Figure 39.7: (a) Variations of total system pressure, (b) target volatge with reactive gas in a reactive sputtering system.

(4) Reactive sputtering technique:

Thin films of compounds such as Oxides (Al₂O₃, In₂O₃, SiO₂, Ta₂O₅), Nitrides (TaN, AlN, Si₃N₄), Sulfides (CdS, CuS, ZnS), Carbides (TiC, WC, SiC), and OxyCarbides and Oxynitrides of Ti, Ta, Al, and Si, are deposited on substrate by sputtering from metallic target in the presence of reactive gas, usually mixed with the inert working gas. The process of reactive sputtering is describved in Figure 39.7. Let us now consider the dotted lines: Clearly as Q_i increases, P increases because of the constant pumping speed. When reactive gas introduced: As Q_r increases from Q_r(0), the system pressure essentially same at the initial pressure P₀, because the reactive gas (N2) reacts with target (Ta) and its removed from the gas phase. Beyond critical pressure Q_r, the system pressure jumps to a new value P₁. If no reactive sputtering takes place, then the pressure would be P₃. Once the equilibrium value of P is reached, subsequent changes in Q_r cause P to increase or decrease linearly.

As Q_r decreases sufficiently, P again reaches the initial pressure P₀. This hysteresis behavior represents two stable states "A" and "B". In state "A", the pressure changes very little, while in "B", the pressure changes linearly with reactive gas flow. Clearly all of the reactive gas is incorporated into the deposited film in state "A". A transition from "A" state to "B" state is triggered by compound formation on the metal target. This effect is reflected in the hysteresis of the target voltage with reactive gas flow rate as shown in Figure 39.7b. The sputter rate drops dramatically when compounds form on the metal targets. This is very much dependent on the reactive gas. The variation in the composition and physical properties of reactively sputtered film is possible depending on the operation condition.