(A) study on the low-temperature crystallization of $LiCoO_2$ and Carbon thin films for the application of microbatteries

Abstract

Power source can be classified into three classes with respect to their capacity : (a) high-power batteries with a capacity of over 50Ah, for locomotive, submarines, or electric vehicles applications, (b) miniature batteries with a capacity range of 0.2 to 2Ah for telephone, computer, and other widely used products, (c) microbatteries with a typical capacity of 20μAh. During the past few decades, thin-film based lithium microbatteries, in particular, have been actively investigated not only for the viable applications to microelectronics, but also for the academic interests to know $Li^+$ ion diffusivity in various electrode materials or electrode/electrolyte interfacial phenomena. For the electrodes of the thin-film microbatteries, carbonaceous materials and lithium transition metal oxides, e. g., $LiCoO_2$, $LiNiO_2$, $LiMn_2O_4$, and $LiV_2O_5$, that are in principle same materials as those used for bulk form batteries, are required to have the form of thin film. Generally, thin-film fabrication technology of semiconductor industry, such as sputtering, chemical vapor deposition (CVD), and pulsed laser deposition, can be used for that purpose. The fabricated thin films in as-deposited state, however, do not satisfy the specific structural framework (layered or spinel structure) essential for the reversible intercalation/deintercalation reaction of $Li^{+}$ ion. Thus, the conventional post-deposition heat-treatments at about 800℃ should be followed like for the bulk batteries. However, the processing temperature is too high to apply to the thin-film microbatteries for microelectronics. In the current work, as alternatives for the low-temperature crystallization of $LiCoO_2$ thin films prepared by radio frequency (rf) reactive magnetron sputtering, two different plasma treatments, microwave and rf plasma irradiation, are introduced for the first time. The films deposited at 350℃ show (003) preferred orientation corresponding to a hexagonal layered structure. ...