Generally, a PVD process includes removing atoms or clusters from a target material that travel through a low-pressure chamber and impinge on a surface. Thermal evaporation is one of the simplest PVD processes, which includes heating of a crucible or boat with the target material to be deposited. When the material evaporates, it travels through the low-pressure chamber, condensing on the substrate surface, ultimately producing a thin film.
Nearly all metal layers in the early semiconductor technologies were deposited by thermal evaporation. Although evaporation is still used in many research labs, the technique is today replaced by sputtering in most silicon technologies for two reasons. The first is the problem of covering steps. As the lateral dimensions of transistors have increased the thickness of the metal layers has remained nearly constant. As a result of this, the topography that the metal must cover has become more severe. Thermal evaporated films have a very poor ability to cover these structures which yields a discontinued film on the vertical walls. Besides this, it is also difficult to produce well-controlled alloys by evaporation. The poor ability to cover steps can be used as an advantage when using thermal evaporation for e.g., lift-off processes.