SCHOTT solutions no. 1/2013 > Microlithography

Due to the short-wave EUV light, the wafers are exposed under high vacuum using high-precision mirrors. Photo: ASML

The Finest Photon Brush

Extreme ultraviolet technology (EUV) represents a groundbreaking technological change in microchip production. ASML, the leading manufacturer of photolithographic systems for the semiconductor industry, has begun serial production of its EUV wafer scanner family, the ”TWINSCAN NXE” series, and will ship the first system later this year. SCHOTT will provide components and know-how.

Thomas H. Loewe

The future is now, thanks to silicon microchips. Smartphones and tablets are prime examples: the demand for increasingly powerful, yet tiny, processors and memory modules is growing rapidly. At the same time, current manufacturing systems, so-called wafer scanners, have reached their limits. The circuit components on each chip are already smaller than the rays of light that print them. ”We needed a finer brush to continue the advancing ­miniaturization in the semiconductor industry,” says Jos Benschop, ­Vice President Research at ASML, the world’s largest supplier of semiconductor printers, or “photolithographic systems.” Now, the Dutch company has brought the long sought EUV technology to the point of marketability. It allows the production of superchips with extremely reduced pattern scales of 18 nanometers (1.8 x 10– 5 mm) or less. In daring this step, ASML also relied on highest ­precision by SCHOTT. ”ASML has developed a high-tech apparatus that really pushes the limits of physics. We are proud to assist with our know-how and products,” says Antoon Wesselink, General Manager SCHOTT Benelux.
SCHOTT supplies, among other things, light guides several meters long that are made of high-purity quartz glass for the latest generation of EUV wafer scanners of the ASML ”TWINSCAN NXE” series. Photo: ASML
Today’s microprocessors have over one billion transistors. Even now, very short-waved light is needed to fit all of the tiny components onto a very confined space. The manufacturing process of microchips is comparable to a slide projector: It begins with light traveling through a slide or photo mask, which contains the blueprint of the pattern to be printed. Lenses – or mirrors, in the case of EUV – focus the pattern onto silicon wafers. The wafers are pre-coated with a light-sensitive substance. When the unexposed parts are etched away by chemicals, the projected pattern is ­revealed in three dimensions. Foreign atoms and oxide or metal layers can also be added in later stages of the production process. In total, the technique involves over 20 different steps. Depending on the size of the circuit pattern, up to 50,000 chips can come out of a single wafer.

Current machines use ultraviolet light from Argon-Fluor-­Excimer lasers to expose the wafers. This light has a wavelength of 193 nanometers. The new technology uses EUVL – short for ­extreme ultraviolet light – with a wavelength of 13.5 nanometers. It is emitted by laser-produced plasma (LPP) and laser-assisted ­discharge plasma (LDP) sources. Working at these wavelengths poses new and difficult challenges: ”EUV light is very shortwave. It is easily absorbed, even by air. So the wafer’s exposure has to be done in a high vacuum environment,” explains Jürgen Meinl, developer at SCHOTT Lighting & Imaging, who has already helped in the development of ASML’s previous machine models with his

EUVL’s properties also forced ASML to substitute the wafer scanner’s conventional lens system with mirrors. Optical lenses would also immediately absorb the EUV photons. But even the mirrors need to have special properties for the system to work. According to the experts, they are so smooth that if one of them were to be blown up to the size of Germany, the biggest bump would reach less than a millimeter.
EUVL is the future of semiconductor technology. This new technology allows for superchips with highly reduced structural gaps of 18 nanometers and less. This not only means higher performance, but also more cost-effective chip manufacturing. Photo: ASML

Volume production to start in 2014

Highest precision is also essential in determining the silicon wafer’s exact position and the alignment of the photo mask inside the vacuum chamber. For this, ASML integrated dozens of sensors into its EUV prototypes that use light. The sensors must be prevented from affecting the lithography system. Since the huge chip machine is calibrated in the range of a few milli-Kelvins, a complex guiding system made of very fine glass fibers is necessary to channel the light into the sensors.

For this, SCHOTT developed new glass fiber bundles made of extremely pure quartz. Even the slightest trace of contaminants would disturb the entire ­production process at these scales. Despite being only 3.5 millimeters thick, every light guide bundle consists of one thousand ­individual fibers – each no thicker than a human hair. The new wafer scanners are outfitted with several meters of them. SCHOTT also supplied special glass-metal seals that guarantee an airtight transition of the light guide bundles into the vacuum chamber. And further important components of the high-tech project come from SCHOTT: A material with almost non-existent expansion properties was needed for the photo mask fixtures and the silicon wafer fixtures. The glass-ceramic ZERODUR® meets these requirements. It has an extremely low coefficient of expansion and is perfectly suited for applications where highest precision is a prerequisite.

The first ASML ”twinscan NXE” systems have reached their buyers and have meanwhile exposed more than 30,000 wafers. ­Volume production will start in 2014 and the company has already received more than a dozen additional orders. <