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Active monitoring and control of electron-beam-induced contamination

The vacuum systems of all scanning electron microscopes (SEMs), even in the so-called clean instruments, have certain hydrocarbon residues that the vacuum pumps do not effectively remove. The cleanliness of the vacuum and the amount and nature of these residual molecules depends on the type of the pumps and also on the samples moved through the system. Many times, the vacuum readings are quite good but the electron beam still leaves disturbing contamination marks on the sample. This means that in a critical dimension (CD) SEM, repeated measurements cannot be done without extra, sometimes unacceptably high measurement errors resulting from carry-over. During the time necessary for even one measurement, the sample dimension can change, and the extent of this change remains unknown unless a suitable contamination deposition measurement technique is found and regular monitoring is implemented. This paper assesses the problem of contamination of carbonatious materials in the SEM, shows a possible method for its measurement and presents a promising solution to the contamination deposition problem.