Optimization of Source Distribution for half-wavelength DOE

Inhalt:
A lot of source shapes have been proposed for resolution enhancement in semiconductor exposure field, and so-called OAI (OAI: Off Axis Illumination) in them improves not only the resolution but also defocus behavior. Such kind of small window illumination was realized aperture filter at first stage but there were issues of efficiency of light source and complex OPC due to higher coherency. The advantages of use DOE (DOE : Diffractive Optical Element) are not only the flexible illumination shape available but also the controllability of intensity profile in addition to the higher efficiency of light source. However DOE design and fabrication to obtain enough resolution are difficult due to the huge design load and leading-edge fabrication. Not enough design and fabrication error lead unintended intensity distribution, and the distribution degrades the resolution and makes OPC less effective so that photomask specification shall be tighter. In this paper, as one example, dipole source shape named “Soft-Dipole” is optimized considering intensity distribution targeting 90nm with simulator and is estimated the impact to resolution and OPC. Then actual DOE is fabricated for the intended distribution and evaluated the behavior with the simulator of the DOE using captured intensity distribution. The result showed Soft-Dipole illumination had possibilities to reduce OPC load with enough resolution. Then the DOE design, the fabrication and the evaluation are discussed in this paper.