The projection systems, commonly known as ‘steppers’, exhibit several advantages as a higher resolution and prevention of photo-mask damage. In these systems, the image of the patterns in the masks is projected onto the photoresist-covered substrate at a distance of several centimeters. To achieve a high resolution, only a small portion of the mask is transferred. In the case of the projection “by steps”, the photo-mask only contains one chip or group of small chips that will be repeated across the whole wafer, and whose size has been increased by a factor 5 or 10. This increased image of the pattern repetition unit is then reduced back to its real size and projected to the surface by a highly precise optical system. After this projection, the substrate is moved in a prefixed step and the operation is repeated. Thanks to this ‘step-and-repeat’ systems, resolutions lower than 1 micron are easily achievable.

Usually the source of light that is used for the photoresist exposition is UV light produced by a Hg lamp, in the wavelength region of 366 (i-line) and 436 nm (g-line). Several variables must be regularly controlled as are the light collimation and diffraction, and its intensity variation with time, the exposition spectrum, the mask reflection, the uniformity of light absorption in the photo-resist…