High-resolution transmission electron microscopy on aged InP HBTs

Abstract We have used high-resolution transmission electron microscopy (HRTEM), with focused-ion-beam preparation of foils, to study InP HBT devices, both as-fabricated and after aging in life tests. The technology is HRL's G1 process. We found that even after aging for the equivalent of 2 × 107 h under normal operating conditions, the visible damage is extremely benign: the base and emitter contacts are completely intact, there is no evidence of significant Au or Pt migration into the semiconductor, and minor crystal disorder that develops does not extend more than 100 nm below the metal–semiconductor interfaces. Except for these regions, the intrinsic devices are completely devoid of any anomalous features, to within the 0.3 nm resolution limit of the measurements. These observations provide strong evidence that failure mechanisms involving migration of metal from the ohmic contacts, or extended crystal defects, do not limit the reliability of this technology. The small disorder under the base contacts can probably explain the increases of gain (β) observed early in our life tests, and small increases in the base-collector leakage occurring later in the life tests. The minor disorder under the emitter contacts can probably explain the moderate increase of emitter resistance observed late in our life tests. But the mechanism for the eventual decline in β, observed after a long period of stress, for the equivalent of 107 h of normal operation, is not apparent from the HRTEM images.