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EUV microscope explores nanoscale

21 Jun 2005

Researchers demonstrate a high-resolution EUV microscope for use in the microelectronics industry.

A team of US, Russian and Ukrainian scientists are using a table-top EUV illumination source to create an optical microscope that can image features as small as 100 nm. Operating in reflection mode and requiring little sample preparation, the EUV microscope can rapidly characterize the topography of microelectronic circuits, lithography masks and other material surfaces. (OPTICS EXPRESS 13 3983)

At the heart of the imaging apparatus is a pulsed EUV capillary discharge laser emitting at 46.9 nm developed by researchers at Colorado State University, US.

"Materials have low reflectivity at this wavelength, making it a challenge to obtain images," Fernando Brizuela from Colorado State University told Optics.org. "[However], the brightness of the source allowed us to produce very sharp images with short exposure times."

Completing the microscopy set-up is a Schwarzschild condenser, developed by a team from Lebedev Physical Institute, Moscow, and National Technical University, Ukraine, and a zone plate objective. The condenser focuses light from the EUV source on to the sample and the zone plate objective magnifies the reflected sample image on to a CCD detector.

Scientists from Lawrence Berkeley National Laboratory, US, came up with a substrate-free zone plate by attaching a 200 nm nickel film to a silicon frame. The design helps to minimize attenuation of the 46.9 nm emission, which is readily absorbed by most materials.

To reduce image-degrading effects such a speckle and interference, the team shortened the laser's capillary tube length from 36 to 18 cm to give a low-coherence beam with a pulse energy of around 0.1 mJ.

The team trialled its EUV microscope by imaging a silicon test structure and patterned nickel film. Exposure time varied between 20 and 70 seconds.

"The next step is to develop a sub 100 nm resolution microscope that will fit on to a small desk," Brizuela said. "We are interested in cooperating with industrial partners to develop and commercialize the technology."

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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