In the ultrathin regime, even atomic roughness at the interface will induce a large local inhomogeneity of the film. It follows that characterization of the dielectric films by methods based on macroscopic area-averaged information or indirect investigation techniques would probably lose important local details. It is highly desirable to carry on local microscopic studies at high temperatures in situ.

In situ investigations

Previously, Kun Xue and colleagues from the Chinese University of Hong Kong (CUHK) have reported that the thermal decomposition of ultrathin SiO2 on silicon is a peripheral reaction and similar to that of thick films. Now the team has examined the thermal decomposition behaviour (especially the thickness dependent behaviour) of ultrathin HfO film on silicon by direct in situ investigation at the nanometre scale.

The team monitored the surface morphology using high-temperature UHV-STM up to ~800 °C. Sequential scan images on the same site (up to 510 × 510 nm2) are used as frames to construct STM time-lapse movies to visualize the decomposition process directly. A total thermal drift of only 40 nm was achieved thanks to a special bias compensation set-up that automatically compensates the voltage drop across the sample during the heating by direct current.

Non-linear relationship

Two distinct decomposition behaviours are identified: instant and gradual reactions, which depend on the film thickness. This finding highlights the non-linear relationship between the decomposition time and film thickness in the ultrathin regime (<1.2 nm).

"This work shows that the formation and desorption of SiO at the interface plays an important role in determining the reaction mode, and suppression of SiO formation and its desorption should be critical to improve the thermal stability of HfO2 dielectric films in the ultrathin regime," explained Dr Kun Xue and Prof. Jian Bin Xu, leader of the research team at CUHK.

The findings are reported in the journal Nanotechnology.