Secure chips are in permanent risk of attacks. Physical attacks usually start removing part of the package and accessing the dice by different means: laser shots, electrical or electromagnetic probes, etc. Doing this from the backside of the chip gives some advantages since no metal layers interfere between the hacker and the signals of interest. The bulk silicon is thinned from hundreds to some tens of micrometers in order to improve the performance of the attack. In this paper a backside polishing detector is presented that is sensitive to the thickness of the bulk silicon existing below the transistors, a numerical signature is generated which is related to this. The detector implements built-in self-surveillance techniques which protect it from being tampered.