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Pinholes offer cheap EUV calibration

17 Jun 2002

Two pinholes are the key to an elegant way to characterise extreme-ultraviolet (EUV) light beams.

Simple interferometry could provide an inexpensive way to calibrate the wavelength of extreme-ultraviolet (EUV) light sources, according to US researchers.

EUV light sources are set to play a key role in making future generations of smaller scale, more powerful semiconductor chips. The problem is that calibrating their emission spectra is notoriously difficult and requires expensive grating-based spectrometers.

Now, however, Randy Bartels and colleagues at the National Institute of Standards and Technology in Boulder, Colorado, have come up with a calibration method that is based around two pinholes and interferometry (Optics Letters 27 707). "Our approach is much cheaper and simpler than conventional spectrometers - particularly those used in the EUV region," said Bartels.

Bartels says that while the conventional EUV spectrometer in his laboratory costs USD 120 000, the pinhole pair setup costs only USD 10 000 to build. The method uses a Young's double-slit arrangement. Two 20 µm diameter pinholes create an interference pattern that is detected with an EUV-sensitive CCD camera 2.85 m away. Analysis of the pattern reveals the wavelength of the EUV light.

A big benefit of Bartels' system is that it can be calibrated with a simple optical-frequency source that has a well-defined frequency, such as a helium-neon laser. "We do not require a calibrated wavelength source in the EUV," he said.

Michael Hatcher is technology editor of Opto & Laser Europe magazine.

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