Fig. 1 shows the SEM secondary electron image of the surface morphology of the film prepared at different arc source current intensities. It can be seen that as the arc source current increases, the number of droplets on the surface of the film increases, the size also becomes larger, and the surface quality of the film decreases.
a) I=40A, b) I=50A, c) I=70A, d) I=80A, e) I=90A, f) I=100A
Fig.1 Surface morphology of TiN films produced under different arc source currents
The arc source current has a greater impact on the number and size of the droplets on the surface of the film. From the basic principle of arc ion plating, it can be known that under vacuum conditions, the metal cathode (arc source target) and the trigger electrode trigger discharge at a pulse voltage of 10 kV, while the cathode arc has a strong discharge current density (106 A/cm2~108 A/cm2) and focuses on a very small arc spot area of 5μm to 6μm (Fig. 2), resulting in a high temperature above 6000°C, allowing the rapid evaporation of the cathode material while generating intense thermal field emission and ionization to form high-density metal plasma. Because the power density of the arc spot is too concentrated, the arc pool is deeper and forms an excessive liquid volume. When the arc spot emits particles (electrons, ions, atoms, atom groups, etc.), the particles also have an anti-corrosion effect on the liquid surface of the arc spot. It make ions accelerate through the sheath potential to the liquid surface while it bombards the liquid with a large kinetic energy, so that a large number of liquid atoms in the bath simultaneously receive much more energy than the binding energy, resulting in a large number of atoms concentrated emission to form droplet emission. The greater the discharge power density of the arc source target, the deeper the the weld pool formed on the surface of the arc source target, and the larger the spot diameter, so the size of the discharge power will directly affect the generation of droplets. The expression is:
P = I U/S
I—Average discharge current; U—Discharge voltage; S—Surface area of cathode target
From equation, it can be seen that the arc source current increases, the arc source target discharge power density increases accordingly, the quantity and size of generated droplets also increase to make the surface quality of the film decrease.
N: Nitrogen, M: Ti droplets. The main process: A- positive particle move to the substrate, B-neutral particles deposits on the substrate, secondary sputtering of C-neutral particles, secondary sputtering of D-Ti droplets, E-particle sputtering targets . Main reactions: X+e-1 → X*+ e-1, X*→X+hv, X-neutral particles, X*-charged particles, hv-energy, when Ti ions and N ions meet on the substrate, TiN is formed.
Fig.2 Formation process of multi-arc ion-plated TiN film and surface droplets
During the flight from the arc source target to the substrate (sample) of the droplets sputtered from the cathode arc source target, some of particles will collide with others and become smaller, but some of them are still big, so there are many different sizes of droplets in the TiN film surface. In addition, with the increase of the arc source current, some pits appear on the surface of the film. The larger the current is, the more obvious this phenomenon is, it can be seen from Fig. 1. These pits are formed by the drop of the droplets that sputtered on the surface. When the current of the arc source increased, the flying speed of sputtered droplets is large, and they will not directly collide with other particles in the plasma atmosphere and reach the surface of the substrate (sample) directly. If these droplet particles cannot be sputtered by the secondary reverse sputtering (D in Fig. 2), they will remain in the film, and some of them even penetrate the entire film from the substrate (as shown by arrows A and B in Fig. 3). The larger the arc source current, the more obvious this phenomenon will be.
Fig. 3 Large droplets that penetrating TiN film