Charan Srinivasan

Scanning electron microscopy of nanoscale chemical patterns.

A series of nanoscale chemical patterning methods based on soft and hybrid nanolithographies have been characterized using scanning electron microscopy with corroborating evidence from scanning tunneling microscopy and lateral force microscopy. We demonstrate and discuss the unique advantages of the scanning electron microscope as an analytical tool to image chemical patterns of molecules highly diluted within a host self-assembled monolayer and to distinguish regions of differential mass coverage in patterned self-assembled monolayers. We show that the relative contrast of self-assembled monolayer patterns in scanning electron micrographs depends on the operating primary electron beam voltage, monolayer composition, and monolayer order, suggesting that secondary electron emission and scattering can be used to elucidate chemical patterns.

Microcontact insertion printing

The authors describe a chemical patterning technique, “microcontact insertion printing,” that utilizes conventional microcontact printing to pattern isolated molecules diluted within a preexisting self-assembled monolayer. By modifying the preexisting monolayer quality, the stamping duration, and/or the concentration of the patterned molecule, they can influence the extent of molecular exchange and precisely control the molecular composition of patterned self-assembled monolayers. This simple methodology can be used to fabricate complex patterns via multiple stamping steps and has applications ranging from bioselective surfaces to molecular-scale electronic components.

Sub-30-nm patterning on quartz for imprint lithography templates

A parallel and economical method for obtaining nanoscale features on large-area quartz substrates has been developed for use in nanoimprint lithography template fabrication. Self-assembled multilayer films (molecular rulers) are used in conjunction with photolithographically defined metallic features to generate precise nanogaps with sub-30-nm resolution on quartz substrates. These nanopatterns are then transferred to the quartz substrates using the metallic thin films as etch masks.

Extensions of molecular ruler technology for nanoscale patterning

By combining optical lithography and chemical self-assembly, the authors circumvent the limitations of photolithography and provide a parallel, low-cost alternative to fabricate sub-50nm features. Self-assembled multilayers, composed of alternating layers of α,ω-mercaptoalkanoic acids and copper (II) ions (“molecular rulers”), are used as an organic sidewall spacer resist on initial lithographic structures enabling the precise, proximal placement of a secondary structure via lift-off. Here, the authors implemented a positive-tone bilayer resist for improved line-edge characteristics of the secondary structure and evaluated the lithographic and electrical performance of nanostructures fabricated using this approach. Additionally, they describe extensions of this technique by which planar nanojunctions were created, and the generated nanometer-scale pattern was transferred to the underlying substrate.

Nanostructures using self-assembled multilayers as molecular rulers and etch resists

Self-assembled multilayers, composed of alternating layers of α,ω-mercaptoalkanoic acids and Cu2+ ions (“molecular rulers”), are used as organic sidewall spacers and etch resists for the fabrication of registered microstructures with precisely tailored nanometer-scale spacings. The method outlined here eases the stringency of the lithographic processing for patterning second-generation features. Additionally, a new method to lift off the self-assembled multilayered films for the generation of the tailored nanogaps is demonstrated. The advantages of these techniques are discussed.

Molecular-ruler nanolithography

Molecular-ruler nanolithography uses individual molecules as building blocks to create nanometer-scale features in a low-cost, high-throughput process. Self-assembled multilayers are used in combination with radiation-sensitive polymeric resists to interface nanometer-scale features with structures fabricated using conventional lithographic methods. This technique is advantageous for its high precision, parallel processing, and low capital investment. Here, we provide an overview of molecular-ruler nanolithography and describe how this technology is being applied to the creation of nanometer-scale devices, patterning of large-area sub-200-nm grating structures, and the fabrication of quartz templates for use as molds in imprint lithography.