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Taking Control to the Extreme

ASML's Newest Extreme UV Litho Tool Requires High Precision

ASML was started in 1984 as a joint venture between Philips and Advanced Semiconductor Materials International (ASMI), tucked away in a wooden building next to one of Philips' factories in Eindhoven, the Netherlands. Today the lithography toolmaker has its headquarters in Veldhoven, the Netherlands, and employs more than 9,000 people.

As a major player in the semiconductor manufacturing industry, ASML's machines are used to print the tiny circuits inside the chips that make everything run. Early in its history, leading-edge circuits had details close to a micron wide, printed with 436 nm ultraviolet light—easy science, compared with what the tools are asked to do today.

Over the years, lithography toolmakers have had to shift the light they use to 365 nm lamps, then 248 nm krypton fluoride excimer lasers, on to today's 193 nm argon fluoride excimers, which are printing linewidths several times smaller than their wavelength. To increase the difficulty level a bit more, today's cutting-edge lithography tools must image through water (called immersion lithography) instead of air to get a resolution boost from the higher refractive index.

The precision with which semiconductors must be made puts high demands on the tools' control systems. ASML designs most of its control hardware and software modules in-house because it is such a complex system—with wafer positioning systems, subsystems loading and unloading wafers, a linear axis to make sure all cables follow the wafer positioning stage, numerous actuators and sensors to compensate for the dynamics of the optics, and more. "All of those servo loops have to interact," says Gerard van Engelen, groupleader, waferstage, for ASML's newest tool, the NXE. "If we have an error in the servo loop of the wafer stage, it feeds forward to the reticle stage, and can be corrected there. There are numerous interactions like that."

The reticle is the patterned mask the lightsource shines through before being shrunk 4x through a series of optics to print a smaller version of that pattern onto the wafer. At least, that's how it works in the tools being used in production by chipmakers today. The new NXE platform, however, is the technology for tomorrow, whose source makes a leap from a 193 nm wavelength to 13.5 nm, down to what is referred to as extreme ultraviolet (EUV), previously known as soft X-ray.

It's a game changer, to say the least, rife with complications that engineers throughout the semiconductor industry are still working their way through in order to bring the technology to volume production capabilities. Because the EUV wavelength doesn't pass through any media, including air, it requires that the tool operate in a vacuum. It also means that all the optics—lenses and reticles—must now be reflective rather than transmissive. With resolution capabilities getting ever tighter, ultimately aiming below 20 nm linewidths, the tolerance for contamination has gone to almost nil. Overlay—how well each printed layer lines up with another—also allows hardly a single nanometer of wiggle room.

"For the wafer stage, we're using a very methodical way of getting rid of all the disturbances that we know of," van Engelen says. "While integrating functionality, we may find dynamical problems, which could be, for example, disturbances from wires and hoses that have to be connected. We solve those problems by changing the hardware if it's early in the design project, or adding intelligence to the control software."

ASML's engineers use multi-input, multi-output, feed-forward and feedback systems, making use of algorithms that can cope with the position dependency of the wafer stage. "We need to compensate for dynamical modes for, for example, the mirror block," van Engelen says. "Ideally, it would be a rigid body, but in practice it's a flexible body. One method is to make the body as rigid as possible. Another is to put intelligence in the control."

Not only must the control run with extreme precision, but it must do so in a vacuum. The change to a vacuum environment meant that there were a lot of new requirements for materials, van Engelen says. The mirror blocks, motors, connectors, hoses, wires—all need to be vacuum-compatible. There can be no outgassing of any kind from the materials, because that would contaminate the mirrors. "This limits the number of materials and suppliers who are qualified for use in vacuum," van Engelen adds.

The positioning function of the wafer stage doesn't change on account of the vacuum, but every new generation of tool requires improved accuracy. "There were specific areas of control where changes were needed," van Engelen says. "Temperature control is quite different because there is no air."

Finally, the EUV tools inherited a basic architecture from their optical lithography brethren that include twin stages—two stages moving wafers around the machine to be measured and printed. The individual stages themselves need to be controlled with extreme precision, van Engelen says. "Additional challenges arise from the fact that these stages have to switch places. We have to avoid the stages colliding into each other, so there's a lot of functionality that has to be designed and programmed in the firmware. It has quite a big impact on the software."

For machine design, ASML employs roughly 40-50 control engineers, van Engelen says. In the mechatronics department, the control engineers graduated mainly in control theory and system analysis, though a few are more specialized in sensors and measurement systems, and some have more general mechatronic backgrounds. "We have a very large system of software modules which enable control engineers to implement algorithms," van Engelen says. "We don't use off-the-shelf hardware modules and control software. It's quite complicated, and we need a lot of people to manage that system."

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