Microstructure, growth, resistivity, and stresses in thin tungsten films deposited by rf sputtering

Abstract

The growth process and the microstructure of very thin W films (80–500 Å) deposited by rf sputtering on SiO₂ and Si substrates have been observed by transmission electron microscopy (TEM). The resistivity and stress in these films have been related to the film microstructure, composition, and to the deposition conditions (substrate bias and rf deposition power). Thin W films deposited on silicon dioxide substrates under zero or positive bias have been found to grow in two distinct growth stages. Stage I corresponds to the formation of a thin continuous film (80–100 Å thick) of β‐W. The β‐W phase has the A‐15 crystal structure and has been identified as a faulted W₃W compound. A small grain size (50–100 Å) is characteristic of the β‐W film. Stage II corresponds to the transformation of the β‐W film into a pure α‐W film with the bcc crystal structure. This thermally activated phase transformation takes place in the temperature range 100–200 °C. It is characterized by the growth of α‐W nuclei until complete coalescence of the α‐W islands; the resulting α‐W film consists of large grains (1500–2500 Å) which are free of dislocations. The end of stage II occurs for a critical film thickness t_c beyond which the film is a continuous α‐W film. The value of t_c is controlled by the rf deposition power and the substrate temperature. On the other hand, films deposited on negatively biased substrates do not contain the β‐W phase. These films consist of large α‐W grains (1500–2000 Å) with a high dislocation density. The resistivity of thin W films deposited under zero or positive bias is controlled by the amount of β‐W present in the film. The pure β‐W films have a high resistivity (100–300 μΩ cm); after the complete transformation β‐W→α‐W the large resistivity (30–40 μΩ cm) of these films is attributed to scattering by impurities. In particular, the lower resistivity of W films deposited under negative bias is related to their lower oxygen content. The sign and magnitude of the stress in these films are also controlled by the film microstructure, It is found that the stress in the films containing the β‐W phase is always tensile with a σ of (6–12) × 10⁹ dyn/cm². The films consisting of α‐W are always compressively stressed in the range (2–12) × 10⁹ dyn/cm².