Atomic Layer Deposition of the Solid Electrolyte LiPON

Abstract

We demonstrate an atomic layer deposition (ALD) process for the solid electrolyte lithium phosphorousoxynitride (LiPON) using lithium tert-butoxide (LiO t Bu), H 2 O, trimethylphosphate (TMP), and plasma N 2 ( P N 2 ) as precursors. We use in-situ spectroscopic ellipsometry to determine growth rates for process optimization to design a rational, quaternary precursor ALD process where only certain substrate−precursor chemical reactions are favorable. We demonstrate via in-situ XPS tunable nitrogen incorporation into the films by variation of the P N 2 dose and find that ALD films over approximately 4.5% nitrogen are amorphous, whereas LiPON ALD films with less than 4.5% nitrogen are polycrystalline. Finally, we characterize the ionic conductivity of the ALD films as a function of nitrogen content and demonstrate their functionality on a model battery electrodea Si anode on a Cu current collector. W hile planar thin film solid-state microbatteries are in commercial production, the push for higher energy and power density necessitates development of 3D device geometries, realized by improvements in device fabrication processes. 1,2 Moving from planar layer structures to high aspect ratio 3D electrode structures holds promise for significant power enhancement without much loss of energy density, or alternatively a tunable optimization and trade-off between power and energy to fit the application.Since its discovery in the early 1990s, 3 LiPON (lithium phosphorus oxynitride) has been one of the most popular solid state electrolytes used for planar lithium ion microbatteries. LiPON thin films are commonly deposited using reactive sputtering of a Li 3 PO 4 target in an N 2 atmosphere. 4−8 Generally, sputtered LiPON films are ∼1 μm thick, but sputtering of much thinner LiPON films (12 nm) has recently been demonstrated. 9 As a physical deposition technique, sputtering is generally unable to deposit high quality films on 3D geometries. 10 Also, the low reactivity of the N 2 gas during the sputtering process makes it difficult to dope these films with >2% N.Highly tunable N doping of LiPON is possible through ebeam evaporation of Li 3 PO 4 coupled with a N 2 plasma discharge above the substrate; 11 however, this technique is also limited to planar substrates.More recently, Kim et al. developed a MOCVD process for LiPON, 12 but the high deposition temperatures reported (500°C ) are undesirable for coprocessing with many battery materials and packaging components, precluding deposition on materials such as (i) Li 2 CoO 3 cathodes without degradation during the deposition process or (ii) metallic Li metal anodes without melting them.ALD has emerged as the premier deposition process for fabrication of uniform, thin, conformal films on high aspect ratio scaffolds, 13−16 and ALD has previously been used to fabricate the solid electrolytes (Utilizing a unique integrated high-vacuum deposition, surface characterization, and battery assembly system, 22 we have developed a quaternary ALD process for the solid electrolyte LiPON. We ...

Metal Fluorides as Lithium-Ion Battery Materials: An Atomic Layer Deposition Perspective

“…Recently, the deposition of LiPON was achieved both by thermal and plasma-enhanced ALD [108,109,130]. In the PEALD process, LiO t Bu was used as the lithium source combined with a pulsing sequence of water, TMPO and nitrogen plasma [109].…”

“…In the PEALD process, LiO t Bu was used as the lithium source combined with a pulsing sequence of water, TMPO and nitrogen plasma [109]. Deposition of Li 2 O/LiOH before exposure to TMPO resulted in less carbon impurities as compared to the process used by Hämäläinen et al for Li 3 PO 4 [114].…”

“…By using DEPA with LiO t Bu, nitrogen contents as high as 9.7 at.% were achieved. However, the thermal ALD process led to high carbon impurities from 9.9 to 13.3 at.% compared to virtually none in the PEALD process [109]. Both processes deposited conformal coatings on demanding substrates as required from a potential solid electrolyte material.…”

“…Many materials, both crystalline and amorphous, have been deposited and ionic conductivities of the order of 10 −7 S/cm have been obtained with various materials. Of the traditional solid electrolytes, LiPON has been deposited using both ALD and PEALD [108,109]. Table 6.…”

Plasma - Assisted ALD of Lipo(N) for Solid State Batteries

“…Crystalline Li 3 PO 4 on the other hand only provides a conductivity of 10 -18 S/cm. Recently, ALD depositions of both Li 3 PO 4 and LiPON have been reported in literature (6)(7)(8)(9).…”