N-alkanethiolate Monolayers Research Articles

Selective Cu Electrodeposition for Sub-Micrometer Redistribution Layers

The density of integrated circuit (IC) was doubled by shrinking the feature size in semiconductor industry. An improvement in density of standard interconnection and the packaging technology is much slower than the IC trends. To overcome the increasing I/O interconnect gap, the semiconductor packaging technology has been driven by many researchers and companies over the many years. The redistribution layers (RDL), one of the important parts for the package process, play a role to rearrange I/O connections from the die to substrate. For fine line pitch below 10μm, the damascene RDL process was proposed. The flow of damascene RDL process was composed of photoresist patterning, a metal layer deposition, Cu electrodeposition and chemical-mechanical planarization (CMP). During the Cu electrodeposition for the damascene RDL process, Cu overburdens were formed on top surfaces of sub-micrometer trenches. Typically the CMP process has been used to remove these overburdens in semiconductor industry, however, this surface planarization process caused an increase in process cost. For prevent the Cu overburden at low cost, the selective Cu electrodeposition which is process filling only trench is required. The selective Cu electrodeposition can be achieved by μCP with various additives. The microcontact printing (μCP) process which transfers the self-assembled monolayers (SAMs) on the top surface of trenches can be one of the solutions preventing the Cu deposition on these top surfaces.[1-2] SAMs are organic molecules that formed spontaneously on the solid surface by chemisorption and organization of alkanethiol, silane, and so forth.[3] In this research, flat polydimethylsiloxane (PDMS) stamp is used for providing on the top surface of patterned substrate. Selective Cu electrodeposition was proceeded on trench in CuSO4 1 M/H2SO4 0.58M electrolyte with various additives. In this study, electrochemical behavior of additives was studied by cyclic voltammetry and selective electrodeposition process was investigated with various additives, applied potential, charge density. The µCP process show the effective suppression of Cu growth on the top surface of trenches so that Cu overburden thickness was significantly reduced. This result reveals the feasibility of Cu trench-selective deposition below 10 μm. The μCP method with various additives shows the possibility of replacing surface planarization process which removes overburden of Cu for the RDL process.. [1]Y.Xia and G. Whitesides, "Soft lithography", Annu. Rev. Mater. Sci., vol. 28, no. 1, pp. 153-184, 1998. [2]S.Alom Ruiz and C. Chen, "Microcontact printing: A tool to pattern", Soft Matter, vol. 3, no. 2, pp. 168-177, 2007. [3]P.Laibinis, G. Whitesides, D. Allara, Y. Tao, A. Parikh and R. Nuzzo, "Comparison of the structures and wetting properties of self-assembled monolayers of n-alkanethiols on the coinage metal surfaces, copper, silver, and gold", J. Am. Chem. Soc., vol. 113, no. 19, pp. 7152-7167, 1991.

Friction anisotropy and asymmetry in self assembled monolayers

Experiments with molecularly thin self assembled monolayers have suggested existence of friction force anisotropy and asymmetry. Here molecular dynamics simulations are used to study friction anisotropy and asymmetry between two self assembled monolayers (SAM) of n-alkanethiols [CH3(CH2)n−1S, n=14] attached on Au (111) substrates. The simulations show the existence of friction anisotropy and asymmetry in agreement with some of the experimental works in literature. Friction shows dependence on the relative direction of sliding and monolayers tilt direction. The origin of this friction anisotropy is discussed. The friction asymmetry depends on the direction of sliding and is significant in the direction of the tilt. On this “molecular carpet” interestingly, it is easier to slide against the nap (tilt direction) than in the direction of the nap (tilt direction).

Defect Scaling with Contact Area in EGaIn-Based Junctions: Impact on Quality, Joule Heating, and Apparent Injection Current

Although the tunneling rates decrease exponentially with a decay coefficient β close to 1.0 nC–1 across n-alkanethiolate (SCn) monolayer based tunneling junctions determined over a multitude of test beds, the origins of the large spread of injection current densities—the hypothetical current density, J0 (in A/cm2), that flows across the junction when n = 0—of up to 12 orders of magnitude are unclear. Every type of junction contains a certain distribution of defects induced by, for example, defects in the electrode materials or impurities. This paper describes that the presence of defects in the junctions is one of the key factors that cause an increase in the observed values of J0. We controlled the number of defects in AgTS-SCn//GaOx/EGaIn junctions by varying the geometrical contact area (Ageo) of the junction. The value of J0 (∼102 A/cm2) is independent of the junction size when Ageo is small (<9.6 × 102 μm2) but increased by 3 orders of magnitude (from 102 to 105 A/cm2) when Ageo increased from 9.6 × ...

Effect of Pressure on Dry and Hydrated Self Assembled Monolayers: A Molecular Dynamics Simulation Study

Molecular dynamics simulation was used to study the effect of pressure on self-assembled monolayers (SAM) of n-alkanethiols [(CH3(CH2) n-1, n=14, 15] on Au (111) for dry systems and in the presence of water. The stress-strain behavior and effects of compression on structural characteristics under various normal pressures have been investigated. We found the effect of compression on tilt and tilt orientation angles for dry and hydrated SAM system. Furthermore, a comparison of the results obtained for tilt and tilt orientation angles for hydrated C14 and C15 indicates a more stable structure for C15 (an odd system) under high pressures. We also found excellent elastic recovery of SAM monolayers with and without water is evidence of their exceptional potential to be used under compression in various conditions. Young’s moduli are calculated for various systems under uniaxial compression.

Effect of Water on Structural and Frictional Properties of Self Assembled Monolayers

Self-assembled monolayers (SAMs) of n-alkanethiols [(CH3(CH2)(n-1), n = 14, 15] on Au(111) in the presence of water have been simulated by molecular dynamics simulation. The behavior and effects of compression on structural characteristics and water penetration into monolayers under different ranges of normal pressures have been investigated. Frictional properties of hydrated SAM systems under various sliding velocities, and loading conditions are examined to explore correlation between the amount of water penetration and friction. Simulations for one odd and one even SAM (C14 and C15) systems have revealed interesting odd-even effects in water penetration and frictional properties. We have also compared the frictional and structural properties of hydrated systems to that of dry SAM-Au (one surface of gold is covered by SAM) and SAM-SAM (both gold substrates are covered by SAM) contacts. The results reveal that the even hydrated SAM (C14) shows lower friction coefficient compared with the odd hydrated SAM (C15). We found the presence of water reduces the friction only at lower pressures; and at higher pressures, dry SAM-Au contacts offer lower friction. It was interesting to see that the lubricity effect of water was much stronger for the odd system and persisted to slightly higher pressures (300 MPa for the even SAM and 700 MPa for the odd SAM). At higher pressures, for both odd and even systems, the presence of water increased the friction. We also found that at low sliding velocities and higher pressures apparent water viscosity was enhanced by up to 3 orders of magnitude, indicating possible solidification.

Evolution of Conformational Order During Self-Assembly of n-Alkanethiols on Hg Droplets: An Infrared Spectromicroscopy Study

This Letter describes Fourier-transform infrared spectroscopy evidence for the evolution of conformational order and coverage during the formation of n-alkanethiol monolayers on microdroplets of mercury from the solution phase. At the highest coverages obtained by self-assembly, the monolayer is characterized by predominantly all-trans conformational order. For partial monolayers obtained at arbitrarily quenched incubation periods, we find a continuous evolution of the chain conformational order with monolayer coverage. Analyzing these results in light of previously reported models from X-ray scattering reveals a complex self-assembly process in which the density-dependent evolution of the chain conformational order is coupled with that of molecular orientation and density.

Alkanethiols as Inhibitors for the Atmospheric Corrosion of Copper Induced by Formic Acid: Effect of Chain Length

Self assembled monolayers (SAMs) of n-alkanethiols of different chain length (4, 6, 8, 12, and 18 carbons in the chain) have been explored as corrosion inhibitors for copper exposed to humidified air containing formic acid, an environment used to mimic accelerated indoor atmospheric corrosion. Near-surface sensitive in-situ infrared reflection/absorption spectroscopy combined with interface sensitive vibrational sum frequency spectroscopy revealed unique molecular information on the role of each SAM during ongoing corrosion. All SAMs protect copper against corrosion, and this ability increases continuously with chain length. Their structural order is high prior to exposure, but an increased disorder is observed as a result of the corrosion process. The protection ability of the SAMs is attributed to a selective hindrance of the corrosion stimulators water, oxygen gas, and formic acid to reach the copper-SAM interface through each SAM, which results in different corrosion mechanisms on SAM protected copper and unprotected copper. This significantly retards the formation of the corrosion products copper hydroxide and copper formate, and results in essentially no formation of cuprite.

Effect of compression on self-assembled monolayers: a molecular dynamics study

Molecular dynamics simulation has been used to study the effect of pressure on self-assembled monolayers (SAM) of n-alkanethiols [CH3(CH2)n−1, n = 12–15] on Au(1 1 1) substrates. Mechanical properties of the monolayer compressed by another bare Au substrate (SAM–Au contact) or by another SAM (SAM–SAM contact) have been calculated. The effects of pressure and molecular length on structural and physical properties such as tilt angle, radius of gyration and gauche defects have been studied. We find odd–even effects in response to pressure for SAM–Au contacts where even alkanethiols show larger resistance against compression. For SAM–Au contacts, odd alkanethiols compared with even ones show considerably larger gauche defects at the functional end of the chains. However, for SAM–SAM systems the odd–even effect is weak and molecular structural deformations are lower than the SAM–Au contacts. For all SAMs examined here we found a fully elastic response in compression and decompression stages under pressures between 0.5 and 4 GPa. The calculated Young's moduli (E) are reported for C12, C13, C14 and C15 for both SAM–Au and SAM–SAM contacts. The results show that the modulus is mostly independent of the chain length and of SAM being a SAM–Au or SAM–SAM contact.

Frictional properties of two alkanethiol self assembled monolayers in sliding contact: Odd-even effects

Using molecular dynamics simulation, we have investigated the structural effects on the frictional properties of self assembled monolayers (SAM) of n-alkanethiols [CH(3)(CH(2))(n-1)SH, n = 12-15] in SAM-SAM contacts attached on Au (111) substrates. We have observed an odd-even effect where friction coefficient for SAM-SAM contacts with n = odd showed consistently higher values than those with n = even. This odd-even effect is independent of the sliding velocity and the relative tilt directions of the SAMs, and persists over a much higher pressure range than that reported before for SAM-Au contacts [L. Ramin and A. Jabbarzadeh, Langmuir 28, 4102-4112 (2012)]. For odd systems higher gauche defects were shown to be the possible source of high friction coefficient. Under the same load and shear rates (comparable sliding velocities), SAM-SAM contacts show mostly higher friction compared to SAM-Au contacts. For SAM-SAM contacts, a more significant increase of friction occurs at higher shear rates due to a shift in the tilt orientation angle. We show SAM-SAM contacts with misaligned relative tilt orientation angle (∼45°-90°) have considerably lower friction compared with those whose tilt orientation angles are almost aligned in the opposite directions and parallel to the shear.

High-fidelity chemical patterning on oxide-free germanium

Oxide-free germanium can be chemically patterned directly with self-assembled monolayers of n-alkanethiols via submerged microcontact printing. Native germanium dioxide is water soluble; immersion activates the germanium surface for self-assembly by stripping the oxide. Water additionally provides an effective diffusion barrier that prevents undesired ink transport. Patterns are stable with respect to molecular exchange by carboxyl-functionalized thiols.

Adatoms at the Sulfur–Gold Interface in 1-Adamantanethiolate Monolayers, Studied Using Reaction with Hydrogen Atoms and Scanning Tunneling Microscopy

Scanning tunneling microscopy is used to study monolayers of 1-adamantanethiolate as they are exposed to gas-phase atomic hydrogen. H-atom reaction results in complete removal of the organic monolayer. The relaxation of the reconstruction present at the gold–sulfur interface results in the formation of gold-atom islands, as well as the addition of gold atoms to extant surface defects such as steps and pits. Characterization of these changes shows that for 1-adamantanethiolate monolayers, 0.18 ± 0.033 monolayers of gold adatoms participate in bonding with thiolate sulfur atoms. This results in a 1:1 Au:S ratio, in contrast to the 1:2 Au:S ratio reported for n-alkanethiolate monolayers. The difference in adatom density implies a qualitative difference in binding between n-alkanethiols and 1-adamantanethiols.

Electrochemical detection of the thermal stability of n-alkanethiolate monolayers on Au(1 1 1)

The thermal stability of alkanethiolate monolayers of different chain lengths was investigated on Au(1 1 1) by means of electrochemical reductive desorption and AC impedance measurements. The thermal desorption was performed by heating the freshly prepared monolayers between 298 and 573 K in a home-built device under a nitrogen atmosphere. The samples were then transferred to an electrochemical cell and were examined by means of AC impedance spectroscopy and reductive desorption experiments at 298 K. The electrochemical experiments allowed an accurate detection of the coverage of alkanethiolate molecules as a function of heating temperature. The electrochemical detection of the thermal stability allowed us to determine the desorption temperature and the activation energy barrier for the desorption process without the need for ultra-high vacuum (UHV) conditions. The energy barriers for the desorption process are nearly insensitive to the chain length. Density function theory calculations including van der Waals (vdW) interactions were performed to obtain some insights into the nature of the desorption process. We conclude that the monolayer is in a disordered liquid state prior to desorption with most vdW interactions broken.

Comparison of Oligo(ethylene glycol)alkanethiols versus n-Alkanethiols: Self-Assembly, Insertion, and Functionalization

We describe the self-assembly and chemical functionalization of oligo(ethylene glycol)alkanethiol (OEG) molecules. Insertion of OEGs into n-alkanethiolate monolayer matrices depends considerably on terminal functionality, unlike insertion of n-alkanethiols. Thus, inserted fractions of OEGs cannot be inferred from related systems, yet tuning, to some extent, is possible by controlling insertion parameters. Furthermore, while the in situ reactivities of dilute inserted carboxy- or amine-terminated OEGs versus n-alkanethiols protruding from the surrounding matrix are similar in amide bond formation reactions, complete monolayers of OEGs react to a greater extent compared to n-alkanethiols with similar terminal functionalities. We interpret these differences in terms of the reduced crystalline packing of terminal ethylene glycol groups of OEGs.

Odd−Even Effects in Charge Transport across Self-Assembled Monolayers

This paper compares charge transport across self-assembled monolayers (SAMs) of n-alkanethiols containing odd and even numbers of methylenes. Ultraflat template-stripped silver (Ag(TS)) surfaces support the SAMs, while top electrodes of eutectic gallium-indium (EGaIn) contact the SAMs to form metal/SAM//oxide/EGaIn junctions. The EGaIn spontaneously reacts with ambient oxygen to form a thin (∼1 nm) oxide layer. This oxide layer enables EGaIn to maintain a stable, conical shape (convenient for forming microcontacts to SAMs) while retaining the ability to deform and flow upon contacting a hard surface. Conical electrodes of EGaIn conform (at least partially) to SAMs and generate high yields of working junctions. Ga(2)O(3)/EGaIn top electrodes enable the collection of statistically significant numbers of data in convenient periods of time. The observed difference in charge transport between n-alkanethiols with odd and even numbers of methylenes--the "odd-even effect"--is statistically discernible using these junctions and demonstrates that this technique is sensitive to small differences in the structure and properties of the SAM. Alkanethiols with an even number of methylenes exhibit the expected exponential decrease in current density, J, with increasing chain length, as do alkanethiols with an odd number of methylenes. This trend disappears, however, when the two data sets are analyzed together: alkanethiols with an even number of methylenes typically show higher J than homologous alkanethiols with an odd number of methylenes. The precision of the present measurements and the statistical power of the present analysis are only sufficient to identify, with statistical confidence, the difference between an odd and even number of methylenes with respect to J, but not with respect to the tunneling decay constant, β, or the pre-exponential factor, J(0). This paper includes a discussion of the possible origins of the odd-even effect but does not endorse a single explanation.

ChemInform Abstract: Attenuation of Photoelectrons in Monolayers of n-Alkanethiols Adsorbed on Copper, Silver, and Gold.

and 22 A at 554 eV. Estimates of ), obtained by using SAMs as scattering layers are more consistent than those obtained by using thin organic films fabricated by other methods. The relationship between tr and the kinetic energy (KE) of photoelectrons, over the energy range studied by using SAMs, can be fit equally well by the expressions = 9.0 * 0.022K8 or tr o KEp where p = 0.67 + 0.11. Bethe's equation for electron scattering in matter also relates A to KE.

Molecular self-assembly and passivation of GaAs (0 0 1) with alkanethiol monolayers: A view towards bio-functionalization

Properties of as prepared or nanoengineered III–V semiconductor surfaces provide attractive means for photonic detection of different adsorbants from surrounding gaseous or liquid environments. To be practical, this approach requires that the surface is made selectively sensitive (functionalized) to targeted species. In addition, such surface has also to stay stable over extended period of time to make it available for rapid testing. Numerous reports demonstrate attractive properties of GaAs for sensing applications. One of the most fundamental issues relevant to these applications concerns the ability to functionalize chemically, or biologically, the surface of GaAs. The most studied method of GaAs surface functionalization is based on formation of self-assembled monolayers (SAMs) of various n-alkanethiols, HS-(CH 2) n -T (T = CH 3, COOH, NH 2, etc.). In spite of multi-year research concerning this issue, it has only been recently that a comprehensive picture of SAMs formation on GaAs and an understanding of the natural limitation of the SAM–GaAs interface in some bio-chemical sensing architectures has begun to emerge.

Electron tunneling through monolayers of alkanethiols self-assembled on a hanging mercury drop electrode in the presence of aliphatic alcohols

The effect of aliphatic alcohols (1-pentanol, 1-hexanol, 2-hexanol, and 1-heptanol) on the rates of electrochemical electron tunneling across monolayers of n-alkanethiols (CH 3(CH 2) n –SH, n = 8, 9, 11) self-assembled on an expandable hanging mercury drop electrode (HMDE) is described. The electron transfer between the solution dissolved Ru ( NH 3 ) 6 3 + and mercury electrode across the n-alkanethiol monolayer occurs via super-exchange through-bond tunneling (ET) with a tunneling coefficient decay constant β = 1.1/CH 2. Gradual additions of aliphatic alcohols to the solution result in a decrease of the long-range electron transfer rate, by a maximum of 7-fold, dependent upon the alcohol concentration, but independent of the thickness of the n-alkanethiol monolayer (SAM) and the alcohol’s chain length. The mechanism involving competitive adsorption of the alcohol on the SAM surface is proposed to interpret the kinetic data. The previously reported, liquid-like nature of n-alkanethiol monolayers on mercury, allows stepwise expansion of the mercury drop electrodes coated with n-decanethiolate monolayer to ca. 125% of the initial surface area, resulting in an increase of the interfacial capacitance consistent with the overall decrease of the film thickness due to hydrocarbon tilt. The rate of ET, in the absence of alcohols, increases upon the hydrocarbon tilt consistent with the 2-pathway electron tunneling model. In the presence of aliphatic alcohols, the alkanethiol-coated Hg drop can be expanded up to ca. 150%, resulting in Ru ( NH 3 ) 6 3 + electron transfer rates about 10-fold smaller than expected by the 2-pathway model. This result is interpreted in terms of alcohol partitioning into the n-alkanethiol monolayer leading to partly disorganized films.

Effect of Headgroup on Electrical Conductivity of Self-Assembled Monolayers on Mercury: n-Alkanethiols versus n-Alkaneselenols

The relative efficiencies of electron tunneling across self-assembled monolayers (SAMs) of n-alkanethiols and n-alkaneselenols, CH(3)-(CH(2))(n)-XH, where n = 8, 9, 11, and X = S or Se, deposited on mercury electrodes were measured via electroreduction of Ru(NH(3))(6)(3+) in aqueous solutions. Electron tunneling rates across the monolayer films decay exponentially with the monolayer thickness with a tunneling coefficient, beta = 1.1 +/- 0.1 per CH(2) irrespective of the identity of the -XH headgroup. Electron tunneling rates across n-alkanethiol monolayers are ca. 4-fold larger than the rates measured across n-alkaneselenol monolayers containing the same number of carbon atoms, signifying the importance of headgroup/metal contact resistance in electron transfer through SAMs on mercury.

Impedance differences found with n-alkanethiol monolayers prepared by contact printing and solution adsorption

Self-assembled monolayers (SAMs) of n-octadecanethiol, n-dodecanethiol and n-pentanethiol formed by contact printing and solution adsorption are characterized by electrochemical impedance spectroscopy (EIS). Impedance measurements show that monolayers prepared by contact printing are on the order of 5 times less resistive to charge transfer and have greater defect density than monolayers prepared by solution adsorption. Modeling of the average defect size and density show that the size of monolayer defects stays relatively constant independent of the monolayer preparation conditions; however, the density of defects decreases with increasing concentration for the contact printed monolayers. The impedance data also show that monolayers prepared by solution adsorption have fewer defects than those prepared by contact printing.

Modification of copper with self-assembled organic coatings

Self-assembled monolayers of n-alkanethiols (CH 3(CH 2) n−1 SH; n = 12 and 18) were prepared on a copper surface, and the quality and protection efficiency of resulting coatings against aqueous corrosion of copper were investigated. A combination of physical (scanning electron microscopy, energy dispersive X-ray spectroscopy and contact angle measurements) and electrochemical (cyclic voltammetry and impedance spectrometry) methods was used to correlate the structure of the coatings with their barrier properties during exposures in an aqueous solution. Impedance results reveal that the coatings behave almost like an ideal dielectric, with a resistance of several MΩ cm 2 and a coating capacitance that agree well with the value calculated according to the theory of dielectrics. Protection efficiency of chemisorbed alkanethiol coatings determined from dc and ac measurements were above 99%.