The stress of Coating Film
I. classification of film stress and causes of stress
The internal stress can be divided into deposition internal stress and additional internal stress. In the process of film forming, the structural defects and thermal effects formed in the film when the crystal nuclei merge with each other are induced
..This amount of heat is equivalent to a quenching of the substrate, resulting in the generation of stress, and at the same time
Since the steam forms the nucleus at the initial stage of substrate deposition, the surface tension of the grains makes the adjacent grains coalesce and form larger Grain, this coalescence, will make its surface energy decreases, surface area decreases, grain shrinkage and substrate Will prevent it from coalescing and shrinking, thus causing the film to produce condensation internal stress.
Exposed to the atmosphere or the atmosphere into the coating chamber, the film produced by oxidation. Whether the force ACTS in the form of tensile stress or compressive stress, it will occur at the membrane base interface
Generating shear stress.
2. the method of obtaining low stress film
In order to obtain films with low stress, the following measures can be taken in membrane preparation:
(1)Select the Correct substrate temperature to reduce thermal stress
Deposition from reducing thermal stress should be chosen when the substrate temperature, but also should choose from decreasing internal stress higher due to the low melting point metal film structure is neat small internal stress, thermal stress plays a main role at this moment, such as the preparation of the superconducting films such as indium tin lead, substrate at liquid helium temperature thermal stress can be zero, so the low melting point metal, should choose a lower substrate temperature on other kinds of metal, substrate temperature should choose some taller, so as to achieve the purpose of reduce internal stress
In addition, reasonable selection of membrane and substrate, so that the thermal expansion coefficient of the two materials is close to the film is also a way to reduce the thermal stress
(2) Correct selection of residual gas pressure
Film deposition, the residual gas pressure is too high, the collision probability between steam and residual gas molecules will increase, which not only affects the deposition rate, but also caused by collision scattering phenomenon produced by membrane structure random arrangement and make the porous membrane layer, easy to oxidation of the membrane, even within the membrane layer to generate bubbles so coating indoor residual gas pressure is unfavorable and exorbitant, choose in 10-3-10-4Pa is appropriate
(3) Choice of deposition rate
The deposition rate depends on the temperature, shape, size, distance and evaporation capacity of evaporation source Element.
Requirements. Compact structure, weak oxidation, bright and smooth surface, good electrical conductivity. Proper oxidation of the membrane is necessary for the stability of the product. Some. Or with heater produce chemical reaction, both are concerned with evaporative source temperature, and the heat conduction of dielectric film is poor, suffer when evaporative heat is uneven, should use slower deposition rate accordingly.
(4) Choice of film thickness and steam incidence Angle
The relationship between the thickness of the film and the average residual stress is shown in figure1-2-7. The thickness of the film exceeds 100nm When the stress does not change.
It is directly proportional to the film thickness, so the shear force may be greater than the adhesion when the film thickness is too large, resulting is loss the film.
It is also very important to select the incidence Angle of steam, for equipment with small evaporation distance, the incidence Angle
(5) Appropriate control and elimination of additional internal stresses
The additional internal stress is mostly compressive stress, which may be appropriate according to the stress properties of the film produced during deposition
In addition, after film deposition, appropriate thermal insulation can be carried out in the vacuum room, so as to stabilize the internal structure of the film and form a very thin purified film on the surface. It is also important to keep the newly deposited film as little or no exposure to the atmosphere as possible, especially when the substrate temperature is much higher than room temperature