Nanostructures Fabrication Methods bottom-up methods ( atom by atom ) In the bottom-up approach, atoms, molecules and even nanoparticles themselves can be used as the building blocks for the creation of complex nanostructures; the useful size of the building blocks depends on the properties to be engineered. By altering the size of the building blocks, controlling their surface and internal chemistry, and then controlling their organization and assembly, it is possible to engineer properties and functionalities of the overall nanostructured solid or system. These processes are essentially highly controlled, complex chemical syntheses. top-down processes (removal of reformation of atoms to create the desired structure) Top-down approaches are inherently simpler and rely either on the removal or division of bulk material, or on the miniaturization of bulk fabrication processes to produce the desired structure with the appropriate properties.
Top-down processes Top-down processes are effectively examples of solid-state processing of materials milling lithographic processes machining
Milling Mechanical attrition or mechanical alloying Microstructures and phases produced in this way can often be thermodynamically metastable
Conventional lithographic processes are akin to the emulsion-based photographic process and can be used to create nanostructures by the formation of a pattern on a substrate via the creation of a resist on the substrate surface. Visible or UV light X-rays Electrons or ions to project an image containing the desired pattern onto a surface coated with a photoresist material The resist material, typically a polymer, metal halide or metal oxide, is chemically changed during irradiation, often altering the solubility or composition of the exposed resist.
Pattern transfer processes Solution-based wet chemical etching procedures Dry etching in a reactive plasma (reactive ion etching RIE, chemically assisted ion beam etching CAIBE) Doping using ion implantation techniques Thin film deposition Fundamentally, the wavelength of the radiation used in the lithographic process determines the detail in the resist and hence the final planar nanostructure; additional considerations may involve the limitations of the projection optics and the nature of the interaction of the radiation with the resist material. Typically the resolution ranges from a few hundred nanometres for optical techniques to tens of nanometres for electron beam techniques. Phenomenologically, throughput and resolution of lithographic techniques broadly follow a power-law dependence; the resolution is approximately equal to 23A 0.2, where A is the areal throughput.
Schematic representation of the photolithographic process sequences, in which images in the mask are transferred to the underlying substrate surface. d The theoretical resolution capability of shadow photolithography with a mask consisting of equal lines and spaces of width b due to diffraction is given by: s =400 nm, d=1 μm, resolution slightly less than 1 μm In contact-mode photolithography, the mask and wafer are in intimate contact, and thus this method can transfer a mask pattern into a photoresist with almost 100% accuracy and provides the highest resolution. However, the maximum resolution is seldom achieved because of dust on substrates and non-uniformity of the thickness of the photoresist and the substrate. Such problems can be avoided in proximity printing, in which, a gap between the mask and the wafer is introduced. However, increasing the gap degrades the resolution by expanding the penumbral region caused by diffraction. In projection printing techniques, lens elements are used to focus the mask image onto a wafer substrate, which is separated from the mask by many centimeters. Because of lens imperfections and diffraction considerations, projection techniques generally have lower resolution capability than that provided by shadow printing
Deep Ultra-Violet lithography (DUV) - wavelengths below 300 nm Technical challenges: Lower output in DUV (10-20 watts, KrCl and KrF excimer lasers 222 nm and 249 nm) With DUV, optical lithography allows one to obtain patterns with a minimal size of loonm Extreme UV (EUV) lithography with wavelengths in the range of 11-13 nm has also been explored for fabricating features with even smaller dimensions and is a strong candidate for achieving dimensions of 70nm and below. Problems: refractive in this wavelength regime is very strong, and refractive lens can not be used. Phase-shifting lithography
Parallel lines formed in photoresist using near field contact-mode photolithography have widths on the order of 100 nm and are -300 nm in height as imaged by (A) AFM and (B) SEM. [J.A. Rogers, K.E. Paul, R.J. Jackman, and G.M. Whitesides,.J Vac. Sci. Technol. B16, 59 (1998).]
Electron beam lithography Electron beams can be focused to a few nanometers in diameter and rapidly deflected either electromagnetically or electrostatically. Electrons possess both particle and wave properties; however, their wavelength is on the order of a few tenths of angstrom, and therefore their resolution is not limited by diffraction considerations. Resolution of electron beam lithography is, however, limited by forward scattering of the electrons in the resist layer and back scattering from the underlying substrate. Nevertheless, electron beam lithography is the most powerhl tool for the fabrication of feathers as small as 3-5 nm. Four typical subsystems: (i) Electron source (gun) (ii) Electron column (beam forming system) (iii) Mechanical stage (iv) Control computer
X-ray lithography X-rays with wavelengths in the range of 0.04 to 0.5 nm represent another alternative radiation source with potential for high-resolution pattern replication into polymeric resist materials (a) 35 nm wide Au lines grown by electroplating using a template fabricated by X-ray lithography. The mean thickness is about 450 nm, which corresponds to an aspect ratio close to 13. (b) 20 nm wide W dots obtained after reactive ion etching of 1250nm thick W layer. [G. Simon, A.M. Haghiri-Gosnet, J. Bourneix, D. Decanini, Y. Chen, F. Rousseaux, H. Launios, and B. Vidal,.I Vac. Sci. Techno/. B15, 2489 (1997).]
Focused ion beam (FIB) lithography FIB lithography is capable of producing electronic devices with submicrometer dimensions Ions with energy in the MeV range, scattering is much more less But: lower throughput, substrate damage SEM image showing a regular array of 36 gold pillars in each corresponding to an individual ion beam spot created using chemical assisted FIB deposition. [A. Wargner, J.P. Levin, J.L. Mauer, PG. Blauner, S.J. Kirch, and P. Longo,J. Vuc. Sci. Technol. B8, 1557 (1990).]
Neutral atomic beam lithography In neutral atomic beams, no space charge effects make the beam divergent; therefore, high kinetic particle energies are not required. Diffraction is no severe limit for the resolution because the de Broglie wavelength of thermal atoms is less than 1 angstrom. These atomic beam techniques rely either on direct patterning using light forces on atoms that stick on the surface or on patterning of a special resist Schematic illustrating the basic principles of neutral atom lithography with light forces. [B. Brezger, Th. Schulze, U. Drodofsky, J. Stuhler, S. Nowak, T. Pfau, and J. Mlynek, J. Vac. Sci. Technol. B15, 2905 (1997).] SEM image showing chromium nanowires of 64nm on silicon substrate grown by neutral atomic beam deposition with laser forces [ibid.]
Soft lithography techniques pattern a resist by physically deforming (or embossing) the resist shape with a mould or stamp, rather than by modifying the resist chemical structures with radiation as in conventional lithography. Additionally the stamp may be coated with a chemical that reacts with the resist solely at the edges of the stamp. These methods circumvent many of the resolution limitations inherent in conventional lithographic processes that arise due to the diffraction limit of the radiation, the projection or scanning optics, the scattering process and the chemistry within the resist material. Ultimately, nanoimprinting should represent a cheaper process for mass production. Currently, these soft lithography techniques can produce patterned structures in the range 10 nm and above. One of the main limitations on resolution arises from plastic flow of the polymeric materials involved. Master moulds may be fabricated using either conventional lithographic techniques, micromachining or naturally occurring surface relief on the substrate materials.
Machining Lithographic techniques essentially consist of a two-dimensional chemical or mechanical patterning of the surface of a material. Three-dimensional patterning of a material can be achieved by techniques analogous to more conventional machining. Currently resolution limits are of the order 5 μm, but in recent year focused ion beams (FIB) and highintensity lasers have been used to directly pattern or shape materials at micron and submicron levels. Scanning electron microscope image of a multilevel gear structure created by focused ion beam sputtering of silicon