Off-Axis Illumination of a Photomask Using Holographic Condenser Lens for Visible Light Lithoaraphy

Photolithography has been indispensable part of modern VLSI industry. In photolithography desired fine feature of photo mask is projected on semiconductor chips using costly optical system. To achieve minimum feature size extreme ultraviolet lithography is used. However, cost of such system is very high and require high vacuum environment. The present high cost of extreme ultraviolet lithography has provoked the scientific community to search out alternative methods. Photolithography using visible light and properly designed holographic lenses to achieve desirable resolution at low cost is an attractive proposition for present VLSI industry. However, such holographic lenses are highly wavelength as well as angular selective. Wavelength selectivity means that if recorded holographic lenses are illuminated with radiation of different wavelength other than that of recording wavelength, their diffraction efficiency falls. Diffraction efficiency of such holographic lenses also falls rapidly with deviation in angle of reconstruction. Taking these facts into consideration, holographic lenses have been designed to give appreciable diffraction efficiency over desired wavelength range with large tolerance angle. All the analysis of this work has been performed using MATLAB software. Further such lenses may provide ease of off-axis illumination to improve imaging performance.