFM) as a minute electrode. The samples were prepared by coating oxide-covered p- and n-type Si substrates with an octadecylsilyl (ODS) SAM. The chemical and electrical properties of the modified regions were characterized by lateral force microscopy (LFM), Kelvin-probe force microscopy (KFM), and scanning capacitance microscopy (SCM). At a sample bias range higher than 6 V, the ODS-SAM was degraded and the probe-modified regions became more frictional, while there were no LFM contrasts on regions probe-scanned at a lower sample bias voltage. Nevertheless, the regions showed clear KFM and SCM contrasts. Such contrasts could not be ascribed to chemical changes in the ODS-SAM, but to charge trapping and deposition in the ODS-SAM and SiO2 layer. In addition, the charge trapping behavior was found to be different depending on carrier type of the Si substrates through the SCM characterization of the modified regions. The complementary use of KFM and SCM was crucial for the electrical characterization of organic-monolayer-covered semiconductor substrates.

Contribution of Primary Chemical Bonding States of Amorphous Carbon Nitride to Hardness

The hardness of amorphous carbon nitride (CN) was investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FTIR). The results of NMR measurements showed that sp 2 and/or sp hybrid orbitals controlled the hardness of amorphous-CN films, which had been determined by nanoindentation tests. The intensity of the C-H and CN stretching modes observed in the FTIR spectra was the weakest in the hardest sample. The decreases of -CH 3 and -CH 2 groups indicated that the amorphous network had increased. The sp 2 hybrid orbital forms a cross-linker which plays an important role in improving of the hardness of the film.