EUV Light Sources
Keeping Moore’s Law Alive In 1965, Intel co-founder Gordon Moore predicted that the number of transistors that can be placed onto a single IC would double approximately every eighteen months to two years, dictating what is now known in the industry as “Moore’s Law.” Each year, device manufacturers have achieved the advances predicted by Moore's Law, bringing about a next-generation chip to boost computing powers and memory storage capability - from computers and MP3 players to video games, home electronics and more. To keep pace with Moore’s Law in the rapidly advancing computer age, the semiconductor equipment industry has invested heavily in research and development on advanced alternatives to DUV lithography to keep the progress and economics of smaller faster electronic devices from slowing down.
To maintain the advancements of Moore’s Law, Cymer is leading the industry with its decade-long research and development of an extreme ultraviolet (EUV) lithography light source, the best viable long-term option for the continued creation of advanced semiconductor chips.
Today’s current-generation DUV lithography patterns circuitry at critical dimensions of ~250 nm to 45 nm, and is being used to develop next generation manufacturing processes of 32 nm. The shorter the wavelength, the more powerful the final integrated circuit - meaning smaller, faster, and more innovative electronic devices for mankind.
With EUV lithography, devices can be patterned from 32 nm to estimated single digit nm dimensions, keeping shrink progress and Moore's Law alive to enable future generations of electronic products. Due to the fact all matter absorbs EUV light, the lithography process must take place entirely in a vacuum, including all optical elements including the photomask.
The EUV light source operates by tareting a high power laser directed at droplets of tin (Sn), which when hit by the pulsed laser heats the fuel to create a plasma. This process spawned the common EUV industry term “laser-produced plasma,” or LPP, which refers to the source of the EUV system. (Though other techniques have been investigated such as discharge produced plasma (DPP), Cymer has implemented LPP as the most viable commercial EUV approach since 2004.)
Once the laser-produced plasma is created, a large collector mirror collects and focuses the 13.5 nm light. The EUV light is then delivered to the lithography system. As the light projects onto the mask, it is reflected onto a series of mirrors, which reduce the size of the image, and focus it onto a silicon wafer—similar to the way in which lenses in a camera use light to form images onto film or CCD chip.
The mirrors used by the EUV source are critical to the technology. The EUV source utilizes both concave and convex mirrors coated with several layers of molybdenum (Mo) and silicon (Si), in order to reflect the maximum possible amount of 13.5 nm EUV light. About thirty percent of the light is absorbed by each mirror. Without the coating, the light would be absorbed before reaching the wafer. Additionally, mirror surfaces must be nearly defect-free, as even the smallest imperfections can destroy the shape of the optics and distort the printed circuit pattern.
The lifetime of the collector mirror is a critical parameter in the development of LPP EUV sources. Deposition of target material as well as sputtering or implantation of incident particles can reduce the reflectivity of the mirror coating during exposure. Debris mitigation techniques are used to stop damage from occurring.
Probably the most important factor of an EUV lithography light source is the power at which it can continuously run. Cymer has developed techniques to increase the continuous power while running under typical operating conditions used for lithography applications. High-volume commercial production systems are expected to be required to operate at greater than 200W of power, 24 hours per day, in order to get the wafer throughput required by semiconductor device manufacturers.
The pilot light source in 2010 was the world’s first, commercial LPP EUV source for lithography applications, continuing Cymer’s legacy of leading the “light generation.”