We went to our Process Engineer, Nick Franzer, to get his recommendations.
Like all ceramic targets, the Lesker Company recommends that brittle materials be bonded to a copper backing plate. This is typically done after proper metallization of the back side of the target and the bond is made with indium metal solder. In the case of a 3” diameter x 0.125” target the maximum power density for the bonded assembly is 20 watts/in2 which correlates to a maximum power of 140 watts. It is critical that the operating temperature of the target be kept well below the softening temperature of the solder. The power should be slowly ramped up and down on the order of 20 watts/minute or 0.3 watts/sec.
TiO2, like many oxides, tends to loose oxygen during the deposition process. Accordingly O2 needs to be added to the argon process gas stream in order to make stoichiometric films. The amount of oxygen to add to the process is dependent on the specific vacuum chamber base pressure before sputtering. Higher base pressures would typically require less oxygen; alternatively, lower base pressures would typically require more oxygen. For this material Nick recommends a base pressure of 5 x 10-6 Torr or lower. At low base pressures smaller quantities of contaminants, like water, CO, CO2 and other gases found in ambient air get incorporated into the thin film.
Because thin film properties are also effected by system specific issues, such as where the process gas is injecting in the system, throw distance between the target and substrate, and substrate temperature, as a base line Nick suggests that users first deposit a film using only pure argon and analyze that film to determine if oxygen is needed for the process. A good working pressure for this deposition will be in the range of 10-3 Torr.. A lower pressure is better if the sputter cathode can maintain a plasma. Lower pressures imply longer mean free paths, more energy for the adatoms arriving at the substrate and more line-of-sight deposition.
This ceramic is also an insulator so it must be deposited using radio frequency, or RF sputtering. RF deposition is generally a slow process. With a 6” throw distance, using 140 watts of RF power on the target, and a room temperature substrate, the deposition rate could be ~0.2 Angstrom/second. Users will have to empirically determine the deposition rate in their system with the particular process parameters they use.
Figure 1: The effect of substrate bias on compressive stress and phase formation for TiO2 films (Ref 1)
It is also possible to change the phase of TiO2 by adding energy to the substrate. It is common for heat and/or RF bias to be applied to the substrate during deposition of TiO2. The added energy can change the phase of TiO2 film from anatase to rutile. These two phases of TiO2 have very different photocatalytic, band gap, and oxidation characteristics. Other options on phase formation include chamber pressure and O2 input. (Ref. 2)
References:
1. Review of thin film materials deposition by the filtered cathodic vacuum arc process at CSIRO, A. Bendavid* and P.J. Martin, CSIRO Material Science and Engineering, PO Box 218 Lindfield, NSW
2. Sputter deposition of polycrystalline and epitaxial TiO2 films with anatase and rutile structures, Lei Miao, Ping Jin, Kenji Kaneko, Sakae Tanemura, Presented at the 8th International Conference on Electronic Materials (IUMRS-ICEM 2002, Xi’an, China, 10–14 June
Category: Deposition Materials
Sub-Category: Sputtering Targets
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