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New Cantilever Boosts SFM Resolution

Photonics Spectra
Jun 1997
R. Winn Hardin

STANFORD, Calif. -- Stanford University researchers have combined an interferometric diffraction grating with light-reflecting scanning force microscope cantilevers in a way that could result in a tenfold increase in the resolution of commercially available instruments. Industry experts say the cantilever could open doors to mass microlithography and ease silicon substrate inspections within the semiconductor industry.
Since the introduction of scanning force microscopes, also called atomic force microscopes, in the 1980s, developers have looked to new cantilever and probe designs as the most efficient way to increase resolution. Along the way, some trade-offs were made. While reducing the resolution, these made the instrument more user-friendly.
Evidently, the compromises have worked. Today, the microscopes are finding applications across the gamut of research, from small colleges to the best-stocked labs.
The evolution of the cantilever reached a plateau a few years ago. Commercially available scanning force microscopes use an optical deflection system known as the optical lever to make measurements between the sample and the microscope's probe tip. As the tip nears the sample surface, it is deflected or attracted. A simple optical system detects the tip's movements by bouncing a beam from a diode laser off the back of the cantilever arm. Electronics turn that data into microscopic measurements with vertical resolutions on the order of 0.1 Å.

Add a grating
Recently, Scott Manalis and his associates at Stanford University developed a sensor that combines the higher resolution of interferometric cantilevers with the simplicity of the optical lever. Their design, the interdigital cantilever, incorporates an optical grating on a scanning probe tip.
The cantilever, micromachined from silicon, contains two longitudinal sets of interlocking fingers split by a central supporting arm and surrounded by a supporting rim. Half the fingers are attached to the outer ring, which also supports the scanning probe tip; the other half to the central arm. As the tip is repelled or attracted, the cantilever bends, displacing half the fingers.
In their experiments, the group focused a 670-nm laser diode from a scanning force microscope head onto one set of interdigitated fingers. When the cantilever bent, the fingers created a diffraction grating. Based on the interference pattern caused by the light reflecting from both sets of fingers, the device detected vertical movements of the arm within 0.02 Å in the 10-Hz to 1-kHz range.
Because the interdigital cantilever does not require additional devices above the probe, it should work just as well in fluid or vacuum environments, Manalis said.
Officials from Digital Instruments of Santa Barbara, Calif., said that if Manalis' cantilever were commercialized it could find use in the semiconductor industry, where 1 Å variations in the surface of a silicon substrate pose problems for microchip designers. Also, the cantilever would lend itself to a microlithography system that would use a charge-coupled device camera to control hundreds or thousands of the cantilevers, each etching structures on a nanometer-scale assembly line.

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