Plasma-Enhanced Metalorganic Chemical Vapor deposition of Lipon Thin Films
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Plasma-Enhanced Metalorganic Chemical Vapor deposition of Lipon Thin Films Lamartine Meda Department of Chemistry, Xavier University of LA, 1 Drexel Drive, New Orleans, LA 70125, U.S.A. ABSTRACT Lithium phosphorus oxynitride (Lipon) thin films have been deposited by a plasmaenhanced metalorganic chemical vapor deposition (PE-MOCVD) method using triethyl phosphate [(CH2CH3)3PO4] and lithium tert-butoxide [(LiOC(CH3)3] precursors. Growth rates were between 100 and 415 Å/min, and thicknesses ranged from 1 to 2.5 µm. X-ray powder diffraction showed that the films were amorphous, and X-ray photoelectron spectroscopy revealed approximately 6.9 at.% carbon in the films. The ionic conductivity of Lipon was measured using electrochemical impedance spectroscopy (EIS) and approximately 1.02 µS/cm was obtained, which is consistent with the ionic conductivity of Lipon deposited by radio frequency magnetron sputtering of Li3PO4 targets. An all-solid-state thin-film lithium microbattery such as Li/Lipon/LiCoO2/Au/substrate was successfully fabricated with Lipon deposited by PE-MOCVD. The battery has a capacity of ca. 22 µAh/cm2µm. INTRODUCTION Miniature electronic devices such as smart cards, complementary metal oxide semiconductors (CMOS), implantable medical devices, microelectrochemical systems, and biosensors will benefit from the development of small, light-weight, all-solid-state lithium rechargeable microbatteries [1-3]. The most commonly used solid electrolyte for all-solid-state rechargeable microbatteries is lithium phosphorus oxynitride, also known as Lipon. Lipon was developed by Bates et al. [1] almost two decades ago and is electrochemically stable when in contact with Li/Li+ up to 5.5 volts. This stability window makes Lipon a very attractive option for fabricating high voltage rechargeable microbatteries. Lipon thin films have been primarily deposited by radio frequency (rf) magnetron sputtering of Li3PO4 targets in either mixed Ar-N2 or pure N2 atmospheres; but there is a need for a new method. For example, Qin et al. [4] deposited Lipon in a N2 plasma-assisted deposition of electron-beam reactive evaporation of Li3PO4. One of the challenges with this process is how to controlling the nitridation of the Li3PO4 precursor. Goldner et al. [5] use ion-beam-assisted deposition (IBAD) to deposit Lipon by evaporating Li3PO4 in a N2 environment. However, using this method, thermal residual stresses can cause the films to crack under tensile stress, which leads to cell failure. Lipon thin films have also been deposited by Wadley et al. [6] using a plasma-assisted direct vapor deposition (PA-DVD) approach. This approach produces localized crystalline structure, which decreases the ionic conductivity. In this article, we report the deposition of Lipon thin films by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) from triethyl phosphate (TEP), (CH2CH3)3PO4, and lithium tert-butoxide (LiOBut), LiOC(CH3)3, in mixed N2-H2 atmospheres. The ionic conductivity (σ) of the films was approximately 1.02 µS/cm, which i
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