Einblick in die optischen Systeme der EUV Lithographie der ZEISS SMT

New light for digitalization: Without EUV lithography, modern microchips would be inconceivable – and without optics from ZEISS, there would be no EUV lithography.

The light of the future: How does EUV lithography work?

Microchips are manufactured using the lithography process. This means that the structures desired on the chip are first transferred from a mask onto a light-sensitive layer – as in the projection of a slide. In the next step, the exposed structures are chemically fixed and then the unexposed areas are washed out. This process is repeated up to a hundred times with different masks, gradually creating an increasingly complex three-dimensional structure of tiny electrical components such as transistors and conductor tracks – a functioning microchip.

Moore's law is alive and kicking

To increase the computing power of a chip, engineers are packing more and more transistors onto the wafers. The microchips in the Apollo 11 moon capsule had only about 1,000 transistors; today, the central chip of a humble smartphone has more than 57 billion transistors. Gordon Moore, co-founder of Intel, predicted this continuous increase in performance as early as 1965. The "Moore's Law" he postulated states that the performance of each current chip generation doubles every two years – this increase being driven by the miniaturization of chip structures. A prediction that has become the benchmark for the semiconductor industry.

Portrait of Dr. Frank Rohmund, President ZEISS Semiconductor Manufacturing Optics

The microchips in the Apollo 11 lunar capsule had only about 1,000 transistors; today, thanks to EUV, the central chip of a smartphone has more than 57 billion transistors.

Dr. Frank Rohmund

President ZEISS Semiconductor Manufacturing Optics

Small waves, big effect

The size of the chips has not grown over time – but the number of transistors has. This has been made possible by the increasing efficiency of the lithography process, which exposes ever finer structures onto the wafers. The quality of these structures depends on many factors, such as the mask (or to stay in the picture: the slide), the mechanics and optics used – but above all the light used. The shorter the wavelength, the smaller the exposed structures.

Optical representation of the size ratios of EUV - wavelengths

A ray of hope for the future

Over time, increasingly shorter wavelength light sources have therefore been used for microchip lithography. While a wavelength of 193 to 365 nanometers is still used for many commercially available microchips, 13.5 nanometers are already used for high-performance chips (for instance, for graphics cards or high-end smartphones). At 13.5 nanometers, we enter the range of extreme ultraviolet radiation that is invisible to the human eye – hence the name EUV lithography. This technology increases transistor density enormously compared to current microchips. This is because the conductor paths created in this way are only a few nanometers wide. By comparison, a human hair is about 70,000 nanometers wide.

Hard at physical limits

The use of EUV light requires an immense technical effort – from the light source to the entire optics used to the mechatronics. The EUV light is generated within a globally unique light source. A tiny drop of tin is hit by a laser beam in a vacuum, causing it to swell. After this so-called pre-pulse, the subsequent main pulse heats the tin to 200,000°C (approx. 360,000°F), transforming it into a hot plasma that is almost forty times hotter than the surface of the sun. The ignited tin plasma then emits the desired EUV radiation – this process is repeated 50,000 times a second. The laser used for this is the most powerful pulsed industrial laser in the world – ten times more powerful than all currently available systems used for steel cutting.

EUV technology in the semiconductor industry

High-precision optics with two functional modules

Two components are needed to produce EUV technology: the projection optics and the illumination system.

Without air and glass

Because EUV radiation is absorbed by air and glass, ZEISS has designed an optical system specifically for EUV lithography that operates in a vacuum and uses only mirrors. The entire system is about one and a half meters high, weighs about 3.5 tons and consists of more than 35,000 individual parts. The high-precision optics enable near-perfect imaging and are divided into two functional modules: the illumination system and the projection optics. The illumination system has the task of illuminating the mask as uniformly as possible from adjustable directions. For this purpose, a large mirror (the so-called collector) collects the EUV light emitted by the tin plasma and shapes it via the next mirrors in a beam path to precisely fit the mask. With the projection optics, the extremely small mask structures are precisely projected onto the photoresist-coated wafer via further mirrors. In the end, billions of individual transistors the size of a fingertip are created.
The optical and mechatronic requirements for this system are enormous. It works so precisely that it could aim a golf ball at the moon from the Earth. The mirrors are masterpieces of optics: If they were scaled to the size of Germany, the highest unevenness would be just a tenth of a millimeter high. With these requirements, these are the most precise mirrors in the world – and it takes months to produce an EUV mirror. Another extreme requirement is that the mirrors reflect EUV light with as little loss as possible. This is achieved by a layer system consisting of more than 100 atomically precise individual layers, each only a few nanometers thick.
  

Insights into the ASML waferscanner

The heart of EUV technology beats in Oberkochen

Does all this sound complicated? It is. Mark Phillips, Intel Fellow and Director of Lithography, sums it up: "The EUV scanner is the most technically advanced tool of any kind that's ever been made." And indeed, an ASML EUV lithography system is as big as a school bus, weighs around 200 tons, and consists of hundreds of thousands of individual parts. Research for what is now EUV lithography began more than 25 years ago. During this time, more than 2,000 ZEISS patents have been applied for that are related to EUV. The market launch was a huge success: EUV microchips are now installed in all leading smartphones. What's more: powerful and energy-efficient EUV chips enable life-saving medical technology, support autonomous driving and form the backbone of high-performance computers, smart cities and artificial intelligence.

The High-NA EUV mirror in the clean room at ZEISS SMT

The way to new shores leads over narrow paths

Together with our strategic partner ASML for the system architecture, ZEISS is already working on the next EUV generation High-NA – which will enable even more powerful microchips. From today's perspective, their areas of application are visionary. Whatever is to come: ZEISS is working on pushing the boundaries of what is technically feasible.

Portrait of Dr. Frank Rohmund, President ZEISS Semiconductor Manufacturing Optics
Author

Dr. Frank Rohmund

President ZEISS Semiconductor Manufacturing Optics


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