Cathode sputtering

The cathode sputtering is a method of deposit of thin layers.

Principle

The cathode sputtering (or sputtering ) is a technique which authorizes the synthesis of several materials starting from the Condensation of a metal vapor resulting from a solid source (target) on a Substrat.

The application of a difference of potential between the target and the walls of the engine within a rarefied atmosphere allows the creation of a cold plasma, composed of electron S, of Ion S, Photon S and neutral in a fundamental or excited state. Under the effect of the Electric field, the positive species of plasma are attracted by the Cathode (target) and enter in collision with the latter. They then communicate to them Quantité of movement, thus causing the pulverization of the Atome S in the form of neutral particles which condense on the Substrat. The formation of film is carried out according to several mechanisms which depend on the forces of interactions between the substrate and film.

The discharge is self-sustained by the emitted secondary electrons of the target. Indeed, those, at the time of inelastic collisions, transfer part of their kinetic energy in potential energy to the atoms of Argon which can ionize.

; Cathode sputtering magnetron

In order to increase the ionic density in the vicinity of the target, this one is equipped with a device Magnétron, which consists of two Aimant S permanent of opposite polarity located under the target. They create a Magnetic field B parallel at the surface of the target and orthogonal to the electric field E. the combination of these two fields gives rise to lines of field which trap the secondary electrons. The induced Force of Lorentz causes a helicoid movement of the electrons thus increasing their trajectory and, so their effectiveness of ionization. The magnetron effect thus makes it possible to maintain the discharge for moreover low pressure of work, improving consequently quality of the coatings.

Ceramic film synthesis

In order to carry out ceramic films (Oxide S, Nitride S, etc), a species reactivates, the Oxygène for oxides, the Azote for nitrides, is added to carrying gas, generally of the Argon. The Stœchiométrie of the layers is in direct relationship with the pressure partial of introduced reactive gas. The synthesis of a stoechiometric coating requires, often, the formation of composed on the surface of the target. The reactive gas reacts with various surfaces present in the enclosure (target, substrate, walls) and takes part in pulverization by modifying the electric characteristics of the discharge. This can result in phenomena of instabilities electric and mode of pulverization.

Electric instability

The reactive gas recovers partially surface of the target to form an insulating layer. The ions argons which come to strike it cannot be evacuated any more, generating an accumulation of positive loads in the zones where the process of pulverization is weakest. These loads are eliminated by breakdown from the Diélectrique causing the ejection from microphone-droplets which damage the quality of the coating.

Instability of mode of pulverization

The presence of an instability of the mode of pulverization is represented by a Hystérésis on the curve of follow-up of the reactive gas pressure according to the quantity of introduced reactive gas. For the low outputs gas reactive, the evolution of the pressure partial of oxygen remains weak and the target is primarily metal. The system is in elementary mode of pulverization (RPE). The deposit rate is important but does not lead to the formation of stoechiometric compounds. When the reactive gas output reaches D2, the sites of adsorptions of the walls are saturated, which results in an abrupt increase in the partial pressure, involving a more important contamination of the target. The system rocks in mode of pulverization of made up (RPC) where the deposit rate is lower but allows the realization of stoechiometric compounds. The difference between the curves with and without discharge represents the quantity of gas consumed by the walls. The reverse of the slope of the right-hand side without discharge corresponds at the speed of pumping.

Comparison with other methods of deposit

An important advantage of the pulverization as technical of deposit which the films deposited have the same composition as the material source. The stoechiometric identity between film and the target could surprise owing to the fact that the output of pulverization depends on the Atomic mass of the atoms in the target. One could thus expect that a component of a Alliage or a mixture pulverizes more quickly than the other components, driving with an enrichment of this component in the deposit. However, as only the atoms on the surface of the target can be pulverized, the faster ejection of an element leaves the surface enriched with the others, which effectively compensates for the difference in speeds in pulverization. This contrasts with the thermal techniques of evaporation, where a component of the source can have a pressure more raised vapor, it results from it a film deposited with a composition different from the source.

The deposit by pulverization has also an advantage on the epitaxy by molecular jet beam epitaxy '' (MBE) because its speed. Higher speed has as a consequence the lower incorporation of impurity because less impurities can reach the surface of the substrate in the same quantity of time. The methods of pulverization can consequently use gases of process with concentrations in impurities much higher than the pressure of vacuum which can be tolerated by the methods of MBE can tolerate. During the deposit by pulverization the substrate can be bombarded by ions with great energy and atoms neutral. The ions can be deviated with a polarization of the substrate and the bombardment can be minimized while pulverizing far from the axis but with depend on the deposit rate. The plastic substrates cannot tolerate the bombardment and are generally treated by evaporation.

Types of deposit per pulverization

The sources of pulverization are usually magnetrons which use extremely electric and magnetic fields to imprison electrons close to the surface of the magnetron, which is known like the target. The electrons follow helicoid trajectories around the lines of magnetic field undergoing more ionizing collisions with the gas neutral elements close to the surface of target, than that would not occur differently. The gas of pulverization is inert, typically argon. The supplement of ions argon created in consequence of these collisions leads to a higher deposit rate. It also means that plasma can be maintained with a lower pressure. The pulverized atoms are electrically neutral and thus insensitive with the magnetic trap. The accumulation of loads on insulating targets can be avoided by the use of the pulverization RF ( RF Sputtering ) in which the sign of polarization anode-cathode is changed at high-speed. Pulverization RF function well to very produce films of an oxide insulator but only with the additional expenditure of the food for pulverization RF and the networks adapting to the Impédance.

Parasitic magnetic fields fleeing of the ferromagnetic targets disturb also the process of pulverization. Guns of pulverization especially designed with very powerful permanent magnets must often be used in compensation.

Pulverization by beam of ions Sputtering '' (IBS)

A method in which the target is external with the ionic source. A source can function without any magnetic field like in the case of a hot measurement of ionization of filament (hot cathode). In a Kaufman source of the ions are generated by the collision with electrons which are confined in a magnetic field as in a magnetron. They are then accelerated by the emanating electric field of a grid, towards a target. When the ions leave the source they are neutralized by the electrons coming from a second filament IBS has an advantage because the energy and the flow of ions can independently controlled beings. Like the flow which strikes the target, is composed of neutral atoms, the targets conducting or insulating can be pulverized. IBS found an application in the manufacture of the heads in thin layer for disk drives. The principal disadvantage of IBS is important maintenance necessary to keep the source of ions in operating condition.

Reactive pulverization

Refers to a pulverization where the film deposited is formed by chemical reaction between material of the target and a gas introduced into the vacuum chamber. Nitride and oxide films are often manufactured by using reactive pulverization. The composition of film can be controlled while varying the relative pressures of inert and reactive gases. The stoichiometry of film is an important parameter for functional properties such as the constraint in SiNx and the index of refraction of SiOx. The transparent oxide driver of Indium and tin is used in Optoélectronique and in the solar cells carried out by reactive pulverization.

Deposit assisted by gun with ion deposition '' (IAD)

In this technique the substrate is exposed to a secondary beam of ions operating with a pressure lower than that of the gun with pulverization. Usually a source of Kaufman like that used in IBS provided the secondary beam. Technique IAD can be used to deposit carbon in its form diamond on a substrate. All the carbon atoms which land on the substrate without binding correctly to the crystalline structure of diamond will be driven out by the secondary beam. NASA used to make an experiment by depositing diamond films on blades of turbines in the years 1980. The IAD is used in other important industrial applications such as the surface treatment with tetrahedral amorphous carbon on the plates of the hard drives and the hard transitions from coating nitride metal on the medical implants.

HiTUS ( High Target Use Sputtering /Haute use of the targets of pulverization)

This technology is a major change of traditional technology magnetron of deposition of thin layers largely used in the fields of industry and the required one. A process based on the remote generation of a plasma with high density. Plasma is generated in a side room opening on the principal room containing the target and the substrate to be covered. To increase adherence to prepare the substrate, from the volatile contaminants on the surface of the substrate are removed, while directing the jet beam of plasma on the substrate. Before the deposit, the target is cleaned by pulverization in a pure argon plasma to eliminate from oxides/contamination. As plasma is generated remotely, and not starting from the target itself (as in conventional pulverization using the magnetron), the current of ions to the target is independent of the tension applied to the target.

The process offers a multitude of comparative advantages with the traditional techniques of pulverization such as:

use of the target to 95 % in a homogeneous way on all its surface (not of furrow dug in the target (racetrack)) ;
increase deposit rate for the dielectric ones deposited in particular by reactive pulverization;
process of reactive pulverization controlled very simply - absence of system of regeneration (the ionic current of the target is independent of the tension applied to the target);
better precision of the deposits;
better control of the characteristics of film with properties close to those of material deposited in the mass;
better control of the surface quality: smoothing;
elevated levels of reproducibility this of repeatability;
more high-speed of production;
possibility of production on line or roll-to-roll with the possibility of multi-layer;
forced in the deposit easily controllable, of compression to the tension, or null between these two possibilities;
process at low temperature authorizing a deposit on organic substrates;
the process can easily be integrated in many existing installations of pulverization.

Very precise deposit of the thin layer on substrates of great dimension

One of the main issues met at the time of the deposit of the thin layer is its aptitude to be covered with the substrates of great dimension while obtaining the very precise results of the mono or multi-layer deposits. Technology plasma cathode sputtering ( sputtering) HiTUS (High Target Use Sputtering /Haute use of the targets of Sputtering ) associated with that of the linear target showed a major improvement in the results anticipated such as the precision, the uniformity, the control of the constraint as well in compression as in tension while passing by the null constraint, and roughness on substrates measuring until and even beyond 50 to 60 cm. The linear target allows the development of a linear process on a broad surface with the same advantages as technology HiTUS for the processes roll-to-roll or in line.

external bonds

'' Techniques of the engineer '', “treatment of metals”, M1654
'' DEVELOPING COUNTRY coatings ''
'' HiTUS - High Target Use Sputtering '']
'' Linear Target Technology ''

{'' Surcotec S.A. - Geneva: Specialist in deposit in thin layers by method sputtering ''

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