The temperature of a substrate, while the covering is drawing on it’s surface, страница 4

Fig. 2.1 The scheme of magnetron sputtering system with a flat cathode - target: 1 – the cathode - target; 2 - magnets; 3 – the power supply; 4 – the anode; 5 – the trajectory of electron movement; 6 – the zone of dispersion; 7 – the field line of magnetic field.

At submission of a constant voltage between the cathode - target (negative potential) and the anode (positive or small negative potential) the nonuniform electric field and the abnormal glow discharge is initiated. Presence of the closed field lines of magnetic field at the sprayed surface of the target allows to locate plasma of the discharge directly at the target. Electrons, emitted from the cathode under action of ionic bombardment are grasped by the magnetic field, the complex cycloidal movement on the closed trajectories at the target surface is imparted to them. Electrons appear in a trap created on the one hand by the magnetic field, returning electrons on the cathode, and on the other hand - the target surface, repellent electrons.

Fig. 2.2 The scheme of magnetron sputtering system:

1 - the cathode - target; 2 - magnets; 3 - the anode; 4 – the area of electron drift; 5 - the sprayed atoms; 6 - the substrate

Presence in the cathode area of primary power electrons emitted by the cathode which, moving along the field, collide with atoms of gas, results in its ionization and the subsequent bombardment of the cathode - target by positively charged ions. Result of this bombardment is dispersion of the cathode surface and emission of secondary electrons, which also participate in ionization of working gas.

3. Sources of heat in magnetron sputtering systems

In order to define a thermal mode of a substrate when drawing coatings with the help of magnetron sputtering systems, it is necessary to understand those mechanisms, which define generation of heat in magnetron sputtering systems.

The heat generation resulting to heating the substrate in magnetron sputtering systems, occurs due to such parameters, as:

1.  Energy of condensation of the sprayed atoms;

2.  Kinetic energy of precipitable atoms;

3.  Energy of the neutral atoms, reflected from the target;

4.  The  thermal stream, transferable by radiation of plasma.

The scheme of dispersion process in magnetron sputtering systems is presented below.

Figure 3.1. The scheme of dispersion process in magnetron sputtering systems:

1 - the source of plasma; 2 - the substrate.

Q_in – energy, which is brought to the substrate;

Q_out - energy removed from the substrate.

When drawing a coating with help of magnetron sputtering systems, there is a heat generation, owing to four just listed factors, and heat transfer to the substrate, causing its heating as it’s shown in the scheme, presented above. The task of the given work is development of a technique of a substrate thermal mode estimation, hence it is necessary to establish, how the heat transfer is carried out, what mechanisms are involved in this process and what factors influence on it. For this purpose it is expedient to work out the equation of thermal balance of the substrate and to analyse all its components.

The quantity of energy brought to the substrate is equal to the quantity of energy removed from it. Then the heat balance can be presented as

Qin= Qout,                                                   (3.1)

Qin= Q₁+ Q₂ - Q₃+ Q₄,                                (3.2)

Where Q₁ – energy of condensation of the sprayed atoms;

Q₂ - kinetic energy of precipitable atoms;

Q₃ - energy of the neutral atoms, reflected from the target;

Q₄ - the thermal stream, transferable by radiation of plasma.

Then

Q₁+ Q₂ - Q₃+ Q₄= Q'₁+ Q'₂ - Q'₃,                                               (3.3)

Where Q '₁ - energy removed from the substrate by radiation;

Q '₂ - energy spent for heating of equipment;

Q '₃ – energy, which is carried away by cooling.

Let's consider in detail each of components of the equation (3.3).