What are the factors affecting quality of coating film?

[Julie Guo]

Factors Affecting the Stress of Film

Different materials have various coefficients of thermal expansion. The smaller the coating materials’ and platings’ coefficients of thermal expansion are, the better the toughness of metal or alloy will be. Therefore, small stress of the coating film will lead to high quality. The stress of the film decreases with the increase of the vacuum degree. When the partial pressure of argon is less than 0.13Pa, the film usually does not produce stress. If the incident angle of the sputtered particles and the plating elements is less than 15 °, no stress cracks will be generated. However, if the angle is more than 15 °, the stress crack will become larger with the increase of the angle. Plating heating is one of the main causes of stress cracking, so the temperature of the plating should be controlled below the critical temperature at which the crack is generated.

Factors Influencing Film’s Thickness Uniformity

If the sputtering of the magnetron source is not uniform on the target surface, the film thickness will not be uniform. Therefore, during the design of magnetron source, reasonable arrangement of the magnet structure will make the magnetic field strength on both ends stronger than that of the middle. In such a condition, the distribution of film thickness can be improved. Plating parts have different modes of motion depending on the shapes of magnetron source. Planetary motion mechanism is usually the most popular one being used, because it leads to even coating, good metal step coverage deposition and large plating loads. The magnetic source should be put at proper relative positions to the plating so that the surface will receive even sputtering during the movement of the plating.

Factors Influencing Film Reflectivity

Usually, the higher sputtering rate is, the higher film’s reflectivity is. However, the reflectivity of different metal films varies with different sputtering rates. The optimum sputtering rate should be determined by experiment. The smoother the surface of plating is, the greater reflectivity of the film is, and vice versa. Film’s reflectivity decreases with the increase of the film’s thickness. The influences vary between different metals. Reflectivity of the film decreases sharply with the increase of argon pressure. Therefore, the argon partial pressure should be controlled in the range of 0.40 ~ 0.53Pa.