Figure 2 shows the variation of deposition rate under different reaction pressures. The process parameters are: flow ratio of O2 and Ar is 3/20, deposition temperature is 200°C, and sputtering power is 140 W. It can be seen that the deposition rate increases first and then decreases with increasing of reaction gas pressure, and there is a maximum deposition rate corresponding to an optimal reaction gas pressure. The analysis is as follows: On the one hand, as the reaction gas pressure is located in a lower working range, as it rises, the density of gas molecules increases, and the probability of collision between electrons and gas molecules increases, so that the energy of electrons can be more fully exchanged among gas molecules, and the energy of gas sputtering particles increases accordingly. On the other hand, the increasing of the reaction gas pressure help increase the ionization degree and decrease the plasma resistance, also, the discharge increases as well as the ion current, and energy of the accelerated initial electrons that pass the cathode dark space is decreased, so the electron capture efficiency of the cathode to electron increase and then the deposition rate increases. However, when the reaction gas pressure is in a higher working range, the deposition rate will decrease with the increase of the reaction gas pressure. Because the mean free path of atoms and ions in the glow discharge drops, and the probability of back-reflection and the probability of scattering by gas atoms (Ar) of sputtered target atoms increases, causing the particle energy decreasing, and the particles escape from the deposition area and return to the surface of the cathode after multiple collisions, which reducing the collection efficiency of the substrate to sputter atoms. This leads to a decrease of the deposition rate. From Fig.2 we can see that the optimum reaction pressure in this experiment is about 0.7 Pa.
Fig.2 The relationship between reaction pressure and deposition rate
Fig.2.1 The relationship between O2 flow rate and deposition rate under different reaction pressure
On the curve of Figure 2, two points 0.5 Pa and 0.9 Pa are chose and plotted as Figure 2.1, and the other process parameters remain the same. It can be found that as the pressure increases, the two inflection points move in the direction of high flow, the proportion of metal patterns increases, and the phenomenon of target poisoning is alleviated, but the deposition rate is still reduced rapidly.