Helium Bubbles Formed in Si(001) Layers after High-Dose Implantation and Thermal Annealing

In this paper, we show that high-dose low-energy plasma-immersion ion implantation H+ (E = 2–5 keV) leads to the formation of hidden layers of helium bubbles in an Si(001) substrate. The structural studies of the bubbles’ layers are carried out by using X-ray reflectometry, small-angle scattering, and transmission electron microscopy. Changes in the structure of the layers of the bubbles during thermal annealing are also studied. We have shown that as a result of implanting helium ions with a dose of 5 × 1017 cm–2, a multilayer structure is formed in the near-surface layers of the silicon substrates that generally consists of amorphous, porous amorphous, and porous crystalline sublayers. The structural parameters of the sublayers (density, thickness, and size of the boundaries) and the concentration and size of the bubbles after annealing from 580 and 800°C are determined. It is shown that annealing leads to a bimodal distribution of the bubbles with the average size of 2 to 3 nm and 7 to 8 nm. We establish that the upper amorphous layer is 15 nm thick and can be considered as a protective layer for further processing.