Vibrating nanorods measure thin films for microcircuits

Schematic illustration of the measurement of in-plane and out-of-plane vibrational modes of a silicon cantilever. Modulated blue laser excites the NEMS structure and laser interferometry monitors the cantilever motion. Spectral characteristics of the in-plane (blue) and out-of plane (red) modes change when a thin-film coating, shown in green, is applied.

A key step in many nanofabrication processes is to create thin films, sometimes only one molecule thick, by a method known as atomic layer deposition. Researchers at Cornell and Tel Aviv University have developed a new tool for nanofabricators to test the physical properties of such films.

Ultrathin films are increasingly important in constructing microcircuits. Their physical characteristics often determine their electronic behavior as well as their resistance to wear.

The researchers have shown that tiny resonant cantilevers -- silicon rods anchored at one end, like a tiny diving board -- can determine the density of a film and its Young’s modulus, a measure of resistance to bending. The method offers several advantages over other methods of measuring these characteristics of thin films, the researchers said, and can be used by any researchers with access to nanofabrication capabilities comparable to those at the Cornell Nanoscale Facility.

The work was reported in the Aug. 15 issue of the Journal of Applied Physics by Cornell research associate Rob Ilic, Slava Krylov, senior lecturer at Tel Aviv University and former visiting professor at Cornell, and Harold Craighead, the C.W. Lake Jr. Professor of Engineering at Cornell.

Cornell researchers have previously used tiny vibrating cantilevers just a few nanometers (billionths of a meter) thick to detect the mass of objects as small as a virus. Just as a thick guitar string vibrates at a lower note than a thinner one, adding mass to a vibrating rod changes its frequency of vibration. Coating the rod with a thin film adds detectable mass, and from the mass and thickness of the film, density can be determined.