How is defectivity on the photomask specified?
Defects are any instance where the completed mask does not match the data that was used to write the mask. The most common cause of defects are any impurities in the materials used to make the mask blank, or the effects of the chemical processing of the mask, as it is developed and etched.
There are several different types of defects, however for all but the most advanced masks, customers usually have a single specification that is applied to all types of defects. There are three common methods of specifying a defect specification:
1) No defects on the mask greater than a specified size.
e.g. 0 defects greater than 1 μm
This type of defect specification is usually used for masks that are reticles or contain a small number of die, where no defects can occur on the mask. This is the most demanding specification and is the most expensive specification to apply.
2) An acceptable density of allowable defects at a certain size.
e.g. 1 defect per square inch at 2.0 μm
This type of specification is usually used for masks where the image is an array of identical chips, where defects affecting a small number of the chips are not significant for the wafer yield. This type of defect specification can also be combined with the previous one,
e.g. 1 defect per square inch at 2.0 μm and 0 defects > 10 μm
3) No automatic or microscope inspection.
e.g. Strip and Ship.
Where defectivity is non-critical, then the mask can be given a visual inspection under a bright monochromatic light. The mask will be shipped to the customer if there are no defects large enough to be spotted by the human eye.
In this case there are no automatic inspection or microscopes used to check the defectivity and, as a result, such masks are cheaper.
The Strip and Ship option provides a cost effective method of prototyping where a high yield is of less importance.