Investigation of Thermal Stress Degradation in Indium-Gallium-Zinc-Oxide TFTs

The performance of IGZO TFTs has improved significantly in recent years, however device stability still remains a significant issue. Thermal stability of IGZO TFTs be- comes very crucial to ensure desired performance of end-product. Both bottom-gate (BG) and double-gate (DG) TFTs were observed to degrade with hotplate treatments under 200◦C. Such events are rarely reported in the literature, and thus became the primary focus of this work. The mechanism causing the instability is not completely understood, however experimental results indicate the instability occurs either di- rectly or indirectly due to the influence of H2O within the passivation oxide above the IGZO channel region. DG TFTs saw more pronounced degradation, which led to the hypothesis that there may be a reaction of the top gate metal with H2O molecules in the passivation oxide, liberating monatomic hydrogen. Both H2O and hydrogen behave as donor states in IGZO, thus rendering the channel more conduc- tive. The thermal stability also demonstrated a dependence on channel length, with shorter channel devices showing greater stability. This may be due to the metalized source/drain regions acting as effective getter to water during a 400◦C passivation anneal which is performed prior to top-gate metal deposition. This hypothesis led to an investigation on atomic layer deposition (ALD) of capping layers over the passiva- tion oxide of IGZO TFTs to act as an effective barrier to water/hydrogen migrating to the underlying IGZO channel