EUV lithography optics from ZEISS
New light for digitalization
The light of the future
In 1970, there was room for about 1,000 transistors on a microchip. Today there are 57 billion (semiconductor) components on an area only slightly larger than a fingertip. Mikrochips show structures 5,000 times finer than a human hair and are produced with light of the extremely short wavelength of 13.5 nanometers. For this purpose, EUV lithography optics from ZEISS SMT are used in production (no distribution in Germany). EUV technology is pushing the boundaries of what is technologically possible. For the next technological breakthrough. For future trends such as autonomous driving, artificial intelligence and 5G. For a digitalized life and work.
Smaller size, more power, more energy efficient
Transistors are the crucial component in the manufacture of microchips. The more of these switching units there is in a computer chip, the more powerful the processor. And the development is rapid. Intel co-founder Gordon Moore established the law named after him in 1965, according to which the number of transistors on a microchip doubles every two years. A challenge that ZEISS SMT has been facing for more than 50 years – with success. Most recently, in 2019, together with strategic partner ASML, TRUMPF, the Fraunhofer Institute IOF and around 1,200 other partners, a further technological leap was achieved. This perpetuates Moore's Law: EUV lithography. This was awarded the German Future Prize by German Federal President Frank-Walter Steinmeier in 2020.
German Future Prize for EUV lithography
Together with TRUMPF and the Fraunhofer Institute IOF, we won the 2020 award
Shorter, more precise, finer
EUV stands for "extreme ultraviolet" light. The light visible to humans has wavelengths between 400 and 800 nanometers. The range of ultraviolet light begins below 400 nanometers. The leading lithography process to date using "deep ultraviolet light" (DUV) operates at a wavelength of 193 nanometers. This makes structures with dimensions of 40 nanometers possible. EUV lithography uses light with an extremely short wavelength of 13.5 nanometers. Thus enables structures with dimensions of less than 20 nanometers.
The world's most powerful pulsed industrial laser
To produce light with this wavelength, a special light source is needed. First of all, this is a high-power CO2 laser from TRUMPF. With 30 kilowatts of power – about twice as much as classic industrial lasers that cut through centimeter thick steel – it is the most powerful pulsed industrial laser in the world. But the laser itself does not yet produce extreme ultraviolet light.
This is how extreme ultraviolet light is created
In order to generate the EUV light, ASML and TRUMPF designed a unique light source. In a plasma source developed by ASML, 50,000 droplets of tin are fired into a vacuum chamber every second. In there they are struck by two consecutive pulses from a high-power CO2 laser from TRUMPF. The so-called pre-pulse hits the tin droplets so that they virtually swell up. The trailing main pulse now hits the droplet at full power. This ignites the tin plasma, which emits the EUV radiation. To generate EUV light, the plasma has to be heated to a temperature of nearly 220,000 degrees Celsius. This is almost 40 times hotter than the average surface temperature of the sun.
Optics with extreme precision
Since ultraviolet light is absorbed by all materials – including air, ZEISS SMT created an optical system for the EUV lithography machine. This operates in the vacuum chamber and is made up of curved mirrors. Even the smallest irregularities lead to imaging errors. Therefore, the world's most precise mirror with a multilayer coating (so-called Bragg mirror) was developed for EUV lithography. If you were to enlarge such a mirror to the size of Germany, the largest unevenness – the Zugspitze, so to speak – would be a whole 0.1 millimeters high.
Extremely thin layers of silicon and molybdenum – only a few atomic layers thick – are vapor-deposited onto the glass surface. For this, up to 100 layers lie on top of each other here. A single layer would only reflect a good one percent of the light – the loss would be far too great. To increase the efficiency of the mirrors, ZEISS SMT has developed a unique coating system together with the Fraunhofer Institute IOF that requires atomic-based precision. The layer thicknesses are only a few nanometers thin. The result is a reflectivity that makes up to 70 percent of the light usable. This happens through constructive interference: the EUV light is reflected by individual layers in each case. When these are precisely superimposed, the light is amplified because the individual radiation waves are perfectly superimposed.
Precision to the moon
Because the mirrors have to be held in position as precisely as possible during the exposure process, an entirely new mechatronics concept was required for maximum tilt stability. The results speak for themselves. If one of these EUV mirrors were to redirect a laser beam and aim it at the moon, it would be able to hit a ping pong ball on the moon’s surface.
Mirrorblock enables precise wafer positioning
The mirrorblock is part of the wafer stage and has precisely manufactured support structures for wafers and optical sensors. It enables precise alignment of the wafer to the mask and projection optics for the wafer exposure. Despite thermal loads and high dynamic stress in the wafer scanner, the mirrorblock keeps its shape almost perfectly.
The lithography process: like a slide projector
As with a slide projector, the light passes through the photomask on which the blueprint – the template – is located. Instead of being enlarged in size, it is reduced. The structures are thus imaged on the wafer coated with a light sensitive photoresist film. In the next step, the exposed parts are etched away. The free areas are filled with copper and the wafer is polished. Then a new silicon layer and photoresist film are applied – and the lithography process starts all over again. This is repeated up to 100 times. In the end, the processed wafer is then cut into many small pieces. The microchips are ready.
How microchips are manufactured – with optics from ZEISS
EUV technology highlights
EUV lithography optics from ZEISS SMT: No sales in Germany
The optical EUV system from ZEISS SMT consists on the one hand of the illumination system. In it, the EUV light from the source is converted into the appropriate illumination for the structures on the mask. It consists of 15,000 individual parts weighing 1.5 tonnes.
On the other hand, the optical EUV system consists of the projection optics - with six mirrors. These are the world's most precise mirrors for imaging the mask structures in the nanometer range onto the photoresist-coated wafer. This requires around 20,000 individual parts weighing 2 tonnes.
The large numerical aperture of the high-NA optics requires larger mirror surfaces with even more extreme curvatures and therefore even more precision. ZEISS SMT is setting new standards for metrology with its measuring machines specially developed for the production of high-NA optics.
The mirrorblock is part of the wafer stage and has precisely manufactured support structures for wafers and optical sensors. The mirrorblock thus enables the wafer to be precisely aligned with the mask and projection optics for wafer exposure. Despite thermal and high dynamic loads in the wafer scanner, the mirrorblock remains almost perfectly dimensionally stable.