MAGNETRON PLASMA

A magnetic field is introduced with the purpose:

• to increase the length of reactor (it actually increases the distance that an electron can travel before colliding with the walls) - when the magnetic field is parallel with the electric field
• Lorentz force, caused by the magnetic field curbs the electron trajectory, the resultant force will be:
• The electron trajectory will look like in the picture below:
• The effect is that during this long path, the electron has time to absorb more energy from the RF source and the plasma efficiency increases
• It is used especially to ignite plasma at low pressure, when fewer collisions causes the electron to lose its energy at the walls before having the chance to ionise a molecule
• confide the electron movement close to the cathode - when the magnetic field is perpendicular on the electric field, as in the picture below:

• The plasma is very dense (high concentration of electrons, ions) around the cathode
• Application: sputtering purposes, plasma at low pressure, etc.

• ECR (electron cyclotron resonance) is an example of plasma in which magnetic field is used to maintain plasma at low pressure (low pressure is desired for some applications)

  • is an electrodeless plasma generated by superposing a static magnetic field and a microwave power source. The microwave energy is used to accelerate the electrons. The electric field of a microwave can be written as in the case of RF plasma: , where w₀ is the microwave frequency. In vacuum, without any collisions, the electron motion in the electric field will be similar to a resort, half of the period accelerated, the other half deccelerated.

  • In the presence of a magnetic field, electrons experience a force perpendicular (Lorentz force) to their direction of motion, which will make them rotate around the magnetic field lines with the cyclotronic frequency (w_c): , where B is the intensity of the magnetic field and e and m_e the charge and the mass of the electron, respectively. Therefore the electron phase will be modified. If w₀ is equal to the w_c, then electrons will move in phase with the electric field and the field will accelerate them continuously. This phenomenon is called electron cyclotron resonance.

  • Because of the long electron trajectory around the magnetic field lines, the electrons can collide with gas molecules and ionise them before loosing their energy at the walls, even at low pressures when the mean free path is similar to the reactor size. Therefore ECR is efficient for low pressures; at high pressures due to many collisions, the resonance is lost.

  • In the lower picture, it is shown the multipolar magnetic field created by eight permanent magnets distributed outside the plasma dome. In the ECR layer shown in the figure, the intensity of the magnetic field is 875 Gauss, and the corresponding cyclotronic frequency is 2.45 GHz, equal to the frequency of the microwave source. Therefore, this is the area where the electrons are highly accelerated and the dissociation and ionisation of the gas occurs.

• ICP (inductively coupled plasma) is another type of plasma . The magnetic field is created by passing an RF current through a coil.