Manufactured in what’s called a mask shop, the photomask is a template or master copy of what will be printed on the final wafer. Generally, an optical mask consists of an opaque layer of chrome on a glass substrate.
In the mask production flow, the first step is to obtain the data from a given IC design. Then, the data is fed into an e-beam mask-writing tool, which converts it into a pattern on the mask.
To deal with diffraction issues in optical, mask makers must use various reticle enhancement techniques (RETs) on the mask. One RET, called optical proximity correction (OPC), is used to modify the mask patterns to improve the printability on the wafer. OPC makes use of tiny sub-resolution assist features (SRAFs) on the mask.
After the mask is patterned, the mask goes the following steps in order—etch, mask repair, inspection and metrology. Inspection and metrology are the key parts of the flow. If there is a defect or a problem on the mask, it will get printed on the final wafer.
In fact, the total time for inspection and metrology for a mask could run some 18 hours today, which is roughly double the amount of time as compared to several years ago.
Generally, a leading-edge mask is inspected for defects at least three times during the flow. Then, in the metrology flow, the global and systematic error sources are addressed. Then, focus shifts to the local and/or hot spot errors on the mask. All told, a large amount of measurement data is required to sort out the problems.