Deep-UV Excimer Lasers

The recent trend in microelectronics towards patterning critical feature sizes of 0.25 m and below has motivated the development of microlithography at the deep ultra-violet (DUV) laser wavelengths of 248 and 1 93 nm. In recent years the performance, reliability, and cost of ownership of excimer light sources have improved. Some key technologies needed for excimer lasers in microlithography include materials issues, gas lifetime, higher repetition rates and improved pulse-to-pulse energy repeatibility. As dimensions of circuit elements shrink, new wavefront engineering technologies such as phase shifting techniques, off-axis illumination, and other modifications to extend the lifetime of optical lithography are required [1-5]. Simultaneous improvement of the resolvable linewidth (CD) and the depth of focus (DOF) is an important issue. Both CD and DOF are limited by the familiar scaling laws CD = k1X/NA and DOF = k2\/NA2, where ) is the wavelength and NA the numerical aperture of the projection lens. The parameter k1 depends upon the imaging technology and process control, while k2 has been within range of 1-2 for many years. The key innovations required for microlithography are those that reduce k1. The 0.35 im critical feature sizes required for the 64 Mb DRAM can be achieved with wavefront enhancement technology developed for i—line. Similarly, such techniques can be added to 248 nm DUV stepper systems to achieve the necessary resolution for 0.25 m features needed for the production of the 256 Mb DRAM chip slated for initial production in 1998. The practical limit of optical lithography appears to be 0.12 m required for the 4 Gb DRAM generation. In 1987 Durnin [6] showed that the field described by E(r, z, t) = A Jo(k1r) . ei(kiiz_t) is an exact solution of the wave equation where k + = w2/c2, and Jo is the zero-order Bessel function of the first kind. This field represents a nondiffracting beam, because the transverse intensity distribution is independent of the propagation distance z. However, such an ideal beam cannot be realized experimentally over large values of z and r, because the