Depth of focus and resolution enhancement of i-line and deep-UV lithography using annular illumination

Annular illumination has been studied as a method for improving depth of focus (DOF) in microlithographic systems. A 2X increase in DOF for 0.25 micrometers dense line/space features has been demonstrated using a deep-UV exposure tool with annular illumination. The same increase in DOF for 0.35 micrometers dense line/space patterns has been demonstrated using an i- line exposure tool employing annular illumination. No improvement in isolated features has been found. Annular illumination exhibits no degradation in isolated feature DOF, but the critical dimension (CD) split between dense and isolated features is affected when using annular illumination. Prototype i-line and deep-UV annular illumination systems have been built and tested which minimize the reduction in intensity and loss of uniformity control when using annular illumination. We have employed the use of conical optics as a high efficiency method of producing ring-shaped illumination in an i-line illumination system. The deep-UV prototype system uses a pre-uniformizer device to convert the collimated excimer laser light into a flat-top pupil fill which is then centrally obscured to produce annular illumination.