Alignment System for X-Ray Lithography.

Abstract

Among the various contenders for VLSI lithography below 0.1 $\mu$m, proximity x-ray printing is the most promising one and has been investigated for many years. It will require overlay accuracy far below 100 nm, 3$\sigma$, typical of today's systems. In addition, it should use wafer marks that permit planar resist flow, so their apparent position is not distorted. We have demonstrated an alignment system that uses a linear zone plate on the mask to focus laser light into a line on the wafer. The wafer mark is the boundary between two adjacent fine pitch gratings. These gratings diffract the light into photodiodes. The gratings either differ slightly in pitch, or by 180$\sp\circ$ in phase. This configuration minimizes the disturbance to the resist as it flows over the alignment mark. In the case of gratings with slightly different pitch the light from each grating is detected by one half of a split photodiode. This arrangement has good sensitivity and a very wide capture range. The phase shift gratings have reduced capture range, but enhanced sensitivity, because of the improved resolution from the phase shift. We observe that the FWHM of the alignment signal is 0.6 $\mu$m when the FWHM of the focused laser line is 1.0 $\mu$m. By collecting all of the light focused by the zone plate and diffracted by the gratings we obtain strong alignment signals with signal to noise ratio of more than 1000:1. We routinely obtain a repeatability in a bench setup equivalent to less than 7 A, 3$\sigma$. This alignment technique is currently undergoing installation in the CAMD x-ray exposure tool.