Multilayer Coating Shows Atomic Precision
Daniel C. McCarthy
Extreme-ultraviolet wavelengths don't transmit well, even through gaseous materials like air. Consequently, the EUV lithography prototype stepper being developed by a consortium of government and semiconductor industry partners steers its 13.4-nm beam using nine reflective optics: four condenser optics, a reflective mask and four projection optics, which image the mask pattern onto the wafer.
The thickest and thinnest parts of the multilayer coatings on these mirrors cannot differ by more than one extra layer of atoms. This degree of thickness control, essential to optics for extreme-UV lithography, has been demonstrated and is on its way to becoming commercial. Courtesy of Lawrence Livermore National Laboratory.
In order to reflect the EUV radiation, systems engineers apply 81 alternating layers of molybdenum and silicon to each of the mirrors. Controlled with enough precision, the thickness of these multilayers can create constructive interference, increasing the mirrors' normal-incidence reflectance to 70 percent. But the thickness must be uniform within 0.1 nm rms, which translates as 0.04 percent rms for a coating that is 280 nm, or 1000 atoms, thick.
Condenser optics require a precise thickness to reflect the proper wavelength. If the coating is too thick, reflectance shifts to longer wavelengths. If it's too thin, reflectance shifts to shorter wavelengths. Nonuniform coatings on the projection optics distort their optical surface and effectively degrade the image quality of printed features. Adding to the challenge is the fact that all but one projection and one condenser optic are aspheres.
Three years ago, engineer James A. Folta and his team at Lawrence Livermore National Laboratory in Livermore, Calif., overcame many of these challenges and established total thickness control within one atom over a 150-mm area. More recently, in partnership with Veeco Instruments in Fort Collins, Colo., they scaled up the technology and demonstrated it on production-scale mirrors with diameters as large as 400 mm.
The deposition process employs a DC magnetron sputter system that places the substrates face up on spinner assemblies mounted on a rotating platter. The rotation velocity determines layer thickness. To handle larger optics, the group stiffened the rotating substrate platter drive system and replaced the original stepper motor with a servomotor that had larger torque and better resolution.
The new system not only can handle manufacturing scale mirrors, but also can coat four optics, each with different prescriptions. The system is also more stable -- most notably in thickness control across the diameter of the optics, which is required to preserve the image quality.
"We coated one optical system last year and six months later had a [second] set of optics to coat," said Folta, now a deputy program leader for EUV lithography. "We didn't have to tweak the process at all, except for the overall rate."
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