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Tamper Evidence of SEM Imaging Attack in Phase Change Memory Nanodevices

Breach of security due to unauthorized access to electronic hardware devices or chips has recently become a serious concern for the internet-connected daily activities. Imaging with electron microscopy is one of the invasive techniques used to gain knowledge about a chip layout and extract secret information by the attackers. Automatic destruction or disturbance of the secret key during such invasive attacks are required to ensure protection against these attacks. We have characterized the disturbance caused to programmed phase change memory (PCM) cells by the imaging electron beam during scanning electron microscopy (SEM) in terms of the measured cell resistance. A sudden increase of resistance is observed on all imaged amorphous cells while the cells programmed to intermediate states show either abrupt increase or erratic decrease. These erratic disturbances of state are promising to mislead an attacker that is trying to acquire a stored key and leave indelible marks of tampering. Since PCM is recently being considered for implementation of various hardware security primitives, these beam-induced state change and tamper-evidence features enhance security of PCM devices against physical attacks.