End point of silicon milling using an optical beam induced current signal for controlled access to integrated circuits for backside circuit editing

Integrated devices increasingly use flip chip packaging, which complicates modification of the circuitry using focused ion beam (FIB) systems, as the thickness of the silicon must be reduced to a few micrometers before work can begin. A technique for end pointing the milling of bulk silicon from the backside of flip chip devices, using laser illumination to produce an optical beam induced current (OBIC), is described. System design and operation are described and results shown. The OBIC signal varies strongly with the thickness of the silicon above the bulk to well junction—by monitoring the OBIC signal in real time and end pointing on a characteristic feature the technique can reliably end point the bulk-well interface. It is shown that this system represents a significant advance over previous techniques and provides a robust and reliable way of end pointing. Theory of operation and reasons for the improved performance are discussed.