(Karl Süss MA 150ML)

This is a high precision positioning system for masks that are used in photolithography and for microchip fabrication. The mask contains the pattern for the whole wafer and in a single illumination process usually with UV light this pattern is reproduced on the wafer. The wafer is then processed with wet chemistry. Almost any micro scale device or structure requires more than one photomask step. This requires a positioning system of high accuracy. Our mask aligner is capable to position with accuracy of one micrometer.

Our instrument is designed for UV-lithography on 100 mm diameter substrates. The system is equipped with a 350 W Hg lamp and features both front- and backside alignment in proximity and contact printing.

Contact printing refers to the light exposure process wherein the photomask is pressed against the resist-covered wafer. The contact pressure is adjustable and light with a wavelength of about 400 nm is used.

Contact printing is capable of attaining resolutions of less than 1 micron. However, the presence of contact between the mask and the resist somewhat diminishes the uniformity of attainable resolution across the wafer. To alleviate this problem, masks used in contact printing must be thin and flexible to allow better contact over the whole wafer.

Proximity printing is another optical lithography technique. As its name implies, it involves no contact between the mask and the wafer, which is why masks used with this technique have longer useful lives than those used in contact printing. During proximity printing, the mask is usually only 20-50 microns away from the wafer.

The resolution achieved by proximity printing is not as good as that of contact printing. This is due to the diffraction of light caused by its passing through slits that make up the pattern in the mask, and traversal across the gap between the mask and the wafer.

This type of diffraction is known as Fresnel diffraction, or near-field diffraction, since it results from a small gap between the mask and the wafer. Proximity printing resolution may be improved by diminishing the gap between the mask and the wafer and by using light of shorter wavelengths.

(from www.siliconfareast.com)