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  • The stress of Coating Film
    Dec 08, 2018

    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.The former is 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

    .When the gas phase atoms are injected into the substrate, a large amount of heat is released during the film forming process.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.The latter is due to film forming after burst

    Exposed to the atmosphere or the atmosphere into the coating chamber, the film produced by oxidation.The above three responses Whether the force ACTS in the form of tensile stress or compressive stress, it will occur at the membrane base interface

    Generating shear stress.When the shear stress is large enough to overcome the adhesion between the membrane base interfaces, the membrane will crack.

    Warping or shedding, so as to properly match the film and substrate, reduce the thermal stress of the film, the correct formulation of the film In the process of deposition, The key to strength is reducing the internal stress or making the two stresses compensate each other is also to improve the film adhesion.

     

    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.The choice of deposition rate should consider not only the performance requirements and stress of the film, but also the process

    Requirements.For conductive metal films, the deposition rate can be selected to be larger, such as the film, grain size as small as possible, knot Compact structure, weak oxidation, bright and smooth surface, good electrical conductivity.On the resistance film in order to increase the film sink Proper oxidation of the membrane is necessary for the stability of the product.Therefore, the deposition rate can be slowed downSome.Because most dielectric membranes are oxide or other compound membranes,they break up when oxidized.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.However, the shear force due to the stress at the membrane - base interface

    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.

    image

    Figure1-2-7 The average residual stress during evaporation and sputtering of silver films

    It is also very important to select the incidence Angle of steam, for equipment with small evaporation distance, the incidence Angle

    In order to reduce the incidence Angle, it is better to deposit less substrates

    Is necessary.The Angle of incidence should generally not exceed 15°.Of course, also can pass the reasonable setting of base piece frame.To solve this problem.

    (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

    Control and adjustment, such as the film has a larger tensile stress, can make the additional stress to achieve a greater variety.The mutual compensation of stresses.

     

    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