Expensive lithography for atom-sized transistor

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The structures of future processor generations will hardly be bigger than a single atom. To capture them on a wafer, revolutionary new exposure machines are working with soft x-ray radiation and “intolerant” mirrors.

In 1971, the first microprocessor from Intel had to settle for just 2,300 transistors. Some 20 years later, the Pentium increased that to 3.1 million. On Intel’s Xeon Broadwell-E5, now 7.2 billion transistors are crowded onto 456 mm² of space.

In the beginning, the UV light of a mercury-vapor lamp with a light wavelength of 434 nm was used to lithographically transfer the processor layout onto a wafer. Starting in the 1990s, an excimer laser with 193 nm was used. Today, that wavelength is still used to produce structure sizes of up to 14 nm.

The so-called Abbe resolution limit says that a light source cannot depict structures that are smaller than its wavelength. But there are plenty of tricks and “contortions” that can be used to overcome that law.

For instance, right now the most important exposure process—so-called immersion lithography—uses a liquid to fill the space between the last projection lens and the wafer. Its higher refraction index automatically results in better resolution because the two are directly proportional to one another.

Even though Intel still appeared optimistic recently, being able to conquer even 7-nm structures using this form of “wet” lithography still means that a technological and economical limit has been reached.

Lithography using tin droplets

In EUV lithography, high-precision reflectors direct the x-ray light onto the wafer. (Photo: Fraunhofer IWS)

Which is why EUV (extreme ultra violet) lithography is already waiting in the wings as a candidate for the smallest structures starting at 10 nm. Its extremely short wavelengths of some 13.5 nm are produced by using a high-power laser to bombard tin droplets in a vacuum chamber. Unlike classic optical lithography, they are captured and bundled by a system of high-precision mirrors. After all, refractive optics such as lenses, for example, would absorb these short wavelengths just as much as air.

The tolerances for the thickness of the reflective layers are in the 10 picometer range. That is less than the diameter of an atom. But that’s not all. Because generating the EUV radiation is complicated and expensive, when it comes to the mirrors, every percentage of reflectivity counts. Nanometer-thin layers of various materials—each responsible for reflecting a tiny frequency band—ensure that up to 70 percent of the X-radiation they receive is reflected back again.

EUV—The next generation

While IBM presented a first test chip using EUV lithography at 13.5 nm at the end of last year, scientists at Fraunhofer IOF and ILT are already working with industrial partners in the “Beyond EUV” project to develop key components for a new wavelength of approximately 6.7 nm. Tin as a target material would have to step aside in favor of gadolinium or terbium alloys. And the structures are hardly thicker than individual atoms. But then, small is never small enough.

EUV Fraunhofer ILT

EUV radiation is created by using a laser to bombard droplets of liquid metal in a vacuum. (Photo: Fraunhofer ILT)