Knowledge Center
Knowledge Center

CD-SEM: Critical-Dimension Scanning Electron Microscope

CD-SEM, or critical-dimension scanning electron microscope, is a tool for measuring feature dimensions on a photomask.


A scanning electron microscope, or SEM, takes measurements by sending out an electron beam, which interacts with electrons in the material being scanned. That sends back signals, which are mapped by the equipment. The more critical dimensions that need to be mapped, the greater the amount of data that needs to be processed and stored.

In general, critical-dimension scanning electron microscopes (CD-SEMs) are the workhorse metrology tool in the mask shop.

The CD-SEM is sufficient to measure traditional mask shapes. But the CD-SEM could struggle, or may not work, amid the shift to more complex patterns and shapes on the mask. Most complex mask patterns do not have uniform CDs after optical proximity correction (OPC) or inverse lithography technology (ILT).

Indeed, there are several challenges in mask metrology, particularly with the CD-SEM. For one thing, mask makers must take more measurements than ever before with the CD-SEM. In addition, the number of mask defect issues flagged during mask inspection increases, but not all of these will actually result in yield problems on the wafer.

Additionally, CD-SEM files are measured in tens or hundreds of gigabytes. Just being able to decipher this much data requires a higher level of abstraction. There is no equipment powerful enough to process it within a reasonable time frame, and no way to retrieve that data quickly. In data terminology, it has to be mined just to be useful.

This problem grows worse with finFETs, which has stretched CD-SEM to its limits. About 75% of the inspection on a 3D transistor can be done using conventional CD-SEM, according to IBM. But that still leaves a large amount of information for which there is no solution using an image-based tool. As device geometries continue to shrink, and as more features and functionality are added onto a die, that becomes increasingly troublesome.

This has led to a search for new approaches, such as atomic-force microscopy and some hybrids of different inspection approaches.


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