Resistance evaporation vacuum coating equipment mainly consists of a vacuum chamber and a vacuum pumping system. The vacuum chamber contains an evaporation source (i.e., an evaporation heater), a substrate and substrate holder, a substrate heater, and an exhaust system. The coating material is placed in the evaporation source within the vacuum chamber. Under high vacuum conditions, the evaporation source heats the material, causing it to evaporate. When the mean free path of the vapor molecules exceeds the linear dimension of the vacuum chamber, the atoms and molecules of the vapor escape from the surface of the evaporation source with minimal collisions or obstruction from other molecules or atoms, allowing them to directly reach the surface of the substrate. Due to the lower temperature of the substrate, the vapor particles condense on it to form a film.
To improve the adhesion between the evaporated molecules and the substrate, the substrate can be activated by appropriate heating or ion cleaning. The physical processes involved in vacuum evaporation coating, from material evaporation and transport to film deposition, are as follows:
(1) Various methods are used to convert other forms of energy into thermal energy, heating the film material to cause evaporation or sublimation, resulting in gaseous particles (atoms, molecules, or atomic groups) with a certain energy (0.1~0.3 eV);
(2) The gaseous particles leave the film surface and are transported to the substrate surface in a relatively straight line with minimal collisions at a considerable speed;
(3) The gaseous particles reaching the substrate surface condense and nucleate, growing into a solid-phase thin film;
(4) The atoms constituting the thin film rearrange or form chemical bonds.
Resistance heating evaporation is the simplest and most commonly used heating method, generally suitable for coating materials with melting points below 1500℃. Typically, a high-melting-point metal (W, Mo, Ti, Ta, boron nitride, etc.) in the form of a wire or sheet is made into an evaporation source of appropriate shape, onto which the coating material is loaded. The Joule heating of the current melts, evaporates, or sublimates the coating material. The shapes of the evaporation source mainly include multi-strand spiral, U-shape, sinusoidal waveform, thin plate, boat shape, and conical basket shape.
Simultaneously, this method requires the evaporation source material to have characteristics such as high melting point, low saturated vapor pressure, stable chemical properties (not reacting chemically with the coating material at high temperatures), good heat resistance, and small power density variation. A large current is used to pass through the evaporation source to heat and evaporate the coating material directly, or the coating material is placed in a crucible made of graphite or certain high-temperature resistant metal oxides (such as Al₂O₂, BO) for indirect heating and evaporation.
Resistance evaporation vacuum coating equipment has limitations in film deposition. Refractory metals have low vapor pressures, making them difficult to form thin films; some elements easily form alloys with the heating wire; and it is difficult to obtain alloy films with uniform composition. However, resistance heating evaporation is a widely used evaporation method due to its simple structure, low cost, and ease of operation.
