The EUV lithography is the most promising candidate for the next generation semiconductor exposure technology to
fabricate fine patterns of 32nm and below 32nm, now that ArF immersion technology is expected to cover the 45 nm
node these days. This paper focuses on the optical performance of EUV projection optics that will be the lithography tool
under 32nm node. EUV projection optics is consists only of the mirrors, and the number of mirrors varies depending on the numerical
aperture (NA) of projection optics. As the NA becomes higher, more mirrors are required. For example, it is expected
that 6-mirror projection optics (6M-PO) could have 0.25NA, and 8-mirror projection optics (8M-PO) could have over
0.35NA. Assuming a process factor K1 of 0.6, EUV projection optics with NA of 0.25 can achieve 32nm resolution, and
projection optics with NA of 0.35 can achieve 22nm resolution. Therefore, 6M-PO and 8M-PO are suitable for 1st EUV
generation and 2nd EUV generation, respectively.
In EUV optics, each mirror has multi-layered Mo/Si coating to get high reflectivity. The reflectivity is extremely
sensitive to the layer thickness of coating, the exposure wavelength, and the ray incidence angle, so the multilayer
coating is designed to give a best performance at exposure wavelength of 13.5nm. This means that thickness and optical
indices in the multilayer coating cause serious degradations on the imaging performance. Therefore, more mirrors might
cause greater degradation on the image performance.
This paper discusses the relationships between the multilayer coatings and the imaging performance in order to apply
EUV lithography for below HP32nm technology node. We compare 6M-PO to 8M-PO from the point of view of the
multilayer coatings numerically.
1. Imaging performance with error-less multilayer coating in consideration of chromatic aberration
Even If the multilayer coating without any coating errors can be coated on all of the mirrors, the chromatic aberration
cannot be avoided. We compare the imaging performance of 6M-PO to 8M-PO in consideration of chromatic aberration.
2. Influence of multilayer coating errors on transmittance of optical system and aberration
There are some kinds of multilayer coating errors. In this paper, we discuss thickness errors that have a great influence
on transmittance and aberration.
3. Influence of contamination on aberration
We discuss the influence of carbon contamination films that deposit on the top surface of the multilayer coatings.