can be etched away, leaving a clear path for the illumination light in the stepper/scanner system to travel through. Resolution in projection systems The ability to project a clear image of a small feature onto the wafer is limited by the wavelength of the light that is used, and the ability of the reduction lens system to capture enough diffraction orders from the illuminated mask. Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light from excimer lasers with wavelengths of 248 (KrF) and 193 (ArF) nm (the dominant lithography technology today is thus also called "excimer laser lithography"), which allow minimum feature sizes down to 50 nm. Excimer laser lithography has thus played a critical role in the continued advance of the Moore's Law for the last 20 years (see below). The minimum feature size that a projection system can print is given approximately by: where is the minimum feature size (also called the critical dimension, target design rule, or "half-pitch"). (commonly called k1 factor) is a coefficient that encapsulates process-related factors, and typically equals 0.4 for production. The minimum feature size can be reduced by decreasing this coefficient through computational lithography. is the wavelength of light used is the numerical aperture of the lens as seen from the wafer According to this equation, minimum feature sizes can be decreased by decreasing the wavelength, and increasing the numerical aperture (to achieve a tighter focused beam and a smaller spot size). However, this design method runs into a competing constraint. In modern systems, the depth of focus is also a concern: Here, is another process-related coefficient. The depth of focus restricts the thickness of the photoresist and the depth of the topography on the wafer. Chemical mechanical polishing is often used to flatten topography before high-resolution lithographic steps. From classical optics, k1=0.61 by the Rayleigh criterion. The image of two points separated by less than 1.22 wavelength/NA will not maintain that separation but will be larger due to the interference between the Airy discs of the two points. It must also be remembered, though, that the distance between two features can also change with defocus. Resolution is also nontrivial in a two-dimensional context. For example, a tighter line pitch results in wider gaps (in the perpendicular direction) between the ends of the lines. More fundamentally, straight edges become rounded for shortened rectangular features, where both x and y pitches are near