Reactive evaporation

Reactive evaporation is a process defined by the evaporation of a metal nnder slight pressure from an oxidizing gas, like oxygen or steam, and its subseqnent condensation on an adapted stand. This process allows us to obtain oxide films directly. 

Because of their porous character, the films obtained do not necessarily belong to the class of film considered compact. The choice of such a reactive process stems from the fact that direct evaporation or sublimation emerges as an important technological problem. 

This material is very oxidizing at high temperatures. It turns out to be extremely aggressive towards the cracible, which is generally made using metallic filaments. 

An intermediate solution consists of evaporating the tin and then oxidizing the obtained metallic film. 

In fact, after oxidizing, this process yields irregular films with veiy poor adhesion to the support. 

The three principal PVD subprocesses, evaporation, sputtering, and ion plating, are described belowl lPl 

To evaporate the source material, various heating methods are used such as resistance heating, electron beam, pulsed excimer laser, or cathodic arc (where the source is the cathode). 

Compounds such as the refractory carbides, nitrides, and oxides have extremely high boiling points and generally dissoci- 

As with CVD, the reaction must have a negative free energy of formation (-AG°) in order to proceed. This is usually the case as shown by the following typical reactions for the deposition of TiC and TiN (at 298 K)  

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In direction evaporation, the evaporant is the refractory compound itself, whereas in reactive or activated reactive evaporation (ARE), a metal or a low valency metal compound is evaporated in the presence of a partial pressure of a reactive gas to form a compound deposit, eg, Ti is evaporated in the presence of N2 to form TiN, or Si or SiO is evaporated in the presence of O2 to form Si02. 

Reactive Evaporation. In reactive evaporation (RE), metal or alloy vapors are produced in the presence of a partial pressure of reactive gas to form a compound either in the gas phase or on the substrate as a result of a reaction between the metal vapor and the gas atoms ... 

In some cases, the deposition rate can be increased by the action of a plasma in a process known as activated reactive evaporation (ARE). PI The plasma enhances the reactions and modifies the growth kinetics of the deposit. 

Like reactive evaporation, reactive sputtering is used in the deposition of refractory compounds by providing a small partial pressure of hydrocarbons, nitrogen, or oxygen. A problem is target poisoning caused by the reaction of the target with the reactive gas. 

Coatings of refractory compounds can be deposited reactively in a manner similar to reactive evaporation and sputtering by spraying the pure metal in an atmosphere of either a hydrocarbon or nitrogen. 

Mattox, D., Vacuum Deposition, Reactive Evaporation and Gas Evaporation, m ASM Handbook, Surface Engineering, 5 556-572, ASMPubl. (1994)... 

O. Marcovitch, Z. Klein, and I. Lubezky, Transparent conductive indium oxide films deposited on low temperature substrates by activated reactive evaporation, Appl. Opt., 28 2792-2795 (1989). 

Reactive Sputtering. Reactive sputtering is similar to reactive evaporation and reactive-ion plating in that at least one coating species enters the system in the gas phase. Examples include sputtering Al in 02 to form A O Ti in 02 to form Ti02, In—Sn in 02 to form tin-doped ln203, Nb in N2 to form NbN, Cd in H2S to form CdS, In in PH3 to form InP, and Pb—Nb—Zr—Fe—Bi—La in 02 to form a ferroelectric oxide. 

This supplemental activation of the gas mixture is established by placing a positive electrode in the gas volume. This technique is called Activated Reactive Evaporation (ARE) and is illustrated in figure 13.7. 

Figure 13.7 Activated reactive evaporation (G= gas supply S= substrate V= source TE= positive electrode P= pumps).

Rare earth oxides, sueh as EuO, can be prepared by reactive evaporation, with the same capability to control the oxidation state as above. ... 

Films of IVA-VIA compounds have been prepared by the aqueous reactions of group IV nitrates with thio- or selenourea, in basic solution. More recently, bulk crystals, especially of the alloys, have been made by direct reaction. Control of stoichiometry is always difficult. At present, molecular beam epitaxy (precise evaporation of the elements) has become preeminent, because alloys of PbTe with both SnTe and EuTe can be made. It is surprising that a rare earth atom can be substituted into such a lattice, and even more surprising that its electronic behavior appears to be that of a substituent with a valence of + 2. Sn02, while differing widely from the lead salts , is also a IV-VI compound that can be prepared as films by spray pyrolysis of the chloride, or by reactive evaporation or sputtering. 

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