This article represents the first in a series of briefs on the directed self assembly of nanostructures. Directed assembly can be expressed in the guided synthetic, stereochemical, and 3D configurational control of complex molecules, such as the rhibosome-assisted assembly of proteins. For these types of reactions, the final placement of these complex materials may not be controlled. Alternatively, a substrate can be preconditioned to preferentially guide the growth and/or assembly of nanostructures with desired morphologies and at desired locations. This class of directed assembly includes the phase segregation of diblock copolymers into well-defined domains on patterned substrates. This month’s article addresses recent advances in a third class of directed assembly processes, deterministic field assisted assembly. This approach leverages external fields and local topography to guide the placement of remotely synthesized nanostructures.
Deterministic Field Assisted Assembly
by Theresa S. Mayer and Christine D. Keating, Pennsylvania State University
Silicon electronics could be made to run cooler or to interact with the environment by adding new materials and devices atop the circuitry. Attractive new materials include narrow bandgap semiconductors that operate at lower voltages than silicon, giving lower power consumption without sacrificing speed. Molecular and metal oxide materials, which can produce a large electronic response to chemical vapors or biological molecules, offer sensing capabilities. Unfortunately, the high temperatures and harsh chemicals used in conventional fabrication processes generally destroy these materials or the chips themselves, which up to now has made it impossible to effectively couple them.
Deterministic field assisted assembly begins by fabricating hundreds of millions of uniform nanoscale elements—wires, spheres, or sheets of a desired material—under optimal conditions, and suspending them in a fluid. Different populations of these nanocomponents are then delivered, one by one, to a functioning silicon chip. A programmable voltage created by the silicon circuit itself directs individual parts to a specific region of the chip, and then snaps them into place with the submicron accuracy needed to connect a specific part to a transistor on the chip. Assembly follows seconds after delivery, and component densities can exceed a million per cm2. Direct coupling of this unprecedented flexibility in materials with silicon electronics has the potential to transform many diverse technologies, from ‘green’ computing to integrated sensing.
Traditional chip fabrication relies on multilevel photolithography to define device features. It gives excellent control of feature geometry and registration over different levels, but places considerable limits on the materials and molecules that can be incorporated. Alternatively, nanocomponents can be synthesized prior to assembly onto the chip, providing much greater diversity in materials often at the expense of accurate placement. The deterministic field assisted assembly integration strategy solves this problem by using electric field forces to direct different components to specific regions of the chip, while providing accurate registry between each individual part and the lithographic features in that region. This allows us to add entirely new capabilities on top of the existing powerful, inexpensive processing and storage functions of silicon.
The International Technology Roadmap for Semiconductors (ITRS) highlights not only the need to continue silicon CMOS miniaturization, but also the growing importance of expanding its capabilities by adding new materials and devices. The seamless integration of power sources, RF communication, sensors, actuators, and biological functions will allow future chips to interact directly with people and the environment. However, inherent incompatibilities between the CMOS circuit and nanomaterial fabrication conditions must first be overcome. Deterministic field assisted assembly meets this challenge using the powerful concept of programmed assembly, which can be generalized to connect almost any nanomaterial component directly to the transistor circuitry on the chip. Off-chip component fabrication allows the luxury of optimizing process conditions for a given material (temperatures, chemicals, mixing, etc.), while still employing conventional methods to build the electronic circuitry.
This new nanofabrication technology that has the potential to add new materials and devices after silicon chips are made. Our recent breakthroughs in programmed nanomaterial assembly have shown that this is possible. We have directed different populations of DNA-coated nanowires to specific regions of the chip with the submicron placement accuracy that enables fabrication of electrical connections between individual nanowires and specific transistors on the chip. Individual nanowire assembly yields of greater than 80% were achieved for arrays containing several thousand nanowires integrated at densities exceeding 106/cm2. The experiments confirmed the retention of binding selectivity for the probe DNA on the assembled nanowires despite exposure to electric fields, photoresist coatings, and solvents.
We have also fabricated active devices from diverse nanomaterials including semiconductor, conducting polymer, and metal nanowires. Notably, the performance of cantilevered semiconductor resonators, polythiophene nanowire chemical sensors, and axially-doped silicon nanowire field effect transistors matched or exceeded similar stand alone devices fabricated by conventional methods.