Hans Biebuyck

Lithography beyond light: microcontact printing with monolayer resists

We describe high-resolution lithography based on transfer of a pattern from an elastomeric stamp to a solid substrate by conformal contact : a nanoscale interaction between substrate and stamp on macroscopic scales that allows transport of material from stamp to substrate. The stamp is first formed by curing poly(dimethyl siloxane) (PDMS) on a master with the negative of the desired surface, resulting in an elastomeric solid with a pattern of reliefs, typically a few microns deep, on its surface. The stamp provides an ink that forms a self-assembled monolayer (SAM) on a solid surface by a covalent, chemical reaction. Because SAMs act as highly localized and efficient barriers to some wet etches, microcontact printing forms part of a convenient lithographic system not subject to diffraction or depth of focus limitations while still providing simultaneous transfer of patterned features. Our study helps to define the strengths and limitations of microcontact printing with SAMs, a process that is necessary to assess its worth to technology. We used lithography based on scanning tunneling microscopy (STM) to demonstrate that disruption of SAMs on gold allowed the formation of etched features as small as 20 nm using a CN - /O 2 etch. This result implied that etching occurred where damage of a few molecules in the ordered SAM allowed passage of cyanide, whereas adjacent molecules in the SAM remained unperturbed at this scale. Features as small as 30 nm 2 etched in gold over areas greater than 1 cm 2 resulted from microcontact printing with replicas of electron-beam-formed masters, with the transfer of these printed SAMs requiring only 1 s. STM studies of these transferred SAMs revealed an achievable order indistinguishable from that found for SAMs prepared from solution. Facile alignment of printing steps at submicron scales may result from new designs of stamps that exploit their limited deformability and lock-and-key-type approaches to mate stamp and substrate.

Affinity capture of proteins from solution and their dissociation by contact printing

Biological experiments at the solid/liquid interface, in general, require surfaces with a thin layer of purified molecules, which often represent precious material. Here, we have devised a method to extract proteins with high selectivity from crude biological sample solutions and place them on a surface in a functional, arbitrary pattern. This method, called affinity-contact printing (αCP), uses a structured elastomer derivatized with ligands against the target molecules. After the target molecules have been captured, they are printed from the elastomer onto a variety of surfaces. The ligand remains on the stamp for reuse. In contrast with conventional affinity chromatography, here dissociation and release of captured molecules to the substrate are achieved mechanically. We demonstrate this technique by extracting the cell adhesion molecule neuron-glia cell adhesion molecule (NgCAM) from tissue homogenates and cell culture lysates and patterning affinity-purified NgCAM on polystyrene to stimulate the attachment of neuronal cells and guide axon outgrowth.

LIGHT-COUPLING MASKS FOR LENSLESS, SUB-WAVELENGTH OPTICAL LITHOGRAPHY

Light-coupling masks (LCMs) based on structured organic polymers that make conformal contact with a substrate can constitute an amplitude mask for light-based lithographies. The LCM is exposed through its backside, from where the light is differentially guided by the structures towards the substrate. Images of arbitrarily shaped features having dimensions much smaller than that of the vacuum wavelength of the exposing light are formed in the resist in a 1:1 correspondence to their size in light-guiding portions of the mask. LCMs allow pattern replication at high resolution and densities over large areas in photoresist without the need for elaborate projection optics.

X-ray Damage to CF3CO2-Terminated Organic Monolayers on Si/Au: Principal Effect of Electrons

The relative importance of x-rays alone and of x-ray-generated primary and secondary electrons in damaging organic materials was explored by use of self-assembled monolayers (SAMs) on multilayer thin-film supports. The substrates were prepared by the deposit of thin films of silicon (0, 50, 100, and 200 angstroms) on thick layers of gold (2000 angstroms). These systems were supported on chromium-primed silicon wafers. Trifluoroacetoxy-terminated SAMs were assembled on these substrates, and the samples were irradiated with common fluxes of monochromatic aluminum Kα x-rays. The fluxes and energy distributions of the electrons generated by interactions of the x-rays with the various substrates, however, differed. The substrates that emitted a lower flux of electrons exhibited a slower loss of fluorine from the SAMs. This observation indicated that the electrons—and not the x-rays themselves—were largely responsible for the damage to the organic monolayer.

Monolayers of 11-Trichlorosilylundecyl Thioacetate: A System that Promotes Adhesion Between Silicon Dioxide and Evaporated Gold

Abstract : This chapter describes the use of sulfur-containing organic monolayers to improve the adhesion of gold to silicon substrates having a native silicon dioxide surface layer. Gold adheres to clean silicon, but not to silicon dioxide. The affinity of gold toward silicon dioxide can be improved by coating with chromium or titanium films or by adding interlayers containing fluoride salts. Bombardment of gold-covered silicon dioxide with electrons or heavy ions also enhances adhesion. Thin covalently-bonded alkylsiloxane films containing amines or epoxides improve the adherence of gold to glass. Here we demonstrate that covering a Silicon/Silicon dioxide substrate with a covalently attached organic monolayer film containing thiol groups (and possibly disulfides derived from them) or thioacetate groups improves the adhesion of gold to the substrate.

Light-coupling masks: An alternative, lensless approach to high-resolution optical contact lithography

We describe an approach to optical lithography using light-scattering contact masks with protruding elements that couple light into a photoresist. This method differs from conventional contact lithography in two important ways. First, because portions of the light-coupling mask (LCM) are made from a polymer, intimate contact with the resist occurs over large areas without additional load. This contact is readily reversible, and causes no observable damage or contamination of the LCM or substrate. Second, the structure formed by the protruding parts of the LCM in contact with the resist can define local optical modes that impart directionality to the light propagating through the LCM and amplify its intensity. We provide an experimental realization and theoretical description of the method, demonstrating its use for the formation of 100 nm features with light having a wavelength of 256 nm.

Acid-base interactions in wetting

The study of the ionization of carboxylic acid groups at the interface between organic solids and water demonstrates broad similarities to the ionizations of these groups in homogeneous aqueous solution, but with important systematic differences. Creation of a charged group from a neutral one by protonation or deprotonation (whether -NH3 + from -NH2 or -CO2- from -CO2H) at the interface between surface-functionalized polyethylene and water is more difficult than that in homogeneous aqueous solution. This difference is probably related to the low effective dielectric constant of the interface (e≃9) relative to water (e≃80). It is not known to what extent this difference in e (and in other properties of the interphase, considered as a thin solvent phase) is reflected in the stability of the organic ions relative to their neutral forms in the interphase and in solution, and to what extent in differences in the concentration of H+ and OH- in the interphase and in solution. Self-assembled monolayers (SAMs)-esp...

Energy flow in light-coupling masks for lensless optical lithography

We illustrate the propagation of light in a new type of coupling mask for lensless optical lithography. Our investigation shows how the different elements comprising such masks contribute to the definition of an optical path that allows the exposure of features in the 100-nm-size range in the photoresist.

Assays for High‐Throughput Screening of E2 and E3 Ubiquitin Ligases

We developed a series of assays for biochemical activities involving ubiquitin. These assays use electrochemiluminescence detection to measure the ubiquitylation of target proteins. To enable electrochemiluminescence detection, the target proteins were prepared as bacterially expressed fusion proteins and captured on the surface of specially designed microtiter plates having integrated electrodes. Ubiquitylation was quantitated directly, through the use of ubiquitin labeled with an electrochemiluminescent label, or indirectly, through the use of labeled antiubiquitin antibodies. Assays were carried out in both 96-well and 384-well plates. The success of the assay with this variety of formats allowed the selection of optimal work flows for specific applications on the basis of ease of use and overall reagent consumption and availability. We used our ubiquitylation assays to measure the activities of E2 ubiquitin-conjugating enzymes and E3 class ubiquitin ligases. Signal/background ratios for many of our assays were greater than 50, significantly facilitating their conversion to high-throughput practice in a convenient manner. The speed, sensitivity, and convenience of the assay formats makes them well suited for comprehensive interrogations of libraries of compounds or genes in applications like drug and substrate discovery for ubiquitin ligases.

Characterization of electron beam induced modification of thermally grown SiO2

We used local probe techniques to characterize electron beam (e‐beam) induced changes in thin oxides on silicon. Primary effects of the 1 nm wide, 300 keV e beam included the formation of positive charges trapped in the SiO2, physical restructuring in the oxide, and deposition of carbonaceous compounds. Charges remained stable in thicker oxides (460 nm) and appeared as changes in the contact potential or microwave response with widths down to 100 nm. In thinner oxides (20 nm) the amount of charge was smaller and less stable; below 7 nm no charge was detected. Physical changes in the oxide, evident as a swelling of irradiated areas, accounted for the etching selectivity of these regions.