P. Polcik¹, M. Wolff¹, M. Eusterholz¹, C. Linke², D. Beisenherz³, S. Hübner³, N. Salvadores⁴, H. Riedl^4
1Plansee Composite Materials GmbH, Lechbruck, Germany
²Plansee SE, Reutte, Austria
³Singulus Technologies AG, Kahl am Main, Germany
⁴TU Wien, Wien, Austria
Target materials made from lithium compounds, such as lithium phosphate (Li₃PO₄), are used in PVD systems for the deposition of lithium-ion electrolyte layers. These layers are utilized in the production of thin-film batteries, where the deposited lithium-ion electrolyte has very low electrical conductivity and is ideally electrically insulating since its effectiveness as an electrolyte depends on its ionic conductivity. Typically, the deposition of such lithium-ion electrolytes is carried out using a reactive RF/HF sputtering process. For example, when sputtering a lithium phosphate target with additions of nitrogen and/or oxygen during the deposition process, nitrogen-containing LiPON layers characterized by high ionic conductivity, are formed.
The low electrical conductivity of traditional target materials made from lithium compounds significantly restricts the use of DC and pulsed DC sputtering techniques. This limitation, in turn, affects the deposition rates of lithium-ion electrolyte films, as fully insulating target materials cannot be sputtered in power-intense processes.
In the present work, carbon enriched electrically conductive lithium phosphate targets were tested using DC sputtering for the deposition of lithium-ion electrolyte layers. The carbon in the targets forms a separate component within the microstructure and results in a significantly increased electrical as well as thermal conductivity of the target material. The carbon content was chosen to be in the range of 3 to 7 wt% to achieve the optimal combination between capable high-power input and deposition rate. Due to the increased electrical and thermal conductivity, compared to pure Li₃PO₄ targets, higher average power densities are possible during DC sputtering. The generated heat within the target material can be dissipated more quickly and the temperature is reduced. Consequently, higher deposition rates compared to RF sputtered Li₃PO₄ target materials are achieved.
The lithium-ion electrolyte films deposited with the carbon-enriched targets obtain very low electrical conductivity compared to the used targets. The carbon present in the target materials must essentially not be incorporated into the grown LiPON film. The released carbon is bound by the oxygen present in the residual gases of the deposition chamber forming CO or CO₂. In addition, the carbon content can be further reduced by an optional reactive sputtering step which ensures that the function of the grown lithium-ion electrolyte film is not impaired by the carbon originating from the target material.
For sputtering in DC mode, the microstructure of the LiPO/C target materials plays a very important role. The homogeneity of the graphite powder distribution, which serves as the carbon source, is crucial for the stability of the coating process. For this reason, both the particle size distribution of the powders used for target manufacturing and the powder mixing technology are decisive factors. Poor mixing of the raw materials leads to electrically insulating areas, which in turn can cause arc discharges and destruction of the target during the coating process. In the present study, different versions of the targets were tested in successive generations of material development within the DCMS process and evaluated for application in industrial coating systems.