Channel-Length Dependent Performance Degradation of Thermally Stressed IGZO TFTs

The focus of this work is on the performance degradation of thermally stressed IGZO TFTs with SiO2 for both the gate dielectric and back-channel passivation material. The TFT fabrication process included a post-passivation 400 °C O2 anneal to adjust the IGZO semiconductor properties. I-V characteristics of TFTs with bottom-gate (BG) and double-gate (DG) electrode configurations were observed to left-shift and degrade with 200 °C hotplate treatments as shown in Fig. 1. The mechanism causing the instability is not completely understood, however experimental results indicate the instability occurs either directly or indirectly due to the influence of H2O within the passivation oxide above the IGZO channel region. The instability of DG devices was consistently more pronounced, suggesting a reaction between H2O and the aluminum top-gate electrode resulting in the liberation of monatomic hydrogen. This interpretation is supported by complementary experiments on MOS capacitors fabricated on bulk silicon substrates. The application of an alumina capping layer deposited by atomic layer deposition (ALD) was successful in improving thermal stability, with representative results shown in Fig. 2. While most BG capped devices remained stable over several successive thermal treatments, DG devices demonstrated the onset of degradation with noted variation in both the time required and occurrence frequency. The analysis revealed an unanticipated dependence on channel length, with long-channel devices (L = 48 µm) being much more prone to an early onset of degradation compared to shorter devices (L = 12 µm) as shown in Fig. 2. With H2O remaining as the expected origin of thermal instability, a hypothesis has been developed to explain the observed channel length dependence. The results are consistent with the molybdenum source/drain contact regions acting as gettering sites for H2O molecules during the 400 °C annealing process. The removal of H2O from the back-channel region is a two dimensional process limited by the diffusivity of water, with the process more complete in short channel devices. Remaining H2O molecules at or near the back-channel interface in long-channel devices are able to migrate during thermal stress treatments, establishing complete coverage on devices that experience degradation. Experiments which explore the addition of getter features on or adjacent to the back-channel region, as illustrated in Fig. 3, in addition to localized materials analysis techniques such as secondary ion mass spectroscopy (SIMS) will provide additional evidence either in support or denial of the proposed hypothesis. Figure 1

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