The use of Cu to link transistors in chips brings commercial benefits but its high reactivity to silicon creates technological difficulties. Ta and Ta-based compounds are proven to be effective barrier layers against Cu penetration. However, very little is known about the initial stages of Ta/Si interface formation. Early studies showed the formation of disordered phase for Ta coverage below 5 ML. Annealing above 650°C leads to interdiffusion and surface atomic ordering, depending on the thickness.
We studied the initial growth process of Ta on the Si(111)/(7x7) surface at a coverage ≤ 1ML. Ta layers were deposited at room temperature (RT) and annealed at T ≤ 600°C, or were directly deposited on hot surfaces (100°C-400°C). STM images acquired with OMICRON VT STM, showed a disordered surface at RT. Annealing at 80 °C results in the formation of clusters. They have a butterfly-like shape and occupy a half unit cell of the 7x7 phase. They are randomly distributed on a surface and prefer faulted halves with the ratio 2:1. A single cluster consists of four well-resolved protrusions. Two of them cross the border of a unit cell extending to two neighbour halves. Deposition on a hot surface (80-100 °C) results in a layer exhibiting three types of clusters. The first is identical to the previous butterfly-like cluster, the second one fully occupies a half of the 7x7 unit cell as well, but does not exceed its border. The centre of the third type of cluster is located at a corner hole of the 7x7 unit cell. Furthermore, it is surrounded by six protrusions, two of which are situated in one of the three bordering unit cells. All clusters are mixed randomly on a surface where, the 7x7 reconstruction is still resolved. Depositing on the surface of 400 °C, has a dramatic effect on the interface. The first two types of Ta-Si cluster disappear and the size of the third one increases substantially exhibiting an irregular shape. The quality of the 7x7 reconstruction between clusters was improved, no significant defects were observed. However, domains on terraces were identified with miscellaneous ordering of atoms in the borders. Cluster atomic arrangements are discussed based on DFT calculations.
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