Photomask

From ArticleWorld

A photomask is a transparent quartz blank fitted with a chrome metal that is provided as an absorbing film during semiconductor device fabrication using photolithography. These are used at wavelengths of 365nm, 248nm and 193nm. Other forms of radiation can also be used in such materials. Typical among these include 157nm, 13.5nm, electron and ion beams.

A photomask essentially consists of a glass plate which is selectively patterned with an opaque material like chrome. The mask is used to transfer a master image to a silicon wafer’s substrate. For the purpose of defining a pattern layer during integrated circuit fabrication a set of photomasks may be sent into a photolithography stepper and brought out one by one for exposure.

Large-scale production

During the large scale production of integrated circuit devices, reduction optics is invariably used, with the photomask often referred to as a ‘reticle’ or a ‘photoreticle’. The reticle has just a single layer of the chip to be processed. Until complete exposure is achieved, the wafer to be produced is moved between different levels in the optical column.

Enhancements of wafers

There may be an attenuated phase-shifting background film on the photomask to enhance the contrast of small intensity peaks. This technique is referred to as ‘attenuated phase-shifting.’ Alternatively, the quartz which is exposed is etched so that the edge between etched and unetched regions can be used to image nearly zero intensity. This technique is called ‘alternating-aperture phase-shifting’. The second method is considered stronger.

A cause for concern in recent times has been the drastic shrinking of leading-edge features of semiconductors. This has lead to the reduction in opacity in absorber film, resulting in poorer image contrast. It has been suggested to completely do away with absorbers and bring to use ‘chromeless’ photomasks by using the technique of phase-shifting alone.