Physical Identification of Gate Metal Interdiffusion in GaAs PHEMTs

Abstract

The Ti metal interdiffusion of Ti/Pt/Au gate metal stacks in 0.15-m GaAs PHEMTs subjected to high-temperature accelerated lifetest has been physically identified using scanning transmission electron microscopy. Further energy dispersive analysis with X-ray (EDX) analysis confirms the Ti diffusion into the AlGaAs Schottky barrier layer and the decrease of Schottky barrier height suggests the Ti-AlGaAs intermetallic formation, which is consistent with previous Rutherford backscattering spectroscopy/X-ray photoelectron spectroscopy studies. The Ti metal interdiffusion reduces the separation of the gate metal and InGaAs channel, thus leading to a slight Gm increase, positive shift in pinchoff voltage, and S21 increase during the preliminary portion of the lifetest. Accordingly, the Ti interdiffusion effect implies that the lifetime of GaAs PHEMTs subjected to high-temperature accelerated lifetest could be dependent upon the initial thickness of the Schottky layer underneath the gate metal.Index Terms-Focused-ion-beam (FIB), gate metal sinking, HEMT, interdiffusion, scanning transmission electron microscopy (STEM).

Electrical degradation mechanisms of RF power GaAs PHEMTs

“…In the previous section, although no hydrogen-induced VT shifts were observed, a positive shift in VT is observed under high-temperature storage, which we attributed to gate sinking, a previously identified mechanism [18]. This mechanism could explain the behavior of VT in our off-state stressing experiments performed at high temperatures.…”

“…The effect is a positive shift in the threshold voltage VT, which thus causes a decrease in ID (and therefore a decrease in output power). Due to the smaller distance between the gate metal and the channel, an increase in the peak transconductance gmpeak is sometimes also observed [18]. Since the interdiffusion mechanism typically has an activation energy around 1.4-1.6 eV, its presence usually requires relatively high temperatures [11].…”

Degradation mechanisms of GaAs PHEMTs in high humidity conditions

“…V th shifts after several types of accelerated tests have been investigated, and several mechanisms have been reported: gate metal sinking during high temperature stress [8,9], reversible V th change due to traps under gate via electric field stress [10,11], and V th shift by piezoelectric effect [12] due to stress change near the gate by hydrogen poisoning [13]. Gate sinking or other damage under the gate was not observed in our TEM images.…”

Reliability model for predicting long-term DC/RF performance in GaAs PHEMTs

“…As substantiated by Y. C. Chou et al [9], Ti interdiffusion into the AlGaAs Schottky barrier layer is the primary degradation mechanism of GaAs PHEMTs under elevated temperature lifetest. As a result, gate sinking depth evolves as a diffusion parabolic rate law [10], as in Eq (1) ) exp(…”

“…In lifetest, the devices are stressed at V DS of 5.2V and I DS of 250 mA/mm in air atmosphere at T channel of 300 scanning-transmission-electron-microscopy (STEM) imaging investigation. The details of failure analysis of GaAs PHEMTs using FIB and STEM have been reported elsewhere [9]. Figure 2 shows the representative STEM micrograph of a degraded GaAs PHEMT after elevated temperature lifetest, illustrating the physical evidence of Ti gate metal diffusion into AlGaAs Schottky barrier region.…”