Traditional lenses cannot be used with EUV as these absorb light. A mirror with a highly specialised coating, called multilayer mirror, must be used. Even these special mirrors absorb about 30 per cent of the light, so it is advantageous to use as few as possible. Any gas in the light path such as air or nitrogen will also absorb the light, thus the entire light path is inside a vacuum chamber. Primary EUV tool maker, ASML, projects EUV at 5nm node would require a higher numerical aperture than currently available and multiple patterning to a greater degree than immersion lithography at 20nm node.
Immersion lithography is still more than four times faster than EUV (275 WPH versus 65 WPH as detailed below), due to source power limitations. Hence, multiple patterning with immersion lithography has already been used where EUV had previously been expected to be used. However, it is currently recognised that EUV cannot practically realise 40nm to 50nm pitch due to stochastic effects in resist exposure, so even 10nm node is off limits. End of the road for silicon is nearing as alternative materials are now required for 7nm node and beyond.
Use of EUV lithography in a manufacturing plant today is almost as innovative as making 7nm SiGe based transistors. These use light in the DUV range at about 248nm wavelength to print 150nm to 120nm-size features on a chip. Beyond that point, smaller features require wavelengths in EUV range.
Light at 193nm and 157nm wavelengths is absorbed instead of transmitted by conventional lenses. The result is no light, no image and no circuit. To reach these small geometries, manufacturers use self-aligned quadruple patterning and EUV lithography. EUV lithography is a next-generation lithography technology using EUV wavelength, currently expected to be 13.5nm.
Current lithographic techniques utilise DUV light sources that produce wavelengths of 248nm or 193nm. EUV light used in EUV lithography has a wavelength of 13.5nm, a full order of magnitude shorter. This is important because imaging capabilities or resolutions of a lithography system are proportional to (and limited by) the wavelength of light used.
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V.P. Sampath is a senior member of IEEE and a member of Institution of Engineers India. He is a regular contributor to national newspapers, IEEE-MAS section, and has published international papers on VLSI and networks