Susan L. Brandow

Low voltage electron beam lithography in self‐assembled ultrathin films with the scanning tunneling microscope

With a scanning tunneling microscope (STM) operating in vacuum, we have studied the lithographic patterning of self‐assembling organosilane monolayer resist films. Where the organic group is benzyl chloride, the resist layer can be patterned with electrons down to 4 eV in energy. The patterned films have been used as templates for the electroless plating of thin Ni films. Linewidths down to ∼20 nm have been observed in scanning electron micrographs of the plated films. Still smaller features are observed in STM images of the exposed organosilane films.

Selective adhesion of functional microtubules to patterned silane surfaces

We show that microtubule polymers can be immobilized selectively on lithographically patterned silane surfaces while retaining their native properties. Silane films were chemisorbed on polished silicon wafers or glass coverslips and patterned using a deep UV lithographic process developed at the Naval Research Laboratory. Hydrocarbon and fluorocarbon alkyl silanes, as well as amino and thiol terminal alkyl silanes, were investigated as substrates for microtubule adhesion with retention of biological activity. Microtubules were found to adhere strongly to amine terminal silanes while retaining the ability to act as substrates for the molecular motor protein kinesin. Aminosilane patterns with linewidths varying from 1 to 50 microns were produced lithographically and used to produce patterns of selectively adhered microtubules. Microtubules were partially aligned on the patterned lines by performing the immobilization in a fluid flow field. Patterns were imaged with atomic force microscopy and differential interference contrast microscopy. Motility assays were carried out using kinesin-coated beads and observed with differential interference contrast microscopy. Kinesin bead movement on the patterned microtubules was comparable to movement on microtubule control surfaces.

The Morphology of Electroless Ni Deposition on a Colloidal Pd(II) Catalyst

The surface morphology of a surface-bound colloidal Pd(II) catalyst and its effect on the particle sizes with the largest particles reaching approximately 50 nm in diameter. Catalyst surface coverages as low as 20% are found to be sufficient to initiate complete and homogeneous metallization. The distribution of particle sizes for the electroless metal deposit, found to be a function of plating time, is broad with the maximum Ni particle size exceeding 120 nm. Results indicate controlling the size of the bound catalyst is the principal determining factor in controlling the particle size of the electroless deposit. Modification of the surface by depleting the concentration of surface functional groups capable of binding catalyst is used to shift the size distribution of bound catalyst to smaller values. A resulting three-to fourfold reduction in the particle size of the electroless deposit is demonstrated.

Effects of molecular properties on nanolithography in polymethyl methacrylate

High-resolution lithographic performance of polymethyl methacrylate (PMMA) of molecular weights (MWs) of 50, 100, 496, and 950 K is compared. A chain scission model is used to analyze the behavior of the four molecular weight resists. The chain scission model is combined with an empirical dissolution model to successfully describe the edge profile of a bar pattern. Isolated linewidth data for the 100 and 496 K resists both fit a Monte Carlo code generated linespread function that was convolved with a Gaussian of standard deviation 9 nm. The width was comparable to that in the 950 K resist, but a factor of 3 narrower than that found for the 50 K resist. The higher molecular weight, 496 and 950 K resists showed more developer induced swelling than the lower molecular weight resists. In fact, the developer induced swelling limited the ability to develop 40 nm gratings in the 496 and 950 K resists. Reduction in developer strength produced some improvement. Etching of the supporting resist structure in the gratings was also observed, particularly in the 50 and 100 K resists. The 50 K MW resist exhibited the worst grating contrast upon development. Grating enhanced etching relative to 10 μm bar areas exposed with comparable area dose was observed. A 40 nm period grating was defined in the 100 K resist.

Channel‐Constrained Electroless Metal Deposition on Ligating Self‐Assembled Film Surfaces

Channel-constrained metallization is described as a novel process for fabrication of metal features useful as etchmasks and electrical interconnects in microelectronics applications. The method creates a requisite surface reactivity template through patterned exposure and development of photoresist films to open channels to an underlying ligand self-assembled film. Subsequent electroless metal deposition occurs selectively at exposed ligand sites in the channels, which constrain lateral metal growth detrimental to feature critical dimension (CD) control during plating. A characterization of the individual process steps is presented using a positive tone photoresist system as an example. Determination of the exposure and development conditions that promote clearance of photoresist residues from the channels while maintaining adequate feature CD control is identified as an important issue in successfully performing the process. The process has been successfully demonstrated using optical exposure sources and is compatible with a range of substrates relevant for electronics applications, including Si. The high plasma etching selectivity of a thin Ni metal masking layer was used in the fabrication of high aspect ratio structures (≤5:1) in Si.

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

Characterization of a colloidal Pd(II)-based catalyst dispersion for electroless metal deposition

Abstract An aqueous Pd(II) dispersion, useful as a catalyst for the selective electroless deposition of nickel metal at ligand-bearing surfaces, is prepared by the hydrolysis of PdCl 4 2− at pH 5 in an approximately 0.01 mol dm −3 NaCl solution. The catalyst dispersion is characterized by UV-visible absorption spectroscopy, electroless metallization, ultracentrifugation, and electrophoresis. The dispersion is found to consist of a distribution of anionic and uncharged Pd(II) species ranging in type from monomeric to colloidal. The species responsible for the initiation of electroless metal deposition at the ligand surface are identified as colloidal. Atomic force microscopy indicates that the colloidal catalysts are bound at the surface and range in diameter from approximately 4 to 53 nm with an avarage size of 30 ± 12 nm. The behavior of the catalyst dispersion is consistent with a model in which colloid formation is initiated by polymerization of monomeric precursors generated by successive hydrolytic Cl − loss from PdCl 4 2− , and deprotonation of the corresponding aquo-Pd(II) complex(es).

Fabrication of 15 nm wide trenches in Si by vacuum scanning tunneling microscope lithography of an organosilane self‐assembled film and reactive ion etching

Organosilane precursor molecules, here (aminoethylaminomethyl)phenethyltrimethoxysilane, or PEDA, are chemisorbed onto a Si surface forming a monolayer thick film. These films are patterned using the scanning tunneling microscope to locally modify the chemical reactivity and are then used as a template for selective electroless plating of a thin Ni film. Reactive ion etching in a C2F6/O2 mixture transfers this pattern into the substrate. Improvement over previous lithographic performance is achieved by growing the films on a passivated and lightly oxidized Si surface, optimizing the patterning conditions, and improving the metallization and etch chemistry. In this way, we have generated deep trenches on the order of 15±4 nm width, with edge roughness of 3 nm. We believe this demonstrates the resolution limiting factors of this lithographic process.

Imaging layers for 50 kV electron beam lithography: Selective displacement of noncovalently bound amine ligands from a siloxane host film

We report the development of an imaging layer technology for 50 kV electron-beam lithography based upon the displacement of noncovalently bound amine ligands from a siloxane host film. The patterned films were used as templates for the selective deposition of an electroless nickel film resulting in a positive tone imaging mechanism. The deposited nickel was sufficiently robust to function as an etch mask for pattern transfer by reactive ion etching. Metallized and etched patterns with linewidths to approximately 40 nm are demonstrated using an exposure dose of 500 μC/cm2.

PROXIMITY X-RAY LITHOGRAPHY OF SILOXANE AND POLYMER FILMS CONTAINING BENZYL CHLORIDE FUNCTIONAL GROUPS

Silicon wafers coated with films of (p-chloromethyl)phenyl-trichlorosilane or spun coated polyvinyl benzyl chloride were exposed at the University of Wisconsin synchrotron x-ray source using 0.9385 nm radiation (800 MeV) at doses ranging from 50 to 1500 mJ/cm2. Exposure resulted in changes to the surface energy and chemical reactivity of the films. The loss of chlorine and formation of oxidized carbon photoproducts upon exposure was followed as a function of dose using x-ray photoelectron spectroscopy. A corresponding change in surface energy, as monitored by static water contact angle, was also observed. The selective chemical grafting (reductive amination) of amine ligands to the portions of the siloxane and polymer films which have been exposed to proximity x rays definitively establishes the formation of surface aldehyde or ketone groups as an important photochemical pathway. The resulting surface amine was used to covalently bind either a fluorescent tag or a colloidal Pd (II) nanoparticle capable of...