Microelectronically fabricated LiCoO2∕SiO2/polycrystalline-silicon power cells planarized by chemical mechanical polishing

Abstract

Effects of manganese oxide-mixed abrasive slurry on the tetraethyl orthosilicate oxide chemical mechanical polishing for planarization of interlayer dielectric film in the multilevel interconnection J. Vac. Sci. Technol. A 26, 996 (2008); 10.1116/1.2936225 Controlled chemical mechanical polishing of polysilicon and silicon dioxide for single-electron device Monolithic integration of different electronic circuit elements onto a single chip is beneficial for the purpose of reducing overall system cost as well as improving performance and reliability. Integrating a power unit onto a silicon chip requires the implementation of a thin-film solid-state battery that is compatible with existing silicon integrated circuit technology in terms of manufacturing methods, materials, and performance. One of the materials in the battery industry, studied by us, is silicon dioxide ͑SiO 2 ͒. The high level of process control for thin SiO 2 layers which is a result of microelectronic advances, its insulating properties, and its past reported permeability to light ions have motivated us to exploit SiO 2 as a solid-state electrolyte in our integrated thin-film battery. This SiO 2 electrolyte layer is thermally grown from an amorphous silicon ͑a-Si͒ thin layer deposited on doped polycrystalline silicon ͑polysilicon͒ which serves as the anode against a thin-film-sputtered LiCoO 2 cathode. We have fabricated and characterized ultrathin solid-state thin-film power cells consisting of LiCoO 2 , SiO 2 , and polysilicon. Cells containing an ultrathin SiO 2 lithium-free electrolyte with a thickness range of 7 -40 nm and active-area sizes of 5 ϫ 5, 2 ϫ 2, 1 ϫ 1, and 0.5ϫ 0.5 mm 2 were created using established microelectronics processing and expertise. Ultrathin cells require smooth surfaces ͑nanometer-scale roughness͒ as opposed to higher roughness encountered in bulk batteries and microbatteries. To ensure a very thin and flat electrolyte, this work demonstrates the implementation of a planarization step by using chemical mechanical polishing ͑CMP͒ in the fabrication of the integrated solid-state thin-film lithium-ion battery. Polishing the polysilicon layer reduced its 1 ϫ 1 m 2 root-mean-square roughness from 8.06 to 0.53 nm and led to smoother interfaces and to higher quality of the SiO 2 grown on top of it. The cells were charged and discharged using conventional microelectronic electrical testing equipment and exhibited improved performance when prepared with the additional CMP planarization step. Up to 40% of the charge was retrieved from the planarized cells compared to a maximum of 14.5% retrieved from the nonplanarized cells. The open circuit voltage ͑V OC ͒ of the LiCoO 2 / SiO 2 /polysilicon cell was estimated by comparing the initial charge voltage values obtained with different electrolyte thicknesses and was found to be 2.19± 0.02 V. The results presented in this work show the importance of interfacial quality in the process of moving to integrated power on a chip.

Ionic Transport Across Interfaces of Solid Glass and Polymer Electrolytes for Lithium Ion Batteries

“…The silica layer is an effective barrier to Li þ transport; previous studies have shown the Li þ conductivity of SiO 2 to be approximately 2.3 Â 10 À12 S cm À1 . 17 Moreover, the electrical resistances of the silica films were well above the resolution of the Solartron 1260; thus measurement of charge transport was limited to the intersecting electrode areas. Strips of aluminum foil were permanently attached to the electrodes with Ag-filled epoxy; extreme care was taken to avoid contact of the epoxy with the electrolyte materials.…”

Lithium Transport through Nanosized Amorphous Silicon Layers

“…Lithium diffusion in electrode and electrolyte materials is a key process for the performance of lithium-ion batteries, important for charging and discharging rates, power density, self-discharge, and cycling stability. − A promising negative electrode material is silicon due to its high theoretical capacity to store Li ions of about 4000 mAh/g. However, silicon cannot be used properly in its bulk crystalline form due to the extreme structural degradation that occurs during operation. − This problem might be solved using nanostructured silicon, such as thin films, nanowires, nanotubes, or nanoparticles, which are more resistant to structural degradation by virtue of their small size. − Experimental research has shown that fracture is minimized when the silicon material has dimensions in the nanometer range. − ,− The high surface-to-volume ratio of silicon nanostructures causes a better accommodation of the volume expansion/contraction during lithiation/delithiation cycles. In addition to strain accommodation, a decrease of the lateral dimensions of silicon electrode material gives also a more effective electrical contact and has additional benefits for enhanced mass transport due to shorter diffusion distances.…”

“…Recently, excellent performance of Li batteries was achieved using negative electrode materials made of amorphous silicon with dimensions in the nanometer range. Often electrode dimensions of less than 50 nm in the confined direction are used. − ,− …”

“…This is especially the case for on-chip integrated lithium ion-power microcells. Recently, it was shown that an energy supply unit can be integrated on silicon integrated circuits (IC) in the form of thin film solid-state batteries, using silicon IC fabrication methods. , Nanometer sized (e.g., 9 nm ultrathin) SiO 2 and Si layers were successfully used as electrolyte and negative electrode.…”