Glow Discharge With Plasma

- Dec 28, 2018-

Glow discharge with plasma

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1. Glow discharge


The physical phenomenon that gas is broken down and conducts electricity under the action of electric field is usually called gas discharge. Gas discharge has "glow discharge" and "arc discharge" two forms. Glow discharge is divided into "normal glow discharge" and "abnormal glow discharge", they are the basic link of plasma generation in magnetron sputtering coating process.


Glow discharge (or abnormal glow discharge) can be generated by direct current or pulse direct current target power through gas discharge, or by alternating current (rectangular wave bipolar pulse if power, sine wave if and RF) target power through gas discharge in a vacuum chamber.


During gas discharge, various factors such as working gas, pressure, current density, distribution and height of electric field and magnetic field intensity, different materials, shapes and position characteristics of electrodes will affect the process and properties of discharge, as well as the properties and colors of radiation light during discharge.


(1) DC glow discharge

1) add dc voltage between anode and cathode, the cavity working gas in the residual electrons and ions under the action of the electric field for directional movement, so the current from zero increase;

2) when the voltage between the poles is large enough, all charged ions can reach their respective electrodes, then the current reaches a certain maximum value (that is, saturation value);

3) continue to increase the voltage, resulting in the increase of charged ions, discharge current increases accordingly; When the discharge voltage between electrodes is greater than a certain critical value (ignition initiation voltage), the discharge current will suddenly rise rapidly, and the voltage between anode and cathode will drop sharply and maintain a low stable value. Working gas is broken through, ionized, and generates plasma and self-sustaining glow discharge, which is the basic process of "townsend discharge", also known as small current normal glow discharge.

4) the cathode of the magnetic control target is connected to the target power supply negative pole, anode is connected to the target power supply positive pole, enter the normal sputtering, must be in the gas discharge volt - ampere characteristic curve "abnormal glow discharge section" operation. The characteristic is that the sputtering current should rise slowly and synchronously with the increase of the working voltage of the magnetron target output of the power supply.


(2) Pulsed DC glow discharge

The gas discharge of a single pulse of pulse or sinusoidal half-wave medium frequency target power supply should be consistent with the abnormal glow discharge section and the change law of the previous section of dc gas discharge volt-ampere characteristic curve. It can be regarded as the repetition of the volt-ampere characteristic of gas discharge in the discharge of a single pulse. The pulsed dc target power supply sputters during the pulse period and deglows naturally in the pulse gap (due to high frequency, it is difficult to be distinguished by the naked eye).


After the spark discharge of the sputtering target, when the repetition frequency of the output pulse of the power supply is high enough, as the conductive ions in the vacuum cavity have not been completely neutralized, the reillumination voltage of the second (later) repeated pulse is close to or the same as the working voltage of the sputtering target. When the power supply output pulse repetition frequency is very low (for example, a few hundred HZ) or arcing time is too long (more than more than 100 ms), sputtering target after glow discharge, as a result of the conductive ion in the vacuum chamber is basic be neutralised, the second (after) repeat the recalescence pulse voltage to restore to a higher value, and the ignition of fai when high voltage is close to or the same.


2. Plasma


(1) plasma in vacuum magnetron sputtering coating technology, is generally formed by the action of electric field through the discharge of working gas. The atoms that make up the molecule gain enough kinetic energy to start separating from each other, and the outer electrons of the atom break free from the nucleus and become free electrons, and the atoms that lose electrons become positive ions. This process is called ionization. Plasma is a kind of ionized gas, which is a collection of ions, electrons and high-energy atoms. Positive ions and electrons always appear in pairs, the total number is roughly equal, the whole is quasi-neutral, it is a kind of ionized state composed of charged particles, called the fourth state of matter - plasma.

(2) on the gas discharge forming plasma voltage or electric field, along with conductive ions, particles, electrons and other movement, in the plasma will flow current, this is the conductivity of the plasma

(3) in the process of gas deposition, the working gas and the metal atoms of the target material are ionized by high-energy electrons into plasma composed of electrons, gas ions, metal ions and other conductive particles.


3. Composite of gas and target ions


(1) the electron produced by gas ionization after several collisions, energy gradually decreased, gradually away from the target surface; Some of them fall on the vacuum chamber wall (i.e., target power anode) with very low energy. The other part is compounded with the positive ions of the gas or metal that migrate and pass through the plasma region to form neutral molecules. The disappearance of such charged particles is also called "deionization".

(2) ionization of the working gas and the ionization of the target material; The recombination of positive ions and electrons with positive and negative charged particles makes the plasma in the vacuum cavity in the dynamic balance of ionization, ionization and recombination (deionization), which occur continuously above.


4. Luminescence of excited atoms


(1) many normal atoms in the electron is absorbed by the collision of the incident electron energy, the atom from the low energy level to the high energy level, become excited atoms. Excited atoms are unstable, and will emit the energy within 10-7~ 10-8s. When they return to the low-energy ground state, they will emit photons and release the excess energy in the form of luminescence. In the process of vacuum magnetron sputtering, we can see the luminescence phenomenon of target atoms and gas atoms.

(2) after target atoms and gas atoms gain energy, sputtering on the target surface is completed at the same time, the formation of discharge glow and aperture; The color and depth of characteristic light emitted by gas discharge are related to the type of working gas and target atom, pressure and discharge current. The color and depth of discharge glow and aperture vary to a certain extent depending on the current or working gas pressure. Such as:

Argon gas discharge to lavender blue light;

Nitrogen discharge to pink light color;

Helium discharge to yellow to orange;

Neon discharge to dark red to orange;

Krypton gas discharge to white or gray, low pressure green;

Xenon discharge to blue white or blue gray.

(3) gas discharge characteristics of the color of light and cathode sputtering target material. Such as:

Argon ionization discharge, copper target atoms are sputtering out, emitting green floodlight;

Argon ionization discharge, aluminum target atoms are sputtering out, emitting blue and white floodlight;

Argon ionization discharge, titanium target atoms are sputtering out, emitting blue flood light;

Argon ionization discharge, nickel target atoms are sputtering out, to emit light yellow pink floodlight;

Argon ionization discharge, chromium target atoms are sputtering out, to emit light green floodlight;

Argon ionization discharge, titanium target through nitrogen reaction deposition to produce titanium nitride, fuchsia flash;

Argon ionization discharge, silicon target through nitrogen reaction deposition to generate silicon nitride to pink (peach) color flood light.

(4) in the process of magnetron sputtering coating process test, magnetron target before a certain color of the characteristic flash, that the existence of the target material ions, the characteristic flash brightness and intensity can indirectly reflect the relative number of the target material ions sputtering out. The color difference of gas discharge luminescence before magnetron target is often taken as one of the important criteria for whether a target ion is sputtered or not.