**3.1 Elements influencing the sputter yield**

The interactions of incident ions with target surface atoms create sputtering. The following elements/factors will influence the sputter yield: *1) The bombarding particle's angle of incidence:* The angle of Ar atoms from the target metal's surface affects the sputter yield as well. As the incident angle increases, sputter yield rises. Between 60° and 80° angles, the sputter yield achieves the highest, the deposition rate will also be highest, resulting in a thicker coating. Further increasing the angle will cause the sputtering yield and film thickness to decrease rapidly. *2) Sputtering Voltage through which the ion is accelerated:* The applied voltage regulates the maximum energy that the atoms that are expelled from the target can have. Ions' Kinetic energy (KE) that collide with the target surface is controlled by the applied voltage. The energy of the ions will be greater due to the higher cathode voltage, which will cause a greater number

of atoms to sputter from the target. The more atoms that are sputtered, the more material will be deposited and sputter yield will rise as a result. The term "Threshold Voltage" refers to the required minimum voltage for the sputtering process. Ions do not have enough energy to knock out the target's binding energy atoms below this voltage. Its value ranges from 0 to 100 eV. To have an appropriate film thickness, the normal voltage range is between 100 and 1000 V. Unfortunately, raising power or voltage has a lot of negative consequences such as any energy used to operate the gun will eventually be lost, about 75% of it goes up heating the cooling water for the gun. Thermal conductivity, melting point, thermal coefficient of expansion and mechanical strength properties of the target are undoubtedly important factors. *3) Sputter Gas Pressure:* The mean free path will be shorter and there will be more collisions before the sputtered atoms deposit on the substrate as the gas pressure rises. A low deposition rate brought on by increasing number of collisions will result in a reduction in film thickness. As a result, modest/low pressures between 10−5 and 10 torr are used during the sputtering process. A small increase in deposition rate is produced by lowering the sputter gas pressure through two mechanisms: I- There will be fewer thermal collisions for sputtered atoms that are leaving the target. They are more likely to propagate to the substrate, less likely to scatter laterally and enhancing the deposition rates slightly. II- The plasma-to-target voltage will slightly rise in power control mode when using RF or DC power. As a result, the energy of the ions that collide with the target will be higher, somewhat increasing the sputter. A change in film homogeneity is one potential adverse effect of lowering the gas pressure. Deterioration is usually unpredictable because many factors play a role. The quantity of thermalizing impacts is lessened, though, and this is an obvious aspect. Arcs are more likely to form close to the target as a result of the combination of lower gas pressure or higher plasma to target voltage. According to the experimental results described by Chargui A, et al. [7], higher pressure results in thinner and less crystallinity in the tungsten films produced. Tungsten foil exhibit the best electrical and elastic properties at low pressures.

*4. Increasing Target Size:* The sputter rate increases with target diameter. This can be explained easily. For a given power density, a bigger target diameter results in a larger sputter trench area, and a larger trench area results in a higher sputter rate. *5. Number of Guns:* Most R and D deposition systems are equipped with multiple sputter guns. The user often installs several target materials in each pistol. The sputter rate and subsequent deposition rate can be doubled, tripled, etc. when the identical target material is added into two or more guns and they are all fired at the same time. The disadvantage is that many multi-gun systems only have one power source and were not designed for simultaneous deposition operations. This method could be more expensive if additional supplies are needed for concurrent operation. *6. Atomic number of element:* By reducing the target element's atomic number, the spatter yield is increased. *7. Reducing the target-to-substrate distance:* A quick, easy technique to boost deposition rate is to shorten the throw distance—the distance between the target and the substrate. The flux distribution of a sputtered material is described by terminology like over-cosine and under-cosine. Material is ejected from a circular 'trench' around the target. For these remarks, the arrival rate of the sputtered particles (per unit area of the substrate) varies as the inverse square of the throw distance. This means that halving the throw distance will quadruple the rate at which the material arrives at the substrate and the layer thickness will be four times the previous rate.

It is crucial to take into account how the shorter throw distance would affect the uniformity of the film's (thickness). The number of thermalizing collisions between

#### *Sputtering Deposition DOI: http://dx.doi.org/10.5772/intechopen.107353*

sputtered atoms and sputter gas atoms increases with throw distance, for example, if material departs the target in an approximately cosine distribution pattern. The cosine distribution tends to "flatten out" as a result of these encounters, which makes the deposition more uniform across the substrate. Film homogeneity may be worse at shorter distances because there are fewer impacts at shorter throw distances. Additionally, substrates may experience greater energy sputter particles, more stray electrons, more plasma ions and "hot" neutrals, as well as increased thermal radiation heat transfer from the plasma and target surface, when throw distances are shorter. Excessive outgassing of the substrate, an increased compressive stress of the growing membrane/film, substrate melting, substrate films/ membranes under the film destroyed by electron bombardment and other negative effects are caused by shorter throw distances. There are also advantageous effects of shorter throw distances (higher substrate temperatures) such as tensile stress of film may be lowered, the high energy of the incoming atoms improves the adhesion of the film and the membrane can be "densified" by colliding high-energy plasma ions with "high temperature" neutral particles. *8. Temperature:* The system's temperature has an impact on the thickness of the film as well. The atoms on the surface will be more mobile as the temperature rises. The larger particle size and smoother film will result from this higher mobility. The larger the particle size, the thicker the film and greater the sputter rate. The substrate is often placed on a heating stage for the higher temperature because of this. However, the act of sputtering itself generates heat as a result of collision between the atom and the surface. The deposited film might be harmed by excessive heating. As a result, cooling is required in cases of severe heat [8–10].

### **3.2 Cosine sputtering law**

The angular distribution of the sputtered particles ejected from the target surface can often be estimated by a cosine distribution as shown in **Figure 7** in circumstances of normal incidence of the projectile atoms on the surface of the target.

$$J\Omega(\boldsymbol{\theta}) = \text{Y}\Phi\left[\frac{\cos\theta}{\pi}\right] \tag{2}$$

$$\mathbf{O} = \underbrace{\int\_{\text{-}\infty}^{\text{-}\infty\text{-}\infty} \underbrace{\int\_{\text{-}\infty\text{-}\infty}^{\text{-}\infty\text{-}\infty}}\_{\text{-}\infty\text{-}\infty}}\_{\text{-}\infty\text{-}\infty}$$

**Figure 7.** *Cosine law angular distribution.*

Here, jΩф is the angular distribution of the emission flux as a function of the angle ϴ into the differential solid angle dΩ(ф), Y is the sputter yield, and ф is the local ion flux incident on the surface.

If the recoil velocities of the sputtered atoms are considered to be isotropic, this conclusion can be calculated analytically. Low sputter-ion energies (undercosine) and high ion energies (overcosine) show deviations from the ideal cosine distribution. Undercosine means the flatter distribution and overcosine means the sharp forwardpeak distribution [11].
