Material Integration for Flexible Electronics for Energy Application Using Nanoparticulated Dense and Stress-Free Ceramic Thick Film (2016)

Recently, the miniaturization and multifunction capabilities of mobile terminals such as smartphones, tablet PCs demand thin electrical element modules. Accordingly, passive integration, including energy-generation devices using thermoelectricity [1] and energy storage such as solid-state lithium ion batteries [2] in a single-module package, is of demand [3]. This demand has expanded to include the R&D of flexible and stretchable electronics [4, 5]. Passive components may consist of ceramics such as BaTiO3, Al2O3, ZnO, Li compounds, and so on, and metal for wiring. Because ceramics generally require firing at temperatures greater than 1000 °C, it is difficult to incorporate ceramics with low-resistivity and low-melting-point metals, such as Cu with a melting point of 1083 °C, and to embed them in a resin package or sheet.In this study, we demonstrate that dense, thick, and organic-binder-free ceramic green-state film made of stress-free nanoparticles can be obtained at room temperature by making use of the unstable and high-surface energy amorphous phase on the surface of the particles produced in the deposition process for bonding the nanoparticles and propose the process for obtaining multilayer ceramic structure at room temperature.Previously, we reported that dense BaTiO3 films with a relatively high dielectric constant on Cu films of printed wiring boards (PWB) can be deposited using aerosol deposition at room temperature and can be also passively embedded [6]. Aerosol deposition is carried out using room-temperature impact consolidation (RTIC), which converts the kinetic energy of the particles into bonding energy between the substrate and the fine particles, and between the fine particles themselves during collision of the fine particles with the substrate. Therefore, the crystal structure of the particles hitting the substrate is destroyed and large amounts of residual stress are introduced in the as-deposited film. Because of the large amounts of internal stress in the as-deposited film, obtaining the inherent film properties is difficult [7]; furthermore, macrocracks can form in thick films and annealing is required to release the stress. By revising the abovementioned deposition method, we succeeded in forming a multilayered structure consisting of nanoparticulated dense and stress-free ceramic thick film on Cu foil at room temperature [8]. We also verified that the ceramic film can be densified at low temperatures, and coannealed and integrated with different ceramic films and Cu wiring [9].Figure 1 shows our deposition method; that is, an aerosol-type nanoparticle deposition (NPD) system for ceramic films. Only aerosol composed of dry ceramic powder and carrier gas is transferred to the nozzle in the deposition camber, which is evacuated with a rotary pump. The film is deposited by spraying the injected aerosol from the nozzle. No heater is needed in this system and the deposition process takes place at room temperature.The particle size of the original raw ceramic material powder is approximately 100 nm. However, because only particles floated by gas are carried to the nozzle in the deposition system, particles reaching the nozzle are smaller than the original powder. Subsequently, the transferred particles collide inside the nozzle and smaller particles are produced. In this process, the high-surface cleavage surfaces in each particle can be exposed.Figure 2(a) shows the cross-sectional HAADF–STEM image of the deposited 4 μm BaTiO3 film. The dense and close-packed particulated ceramic film can be obtained only by spraying the particles on the foil. The as-deposited film is polycrystalline, and is unlikely amorphous as observed in the sputtering and CSD processes. As described below, because the as-deposited film has excellent properties, it can find use in several applications. In the magnified image of Fig. 2(a), grains around 40 nm in a black matrix are observed as shown in Fig. 2(b)-1. The magnified image of the interior of the 40 nm grain in Fig. 2(b)-2 and Fig. 2(c)-1 has a defect-free periodical atomic order structure. Figure 2(c)-2 is the fast Fourier transform (FFT) image of the 40 nm grain in the as-deposited film. The observed zone axis is [001]. The indexed data of every spot do not match the cubic BaTiO3 and tetragonal BaTiO3 JCPDS data. The crystal seems to be pseudocubic between the cubic and tetragonal structure. This is consistent with earlier work showing that BaTiO3 particles with several tenth nm diameter have a pseudocubic structure while maintaining a tetragonal structure [10]. The magnified image of the black matrix in Fig. 2(d) shows the aggregate texture consisting of a 5 nm grain with a 1–2 nm disrupted crystal layer, which is amorphous. We assume that the aggregation originating from this amorphous-like disrupted layer is created by the particles bombardment and promotes the densification of the as-deposited film.As-deposited film based on the hard X-ray analysis (X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), hard X-ray photoemission spectroscopy (HAXPES)) by synchrotron radiation is thus summarized as follows:The As-deposited film contained an amorphous phase, which consisted of 2 nm crystalline elements scattered on the interface of 10-nm-class particles with an excellent crystalline property, as shown in the TEM observation (Fig. 2).Figure 3 represents a micro-structure of the As-deposited film, constructed based on the above findings. Highly crystalline particles of approximately 10 nm in size are randomly arranged, with crystals having a diameter of approximately 2 nm in a disorder-like state forming grain boundaries and bonding the particles. This is a minutely fine structure of a unique configuration. The grain boundary is composed of the disordered phase in which there are many crystals of lattice constants disordered mainly by displaced Ba atoms as a result of the structural relaxation in relation to BaO and BaTiO3 molecules that are formed due to distorted Ba lattices.Because the as-deposited BaTiO3 NPD film is dense, strain- and organic-binder-free, wet photolithography processes such as electric plating and acid–alkali process can be applied to the film before annealing despite the green-state. The NPD film on the polyimide/Cu sheet can be bent easily and is suitable for flexible electronics applications as shown in Fig. 4.Figure 5 shows our developed typical manufacturing flow for a multilayered structure with material integration on a Cu foil. First, a ceramic film is deposited on the Cu foil using NPD. The via hole on the as-deposited film can be shaped by chemical etching on the patterned resist using HF/HNO3, dry etching using reactive ion etching (RIE), etc. The via conductor can be filled using Cu plating, Cu sputtering, and so on. The Cu internal electrode is formed by wet etching a Cu sputter film designed using patterned resist, after the Cu blanket sputter deposition on the as-deposited film formed the via hole. These processes deposit different ceramic films and form via and internal electrodes, and are repeated to attain and complete the multilayered structure. The multilayered ceramic structure can be coannealed at around 900–1000 °C in nitrogen atmosphere, as one of the option.This methodology can be expanded to multilayered solid-state lithium ion batteries consisting of ceramic films for anodes, solid electrolytes, cathodes, and metal electrodes with serial and parallel connections. Because the green-state ceramic film produced at room temperature has apparently excellent electrical properties, the electrical circuit with passive ceramic elements can be constructed on polymer sheets for flexible electronics applications.The aerosol-type nanoparticle deposition (NPD) equipment consist of an aerosol generation system, a deposition chamber, and a vacuum system. No heat source is used. More than 99.9% pure raw powder BaTiO3 (specific gravity: 5.85; Young's modulus: 280 GPa) with an average particle size of 200 nm was placed in the vessel of the aerosol generation system and vibrated at 10 Hz. Then, 99.9% pure oxygen at a gas pressure of 0.2 MPa and a flow rate of 5l min−1 was introduced into the vessel and transferred into the aerosol by the nozzle in the deposition chamber. The in-chamber base pressure was controlled to less than 10 Pa using a mechanical booster and a vacuum pump. The substrate in the chamber was bombarded for 10 min with aerosol particles by means of the nozzle at a speed of 50 m s−1. Thus, the NPD film was deposited at room temperature with a thickness is 4 μm.This research was partially supported by New Energy and Industrial Technology Developement Organization (NEDO) projects for “Innovation Promotion Program” in Japan.