Degradation of Au–Ti contacts of SiGe HBTs during electromagnetic field stress

Abstract

This paper addresses electromagnetic field stress effects on SiGe heterojunction bipolar transistors (HBTs)' reliability issues, focusing on the relationship between the stress-induced current and device structure degradations. The origin of leakage currents and electrical parameter shifts in failed transistors has been studied by complementary failure analysis techniques. Characterization of the structure before and after ageing was performed by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). For the stressed samples, interface deformations of the titanium (Ti) thin film around all gold (Au) contacts have been clearly detected. These degradations include localized interface reaction between Au and Ti layers as well as their lateral atomic migration causing a significant reduction of Ti thickness. EDS analysis of the disordered region which is near the Si 3 N 4 interface has shown significant signals from Au. These observations could be attributed to the coupling between high current densities induced by stress and thermal effects due to local heating effects.

STEM nanoanalysis of Au/Pt/Ti-Si3N4 interfacial defects and reactions during local stress of SiGe HBTs

“…We consider that a unit cell is composed of collector, emitter, and base with its intrinsic 40-nm SiGe layer under the n-doped polysilicon emitter and extrinsic layer of p-doped polysilicon under the base contact [ 7 ]. All the device fingers have been analyzed before and after stress, but the emitter results will be essentially presented in the following sections (Figure 1 ).…”

“…In our previous study [ 7 ], TEM observations and EDS analyses have used conventional JEOL 2000FXII equipment (JEOL Ltd.). A first approach has been then given on the structural and elemental characteristics of the device and its degradation.…”

“…The stress has been applied on a minimum set of five devices in order to minimize the technological dispersion effects. As discussed in our previous work [ 7 ], the electromagnetic coupling phenomenon between the induced field and the micro-strip line connecting the base of the HBT is responsible for the performance degradations after stress. To understand aging impact coming from the external electromagnetic disturbance, the evolution of the current induced by coupling phenomenon in front of HBT has been studied.…”

“…To understand aging impact coming from the external electromagnetic disturbance, the evolution of the current induced by coupling phenomenon in front of HBT has been studied. We found that the value of the induced current in front of the base reaches 30 mA, whereas the base breakdown current is around 200 μA [ 7 ]. In fact, temperature rise of component metal layers due to increasing current densities with associated self-heating effects can strongly affect metal connection reliability [ 8 ].…”

Modeling of the coupling phenomenon between a transmission line and a near-field excitation

“…The deviation of this frequency at low current is consistent with the increase of both capacitances after stress. Noting that, an eventual degradation of the collector and emitter series resistance can be inferred [10]. …”