Optical lithography is a key technology to improving resolution.

THE SEMICONDUCTOR INDUSTRY has doubled the number of transistors designed on a chip every two to three years (Moore’s Law) primarily by shrinking the circuit linewidths (improving resolution). Optical lithography proves to be a key technology enabling this achievement. To print the finer lines, the illumination optics have relied on the smaller and smaller wavelength light source, which currently is 193 nm ArF Excimer laser targeted to the 65 nm features (linewidth). Figure 1 shows the chronology of linewidth and wavelength of lightsource over the past twenty years.

The industry is gearing up to switch to 193 nm immersion lithography from 193 nm dry lithography by using ultra-pure water (UPW) in order to extend the utility of 193 nm illumination beyond 65 nm. UPW immersion lithography is an emerging, enabling technology for extending the utility of 193 nm ArF excimerillumination beyond 65 nm technology node to 45 nm. [1,2]

Ultimate LW (resolution) in optical lithography is given by:

To increase n and numerical aperture (NA) for finer resolution, immersion lithography employs a higher refractive index liquid (e.g. UPW water, n = 1.44) that is placed between the final lens and the wafer (replacing the lower index air, n = 1). The liquid’s refractive properties increase NA creating higher resolution images and increased depth-of-focus (of up to 50%) for printing the finer circuit lines onto wafers than a “dry” lens system willallow (Figure 2). [1,2]

Immersion lithography must successfully address critical contamination control challenges. Control of defects in immersion environment and preventing haze formation in exposure lens are critical for high yields. No particles should be added to the wafer during the lithography process. The fluid must be free from bubbles that may be caused by the scanning process, exposure, or the fluid delivery, recovery, and recirculation system. Dissolved materials or particles may cause staining if fluid remains on the wafer after exposure.