Extreme-Ultraviolet Microscope Provides Record Resolution

At next week's Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference (CLEO/QELS) in California, Courtney Brewer of Colorado State University (brewerca@holly.colostate.edu) and her colleagues will present a tabletop optical imaging system that can reveal details smaller than 38 nanometers (billionths of a meter) in size, a world record for a compact light-based optical microscope.

The microscope can keenly inspect nanometer-scale devices designed for electronics and other applications. It will also be capable of catching subtle manufacturing defects in today's ultra-miniaturized computer circuits, where defects just 50 nm in size that were once too small to cause trouble could wreak havoc in the nanometer scales of today's computer chips.

Except for some high-tech details, the microscope works very similarly to a conventional optical microscope. Light shines through the sample of interest. The transmitted light gets collected by an "objective zone plate," which forms an image on a CCD detector, the same kind of device that records images in a digital camera.

However, in the case of the sub-38-nanometer microscope, there are some advanced technological twists. The microscope uses a laser that produces light in the extreme-ultraviolet (EUV) spectrum, whose very small wavelength makes it possible to see a sample's tiny details.

The EUV light is created by ablating (boiling away) the surface of a silver or cadmium target material so that the vaporized material forms a plasma (collection of charged particles) that radiates laser light. To focus this light, the researchers avoid standard lenses because they strongly absorb EUV radiation. Instead, the microscope uses "diffractive zone plates," structures containing nanometer-spaced concentric rings that focus the light in the desired fashion.

Other state-of-the-art optical microscopes have achieved resolutions as low as 15 nanometers, but they required the use of large synchrotrons. This more compact and less expensive system has the potential to become more widely available to researchers and industry. In addition, since the extreme ultraviolet laser produces light pulses with very short duration (4 picoseconds, or trillionths of a second), the researchers believe it may be possible to create picosecond-scale snapshots of important processes in other applications.

Paper CME4 at the Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference (CLEO/QELS)
Contact Courtney Brewer, Colorado State University, brewerca@holly.colostate.edu