Gary S. Calabrese

Covalent Binding of Pd Catalysts to Ligating Self‐Assembled Monolayer Films for Selective Electroless Metal Deposition

A new approach for the selective electroless (EL) metallization of surfaces is described. Surfaces are modified with a chemisorbed ligand‐bearing organosilane film, and then catalyzed with an aqueous Pd(II) catalyst solution. The catalyzed substrate is then immersed in an EL metal deposition bath to complete the metallization process. The ligating surfaces are produced by molecular self‐assembly of 2‐(trimethoxysilyl)ethyl‐2‐pyridine (PYR) on silicon or silica substrates. The catalyst consists of chloride‐containing aqueous Pd(II) solutions buffered at pH 5.0 to 6.4; oligomeric chloro and/or hydroxo‐bridged Pd(II) complexes act as the catalytic species at the surface. The activity of the catalyst has been characterized and modeled as a function of solution pH, [Cl−], and time from preparation. Adhesion of the Pd(II) EL catalyst to the substrate involves covalent bond formation with the surface ligand. An average minimum Pd(II) level on the surface of ~1015 Pd atom cm2 is shown to be necessary to initiate EL metallization of the substrate with an EL Co bath. This process involves fewer steps and displays improved selectivity compared to processes that involve a conventional Pd/Sn catalyst. Fabrication of high resolution metal patterns using the new metallization chemistry in conjunction with deep UV patterning of PYR films is demonstrated.

Characterization of n‐Type Semiconducting Tungsten Disulfide Photoanodes in Aqueous and Nonaqueous Electrolyte Solutions Photo‐oxidation of Halides with High Efficiency

Synthetic, single crystal, n-type semiconducting WS/sub 2/ (bandgap /approximately equals/ 1.3 eV) has been characterized as a photoanode in aqueous and nonaqueous electrolyte media. The WS/sub 2/ was synthesized from the elements by bromine and chlorine transport to yield plates up to 3* 3 mm in dimension. Interface characterization includes (i) cyclic voltammetry in the presence of a large number of fast, one-electron redox couples in CH/sub 3/CN/0.1M(/eta/-Bu/sub 4/N)ClO/sub 4/ solutions; (ii) steady-state photocurrent-voltage properties in aqueous and nonaqueous solutions of X/sup -/ (X/sup -/.Cl/sup -/, Br/sup -/, I/sup -/); (iii) tests of durability; (iv) wavelength dependence of photocurrent and photovoltage; and (v) high resolution (approximately 5 /mu/m) laser mapping of the surface to reveal surface inhomogeneity with respect to output photovoltage. 27 refs.

Size‐Controlled Colloidal Pd(II) Catalysts for Electroless Ni Deposition in Nanolithography Applications

A new Pd(II) electroless metal deposition catalyst dispersion, PD2, prepared by quenching a PdCl 4 2 solution with HCl and excess NaCl following rapid hydrolysis at pH ∼ 7 and ∼0.8 mM NaCl is described. The precursors to the catalytic Pd(0) species are shown to be chloride-rich Pd(II) colloidal particles having negative surface charge by x-ray photoelectron spectroscopy, UV-visible spectroscopy, centrifugation, and chemical tests. The particles bind selectively and covalently at ligand-modified surfaces with complete surface coverage occurring for treatment times ≥2 min. Atomic force microscopy indicates that the average and maximum sizes of the bound particles are 9 ± 3 and 18 nm, respectively. A correspondingly narrow distribution (15 to 33 nm) of Ni particles of average size 21 ± 5 nm is obtained following metallization of catalyzed surfaces. The ability to control Ni particle morphology using PD2 is successfully exploited in the selective metallization of ∼15 nm features patterned by scanning tunneling microscopy. Metallization occurs with minimal distortion of feature geometries and no pattern degradation due to Ni overgrowth or bridging of adjacent features. Catalyst behavior is well described by a model in which domination of particle nucleation events and dispersion medium chemistry during colloid formation determine particle surface binding, stability, size, and dispersity

Chemical derivatization of electrode surfaces with derivatives of N,N,N′,N′-tetraalkyl-1,4-benzenediamine

Abstract Two N,N,N′,N′ -tetraalkyl-1,4-benzenediamine surface derivatizing reagents have been synthesized and used to derivatize Au, Pt, SnO 2 , and n-Si electrodes. The two new reagents are N,N -dimethyl- N′ -ethyl- N′ -(trimethoxysilyl-4-butyl)-1,4-benzenediamine (I) and N,N,N′,N′ -tetrakis-(trimethoxysilyl-3-propyl)-1,4-benzenediamine (II). Either reagent can be used to modify electrode surfaces with greater than monolayer quantities (>10 −8 mol/cm 2 ) of electroactive material that has redox behavior consistent with a surface-bound N,N,N′,N′ -tetraalkyl-1,4-benzenediamine derivative. Oxidation of the surface-bound material from I or II occurs chemically reversibly at ca. +0.1 V vs. SCE in MeOH or CH 3 CN electrolyte solution and the oxidation is accompanied by large visible spectral changes studied quantitatively using derivatized, optically transparent SnO 2 electrodes. The visible spectral changes are similar to those found for the solution species I and II upon oxidation in terms of molar absorptivity (∈≈12,000 M −1 cm −1 ) and absorption maxima (ca. 620, 580, and 530 nm). The redox potentials, E o′ , for the surface-bound reagents are within 100 mV of the solution species. Additionally, the E o′ for the surface-bound materials in aqueous media are pH dependent as for solution species; at pH=7, the E o′ =+0.04V vs. SCE. Derivatized electrodes are fairly rugged in MeOH, CH 3 CN, or H 2 O electrolyte solutions, provided potentials negative of that necessary to effect the second oxidation are employed; the second oxidation is observed at ca. +0.7 V vs. SCE in CH 3 CN electrolyte solution for either surface-bound or solution redox material. The two-electron oxidized species is very hydrolytically unstable. Electrodes modified with I or II show improved voltammetric response to horse heart ferri-and ferrocytochrome c, E o′ =+0.02 V vs. SCE, at pH=7 compared to the naked electrodes, consistent with the expectation that the surface-bound form of I or II should be a redox mediator for such a biological reagent. Also, derivatized electrodes show improved response for the oxidation of ascorbic acid; the rate constant for oxidation of ascorbic acid by the one-electron oxidized species on the electrode is ca. 10 2 M −1 s −1 . Unfortunately, the derivatized the electrodes do not seem sufficiently durable to be useful in long term practical applications.

Photoelectrochemical Reduction of 2‐t‐Butyl‐9,10‐Anthraquinone at Illuminated P‐Type Si: An Approach to the Photochemical Synthesis of Hydrogen Peroxide

The electrochemical behavior of 2‐t‐butyl‐9,10‐anthraquinone, BAQ, has been investigated at illuminated (632.8 nm, ~50 mW/cm2) p‐type semiconducting Si in with and without added . In the absence of the BAQ is photoreducible at the photocathode to form . Relative to vs. SCE the photovoltage, at open circuit is ~0.5V. An efficiency for a regenerative photoelectrochemical cell based on the is ~4% for the conversion of 632.8 nm light. In the presence of the BAQ is photoreducible to 2‐t‐butyl‐9,10‐dihydroxyanthracene, , with a measured current efficiency of >90% when the p‐type Si photocathode is held at −0.3V vs. SCE in a two‐compartment cell. Preparative, controlled‐potential photoelectrochemical reduction of BAQ to in has been demonstrated for >90% conversion of 0.2M BAQ to . Reaction of the photoelectrochemically generated with from air yields solutions containing . The has only ~2% photoelectrochemical energy conversion efficiency. The low efficiency is apparently a consequence of sluggish electrode kinetics or adsorption problems that lead to a low with respect to the vs. SCE in the presence of .

Novel surface imaging masking technique for high-aspect-ratio dry etching applications

G- and i-line diazonaphthoquinone/novolak photoresist films are surface imaged with g-line, i-line and deep-UV steppers. Following optical exposure, the resist film is treated with aqueous solutions which deposit a catalyst for electroless metal deposition. Wet development of the exposed and catalyzed photoresist results in selective removal of catalyst along with the exposed portion of the underlying photoresist. Upon immersion in an aqueous electroless plating solution, metal is selectively deposited on the unexposed photoresist which is still bearing catalyst to yield a positive-tone plasma etch mask. Oxygen magnetron-enhanced reactive ion etching (O2 MERIE) provides high polymer etch rates (approximately equals 1 micrometers /min) with excellent selectivity (> 300:1) to 70-170 angstrom Ni films. In addition, large ion fluxes produce highly anisotropic etch profiles for faithful pattern transfer. The process has achieved 0.30 micrometers resolution with a 6:1 aspect ratio at 248 nm (0.35 NA). Printing of 0.40 micrometers lines and spaces has been achieved at i-line (0.45 NA) over Al steps.

Positive-tone surface imaging: methodologies for analysis and process control

A variety of analytical and process control techniques have been employed during process development activities for a 0.5 micrometers deep UV positive tone surface imaging process. Examples of applications of these methods for identification of primary positive tone surface imaging issues and process optimization for enhancement of ultimate resolution are described. Advantages and limitations for each technique are discussed.