Abstract

Projection printing is the imaging of a mask pattern onto a wafer by means of a projection system. Because it combines high resolution with low defect density it is the favored optical method. For a given number of smallest features resolvable by the system (typically 10⁸ to 10⁹), smaller features are obtained in the image if the image field is kept small. Today the favored projection printer is the 5× or 10× reduction wafer stepper which exposes typically 20 mm diameter fields using 436 nm (or 405 nm) light and having a numerical aperture, NA ≈ 0.4 The Rayleigh resolution of such a system is R = λ/2NA = 0.6μm and Raleigh depth of focus W = ±λ/2(NA)² = ±1.4 μm. In a production environment the available resolution is somewhat worse. New steppers operating at λ = 365 nm are being introduced for better solution. Under development are steppers using excimer laser light sources operating at λ = 248 nm for even higher resolution.¹ Reducing the wavelength is preferable to increasing the numerical aperture, because the depth of focus is more sensitive to changes in NA, and is already quite small. The limiting resolution of a stepper with a conventional transmissive mask is about 0.3 μm. From figure 1 it can be seen that such high resolution will be needed for chip production before the end of the century.

© 1988 Optical Society of America