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Particle-free manoeuvring with EUV masks

Remove all the bacteria from a football pitch without snapping one blade of grass, such is the precision of a chip nowadays. Particles of 20 to 50 nanometres on an extreme ultraviolet mask (EUV mask) measuring 15 x 15 cm can disturb the process and render the chip useless. TNO helps manufacturers of chip production machines to prevent such production losses.

Recent decades have seen chips become increasingly smaller. The lithographic process required lasers with ever shorter wavelengths, but once the boundaries of laser technology had been reached, new methods were sought. The solution lay in lithography with extreme ultraviolet light. EUV enables even smaller structures to be produced, but not without consequences. Strong absorption of light at these very short wavelengths, for one. This is why lithography machines have to work in a vacuum and lenses had to make way for mirrors. Moreover, the old protection trick for masks with a protective membrane (pellicle) placed 5 millimetres above the mask no longer worked. Once a particle comes onto that pellicle, it is no longer in focus and is not imaged on the chip. And since the pellicle is not sufficiently transparent, a lot of light is lost. EUV masks therefore have no pellicle.

Shift

The consequences of working without a pellicle are significant. Previously particles of 3 micron caused a distorted image but now particles of 20 nanometres, a factor of 100 smaller, can be a risk. Furthermore, particles on the rear side of the reticle can now cause a shift of the image, which produces and overlay-error: the various layers of the IC no longer align. If a particle remains behind in the lithography machine, the machine has to be opened and cleaned. This means down time, something that has to be prevented. This is why equipment that operates EUV reticles are very clean.

Alpha Demo Tools

ASML asked TNO to develop, build and test key units of the first EUV machines, the Alpha Demo Tools. The 'reticle handler' ensures that the mask is brought from the atmospheric environment to the vacuum of the EUV machine as particle-free as possible, and there transported and clamped for exposure using a robot arm. In addition to other creative design solutions, for instance for the air circulation, the number of contact moments – read: risk moments – in the unit is reduced by moving the mask on a handling frame through the production machine. Currently more and more equipment for EUV reticles are coming on the market. All this equipment must comply with very strict cleanliness requirements. TNO helps companies to develop this equipment so that they function free of particles. And should a particle manage to slip in somewhere, then the measuring technology developed and continually refined by TNO will still track it down.

Dr. Jacques van der Donck

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