Bare Silicon Surface Research Articles

ACID CHLORIDES ON SILICON SURFACES

ANOVEL METHOD FOR PATterning and functionalizing silicon that will enable researchers to selectively deposit amines, alcohols, and proteins on silicon surfaces could find applications in nanotechnology and sensors, according to chemists at BrighamYxing University Provo, Utah ( Langmuir 2005,21,2093). We have shown for the first time that acid chlorides react directly with bare silicon surfaces to create chemisorbed methyl-terminated or acid chloride-terminated monolayers, says assistant professor Matthew R. Linford, who led the team. Thus, we have a fast, straightforward method for patterning silicon with an important functional group—an activated carboxylic acid. The researchers also have demonstrated that the chemisorbed acid chlorides react with the amino groups of amines and proteins. The technique uses scribed silicon—that is, silicon that has been written on or mechanically marked with a tip that moves across the surface and at the same time activates the surface chemically. The surface-adsorb...

First Reaction of a Bare Silicon Surface with Acid Chlorides and a One-Step Preparation of Acid Chloride Terminated Monolayers on Scribed Silicon

Methyl-terminated and acyl chloride terminated monolayers are produced when silicon is scribed under mono- and diacid chlorides, respectively. To the best of our knowledge, this is the first report of the reaction between a bare silicon surface and acid chlorides. This reaction takes place by wetting the silicon surface in the air with the acid chloride and scribing. Scribing activates the silicon surface by removing its passivation layer. We propose that scribed silicon abstracts chlorine from an acid chloride to form an Si-Cl bond and that the resulting acyl radical diffuses back to the surface to condense with the surface and form an alkyl monolayer. X-ray photoelectron spectroscopy (XPS) confirms the presence of chlorine and shows a steady increase in the amount of carbon with increasing alkyl chain lengths of the acid chlorides. Time-of-flight secondary ion mass spectrometry shows SiCl(+) species and a steady increase in representative hydrocarbon fragments with increasing alkyl chain lengths of the acid chlorides. XPS indicates that diacid chlorides react primarily at one of their ends to create acyl chloride terminated surfaces in a single step. The resulting surfaces are shown to react with various amines (piperazine, morpholine, and octylamine) and a protein. Calculations at Hartree-Fock and density functional theory levels are consistent with the proposed mechanism.

Electrochemical polymerization films of patterned polyaniline on Si(1 0 0) surface with microcontact printing

Patterned polyaniline (PANI) films on Si(1 0 0) surface were achieved by microcontact printing technique and electrochemical polymerization in the aqueous aniline solution. Optical microscope, atomic force microscopy (AFM), lateral force microscopy (LFM) and X-ray photoelectron spectroscopy (XPS) were used to examine the topography and properties of patterned PANI films. Optical microscope and AFM results showed that the PANI microstructure has regular pattern and clear boundary, while LFM and XPS results implied that this microstructure was completely covered by PANI. The data suggested that there are different polymerization rate in concordance with the different interfacial properties. That is to say, the interfacial properties control the rate of electrochemical polymerization, and the rate on the modified silicon surface is higher than that on the bare silicon surface. In our experiments, cyclic voltammetic method was employed to deposit patterned PANI films on octadecyltrichorosilane modified Si substrate.

Antibody immobilisation on the metal and silicon surfaces. The use of self-assembled layers and specific receptors

The use of Staphylococcal protein A and lectins as intermediate immobilising agents allows operators to orient antibodies (Ab) towards the solution due to the presence of a specific binding sites of immunoglobulin (Ig) molecules. Antibodies of different species of animals have unequal affinities to individual lectins. The effective thickness of immobilised Ab's depends on the type of substrates used and increases in the following sequence: bare gold or silicon surface, the surface treated with self-assembled polyelectrolytes (PESA) or with protein A or some lectins deposited on the preliminary formed polyelectrolyte layer. The glycolysated protein of jp51 may be selectively immobilised from the mixture of retroviral proteins (p24 and jp51), if it is necessary to distinguish infected animals from preliminarily immunised ones by means of a vaccine based on p24 protein. It was shown that the use of Staphylococcal protein A, instead of some lectins as intermediate layer for the Ab immobilisation, does not lead to a more sensitive determination of such low-weight toxins as 2,4-dichlorophenoxyacetic acid (2,4-D). The above-mentioned results were obtained with surface plasmon resonance (SPR) technique.

A comparative infrared study of H 2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H 2O → H–S–Si–OH + H 2, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm −1. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H 2O → H 2Si ⋯ HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.

Electrografting of polymers onto AFM tips: a novel approach for chemical force microscopy and force spectroscopy.

Grafting makes versatile probes: Polymer-modified AFM tips have been prepared through an easily implemented and versatile electrochemical method (see graphic). The authors describe the electropolymerization process of acrylic-based monomers onto gold-coated tips (see graphic) and show force curves obtained during approach–retraction cycles between the modified tips and a bare silicon surface. This direct chemical grafting creates new possibilities for single-molecule force spectroscopy.

Competitive adsorption of plasma proteins on polysaccharide-modified silicon surfaces

ABSTRACTThe initial response of blood exposed to an artificial surface is the adsorption of blood proteins that triggers a number of biological reactions such as inflammation and blood coagulation. Competitive protein adsorption plays a key role in the hemocompatibility of the surface. The synthesis of nonfouling surfaces is therefore one of the major prerequisites for devices for biomedical applications. Polysaccharides are the main components of the endothelial cell glycocalyx and have the ability to reduce nonspecific protein adsorption and cell adhesion and, therefore, are generally coupled with a wide variety of surfaces to improve their biocompatibility. We have developed a procedure for covalently binding dextran and sodium hyaluronate (HA) on silicon wafers and we have been able to achieve a high level of control over the surface properties of the coatings. In the present research effort we focus on a detailed investigation of competitive bovine serum albumin (BSA) and bovine fibrinogen (Fg) adsorption on dextran- and HA-modified silicon surfaces. Polysaccharide based biomimetic layers preferentially adsorb BSA and, in general, strongly suppress protein adsorption with respect to bare silicon and APTES-activated silicon surfaces used as control.

Amine-Reactive Monolayers on Scribed Silicon with Controlled Levels of Functionality: Reaction of a Bare Silicon Surface with Mono- and Diepoxides

An „nackten“ Silicium-Oberflächen werden Epoxide unter Ringöffnung gebunden (siehe Schema). Mit Diepoxiden entstehen Monoschichten, die mit Aminen reagieren können. Das Ausmaß der Epoxyfunktionalisierung kann durch Verwendung von Gemischen aus 1,2-Epoxyoctan und 1,2,7,8-Diepoxyoctan eingestellt werden.

Amine-reactive monolayers on scribed silicon with controlled levels of functionality: reaction of a bare silicon surface with mono- and diepoxides.

Amine-reactive monolayers on scribed silicon with controlled levels of functionality: reaction of a bare silicon surface with mono- and diepoxides.

Ultrathin HfO 2 films grown on silicon by atomic layer deposition for advanced gate dielectrics applications

We report on growth behavior, structure, thermal stability and electrical properties of ultrathin (<10 nm) hafnium oxide films deposited by atomic layer deposition using sequential exposures of HfCl 4 and H 2O at 300 °C on a bare silicon surface or a thin thermally grown SiO 2-based interlayer. Compared to good quality continuous films deposited on SiO 2 surfaces, HfO 2 deposited on HF-last treated Si surfaces show a non-uniform, island-like morphology and poor electrical properties due to poor nucleation on H-terminated Si. As-deposited films have a significant amorphous component and undergo crystallization to a monoclinic phase above ∼500 °C. Crystallization behavior is found to be dependent on film thickness with higher crystallization temperatures for thinner films. HfO 2 on an ultrathin SiO 2 interlayer shows good electrical properties with gate leakage current reduced by a factor of 10 3–10 4 with respect to conventional SiO 2 gate dielectrics which justifies its consideration as a candidate for high-K dielectric for future CMOS devices.

Theoretical study on the initial processes of nitridation of silicon thin film

The initial processes of nitridation of silicon thin film have been examined by means of the first-principle calculations. The nitrogen atom on the bare silicon(1 0 0) surface stays around the surface due to the dangling bond of the first-layer silicon, and the large stabilization on the bare silicon surface raises the energy barrier of the penetration of the nitrogen atom into the surface. The nitrogen atom on the H-terminated silicon surface can penetrate into the surface by crossing over the small energy barrier. In the nitridation by using nitrous oxide (N 2O) molecule, N 2O can be adsorbed only with the large NN bond length, and then an exothermic elimination of nitric oxide (NO) molecule from the adsorbed nitrogen atom on the bare silicon surface takes place. The electronic processes have been discussed in terms of the quantum mechanical energy densities, which give new images of microscopic electronic stresses in the interaction system.

Silicon dioxide thin film removal using high-power nanosecond lasers

High intensity nanosecond laser pulses of different wavelengths are used to remove thin film silicon dioxide coatings from the silicon surface. Films as thick as 0.7 μm can be removed from the surface in one shot with very minor or no damage to the bare silicon surface. The accumulated effect of multiple-pulse irradiation is studied. It is found that multiple-pulse irradiation results in some reduction of the threshold for coating removal. However, the surface quality gets significantly worse with the increasing number of shots.

Interfacial oxide formation and oxygen diffusion in rare earth oxide–silicon epitaxial heterostructures

We report on controlled interfacial oxide formation within epitaxial (LaxY1−x)2O3/Si(111) heterostructures under UHV environments. Results indicate that exposure of these epitaxial films to molecular oxygen right after deposition results in the formation of an amorphous interfacial layer thicker than that expected when a bare silicon surface is exposed to molecular oxygen under the same conditions. The results imply significant oxygen diffusion through the epitaxial dielectric and reaction at the silicon–oxide interface. Arguments have been developed to explain these observations.

Hyperbranched Molecules with Epoxy-Functionalized Terminal Branches: Grafting to a Solid Surface

We report the fabrication of a molecular surface coating from an epoxy-functionalized hyperbranched polyester functionalized by secondary epoxy groups. These groups were presented in the fraction of terminal branches of the hyperbranched shell with the ratio of epoxy-containing branches and alkyl branches of 1:2. We demonstrated that a uniform monolayer with the thickness of 4.5 nm could be fabricated by melt grafting of functionalized hyperbranched polymers to a bare silicon surface. Steric constraints imposed by the chemical attachment of alkyl and epoxydized branches to a single core prevented microphase separation of dissimilar segments and allowed the fabrication of uniform monolayers with surface exposure of the functional groups and high adhesion. An estimated 3−4 epoxy groups per molecule were located in the uppermost surface layer and provided residual functionality sufficient to graft another polymer layer. Grafted layers were extremely robust and sustain high compression and shear stresses whil...

Fermi-level pinning and passivation on the oxide-covered and bare silicon surfaces and interfaces

The properties of the bare (oxide-free) and oxide-covered silicon have been discussed in details in the literature. Positive charge is usually assumed at the insulator–semiconductor interface, thus depletion or inversion layer develops in the case of p-type, and accumulation in the case of n-type semiconductor. However, the semiconductor (Si) covered by an ultra-thin tunnelable insulator (native SiO 2) layer has some peculiarities, i.e. connection between surface charge and the interface charge carrier density. Theoretical considerations and vibrating capacitor experiments show that the surface charge tends to shift the surface nearer to the intrinsic condition in the case of the ultra-thin insulator-covered semiconductor surfaces.

Improved copper detection in hydrofluoric acid by recombination lifetime measurements on dedicated silicon substrates

Copper (Cu) adsorption from diluted hydrofluoric acid (DHF) onto bare silicon surfaces strongly depends on the substrate doping. On highly phosphorus-doped silicon, the adsorption rate is up to three orders of magnitude larger than on moderately doped silicon. This may open a gap between Cu-induced semiconductor device degradation and the detection of Cu contaminations in DHF by minority carrier lifetime measurements. Using dedicated copper monitor wafers where a highly phosphorus-doped backsurface ensures strong Cu adsorption while a moderately doped frontsurface enables minority carrier lifetime measurements, we are able to improve the limit of detection for Cu in DHF by two orders of magnitude.

Investigation of the tribological behavior of octadecyltrichlorosilane deposited on silicon

In this work, the tribological behavior of octadecyltrichlorosilane (OTS, CH 3(CH 2) 17SiCl 3), which is a self-assembled monolayer (SAM) known for its low surface energy and hydrophobic property, deposited on silicon was investigated. Since the state of OTS depends heavily on the coating time, the effect of coating time on various tribological characteristics of OTS deposited on silicon (1 0 0) was of primary interest. The macroscopic frictional force of OTS deposits was also measured using a pin-on-disk type tribotester. The OTS deposited silicon specimens were slid against a 1.6 mm diameter STB2 steel ball under a normal load of 50 mN. It was found that the macroscopic friction coefficients for coating times >1 h ranged between 0.1 and 0.2 whereas the value for shorter coating time was as high as 0.7. We also found that the frictional behaviors of OTS coated silicon surface were less sensitive to humidity changes compared with bare silicon surface. Furthermore, the stiction of OTS coated silicon was much less sensitive to apparent area of contact, applied load, and soaking time. This work demonstrates the effectiveness of OTS film in reducing sliding friction of silicon micro-systems when deposited properly.

Hyperbranched Polyesters on Solid Surfaces

The interfacial behavior of third and fourth generations of hyperbranched polyesters (HBP3 and HBP4) with 32 and 64 hydroxyl-terminal groups was studied with scanning probe microscopy. The molecular adsorption on a bare silicon surface of both hyperbranched polymers was described in the terms of the Langmuir isotherm. A higher adsorption amount under an identical adsorption condition was found for lower generation HPB3. The shape of HBP3 molecules within an adsorbed layer evolved from a pancake with a thickness less than 1 nm for very low surface coverage to densely packed wormlike bilayer structures with a thickness of about 3 nm for the highest surface coverage. The molecules of the fourth generation, HBP4, hold a stable, close-to-spherical shape with a diameter of 2.5 nm throughout the entire range of surface coverage including both dense monolayers and isolated molecules. High intramolecular flexibility of HBP3 molecules as compared with constrained mobility of bulkier branches of HBP4 is considered t...

Multi-Scale Simulations of Silicon Etching by Halides: Effects of Surface Reaction Rates.

ABSTRACTWe investigate the reactive ion etching of amorphous silicon by halides using a hierarchy of models on different time and length scales. The feature evolution is modeled using a two- dimensional cell based Monte-Carlo feature scale simulator. The fluxes, the energy distributions, and the angular distributions of the wafer-incident particles are provided by a hybrid plasma sheath simulator. The relevant surface reaction rates are calculated by a molecular dynamics simulator using a Stillinger-Weber representation of the interatomic potential. Our investigations show that the surface reaction rates are strongly determined by the particular surface morpho- logy, which, in turn, is strongly influenced by the kinetic properties of the impinging particles. Thus, we link the molecular dynamics simulator into the model as a whole.As results, we present calculations for the etching of amorphous silicon by fluorine, chlorine, and bromine. A Stillinger-Weber representation of the bromine and the silicon-bromine potential which was not yet available in literature is additionally developed. We discuss the different morphologies of halogenated silicon surfaces as a consequence of the energy distri- bution and the angular distribution of the impinging particles. Comparisons of the sputter yield functions of bare amorphous silicon surfaces and corresponding halogenated surfaces exhibit considerable differences, qualitative as well as quantitative.

Microtribological properties of silicon and silicon coated with diamond like carbon, octadecyltrichlorosilane and stearic acid cadmium salt films: A comparative study

A comparative study of the microtribological properties of native oxide covered Si(100) and Si(100) coated with chemical vapor deposition grown diamond like carbon (DLC) films, stearic acid cadmium salts (STCd) Langmuir–Blodgett films and octadecyltrichlorosilane (OTS) self-assembled monolayers is presented. The frictional properties between these samples and a sapphire ball were studied using a micro friction tester as a function of the normal force, sliding speed and relative humidity. While all coatings significantly reduce friction, OTS and STCd coatings show the largest reduction. The bare silicon surface exhibits hydrophilic surface properties with a significant jump-into-contact at zero normal load. All the coated Si samples show negligible dependence on humidity, which is in agreement with their hydrophobic nature, while friction on the oxide covered Si surface rapidly increases with a rise in humidity. The friction vs. normal load curves of oxide covered silicon can be described by the Johnson–Kendall–Roberts model with a pressure-dependent shear strength. The monolayer samples and DLC coated samples appear to possess contact area vs. load characteristics that are approximated by the Hertz contact theory. While the oxide covered Si surface shows a large reduction in friction with sliding speed, all the other samples show a relatively weak dependence.