Bottom-up Nanofabrication

Abstract

A bottom-up approach to nanofabrication can be looked upon as a synthesis approach mimicking biological processes in which individual atoms are piled up one at a time on the substrate to form molecules. These molecules arrange themselves on their own into the desired form to yield the required nanostructures. The driving mechanisms for this molecular arrangement are the physical and chemical forces operative at the nanoscale. These mechanisms have been perfected by Mother Nature over a period of several millennia. Of particular interest to nanoelectronics are techniques such as sol-gel synthesis, vapour deposition, atomic layer deposition, molecular self-assembly, DNA-assisted assembly and many others. Sol-gel technique offers a simple process to produce nanoparticles. Two forms of vapour-phase techniques are physical vapour deposition in which the active species is evaporated into the vapour phase and chemical vapour deposition in which, a precursor is used which decomposes into the required species via a chemical reaction. Based on successive, self-restricting reaction cycles, atomic layer deposition provides thickness adjustment at nanometer level along with composition control. Molecular self-assembly exploits the organizational capability of matter to form homogeneous monolayers. Physical and chemical vapour deposition constitute self-assembly from gaseous phase. Artificial DNA nanostructures are used to arrange functional nanomaterials into nanoelectronic circuits.

Review Exercises

• 24.1 Define the following terms: (i) Sol and (ii) Gel . Why is the term ‘sol-gel ’ misleading?

• 24.2 What is meant by TMOS and TEOS ? What is hydrolysis? Write the equation for the hydrolysis of silicon alkoxide precursor.

• 24.3 What is meant by polycondensation ? Write the equation for water condensation of silicon alkoxide precursor. Also write the equation for alcohol condensation of this precursor.

• 24.4 After the polycondensation of silicon alkoxide, what process takes place inside the mixture during gelation?

• 24.5 Name and describe the three sub-processes that occur during aging of the silicon alkoxide mixture after gelation.

• 24.6 What is vapor deposition ? Differentiate between physical vapor deposition and chemical vapor deposition .

• 24.7 What are the two common types of physical vapor deposition? In which type, the material is liberated by striking an electron beam on it? In which type, the material is released by bombardment of a target with inert gas ions? What is the difference between reactive sputtering and co-sputtering?

• 24.8 What are the main steps in chemical vapor deposition of a material? Describe the main features of hot wall and cold wall reactors.

• 24.9 Is “atomic layer deposition” a chemical vapor deposition process? For atomic layer deposition, is there any restriction on substrate shape, size or surface topography?

• 24.10 How does atomic layer deposition process ensure deposition of atomically thin layers of materials? How does it differ from traditional CVD?

• 24.11 Can ALD provide conformal coatings on irregular shaped substrates? How is thickness of deposited film controlled? How is its composition controlled?

• 24.12 (i) ALD is a fast process. True or false? (ii) What is spatial ALD ?

• 24.13 Give a few examples of metallic and insulating films which can be grown by ALD. Can it be applied for depositing polymers?

• 24.14 Define molecular self-assembly and illustrate it by lipid bilayer formation. How does the phospholipid bilayer evolve into a spherical shape to avoid water by its water fearing tail?

• 24.15 How does molecular motion aid in molecular assembly? What is the role of intermolecular forces in molecular assembly?

• 24.16 What is a polyelectrolyte? How is it used in layer-by-layer self assembly of molecules? Give an example of application of this technique in sensors.

• 24.17 Define the following terms: (i) Surfactant and (ii) Self-assembled monolayer .

• 24.18 What is an alkanethiol ? What are the three parts of an alkanethiol molecule? Write the recipe for using an alkanethiol for SAM formation?

• 24.19 How are patterned mixed monolayers formed by microcontact printing ?

• 24.20 Why are studies on SAMs important? What has encouraged widespread utilization of SAMs? What are reasons of the increasing trends of SAMs formed by thiols on gold?

• 24.21 What is the full form of DNA ? What are the four types of the nitrogen bases in the nucleotide of DNA? What does the order of the nitrogen bases in a DNA sequence indicate?

• 24.22 What is the meaning of ‘Origami ’? What is done in DNA origami? Distinguish between scaffold strand and staple strand in DNA origami.

• 24.23 How are the strands bound to each other in DNA origami ? How is the double helix structure of DNA realized from complementary base pairings?

• 24.24 Starting from computer-aided design, describe the sequence of steps in the implementation of the DNA origami process?

• 24.25 How is the DNA scaffolding assembled from DNA strands? How are nanoelectronic components self-assembled into rows on DNA scaffolding?

• 24.26 How are self-assembled DNA scaffolds prepared for use as circuit boards for computer chips?