Wafer Fab:  Thin Films                                                             

A thin films, as the name implies, is a layer with a high surface-to-volume ratio.  Thin films are extensively used in wafer fabrication, and can be a resistor, a conductor, an insulator, or even a semiconductor.  Thin films can be deposited on a substrate by thermal growing or by vapor deposition.

Thins films behave differently from bulk materials of the same chemical composition in several ways.  For instance, thin films are sensitive to surface properties while bulk materials generally aren't.  Thin films are also relatively more sensitive to thermomechanical stresses.

The integrity of thin films is influenced by the quality of its adhesion to and conformal coverage of the underlying layer, residual or intrinsic stresses after deposition, and the presence of surface imperfections such as pinholes. 

The adhesion of a thin film to the substrate or underlying layer is of great concern in ensuring the reliability of the thin film.  A thin film that is initially adhering to the underlying layer but lifts off after the device is subjected to thermomechanical stresses can result in field failures.  Reliable thin film adhesion depends greatly on the cleanliness of the surface upon which the film is deposited. Optimum substrate roughness also affects thin film adhesion.  A very smooth substrate decreases adhesion tendency.  A very rough substrate on the other can result in coating defects, which can also lead to thin film adhesion failures.

Regardless of the deposition process, thin films always end up with an intrinsic stress which can either be tensile or compressive.  High residual stresses can lead to adhesion problems, corrosion, cracking, and deviations in electrical properties. Thus, proper deposition is critical to minimize intrinsic stresses in thin films.