High Resolution Auger Imaging Combined with Focused Ion Beam for the Investigation of Metal/GaAs Contacts in High Power Transistors

ABSTRACTInspecting and understanding the thermal stability of metal contacts in the context of reliability testing on high power transistors (especially in the GaAs technology) often necessitates to view atomic concentration profiles at deep interfaces. One common method for this purpose is the combined Auger spectroscopy/ion etching technique, where the region to be investigated is etched sequentially (with a “low” etch rate, typically in the Å/second regime) and Auger electron spectra (AES) are recorded at each step of the etching process. The 3 major resolution drawbacks of this approach are first that it is time-consuming, second that the spatial resolution is limited (atomic profiles on contact regions with lateral dimensions smaller that I R~m are difficult to obtain) and third that the interface of interest is “smoothed” during the etch process. The third point yields atomic concentration profiles with an apparent inter-mixing, which sometimes hinders observation of the details of interest for the interface under investigation.We have used a different approach for this aspect of metal contact testing, in which the interface to be studied is first “revealed” by focused ion beam (FIB) micro-sectioning. After this preparation step, chemical images of the interface are directly obtained by high resolution Auger electron spectra mapping. The different operating conditions for the FIB process (orientation of the cross-section with respect to the transistor surface, preliminary procedures to eliminate residual roughness as well as surface contamination) have been optimized in order to produce Auger spectra free of any artifact. The approach is demonstrated on Au/GeNiAu ohmics contacts to the emitter electrode of GaAs-based hererostructure bipolar transistor, designed for high power amplification in the microwave regime. Ultimate spatial resolution of 20 to 30 nm on the Ayuger chemical images is demonstrated on an Auger microscope equipped with a Schottky field emitting tip.