Domande d'esame surface technology

Theory indicates that the kinetic energy and angular spread of neutral atoms sputtered from a surface are given by the distribution function:

where U is the binding energy of surface atoms and θ is the angle between sputtered atoms and the surface normal.

As we can see from the graphic, the maximum distribution we can have for sputtered atoms is from E=U/2.21.

What is the purpose of sheath around the cathode?

The sheath is needed in order to enhance the electric field so the ions are attracted with a higher force and at the same way e- are emitted from the cathode, avoiding in this way the accumulation of discharge and the possibility of having arching.

Can insulating material be used as a target in sputtering?

Sputtering can be used as a technique for having the deposition of insulators if we are going to consider RF sputtering. In this case, even if the material is not conductive and it has a very high impedance inside it, the charge in this case is all accumulated on the target surface.

How can we activate the plasma if the cathode is an insulator?

Using radio frequency system for a separate plasma generation. Because when the cathode is an insulator, you cannot pass voltage through it like you probably would in a conductor to start the ionization. We can start the plasma using external source of ions or if we can use the characteristic that our cathode is insulator it cannot conduct current. But definitely it resists current (high impedance). When a material resists current flow after a certain time the material gets heated due to the resistance action. So, when it gets heated the electrons in the atom of the materials goes to higher energy state and will get released from the atom leaving the atom either positive / negative ion. This released electrons & ions again hit back the cathode and this causes the change in the energy of particles / atoms inside the cathode due to internal collision between them again it releases the electron and ions ( let's say second).

(release).V= IR so resistance if the material heats that materials ..so sputtering depends on how much itresists the current flow..R = V/I ---- we couple the cathode / electrode with Radio frequency and we give AC (alternatingcurrent) ..

24. How can we stop the substrate from sputtering in case of self-bias negativevoltage passing through it due to RF sputtering?

We can decrease the system temperature and stop applying current so that the energizedparticles hitting the source electrode will not have enough energy to initiate the internal collisionbetween the atoms to cause the emission of atoms from the electrode. To reduce the temperature,we can use cooling system attached to the back of the source which cools immediately the source.Controlling the ratio of area of anode/ area of cathode. The lower is the area of the anode, thelower the self-bias voltage and thus the lower the sputtering. While a large ground A is effective inraising the target sheath potential while minimizing the ion

bombardment of the grounds. 25. How it's possible to apply sputtering with an insulator and how to create a "fixed" voltage with RF sputtering? In order to use an insulator as the target during the sputtering process, we have to use an RF voltage, which can be coupled through any kind of impedance. To establish fixed values of the voltage, we have to work on the area of the target and on the ground (substrate, chamber walls, baseplates, etc.) with q=2.5. The higher the Ag (silver) content, the higher the Vt (target voltage), but the lower the ion bombardment of the substrate. 26. Reactive sputtering. Sputtering from metallic targets in the presence of a reactive gas (O, N, C, etc.), usually mixed with the inert working gas (Ar). The most common compounds reactively sputtered are oxides, nitrides, carbides, sulfides, oxycarbides, and oxynitrides. At high flow rates, a condition is reached which corresponds to a completely covered cathode. Sputter rates of metals drop dramatically when compounds form on the surface.

Conditioning of the target in pure Ar is required to restore the pure metal surface and desired deposition rates. In some configurations, the inert gas is introduced around the sputtering target and the reactive gas is introduced near the substrate surface. A gas flow rate has to be optimized. Cleaning the target needs very low flow rate due to hysteresis behaviour²⁷.

Efficiency of sputtering. Is not an efficient process because most of the sputtering energy is converted to heat in the target, which has to be cooled. Also, the deposition rates are actually low compared to those achieved by thermal evaporation²⁸.

Cathodic arc and filtered cathodic arc. Is a PVD technique of deposition where the vaporized material is ejected from an electrode under arcing conditions. The arc consists of a high-current low-voltage electrical discharge passing through a gas or a vapor of the electrode material. On the cathode, a spot is formed that has a current density of 10⁴-10⁶ A/cm². This current

densitycauses arc erosion by melting, vaporization and ejection of molten or solid particles. Minimum arc currents required to form a stable arc vary from about 50-100 A for low-melting materials (Cu, Ti, etc.) to 300- 400 A for refractory materials (e.g. W). Most of the arc voltage drop will occur near the cathode surface. The arc voltage can be from 15 V to 100 V. Macros are formed by ablation of molten or solid particles by thermal shock and hydrodynamic effects in the molten spot on the cathode surface. The number of macros can be reduced by decreasing the arc current and increasing the source-substrate distance. The presence of macros in the arc plasma is not wanted, because they can ruin the uniform deposition of the substrate. So in order to minimize their presence we can enhance the distance between substrate and target, but also the deposition time will increase or the filtered cathodic arc technique is used, this uses a plasma duct, here using a magnetic field the ions of the coating.

1. material are accelerated to the substrate
2. macros are deposited on the walls of the duct

The only negative aspect of this technique is the reduction of the deposition rate.

29. How does plasma work? Why do I sometimes accelerate ions even on anode?

A plasma is a partially ionized gas composed of ions, electrons, and neutral species that is electrically neutral when averaged over all the particles contained within. Plasmas can be generated and sustained by imparting energy to electrons through an electric field. Energetic electrons collide with atoms or molecules of the gas present in the chamber where we want to achieve the plasma and create ions by the emission of new electrons. These e- can be originate from ion bombarded surface (secondary e- emission) in this case we have an external source of ions, ionizing collisions or thermo-electron emittingsources (hot cathode).

For the post part of the deposition techniques the atoms to be deposited are neutrals, but there are some techniques

caso, l'argon viene utilizzato come gas di processo per la tecnica del plasma. L'argon è un gas inerte che viene utilizzato perché è facilmente ionizzabile e ha una buona conducibilità elettrica. Inoltre, l'argon ha una bassa reattività chimica, il che significa che non reagisce facilmente con il materiale da depositare, consentendo una migliore purezza del film depositato. In alcune tecniche di deposizione reattiva, come ad esempio la deposizione di film di nitruro o ossido, viene utilizzato un gas reattivo come l'azoto o l'ossigeno insieme all'argon per ottenere la reazione chimica desiderata durante il processo di deposizione.

waywe will have the formation of very hard and wear R compounds at the surface of the substrate.

The reason of which we use inert gases is that they are easy to ionize and are sure that they don't react with the target/substrate creating refractory compounds.

We have to pay attention in all the techniques in which we have the acceleration of ions also on the anode because if the concentration Ar+ is higher may cause supplantation of the Ar atoms in the film, leading to a generation of lattice defects, high residual stresses, a deterioration of the quality of the film/substrate interface, and a poor film adhesion.

Cathodic arc + High Power Pulsed Magnetron Sputtering and comparison.

Cathodic arc we have already said before.

HPPMS is a PVD technique which uses a high power the power is applied to the target in pulses of low duty cycle (<10%) and frequency (<10 kHz) leading to pulse target power densities of a few kW cm−2 which are much higher than the DC MS with 10 W.

What is important to figure out is that the time-averaged target density values are similar to those of the DCMS, so what we have is very high power density in very short time but in the all average time process the power density in not high. This because we want to avoid overheating of the cathode or damage to the magnets. The plasma we obtain is very dense, with a very high degree of ionization, and the substrates will be with a uniform thickness, dense and with a fine-grained microstructure.

It is very important to figure out that Cathodic arc and high power pulsed magnetron sputtering (HPMS) are prime examples of ionized physical vapor deposition (i-PVD), where films are deposited from atoms and ions, and where the fraction of ions is very significant. Dense, smooth, well-adherent metal, ceramic, or diamond-like carbon thin films or nanostructured coatings are obtained.

The main differences between the 2 is the degree on ionization in the plasma, fully for the cathode while only 10% for

In the HPPMS, the average Kinetic E is much higher in the Ca (20-200eV) with respect to HPPMS (3-5ev, because the current density on the cathode as to be maintained constant with a very intense flux in both cases. Very important also is to consider the macroparticles, which are very few in the Ca if there is a filtered system used, while in the HPPMS there are none usually.

32. How to obtain same coatings without macros problems?

We can enhance the distance between electrodes, we can use an external B to obtain a steered arc (less residence time of the spot at any cathode site) or the most used is using the filtered cathodic arc deposition.

33. Considerations on the effect of a voltage rise on the cathode and on ionization current in cathodic arc deposition.

Higher is the voltage and lower will be the current on the cathode. In the arc cathodic deposition, we have low voltages and high currents, most of the V has a drop near the cathode surface while is much higher in between the two electrodes.