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New process solutions were developed to overcome yield loss due to process margin issue, coming from enhanced Emitter Base leakage in a mixed-signal BiCMOS technology. This paper presents failure analysis in a band gap reference circuit malfunction, in a SiGe BiCMOS Technology currently ramped to production. The associated yield loss was significant. In-circuit fault analysis identified leakage at the Emitter Base junction of the NPN device with SiGe Base, as electrical root cause. Isolation and characterization of the NPN device and high resolution TEM showed tungsten extrusion from the contact plug process as being the cause of enhanced emitter base leakage. Process margin improvements were made to the emitter window module of the NPN device, resulting in yield improvement of the band gap reference circuit.