OBIC Endpointing Method for Laser Thinning of Flip-Chip Circuits

Laser microchemical etching systems provide enhanced through-wafer IR viewing and provide access for focused ion beam (FIB) tools and e-beam testers on flip-chip packaged die [1]. In demanding applications, laser etching is directed at rates of 100,000 cubic micrometers per second and must be stopped within 10 to 15 micrometers (thickness remaining) of the active flip-chip circuit. In cases where the initial die thickness is known, the laser process is sufficiently reproducible and system depth of focus is sufficiently narrow to place the laseretched floor within an accuracy of about plus or minus 5 micrometers relative to the initial surface of the die. However, greater accuracy is often desired to minimize FIB etch time. In addition, the laser step is often proceeded by a mechanical thinning operation on the die. This mechanical process introduces an uncertainty in initial part thickness, as well as part wedge and bowing. In this paper we describe an optical beam induced current (OBIC) method for accurate closed-loop endpointing with direct reference to the active device surface on the flipped die. The method relies on an exponentially increasing current that is induced by the laser as the device is thinned. Because of the strong absorption of the silicon bulk at visible wavelengths, the signal is sensitive to submicrometer thickness changes and, hence, may be used to stop the laser etching process with high accuracy at the desired 10 to 15 micrometer distance from the active circuit. The new technique has been studied on commercially available devices and shown to be insensitive to localized device junction density. Hence, endpointing is not highly dependent on the circuit design or exact placement of circuit elements. We outline the substrate and circuit properties that are most relevant to accurate implementation of the technique. The laser-etch process dependency of the OBIC signal has also been characterized. Simple high-speed closed loop electronics have been developed in order to apply the method for in situ endpointing New failure analysis/circuit debug techniques, including spectroscopic photoemission and picosecond time-resolved methods rely on observation of weak optical signals through the wafer. These would optimally be viewed though a remaining silicon thickness of a few micrometers or less. The limits of the OBIC endpointing method have been explored for the high-speed preparation of ultra thin local viewing windows in support of these new techniques.