AlGaN detectors suit EUV lithography

09 Nov 2007

Devices being developed for extreme UV detection in space could replace silicon-based detectors in the latest lithography tools.

The shortcomings of silicon for detecting the ultraviolet wavelengths used in sub-45 nm lithography look set to open up a new application for AlGaN detectors.

This is the hope of Joachim John, who says he's demonstrated robust 13.5 nm UV detection with his GaN-based devices for the first time.

“This is the first time that we could show, in measurements, that we have a radiation-hard detector, which is superior in comparison to a reference silicon diode,” John said.

13.5 nm is the UV wavelength employed in the latest sub-45 nm lithography tool from ASML, the leading system supplier. One of only two existing €50 million ($73 million) sub-45 nm ASML prototypes is in place at the Interuniversity Microelectronics Centre (IMEC) in Leuven, Belgium, where John is performing his research.

Inside ASML's tool, detectors measure the light every time the UV radiation interacts with a lens or a mirror, allowing the position, intensity, and other beam characteristics to be monitored.

Given that there are up to 15 such stages, a lot of detectors are required. However, at wavelengths of 13.5 nm, and when the flux - the number of photons per cm² area - is high, standard-issue silicon devices begin to degrade.

“We know from ASML that they are very unhappy with the detectors that they have inside [the lithography tool] now, but there is just nothing else,” John explained.

John and his colleagues have been working towards AlGaN detectors for wavelengths between 10 and 100 nm. The devices are intended to be “solar blind” - meaning that they do not respond to other wavelengths produced by the sun. The IMEC group is therefore hoping to exploit AlGaN to make detectors for the European Space Agency’s Solar Orbiter mission.

The IMEC team has already described AlGaN metal-semiconductor-metal diodes grown by MBE on silicon substrates for this purpose, using gold as the metal contact, at the Workshop on Compound Semiconductor Devices and Integrated Circuits (WOCSDICE) in Padova in May this year.

It was other delegates at the conference who pointed out to John that there is also a need for improved EUV detection in the latest lithography systems.

Now more focused on lithography applications, John says he and his colleagues have successfully completed a UV-radiation hardness test on AlGaN diodes at 13.5 nm, below the 121 nm lower limit previously measured for silicon diodes. These measurements were also performed at fluxes 100 times greater than the values reported for silicon.

The IMEC group will now meet with its partners at the end of November to decide whether to continue with the patent process it has started for this technology, or share its knowledge with the wider community.