Abstract

The drive toward high density circuitry in microelectronic devices has increase interest in a variety of high resolution lithographic techniques that produce finer resolution patterns at high production rates. The resolution of a lithographic lens system is a function of the wavelength of the exposure light and the numerical aperture of the projection lens system. The projection lens system is used to transfer or image the integrated circuit pattern from a reticle or mask onto a wafer or semiconductor substrate upon which the semiconductor device is to be formed. Because of the difference of the relative sizes of the reticle and the resulting semiconductor device, a reduction projection optical system, typically at 5x or 4x, is used. The resolution (R) or minimum resolvable feature size is directly proportional to the wavelength (λ ) and inversely proportional to the numerical aperture (NA) of the microlithographic lens. Thus, where k₁ is a proportionality constant. Equation (1) implies that the better resolution can be achieved if the NA of the projection lens is increased and or by reducing the wavelength of exposure light. However, there are tradeoff that must be considered when the numerical aperture of the projection lens is increased. One such tradeoff is that the depth of focus (DOF) of the projection lens decreases with an increase in numerical aperture, since where k₂ is another proportionality constant. The optimization of the optical parameters such as the NA and DOF, together with the illumination conditions and properties of the photoresist, is performed by simulation and is beyond the scope of this paper.

© 1998 Optical Society of America