Spectrometer Aims to Further Refine Lithography

To improve the quality of microchips and make the chipmaking process more efficient, researchers at the University of Twente’s MESA+ Institute of Nanotechnology developed an extreme ultraviolet (EUV) broadband imaging spectrometer. The new spectrometer simultaneously measures the size and color of the light emitted by a plasma source.

The ability to measure the size and color of the light at the same time could improve a crucial step in chipmaking — lithography — by enabling lithography machines to make smaller, faster chips. To produce the microsize chips used in most electronic devices, lithography machines need precision-engineered lenses, mirrors, and light sources.

“Traditionally, we could only look at the amount of light produced, but to further improve the chipmaking process, we also want to study the colors of that light and the size of its source,” professor Muharrem Bayraktar said.

A plasma source for EUV light is produced by aiming lasers at metal droplets. To create microchips, the plasma source is directed at a silicon wafer via special sets of mirrors. “We want to make the plasma as small as possible,” Bayraktar said. “Too large and you ‘waste’ a lot of light because the mirrors cannot catch all the light.”

In addition to the size of the light source, the color that is emitted from the source can also affect the microchip-making process. “The plasma does not only emit extreme ultraviolet light, but also other colors,” Bayraktar said. The new spectrometry tool will make it possible to investigate the relationship between the size of the source and the color it emits, by allowing researchers to look at the size and color of the plasma source together.

To build the spectrometer, the team used a tapered zone plate that was matched to the dispersion of a transmission grating. The tapered zone plate enabled the researchers to manipulate the EUV light emitted from the plasma source, in order to precisely image the source.

The researchers used the transmission grating to disperse the light into individual colors. This allowed the researchers to measure each color separately.

The tapered zone plates were designed and fabricated in the 5- to 80-nm wavelength regime, with a spatial resolution of about 10 µm and a spectral resolution of about 0.8 nm.

The researchers benchmarked the imaging spectrometer with a solid tin, target laser-produced plasma source. They further showed that plane wave propagation simulations qualitatively matched the experimental results, confirming the device’s performance.

The research was published in Optics Letters (www.doi.org/10.1364/OL.496995).