Influence of axial chromatic aberration in projection printing

The computer simulation program SAMPLE is used to study the effects of longitudinal chromatic aberration on the resolution of projection mask aligners for microelectronics. It is shown that source spectral characteristics, filter bandwidth, and lens parameters all play a significant role in limiting image contrast in both single- and two-wavelength systems. Techniques for estimating the loss of image contrast as a function of lens and illumination parameters as well as examples including resist sensitivity and filter characteristics are given. For single-wavelength systems, appropriate filtering can make the effect of dispersion negligible usually at a slight cost of intensity available for exposure. However, in the case of two-wavelength systems, chromatic aberration can lead to a significant reduction in contrast below the diffraction-limited value. The loss in contrast can, however, be minimized by designing the lens for minimum chromatic slope at the two operating lines and by choosing a source with minimum spectral line bandwidth and minimum continuum between the two spectral lines or by using notch filtering.