|
Optical lithography in the deep ultraviolet (DUV) region is being pushed to reach the limits of printing resolution. The
effort required to achieve the 32 nm structure with this technology puts very hard conditions and requests on the
illumination optics. Different kinds of illumination modes are combined to get into a regime of extreme numerical
aperture (hyper NA). Arrays of refractive micro-optics have been and continue to be the ideal solution for high
transmission homogenizing elements for several tool generations.
Illumination of the masks with high numerical aperture is critical for achieving the smallest structure on the
semiconductor material. Exposure tools use different illumination modes to get better imaging of certain mask structures.
The beam shaping necessary to create these illumination modes is achieved mostly with diffractive elements. Most of the
currently used modes can also be created with arrays of refractive micro-optics, manufactured from fused silica and
calcium fluoride. The advantage over the diffractive optical elements lies in efficiency, which comes close to 90%.
An important prerequisite for these special types of optical elements is LIMO's unique production technology which can
manufacture free form surfaces on monolithic arrays exceeding 200 mm edge length with high precision and
reproducibility. These homogenizing elements in the illumination optics can provide a custom designed intensity
distribution, and offer the possibility to correct the failure of other optical elements. Each lens can be designed
individually and can also be shaped asymmetrically. Thus unusual lens sizes and shapes can be produced, and various far
fields such as rectangles, lines, hexagons or multi-poles can be achieved.
In this paper we present novel refractive micro-optical elements which create rectangular dipole illumination. They can
also be combined in such a way as to create a quadrupole with variable intensity ratio between the vertical and horizontal
poles. The huge advantage of such a multipole illumination is polarization control and variable intensity in poles. Working on this combination, the resolution can be enhanced even further.
|
