In photolithography we have seen that photomask contains pattern of circuits that are then imaged on wafers. In these patterns the width of certain features is very critical for larger performance of circuits these feature are called as critical dimensions (CD) that must be measured on mask and later on the wafers , this process is called as CD metrology.
Along with the dimensions of these features it is equally important that all features are at the right locations because the circuits made on wafer consists of many layers which should match to each other. This can be possible if all features are at their right locations as prescribed by their design rules. Measurement of the feature positions is known as image placement (IP) measurement or IP measurement.
Hence metrology in mask making is classified as CD metrology and IP metrology.
The different types of CD metrology used in marks are :
(1) Optical CD metrology
(2) SEM (Scanning Electron Microscope) CD metrology
(3) AFM CD metrology.