A two-dimensional array of nanometer-scale chromium features on a silicon substrate has been fabricated using laser-focused atomic deposition by researchers at the National Institute of Standards and Technology (NIST, Gaithersburg, MD). Feature heights of 13 ± 1 nm with full widths at half maximum of 80 ± 10 nm were fabricated in an array covering an approximate area of 100 × 200 µm.
Laser-focused atomic deposition makes use of a laser light field to control the motion of atoms as they deposit onto a surface. The technique has several putative advantages for fabrication of nanometer-scale structures when compared with conventional fabrication methods--such as optical or electron-beam (e-beam) lithography--that are used to pattern and interconnect devices and circuits.
According to the researchers, optical techniques are fundamentally limited by diffraction to a minimum feature size of about half the wavelength of light used; for UV light this corresponds to about 100 nm. Free-flying thermal atoms, however, have De Broglie wavelengths on the order of 10 pm, which means diffraction effects are essentially negligible. Although the resolution limits of laser-focused atomic deposition are still being investigated, 65-nm structures have been fabricated and features as small as 5-10 nm may be possible.
In the case of e-beam lithography, which can achieve comparable feature sizes, the process is much slower because it requires multiple step-and-repeat operations while the laser-focused deposition method makes a wide area exposure in a single shot. Other factors, including thermal drift, can also be problematic with e-beam lithography.
Deposition method
To produce the array with laser-focused deposition, chromium atoms are collimated to an angular divergence of 0.25 mrad by laser cooling, then directed at the substrate (see Laser Focus World, June 1993, p. 33). Before deposition the atoms must pass through a laser field created by two one-dimensional standing waves at 90° to each other across the substrate. The standing waves are produced by a single-frequency CW dye laser tuned 500 MHz (100 natural linewidths) above the atomic resonance line in chromium at 425.55 nm.
With this tuning a dipole force is exerted on the atoms toward the low intensity regions of the light field such that a concentration of atoms occurs on the substrate at the nodes of the standing waves. These waves occur at an integral number of half-wavelengths from each standing-wave mirror. The result is a series of chromium features on a two-dimensional square lattice with spacing of 212.78 nm.
The ability to fabricate a uniform two-dimensional nanometer-scale pattern on a substrate is, according to NIST`s Jabez McClelland, a significant step toward the possibility of creating an array of isolated nanoscale metal dots on a surface. Such an array would be useful in studying quantum dot effects on semiconductors, for example, and could be used as an etch mask to transfer the pattern to another substrate, thereby extending the fabrication technique to other materials. Other possibilities include making complex arbitrary patterns by creating a more generalized interference pattern of light waves from many standing waves incident from a range of angles with controlled phase.