Metal oxide films can be sputtered from either an elemental metallic (M) target or from a metal oxide (MOx) target. However, as one might expect, the deposition parameters are significantly different depending on the starting material composition. However, it is certainly possible to produce high quality stoichiometrically correct metal oxide resultant films either way.
When sputtering with a pure M target it is necessary to add a certain partial pressure of oxygen in with the argon working gas to initiate a reactive deposition. This will require a second channel in the mass flow controller associated with the deposition system. One channel would be used for argon and one for oxygen. For a reactive deposition to produce stoichiometric MOx films from an M target, a certain percentage of the emitted atoms of M collide and react with the added oxygen molecules in the plasma to form MOx within the mean-free-path between the target source and the substrate. The more collisions that occur the higher the oxygen content will be in the resultant films. It will be necessary to strike a balance between the amount of oxygen being added into the system and the rate of M atoms being ejected from the target. This is essentially a balance between the power density applied to the cathode assembly and the flow of oxygen being added through the mass flow controller. The actual amount of oxygen to be added is heavily dependent on the deposition parameters and the system configuration. If the ratio of power to oxygen content is to high, the resultant films will be metal rich or oxygen depleted, too low and they will be oxygen rich. Achieving the desired film characteristics in the resultant films may be a bit of a trial and error process at first but measuring and plotting the physical characteristics of the films after deposition can be a guide as to which directions to vary power and gas flow levels. Measurements of such properties as resistivity, band gap, density, refractive index, etc. can all be used to assist in determining the proper deposition parameters for a given system. This will depend a lot on the system characteristics associated with a specific deposition tool. There also may be a limitation as to what the practical ramifications are if one is looking to put a particular process into a full scale production mode. The reactive deposition rate from an elemental M target may be too slow as to be economically feasible. Most M targets can be sputtered with either a DC power supply or an rf generator.
Typically MOx materials are not sufficiently conductive enough to deposit from a DC power supply. Therefore it is necessary to utilize an rf generator with an associated matching network. Make certain that the reflected power is always kept at absolute zero during the deposition process so as to not overheat the target or cause arcing in the plasma. During the deposition process some of the oxygen from the MOx sputtering target will be disassociated. As the impinging ions strike the MOx molecules on the target surface, some of the MOx bonds will be broken along with the MOx-MOx bonds. Some of this free oxygen will be pumped away in the vacuum chamber. This will result in oxygen depleted resultant films. To avoid this it will also be necessary to add a certain amount of oxygen into the argon plasma to form a reactive deposition as described above – although a lot less oxygen is likely to be required than when sputtering with a pure M target to produce MOx films. Essentially starting with an MOx target should make it possible to run at higher deposition rates than what can be achieved with an M target and thus increase the product throughput accordingly.
