Bending the rules

Chipmakers are turning to water as the way to delay a massively expensive and difficult transition to lithography based on soft X-rays. The overall resolution of the optical system is defined by the wavelength of the illuminating light divided by the numerical aperture (NA) of the lens system; the higher the NA value, the greater the possible resolution. The NA of the illumination optics is defined by the sharpness of the cone of light that is focused by the final lens onto the wafer and the refractive index of the material it passes through on the way. One way to increase the NA and thus the overall resolution is to use a medium between the lens and the wafer with a refractive index that is higher than air's value of 1. Water, with a refractive index of 1.33, pushes up the NA of the lens system and also helps avoid internal reflections in the lens and reflections from the resist. Going from dry lithography to wet lithography will improve minimum resolutions. It also reduces the angle of incidence and therefore increases the depth of focus.