Diode Sputtering

The fundamentals of sputtering were introduced in sputtering basics. Here we elaborate on Diode Sputtering which is one of the sub-categories of sputtering and some of the subtleties of the techniques.

Diode Sputtering

This describes sputtering without magnetic enhancement. Both DC and RF can be used in the diode arrangement. Without the benefit of the magnetic containment of the electrons in either style the cathode voltage tends to become large.

DC Diode Sputtering

Firstly, consider the DC case. At low applied voltage relatively little current will flow. With increasing voltage the current increases steadily. Eventually, a point is reached where an avalanche occurs. At this point the cathode contributes secondary electrons released following ion impact at the target surface. These produce more ions through collision with the background (neutral) gas. Once the number of electrons generated is sufficient to produce enough ions, that in turn regenerates the same number of electrons a steady state is achieved. At this point the discharge becomes self-sustaining. A glow becomes visible, the voltage drops, and current rises abruptly. This state of operation is called the normal glow. The current density at this point is still relatively low. In the apparatus used for practical applications the current density is substantially higher. The secondary emission of electrons following ion impact at the target surface is key to achieving a steady state discharge. The secondary electron emission coefficients of most materials are such that more then one ion must strike a given area of cathode to produce another secondary electron. The area of bombardment at the cathode in the normal glow region self-adjusts to accomplish this. The area of bombardment may be uneven and concentrated near the edges of the cathode, or any aspirates across the surface. With increasing power the bombardment becomes almost uniform.

Once uniform bombardment of the cathode area is achieved further increase in power is accompanied by both an increase in voltage and current density in the discharge. The discharge has now entered the abnormal glow region. It is this regime that is used in sputtering. At this point if the cathode is not cooled, as it heats, a further electron flux arises from thermionic emission. This may lead to a further avalanche. The output impedance of the power supply limits the voltage, and a low-voltage high-current arc discharge forms.

The classical diagram of a DC glow discharge tube (diagram A)

A

illustrates a number of features that are not exhibited in a sputtering system. This is because the Faraday dark space and positive column are consumed and disappear as the cathode and anode are brought together.

All that is seen with a practical sputtering system are the negative glow and the dark spaces adjacent to the electrodes, (diagram B). In practice, DC diode sputtering is seldom used industrially and the field is dominated by DC magnetron sputtering for the deposition of metals.

RF Diode Sputtering

Why use RF? The simple answer is that it allows the sputtering of dielectric (electrically non-conducting) materials. Adoption of an AC (alternating current) discharge makes the two electrodes equivalent; one electrode is bombarded in one part of the cycle and the other in the remaining part of the cycle. To achieve essentially continuous ion bombardment of the electrodes the AC frequency needs to be above 1 MHz (one million cycles per second). In practice a frequency of 13.56 MHz is commonly used. This is not a magic frequency, it is simply one that has been assigned by the international communications authorities at which one can radiate a certain amount of energy without interfering with communications. Power supplies and impedance transformers have been developed for use at these frequencies and have become less expensive than low volume units at other specific frequencies.

Due to the large difference in the mass between an electron and an Argon ion, at high frequency the electrons respond to the alternating electric field and the ions do not. As the electrons are initially lost from the plasma the discharge attains a positive potential, the electrodes self bias and an RF induced DC bias is established at each electrode. As a result there is continuous ion bombardment of both electrodes with ions that are accelerated across the dark space sheaves adjacent to each of the electrodes.

The RF power delivered to the electrode needs to pass through an impedance matching transformer. This will contain both capacitors and an inductor and results in the power delivery through a DC blocking capacitor.

A typical arrangement is illustrated, (Diagram C)  and shown diagrammatically (Diagram D) . The inclusion of this blocking capacitor is important.

The classical treatment of voltage division in an RF discharge has been described by Koenig and Maissel (1970) and Koeing (1972). For a powered electrode of area A1 and voltage V1 and a counter electrode of area A2 and voltage V2, then the division of voltages is given by the expression:

 

This is significant and very convenient. Imagine two parallel electrodes, of equal area within a glass vacuum chamber. One is bonded to earth potential and the other connected to an RF power supply. The voltage at each electrode would be equivalent. Where the two electrodes were clad with the same material no net deposition at the earthed electrode would be attained. For a practical sputtering system the chamber is metal and is bonded to earth. Here the earthed area in contact with the glow discharge will be many times the area of the RF powered target. The result is that a far greater voltage will develop at the powered electrode and very small voltage at the counter electrode; resulting in net deposition of material at the earthed electrode with no contamination from the chamber walls that also exhibit a very low sheath voltage. Other studies have suggested that the general relationship may be true but the ratio may not be divided by as much as the fourth power.

Adoption of an RF power system allows the sputtering of insulating surfaces. An RF discharge is more efficient than its DC counterpart and promotes a high level of ionisation to sustain the discharge. This is evidenced in that for an equivalent power a glow discharge can be sustained at lower pressure by RF at 13.56 MHz than by DC.