Hydrogen Absorption Method Using HfOx in Crystalline In-Ga-Zn Oxide FETs for NVM Applications

We fabricated and evaluated an oxide semiconductor field effect transistor (OSFET) with a channel of c-axis aligned crystalline In-Ga-Zn oxide (CAAC-IGZO) in order to examine the availability of the OSFET in nonvolatile memories (NVM). A featured extremely low leakage current of the OSFET largely depends on the threshold voltage, and thus controlling the threshold is a key issue. In particular, reducing the hydrogen concentration in and around the CAAC-IGZO layer as much as possible is one of the most important factors leading to threshold controllability and stability improvement in the OSFET. Accordingly, we employed a structure in which the whole OSFET is sealed with a hydrogen barrier film (SiNx) to prevent hydrogen entry from the outside and provided a modified HfOx film that we found serves as a hydrogen absorption layer inside the encapsulation structure. The HfOx film having a high hydrogen absorption capability inside the encapsulation structure resulted in a significant improvement in OSFET reliability. Specifically, the prototype OSFET with a gate length of 43.9 nm had a suppressed threshold variation for 500 hours in the positive gate-bias temperature (+GBT) stress test (150°C, Vgs = 3.63 V, Vds = Vbgs = 0 V). This process enables the control of the hydrogen concentration in the CAAC-IGZO layer and increases the expectation for OSFET mass production.