Thermal Evaporation,Electron beam evaporation

Thermal Evaporation:

  Typically, physical vapor deposition (PVD) processes involve removing atoms or clusters from a target material, which travel through a low-pressure chamber and impact a surface. Thermal evaporation is one of the simplest PVD processes and involves heating a crucible or boat containing the target material to be deposited. As the material evaporates, it travels through a low-pressure chamber, condenses on the substrate surface, and ultimately forms a thin film.

  Almost all metal layers in early semiconductor technology were deposited by thermal evaporation. Although evaporation is still used in many research labs, it has been replaced by sputtering in most silicon technology today for two reasons. The first is the problem of the blanketing step. As the lateral size of the transistor increases, the thickness of the metal layer remains nearly constant.As a result, the topography that the metal has to cover becomes more severe. Thermally evaporated films have a very poor ability to cover these structures, which results in discontinuous films on vertical walls. In addition to 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 thermal evaporation is used, for example, in lift-off processes.

Electron beam evaporation:

  Electron beam deposition is a physical vapor deposition process similar to thermal evaporation. The source material is heated above its boiling/sublimation temperature and evaporated to form a thin film on a surface impinged by a stream of atoms in a low pressure environment. This evaporation method shares the core capability of a capping step with thermal evaporation, which also makes this method ideal for lift-off processes.A significant advantage of electron beam evaporation over thermal evaporation is that a large amount of energy can be added to the source material. This produces a higher density film with increased adhesion to the substrate. Because the electron beam heats only the source material and not the entire crucible, contamination from the crucible is lower than in the case of thermal evaporation. By using a multi-crucible electron beam gun, several different materials can be deposited without breaking the vacuum.After the source material is placed in the crucible, a filament below the crucible is heated. By applying a large voltage, electrons are drawn from the filament and focused on the source material in the form of a beam by several bending magnets. The beam is swept across the surface of the source material to heat all of the material.