A new type of X-ray laser could give hope to the semiconductor industry as it struggles to continue its march toward miniaturization. This next-generation chip-making tool was developed at the National Science Foundation’s Engineering Research Center for Extreme Ultraviolet Science and Technology, located at Colorado State University, in Fort Collins.
The laser operates at wavelengths of 18.9 and 13.9 nanometers, the latter fine enough for extreme ultraviolet (EUV) lithography, which will be needed to manufacture the generation of chips that are to become available around 2011. The Colorado team found a way to take a small ”seed” of EUV light, also called soft Xâ''rays, and amplify the seed to produce a beam 400 times as intense. Finding a suitable light source for EUV lithography machines has proved much more difficult than expected, and though the Colorado laser is not yet powerful enough to replace the light sources already in development, its tabletop size and optical quality could accelerate the development of EUV components and materials.
The Colorado group, led by IEEE Fellow Jorge Rocca, generated low-energy seed pulses of EUV light by firing a titanium-sapphire laser through a neon-gas cell. The interaction between the laser and the neon generated harmonics—low-energy laser light at multiples of the original laser’s frequency. The harmonics were fed to an amplifier, which is really a plasma made by irradiating polished molybdenum or silver slabs with pulses from another laser. The amplifier boosts the power only of the desired wavelengths. Molybdenum amplified the 18.9-nm wavelength, and silver boosted the 13.9â''nm wavelength required for EUV.
Chip makers will begin to need EUV for state-of-the-art chips by 2011 or 2012, when the features that make up transistors must be just 22 nm. On most advanced chips today, they are 65 or 45 nm.
The chief challenge in getting EUV lithography ready for its debut has been the light sources [see ”Plans for Next-Gen Chips Imperiled,” IEEE Spectrum, August 2007]. At the moment, the leading light sources generate EUV light by blasting droplets of tin with kilowatt-class carbon-dioxide lasers. The tin becomes a plasma and reradiates some of the laser’s energy at 13.5 nm. Cymer, of San Diego, Calif., recently reported reaching a record 100 watts of light using that process, but only in short bursts.
The problem with the existing EUV sources is that the light produced is not coherent—that is, its waves are not all in phase with one another, according to Stefan Wurm, who manages extreme ultraviolet strategy at Sematech, the independent, nonprofit semiconductor industry consortium responsible for helping the industry develop new chip-making technologies. Coherent light is better for most applications. ”You reduce power if you turn incoherent sources into coherent sources by filtering,” says Wurm. And the lower the power, the longer it takes to form patterns on a chip.
”The most important aspect of this work is the demonstration of an almost fully coherent soft X-ray laser,” says Rocca, head of the Colorado team. ”Coherent soft X-ray light can be used to measure the properties of materials and directly write patterns with nanoscale dimensions. It can also be used to look for extremely small defects in the masks that will be used to print the future generations of semiconductor chips.”
The new EUV lasers demonstrated by Rocca’s group will prove valuable to semiconductor industry research right away, predicts Wurm. In particular, he thinks they’ll be used in developing and testing photoresists—the polymers that harden when exposed to light, thus capturing the light’s pattern on the chip. If chemists can reduce the amount of energy needed to set resists, then EUV-light-source makers have an easier goal to attain for the power level of their sources.
The logical next step is to increase the energy of the output pulses by increasing the energy of the seed pulse and the volume of the amplifier, Rocca says. ”More power will make the lasers easier to use.”