European Society of Anaesthesiology.

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 tc beyond which the film is a continuous α-W film. The value of tc 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.

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In device-manufacturing technology, it is important to understand why dielectric films crack. With this objective in mind, we have constructed an apparatus for measurement of thermal stresses in thin films (25–500 °C), obtained results on various reactively plasma deposited (RPD) Si-N and CVD SiO2 films, and developed a model which quantifies the cracking resistance of different types of RPD Si-N films. Measurements were made of the coefficient of thermal expansion α (T), which increases on going from SiO2→Si3N4→SiN→Si and the intrinsic stress, which is compressive for RPD Si3N4, nearly zero for thermal SiO2 and tensile for RPD SiN and CVD SiO2. The cracking resistance of Si-N film at a given temperature is functionally related to its density, intrinsic stress, thermal mismatch with Si, and the deposition temperature.