Optical lithography has enabled semiconductor manufacturers to increase chip capacity at astounding rates by shrinking circuit line widths. Printing of these finer lines has been achieved using a progressively smaller wavelength light source, which currently isa 193-nm laser targeted to the 65-nm features. To extend the 193-nm illumination to 45-nm and perhaps beyond, the industry will switch to immersion lithography using ultra-pure water (UPW) which is placed between the final lens and the wafer. The availability of 13.5-nm wavelength EUV illumination to print even smaller lines is anticipated by 2012 (see Figure 1).

The higher refractive index UPW, index n = 1.44, replaces the lower index air,index n = 1. The UPW’s higher refractive index creates higher resolutionimages than a “dry” lens system will allow.
Ultimate LW (resolution) in optical lithography is given by:

where is illumination wavelength (193-nm currently), k1 is an optics/process parameter, and NA is the numerical aperture given by NA = n sin ,with n being the fluid refractive index.

The semiconductor industry is actively pursuing various litho options starting with 193-nm immersion using UPW and higher index liquids, double patterning lithography, and 13.5-nm wavelength EUV.^1,2

In UPW immersion lithography, control of defects in the immersion environment and prevention of haze formation on the exposure lens are critical to achieve high yields. Also, the UPW water must be at the highest level of clarity (low light absorbance) and purity (ppt levels of contaminants) to ensure transmission of the imaging radiation through the water. It is evident from the stringent water quality guidelines, shown in Table 1, that all impurities in water must be removed to trace levels at the point-of-use to achieve the high process effectiveness. The DI water temperature and, thus, refractive index, must be strictly controlled to eliminate patterning defects. The water flow rate and pressure must be controlled for a stable process.^3,7 In this article, we describe the effects of various water contaminants and attributes on the immersion process and how to remove these contaminants from DI by purification and conditioning techniques to prevent patterning defects. Also discussed are aspects of system flow and pressure controls that are critical to a seamless interface with the scanners.