HANCOCK, N.H. Using protein membranes as etch masks, a team at the University of Colorado, Boulder, has found what they believe is a practical route to creating precise nanocluster arrays at dimensions of around 10 nanometers. The method beats current lithographic techniques by a factor of 10 and is more precise than electron-beam methods, which also have the drawback of slow serial definition of features. Possible applications being explored for the new technique include high-resolution color displays, dense magnetic recording media and ultraprecise metrology for semiconductor manufacturing.
The masks are derived from the outer protein coat that protects a bacterium known as sulfolobus acidocaldarius, which grows in hot and acidic conditions. "This particular bacteria was first discovered in hot springs in Yellowstone National Park," said Andrew Winningham, a researcher on the project who subsequently moved to the University of Central Florida. "It is able to grow at 80 degrees C in a solution with a pH between 2 and 3-about the same acidity as stomach acid. It's very rugged."
Large volumes of the protein coats are easy to produce using standard biotechnology methods. The bacterium is cultured in a vat and detergents and enzymes are used to remove its innards, leaving a solution containing the spherical outer protein coats. Ultrasound is then used to break the spherical membranes so that they open out into a flat configuration. The protein molecules organize themselves into a regular crystalline structure, but because the bacterial coats are spherical, there are defects in the pattern. However, the important aspect of the structure is a set of pores whose regular spacing is not disturbed by irregularities in the crystalline pattern.
An advantage of the hostile environment that the bacteria love is the absence of any contamination from other organisms that might accidentally be introduced. That makes refinement and extraction of the membranes very easy, Winningham said.
Once the protein coats have been deposited on a substrate and assembled into a patch, they are used as an etch mask. The pores in the protein crystal structure form a regular hexagonal array with a lattice constant of 22 nm. The pores themselves are 18 nm in diameter, and the pore pattern can be transferred to the silicon substrate via a special low-energy reactive ion-etching process the team developed.
Because the protein film is so thin, standard etching processes used in semiconductor fabrication would create too much damage. A typical etching process will leave a 10-nm-thick layer of amorphous silicon-negligible for current electron device sizes, but unacceptable with 18-nm holes. The Colorado team's process, called LE4, uses electrons at 1.5-kilovolt energy to catalyze a reaction between hydrogen ions and the silicon substrate. The energy of the reaction is adjusted so that the etched holes are absolutely uniform, a requirement for creating standard metal nanoclusters.
The recent breakthrough for the re-searchers was the demonstration of a highly uniform array of titanium metal clusters that spontaneously form in the etched holes. Using electron-beam evaporation in a vacuum chamber, the group deposited a thin (1.2-nm) film of titanium over the protein mask. Subsequent LE4 etching removed the areas covered by the protein molecules, leaving a precisely formed hexagonal array of titanium nanoclusters which formed in the silicon holes.
Although the array represents a minimal geometric pattern, its scale and precision could be used in several near-term applications, Winningham said. For example, another research group at the University of Colorado is using the process to create an ultradense magnetic recording medium at a resolution that will soon be needed for disk drives, he said.
In this work, chromium is evaporated onto the protein film and etched so as to create an array of holes 10 nm in diameter. This array then forms a mask for molecular-beam epitaxy deposition of lead and soft iron. Alternate layers of lead and iron are deposited to create a 6.5-nm-high stack in each hole. After etching away the mask, an array of tiny magnetic domains remains. These domains were verified to perform well as magnetic bits in a storage system.
An application that Winningham is pursuing applies the process to metrology for standard silicon device fabrication. A precise array of metal dots can be laid down on a substrate as a known grid. Transmission electron microscope images will reveal moire patterns that can be used to calculate the amount of displacement created when the substrate is subjected to strain, giving very precise information on the behavior of the material.
An application that Winningham feels is a bit further off is the creation of electroluminescent displays using the tiny metal clusters as quantum dots. Using quantum confinement, dots of different diameters would produce different colors, and the display could operate at 10 V rather than the 100 V now required by EL panels.
Winningham, who entered the project as a Colorado graduate student, devised the essential LE4 low-energy etch process that has been a key factor in transferring the precise protein array features into silicon. He is now working with chemists on unraveling the three-dimensional structures of the proteins themselves. That work could find immediate application in the field of proteomics, in which medical researchers are attempting to understand the critical role of proteins in the cell.