*4.2.2 Conventional magnetron/balanced magnetron*

In 1986, papers to describe various magnetic field configurations of the substrates were published by Window and Savvides [14] which gave birth to the terms "balanced" and "unbalanced" magnetrons. Shortly thereafter, unbalanced magnetron technology was used to deposit thin films.

In a balanced magnetron (**Figure 12**), all lines of magnetic force originate from one pole and are closed by the other pole of the target. The magnetic flux strengths via the pole faces of the outer and inner magnets are equal or comparable. As the magnetic field controls the plasma transmission path, the balanced magnetron's plasma is restricted or trapped at the cathode, which is advantageous for high rate cathode sputtering, but resulting production efficiency is relatively low and also prevents the large amount of bias current from reaching the substrate [15].

#### *4.2.3 Unbalanced magnetron*

The balanced magnetron can be unbalanced if the outer or inner set of magnets is strengthened or weakened. As the ion bombardment of growing films supports in

**Figure 12.** *Balanced magnetron.*

the formation of thin films with better characteristics. The technology of unbalanced magnetron sputtering allows to get thin films with such improved properties. Fraser and Cook [16] initially highlighted the possibilities of this technology in 1977. It's been utilized to deposition and ion-bombard films at the same time.

By allowing some of the confined lines of magnetic field, which are parallel to the surface of cathode in balanced magnetron, to become perpendicular to the cathode surface in the unbalanced magnetron, the plasma purposely leaks and collide with the substrate. As a result, some of the electrons discharged from the cathode surface are allowed to exit the cathode region along those normal field lines, causing electrostatic forces to pull ions along with them. This produces a plasma beam that is focused towards the substrate surface, bombarding it as well as the forming film surfaces with ions from the sputtering gas.

As the mobility of electrons is significantly more than ions and their mean free path is also longer, so the surface of an insulating or isolated substrate submerged in this high density plasma will attain a negative charge and potential with respect to the plasma until the electron and ion fluxes become equal. The ions will collide with substrate under the effect of this floating potential [15].

#### *4.2.4 Types of unbalanced magnetron*

Savvides and Window [17] also introduced types of unbalanced magnetron, type I and type II as in **Figure 13**. Magnetic flux is important in both type I and type II.

The inner set magnetic flux in type I is greater than the outer set magnetic flux, whereas the inner set magnetic flux in type II is lower than the outer set magnetic flux. To put it another way, in type I, all field lines originate from the inner set of

**Figure 13.** *Types of unbalanced magnetron.*

**Figure 14.** *DC sputter deposition.*

magnets, with only a few reaching the outer set, whereas in type II, field lines comes out from the outer set of magnets, with only a few reaching the inner set.

#### **4.3 DC and RF sputtering deposition**

The power source in DC sputtering is direct current. Positively charged sputtering gas is propelled towards the target in this approach. As a result, atoms are ejected and deposited on the surface of substrate. Schematic diagram of DC is shown in **Figure 14**.

A cathode (the target) and an anode are connected in series with the blocking capacitor in RF sputtering as shown in **Figure 15**. The capacitor detects that power from the RF source is transferred to a plasma discharge. There are two stages of RF sputtering. The target material is negatively charged in the first cycle. Atoms get polarized as a result of this. The atoms of sputtering gas are drawn to the source that knock away source atoms. Here, the polarization of the target leaves source atoms and ionized gas ions on the surface of the target. The target is positively charged in the second cycle. This causes gas ions and source atoms to be emitted by depolarization. These are propelled towards the substrate, causing deposition [18].
