APPLICATIONS EUV LITHOGRAPHY
Printing at the extreme
Immersion lithography has all but reached the limit of how small it can print features on silicon chips. Extreme UV lithography promises to be the answer, but there’s still a lot of work to be done before the technology is ready for commercial use, as Greg Blackman discovers
Cymer’s EUV source, 3300, has demonstrated 30 to 40W output power incorporating a pre-pulse laser T
he likes of Intel are finding it more difficult to make the smallest features on their silicon chips with current immersion lithography techniques. If microchips keep getting faster at the rate predicted by Gordon Moore, co-founder of Intel, a new means of printing circuits on silicon is needed. Enter extreme UV lithography. EUV lithography has been hailed for some time as the natural successor to photolithography at 193nm, the established method for printing integrated circuits, because its shorter wavelength of 13.5nm gives a better resolution and the ability to print smaller structures. But it represents a step change. EUV is absorbed by pretty much all material and has to be generated in a vacuum. All the optics have to be reflective. This makes building a reliable and efficient EUV scanner a challenge, unlikely to come to fruition in terms of high-volume chip manufacture for at least a couple of years.
The lithography scanner provider ASML announced in October the purchase of EUV light source maker Cymer to help its EUV programme. EUV pilot production tools from ASML are being tested and the company plans to ship its latest generation of pre-production tools, the NXE:3300B, within a few months. ASML is targeting 11 shipments of the system in 2013, worth around €700 million. This is all geared towards allowing chip manufacturers to finalise their EUV processes, with production machines expected to be delivering 70 wafers per hour by mid-2014. It won’t be until 2015 or later before chips are being produced by this technology in any kind of volume.
The source of the problem The EUV light source remains the major stumbling block. Cymer says its current light source operating in the field is running at around 10W of output power; 250W is generally
12 ELECTRO OPTICS l FEBRUARY 2013
considered the benchmark where EUV scanners become economically viable for silicon chip manufacturers. Cymer’s source has demonstrated 30 to 40W output power, and is currently being integrated into ASML’s scanner. At low duty cycle, Cymer can generate around 90W raw power, but a certain amount of energy overhead is needed to keep a reproducible dose at the wafer. Cymer bases its EUV source on a laser produced plasma (LPP)
coated with silicon molybdenum multilayers, that selects the 13.5nm radiation and focuses it to the intermediate focus, the interface with the scanner. ‘This is really the only architecture scalable to the power levels that will be needed for high-volume manufacturing at high throughput,’ says Dr Nigel Farrar, vice president of EUV strategic marketing at Cymer. The alternative is discharge
produced plasma (DPP), which uses 250W is generally considered the
benchmark where EUV scanners become economically viable for chip manufacturers
architecture, whereby a tin droplet is heated to around 500,000°K with a high-power pulsed CO2
generates a plasma that emits EUV light at 13.5nm, along with other wavelengths. The EUV radiation is collected with a large mirror
laser. This
an electrical discharge to generate the plasma, although one problem with the method is its limited scalability to high powers. Xtreme Technologies is developing a hybrid, called laser-assisted discharge plasma (LDP), which it claims combines the
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Cymer
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