Colloquium: Reactive plasmas as a versatile nanofabrication tool

The underlying physics of the application of low-temperature, low-pressure reactive plasmas in various nanoassembly processes is described. From the viewpoint of the “cause and effect” approach, this Colloquium focuses on the benefits and challenges of using plasma-based systems in nanofabrication of nanostructured silicon films, low-dimensional semiconducting quantum structures, ordered carbon nanotip arrays, highly crystalline ${\mathrm{TiO}}_2$ coatings, and nanostructured hydroxyapatite bioceramics. Other examples and future prospects of plasma-aided nanofabrication are also discussed.

Figure 1

Schematics of the reactive-plasma-based nano-fabrication environment. $\mathrm{BU}=$building unit.

Figure 2

(Color in online edition) Simplified concept map of the reactive-plasma-based cause and effect nanofabrication approach.

Figure 3

Transmission electron micrograph (left image), and scanning electron micrograph (right image) of carbon nanofoam, showing its weblike structure. From 102.

Figure 4

Initial stages of epitaxial recrystallization (left image) and burrowing (right image) of a 4876-atom Co nanocluster deposited on a Cu(100) surface. The burrowing is accompanied by formation of well-resolved grain boundaries. From 28.

Figure 5

Reaction kinetics of $\mathrm{Si}{\mathrm{H}}_3$ (left image) and $\mathrm{C}{\mathrm{H}}_3$ (right image) radicals with $a$-Si:H and $a$-C:H surfaces, respectively. $\mathrm{Si}{\mathrm{H}}_3$ is strongly physisorbed by the dipole interaction, whereas chemisorption of $\mathrm{C}{\mathrm{H}}_3$ is controlled by the creation of dangling bonds on the surface by either reactive hydrogen atoms or ion bombardment. From 95.

Figure 6

(Color in online edition) Representative distribution of ion current density over a nanostructured surface in the $\mathrm{Ar}+{\mathrm{H}}_2+\mathrm{C}{\mathrm{H}}_4$ reactive-plasma-assisted nanofabrication of ordered carbon nanotip arrays for electron-field emission applications. From 68.

Figure 7

Schematics of experiment on reactive-plasma-based deposition of positively charged nanocrystals onto liquid-nitrogen-cooled substrates (13; 99): (a) When the building units (BUs) land with very low energy, there is no nanocluster disintegration and the films are predominantly crystalline; (b) when the building units are accelerated to higher energies, nanocrystals break up into atomic/radical fragments and the films are mostly amorphous.

Figure 8

(Color in online edition) Quantum dot deposition and emission properties: (a) Scanning electron micrograph of ${\mathrm{Al}}_x{\mathrm{In}}_{1-x}\mathrm{N}$ quantum dot structures synthesized by the reactive-plasma-assisted rf sputtering deposition technique; (b) dependence of the photoluminescence intensity on quantum dot size/composition (130). Quantum dot size is smaller for smaller values of $x$.

Figure 9

Silicon nanowire growth and interaction with charged clusters: (a) Scanning electron micrograph of silicon nano-wires grown in low-pressure $\mathrm{Si}{\mathrm{H}}_4+{\mathrm{H}}_2$ reactive plasma (130); (b) schematics of the charged nanocluster-nanowire interaction (52). Here, ${\mathbf{F}}_{\mathrm{int}}$ denotes the force of the electrostatic building-unit–nanowire interaction.

Figure 10

(Color in online edition) Characteristic optical emission lines of the carbon dimer in nanofabrication of carbon nanowall-like structures in $\mathrm{Ar}+{\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{H}}_2$ rf plasmas (73).

Figure 11

Various carbon nanostructures synthesized in hydrocarbon-based reactive plasmas: (a) Ni-catalyzed Si(100) surface before the beginning of PECVD; (b) carbon nanotips; (c) pyramidlike structures grown in $\mathrm{Ar}+\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2$ plasmas without any external substrate heating; (d) carbon nanowall-like structures grown in $\mathrm{Ar}+{\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{H}}_2$ rf plasmas. (Figure courtesy of J. D. Long and Z. Tsakadze.)

Figure 12

Nanoparticles (a) attached to the top of carbon nanotip structures and (b) on a nickel-catalyzed silicon surface (119, 120).

Figure 13

(Color in online edition) Hydroxyapatite (HA) fabricated by reactive-plasma-assisted rf magnetron sputtering deposition: left, control of Ca and P content based on the different responses of cationic $\mathrm{Ca}{\mathrm{O}}^{+}$ and anionic $\mathrm{P}\mathrm{O}_4{}^{3-}$ building units to a negative substrate bias variation; right, representative AFM image of nanoscaled surface morphology (129).