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To produce thin film coatings by vacuum deposition, there are mainly two groups of methods: thermal evaporation and ionic sputtering. Thermal evaporation heats the materials in vacuum where the vapor pressure becomes higher than the ambient pressure, then the deposition starts. Ionic sputtering involves high energetic ions to strike the solid target where the atoms are

knocked off the surface. Among the ionic sputtering techniques, magnetron sputtering method is a standard process for the deposition of many important microelectronics fabrication. This technique can deposit metals, alloys to a variety of materials with a thickness of up to a few micrometers. Coatings for hardness, corrosion resistance, low friction, and specific electrical properties are a few examples. Although conventional magnetron was the first technique introduced, the major transformation of this technique was established after the introduction of unbalanced magnetron. Among those, especially the closed-field unbalanced magnetron sputtering is capable of commercially depositing high quality films of a wide range of materials [43].

The main advantages of magnetron sputtering are; high rate of deposition, use of any metal, compounds or alloys, thin films with high purities and uniformity even for large area substrates, and very high adhesion of deposited thin films. Even though there are several sputtering techniques such as RF, DC, diode or reactive sputtering, they fundamentally work on the same principle that molecules or atoms of a material are ejected by high energy particle bombardment. The ejected material is then coated onto the substrates. The first criteria is to generate ions with sufficient energy and direct them to the target to eject atoms from the material. The second criteria is that the ejected atoms must be freely moving towards the substrate to be coated. The plasma can be generated by DC or RF power. DC powered discharges can deposit metallic films such as gold, copper, or tungsten. RF powered sputters can be used for deposition of insulating films such as ceramics and oxides.

The low pressures are necessary to maintain high ion energies and prevent dominating atom-gas collisions. The mean free path, the distance that atoms can travel with no collision with another gas atom, is of critical importance. In order to maintain a reliable atom transfer to the substrate, the pressure of 7.5 Torr or lower are required when the background gas is supplied to

the vacuum chamber. Otherwise, the collisions are high and depositions rates become very low. The best practice is pumping the chamber to ultrahigh vacuum pressures in order to eliminate contaminations. However, there is a certain pressure range to initiate the plasma generation, required for sputtering. When the pressure is further decreased, the mean free path for electrons increases. As a consequence, the ions are generated far from the target and the ionization efficiency is reduced, thus the plasma generation rate gets diminished, and sputtering is terminated. In magnetron sputtering however, the ionization efficiency is increased at lower pressures with the introduction of specifically designed magnets that trap the primary and secondary electrons in a race track so that the collisional processes with gas atoms are greatly increased near the target. The impedance of the plasma is then reduced due to the ionization rate increase, which affects the material removal rate from the target. The schematic for a DC diode magnetron sputter is shown in Fig. 3.4. The cathode is equipped with magnets that produce dc magnetic fields to confine the secondary electrons. These permanent magnets are placed behind the target and magnetic lines enter and leave through the cathode plate. Magnetic field bends the electron trajectories into helices, so that their travel path is increased resulting to higher collision rates. The magnetic field strength for proper magnetron operation is relatively small; 300 G which can easily be achieved by permanent magnets. When a voltage of 200 V or more is applied, the discharge gets formed. The current-voltage characteristic for the magnetron discharge is given as [78]:

𝐼 = 𝛽(𝑉 − 𝑉₀)2 _(3.5)

where V0 is the threshold voltage to maintain the discharge and 𝛽 is the pressure dependent

parameter. For an estimated maximum current density of 0.25 A cm-2 at the target which is mostly

limited to this value due to the cooling and operation limits, the power supply should have a rating that can provide this maximum current with voltage values reaching to several hundreds of volts.

Typical magnetron sputtering chamber operates at 10-3 Torr pressure, 500 V discharge voltage, 20

mA cm-2 _{current density, and 300 G magnetic field strength.}

Fig. 3. 4 DC diode magnetron sputtering schematic. Redrawn from Ref. [79].

The magnetron discharge occurs in the form of a bright glowing circular plasma. However, the electric field is not uniform between the cathode and the anode. The electric field is shielded by the plasma and a cathode sheath is developed with a thickness of ≈ 1 mm. Most of the applied voltage is sustained by the cathode sheath. Sheath thickness is given by Child-Langmuir law [76]:

𝑠2 ₌4ℰ0 9𝑗 √ 2𝑒 𝑚 𝑉3/2 𝑠2 (3.6) where j is the current density of ions for cathode voltage V, e is the electron charge, Ɛ0 is the permittivity of free space, m is the ion mass, and s is the length of the cathode sheath which is called “cathode dark space” in glow discharges. The sheath thickness increases with the cathode voltage and decreases with pressure through the j, the current density of ions.

Argon is commonly used as a background gas in magnetron sputters. The sputtering occurs

when impacting ions have a threshold energy of ≥50 V. For many applications, argon ions with

energies of several hundreds of volts (200-1000 V) are ideal for sputter deposition. Ar ions are accelerated towards the cathode and strike it with high energies which sputters the cathode target and produces secondary electrons emission. Further ionization collisions are sustained by these accelerated secondary electrons emission which maintains the discharge. The sputtered film morphology is mainly determined by the substrate temperature and deposition pressure. The deposition rate is given by [76]:

𝐷_{𝑠𝑝𝑢𝑡} = ɣ_{𝑠𝑝𝑢𝑡}Г_𝑖𝐴_𝑡/𝑛_𝑓𝐴_𝑠 (cm/s) (3.7) where Г_𝑖 is the incident ion flux (cm-2s-1), nf is the deposited film density in cm-3, At is the sputtered target area in cm2_{, As is the substrate area and ɣsput is the sputtering yield (atoms sputtered per} incident ion). The discharge causes erosion track to the cathode substrate material. In this case, the sputtering target should be replaced when the erosion track is comparable to its thickness.

When we consider the glow discharge current-voltage curve for the conduction of electricity in a gas environment at low pressures, the sputtering occurs in a region where the current densities are high. In order to maintain a stable operation, the power supplies driving the sputtering system should be specifically designed to maintain voltage and current values because rapid changes can cause to arching and eventually disrupt the proper operation.