Extreme Ultraviolet Lithography - EUVL Optics

EUVL Optics

EUVL is a significant departure from the deep ultraviolet lithography used today. All matter absorbs EUV radiation. Hence, EUV lithography needs to take place in a vacuum. All the optical elements, including the photomask, must make use of defect-free Mo/Si multilayers which act to reflect light by means of interlayer interference; any one of these mirrors will absorb around 30% of the incident light. This limitation can be avoided in maskless interference lithography systems. However, the latter tools are restricted to producing periodic patterns only.

The pre-production EUVL systems built to date contain at least two condenser multilayer mirrors, six projection multilayer mirrors, and a multilayer object (mask). Since the optics already absorbs 96% of the available EUV light, the ideal EUV source will need to be sufficiently bright. EUV source development has focused on plasmas generated by laser or discharge pulses. The mirror responsible for collecting the light is directly exposed to the plasma and is therefore vulnerable to damage from the high-energy ions and other debris. This damage associated with the high-energy process of generating EUV radiation has precluded the successful implementation of practical EUV light sources for lithography.

The wafer throughput of an EUVL exposure tool is a critical metric for manufacturing capacity. Given that EUV is a technology requiring high vacuum, the throughput is limited (aside from the source power) by the transfer of wafers into and out of the tool chamber, to a few wafers per hour.

Another aspect of the pre-production EUVL tools is the off-axis illumination (at an angle of 6 degrees) on a multilayer mask. The resulting asymmetry in the diffraction pattern causes shadowing effects which degrade the pattern fidelity.

EUVL's shorter wavelength also increases flare, resulting in less than perfect image quality and increased line width roughness.

Heating per feature volume (e.g., 20 nm cube) is higher per EUV photon compared to a DUV photon, due to higher absorption in resist. In addition, EUV lithography results in more heating due to the vacuum environment, in contrast to the water cooling environment of immersion lithography.

Heating is also a particularly serious issue for multilayer mirrors used, because, as EUV is absorbed within a thin distance from the surface, the heating density is higher. As a result, water cooling is expected to be used for the high heating load; however, the resulting vibration is a concern.

A recent study by NIST and Rutgers University found that multilayer optics contamination was highly affected by the resonant structure of the EUV mirror influencing the photoelectron generation and secondary electron yield.

Since EUV is highly absorbed by all materials, even EUV optical components inside the lithography tool are susceptible to damage, mainly manifest as observable ablation. Such damage is a new concern specific to EUV lithography, as conventional optical lithography systems use mainly transmissive components and electron beam lithography systems do not put any component in the way of electrons, although these electrons end up depositing energy in the exposed sample substrate.