Methods Of Synthesis And Fabrication Of Nanostructured Materials

There is increasing interest in producing structures of nanometer size for use in device fabrication and proof of physics laws in nanometer sized structures. This is a growing field and nowadays is very important for investigating new techniques for nanometer device fabrication.

The most used materials for production of nanostructured materials are, in first place, semiconductors and metals and, in second place, dielectrics, magnetic, and superconductor compounds. They are also manufactured using organic materials, polymers, biomaterials, etc.

Nowadays there are so many methods of synthesis and production of nanostructured materials, in a laboratory scale, but the most important are those where high quality quantum nanostructures are achieved. These methods produce materials that have one atomic layer precision, with abrupt grown interfaces with one monolayer roughness and with great control for purity and doped impurities.

In addition to this technology, in order to guarantee the high quality of the nanostructured materials, the investigation laboratories need to count on several characterization techniques in-situ. Independently of the dimensionality of the grown nanostructures, the most used and known techniques for the production or fabrication of the different kinds of nanostructured materials are molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), and lithography, also known as nanolithography.

2.1. Molecular Beam Epitaxy

MBE is a method by means of which a semiconductor, such as GaAs, is grown in an ultrahigh vacuum environment by supplying molecular and/or atomic beams of Ga, As, and other constituents onto a cleaned crystalline substrate [43-51]. Because one can prepare semiconductor films of high quality by MBE with a precision in thickness of one atomic layer or better, it is now widely used in all nano-structured materials. MBE is one of the best methods for the preparation of a very flat surface of one semiconductor on which another semiconductor of an arbitrary thickness can be deposited. This feature allows one to control the composition profile and thereby to implement an artificial potential V(z) as a function of position z along the direction of stacking. A number of studies have shown, however, that interfaces of GaAs-AlAs (or AlGaAs) grown by MBE are generally abrupt but have a roughness A of one monolayer, a = 2.83 A. Hence it is very important to clarify the origin and nature of such roughness and to establish a method for controlling the atomic structure of interfaces. In addition, one should note that the atomic structure of an interface is usually a frozen image of the freshly grown surface and therefore provides valuable information on the microscopic process of epitaxial growth. On the other hand, the high purity of the low-dimensional system obtained by MBE indicates that the mobility is dominated mainly by scattering by phonons. The advantages of epitaxial growth by MBE, with respect to other methods or technologies, are multiple techniques in-situ that permit an absolute control of the ground parameters of the grown structure, such as size, high purity, abruptness and flatness of interface and impurities in doped heterostructures, and the ultrahigh vacuum environment that it possesses.

2.2. Metal Organic Chemical Vapor Deposition

The MOCVD method is the most useful and conventional process for fabricating thin layer semiconductor heterostruc-tures. In general MOCVD has a reaction chamber that is basically dependent upon the heating system, which fits for long growth run. It is possible to use a radio frequency induction coil as an alternative heater; there is a possibility of dissociation of gases, which produces radicals due to a plasma discharge in reduced pressures, and then it is necessary to employ a large-scaled electric power supply in connection with a precise electric circuit. However, in the case of synthesis of the II-VI compound semiconductors, it should be pointed out that there are several problems concerning the difficulty in precisely controlling the II/VI mole ratio because of the high dissociation pressure of VI elements and also the premature reaction taking place in the vapor phase prior to epitaxial deposition on a substrate. In order to avoid such problems, Yamada [52] has used new metal organic sources diluted in a He gas, namely, dimethylzinc (DMZn) with a concentration of 1.06% in a He gas, for the growth of cubic-structured CdxZn1-x S-ZnS strained-layer multiple quantum wells [53-55]. A mixture of 10% H2S in a H2 gas was used as the sulfur source. Its MOCVD apparatus consists of a main reaction chamber and a sample exchange supplemental chamber with a transfer tube. A special feature is that they employ a halogen lamp capable of heating substrates up to 600 °C by infrared radiation. Two chambers are constructed for all VI materials and each inner wall is coated with TiN films, to avoid accidental impurity contamination. In order to control the gas flow ratio of II/VI, a personal computer connected to a relay box linked to an interface which was capable of switching both the piezovalves of the MFC (mass flow controller) and air valves on or off is used. Then, all of gas sources flowed directly into the MFCs, so that the bubbling system by a carrier gas was unnecessary for the metal organic (DMZn and DMCd) gas flow as well as the H2S gas flow [56, 57]. Therefore, the process using the low-pressure MOCVD system with all gaseous sources has proven to be advantageous for precise control of II/VI mole ratio as well as lower temperature growth (350 °C) of the II-VI compound hetrostructures.

2.3. Lithography

It is instructive to consider the conventional but advanced forms of lithography as practiced today. As it has developed for the semiconductor industry, lithography is the creation of a pattern in a resist layer, usually an organic polymer film, on a substrate material. A latent image, consisting of a chemical change in the resist, is created, by exposing the desired area with some form of radiation. The pattern is developed by selectively removing either the exposed areas of resist (for a positive resist) or the unexposed areas (for a negative resist). These processes are shown schematically in Figure 5. The development of a polymeric resist is usually done with a solvent that brings out the pattern based on the solubility difference created by exposure.

2.3.1. Overview of Lithography

Electrons, ions, and photons can all be used for exposure in the lithography process. Because of intrinsic scattering mechanisms, in no case can the energy provided by the exposing radiation be confined to arbitrarily small volumes. First, the radiation or particle beam cannot be confined arbitrarily because of diffraction, Coulomb repulsion

Exposing radiation

Development

Resist Substrate

Positive

Negative

Figure 5. Schematic diagram of the basic lithographic steps of resist exposure and development.

among particles, optical limitations, or other effects. Second, the energy incident on a resist layer spreads in the film, because of intrinsic scattering mechanisms in the solid.

The figures of merit for a resist/developer system include: the sensitivity to the exposure dose dependence of the resist response, the resolution dictating the minimum feature size, and the suitability of the resist for pattern transfer. While the sensitivity and contrast of a system are vital in determining the speed and process latitude, it is the resolution and pattern transfer utility that are most important to research level nanostructure fabrication.

Inorganic resist materials exist and have been applied in the high resolution of the field. Related processes include vapor or plasma developments that operate on difference in gas phase or plasma reactivity rather than liquid development based on solubility differences. Self-developing resists are of some importance in the high resolution of the area. For these materials the exposing radiation volatizes the resist, creating a pattern in the resist without a development step. By this method, structures on the order of unit cell dimensions were made some years ago [58].

2.3.2. Photon Lithography

The use of photons for high resolution of the lithographic exposure is a broad field that includes most of the lithography in use today. This is the technique most widely used by the semiconductor industry and illustrates the general technique of lithography. It can be taken to include X-ray lithography and deep and near-ultraviolet photolithography. Only the shortest wavelengths are directly relevant to high resolution of the fabrication. The projection and proximity printing processes to be described are analogous to processes that could be used at nanometer dimensions for parallel printing with electrons or ions.

In projection lithography a mask containing the desired pattern is demagnified by an optical system and projected on the resist layer. This is desirable since there is no damaging contact between the mask and wafer, and the fabrication of the mask is not so critical since it is demagnified. The resolution is limited, however, by diffraction and the quality of the optics. The diffraction limit for the minimum resolvable grating period is usually taken as the Rayleigh criterion for the overlap of the diffraction peaks, which is approximately where À is the wavelength and NA is the numerical aperture of the optical system, which is of the order of one. The shortest wavelength to be used with conventional optics is about 193 nm from an excimer laser source. The use of wavelengths much shorter than this is limited by absorption in optical materials. While the technology constantly improves, with better optics, improved contrast enhanced resists, and sources of shorter wavelength, the limit of far-field diffraction is a fundamental one that prevents its use for feature sizes smaller than the wavelength of light. Photolithography is essentially ruled out for fabrication at dimensions below 100 nm.

Projection systems employing X-rays would reduce diffraction effects, but the problem of fabricating optics to use in the X-ray region remains. Work in this area is advancing, with the development of multilayer mirrors and X-ray mask technology.

Proximity printing allows the 1:1 replication of a mask pattern. The effects of near field diffraction can be arbitrarily reduced by decreasing the distance between mask and substrate. Qualitatively, the minimum linewidth is d ~ VXs

minimum line resolvable spacing

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