Hydrogen-terminated Silicon Substrates Research Articles

Application of ultra-thin aluminum oxide etch mask made by atomic layer deposition technique

Ultra-thin layers of aluminum oxide (less than 1 nm) were grown by atomic layer deposition (ALD) technique on hydrogen-terminated silicon substrates. A new technique, called ‘‘plasma defect etching’’, was proposed for the continuity evaluation of such a layer. The layer was examined by using it as a mask in silicon etching at cryogenic temperatures in a DRIE reactor. The etch profile was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Island formation during initial cycles was confirmed. Thicker aluminum oxide layers (1-5 nm thick) were patterned by wet or dry (plasma) etching and used as a mask for deep silicon etching at cryogenic temperatures in a DRIE reactor, using SF6 and O2 gas mixture. We found aluminum oxide to be an extremely resistant mask, etched only 0.05 nm/min. The value for the Si to Al2O3 selectivity reached 70 000:1.

Study of Alkyl Organic Monolayers with Different Molecular Chain Lengths Directly Attached to Silicon

Alkyl organic monolayers with different alkyl molecular chain lengths directly attached to silicon were prepared at 160 degrees C from 1-decene (C10), 1-dodecene (C12), 1-tetradecene (C14), 1-hexadecene (C16), and 1-octadecene (C18). These monolayers were characterized on the basis of water contact angle measurement, ellipsometry, X-ray reflectivity (XR), X-ray photoelectron spectroscopy (XPS), and grazing incidence X-ray diffraction (GIXD) to elucidate the effect of the molecular chain length on the molecular arrangement and packing density of the monolayers. Water contact angle and XPS measurements showed that C12, C14, and C16 monolayers have a comparably higher quality, while the quality of C10 and C18 monolayers is worse. GIXD revealed that the alkyl monolayers directly attached to the Si were all amorphously structured regardless of their alkyl chain length. The amorphous structure of the alkyl monolayers could be attributed to the rigid Si-C bonding, low quality of hydrogen-terminated silicon substrate, and/or low mobility of physisorbed molecules.

Controlled grafting of well-defined epoxide polymers on hydrogen-terminated silicon substrates by surface-initiated ATRP at ambient temperature.

Controlled grafting of well-defined epoxide polymer brushes on the hydrogen-terminated Si(100) substrates (Si-H substrates) was carried out via the surface-initiated atom-transfer radical polymerization (ATRP) at room temperature. Thus, glycidyl methacrylate (GMA) polymer brushes were prepared by ATRP from the alpha-bromoester functionalized Si-H surface. Kinetic studies revealed a linear increase in GMA polymer (PGMA) film thickness with reaction time, indicating that chain growth from the surface was a controlled "living" process. The graft polymerization proceeded more rapidly in the dimethylformamide/water (DMF/H(2)O) mixed solvent medium than in DMF, leading to much thicker PGMA growth on the silicon surface in the former medium. The chemical composition of the GMA graft-polymerized silicon (Si-g-PGMA) surfaces were characterized by X-ray photoelectron spectroscopy (XPS). The fact that the epoxide functional groups of the grafted PGMA were preserved quantitatively was revealed in the reaction with ethylenediamine. The "living" character of the PGMA chain end was further ascertained by the subsequent growth of a poly(pentafluorostyrene) (PFS) block from the Si-g-PGMA surface, using the PGMA brushes as the macroinitiators.

Controlled Grafting of Well-Defined Polymers on Hydrogen-Terminated Silicon Substrates by Surface-Initiated Atom Transfer Radical Polymerization

Controlled grafting of well-defined polymer brushes on the hydrogen-terminated Si(100) substrates (the Si−H substrate) was carried out via the surface-initiated atom transfer radical polymerization (ATRP). Surface initiators were immobilized on the Si−H substrates in three consecutive steps: (i) coupling of an ω-unsaturated alkyl ester to the Si−H surface under UV irradiation, (ii) reduction of the ester groups by LiAlH4, and (iii) esterification of the surface-tethered hydroxyl groups with 2-bromoisobutyrate bromide. Homopolymer brushes of methyl methacrylate (MMA), (2-dimethylamino)ethyl methacrylate (DMAEMA), and poly(ethylene glycol) monomethacrylate (PEGMA) were prepared by ATRP from the α-bromoester functionalized silicon surface. The chemical composition and topography of the graft-functionalized silicon surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. Kinetic studies revealed a linear increase in polymer film thickness with rea...

Chemical resistivity of self-assembled monolayer covalently attached to silicon substrate to hydrofluoric acid and ammonium fluoride

Self-assembled monolayers (SAMs) were prepared on hydrogen-terminated silicon substrates through chemical vapor deposition using 1-hexadecene (HD) as a precursor. The HD-SAMs prepared in an atmosphere under a reduced pressure (≈50 Pa) showed better chemical resistivities to hydrofluoric acid and ammonium fluoride (NH 4F) solutions than that of an organosilane SAM formed on oxide-covered silicon substrates. The surface covered with the HD-SAM was micro-patterned by vacuum ultraviolet photolithography and consequently divided into two areas terminated with HD-SAM or silicon dioxide. This micro-patterned sample was immersed in a 40 vol.% NH 4F aqueous solution. Surface images obtained by an optical microscopy clearly show that the micro-patterns of HD-SAM/silicon dioxide were successfully transferred into the silicon substrate.

Nano-scale Morphology and Crystallography of Laser-Deposited TiN Thin Films

Titanium nitride (TiN) thin films were deposited on hydrogen-terminated silicon (100) substrates by pulsed laser ablation of a ceramic TiN target (purity: 99.9%). The crystallography and properties of the thin films are related to the substrate temperature. In the investigation, scanning tunneling microscopy (STM), X-ray diffraction (XRD) and nanoindentation were used. Nano-scale morphology of the thin films deposited at 600°C was observed by STM using a platinum tip. The STM image showed that the TiN embryos have a uniform size of approximately 17 nm and grow into large clusters. The films grown at 600°C have a full-width at half maximum of the TiN (200) peak in the XRD spectrum close to 0.50°. The hardness of the thin films deposited at 600°C was as high as 26 GPa.

Pulsed laser deposition of TiN thin film on silicon (100) at different temperatures

Titanium nitride (TiN) thin films were deposited on hydrogen-terminated silicon (100) substrates by pulsed laser ablation of a ceramic TiN target (purity: 99.9%). A KrF excimer laser with a wavelength of 248 nm and a pulse duration of 23 ns was used as the laser source. The vacuum chamber was maintained at a pressure of 10−5 Torr during the deposition and the substrate temperature ranged from room temperature to 600 °C. X-ray diffraction, Raman spectroscopy, nanoindentation, and surface profile measurements were performed to characterize the properties of the deposited thin films. The dependence of the hardness, stoichiometry, and crystallinity of the thin films on substrate temperature was investigated. X-ray diffraction measurement showed that the full width at half maximum of the TiN (200) line reached 0.24° at 600 °C. The hardness of the thin films deposited at 600 °C was found to be as high as 26 GPa. The influence of the substrate temperature on the electronic properties of the deposited thin films was studied by Raman spectroscopy. Roughness of the deposited thin films was also investigated.

Systematic Studies of Fullerene Derivative Electron Beam Resists

AbstractWe report systematic studies of the response of C60 derivatives to electron beam irradiation. Films of fourteen different mono, tris and tetra adduct methanofullerene C60 derivatives were produced by spin coating on hydrogen terminated silicon substrates. Exposure of the films to a 20 keV electron beam substantially altered the dissolution rate of the derivative films in organic solvents such as monochlorobenzene. All of the derivatives exhibited negative tone resist behaviour with sensitivities between ∼ 8.5 × 10-4 and ∼ 4 × 10-3 C/cm2 107, much higher than that of C60. Features with widths of ∼ 20 nm were produced using these compounds, and the etch ratios of the compounds were found to be more than twice those of a standard novolac based resist (SAL601).

New High Resolution Liquid Crystal Electron Beam Resists

AbstractWe report the development of a new family of electron beam resists based on liquid crystalline polysubstituted derivatives of triphenylene. These new resists show excellent performance in terms of both high resolution and high durability to plasma etching. Films of the derivatives have been produced in a controlled manner via room temperature spin coating on hydrogen terminated silicon substrates. The dissolution behaviour of the derivatives in various organic solvents was altered by exposure to a 20 keV electron beam. The solubility of the derivative hexapentyloxytriphenylene, in polar solvents, was substantially increased by electron doses greater than ∼ 3 × 10-4 C/cm2 (positive tone behaviour). Doses greater than ∼ 2.5 × 10-3 C/cm2 led to negative tone behaviour in both polar and non-polar solvents. Other derivatives also demonstrated a reduction in their dissolution rate for doses between ∼ 1 × 10-3 and ∼ 7 × 10-3 C/cm2. The derivative sensitivity was found to be roughly proportional to the molecular mass. Negative tone patterns were found to have an etch durability ∼ 70 % greater than that of a conventional novolac based negative tone resist (SAL601). The performance of these new resists has been demonstrated by the definition of line and space patterns with a resolution of ∼ 14 nm, whilst structures with an aspect ratio of.∼ 50 to 1 were etched into the silicon substrate.

Detection of Low-Level Copper-Contamination on Silicon Surfaces by Drop Nucleation

ABSTRACTNucleation and growth of liquid drops from the vapor can be used to locate efficiently surface inhomogeneities such as topological defects, oxide patches, metallic impurities, organic contamination, and particles. In this study, nucleation of water drops was used to investigate the surfaces of copper-contaminated silicon substrates. Hydrogen-terminated silicon (111) substrates were dipped into copper-contaminated ultrapure water and exposed to supersaturated water vapor. The amount of copper deposited was varied by changing the strength of the solution. Nucleation occurred at vapor pressures close to saturation. Higher densities of nucleated drops appeared on areas with greater concentrations of copper. Using this technique, it was possible to detect copper concentrations as low as 6×1011 atom/cm2. Below this concentration, treated and untreated substrates could not be distinguished. The extreme sensitivity of the technique to background nucleants shows its potential for efficient screening of surfaces for a large range of inhomogeneities.

Characteristics of titanium nitride films grown by pulsed laser deposition

Laser physical vapor deposition (LPVD) has been used to grow titanium nitride films on hydrogen-terminated silicon(100) substrates at deposition temperatures ranging from room temperature to 600 °C. A pulsed KrF excimer laser (λ = 248 nm, τ = 25 ns) was used with the deposition chamber maintained at a base pressure of 10−7 Torr prior to deposition. Different properties of the films were investigated by x-ray diffraction, Auger electron spectroscopy, Raman spectroscopy, optical, scanning, and high resolution transmission electron microscopy, and measurement of electrical resistivity. When the substrate temperature was low (at and below 500 °C), oxygen atoms from the residual gases were incorporated in the films. The microstructures and resistivities of TiN films were found to be strongly dependent on the temperature of the silicon substrates. The TiN films deposited at 600 °C were oxygen-free, as observed from Auger analysis, and the room temperature resistivity was found to be 14–15 μΩ-cm. Raman spectroscopy of the films showed that the nitrogen-related optical phonon peak increased with deposition temperature in comparison with the titanium-related acoustic peak. Transmission electron microscopy and x-ray diffraction analyses showed that the films were polycrystalline at low temperature with grain size ranging from 300–600 Å, depending on the temperature of the substrate. At 600 °C, the films were found to be single crystals with occasional presence of dislocation loops. The spacing of Moiré fringes in TiN/Si samples deposited at 600 °C established the nearly periodic elastic strain field extending into the TiN and Si at the interface. Although there exists a large misfit between TiN and Si (24.6%), the epitaxial growth of TiN films on Si(100) substrates was explained by means of domain-matched epitaxy with a 4-to-3 match in unit cells for TiN/Si structure, giving rise to a residual lattice misfit of only 4%.

Adhesion of Nanoscale Metal Clusters to Semiconductor Surfaces: A Scanning Tunneling Microscopy Examination

ABSTRACTScanning tunneling Microscopy (STM) is used to investigate the initial stage of metal overlayer formation on semiconductor surfaces. STM scans are presented of preformed nanometer diameter Au clusters deposited at room temperature on molybdenum disulfide and hydrogen terminated silicon substrates. These scans show that the Au clusters are substantially deformed by interfacial forces between the cluster and substrate. The extent of deformation depends on the substrate.

Hydrogen-terminated silicon substrates for low-temperature molecular beam epitaxy

The preparation of hydrogen-terminated silicon surfaces for use as starting substrates for low-temperature epitaxial growth by molecular beam epitaxy is examined in detail. The procedure involves the ex-situ removal under nitrogen of residual oxide from a silicon substrate using a spin-clean with HF in ethanol, followed by the in-situ low-temperature desorption (150°C) of physisorbed etch residues. The critical steps and the chemical basis for these steps are examined using X-ray photoelectron spectroscopy. Impurity residues at the epilayer-substrate interface following subsequent homoepitaxial growth are studied using Auger spectroscopy, secondary ion mass spectrometry, and transmission electron microscopy. Finally, scanning tunneling microscopy is used to examine the effect of cleaning methods on substrate morphology.