In manufacturing process of three-dimensional Si devices, such as MEMS, trench-gate MOSFET and FinFET, both sidewall surface flattening and corner rounding of the three-dimensional structures have been important issues in order to increase the performance and secure the reliability of the devices. We have been studying the sidewall surface flattening and the corner rounding mechanism of micron-size silicon trenches during high-temperature hydrogen annealing, especially in relation to the effect of hydrogen pressure on the trench corner curvature, demonstrating that the rate of corner rounding decreases with increasing hydrogen pressure. We have assumed that the adsorbed hydrogen suppresses the self-diffusion of Si adatoms during hydrogen annealing.
In this study we experimentally observed the atomic-level morphology of Si surface in the early phase of hydrogen annealing, in order to investigate the effect of the adsorbed hydrogen on the surface self-diffusion of Si adatoms. We performed both AFM on trench sidewall surface(011) and UHV-STM on Si(001) substrate surface.
The AFM image of the sidewall surface processed in 1.0x105 Pa hydrogen at 1000°C clearly shows the evanescence of chemical Si dioxide formed by RCA cleaning process, and in the area without chemical oxide the appearance of atomic steps was observed. To investigate the atomic-level configuration of Si(001) surface processed in 2.5x104 Pa hydrogen at 1000°C, the surface was observed by means of UHV-STM with a hydrogen annealing chamber. The 2x1 dimer configuration was observed, which is considered to be a mono-hydride dimer.
From these observations, in the early phase of hydrogen annealing it is expected that Si dioxide reacts with an active Si in the reductive atmosphere of H2 like as SiO2+Si→SiO by thermal energy at the interface between the surface of Si substrate and chemical Si dioxide, and the exposed Si surfaces reconstruct to the 2x1 structure, being hydrogen terminated and stabilized. The suppression of the shape transformation rate by the increase of hydrogen pressure can be concluded to be attributed to the decrease of the self-diffusion constant of Si adatoms by hydrogenation in hydrogen ambient.
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