Nanotechnology

Nanotechnology describes technologies where the component parts can measure just a few atomic diameters (generally around a millionth of a millimeter). The general goal of nanotechnology research programs is to reduce complex and sophisticated machinery into very small operational units.

Nanotechnology involves the development of techniques to build machines from atoms and molecules. The name comes from "nanometer," which is one-billionth of a meter. It involves the development of new electrical devices that depend on quantum effects that arise when the dimension of a structure is only a few atoms across. Because the techniques best suited for fabricating devices on the submicron scale originated in semiconductor processing technology for the production of integrated circuits, nanoscale devices are all based on semiconductors.

The theory of nanotechnology was the brainchild of K. Eric Drexler, who, as a student of genetic engineering at the Massachusetts Institute of Technology in 1971, envisioned the potential for building machines and materials with atoms as basic blocks just as living matter is constructed with DNA. Until the twentieth century, the physics of atoms and molecules were an invisible science. Atoms finally became visible in 1981, when the scanning tunneling microscope was built. The microscope has the ability to scan through the clouds created by electrons around atoms so the individual atom can be seen. The doorway to nanotechnology was opened in 1985 by Richard Smalley, a chemist with Rice University, who found a way to produce carbon in a third form (the two natural forms are graphite and the diamond) that is crystalline and possesses incredible strength properties. The molecule contains 60 carbon atoms and has a shape much like a soccer ball or a geodesic dome built by the futuristic architect, R. Buckminster Fuller, so the molecule was named the "buckyball" in Fuller's honor.

By 1991, experiments with buckyballs led to long strings of the molecules that are tube- or straw-like and are called buckytubes or nanotubes. Many scientists think nanotubes are a fundamental unit for building countless other nanodevices. Nanotubes can be flexed and woven and are being woven into experimental fibers for use in ultralight, bulletproof vests, as one example. Nanotubes are also perfect conductors, and they may be used to construct atomically precise electronic circuitry for more advanced computers and flat panel displays.

Growth stage

In the growth stage, each successive layer has a different composition to impose electrical or optical characteristics on the carriers (electrons and holes). A variety of growth techniques can be used. Molecular beam epitaxy (MBE) and metallo-organic chemical vapor deposition (MOCVD) have proved to be the most useful for nanostructures because they can be used to grow layers of a predetermined thickness to within a few atoms. In MBE, a gun fires a beam of molecules at a substrate upon which the semiconductor crystal is to be grown. As the atoms hit the surface, they adhere and take up positions in the ordered pattern of the crystal, so a near perfect crystal can be grown one atomic layer at a time. The mixture of material in the beam is changed to produce different layers; for example, the introduction of aluminum to the growth of gallium arsenide will result in the production of a layer of aluminum gallium arsenide.

Lithography/pattern transfer stage

Epitaxial growth allows the formation of thin planes of differing materials in one dimension only. The two-stage process of lithography and pattern transfer is used to form structures in the other two dimensions. In the lithography stage (see Figure 1), a film of radiation-sensitive material known as the "resist" is laid on top of the semiconductor layer where the structure is going to be made. A pattern is exposed on the resist using electrons, ions, or photons. The film is altered during the exposure step, thus allowing the resist to be chemically developed as a relief image. This image is then transferred to the semiconductor by doping (adding minute amounts of foreign elements), etching, growing, or lift-off as shown in Figure 1.

The exposure pattern can be written on the resist in a line-by-line manner using an electron or ion beam or it can be imprinted all at once using a mask, much like spray painting a letter on a wall using a template. Exposure by writing the pattern with a beam is especially useful for making prototype structures, because it avoids the expense of making a new mask for each pattern; features as small as a few nanometers can be written this way.

Writing patterns one line at a time is time consuming, however, and expensive when it comes to producing large quantities, so masks are used that expose a number of chips simultaneously. Masks can be used with electron or ion beams or with photons. It is important to note that the wavelength of light for exposing a pattern has to be less than the smallest feature being exposed.

Where nanoscale dimensions are involved, this necessitates the use of vacuum ultraviolet light or x rays. Penetration of the x rays into the semiconductor must be avoided to prevent damage to the crystal, so wavelengths of 1.3 nm or 4.5 nm are preferred because the polymeric resist exhibits an absorption depth of about 1 micron at these wavelengths.

The requirement on the wavelength of the x rays and the obvious need to have the x ray beam directed precisely necessitates the use of a well-controlled x ray source, such as a synchrotron. Laser-based x ray sources are currently being considered as a less expensive alternative to the synchrotron.