**2. Basics of DC reactive magnetron sputtering**

### **2.1 Sputtering**

Sputter deposition is a physical vapor deposition (PVD) process [27] based on the ejection of atoms from a solid target as a result of collisions with energetic particles. However, if the collisions are due to the impact of positive ions, the process in known as cathodic sputtering.

Sputtering allows the deposition of thin films of a variety of materials, including metals and certain compounds such as oxides and nitrides and any type of substrate can be applied for deposition. Simultaneous deposition from various sources permits to develop complex compositions.

It has advantages over other deposition methods when the intended film is a compound (e.g., oxide) or an alloy, avoiding non-stoichiometric films, separation of phases of the constituent elements, and even differences in the desired composition. Coatings deposited generally have good adhesion and exceptional coverage.

The objective is to remove material from a target and bring it to the substrate (e.g., the fiber to be coated). This is achieved by means of ion bombardment in the plasma, usually by Ar<sup>+</sup> ions. Further, the ions that reach the target with enough energy can eject atoms from the target that are then dropped on the fiber (or other substrate) surface placed nearby to the target. The process basically consists of three distinct steps that occur altogether (**Figure 1**):

**Figure 1.** *Schematic diagram of magnetron sputtering. Adapted from [28].*


The widespread use of sputtering is explained by the many advantages of this technique, mainly due to its simplicity of operate and the quality of the thin films through the stoichiometry control in complex compositions, excellent film adhesion to the substrate, uniform deposition over a large area and tailor of the film thickness. Moreover, by the change of deposition parameters such as oxygen partial pressure, working pressure, and sputtering power is possible to achieve desired film parameters, for example, microstructure, composition, step coverage, among others.

## **2.2 Chamber preparation and DC reactive plasma**

The process begins by creating vacuum inside the chamber and thus the air is pumped out. The chamber is then filled with argon, an inert gas, reaching a pressure between 1 and 10 Pa. When a DC voltage is applied between the electrodes (with a gas in between them), a plasma is formed. The applied voltage is high enough to enable that a large quantity of inert gas atoms turns into ions; electrons acquire enough kinetic energy to ionize gas atoms (break gas atoms) and thus, the plasma is formed [29]. The ions and electrons are then accelerated towards opposite electrodes. Plasma is thus a partially ionized gas. Depending on the mean free path in the gas, the accelerated particles can collide with inert gas atoms and give rise to scattering occurrences at a rate that can change with the pressure and nature of the gas. Moreover, these scattering occurrences can lead to ionization of further gas atoms. The probability of ionization (*α*) occurs will depend basically on the threshold voltage to initiate the breakdown of the gas (trigger the gas discharge) which must surpass the ionization potential of the gas species and can be calculated by [30]:

*TiO2 Nanocoatings on Natural Fibers by DC Reactive Magnetron Sputtering DOI: http://dx.doi.org/10.5772/intechopen.110673*

$$a = \frac{1}{\lambda} \exp\left(-\frac{V\_i}{eE\lambda}\right) \tag{1}$$

where λ is the mean free path of the sputtering ion, *Vi* is the ionization potential of the gas in electron volts, *e* is the electron charge, and *E* = *V*/*d* is the electric field between electrodes [30].

The breakdown voltage of a gas, which is the voltage required break a sustained plasma, is established by Paschen's law, which is a function of the product of electrode gap spacing and chamber pressure, according to:

$$V\_b = A \frac{pd}{\ln\left(pd\right) + B} \tag{2}$$

where *Vb* is the breakdown voltage, *d* is the gap electrode distance (cm), *p* is the pressure (torr), and *A* and *B* constants depending on the gas mixture inside the chamber. Paschen's law relationship the breakdown voltage versus the product of the pressure and the gap electrode distance (*pd*) as shown in **Figure 2** [31] and predict a minimum breakdown voltage for any gas.

Once the *Vb* is achieved plasma becomes self-sustaining and plasma reaches a steady state, exhibit enough energy to be used in sputtering.

### **2.3 Principle of sputter deposition**

Typical gases used in the sputtering process are from the group of noble gases because they tend not to react with the target material. Argon (Ar) gas is the most common one in this process. Positively charged argon ions from the plasma (Ar<sup>+</sup> ions) are accelerated by an electrical potential difference toward the negatively biased target (cathode), where the target material, for example, Ti, is placed and hits it.

**Figure 2.** *The breakdown voltage versus gas pressure curve [31].*

With the impact energy, atoms are ejected from the target and diffuse through the vacuum chamber until they are deposited on the substrate to form a thin film (**Figure 1**). This atom ejection is known as sputtering. From a physical point of view, the principles of sputtering are based on a simple momentum transfer model, which allows understanding how atoms are ejected from the surface of a material due to successive collisions. The collision of particles and the transfer of momentum are important aspects of the DC sputtering process. In a plasma, there are various types of particles, such as electrons, ions, and neutral atoms or molecules. When these particles collide with each other or with the target material, momentum is transferred between them.

Because of the bombardment of the target, beyond ejected or sputtered atoms, additional events can occur as shown in **Figure 3**, including the followed briefly underlined: secondary electrons, reflected ions at the target surface, ion implantation in the structural atomic network, lattice defects, and structural rearrangement by trapping ion species.

The mass of the energetic ions is key to the energy and momentum transferred to the film atom during the collision. From the physics laws of the conservation (of energy and momentum), the energy transferred in a collision of an incident particle (i) and a target particle (t) is given by:

$$\frac{E\_t}{E\_i} = \frac{4m\_i m\_t}{\left(m\_i + m\_t\right)^2} \cos^2 \theta \tag{3}$$

where *E* and *m,* are, respectively, the energy and the mass. *θ* is the angle of incidence as measured from a line across the two centers of masses, as shown in **Figure 4**. When the ejected particles reach the substrate, they deposit onto its surface due to the momentum transfer that occurs during the collision. The amount of momentum transferred during the collision depends on the mass and velocity of the particles involved. In general, heavier particles transfer more momentum than lighter particles, and faster particles transfer more momentum than slower particles.

The transfer of momentum is an important factor in determining the quality and properties of the deposited thin film. If the momentum transfer is too low, the deposited film may be porous and have a low density. On the other hand, if the

**Figure 3.** *Events that may occur on the target surface being bombarded with energetic ions. Adapted from [27].*

*TiO2 Nanocoatings on Natural Fibers by DC Reactive Magnetron Sputtering DOI: http://dx.doi.org/10.5772/intechopen.110673*

**Figure 4.**

*Collision of particles and the transfer of momentum. Adapted from [32].*

momentum transfer is too high, the deposited film may be dense but have high levels of residual stress. Therefore, it is important to carefully control the parameters of the DC sputtering process, such as the gas pressure, target material, and substrate temperature, to optimize the momentum transfer and achieve the desired properties of the deposited thin film. The efficiency of the momentum transfer is the highest,

*Et Ei max*, when **cos <sup>θ</sup>** <sup>¼</sup> **<sup>1</sup>** and *mi = mt*, that is, is desired that the atomic weight of the sputtering gas could be identical to that of one of the target.

Ejected atoms must be able to diffuse freely toward the substrate with desirable little opposition to their movement, which explains the necessity of the sputtering to be done in vacuum conditions. To achieve this, a low pressure within the chamber and a suitable large DC voltage applied between the electrodes, in other words, between the target and the substrate, give rise to a glow discharge that allow accelerate the positive ions to the target. Therefore, ions can retain their high energies. Besides atomgas collisions can be prevent after ejection from the target. Still, the initial kinetic energy of the atoms transported through the plasma can be lost by collisions within the plasma, failing the energy needed to deposit themselves on the substrate. Thereby, not all atoms ejected from the target reach the substrate, many are projected in different directions and deposit on any surface they encounter. The atoms that can reach the substrate thereby form a layer called a thin film. So, sputtering is also described by its yield, which is the ratio of the number of atoms ejected to the number of incident energetic ions and depends on the chemical bonding of the target atoms and the energy transferred by impact [27].

The sputtering yield (Y) is an important parameter that characterizes the efficiency of the sputtering process since it determines the rate at which atoms are ejected from the target material and deposited onto the substrate. Therefore, the sputtering yield plays a crucial role in the fabrication and processing of thin films, coatings, and surface modifications using sputtering techniques. Y is defined as the number of atoms or molecules sputtered from the target per incident particle. Y is zero for ion energies below the threshold energy of sputtering, Φ. This means that particles with energy below this threshold are not able to cause sputtering. Mathematically, we can express this as *Y* = 0 for *E* < *Φ*, where *Y* is the sputtering yield and *E* is the ion energy. *Y* is a function of the ion energy and the target material properties, with a threshold energy below which sputtering does not occur and a power law relationship above the threshold energy.

The sputtering yield depends on various factors, such as the energy and flux of the incident ions, the target material properties, and the surface conditions. By controlling these factors, the sputtering yield can be optimized to achieve desired properties, such as film thickness, composition, morphology, and adhesion. The sputtering yield also affects the overall efficiency and quality of the sputtering process, as well as the cost and environmental impact.

Reactive sputtering is a widely used technique for depositing compound films on substrates. In reactive sputtering, a target material is bombarded with ions in the presence of a reactive gas such as oxygen, nitrogen, or hydrogen. The sputtered species react with the reactive gas to form a compound film on the substrate surface.

One of the advantages of reactive sputtering is that it allows for precise control of the stoichiometry of the deposited film by adjusting the flow rate of the reactive gas. This makes it possible to deposit films with desired properties such as optical, electronic, magnetic, or mechanical properties.

Another advantage of reactive sputtering is that it can be used to deposit films on complex substrates, fibers, nanoparticles, films and materials with irregular surfaces, porous materials, etc. This is because the sputtered species have high kinetic energies, which enable them to penetrate the pores and irregularities of the substrate surface. As a result, the deposited film can conformally coat the entire surface of the substrate, including its complex features, such as corners, edges, and high aspect ratio structures.

### **2.4 Magnetron sputtering**

The sputtering process is a relatively simple technology, but it still requires additional support systems, such as efficient cooling of the substrate because the electrons that are repelled by the negative cathode can reach the substrate heating it; and the use of magnets to confine the electron paths towards the cathode surface (magnetron sputtering) to increase the plasma efficiency and therefore the deposition rate. This allows the plasma thus located/confined to improve deposition rates due to the greater number of ions colliding with the target and reduces the temperature of the substrate as less electrons collide with it.

The presence of magnets behind the cathode creates a magnetic field close to the surface of the target. These magnets are positioned to produce a magnetic field near the target is a such way that magnetic field lines are parallel to the cathode surface and perpendicular to the electric field lines (**Figure 5**). This arrangement allows to concentrate the electrons close to the target, as shown in **Figure 5a**, instead of them circulating randomly dispersed around it, while the ion trajectories are not influenced by the deflection due to their greater mass. The combined action of the electric (*E*) and magnetic (*B*) fields near the target generates the *E* � *B* drift phenomenon. The trajectories of electrons, of charge *q* and velocity *v*, captured in this drift are forced to bend and follow helical trajectories around the magnetic field lines (**Figure 5b**), and to follow them because of the Lorenz force (*FL*). The Lorenz force acting on a charged particle is given by the following equation:

$$
\overrightarrow{F}\_L = q \left( \overrightarrow{E} + \overrightarrow{v} \,\textbf{x} \overrightarrow{B} \right) \tag{4}
$$

where *q* is the charge of the particle, *E* ! is the electric field, *v* ! is the velocity of the particle, and *B* ! is the magnetic field. The term *v* ! x *B* ! represents the cross-product of

*TiO2 Nanocoatings on Natural Fibers by DC Reactive Magnetron Sputtering DOI: http://dx.doi.org/10.5772/intechopen.110673*

### **Figure 5.**

*(a) Layout of the DC magnetron sputtering system near the target; (b) helical electron trajectory around the magnetic field line due to the Lorenz force.*

the velocity and magnetic field vectors. In the case of DC sputtering, the magnetic field is typically generated by a permanent magnet. The electric field is created by the potential difference between the cathode and anode. As the electrons move toward the anode, they experience a Lorentz force that causes them to follow a curved path.

This curved path increases the path length of the electrons to the anode, which means they have a larger number of collisions with the argon atoms in the plasma. This, in turn, significantly improves the ionization probability because the collisions between the electrons and argon atoms result in the formation of more ions.

This additionally acting Lorenz force restricts the trajectories of the electrons. Therefore, the path of the electrons to the anode increases which significantly improves the ionization probability because of the larger number of collisions between argon atoms and electrons.
