Functionalization of Silicon Surfaces for Device Applications

Over the past decade, there has been increasing interest in functionalizing silicon surfaces, avoiding the ubiquitous oxide overlayer utilized in most integrated circuits based upon silicon. While silicon is clearly the backbone material of the microelectronics industry, an understanding of its surface chemistry still remains in preliminary stages. The native oxide overlayer on silicon chips has served admirably well for most microelectronic applications but as the features on integrated circuits become increasingly small, the ability to tailor the interfacial properties of the surface is extremely desirable. As the well known Moore's law describes, the number of devices on an integrated circuit has been increasing exponentially since the early 1960's, approximately doubling every year to 18 months. The smallest feature on a Pentium processor chip is now about 300 nm, and both industry and academia look to make even tinier surface features and devices as the state-of-the-art moves from ultra-to giga-scale integration, with over 10 to 100 million transistors per chip. Because the surface properties of these nanoscale devices will have crucial effects on their performance, new surface terminations and the chemical tools to access them are required.     

Gas-phase surface functionalization of multi-walled carbon nanotubes with vacuum UV photo-oxidation

For bulk processing of carbon nanotubes, an important first step in adhesion to the nanotubes is often liquid-phase functionalization through chemical oxidation with acids (e.g., nitric and sulfuric), peroxides and/or potassium permanganate. In comparison, gas-phase photo-oxidation can be an alternative to introduce oxygenated functional groups on the surfaces of carbon nanotubes without the generation of liquid waste. In the present study, vacuum UV photo-oxidation of multi-walled carbon nanotube (MWNT) paper was investigated downstream from an Ar microwave plasma. X-ray Photoelectron Spectroscopy (XPS) was used to detect the carbon- and oxygen-containing functional groups in the top 2–5 nm of the sample's surface. The current results are compared to previous investigations using MWNT powder and single-walled carbon nanotube (SWNT) paper showing decreased levels of oxidation in MWNT samples.     

Role of Hydrogen Peroxide in Selective OH Group Functionalization of Polypropylene Surfaces using Underwater Capillary Discharge

Plasma chemical methods are well suited for introducing functional groups to the surfaces of chemically inert polymers such as polyolefins. However, a broad variety of functional groups are often formed. Unfortunately, for further chemical processing such as grafting of molecules for advanced applications a highly dense monotype functionalized polyolefin surface is needed. Therefore, the main task was to develop a selective surface functionalization process, which formed preferably only a single type of functional groups at the surface in high concentration. Amongst the novel plasma methods, the underwater plasma process (UWP) is one of most attractive options to solve the problem of monotype functionalization. Such plasma is an efficient source of ions, electrons, UV-radiation, high-frequency shock waves, radicals such as hydroxyl radical, and reactive neutral molecules such as hydrogen peroxide. In contrast to established gas phase glow discharge processes, the water phase limits the particle and radiation energies and thus the energy input into the polymer. By virtue of the liquid water environment, which moderates highly energetic plasma species, extensive oxidation, degradation, cross-linking and radical formation on the polymer are more limited as compared to gas plasma exposure. The variety of plasma produced species in the water phase is also much smaller because of the limited reaction possibilities of the plasma with water. The possibility to admix a broad variety of chemical additives makes underwater plasma even more attractive. Hydrogen peroxide and the catalyst (Fe-ZSM5) should influence and increase the equilibrium concentration of OH radicals in the underwater plasma process. It was found that these radicals played a very important role in OH functionalization of polyolefin surfaces. Hydrogen peroxide was identified to be the most prominent precursor for OH group formation in the UWP. The catalyst would affect the steady state of OH radical formation and its reaction with the substrate surface and thus accelerates the functionalization process.     

40—SELF-CROSS-LINKING IN HEATED KERATIN

An account is given of an investigation of the influence of dry heat on wool keratin. It is shown by three different chemical techniques that cross-linking occurs up to a temperature at which the amino-acid decomposition becomes marked. Although Iysinoalanine and lanthionine are shown to be formed, these amino acids do not appear to be major contributors to the cross-linking of the heated protein.

The covalent binding of the ?-amino group of lysine was determined quantitatively, and, from the results obtained, it is deduced that this group is involved to a large extent in the formation of cross-links.

On the basis of these findings, it is postulated that amide cross-links are formed through the ?-amino group of lysine by reaction with carboxylic side chains of aspartic or glutamic acids or their amide derivatives.     

Atmospheric-Pressure Plasma Amination of Polymer Surfaces

Using dielectric barrier discharges (DBDs) in suitable gas atmospheres, appreciable densities of amino groups can be generated on polymer surfaces. After the introduction and a few remarks on analytical methods for the determination of functional groups densities, this paper presents a short summary of recent studies on the mechanism of the polymer surface amination from nitrogen and nitrogen/hydrogen mixtures, and possible relevant precursor species. Combination of chemical derivatization with quantitative FT-IR spectroscopy was employed for the determination of primary amino groups densities introduced on polyolefin surfaces in DBD afterglows in N₂ and N₂ + H₂ mixtures. Owing to the possibility to generate atmospheric-pressure plasmas in sub-mm³ volumes, DBD plasmas can be used to modify polymer surfaces area selectively: a new process termed 'plasma printing' can be applied for the achievement of micropatterned surface modifications, such as hydrophilization/hydrophobization or chemical functionalization. Direct-patterning polymer surface modification processes are of interest for biochemical/biomedical applications as well as for polymer electronics. Two examples are presented in more detail: ? the area-selective plasma amination of carbon-filled polypropylene minidiscs to manufacture microarrays with peptide libraries utilizing parallel combinatorial chemical synthesis, and ?the continuous treatment of polymer foils by means of reel-to-reel patterned plasma amination for the subsequent electroless copper metallization, leading to a fast and highly efficient process for the manufacture of structured metallizations for flexible printed circuits or RFID antennas.     

Capability of Differently Charged Plasma Polymer Coatings for Control of Tissue Interactions with Titanium Surfaces

Titanium surfaces were equipped with positively and negatively charged chemical functional groups by plasma polymerization. Their capability to influence the adhesion of human mesenchymal stem cells (hMSCs) and inflammation processes was investigated on titanium substrates, which are representative of real implant surfaces. For these purposes, titanium samples were coated with plasma polymers from allylamine (PPAAm) and acrylic acid (PPAAc). The process development was accompanied by physicochemical surface analysis using XPS, FT-IR and contact angle measurements. Very thin plasma polymer coatings were created, which are resistant to hydrolysis and delamination. Positively charged amino groups improve considerably the initial adhesion and spreading steps of hMSCs. PPAAm and PPAAc surfaces have an effect on the differentiation of hMSCs, e.g., the expression of osteogenic markers in dependence on culturing conditions. Acrylic acid groups appear to stimulate early mRNA differentiation markers (ALP, COL, Runx2) under basal conditions, whereas positively and negatively charged groups both stimulate late differentiation markers, like BSP and OCN, after 3 days of osteogenic stimulation. Long-term intramuscular implantation in rats revealed that PPAAc surfaces caused significantly stronger reactions by macrophages and antigen-presenting cells compared to untreated control (polished titanium) samples, while PPAAm films did not show a negative influence on the inflammatory reaction after implantation.     

Adhesion of nylon-6 to triazine trithiol-treated metals during injection molding

Abstract —An examination was made of the adhesion of nylon-6 resin to treated metals such as phosphor bronze, brass plates, and electronickel platings during injection molding. No adhesion to any of these metals was noted to occur under ordinary injection molding conditions and an aqueous solution of 1,3,5-triazine-2,4,6-trithiol mononatrium (TTN) was thus used to induce adhesion. Following treatment with aqueous TTN solution under optimal conditions, nylon-6 adhered tightly to all the above metals under ordinary injection molding conditions. The TTN treatment led to the formation of surface films containing metal salts of 1,3,5-triazine-2,4,6-trithiol (TT). Conditions were made optimal with regard to time, temperature and TTN concentration. Adherent films were generally formed when bronze and brass were treated for short periods, at low temperature, and at low TTN concentration, although this was not the case with nickel plating. There was no adhesion to nickel plating even for a prolonged treatment time, high temperature, and high TTN concentration. Adherent and non-adherent films did not differ in the chemical structures of the metal salts of TT but they did differ in morphology. Good adhesion was noted in the case of TT-metal salts present at low density on the metal surface. Some films readily reacted with amino compounds under conditions similar to those generally used for the injection molding of nylon. The adhesion was concluded to be due to the formation of interfacial bonds during injection molding.     

A New Concept for Adhesion Promotion in Metal–Polymer Systems by Introduction of Covalently Bonded Spacers at the Interface

A new concept for molecular interface design in metal–polymer systems is presented. The main features of this concept are the replacement of weak physical interactions by strong covalent bonds, the flexibilization of the interface for compensating different thermal expansions of materials by using long-chain flexible and covalently bonded spacers between the metal and the polymer as well as its design as a moisture-repellent structure for hindering diffusion of water molecules into the interface and hydrolysis of chemical bonds. For this purpose, the main task was to develop plasmachemical and chemical techniques for equipping polymer surfaces with monotype functional groups of adjustable concentration. The establishing of monotype functional groups allows grafting the functional groups by spacer molecules by applying usual wet-chemical reactions. Four processes were favoured for production of monotype functional groups by highly selective reactions: the plasma bromination, the plasma deposition of plasma polymers, the post-plasma chemical reduction of O-functionalities to OH-groups, and the chemical replacement of bromine groups by NH₂-groups. The grafting of flexible organic molecules as spacers between the metal layer and polymer improved the peel strength of the metal. To obtain maximal peel strength of aluminium coatings to polypropylene films and occurrence of cohesive failure in the polypropylene substrate, about 27 OH groups per 100 C-atoms or 6 COOH groups per 100 C-atoms were needed. Introducing C_6–11-aliphatic spacers 1 OH or COOH group per 100 C-atoms contributed about 60% of the maximal peel strength of the Al–PP system, i.e. 2 or 3 spacer molecules per 100 C-atoms were sufficient for maximal peel strength.     

43—THE INTRODUCTION OF AMIDE AND ESTER CROSS-LINKS INTO WOOL

An account is given of the examination of carbodiimides and ethyl chloroformate as cross-linking agents for wool. The level of cross-linking was estimated from the extent of swelling of the treated wool in formic acid. Carbodiimide treatments resulted in the condensation of amino and carboxyl side chains to give amide cross-links; ester cross-links were also formed by condensation of hydroxyl and carboxyl side chains, Hydroxylamine treatments, which cleaved the ester but not the amide cross-links, were used to estimate the relative abundance of the two types of cross-link. N-Hydroxysuccinimide strongly promoted the cross-linking of wool with carbodiimides, most of the extra cross-links introduced being esters.

The treatment of wool with ethyl chloroformate introduced relatively few cross-links. The main reactions are believed, on the basis of indirect evidence, to be esterification of carboxyl side chains and conversion of amino side chains to urethanes.     

12—A CRITIQUE OF SOME HYPOTHESES RELATING TO THE HETEROGENEITY OF THE HIGH-SULPHUR PROTEINS OF WOOL

Joubert and Burns prepared a large number of fractions from the high-sulphur proteins of wool and estimated their molecular weights and amino-acid compositions. Their data have been re-examined in order to look for statistically significant interrelations between amino acids and between the proportion of various amino acids and molecular weight. Statistical analysis of the data is also used to examine the credibility of some hypotheses concerning the mechanism of keratin biosynthesis and to provide further evidence for the existence of families of proteins within the high-sulphur fractions of wool.