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The Materials Science of Thin Films 2011 Part 4

Tham khảo tài liệu 'the materials science of thin films 2011 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 128 Physical Vapor Deposition sputtering of Ta. Now consider what happens when reactive N2 gas is introduced into the system. As Qr increases from Qr 0 the system pressure essentially remains at the initial value Po because N2 reacts with Ta and is removed from the gas phase. But beyond a critical flow rate Q the system pressure jumps to the new value p . If no reactive sputtering took place p would be somewhat higher i.e. P3 . Once the equilibrium value of p is established subsequent changes in Qr cause p to increase or decrease linearly as shown. As Qr decreases sufficiently p again reaches the initial pressure. The hysteresis behavior represents two stable states of the system with a rapid transition between them. In state A there is little change in pressure while for state B the pressure varies linearly with Qr. Clearly all of the reactive gas is incorporated into the deposited film in state A the doped metal and the atomic ratio of reactive gas dopant to sputtered metal increases with Qr. The transition from state A to state B is triggered by compound formation on the metal target. Since ion-induced secondary electron emission is usually much higher for compounds than for metals Ohm s law suggests that the plasma impedance is effectively lower in state B than in state A. This effect is reflected in the hysteresis of the target voltage with reactive gas flow rate as schematically depicted in Fig. 3-22b. The choice of whether to employ compound targets and sputter directly or sputter reactively is not always clear. If reactive sputtering is selected then there is the option of using simple de diode RF or magnetron configurations. Many considerations go into making these choices and we will address some of them in turn. 3.7.4.1. Target Purity. It is easier to manufacture high-purity metal targets than to make high-purity compound targets. Since hot pressed and sintered compound powders cannot be consolidated to theoretical bulk densities incorporation of gases .