EUV (Extreme Ultraviolet) lithography uses an EUV light of the extremely short wavelength of 13.5 nm. It allows exposure of fine circuit patterns with a half-pitch below 20 nm that cannot be exposed by the conventional optical lithography using an ArF excimer laser.1) Putting it into practical use requires a variety of element technologies, including the light source, optics, masks, photoresist, and lithography tools.
Among these element technologies, the biggest challenge is the technology that generates a powerful EUV beam of the extremely short wavelength of 13.5 nm. This EUV beam can be taken out from high-temperature and high-density plasma. Two methods are used for producing plasma: the Laser-Produced Plasma (LPP) method that produces plasma by condensing a strong laser beam onto a certain material, and the Discharge-Produced Plasma (DPP) method that produces plasma by a pulsed high-current discharge between electrodes in an atmosphere of certain materials. The EUV beam exiting from the plasma is collected by the condensing mirror, passes through a point called the intermediate focus (IF), and illuminates a reflection-type mask after it has been reshaped by the illumination optics. The EUV beam reflected by the mask is exposed by the projection optics to form a pattern on photoresist that is coated on a wafer surface. (See figure 1.)
Today, EUV lithography technology including the light source is still under development; it is expected to be put into practical use within several years. The output power required for high-volume production is 200 W or more. But, the present EUV light source for production use has the output power of approx. 10 W, which is still at a low level. The EUV light source for high-volume production requires a great enhancement of the output power and high reliability that allows non-stop operation without maintenance for several months.2)
= References =
- Shinji Okazaki: Optical Society, Vol. 41, No. 3, pp. 116-124 (2012)
- Imec news,