Industrial inspection in the nano range
Miniaturization in semiconductor technology poses a huge challenge for photonics. Technologies for structure widths of less than 10 nanometers are required.
Nowhere else is miniaturization more obvious than in semiconductor technology. On today’s microchips, 1.3 billion transistors work on the same space on which Fairchild installed individually integrated circuits back in 1961. This is possible only with accuracies in the low double-digit nanometer range. And industry has its eyes firmly fixed on the next steps. Production technologies are being developed for 7-nm and 5-nm technologies that will update Moore’s Law.
Development hot spots are Veldhoven (Netherlands) and Oberkochen (Germany), which is where ASML, the world’s largest provider of lithography systems for semiconductor production, and the most important supplier of high-performance optics, Zeiss SMT (Semiconductor Manufacturing Technology), are based. In lithographic structuring, chip manufacturers still work with excimer lasers in the UV range of 193 nm. Optical tricks are needed to expose the tiniest nano structures on photoresist-coated wafers with this “rough” tool. First, the manufacturers produce masks with the exact image of the chip structure. This image is projected onto the wafer, with the light from the argon fluoride laser being directed via a high-precision optical system of mirrors and lenses, and through a fine film with immersion fluid. The image is thus reduced by a factor of four. With multi-patterning, in which the wafers are moved by a few nanometers with nano positioning systems, it is possible to reduce the structure widths even more.
EUV lasers driving miniaturization further
But the optical tricks have all been used up. The wavelength of 193 nm is too rough for structure widths below 10 nm. That’s why ASML and Zeiss are working on a technology leap: Extreme ultraviolet (EUV) laser light with a 13.5 nm wavelength should provide the required precision. At the end of 2016, ASML bought a billion-euro stake in Zeiss SMT and now plans to support the company’s optics development for EUV lithography with an additional 760 million euros. ASML has announced that serial production of the new microchip technology will already commence in 2018. By then, Zeiss SMT will begin operating a new center for lithography optics.
The investments are necessary because EUV technology requires new processes. Since the shortwave light is even absorbed by oxygen atoms in the air, exposure in future will have to take place in a vacuum. High performance lasers (>250 W) with the necessary stability and precision at 13.5 nm are also needed. Because of the absorption problem in the area of EUV, it will hardly be possible to continue using lenses. And, in addition, much more exact masks and mirrors are needed to prevent light scattering. On top of this, the changed photoresists will also have to be applied more precisely.
EUV systems on the market from 2018
ASML has already reported on promising experiments with production rates of 1,000 wafers per day and 80 percent system availability. But until mass EUV chip production exists, manufacturers of measurement and inspection systems still currently face huge challenges. What are wanted are systems that inspect the nanometer-sized structures on the 45 centimeter wafers during production. Measurement resolutions in the sub-nanometer range will be required for future mirrors and masks. This will be possible only with equally precise positioning technology. Speed is also critical: Time windows of just milliseconds are allowed for inspection of the individual process steps in semiconductor production. With planned production rates of 1,400 wafers that are packed tightly with many billions of nano transformers, just one minute production time is allowed per wafer. The measurement technology is given just a fraction of that time. But if it misses defects or deviations, the damage is immediately enormous.
Ultimately, in the nano world, photonics is without peer: Only it can solve the laser technology and optical challenges of EUV lithography. Only it has solutions for inspecting the optics, wafers and photoresist coatings, and the finished microchips. But one thing is certain: With its intelligent light beams, photonics will write the next chapter of Moore's Law.