From an earlier post in another thread, the max SEM voltage needs to scale down with device size. Otherwise the device will be damaged. The paper I cited there is as follows:

http://ieeexplore.ieee.org/xpl/freeabs_all...snumber=4227592

Maximum Permissible EB Acceleration Voltage for SEM-Based Inspection Before Electrical Characterization of Advanced MOS
Mizuno, T. Takahashi, M. Azuma, Y. Yanagita, H. Asayama, K. Nakamae, K.
Renesas Technol. Corp., Tokyo;

Reliability physics symposium, 2007. proceedings. 45th annual. ieee international
15-19 April 2007
p. 618-619